INVESTIGATION OF PLAYA HYDROLOGY AND RECHARGE FLUX TO THE HIGH PLAINS AQUIFER AT EHMKE PLAYA IN WESTERN KANSAS

Groundwater levels of the High Plains aquifer (HPA) are declining in Western Kansas, mainly due to large-scale irrigation pumping beginning in the 1950s. Previous recharge rate estimates in the Central High Plains vary between 5 to 54 mm/yr, requiring a minimum of 270 years for water to penetrate through the 15 to 100 m thick unsaturated zone. Despite this timeframe, agricultural contaminants have been identified in the aquifer, indicating the existence of preferential flow and recharge pathways to the HPA.

Playas, ephemeral lakes with hydric soil floors, are prevalent (>22,000) across the High Plains in western Kansas, but are in decline due to anthropogenic modification. It is hypothesized that playas act as preferential pathways to the HPA mainly because of desiccation cracks that form in the playa floor during dry periods, facilitating fast movement of water during the next rain event. This study aims to determine recharge rates at Ehmke playa in Western Kansas, and deduce whether playas are significant areas of recharge to the HPA.

Geochemical trends through the vadose zone are used in conjunction with matric potential sensors, installed at three depths, to understand fluid and chemical movement beneath the playa. Recharge rates are estimated through chloride mass balance via pore water and water table chloride measurements to determine flux and travel time through the unsaturated zone. Environmental tracers (δ¹⁸O, δ²H, ³H and SF₆) will be collected at the water table and deeper in the groundwater column to determine recharge sources and groundwater residence time. Combined, this data will provide an overview of all hydrologic interactions at the Ehmke playa and constrain rates of recharge. Preliminary chloride results from the upper 5 m of the unsaturated zone at Ehmke Playa range from 2.4 to 15.2 mg/kg of dry soil (0.01 to 0.08 mg/L pore water) in the playa and range from 2.1 to 17.9 mg/kg (0.01 to 0.10 mg/L pore water) in the interplaya. Ultimately, information from this study will be used to better understand playas and their hydrology by providing information on preferential flow, identifying the role of playas in the health of the HPA, improving recharge estimates and providing information to support conservation for the long-term sustainability of the HPA.