The Mezquital valley, 80 km north of the metropolitan area of Mexico City (MAMC), is an example of a low cost Soil-Aquifer-Treatment system, in which untreated sewage and surface runoff collected within the closed basin of Mexico, are used to irrigate mainly fodder crops and maize. The valley is at 2100 m asl, and has a temperate semi-arid climate. Natural vegetation corresponds to xerophytic shrublands, but irrigation has changed the natural hydrologic environment and the land cover. Today more than 90,000 ha are cultivated mainly with lucerne and maize cropped in rotation. The soil moisture regime has changed from ustic to udic, and there is an artificial groundwater recharge of 6 m3/s of the semi-confined upper aquifer. Supply water for more than 500,000 inhabitants of the valley is provided by this semi-confined aquifer.
Our working group manages a critical zone observatory in this area since 1990. The objective is to evaluate the impact of long-term irrigation on the soil, crop, groundwater and air quality in the region. We also collaborate in assessing the impact on human health. We do this following two main strategies:
We have repeatedly sampled fields irrigated for different lengths of time with untreated wastewater in 1990 and 2009, respectively. Fields are distributed among the dominant soil types, namely Leptosols, Phaeozems and Vertisols (Calciustolls and Pellusterts). In these fields we have sampled soils (by genetic horizon and also composite samples from the upper 30 cm) and crops, and have analyzed them for nutrients and pollutants as heavy metals, pharmaceuticals, detergents and salts. We have also studied the microbial communities and their activity.
Since 2008 we are monitoring critical zone processes during single irrigation events to follow groundwater recharge, nutrient and pollutant leaching as well as greenhouse gas emissions. This has been done along a transect crossing the extended piedmont of the valley. We have installed deep piezometers (30 and 25 m deep), observation wells (at 1, 2, 4 and 5 m depth) and suction cup lysimeters in the first 4 dm of the soil. We also installed gypsum blocks, TDR probes, tensiometers to follow moisture contents and water tension; Pt-electrodes to monitor redox potentials and static chambers to measure gas emissions.
We also have sampled fields and monitored processes in rain-fed agricultural fields as well as in plots covered by the natural vegetation for comparison.