FOCUSED RECHARGED AND ENHANCED-CHEMICAL MIGRATION FROM SEASONAL PONDING NEAR IRRIGATION WELLS, HIGH PLAINS AQUIFER

Groundwater recharge and contaminant transport are controlled in part by the inherent hydrogeologic properties of the unsaturated zone, which includes the effects of preferential-flow pathways. We investigated the importance of seasonal ponding near leaky irrigation wells as a mechanism for preferential flow and enhanced-chemical migration through the unsaturated zone to the High Plains aquifer (450,700 km²). Such a mechanism could explain the widespread occurrence of agrichemicals in recently-recharged groundwater despite estimates of advective-chemical transit times through the unsaturated zone that exceed the historical period of agriculture. Using a combination of field observations, unsaturated-zone flow and transport simulations, and probabilistic neural-network modeling, we demonstrate that focused flow and recharge beneath seasonal ponds could affect the entire aquifer, but in only 20% of the areas is it likely to reach the water table in less than 50 years. Transport-simulation results indicate enhanced transit times near irrigation wells ranging from 7 to 50 years, which are 1 to 2 orders of magnitude faster than previous estimates from diffuse recharge settings. These findings support the concept of spatially-distributed fast and slow recharge zones, and help to explain the previous discordant findings of long unsaturated-zone transit times and the presence of agrichemicals at the water table. Using predictions of aquifer susceptibility from probabilistic neural-network models, we delineated areas of the aquifer that could experience advective-contaminant transit times to the water table of less than 50 years owing to preferential flow. The resulting aquifer susceptibility map delineates the spatial distribution of potential focused recharge and may provide a tool to help resource managers prioritize areas for groundwater monitoring or implementation of alternative best-management practices.