UNDERSTANDING THE ROLE OF GEOLOGY ON RECHARGE TO AND PRODUCTION FROM THE HIGH PLAINS AQUIFER IN KANSAS (Invited Presentation)

While it has long been clear that steep declines in groundwater levels in the High Plains aquifer (HPA) are due to overpumping for irrigation, long-term pumping rates have remained at unsustainable levels. The aquifer is commonly characterized as a homogeneous aquifer; however, it is actually a complex stacking/interfingering of highly productive sands and gravels with confining or semi-confining silts and clays. This complex geology affects both recharge and production of water across the aquifer.

The goal of this research is to obtain a better understanding of the role of geology on recharge to and production from the HPA. Detailed, stratigraphically-controlled fluid geochemical analysis is combined with geochemical and age analysis of waters sampled from irrigation and monitoring wells to identify stratigraphically significant units. Water-level analysis provides additional controls on stratigraphically significant units, identifying flow boundary conditions and suggesting further limitations to aquifer lifespan.

Recharge to the aquifer is constrained by surface features, land use, subsurface geology and depth to water. In western Kansas, surface or near-surface loess of variable thickness likely controls recharge rates in many areas. Focused recharge through depressions, ephemeral stream beds, and playas is likely dominant, but has not yet been studied in great detail in Kansas. Increased subsurface moisture beneath irrigated fields relative to grasslands should result in increased recharge rates, but this can be limited by loess, silts, and clays, as well as ever-increasing depths to water. Several studies in upland portions of the western Kansas HPA have yet to identify irrigation return flow reaching the aquifer.

As water levels fall, silts and clays increasingly limit productive zones, restricting production to only a few stratigraphic zones. In some instances, this is now one or two zones of less than 10 m each, even though the reported saturated thickness exceeds 60 m. These zones, identified through a combination of water-level and geochemical analyses, indicate the life expectancy of the aquifer may be more dire in some areas than typically described. Evidence also exists to suggest some aquitards and well screens are leaky also provide water for irrigation.