Density dependent flow occurs in areas where high-salinity groundwater interacts with low-salinity groundwater to create a brine-to-freshwater interface that defies common assumptions about groundwater movement. Yet the geologic and hydrologic factors that impact interface dynamics and migration remain poorly defined. With less than 20 mm•yr-1 of precipitation and with an extremely dense (i.e. 1.2 g•cm-3) naturally occurring brine, Chile’s Salar de Atacama (SdA) provides an excellent analog for exploring interface dynamics in other arid regions. Site-specific 2-D models of the interface in the southeastern region of SdA, with interpretations of the hydrostratigraphic framework, provide an analysis for density-driven response rates to climatic change. A separate parametric, equally probable series of distributions of hydraulic conductivity provides a means for expanding analysis to other similar arid salar (i.e. “salt flat”) environments. Comparing the modeled interface’s geometry and response to perturbations in the rates of lateral recharge in each hydrostratigraphic realization yields insight into the dynamics of interface migration to coupled climatic and geologic conditions. Changes in hydrologic conditions, informed by paleoclimatic interpretations and previously modeled climate predictions, are introduced to each hydrostratigraphic realization following the interface reaching an initial dynamic equilibrium, and the interface’s response is assessed subsequent to it reaching a new dynamic equilibrium. Metrics for model evaluation include migration rate, change in the interface’s areal extent, change in interface slope, and the response rate following the introduction to a perturbation in the aquifer’s hydrology. Model analyses suggest that evaporation rates strongly control the interface’s geometry and sensitivity despite climatic and geologic conditions; continuity of high-permeability pathways controls interface slope; increasing continuity also decreases interface stability in terms of time required to reach a new steady state. While these results have implications for interface dynamics in both salars specifically and arid climates in general, they also indicate the importance of considering hydrostatigraphic continuity for saline water intrusion in coastal regions. They also provide a compelling method for assessing interface dynamics in other climatic and geologic conditions.

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. / text