ACCOUNTING FOR THE HUMAN IMPACTS OF OVER-EXPLOITING AQUIFERS WITH DETERIORATING WATER QUALITY IN SEMI-ARID REGIONS

In many semi-arid regions, aquifers have watered the Green Revolution vastly increasing food security and economic well-being. As water tables fall, however, rising concentrations of anthropogenic and geogenic contaminants make the water toxic for human consumption. Aquifers throughout Central Mexico and the American West are heavily exploited with irrigation consuming up to 90% of extracted water, dwindling supply and deteriorating quality. Over-pumping aquifers, causing water tables to fall to such a low level that they would not be profitable to pump without government subsidies, and water chemistry changes that inflict untold present and future human burden of disease and its associated costs, is not sustainable. This problem is often viewed as politically intractable, however, since people live off, and profit from, the exploitation of aquifers. Therefore, it is crucial that solutions to the problem are sought and discussed. It is important to identify the subsurface processes that drive groundwater quality deteriorations to design and budget for mitigation options to prevent disease and economic loss. Common causes of groundwater quality deterioration include: 1) tapping older, more mineralized groundwater from deeper depths; 2) up-welling of geothermal waters along faults; 3) irrigation return flow as evapotranspiration drives increases in Total Dissolved Solids (TDS); 4) rising aquifer pH which liberates oxyanions like arsenic from metal oxide binding sites; and 5) changing aquifer redox state from an expanding unsaturated zone or the infiltration of treated or untreated wastewater. Case studies will be presented, along with a discussion of the mitigation options and their costs. Then, taking lessons from our work in the State of Guanajuato, Mexico, we describe a framework for a coupled hydrologic-public health-economic model to account for losses and gains incurred by a basin’s residents and stake-holders by various water management and mitigation strategies. We hypothesize that such an accounting model can help inform decisions made by policy makers at all levels (state, municipal, household) and ultimately help curb the over-exploitation of the shared resource by revealing previously hidden feedbacks between pumping, human health, livelihoods, profits and tax revenue.