REVISITING THE CONTRIBUTION OF GROUNDWATER TO PLAYAS AS A FUNCTION OF TIMESCALE AND DEVELOPMENT IN THE WESTERN UNITED STATES

Recent studies to determine the amount of fresh groundwater available in alluvial aquifers surrounding playas in the western United States for new water supplies have utilized different techniques to determine water balances for the basin and available water for human use. Measurements of playa surface evaporation and evapotranspiration from phreatophyte areas surrounding the playas indicate that phreatophytes surrounding some playas extract most up-gradient fresh groundwater, limiting groundwater movement to and evaporation from playas that could be used for water supply in these basins. Other studies that have measured small evaporation over large playa surfaces indicate that groundwater could be available for water supplies. Groundwater flow models have shown that density contrasts as the water moves to the playa center can cause recirculating brines as the dense brine sinks and circulates back to the playa edge. Most of these models and water balance studies assume flow through the model on timescales that are short (years). When viewed on this timescale, groundwater contributions to playas are small and don’t contribute significantly to the overall yearly water budget. Although, when viewed on a one- to ten-thousand-year timescale or more, groundwater contributions become more significant and can control the chemistry of the minerals precipitated in the playa and the chemistry of the brine in the playa. The hydraulic conductivity (K) differences between clays in the playa center versus sands, silts and gravels in the surrounding alluvial fans have also been used to indicate that water in alluvial fans do not interact with the low-K playa center lithologies. However, many playas are not simply low K muds. Brine extraction from wells in many playa settings have been occurring for years, indicating that there is sufficient permeability to extract brines. This presentation revisits the different models of groundwater flow in and adjacent to playas and suggests a revised conceptual model that shows that different timescales and objectives of the modeling effort need to be recognized before one can evaluate the role of groundwater as being important, or not. As a long-time friend of Beth Gierlowski-Koresch, who was an advocate of understanding the hydrology of lake systems, this is a fitting tribute to her legacy.