Semi-Automatic Field Station for Monitoring Agricultural Leaching of Nutrients and Pesticides

Abstract

Leaching of nitrogen, pesticides or other agro-chemicals from agricultural areas have long been investigated due to the increasing awareness of the possible impact on groundwater and the discharge to surface water.

Chemical analyses on soil samples and on groundwater sampled in wells can be performed to get an estimate of the loss of compounds from the top soil. However, a more permanent system to monitor the transfer of compounds from top soils has been developed by the Geological Survey of Denmark and Greenland and the Danish Institute of Agricultural Sciences. The system has been used for more than 8 years on several agricultural sites and is still improved by adding new extensions.

All stations are situated in Jylland, Denmark, covering soil types as coarse sand, sandy loam and loamy till. As the hydraulic conditions are quite different on the field stations the equipment set-up differs from installation to installation.

The basic set-up on all sites consist of a boring (sampling well) drilled to a depth between 5 and 40 m., which in all cases were below the groundwater table. The borings were equipped with sampling screens for groundwater and soil air sampling is possible due to special designed bronze filters placed adjacent to the boring. Soil samples for chemical and physical characterisation and for further experimental use were always collected while drilling.

Below the plough layer and at different depths in the unsaturated profile Teflon suction cups attached to an automatic vacuum or time dependent pump were installed for continuously soil water sampling. In addition, subsurface box-lysimeters with an electronic percolation gauge were installed at 1 meter below soil surface collecting gravitational water. In clayey soils zero-tension samplers have been developed for collecting water conducted in macro pores and fractures.

All sensors and samplers including the groundwater filters, water-tension blocks, thermistors Teflon suction cups, and lysimeter were all installed 8–14 meter from the field boundary, and connections, such as PVC/Teflon tubes and wires, were placed below the plough layer, allowing the farmer to practise the fields unhindered. Sampling was performed from an instrument cottage outside the field, where also a climate station was installed for recording soil-, groundwater- and air temperature, precipitation, lysimeter percolation and groundwater table on hourly base.

With intervals of approximately 15 to 45 days samples from groundwater, lysimeters and Teflon suction cups, soil air and climatic records were collected. Groundwater was extracted by an vacuum pump system through a flow-cell to a sampling container avoiding any contact with the atmosphere. Simultaneous measurements of temperature, oxygen and pH were obtained in the flow-cell attached. Analyses of Fe⁺⁺ and PO₄ was carried out instantaneously in the field.