Geostatistical Survey and Amelioration of an Inland Salt Affected Area in the Lake of Texcoco, Mexico

Abstract

The area formerly occupied by the Texcoco Lake, near Mexico City, was subjected to drainage since the past century, to prevent flooding of the city; however, the progressive desiccation and the geological origin of the area gave place to the appearance of strongly salt and sodium affected soils. Lacking vegetation, these soils are very susceptible to wind erosion, and has been attempted to provide the area with a plant cover, using mostly native species. However, there are extensive areas where the levels of salts and exchangeable sodium percentage (ESP) are so high that the introduction of plants has not been successful; in these places, some kind of amelioration is needed, but since a general application of chemical amendments and leaching would be very costly, a methodology for separating areas with different levels of salts and sodium is needed, and this was accomplished using geostatistics. An undisturbed and severely affected area of 1.1 x 1.4 km was selected, dividing the area in squares of 100 m per side, and taking a total of 180 compound samples twice within a year, in order to know the degree of changes in salts and sodium levels along the year. Samples were subjected to analysis of electrical conductivity (EC) and ESP. Data were geostatistically processed in order to obtain variograms and maps by means of block kriging. From the analysis of the same data for vegetated and unvegetated areas, it was concluded that plants develop only in places where the soil EC is lower than 60 dS/m, and ESP is lower than 90. As the obtained maps show the distribution of different degrees of EC and ESP, they can be used to determine the areas which need some kind of reclamation, to lower the salt and sodium level to a degree acceptable by plants. Using this methodology, several halophytic species were introduced in a highly affected, unvegetated area, previously subjected to reclamation using calcium polysulphide and leaching to lower the EC values to 40 dS/m and the ESP to 80. Several species established successfully, including Suaeda torreyana, Distichlis spicata and Tamarix chinensis.