Hydrology, Fluid Flux, and Chemical Travel Times through the Vadose Zone at Ehmke Playa in the Central High Plains of Kansas

Abstract

Groundwater levels of the High Plains aquifer (HPA) are declining in western Kansas, due mainly to large-scale irrigation pumping since the 1950s. Previous recharge rate estimates in the Central High Plains vary between 5 to 54 mm yr-1, indicating infiltration to the water table takes at least 270 yrs through the 15 to 100 m-thick vadose zone. Despite this, aquifer contaminants related to agricultural practices since the 1950s suggest the existence of preferential flow and recharge pathways to the aquifer. Playas, ephemeral lakes with hydric soil floors, are ubiquitous features (22,000) across the High Plains in western Kansas, but are in decline due to anthropogenic modification. It is hypothesized that preferential recharge pathways to the HPA form in playa basins, primarily through desiccation cracks that develop in the hydric soil floors, which could facilitate rapid movement of water at the onset of rain events. This study aims to determine recharge rates at Ehmke Playa site in western Kansas and determine if this playa acts as significant point of recharge to the HPA. Anion concentrations in the vadose zone are plotted against depth to visualize peaks and troughs, and better understand vertical flow. Vadose zone chloride concentrations are significantly higher beneath the interplaya (30 to 3,900 mg L-1) than the playa (12 to 140 mg L-1), which indicates slower chemical and water movement through the interplaya. Bromide, nitrate, sulfate, and fluoride depth profiles also show higher concentration beneath the interplaya. Some anion concentrations (chloride, bromide, and sulfate) exhibit a maxima at the base of the root zone (2 m), implying greater evapotranspiritive enrichment in the interplaya and lower fluid recharge flux than within the playa. Utilizing the chloride mass balance (CMB) method, a fluid flux and travel time through the vadose zone is calculated using a ratio of chloride concentration input at the surface with chloride concentration in the vadose zone pore water. Fluid flux rates range between from 78 to 118 mm yr-1 for playa dry periods, 1,700 mm yr-1 during playa wet periods, and from 0.1 to 10 mm yr-1 for the interplaya. Travel times through the 11.5 m playa vadose zone (8 to 150 yrs) were at least an order of magnitude faster than through the 16.7 m interplaya vadose zone (4,800 to 5,500 yrs). Matric potential (MP) sensors installed at four near-surface depths beneath the playa bottom indicate gravity drainage during playa ponding events. The uppermost sensor (12 cm below ground surface, bgs) exceeded field capacity (-33 kPa) on March 29 at 11:30 AM, likely from a rain event beginning March 23 at 8:00 PM and totaling 56.9 mm. Deeper MP intervals (47, 92, and 152 cm bgs) increased above field capacity much later (April 12 at 10:30 PM, April 13 at 9:20 AM, and April 14 at 3:20 AM, respectively) after a large precipitation event (79.3 mm) occurred on April 12 causing the playa to become inundated on April 13. This indicates deeper wetting is heavily dependent upon precipitation events resulting in playa inundation. Evidence of macropores was observed during inundation, when MP is higher at 96 cm than 47 cm, resulting in a zero flux plane (ZFP) around the 96 cm depth. Soil saturated hydraulic conductivity (Ksat) was determined using an automated infiltrometer. Measurements in both the playa and interplaya range from 1.62 x10-4 to 7.84 x10-4 cm sec-1, indicative of a silt loam soil. This value is representative of the playa during inundation, or of the interplaya when saturated during rain events. Hourly water level measurements from pressure transducers in three monitoring wells at the site indicate groundwater flow is towards north 66° east and under a hydraulic gradient of 0.0016. Groundwater anion concentrations show the chloride/bromide mass ratio upgradient (0.004) is lower than downgradient (0.01), which indicates greater meteoric water influence on downgradient waters when compared to upgradient waters, again indicating recharge flux through the playa basin. Combined, the data provide an overview of all hydrologic interactions at the Ehmke Playa site and constrains recharge rates. This study advances understanding of playa hydrology in the CHP by improving recharge estimates, analyzing preferential flow paths, and identifying the role of playas in the long-term sustainability and quality of the HPA. Further, data from this study can be used for more realistic water resource estimates, support of playa conservation, and improved management of the HPA.