Groundwater Age and Water Levels in the High Plains Aquifer

Telemetered, transducer-equipped wells were installed at three sites in the HPA in August 2007 to assess water-level change. The first site, located in southwestern Kansas, consists of unconfined and confined aquifer intervals separated by a thick (>34 m) aquitard. Pumping-induced water level changes in the unconfined intervals indicate a closed (surrounded by no- or low-flow boundaries) system, with no indication of vertical inflow. Given the current rate of decline (~1.5 m/yr), most irrigation wells in this interval will be unusable within several years. The dewatering of the unconfined interval has significant ramifications for the sustainability of pumping from the lower confined interval. Hydrograph analysis indicates most of the pumped water from the lower interval appears to be originating as downward vertical leakage induced by the large drawdown (>35m) resulting from interference between irrigation wells in the lower interval. ¹⁴C age of 11,572 BP beneath the aquitard provide an initial confirmation of the physical interpretation of the system, and compare well with data from a HPA site located in west-central Kansas (¹⁴C age = 11474 BP). A second site, located in northwestern Kansas, has an HPA sequence that consists of a single relatively thick unconfined aquifer. Unlike the classic groundwater-mining condition to the south, previously unrecognized vertical inflow (possibly both from above and below) can be discerned despite indications of a closed system laterally. In years of relatively low pumping withdrawals, this inflow can produce year-to-year increases in water levels, unexpected in an aquifer that has been “mined” for decades. Hypotheses regarding the source of this inflow are currently being tested using environmental. Tritium detected (1.12 TU) in one of five wells sampled in the area, combined with younger ¹⁴C age (4,223 BP) compared with the other HPA sites, indicate that modern recharge is mixing with older water.