Analysis of Chemical Storage and Transit Times to Characterize Water Movement Through a Thick Unsaturated Zone Overlying the High Plains Aquifer, Northwestern Kansas

Abstract

The High Plains aquifer (HPA) is one of the largest aquifers in the world and is critical for agricultural production in both the United States and Kansas. Irrigation has had a large-scale effect on the water resources throughout the HPA, causing steep declines in groundwater levels due to extensive groundwater mining. Work conducted as part of the Kansas Geological Survey (KGS) Index Well Program revealed unexpected increases in water levels in the northwestern Kansas portion of the HPA not associated to precipitation or pumping. This indicated a previously unknown source of recharge to the aquifer and the need to further investigate water movement processes through the thick unsaturated zone above it. The roles of irrigation, land use, and subsurface geology as a possible driving force for water and chemical movement to the aquifer is uncertain due to the thick unsaturated zone above it. The goal of this research was to obtain a better understanding of the relationship between these processes occurring in the unsaturated zone and the aquifer by investigating the storage and transit time of chemicals in the thick 65 meter unsaturated zone in Thomas County, Kansas. Climate, land use changes and the conversion from natural to agricultural irrigated ecosystems within the last century are important controls on chemical movement through the unsaturated zone at this site. Detailed geochemical profiles indicated displaced chloride, nitrate and sulfate reservoirs that were likely mobilized by both an increase in downward water flux during the initial clearing of natural vegetation in the 1920s and again by increased drainage due to the onset of large-scale irrigation in 1984. An average annual water flux of 92 mm/yr was estimated using a chloride mass-balance approach and corresponds with other independent estimates in the area. Calculated chemical transit times through the profile are on the order of hundreds of years and longer than the irrigation history at the field site. However, only chloride mass-balance ages above the predevelopment water table (above 36 m) are considered accurate since water in the lower profile appears to be remnant water from before the water table declined. This data establishes evidence for complicated flow paths through the unsaturated zone to the aquifer that are influenced by many factors including lithology, climate, land conversion, and irrigation.