Anthropogenic impacts on recharge processes and water quality in basin aquifers of the desert Southwest : a coupled field observation and modeling study

The development of natural grass/scrubland for agricultural use within the Trans-Pecos basins has altered recharge mechanisms and raised questions about groundwater sustainability. Past efforts focused on recharge in arid basin systems used three main assumptions: there is minimal modern recharge, no widespread recharge on basin floors, and no recharge from anthropogenic sources. However, in the Trans-Pecos, nitrate (NO₃-) concentrations have increased in basin groundwater (up by 3-4 mg/l as NO₃- in 40 yrs), refuting the “classic” model and posing water quality risks. Grazing and irrigated agriculture have impacted basin hydrology by altering vegetation regime and the magnitude and spatial distribution of infiltration. This has increased recharge, Cl⁻, and mobile N flux to basin groundwater. A series of spatially-distributed net infiltration models were used to estimate potential recharge from natural and anthropogenic sources. Between 7-20% of potential recharge results from widespread recharge on the basin floors. Additionally, from 1960-2000, irrigation return flow may have contributed 3.0 × 10⁷ - 6.3 × 10⁷ m³ of recharge. These results are supported by field observations. Cores collected beneath agricultural land document changes in water content and pore water chemistry that imply increased downward flux of moisture and solute, and NO₃- and Cl- inventories beneath irrigated land are distinct in amount and profile from those in natural areas. There are significant implications for sustainability based upon the trends in groundwater NO₃- concentrations, core results, and net infiltration models: more recharge may enter the basins than previously estimated and there is a potential long-term concern for water quality. Due to thick unsaturated zones in the basins, long travel times are anticipated. It is unknown if NO3- and Cl⁻ flux has peaked or if effects will continue for years to come. Further study should be undertaken to examine anthropogenic impacts on basin water quality. Additionally, these impacts may occur in similar systems globally and there is considerable evidence for the re-evaluation of the validity of the “classic” model of recharge in arid basin systems. Future studies and management plans should incorporate potential impacts of changes in vegetation and land use on recharge processes and water budgets in arid basins.