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Hydrogeology Journal

, Volume 27, Issue 3, pp 857–883 | Cite as

Hydrogeological characteristics of the Omaruru Delta Aquifer System in Namibia

  • Brian Matengu
  • Yongxin XuEmail author
  • Eric Tordiffe
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Abstract

Sustainable utilization of groundwater in parts of hyper-arid Sub-Saharan Africa, like the Namib Desert, is always a challenge due to lack of resources and data. For the Omdel Aquifer in the Omaruru catchment, Namibia, issues to investigate include the lack of information on the geology and hydrogeological setting, the hydraulic properties and geometry of the aquifer at the inflow and outflow sections, groundwater recharge conditions upstream of the aquifer, and the impact of artificial recharge. In this desert environment, the methods applied are hydrogeological surveys and site visits, together with interpretation of geological, hydrological and geomorphological data from investigations carried out to define the hydrogeological characteristics of the Omdel Aquifer. The bedrock geometry of the aquifer indicates that the Main channel (one of four palaeochannels) is the largest reservoir of stored fresh groundwater, estimated at 133 Mm3, and it is deeper than the other three channels, with an average sediment thickness of 80 m. All groundwater chemistry facies of the selected boreholes tapping the Omdel Aquifer reveal a NaCl character, indicating a coastal environment. The yield of the Omdel Aquifer is estimated to have increased from 2.8 Mm3/year before construction of a recharge enhancement dam to 4.6 Mm3/year after the construction. This paper focuses on the understanding of hydrogeological characteristics of the Omaruru Delta Aquifer System in terms of groundwater recharge and discharge, groundwater dynamics within the aquifer and groundwater chemistry, in order to assess whether the current abstractions are operating within the hydrogeological limits of sustainability.

Keywords

Hydrogeological characteristics Artificial recharge Coastal aquifer Sub-Saharan Africa Namibia 

Caractéristiques hydrogéologiques du système aquifère du delta d’Omaruru en Namibie

Résumé

L’utilisation durable des eaux souterraines dans les régions de l’Afrique Sub-Saharienne hyper-aride, comme le désert du Namib, est toujours un défi dû au manque de ressources et de données. Pour l’aquifère d’Omdel dans le bassin d’Omaruru, en Namibie, les sujets à étudier incluent l’absence d’information sur le contexte géologique et hydrogéologique, les propriétés hydrauliques et géométriques de la couche aquifère à l’entrée et à la sortie du système, les conditions de recharge des eaux souterraines en amont de l’aquifère, et l’impact de la recharge artificielle. Dans cet environnement désertique, les méthodes appliquées sont des campagnes hydrogéologiques et des visites de terrain, ainsi que l’interprétation des données géologiques, hydrologiques et géomorphologiques à partir des investigations effectuées pour définir les caractéristiques hydrogéologiques de l’aquifère d’Omdel. La géométrie du substratum de l’aquifère indique que le chenal principal (l’un des quatre paléochenaux) est le plus grand réservoir d’eaux douces souterraines, estimé à 133 Millions de m3, et plus profond que les trois autres chenaux, avec une épaisseur moyenne de sédiments de 80 m. Tous les faciès chimiques des eaux souterraines des forages sélectionnés captant l’aquifère d’Omdel traduisent un caractère chloruré sodique indiquant un environnement côtier. On estime que le rendement de l’aquifère d’Omdel a augmenté de 2.8Mm3/an avant la construction d’un barrage pour améliorer la recharge à 4.6 Mm3/an après sa construction. Cet article se concentre sur la compréhension des caractéristiques hydrogéologiques du système aquifère du delta d’Omaruru en termes de recharge et décharge d’eaux souterraines, la dynamique des eaux souterraines dans l’aquifère et la chimie des eaux souterraines, afin d’évaluer si les conditions d’exploitation actuelles opèrent dans les limites hydrogéologiques de durabilité.

Características hidrogeológicas del sistema de acuíferos del delta de Omaruru en Namibia

Resumen

La utilización sostenible del agua subterránea en zonas hiperáridas del África subsahariana, como el desierto de Namib, es siempre un desafío debido a la falta de recursos y de datos. Para el acuífero Omdel en la cuenca de Omaruru, Namibia, los problemas que se investigan incluyen la falta de información sobre el ambiente geológico e hidrogeológico, las propiedades hidráulicas y la geometría del acuífero en las secciones de entrada y salida, las condiciones de recarga del agua subterránea aguas arriba del acuífero y el impacto de la recarga artificial. En este ambiente desértico, los métodos aplicados son los estudios hidrogeológicos y los relevamientos en los sitios, junto con la interpretación de los datos geológicos, hidrológicos y geomorfológicos de las investigaciones realizadas para definir las características hidrogeológicas del acuífero Omdel. La geometría del basamento del acuífero indica que el canal Principal (uno de los cuatro paleocanales) es el reservorio más grande de agua subterránea dulce almacenada, que se estima en 133 Mm3, y es más profundo que los otros tres canales, con un espesor de sedimento promedio de 80 m. Todas las facies de química del agua subterránea de los pozos seleccionados que alcanzan el Acuífero Omdel revelan un carácter de NaCl, lo cual indica un ambiente costero. Se estima que el rendimiento del Acuífero Omdel aumentó la recarga de 2.8 Mm3/año antes de la construcción de una represa a 4.6 Mm3/año después de la construcción. Este trabajo se enfoca en la comprensión de las características hidrogeológicas del Sistema de Acuíferos del Delta de Omaruru en términos de recarga y descarga, dinámica del acuífero y química del agua subterránea, para evaluar si las extracciones actuales están operando dentro de los límites hidrogeológicos de la sostenibilidad.

纳米比亚奥马鲁鲁三角洲含水层系统的水文地质特征

摘要

由于缺乏资源和数据,过度干旱的撒哈拉以南非洲部分地区如纳米布沙漠地下水的可持续利用一直是一个挑战。对于纳米比亚奥马鲁鲁流域的Omdel含水层,调查中的问题包括缺乏地质和水文地质背景信息、流入和流出剖面处含水层的水力特性及几何结构、含水层上游的地下水补给条件以及人工补给的影响。在这个沙漠环境中,应用的方法为水文地质调查、现场查看、以及进行调查中获取的地质、水文和地貌数据的解译,以确定Omdel含水层的水文地质特征。含水层的基岩几何结构表明,主要通道(四个古通道之一)是最大的储存地下淡水的储存地,该储存地估计为133 Mm3,这个通道比其它三个通道都要深,平均沉积厚度为80 m。所选择的Omdel含水层钻孔所有地下水化学相都显示为NaCl,表明这里是为沿海环境。Omdel含水层出水量估计从增大补给大坝建设前的2.8 Mm3/年增加到建设后的4.6 Mm3/年。本文重点就是了解奥马鲁鲁三角洲含水层系统地下水补给和排泄、含水层之内的动力学以及地下水化学等水文地质特征,以评价目前的开采是否在可持续的水文地质范围内运行。

Características hidrogeológicas do Sistema Aquífero Delta Omaruru, Namíbia

Resumo

A utilização sustentável de água subterrânea em partes hiperáridas da África Sub-Sahariana, como no deserto da Namíbia, é sempre um desafio devido à escassez de recursos e dados. Para o Aquífero Omdel na bacia de Omaruru, Namíbia, os problemas de investigação incluem a falta de informações geológicas e de aspectos hidrogeológicos, das propriedades hidráulicas e da geometria do aquífero nas seções de infiltração e escoamento, das condições de recarga das águas subterrâneas a montante do aquífero e dos impactos de recargas artificiais. Neste ambiente desértico, os métodos aplicados são pesquisas hidrogeológicas e visitas de campo, juntamente às interpretações dos dados geológicos, hidrogeológicos e geomorfológicos utilizadas para definir as características hidrogeológicas do Aquífero Omdel. A geometria da rocha matriz que aporta o aquífero indica que o canal principal (um de quatro paleocanais) é o maior reservatório em armazenamento de águas subterrâneas, estimado em 133 Mm3, e este é o mais profundo em relação aos outros três canais, com uma espessura média de sedimentos de 80 m. Todas as fácies hidroquímicas das águas subterrâneas dos poços exploratórios do Aquífero Omdel revelam um carácter NaCl, o qual indica um ambiente costeiro. É estimado um aumento da produtividade do Aquífero Omdel com a construção da barragem para melhoramento da recarga, com valores antes da construção de 2.8 Mm3/ano chegando a 4.6 Mm3/ano após a construção. Este artigo possui foco no entendimento das características hidrogeológicas do Sistema Aquífero Delta Omaruru em termos de recarga e descarga, dinâmica e hidroquímica das águas subterrâneas, a fim de avaliar se as retiradas atuais estão operando dentro dos limites de sustentabilidade hidrogeológica.

Introduction

As one of the coastal aquifers in the Namib Desert in Namibia (Fig. 1), the Omaruru River Delta Aquifer (Omdel Aquifer) supplies groundwater to Henties Bay, Swakopmund, Arandis, Rossing uranium mine, Langer Heinrich mine and many other consumers in the Central Namib area. Since groundwater resources can be used as a stable water supply for coastal areas in Africa, they should be managed appropriately to ensure sustainable water supply, and furthermore they should be protected and monitored regularly (Steyl and Dennis 2010). Omdel Aquifer is described as an alluvial aquifer consisting of four palaeochannels, i.e. the Main channel (MC), Northern channel (NC), Northern elevated channel (NEC) and Southern elevated channel (SEC; Fig. 1). The MC aquifer is located between the two elevated bedrock channels, i.e. the SEC to the south and the NEC to the north. Further to the north from the NEC is the deeper NC. These palaeochannels are defined mainly by their bedrock elevation, groundwater quality and relative position to the current flow-path of the Omaruru River (Fig. 1). The MC is the only channel with potable groundwater. A total of 174 boreholes have been drilled in the Omdel Aquifer, of which 42 are production boreholes, mainly found in the MC. Monitoring boreholes amount to 96, whilst 34 boreholes are reported dry and 2 boreholes are blocked. Water quality from production boreholes in the MC appears to have changed with time; particularly, the total dissolved solids (TDS) is reported to have increased in some of the production boreholes (Seimons and Muundjua 2011). Increase in drawdown usually causes salinization in coastal aquifers (Bocanegra et al. 2010). Some of these boreholes have been in operation for a long time (before 1986). Groundwater resources can be protected from over-utilization and contamination by implementing enhanced groundwater recharge systems (dams; Sargaonkar et al. 2011).
Fig. 1

Map indicating the study area and Omaruru catchment in Namibia

Groundwater recharge to the Omdel Aquifer occurs mainly from occasional ephemeral run-off in the Omaruru River. According to Nawrowski (1994) only 1 Mm3/year of surface water recharges the Omdel Aquifer during occasional flood events and about 14 Mm3/year escapes into the sea, based on mean annual runoff data. It should, however, be mentioned that these figures are based mainly on the one major flood event which occurred in 1985. Later long-term average runoff of the river, based on the flood events of the past 56 years, before the construction of the recharge enhancement dam in 1994, is estimated to be 13 Mm3/year. It was furthermore calculated that groundwater recharge amounts to 2.3 Mm3/year as a result of the runoff events, whilst the remaining 10.7 Mm3/year flows out to sea without contributing to recharge, Consultants for Water and Environment (IWACO 2001). In order to prevent any run-off from entering the sea, and to retain the run-off water, the Department of Water Affairs introduced an artificial recharge project (Omdel Dam Project) in 1989, and the construction of the Omdel Dam in the Omaruru River, about 30 km inland from the coast, was completed in 1994. The Omdel Dam was constructed to enhance groundwater recharge by first impounding the silt-loaded flood waters and allowing the fine suspended sediment to settle. After sufficient time is allowed for the silt settlement, the clear water is then released downstream to infiltrate into the aquifer under controlled conditions. Downstream of the dam, two infiltration sites (sites 1 and 2; Fig. 1) were selected and prepared for the release and infiltration of silt-free water from the dam into the MC. A further advantage of this system is that whilst the silt from any particular flood event settles as a result of the impoundment, some of water already starts infiltrating the MC from the dam.

Nawrowski (1994) found that at site 1 the average hydraulic conductivity (K) is 145 m/day; below the water table the hydraulic conductivity is 142 m/day and above the water table it is 148 m/day. The alluvium at site 1 was also found to be more permeable laterally along the bedding plane than across the bedding plane, indicating an anisotropic behavior with respect to hydraulic conductivity. According to Nawrowski (1994), the effects of aquifer anisotropy are observed in the aquifer behavior during discharge and recharge events, mainly revealed as delayed yield or delayed drainage. The eastern portion of the site 1 aquifer indicates high values of lateral conductivity, with an average of 218 m/day, whilst the western portion of the site 1 aquifer, has K values ranging between 203 and 204 m/day (Nawrowski 1994). Groundwater flows paths revealed by the water level and water quality trends suggest that entry of groundwater from the Omaruru River bed into the MC occurs about 30 km inland from the coast. At site 1 the water flows partly through a sub-channel into a larger secondary channel aquifer and then converges farther downstream into the MC aquifer. Site 1 aquifer is therefore regarded as a favourable infiltration and conduit system for recharging water to be directed into the secondary and MC aquifers for storage and later abstraction (Nawrowski 1994). After the construction of the dam, IWACO (2001) estimated that 6.2 Mm3/year would be the long term average spill, whilst the natural recharge from the river bed was estimated to be 1.1 Mm3/year (17.5%) based on the dam capacity of 38 Mm3. They also estimated the potential recharge volume (PRV) as 7.1 Mm3, defined as the average annual volume available for enhanced recharge over the long term.

Zeelie (2001) estimated that about 18 Mm3 of run-off water were retained in the dam during the 1999/2000 rainy season and flood event. Of this inflow, about 4.9 Mm3 evaporated, 4.8 Mm3 were released for enhanced infiltration, 4.5 Mm3 directly infiltrated the aquifer within the dam basin and about 3.8 Mm3 remained in the dam. Therefore about 9.3 Mm3 effectively recharged the aquifer from this single event, as compared to the previous events (1997/1998) of the same magnitude.

The problems investigated are: the lack of understanding of the geology and the hydrogeological setting of the Omdel Aquifer; little information on the hydraulic properties and geometry of the aquifer at the inflow and outflow sections; no groundwater recharge study done upstream of the Omdel Aquifer in the Omaruru catchment; little information on the impact of artificial recharge; and the effect of flood events for the hydrological seasons of 1996/1997/1998, 1999/2000, 2007/2008, 2008/2009 and 2010/2011 on groundwater levels of the Omdel Aquifer.

The objective of the paper is to integrate artificial recharge with hydrogeological understanding of the Omdel Aquifer to establish a conceptual framework for assessment of groundwater recharge and discharge, water chemistry and balanced water supply.

Study site

Figure 1 is a map of the Omaruru River Catchment Basin, showing the locality of the study area (Omdel Aquifer), the Omaruru Alluvial Plain Aquifer (OMAP), the main tributaries within the catchment, rainfall gauge stations, location of the Omdel Dam and groundwater supply schemes in the Omaruru catchment. The Omdel Aquifer in Namibia is situated about 80 km north of Swakopmund and extends from the Omaruru River mouth at Henties Bay to about 35 km inland, with the elevation reaching 230 m above mean sea level (mamsl) at its eastern extremity across the relatively flat Namib Plain (Geyh and Ploethner 1995; Fig. 1). Since the Omdel Aquifer is located at the outflow end of the Omaruru River, it is important to note, from a hydrogeological perspective, that the total catchment area of the river covers approximately 15,700 km2 and reaches an elevation of 1,450 m amsl at its source, with a peak area of 2,216 m amsl in the Erongo Mountains downstream of Omaruru town (Geyh and Ploethner 1995). It should also be noted that the mountainous region of the catchment, with an average elevation of about 1,000 m amsl receives a mean annual rainfall between 200 and 450 mm (Geyh and Ploethner 1995). In the area around the Omdel Aquifer the climate is, however, hyper-arid with an average precipitation of less than 50 mm/year. Brandberg Mountain (to the north, at 2,278 m amsl), Grootspitskop (at 1,728 m amsl) and Erongo Mountains (at 2,216 m amsl) to the east are the highest protruding peaks near the study area.

Groundwater supply schemes along the Omaruru River flow-path are Omaruru, Okombahe, Nei-Neis, Tubussis, Spitskoppe and Lêe Water. Within the delta area the outline of the production well field with two infiltration sites (sites 1 and 2) are indicated. The only main towns within the catchment are Omaruru and Henties Bay, whilst a number of formal and informal settlements occur within and outside the catchment. The rain gauging stations have sufficient historic rainfall data that can be used to estimate groundwater recharge and station ID numbers are used to identify them. The positions of boreholes used for the water-table-fluctuation calculations are indicated in Fig. 1. Three lines on the map of Fig. 1 indicate that there might be some tectonic geological control influencing the flow pattern of the Omaruru River. This flow pattern may be controlled by the Omaruru, Erongo and Autseib lineaments described by Corner (1983).

Geological setting

The geological setting of the study area is described in more detail here. Geologically the oldest rocks of the Omdel Aquifer and the surrounding area are the Neoproterozoic quartzite of the Naauwpoort Formation (Nosib Group, ±850–750 Ma), phyllite of the Amis Formation (Zerrissene Group, ±740–600 Ma), marble of the Karibib and Arandis Formations and mica schist of the Kuiseb Formation of the Swakop Group (±770–600 Ma), all part of the Damara Orogen within the Namibian age (Geological Survey of Namibia 1997; Miller 2008). Deposition and deformation of the rock-types in the Damara Orogen are attributed to ancient continental rifting, collision and subduction. Miller (2008) states that the beginning of rifting and thus the base of the Nosib Group is not known, but may be as old as 900 Ma. Dates for the deposition of the Swakop Group vary between 711 and 658 Ma, while ages for continental collision and subduction (deformation and metamorphism) are estimated at 650–633 Ma. Associated and followed by the deformation and metamorphism processes were the intrusion of granitic rocks, dated between 550 and 450 Ma. It was also during this period of deformation that the lineaments (Omaruru, Erongo and Autseib) developed. The metasedimentary and metavolcanic rocks are intruded by diorite (early syn-tectonic to post-tectonic), granite (syn-tectonic, post-tectonic to late-tectonic), and red granites (post-tectonic to late-tectonic; Fig. 2). The granites are described as medium to fine grained and coarse grained, whilst the diorite is coarse grained. Mudstone, siltstone, sandstone and shale, overlying the intrusive rocks, are sedimentary rocks of the Gai-as, Huab and Verbrande Berg Formations of the Karoo Sequence (300–180 Ma). Basalt of the Awahab Formation of the Etendeka Group (Cretaceous age, 132 Ma) overlies the sedimentary rocks of the Karoo Sequence. The stratigraphy is further intruded by granite and gabbro rocks of the Cretaceous intrusive complexes, further intruded by parallel north-westerly trending dolerite sills and dykes of Cretaceous age. Finally, surficial deposits (alluvium, sand, gravel, calcrete, scree, gypcrete) of the Quaternary period overlie all the aforementioned rock formations, in places (Fig. 2). There are also Quaternary age salt pan deposits along the coast. The surficial deposits are divided into two main aquifer systems: the Omaruru River Delta Aquifer (OMDEL) and the Omaruru Alluvial Plain Aquifer (OMAP), the latter extending approximately 120 km inland from the coast (Fig. 1). Located in the downstream portion of the Omaruru River, known as the delta area of the Omaruru alluvial bed, the Omdel occurs as a roughly triangular shape (about 526,000,000 m2).
Fig. 2

Summary of geological features of the Omdel Aquifer (After Nawrowski 1990, Geological Survey of Namibia 1997 and Miller 2008)

Materials and methods

To realize the objective set for this research, the following methods are duly considered: Hydrogeological surveys and site visits, together with the detailed interpretation of geological, hydrological and geomorphological of the area, carried out to define the hydrogeological characteristics of the Omdel Aquifer. Two photographs, taken during a site visit, depict the abstraction tower at Omdel Dam and the infiltration ponds at Omdel Aquifer (Fig. 3). Existing data on geology, climate, hydrogeology, hydrology, rainfall, test pumping and hydrochemistry were collected from different sources such as books, reports, maps, remote sensing images and databases. From the existing data, the hydrogeological data such as depth to water table, borehole depth, water-bearing formations and corresponding geomorphological units were determined.
Fig. 3

a Abstraction tower at Omdel Dam and b infiltration ponds at Omdel Aquifer

Geological information obtained from the borehole completion reports were used to draw geological cross sections, using Arc-Map software. The maps were also created using Arc-Map software. Test pumping data were used to determine the aquifer parameters of the Omdel Aquifer by using the Aquifer Test Curve Fitting and Aquifer Test 3.5 analyses. Hydrochemical data were used to determine the groundwater facies, water types and fingerprints, using WISH and HamVer2Dot softwares.

Borehole WW26483 is one of the monitoring boreholes in the Omaruru River bed at Nei-Neis. Historic water levels of borehole WW26483 and the historic rainfall data at both Usakos and Etendero gauge stations were used to estimate groundwater recharge at Nei-Neis, upstream in the Omaruru catchment, where there is significant rainfall (Fig. 4). From the records, there appears to be a clear relationship between the rise in groundwater levels in the borehole and significant rainfall. Also the decline in groundwater levels with low or no rainfall should be noted. It is also important to note that the rainfall peaks indicate sporadic thunderstorms which often result in flood run-off lasting a short period of time. Very few such floods, however, reach the Omdel Aquifer.
Fig. 4

Graph indicating water levels of borehole WW26483 and rainfall of Usakos and Etendero gauge stations

Usakos gauge station has historic rainfall data up to March 2002, while Etendero gauge station has historic rainfall data starting from January 2001 up to present. The historic water levels of boreholes and the historic rainfall data of gauge stations were used to estimate groundwater recharge at Okombahe, Nei-Neis, Omaruru, Tubussis and Spitskoppe. Water-table fluctuation (WTF) and chloride mass balance (CMB) methods were used to estimate groundwater recharge at different localities in the Omaruru catchment (Okombahe, Nei-Neis, Omaruru, Tubussis and Spitskoppe). Once the significant inflow (flood) reaches the Omdel Dam, some water infiltrates into the dam basin, some will be lost in the process through evaporation at the dam. When the water is released through the abstraction tower, some water infiltrates along the Omaruru River part 1, some water infiltrates at infiltration ponds at site 1, and some will be lost through evaporation. If the water goes beyond the infiltration ponds at site 1 (significant inflow), some water will infiltrate the Omaruru River part 2, some water will infiltrate at infiltration ponds at site 2 and some will be lost through evaporation (Fig. 1). Five major run-off events for the hydrological seasons of 1996/1997/1998, 1999/2000, 2007/2008, 2008/2009 and 2010/2011 were used to evaluate the effect of artificial recharge on groundwater of the Omdel Aquifer. Groundwater flow from the upper river bed (upstream of Omdel Aquifer), OMAP and SEC were conceptualized and estimated by using Darcy’s law. This contributes to the water balance of the Omdel Aquifer in question.

The relief of the bedrock of the multi-channel aquifer system in the Omaruru Delta plays an important role for the delineation of flow direction (flow paths), groundwater storage and quality (Nawrowski 1990). It is therefore necessary to identify aquifer boundaries, recharge areas and discharge areas, as well as to describe the composition of the aquifer material. All the data and information gathered are critically reviewed and interpreted to establish a framework within which the hydrogeological characteristics of the Omaruru Delta Aquifer System are evaluated.

Results

Subdivision of Omdel Aquifer

According to lithological borehole logs, nine geological cross-sections were drawn across the Omdel Aquifer, ranging from the dam to the coast, and are located in Fig. 5. Cross sections AB, CD and EF are referred to as downstream; GH, IJ and KL as center/middle; and MN, OP and QR as upstream.
Fig. 5

a Simplified geological map depicting cross-section locations, b geological cross-section A-B, displaying the palaeochannels, c other geological cross-sections

Lithologically this alluvial aquifer consists of unconsolidated coarse sand and gravel (unconfined aquifer), clay rich sand and cemented sand (aquitard) and predominantly coarse sand and gravel (major groundwater reservoir), which were successively deposited within the different palaeochannels that were incised in bedrock of mainly mica schist and granite. The Omdel Aquifer is characterised by lithology (Table 1):
  • Saturated alluvial deposits, mainly sand and gravel, represent the productive aquifer. Interbedded layers of clay, silt and calcareous cemented sand are also part of the alluvial deposits, acting as leaky confining layers above or within the primary aquifer. Furthermore, the surface geology of the Omdel Aquifer comprises alluvial deposits, mainly consisting of coarse sand and gravel (of granitic origin), interbedded with thin layers of clay and locally cemented with calcrete. Minor dune sand partially covers the surface at the coast and in the river bed, forming low mounds.

  • A sand layer covers most of the upper section of the Omdel Aquifer system, with average thicknesses of about 40, 65 and 20 m in the downstream, middle and upstream sections of the aquifer, respectively. Such sand beds have the potential to allow flood water to infiltrate into the aquifer. According to different cross-sections, the sand layer overlies varying layers of clay, sandstone, sand and clay, gravel, sand and gravel, calcrete, sandstone and clay, loam, with granite or mica schist as bedrock (Fig. 5). Gravel, calcrete, gravel and clay, gravel and calcrete, sand and gravel lithologies dominate the MC of the Omdel Aquifer system. Unconsolidated sand with local thin lenses of partially calcareous material of the deepest alluvial layer within the MC, represent the major aquifer. Clay layers found in all four channels, indicate periods of low energy sediment transport.

  • Sandstone within and at the base of the MC is found mainly in the downstream part of the aquifer. In the NEC and NC the sandstone layer occurs in the central part, and in the SEC it occurs in the central and upstream part of the aquifer.

  • Interbedded layers of the alluvial deposits are prominent in the central and upstream part of the Omdel Aquifer. Calcrete layers, interbedded in the alluvial deposits of up to 25 m in thickness, occur mainly in the central part of the MC. The widespread layer of cemented sand and gravel and lenses of unconsolidated sand and gravel are found at depths down to the basement. It also appears that greatest cemented thicknesses of the alluvial deposits occur in the central part of the MC. Further upstream in the MC a decrease in calcareous material is found, whilst the downstream part has varying degrees of calcareous material, less than 30% of the aquifer thickness (Nawrowski 1990). The less permeable calcareous material layer acts as an aquitard (semi-confined horizon) and displays delayed yield effect characteristics. In the centre of the MC, a dolerite dyke was intercepted at a depth of approximately 85 m.

  • The underlying bedrocks of the Omdel Aquifer are quartzite, mica schist and granite of the Damara Orogen (bedrock geometry indicated in Fig. 5). Granite bedrock is mainly found in the downstream part of the aquifer, while mica schist is mainly found in the central and upstream part. Quartzite bedrock is only observed in the central part of the aquifer.

Table 1

Hydrostratigraphy of the Omdel Aquifer

Lithology

Description

Hydrostratigraphic units and Properties

Sand

Unconsolidated, porous sand (medium or fine grained, rounded or angular)

Mostly unconfined primary aquifer

Sandstone

Consolidated and cemented, semi-porous (medium or fine grained)

Aquitard, normally semi-confined

Sand and clay

Unconsolidated sand with subordinate amounts of clay (or otherwise).

Clay between sand grains reduces transmissivity

Sandstone, granite, mica schist, quartzite and dolerite

Bedrock of the aquifer (impermeable layer)

Water-bearing capabilities are only limited to fractured or decomposed portions

Aquifer parameters

The transmissivity (T), hydraulic conductivity (K) and storativity (S) were determined from test pumping data of boreholes in the Omdel Aquifer. Most of these boreholes are located in the MC, with a few of them in the SEC. There are neither pumping test data nor information on the transmissivity and storativity values from the NEC and NC. Transmissivity values range between 17 and 3,916 m2/day, the lowest value (17 m2/day) is recorded at borehole WW35338 in the central part of the MC, whilst the highest value (3,916 m2/day) is recorded at borehole WW35344, located in the upstream part of the MC (Table 2). Borehole WW35344 is located north-west of infiltration ponds site 1 and the Omdel Dam (Fig. 6). The coarse sediments, deposited during high energy transport at that part of the aquifer, are the reason for the high T value. During deposition of the sediments in the Omdel Aquifer, fine materials such as clay were deposited in some parts of the aquifer due to low transport energy conditions, thus accounting for the low T values in these parts of the aquifer (Fig. 6). The boreholes with high T values are WW21490, WW33068, WW33069, WW100049, WW100142, WW100155 and WW100157. Most of them are located in the upstream and central part of the MC, where high energy run-off water deposited the coarser sediments as it flowed towards the ocean. The hydraulic conductivity (K) values range between 1 and 302 m/day with an average value of 20 m/day. Storativity (S) values range between 0.0001 and 0.01. Boreholes with high storativity values (0.01), in this case the specific yield (Sy), are WW21501, WW100095 and WW21490, found in the central part of the MC. The distribution of transmissivity, hydraulic conductivity and storativity of the Omdel Aquifer boreholes from downstream to upstream are indicated in Figs. 7, 8 and 9 respectively.
Table 2

Statistical assessment of aquifer parameters of boreholes at Omdel Aquifer. SD standard deviation; min minimum; max maximum

Aquifer parameters

No. of datasets

Min

Max

SD

Mean

Median

Transmissivity (m2/day)

57

17

3,916

709

463

208

Hydraulic conductivity (m/day)

53

1.0

302

48

20

4.6

Storativity

57

0.0001

0.01

0.002

0.0006

0.0001

Fig. 6

Map depicting the distribution of transmissivity (T) values in MC and SEC

Fig. 7

The distribution of transmissivity values of Omdel Aquifer boreholes (downstream to upstream)

Fig. 8

The distribution of hydraulic conductivity values of Omdel Aquifer boreholes (downstream to upstream)

Fig. 9

The distribution of storativity values of Omdel Aquifer boreholes (downstream to upstream)

Groundwater level characteristics

The groundwater flow direction is towards the Atlantic Ocean (flowing from north east to south west), with the Omaruru River acting as the base-level of drainage. A total of 154 groundwater level measurements of boreholes were available for assessments of the groundwater flow regime. There is good correlation between surface elevation (topography) and the groundwater level elevation (R2 = 0.97, about 97%; Fig. 10). The good correlation indicates that the water table follows a trend of surface topography.
Fig. 10

Correlation between surface topography and groundwater level elevations

The statistical analysis of the 154 groundwater levels is presented in Table 3, revealing a minimum value of 6.31 m below surface and maximum value of 57.46 m with an average groundwater level of 31.0 m below surface.
Table 3

Recent groundwater level measurements. SD standard deviation; min minimum; max maximum

Groundwater level measurements, number

Groundwater level (m below surface)

Min

Max

SD

Mean

154

6.31

57.46

12

31.0

The groundwater-level elevation contours of the Omdel Aquifer boreholes for June 2016 ranges between 235 to 0 (m amsl; Fig. 11). Since it is obvious that the groundwater levels fluctuate with time, it was important to compile Fig. 11 with data within the same timeframe. The highest elevation is for borehole WW100035 located upstream next to the Omdel Dam while the lowest elevations are for the boreholes next to the coast line. Contour lines of the upstream aquifer range between 235 to 135 m amsl, the middle aquifer between 135 to 50 m amsl and the downstream aquifer between 50 to 0 m amsl. Over abstraction of the aquifer is illustrated by the upstream curving of the contour lines in the middle of the MC. The converging contour lines at the downstream section of the middle aquifer suggests a submerged bedrock high between this aquifer and the downstream aquifer. The contour lines of the downstream aquifer appear to have slightly greater spacing, suggesting a flatter water table.
Fig. 11

Groundwater level elevations of Omdel Aquifer boreholes

Groundwater chemistry

The groundwater cation chemistry fingerprint of the Omdel Aquifer is dominated mainly by the concentrations (meq/l) of Na and Ca, whilst other cations such as Mg and K occur in lesser concentrations (Fig. 12). About 22% of the groundwater samples of the aquifer indicate that Na is the predominant cation.
Fig. 12

Groundwater fingerprint of selected boreholes of Omdel Aquifer

The groundwater anion chemistry fingerprint of the Omdel Aquifer is dominated mainly by the concentration (meq/l) of Cl, whilst other anions such as HCO3 and SO4 occur in lesser concentrations. About 70% of the groundwater samples of the aquifer indicate that Cl as the predominant anion.

Boreholes WW25992, WW100060, WW16484, WW100050 and WW100045 mainly contain Cl with no dominant cation. This water type suggests reverse ion exchange according to the expanded Durov diagram, and the boreholes are located in the downstream part of the NEC, as well as in the downstream, central and upstream parts of the SEC. The majority of the groundwater chemical data of the selected boreholes of the Omdel Aquifer plot in the dominance field of Na+ and Cl, usually indicating an end point in a water evolution sequence.

The ratio rCa/(rHCO3 + SO4) of the water samples collected from the Omdel Aquifer during the 1993 to 2012 period shows changes with time (Fig. 13). Most of the water samples had ratios of rCa/(rHCO3 + rSO4) close to 1.
Fig. 13

Average values of rCa/(rHCO3 + rSO4) of Omdel Aquifer showing change over time

The TDS and chloride are important parameters that can be used to study seawater intrusion. Figure 14 indicates the change of TDS and Cl over time from 1993 to 2012. The two parameters shows similar trend patterns. The three highest peaks observed over this period are at 1998, 2000 and 2004. The graph also indicates the average TDS values of freshwater (less 1,000 mg/l) in 1994, 2005 and 2010, while the rest of the data indicate brackish water (more than 1,000 mg/l).
Fig. 14

Chloride and total dissolved solids (TDS) concentrations in groundwater of the Omdel Aquifer between 1993 and 2012

Seven groundwater quality facies of selected boreholes in the Omdel Aquifer are recognized in the Piper diagram (Fig. 15), and these are Na + K-HCO3-Cl + SO4 of borehole WW22188 (located in the centre of the MC), Na + K-Cl + SO4 of Borehole WW21926 (located upstream in the MC, Fig. 16), Ca + Mg-Na + K-Cl + SO4 (some boreholes), Na + K-Ca + Mg-Cl + SO4 (some boreholes), Ca + Mg-Na + K-Cl + SO4-HCO3 (some boreholes), Na + K-Ca + Mg-Cl + SO4-HCO3 (majority of the boreholes) and Ca + Mg-Na + K-HCO3-Cl + SO4 of borehole WW100044, located upstream in the SEC. The absence of HCO3 in the groundwater of boreholes suggests that there is a lack of recharge in that part of the aquifer. The presence of HCO3 in groundwater of borehole WW100044 suggests seepage flow from the dam. All groundwater facies of selected boreholes of the Omdel Aquifer have NaCl indicating a coastal environment.
Fig. 15

Groundwater quality facies of selected boreholes of the Omdel Aquifer (Piper plot)

Fig. 16

Map depicting locations of the selected boreholes of the Omdel Aquifer

Groundwater recharge

Groundwater recharge estimated to take place in the Omaruru catchment was evaluated by the WTF and CMB methods. The rainfall rate decreases towards the coast and increases inland, i.e. a significant amount of rainfall is recorded upstream in the Omaruru catchment. Spitskoppe, Nei-Neis, Okombahe, Tubussis and Omaruru are groundwater supply schemes where groundwater recharge was estimated. Groundwater recharge (R) estimated by the WTF method for the period between 1986 and 2006 was calculated by using Eq. (1) (Shamsudduha et al. 2011) and is discussed in more detail (Fig. 17).
$$ R=\Delta {S}^{\mathrm{gw}}={S}_{\mathrm{y}}\partial h/\partial t={S}_{\mathrm{y}}\Delta h/\Delta t $$
(1)
ΔSgw is the change in groundwater storage, Sy is specific yield, Δh is change in water table head (between minimum and maximum), Δt is time period.
Fig. 17

Groundwater recharge estimation by the WTF method

The WTF method is based on the response of groundwater levels that rise in unconfined aquifers due to recharge arriving at the water table (Healy and Cook 2002). Recharge is determined as the change in water level over time multiplied by the specific yield. It is best applied to shallow water table systems that display sharp rises and declines in groundwater levels.

Historic groundwater data, between 1986 and recent times, are available for most of the groundwater supply schemes. The groundwater levels of borehole WW26483 and rainfall recorded at Usakos and Etendero gauge stations from January 1986 to 2012 used to estimate groundwater recharge at Nei-Neis are indicated in Fig. 4. During the rainfall season when it is significant, groundwater levels rise, and groundwater levels decline during low rainfall periods. For the period 1988 to 2000, Nei-Neis, Spitskoppe and Tubussis localities indicated high groundwater recharge, estimated to be between 5 and 21-mm rise in water level (Fig. 17). Omaruru groundwater supply scheme is the first location upstream in the catchment, and the groundwater recharge estimation is between 2 and 6.5 mm. The average groundwater recharge estimations are 9.54 mm at Nei-Neis, 3.71 mm at Okombahe, 4.9 mm at Omaruru, 5.84 mm at Spitskoppe and 11.69 mm at Tubussis. This actually means more water is recharged at Nei-Neis, Spitskoppe and Tubussis than at Okombahe and Omaruru, which may be due to topographical differences and aquifer dimensions.

Rainfall recharge estimated at Omaruru, Okombahe and Nei-Neis over 1 km2 is 560, 580 and 780 m3 respectively (Table 4). The recharge over rainfall ratio at different localities in the Omaruru catchment is relatively small indicating that rainfall contributed a small portion to the water level rise (recharge). Therefore, runoff plays a very important role in the water level rise (groundwater recharge) in the Omaruru River bed alluvial aquifers.
Table 4

Recharge over rainfall ratio at different localities: Omaruru catchment

Location

Rainfall recharge (m3/km2)

Rainfall (m3/km2)

Recharge/rainfall

Ratio

(%)

Nei-Neis

780

116,000

0.0067

0.67

Okombahe

580

143,000

0.0040

0.41

Omaruru

560

189,000

0.0030

0.30

The groundwater recharge estimated by using the CMB method ranges between 0.19 and 9.23% Eq. (2),
$$ R=\mathrm{Clp}\times \mathrm{rainfall}/\mathrm{Clg} $$
(2)
where Clp is chloride in precipitation and Clg is chloride in groundwater.
Recharge estimates by using CMB method considered TDS increases in groundwater with time after a run-off event. Such delayed TDS increases are considered to suggest retarded groundwater flow with high TDS emanating from the surrounding bedrock and mixing with low TDS groundwater contained in the alluvial aquifers. It appears that direct rainfall contributes more to recharge at Okombahe compared to the other localities (Table 5). Chloride concentration in groundwater increases towards the coast and is higher in tributaries than in the Omaruru River itself. The data are for the year 2000, due to the fact that some rain gauge stations have no recent rainfall data. The concentration of chloride in precipitation was a projected estimate from chloride concentration distribution in precipitation in the northeastern Namibia map by Klock (2001). It should also be noted that run-off has a greater influence on groundwater recharge of the river bed alluvium at these different localities.
Table 5

Groundwater recharge (R) estimation by CMB method at different localities in the Omaruru River catchment (year 2000)

Location

Chloride in groundwater (mg/l)

Chloride concentration in precipitation (mg/l)

Rainfall (mm)

Groundwater recharge R

(mm)

(%)

Nei-Neis

92

1.3

125.1

1.77

1.41

Okombahe

13

1.2

117.1

10.81

9.23

Spitskoppe

700

1.3

125.1

0.23

0.19

Tubussis

169

1.2

161.5

1.15

0.71

There was no rainfall station at Omdel Dam in the past, but at the beginning of 2014 a rain gauge was installed. So far no rainfall has been recorded at the new station. Due to absence of rainfall stations at the coast, there are also no historical rainfall data for the coastal region. The rainfall at the coast is regarded as insignificant to direct recharge in the Omdel Aquifer, since high evaporation still plays a major role. From this perspective, therefore, the Omdel Aquifer is assumed not to receive any direct recharge from local rainfall, but is rather recharged through artificial recharge (significant run-off), seepage underneath the dam and groundwater flow from the upper river bed (Omdel upstream), OMAP and SEC. The groundwater flow is assumed to play a major role in the recharge of the Omdel Aquifer, since significant run-off only reaches the Omdel Dam occasionally. To estimate groundwater flow, Darcy’s law was applied (Kruseman and de Ridder 1994):
$$ Q= KAi $$
(3)

Q is the volume rate of groundwater flow (length3/time), K is a constant proportionality also referred to as hydraulic conductivity (length/time), A is the cross-sectional area normal to the flow direction (length2) and i is the hydraulic gradient which is dimensionless (Kruseman and de Ridder 1994).

By applying Darcy’s law, the groundwater flow from the upper river bed (Omdel upstream) was obtained; K (110 m/day), A (15,000 m2) and i (0.0044), thus Q = 110 × 15,000 × 0.0044 = 7,260 m3/day (July 2013).

Therefore, the annual groundwater flow from the upper river bed (Omdel upstream) according to the preceding calculation amounts to about 2.6 Mm3/year (Table 6). IWACO (2001) estimated a value of about 3.0 Mm3/year. Groundwater flow from OMAP was estimated as follows: K (6.8 m/day), A (40 000 m2) and i (0.0032), thusQ = 6.8 × 40,000 × 0.0032 = 870 m3/day suggesting an annual groundwater flow from OMAP of 0.3 Mm3/year (September 2014). Previous studies estimated Q as 0.5 Mm3/year (IWACO 2001) and 0.2 Mm3/year (Zeelie 2001).
Table 6

Groundwater flow (Q) estimated for Omdel upstream, OMAP, WW16662, Okombahe, Nei-Neis, Tubusis and Spitskoppe

Location

K (m/day)

A (m2)

i

Flow rate Q

(m3/day)

(Mm3/year)

Omdel upstream

110

15,000

0.0044

7,260

2.6

OMAP

6.8

40,000

0.0032

870

0.3

WW16662 (SEC to MC)

4.6

10,581

0.007

341

0.12

Okombahe

29.5

5023.5

0.014

2,075

0.76

Nei-Neis

28.3

1847.2

0.027

1,411

0.52

Tubusis

5.5

976.9

0.007

37.6

0.014

Spitskoppe

0.229

4963.2

0.005

5.7

0.002

Groundwater flow from the SEC to the MC near borehole WW16662 was estimated, using the following values: K (4.6 m/day), A (10,581 m2) and i (0.007), thus Q = 4.6 × 10,581 × 0.007 = 341 m3/day (November 2015), suggesting an annual groundwater flow from the SEC of about 0.12 Mm3/year. Zeelie (2001) estimated it to be about 0.23 Mm3/year.

The average saturated thickness of production boreholes for February 2016 in the central wellfield and downstream wellfield of Nei-Neis Water Supply Scheme is 6.9 m and 7.0 m respectively, whilst the average saturated thickness of production boreholes for May 2011 in the central wellfield was 19.5 m, due to the significant run-off during that period. The estimated groundwater flow at Okombahe and Nei-Neis for the period between April 1996 and April 2016 is presented in Figs. 18 and 19. At Okombahe and Nei-Neis the estimated groundwater flow for April 2016 is 775,990 m3/year (K is 29.9 m/day, A is 4,941 m2 and i is 0.01439, thus Q = 29.9 × 4,941 × 0.01439 = 2,126 m3/day) and 389,455 m3/year (K is 36.9 m/day, A is 1,555.8 m2 and i is 0.01858, thus Q = 36.9 × 1,555.8 × 0.01858 = 1,067 m3/day) respectively.
Fig. 18

Groundwater flow at Okombahe (Q = flow rate)

Fig. 19

Groundwater flow at Nei-Neis (Q = flow rate)

The groundwater flow estimated at Tubusis (a north flowing tributary) and Spitskoppe for November 2015 is 13,724 m3/year (K is 5.5 m/day, A is 976.9 m2 and i is 0.007, thus Q = 5.5 × 976.9 × 0.007 = 37.6 m3/day) and 2,081 m3/year (K is 0.229 m/day, A is 4,963.2 m2 and i is 0.005, thus Q = 0.229 × 4,963.2 × 0.005 = 5.7 m3/day) respectively.

The schematic diagram Fig. 20 indicates the Omdel Dam, Omaruru River and infiltration ponds (sites 1 and 2). If a significant inflow (runoff) reaches the Omdel Dam, the accompanied silt is first allowed to settle. Efforts are also made to prevent silt from being deposited around the abstraction tower, which blocks the outlet valves. Clear water is released from the tower and flows about 6 km (part 1) and 12 km (part 2) downstream before it reaches and directly recharges the aquifer through infiltration ponds at site 1 (ponds A, B, C and D) and site 2 (ponds E and F) respectively; sites 1 and 2 are situated in the MC.
Fig. 20

Schematic diagram indicating Omdel Dam, Omaruru River and infiltration ponds (sites 1 and 2)

Once the significant inflow (flood) reaches the Omdel Dam, some water infiltrates into the dam basin, some will be lost in the process through evaporation at the dam, whilst the remaining water will be available for release through the abstraction tower and enhanced recharge processes. The silt content is estimated to be about 10% of the total inflow.

If the inflow (flood) is significant enough, it will continue to travel down to infiltration ponds at site 2. Some water will infiltrate the river channel along this course, whilst some water will evaporate. During the hydrological season 2008/2009, about 2.506 Mm3 travelled the Omaruru River beyond sites 1 and 2. Of this flow, 0.050 Mm3 infiltrated the river channel and 0.040 Mm3 was lost through evaporation. Excess water flows to the infiltration ponds E and F at site 2 through pond C at site 1. Inflow of 2.416 Mm3 reached the infiltration ponds at site 2 during the hydrological season 2008/2009, 2.398 Mm3 infiltrated and about 0.018 Mm3 evaporated (Muundjua 2010).

According to Table 7 “infiltration” represents the water infiltrated at the dam basin, river channel (parts 1 and 2) and the infiltration ponds at sites 1 and 2. This “infiltration” indicates the artificial recharge for each hydrological season. The artificial recharge for the hydrological season between 1996/1997/1998 and 2010/2011 ranges between 52 to 89% of the total inflow, representing a significant component towards the Omdel Aquifer recharge. It should however be noted that artificial recharge occurs only during major flood events. The most water inflow recharged into the aquifer is for the hydrological season 2010/2011 and the least water inflow recharged into the aquifer is for the hydrological year 1999/2000. However, the hydrological season 1996/1997/1998 has more infiltrated water (9.55 Mm3) compared to other hydrological seasons.
Table 7

Information on the major flood events (after Zeelie 2001, Muundjua 2010 and Mostert 2014)

Hydrological season

Total inflow (Mm3)

Infiltration (Mm3)

% of artificial recharge

1996/1997/1998

18.027

9.55

53

1999/2000

18.0

9.3

52

2007/2008

2.853

2.0

70

2008/2009

10.423

8.61

83

2010/2011

5.716

5.07

89

The sum of infiltration at the dam, river section and infiltration pond(s) is estimated on average to be about 2.262 Mm3/year, whilst the average volume infiltrated at sites 1 and 2 (infiltration ponds) is estimated at about 1.487 Mm3/year (Mostert 2014). These estimations take into account a 10% silt content and an average release rate of 19.2 Mm3/year.

According to the preceding estimates, it is obvious, therefore, that artificial recharge (enhanced) at the infiltration ponds plays a major role at the Omdel Aquifer, since at least 60% of the infiltration occurs there. The effect on groundwater level of the monitoring boreholes (WW33066, WW31366, WW33069 and WW31243) in the surroundings of the infiltration ponds were observed during the recorded flood events (Fig. 21).
Fig. 21

Groundwater levels of selected monitoring boreholes

Discussion

Four palaeochannels of Omdel Aquifer

As can be seen in Fig. 5, the bedrock geometry of the Omdel Aquifer reveals that the MC is relatively deeper than the other three elevated channels (Fig. 5). This confirms a hypothesis by Nawrowski (1990) that the aquifer geometry is defined mainly by the deepest Omaruru River palaeochannel known as the MC, which extends farther north-eastwards into the alluvium bed. According to the bedrock geometry it is clear that the MC is the largest reservoir of stored fresh groundwater. The MC is filled with varying interbedded layers of sand, gravel, calcrete and clay, with a total thickness ranging between 70 m near the coast to 110 m at its upstream limit (Fig. 5). It should, however, be noted that cross-sections IJ (central part of the aquifer) and OP (upstream part of the aquifer) each indicate a sediment thickness of about 70 m, therefore suggesting an average sediment thickness of 80 m for the MC. The saturated thickness of the MC aquifer ranges from 20 to 60 m. Near the coast (downstream part of the MC) the saturated thickness is on the order of 50 m, decreasing towards the central part and increasing again upstream to about 60 m. The IJ and OP cross-sections indicate a saturated thickness of about 30 and 20 m respectively, therefore suggesting an average saturated thickness of about 40 m for the MC. It should be noted that the saturated thickness obviously declines as the groundwater levels in the aquifer decline.

The NC system, with saline groundwater and also described as the deeper channel sub-system, occurs farther northwards of the NEC (Fig. 22). This channel is also filled with sand, clay, sandstone, gravel and calcrete, with total alluvium thicknesses ranging between 20 and 75 m. Sediments deposited in the NC near the coast are about 70 m thick, decreasing towards the centre of the channel to about 20 m and increase again upstream to about 75 m. From these observations it is estimated that the average thickness of the sediments in the centre of the NC is about 35 m, whilst the average thickness of the sediments upstream is about 60 m. Saturated thicknesses of the NC ranges between 5 and 54 m. Near the coast the saturated thickness is about 54 m, decreasing towards the center and increasing again upstream to about 18 m. The EF cross-section indicates a saturated thickness of about 5 m and the average saturated thickness of the NC is estimated at about 20 m, which is half the average saturated thickness of the MC.
Fig. 22

Map depicting the four palaeochannels

Nawrowski (1990) mentioned that the two elevated channels are almost parallel to each other, suggesting that their flow direction may be controlled by north-easterly trending dolerite dykes. Throughout the Namib Desert, in the study area, dolerite dykes appear as prominent ridges. This is due to the fact that the dolerite is more resistant to weathering than the surrounding rocks. The channels in question are therefore perceived to be limited valleys (channels) incised in the bedrock between parallel dolerite dykes and later filled with sediments. Borehole WW100142 intercepts a dolerite dyke before reaching the mica schist bedrock. A conclusion, confirmed by water-table elevations, is that subsurface ridges, caused by dolerite dykes in the basement surface, form partial barriers on both sides of the MC.

River migration, of the pre-Omaruru River, obviously caused the development of the previously mentioned palaeochannels and the subsequent sediment infilling thereof. During the infilling of the channels there must have been also an ingress of sub-surface water flow. It is also conceivable that the perceived river migration was caused by geological tectonics related to crustal uplift and subsidence; the elevated channels described previously, suggest such a process. Since the NEC is reported to contain saline to brackish groundwater (as in the case of the other channels elevated above the MC), the conclusion must be made that they do not currently receive recharge from local flood events, as in the case of the MC. Only during extreme flood events may some recharge to these elevated channels occur. The thickness of sediments deposited in the NEC ranges between 25 and 30 m, with an average thickness of 26 m and near the coast it is about 30 m. In the NEC the saturated thickness ranges between 1 and 9 m, the CD cross-section indicating a saturated thickness of about 1 m, whilst the EF cross-section indicates a thickness of about 9 m. From these observations the average saturated thickness of the NEC is estimated at about 7 m. This channel has the least saturated thickness, compared to the other palaeochannels.

In the SEC the sediment thickness ranges between 20 and 65 m, increasing upstream, the exception being cross-section KL which indicates a thickness of 15 m. From these observations it is, therefore, assumed that the average thickness of the sediments deposited in the SEC is about 40 m. The SEC has a saturated thickness ranging between 5 and 40 m, with the CD cross-section indicating a thickness of about 5 m. From this observation the average saturated thickness of the SEC is estimated at 21 m, which is equivalent to the average saturated thickness of the NC. The volume of groundwater estimated in the MC is about 133 Mm3. Geyh and Ploethner (1995) estimated the total groundwater reserve in the Omdel Aquifer to be 1.6 Mm3, 50% is considered abstractable.

Water-table characteristics

The water table of the MC of the Omdel Aquifer shows a significant decline with time, which is observed from the groundwater levels in the production boreholes. According to the historic borehole production data of the aquifer, the water table has declined ever since abstraction operations started, indicating over abstraction. Hydraulic head loss in the MC ranges from 2.0 to 31.0 m over the period from January 1986 to December 2012 (approximately 26-year period). This severe decline of the water table is observed in the central part of the MC, where excessive abstraction occurs (WW21649, about 31.0 m; Fig. 23). Geyh and Ploethner (1995) mentioned that the oldest groundwater is abstracted from the northeastern part of the Omdel Aquifer with 14C ages of up to 17,000 years BP. Near the coastline the observed decline in water levels is on average 2.0 m (WW16953 and WW21499). It is estimated that the water levels decline by about 0.08 m/year near the coastline; furthermore, it is estimated that the water levels (water table) of the central part of the MC decline by an average of about 1.08 m/year. A plot of time series depicting groundwater level changes of boreholes WW16953, WW21499, and WW21649 presented in Fig. 23 date back from 1986 to May 2018. Masterson and Walter (2009) report that withdrawals of groundwater from the coastal aquifers of southeastern Massachusetts (USA) change the water levels, the flow directions and the groundwater discharge rate into streams and coastal water bodies. They further mentioned that the potential effects of increased groundwater abstraction will result in declines in pond levels, increases in the depth to the water table beneath inland wetlands, reductions in streamflow, reductions in groundwater discharge to the coast and hence increased saltwater intrusion.
Fig. 23

Plot of time series depicting groundwater level changes of boreholes WW16953, WW21499 and WW21649

The average groundwater draw-downs in production boreholes of the Omdel Aquifer range between 0.8 and 20.03 m, the average draw-down being 0.8 m for borehole WW16953, located near the coast, and 20.03 m for borehole WW22186 in the central part of the MC . Production boreholes with average draw-downs greater than 10 m are WW21495, WW21649, WW22186, WW22187, WW22188/100111, WW22192, WW22194, WW22567/100094, WW35336, WW35337, WW35341, WW35343 and WW35346. The significant draw-downs (>10 m) observed in these production boreholes are due to the cemented alluvial thickness (semi-confined horizon) found in the central part of the MC. Average yields of the production boreholes range between 4.7 and 91.5 m3/h; the lowest average yield of 4.7 m3/h is in production borehole WW21495 and the highest average yield of 91.5 m3/h is observed in WW100157, located in the central part of the MC.

Groundwater chemistry

According to Chen and Jiao (2007), the depletion of sodium in the groundwater is believed to be due to cation exchange when seawater intrudes the fresh groundwater. Na+ is taken up by the soil exchanger and replaced by Ca2+ leaving chloride in excess.
$$ {2\mathrm{Na}}^{+}+{\mathrm{Ca}}^{2+}-{\mathrm{X}}_2={\mathrm{Ca}}^{2+}+2\mathrm{Na}-\mathrm{X} $$
(4)
X indicates the soil exchanger; Na+ is taken up the exchanger during the processes and Ca2+ is released into the water. The excess of Na occurs when the fresh groundwater flushes the previously saline groundwater, resulting in Na+ being released to the solution.
$$ {\mathrm{Ca}}^{2+}+2\mathrm{Na}\hbox{--} \mathrm{X}=\mathrm{Ca}-{\mathrm{X}}_2+{2\mathrm{Na}}^{+} $$
(5)

Seven groundwater quality facies of selected boreholes in the Omdel Aquifer are recognized in the Piper diagram (Fig. 15) and are Na + K-Cl + SO4, Ca + Mg-Na + K-Cl + SO4, Na + K-HCO3-Cl + SO4, Ca + Mg-Na + K-Cl + SO4, Na + K-Ca + Mg-Cl + SO4, Ca + Mg-Na + K-Cl + SO4-HCO3, and Ca + Mg-Na + K-HCO3-Cl + SO4. The presence of HCO3 in some of the groundwater quality facies suggests recharge at that part of the aquifer through seepage at the dam, groundwater flow upstream of Omdel Aquifer and through the OMAP. The absence of HCO3 in the groundwater of some boreholes suggests that there is lack of recharge in that part of the aquifer. All groundwater facies of selected boreholes of the Omdel Aquifer have NaCl indicating a coastal environment.

According the expanded Durov diagram, there are six water types for the selected boreholes of the Omdel Aquifer: calcium bicarbonate, bicarbonate sodium, sulphate or (anions) and sodium, chloride and calcium dominant, chloride and no dominant cation, and chloride and sodium (Fig. 24). Calcium bicarbonate water type indicates recharged or recharging water (Usher 2002), and borehole WW100044 located upstream of SEC has this water type (Fig. 16). Bicarbonate sodium water type indicates ion exchanged water; borehole WW22188 located in the centre of MC contains this water type. Borehole WW100061 located downstream of SEC contain sulphate or (anions) and sodium water type, which may be due to mixing influences. Chloride and calcium dominant water type indicates that reverse ion exchange is taking place, and this water type is found in boreholes WW100057 and WW100046 located downstream and upstream of SEC respectively. Boreholes WW25992, WW100060, WW16484, WW100050 and WW100045, located downstream of the NEC as well as downstream, central and upstream of the SEC, have chloride and no dominant cation water type suggesting reverse ion exchange is taking place. The water type of the majority of the selected boreholes of the Omdel Aquifer is chloride and sodium, indicating an end point in a water evolution sequence. About 89% of the selected boreholes of the Omdel Aquifer have chloride concentrations in the groundwater that exceed the World Health Organization (WHO) drinking water standard (250 mg/l).
Fig. 24

Expanded Durov diagram of selected boreholes of the Omdel Aquifer

Spatial distribution patterns suggest that the boreholes located near or along the River Tugela have high concentrations of Na and Cl (Ntanganedzeni et al. 2018). mNa/Cl and chloro alkaline indices (CAI1 and CAI2) indicate that reverse ion exchange reactions are dominating over cation exchange in the Tugela catchment (Ntanganedzeni et al. 2018). The concentration of TDS, total hardness (TH), Na, Ca, and Cl observed in boreholes that are in Tugela catchment exceeded the drinking water standards recommended by WHO (80%) and South African drinking water standards (SAWQG; 90%) according to groundwater suitability assessment (Ntanganedzeni et al. 2018). Offenborn (1999) found that the hydraulic contact between the MC and NEC has been proved at boreholes WW 21501 and WW 22188, while the hydraulic contact between the MC and NC has been provided at borehole WW 22195, due to brackish groundwater observed in these boreholes. Borehole WW21501 was replaced by WW100095 in 2002; therefore, borehole WW100095 indicates the location of borehole WW21501 in Fig. 16.

The study revealed that about 38% of 101 groundwater samples collected in 2004 had ratios of rCa/(rHCO3 + rSO4) > 1 (Fig. 13), which suggests that the Omdel Aquifer suffered the seawater intrusion in 2004 (Chen and Jiao 2007).

The rCa/(rHCO3 + rSO4) ratios were average 0.94 for the groundwater samples collected in 1998, with about 33% of the samples having ratios >1. It is also observed that the rCa/(rHCO3 + rSO4) ratios of the groundwater samples of the Omdel Aquifer collected between 1993 and 2012 are <1, except for 2004, and only a few water samples have a ratio >1.

The decreasing trend of the rCa/(rHCO3 + rSO4) ratios indicates that the Omdel Aquifer experienced gradual freshening after 2004 and between 1993 and 2003. The decrease in the ratio suggests that the saline front moved seaward and as a result the Ca2+ was adsorbed by the aquifer.

The three highest peaks observed over this period are for 1998, 2000 and 2004, however the 2004 sample remains the highest peak of TDS and chloride recorded, and it agrees with the rCa/(rHCO3 + rSO4) ratio for the year 2004, suggesting possible seawater intrusion (Fig. 14). The ionic ratios HCO3/Cl, Na/Ca, Ca/Cl, Mg/Cl and Ca/SO4 can be efficiently used to delineate seawater intrusion (Lee and Song 2007). The Cl and TDS values greater than 316 and 1,260 mg/l respectively, indicate strongly the effect of saline water intrusion (Lee and Song 2007). About 76 and 63% of Cl and TDS values, respectively, of the selected boreholes of the Omdel Aquifer have values greater than 316 and 1,260 mg/l, respectively, suggesting possible seawater intrusion. In coastal areas, a saline water body would intrude the fresh groundwater and forms an interface or a transition zone in the subsurface, even if there is no pumping taking place (Lee and Song 2007).

According to Geyh and Ploethner (1995), the variability of 14C values indicates that the occasional flash-flood recharge and the pumping action together yield a confusing temporal and spatial 14C distribution pattern. Geyh and Ploethner (1995) said that the clustering of the 14C data of the tritium-free water samples indicates that there is flash-flood recharge in the Omaruru catchment in areas with surface geology dominated by volcanic rocks and calcretes. There is no temporal or spatial trend of the 14C values in the MC where groundwater abstraction is taking place and the groundwater is recharged by groundwater flow and ephemeral river runoff (Geyh and Ploethner 1995). However, the distinct values of 14C and δ13C rule out mixing of the groundwater from flash flood events or recharge conditions (Geyh and Ploethner 1995). Geyh and Ploethner (1995) stated that the δ18O value of NEC groundwater at ─6.87‰ differs significantly from the young groundwater abstracted in the production boreholes of MC (─7.38 ± 0.1‰).

Groundwater recharge

The groundwater recharge estimation by the WTF method indicates that more water is recharged at Nei-Neis, Spitskoppe and Tubussis than at Okombahe and Omaruru, and this may be due to topographical differences and aquifer dimensions. Adelana (2010) mentioned that the WTF method is capable of identifying relative changes in seasonal recharge due to differences in rainfall. The WTF method is only capable of estimating recharge when water is arriving at the water table at a higher rate than it is leaving, producing a water level rise (Healy and Cook 2002). It is observed that the recharge over rainfall ratio at different localities in the Omaruru catchment is relatively small indicating that rainfall contributed a small portion to the water level rise (recharge, Table 4). Therefore, runoff plays a very important role in the water level rise (groundwater recharge) in the Omaruru River bed alluvial aquifers. Surface runoff occurs when the soil’s infiltration capacity is exceeded by the precipitation rate and increases with increasing amounts of precipitation (Adelana 2010).

The groundwater recharge estimation by CMB method indicates that direct rainfall contributes more to recharge at Okombahe compared to the other localities such as Nei-Neis, Spitskoppe and Tubussis. The rainfall at the coast is regarded as insignificant to direct recharge in the Omdel Aquifer, since high evaporation is expected and plays a major role. From this perspective, the Omdel Aquifer is assumed not to receive any direct recharge from local rainfall, but is rather recharged through artificial recharge (significant run-off), seepage underneath the dam and groundwater flow from the upper river bed (Omdel upstream), OMAP and SEC. The groundwater flow is assumed to play a major role in the recharge of the Omdel Aquifer, since significant run-off only reaches the Omdel Dam occasionally. The groundwater flow estimated at Okombahe between April 1996 and February 2016 is more than the groundwater flow estimated at Nei-Neis (Figs. 18 and 19), may be due to aquifer dimensions. The significant groundwater flow estimated at Okombahe and Nei-Neis contributes to the significant groundwater flow estimated at Omdel upstream (about 2.6 Mm3/year; Table 6), and hence contributes to the groundwater recharge of the Omdel Aquifer.

The artificial recharge (enhanced) at the infiltration ponds plays a major role at the Omdel Aquifer, since at least 60% of the infiltration occurs there; the evidence is the effect on groundwater level of the monitoring boreholes (WW33066, WW31366, WW33069 and WW31243) in the surroundings of the infiltration ponds (Fig. 21).

Groundwater discharge

The groundwater discharge from the Omdel Aquifer is considered to be abstraction from production boreholes, evapotranspiration and groundwater outflow to the sea. A total average groundwater abstraction from the 42 production boreholes at the Omdel Aquifer amounts to 5.2 Mm3/year. Groundwater is also discharged from the aquifer through evapotranspiration from open water, trees, reeds and other vegetation and is estimated to be about 0.2 Mm3/year (IWACO 2001).

The groundwater outflow to the sea was estimated by Darcy’s law: K (18.5 m/day), A (190,077 m2) and i (0.0024), thus Q = 18.5 × 190,077 × 0.0024 = 8,439 m3/day, therefore suggesting an annual groundwater outflow to the sea to be about 3.08 Mm3/year (Table 8). Zeelie (2001) and IWACO (2001) estimated Q to the sea to be 3.0 Mm3/year, while Bittner et al. (2014) estimated it to be 3.05 Mm3/year.
Table 8

Estimated groundwater outflow to the sea

Location

K (m/day)

A (m2)

i

Q (m3/day)

Q (Mm3/year)

Sea

18.5

190,077

0.0024

8439

3.08

Groundwater balance

The groundwater balance of the Omdel Aquifer was estimated before the construction of the dam and again after its construction. According to the water balance estimated before construction, the total amount of annual recharge was estimated at 5.8 Mm3/year (Table 9). Such estimates considered groundwater flow upstream of Omdel, groundwater flow (OMAP) and natural recharge which was regarded to be 17% of a 13 Mm3 average flood. The total annual groundwater discharge by groundwater outflow to the sea and direct pumping (abstraction) was estimated at 8.0 Mm3/year, whilst evapotranspiration losses were regarded as zero due the water-table depth and sparse vegetation.
Table 9

Groundwater balance of Omdel Aquifer before dam construction (after Zeelie 2001)

Balance components

Q (Mm3/year)

Comments

Main channel inflow

Groundwater flow (Omdel upstream)

3.0

Darcy calculations

Groundwater flow (OMAP)

0.5

Darcy calculations

Natural recharge

2.3

17.5% recharge of 13 Mm3 flood

Subtotal inflow

5.8

Main channel outflow

Abstraction

5.0

 

Outflow to sea

3.0

Darcy calculations at sea interface

Subtotal outflow

8.0

Overall balance components

Over-exploitation

2.2

Sustainable yield

2.8

Average abstraction from the production boreholes was 5.0 Mm3/year, which exceeds the sustainable yield of 2.8 Mm3/year of the Omdel Aquifer by 2.2 Mm3/year.

According to the water balance estimated after the dam construction, the total amount of annual recharge from groundwater flow (Omdel upstream), groundwater flow (OMAP), natural recharge (17.5% of 6.2 Mm3/a long-term average spill), sum of infiltration at the dam, river section and ponds, average volumes infiltrated at sites 1 and 2 (recharge ponds) as well as groundwater flow from the SEC near WW16662, is estimated at 7.87 Mm3/year (Table 10). The total annual groundwater discharge by groundwater outflow to the sea, direct pumping (abstraction) and through evapotranspiration is estimated at 8.48 Mm3/year.
Table 10

Groundwater balance of Omdel Aquifer after dam construction

Balance components

Q (Mm3/year)

Comments

Main channel inflow

Groundwater flow (Omdel upstream)

2.6

Darcy calculations

Groundwater flow (OMAP)

0.3

Darcy calculations

Natural recharge

1.1

Average contribution from spills (IWACO 2001)

Sum of infiltration at dam, river section and pond(s)

2.26

Mostert 2014

Average volumes infiltrated at Sites 1 and 2 (recharge ponds)

1.49

Mostert 2014

SEC

0.12

Darcy calculations near WW16662

Subtotal inflow

7.87

Main channel outflow

Abstraction

5.2

Outflow to sea

3.08

Darcy calculations at sea interface

Evapotranspiration

0.2

IWACO 2001

Subtotal outflow

8.48

Overall balance components

Over-exploitation

0.6

Sustainable yield

4.6

Abstraction from the production boreholes for the past year was 5.2 Mm3, which exceeds the sustainable yield of 4.6 Mm3/year of the Omdel Aquifer by 0.6 Mm3/year. After the construction of the Omdel Dam, the annual recharge increased from 5.8 to 7.87 Mm3/year and the estimated sustainable yield increased from 2.8 to 4.6 Mm3/year. Figure 25 shows the schematic diagram of groundwater balance components.
Fig. 25

Groundwater balance components of the study area

For the past 22 years, since 1994 (after the construction of Omdel Dam), the Omdel Aquifer has been operating at an average yield of 6.3 Mm3/year. This exceeds the sustainable yield of 4.6 Mm3/year by 1.7 Mm3 and such over-abstraction is clearly observed in the continued downward trends in water levels of the monitoring boreholes and production boreholes. The temporal changes of the 14C values observed in the boreholes between the border of MC and NEC, may be due to over-exploitation of the Omdel Aquifer (Geyh and Ploethner 1995). Saltwater intrusion is actually caused by abstracting more groundwater than is sustainable via recharge, and as a result adjacent bodies of saltwater are drawn into the abstraction zone of influence (Ezzy 2005). In order to maintain the sustainable yield of the Omdel Aquifer, the storage capacity of Omdel Dam should be maintained by regular silt removal. Such an exercise may not be possible for practical and economic reasons; furthermore, the sustainable yield could be maintained if significant run-offs are received more frequently. Coastal aquifers can be used as a sustainable source of freshwater if managed correctly and exploited according to recharge, well pattern and local hydrogeological characteristics (Adelana 2010). According to Bredehoeft (2002), sustainability of groundwater development takes place when the pumping captures an equal amount of virgin discharge.
$$ P=\Delta {D}_0 $$
(6)
whereby P is pumping and ∆D0 is a change in the virgin rate of discharge.

Mitigation for the continued over abstraction may be attributed to an increase in water demand by the various consumers and also the opening of new uranium mines such as Langer Heinrich. Enhanced recharge from the Omdel Dam also did not materialize to the extent that it was expected; however, the aforementioned data clearly indicate that drastic measures need to be implemented to reduce over abstraction from the aquifer and at least maintain its groundwater levels above some critical point.

Conclusions

The Omdel Aquifer is an alluvial aquifer with four palaeochannels (MC, NC, NEC and SEC). Bedrock geometry of the Omdel Aquifer indicates that the MC is the largest reservoir of stored fresh groundwater estimated at about 133 Mm3 and is deeper than the other three channels with an average sediment thickness of 80 m.

Aquifer parameters were estimated with high T values associated with coarse sediments and low T values associated with the presence of clay materials. All groundwater chemistry facies of the selected boreholes of the Omdel Aquifer reveal a NaCl character, indicating a coastal environment. The water type of the majority of the groundwater samples of selected boreholes of the Omdel Aquifer is chloride and sodium, indicating an end point in a water evolution sequence.

The recharge over rainfall ratio at different localities in the Omaruru catchment is relatively small indicating that rainfall contributed a small portion to the water level rise (recharge). Therefore, runoff plays a very important role in the water level rise (groundwater recharge) in the Omaruru River bed alluvial aquifers. Recharge estimation confirms that groundwater in the aquifer is replenished mainly by seepage underneath the dam, enhanced by artificial recharge (significant run-off) and groundwater flow from upstream, OMAP and SEC. The major flood events after the construction of the Omdel Dam took place during the hydrological seasons of 1996/1997/1998, 1999/200, 2007/2008, 2008/2009 and 2010/2011 and the artificial recharge ranges between 52 and 89% of the respective flood events (52% in 1999/2000 and 89% in 2010/2011). However, the hydrological season 1996/1997/1998 shows more infiltrated water (9.55 Mm3) compared to the other hydrological seasons.

The total annual recharge to the Omdel Aquifer after construction of the dam is estimated at 7.87 Mm3/year, with a total groundwater discharge rate estimated at 8.48 Mm3/year. Therefore, the total annual recharge increased from 5.8 Mm3/year (before the dam construction) to 7.87 Mm3/year (after construction of the dam). Groundwater abstraction amounts to 61% of the estimated annual discharge. The sustainable yield of the Omdel Aquifer increased from 2.8 Mm3/year (before the dam construction) to 4.6 Mm3/year (after the dam construction), which can be maintained if the storage capacity of Omdel Dam is maintained by regular silt removal. Artificial recharge (enhanced) therefore contributes significantly towards the increase of the sustainable yield of the Omdel Aquifer. The groundwater system will reach a new equilibrium by means of capture, and the principal tool to carry out such investigations is the groundwater model (Bredehoeft 2002). According to Kalf and Woolley (2005), the law of conservation of mass plays an important role when assessing sustainable yield of an aquifer. They also mention that any groundwater system may reach equilibrium at some time. Groundwater was abstracted from the Omdel Aquifer at an average abstraction of 6.3 Mm3/year for the past 22 years (1994–2016), an over-abstraction of 1.7 Mm3/year. As a result, continued downward trends in water levels of the monitoring and production boreholes were observed. Saltwater intrusion is actually caused by abstracting more groundwater than is sustainable via recharge, and as a result adjacent bodies of saltwater are drawn into the abstraction zone of influence (Ezzy 2005). The groundwater flow dynamics along the Omaruru River suggest that it has a different impact on recharge to the Omdel Aquifer with time. Effective groundwater level monitoring, done by the Department of Water Affairs and Forestry, is in place and plays a vital role. It is against this background that the Omdel Aquifer needs to be carefully operated on a sustainable basis in order to strive for a state of equilibrium. The results provide a sound reference for application to similar aquifer systems prevailing in the Namib Desert, e.g. the Ugab River Delta, Swakop River, Kuiseb River Delta.

Notes

Acknowledgements

The authors acknowledge Namibia Water Corporation Ltd., Geohydrology and Hydrology divisions in the Department of Water Affairs and Forestry, Namibia, for their data, and the Namibia Meteorological Services for the historic rainfall data. The valuable comments and suggestions from the two reviewers are highly appreciated.

References

  1. Adelana SMA (2010) Groundwater resource evaluation and protection in the Cape Flats, South Africa. PhD Thesis, University of the Western Cape, South AfricaGoogle Scholar
  2. Bittner A, van Wyk B, Rossouw T (2014) Numerical groundwater flow model of the Omaruru River Delta Aquifer (Omdel). NamWater, Windhoek, NamibiaGoogle Scholar
  3. Bocanegra E, Da Silva GC Jr, Custodio E, Manzano M, Montenegro S (2010) State of knowledge of coastal aquifer management in South America. Hydrogeol J 18:261–267CrossRefGoogle Scholar
  4. Bredehoeft JD (2002) The water budget myth revisited: why hydrogeologists model. Groundwater 40(4):340–345CrossRefGoogle Scholar
  5. Chen KP, Jiao JJ (2007) Seawater intrusion and aquifer freshening near reclaimed coastal area of Shenzhen. IWA, Hong KongCrossRefGoogle Scholar
  6. Corner B (1983) An interpretation of the aeromagnetic data covering the western portion of the Damara Orogen in South West Africa/Namibia. Spec Publ Geol Soc South Africa 11:339–354Google Scholar
  7. Ezzy TR (2005) Integrated approach to characterization of coastal plain aquifers and groundwater flow processes: Bells Creek catchment, southeast Queensland. PhD Thesis. Queensland University of Technology, AustraliaGoogle Scholar
  8. Geological Survey of Namibia (1997) Geological map of Namibia (Sheets 2114-Omaruru and 2214-Walvis Bay). 1:250 000, Geological Survey of Namibia, Windhoek, NamibiaGoogle Scholar
  9. Geyh MA, Ploethner D (1995) Groundwater isotope study in the Omaruru River Delta Aquifer, Central Namib Desert, Namibia. IAHS Publications-Series of Proceedings and Reports-Intern Assoc Hydrological Sciences 232:163-170Google Scholar
  10. Healy RW, Cook PG (2002) Using groundwater levels to estimate recharge. Hydrogeol J 10:91–109CrossRefGoogle Scholar
  11. IWACO (2001) A critical review of artificial recharge procedures in the Omdel Aquifer, Namibia. Consultants for Water and Environment, AmsterdamGoogle Scholar
  12. Kalf FRP, Woolley DR (2005) Applicability and methodology of determining sustainable yield in groundwater systems. Hydrogeol J 13:295–312CrossRefGoogle Scholar
  13. Klock H (2001) Hydrogeology of the Kalahari in North-Eastern Namibia with special emphasis on groundwater recharge, flow modelling and hydrochemistry. PhD Thesis, University Würzburg, GermanyGoogle Scholar
  14. Kruseman GP, de Ridder NA (1994) Analysis and evaluation of pumping test data. International Institute for Land Reclamation and Improvement, Wageningen, The NetherlandsGoogle Scholar
  15. Lee JY, Song SH (2007) Groundwater chemistry and ionic ratios in a western coastal aquifer of Buan, Korea: implication for seawater intrusion. Geosci J 11(3):259–270CrossRefGoogle Scholar
  16. Masterson JP, Walter DA (2009) Hydrogeology and groundwater resources of the coastal aquifers of southeastern Massachusetts. US Geological Survey, Reston, VAGoogle Scholar
  17. Miller RMcG (2008) The geology of Namibia volume 2 (Neoproterozoic to Lower Palaeozoic). Geological Survey of Namibia, Windhoek, NamibiaGoogle Scholar
  18. Mostert AC (2014) A comprehensive assessment of the hydrology of the Omaruru Delta (Omdel) Dam to determine the volume of water expected to be available for recharge of the Omdel Aquifer. NamWater, Windhoek, NamibiaGoogle Scholar
  19. Muundjua S (2010) Water balance for Omdel Dam during the release for artificial recharge for the 2008/2009 hydrological season. NamWater, Windhoek, NamibiaGoogle Scholar
  20. Nawrowski J (1990) A re-examination of the geohydrology and a re-evaluation of the potential of the Omaruru Delta (Omdel) Aquifer. Department of Fisheries and Water, Windhoek, NamibiaGoogle Scholar
  21. Nawrowski J (1994) Report on investigation of artificial recharge experiments, recharging basin design and operational rules in recharging basins at site I. Department of Water Affairs, Windhoek, NamibiaGoogle Scholar
  22. Ntanganedzeni B, Elumalai V, Rajmohan N (2018) Coastal aquifer contamination and geochemical processes evaluation in Tugela catchment, South Africa: geochemical and statistical approaches. Water 2018(10):687CrossRefGoogle Scholar
  23. Offenborn G (1999) Hydrogeological investigation in the Omaruru “Delta” (Omdel) Aquifer north of Swakopmund, Namib Desert/Namibia. Diploma Thesis, Technical University of Hannover, Hannover, GermanyGoogle Scholar
  24. Sargaonkar AP, Rathi B, Baile A (2011) Identify potential sites for artificial groundwater recharge in sub-watershed of River Kanhan, India. Environ Earth Sci 62:1099–1108CrossRefGoogle Scholar
  25. Seimons W, Muundjua S (2011) Memorandum; a brief overview of the results of Omdel stage 2 water quality monitoring. NamWater, Windhoek, NamibiaGoogle Scholar
  26. Shamsudduha M, Taylor RG, Ahmed KM, Zahid A (2011) The impact of intensive groundwater abstraction on recharge to a shallow regional aquifer system: evidence from Bangladesh. Hydrogeol J 19:901–916CrossRefGoogle Scholar
  27. Steyl G, Dennis I (2010) Review of coastal-area aquifers in Africa. Hydrogeol J 18:217–225CrossRefGoogle Scholar
  28. Usher B (2002) Introduction to hydrochemistry and pollution: lecture notes (GHR 612). University of the Free State, Bloemfontein, South AfricaGoogle Scholar
  29. Zeelie S (2001) Report on the numerical model of the Omdel Aquifer system. NamWater, Windhoek, NamibiaGoogle Scholar

Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Earth SciencesUniversity of the Western CapeCape TownSouth Africa
  2. 2.Namibia Water Corporation LtdWindhoekNamibia
  3. 3.Institute of African Water Resources and EnvironmentHebei University of EngineeringHebeiChina
  4. 4.Karst Hydrogeological ConsultantsWindhoekNamibia

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