A conceptual model for increases of sodium, SAR, alkalinity and pH at the independence aquifer in Guanajuato

The Independence Basin, with an area of almost 700 000 hectares (1 729 676 acres), is completely dependent on sources of groundwater for agricultural, urban and industrial uses. Agriculture utilizes 85% of the water that is extracted from the granular aquifer of lacustrine origin that covers another aquifer of a fractured nature in the volcanic rock that dominates Mexico's high plateau. The concentration of sodium, SAR, alkalinity and pH in the groundwater from the Independence aquifer has increased in the past two decades and is progressively damaging soil fertility. A total of 206 samples of groundwater were taken from the 2500 wells that extract water from both aquifers, in order to explain the origin and evolution of the quality parameters of irrigation water for agricultural use. The presence of dominant ions in the groundwater and their control in the quality parameters of irrigation water are consistent with the dissolving reactions of minerals present in the volcanic rocks that exist in the aquifers' recharging zones. In particular, the presence and evolution of sodium, SAR, alkalinity and pH are associated with products from the reaction of the dissolving of the feldspathic mineral known as "albite" (sodium feldspar), which is also related to thermal waters in the fractured aquifer among ignimbrite rocks. This water, rich in sodium, may be migrating towards the granular aquifer through both the effects of density and the excessive extraction of water from the granular aquifer. It was established that the concentration of sodium presents moderate restrictions in approximately 18 000 hectares (44 477 acres) of irrigation by aspersion and 12 000 hectares (29 651 acres) of irrigation by gravity. These quantities may potentially increase in the future if adequate criteria are not established for managing the aquifer and for agricultural planning in the region.