USING ENVIRONMENTAL TRACERS TO IDENTIFY RECHARGE PATHWAYS TO THE HIGH PLAINS AQUIFER

Unsustainable use of the High Plains aquifer (HPA) for irrigation was identified in the 1970s, but continued despite steep water level declines. Yet, understanding of recharge to this resource remains poorly constrained in many places. To improve estimates of aquifer lifespans, it is necessary to identify sources and quantify rates of recharge to this important resource. The purpose of this study is to understand recharge dynamics and groundwater age across the HPA. Here, geochemical data available in large databases (e.g., STORET, NWIS, etc.), literature, and new data are used to constrain recharge rates and groundwater age across the aquifer.

Recharge rates and depth to water in the HPA vary depending on surface land use and subsurface geology; however, rates generally vary between 5 and 100 mm/yr. Considering depth to water, these values represent recharge rates between 2,000 and 40 years, with longer travel times beneath rangeland and faster travel times found beneath irrigated fields and playas. Similarly, the groundwater age, as determined by the presence of agricultural contaminants or environmental tracers such as tritium, within some portions of the aquifer is modern (<50 years), typically in the northern part of the HPA or near ephemeral streams and playas. In areas with the most significant water table declines, there is typically no evidence for modern water, with ages typically between 2,000 and 5,000 years. Combined, these data indicate that while there are areas of focused, and potentially locally significant, recharge, across much of the western HPA recharge and irrigation return flow are diffuse, suggesting the sustained decline in water level in these locations is no accident.