Comparing fluid flux and travel time through an anthropogenically modified and natural playa in Western Kansas

Nerhus, Kaela

Publication

Comparing fluid flux and travel time through an anthropogenically modified and natural playa in Western Kansas

Nerhus, Kaela

Abstract

Playas (ephemeral wetlands in semi-arid regions) are thought to be important contributors of recharge to stressed aquifers like the High Plains aquifer (HPA) in the Great Plains region of the United States. Characteristics of playas and playa watersheds may alter the quantity and quality of water reaching the High Plains aquifer as recharge. The mechanisms and controls of recharge through playas are complex. Currently, there are not studies in Central High Plains comparing recharge through playas varying in land use and area. Playa conservation efforts are in place, but it is not known which characteristics of playas make the efforts successful for recharging the HPA, which may limit the effectiveness or efficiency of these efforts. To better understand recharge through playas, we compare fluid flux rates and travel times at an anthropogenically unmodified (natural) playa and a playa with a history of being farmed-through that was recently enrolled in the USDA Conservation Reserve Program. The fluid flux rates and travel times were compared to a nearby anthropogenically unmodified (natural) playa that was studied by Salley (2018), 1.5 km from the modified playa in western Kansas. The playas also differ in size but retain similar regional characteristics like precipitation and depth to groundwater. Two deep cores were collected from the modified playa in July and September 2021. Water-extractable chloride from the core was used to calculate fluid flux and travel times using chloride mass balance techniques. The results are compared to chloride mass balance calculated fluid flux and travel times for the natural playa in 2018. The depth-weighted average fluid flux rates were 24 to 47 mm/year in the modified playa and 19 to 54 mm/year in the natural playa. Rates vary depending on the amount of run-on from precipitation accounted for in the chloride mass balance calculations. The playa with a higher fluid flux changes depending on the value used for run-on in the calculations, which is derived from the precipitation flux at the site. Chloride travel times to the water table were 79 to 169 years slower in the modified playa compared to the natural playa, though the water table was 3.1 m deeper at the modified playa. Based on increased fluid flux measurements at shallow depths in the modified playa core, it is likely that chloride was mobilized due to the land use change in the modified playa fifteen years prior to sample collection. This study suggests playa conservation may be important for recharging the HPA in addition to maintaining the proven ecological benefits of playas. Due to the recharge benefits, landowners and farmers may have a strong incentive to farm around playas rather than through them. To better guide conservation efforts, future studies should better identify the playa characteristics (such as the watershed area compared to floor area, land use, and depth to water) that may promote recharge, which is increasingly imperative as water levels are declining in western Kansas and much of the HPA.