REVISITING THE HYDROGRAPH IN LARGE, OVER-DEVELOPED AQUIFERS – NEW INSIGHTS ON WATER AVAILABILITY IN THE HIGH PLAINS AQUIFER FROM EASILY COLLECTED DATA

Changes in water levels through time provide an integrated response of an aquifer to groundwater development. These changes are calculated from the water-level elevation in a well measured through periodic manual means or continuous monitoring with in-well sensors. Data from continuous monitoring of water levels are particularly valuable because they provide insights into system behavior in large, over-developed aquifers that are difficult to obtain via other means.

Telemetered, transducer-equipped wells were installed at three sites in the Ogallala-High Plains aquifer complex in Kansas in August 2007 with a goal of producing quantitative estimates of year-to-year changes in water level on scales relevant for aquifer management. Transducer records from nearby wells were available for comparison at the two sites that are the focus here. The southern site consists of unconfined and confined aquifer intervals separated by a thick (>34 m) aquitard, while the northern site consists of a single unconfined aquifer (23 m). At both sites, pumping-induced water level changes in the unconfined intervals indicate a closed (surrounded by no- or low-flow boundaries) system. Identification of the nature of this closed basin response is important, particularly at the souther site where water use is five times as large as at the northern site. Water levels in the unconfined interval at the southern site show no indication of vertical recharge or lateral inflow. Given the current rate of decline in the unconfined interval (approx. 1.5 m/yr), most irrigation wells in that interval will be unusable within the decade. The dewatering of the unconfined interval has significant ramifications for the sustainability of pumping from the confined interval because most of the pumped water from that interval appears to be originating from downward vertical leakage. The nature of the closed basin responses are currently being tested using environmental tracers (such as ³H and ¹⁴C). The results of this study indicate that significant insights into aquifer dynamics and water availability can be readily obtained through careful interpretation of well hydrographs, even in complicated, extensively developed aquifer systems.