Suscep bility to Enhanced Chemical Migra on from Depression-Focused Preferen al Flow, High Plains Aquifer

1172 P in the vadose zone is nonuniform fl ow that can cause water and solute movement to bypass much of the soil matrix, often resulting in greater water fl uxes and shorter solute transit times than expected (Flury et al., 1994). Understanding preferential fl ow is crucial for quantifying aquifer susceptibility, defi ned as the inherent hydrogeologic properties that control the occurrence of nonpoint-source contamination of groundwater without specifi cally addressing the chemical properties of that contaminant. Numerous mechanisms that cause preferential-fl ow patterns (macropore, unstable, and focused-fl ow patterns; Hendrickx and Flury, 2001) have been investigated, including fl ow along cracks or fi ssures, biological eff ects (earthworm or root channels), water repellency, variations in soil hydraulic properties, air entrapment, and collection of surface water in topographic depressions (such as recharge playas [Wood et al., 1997]) (Faybishenko, 2000). We investigated a previously unexplored mechanism of depression-focused preferential fl ow and enhanced chemical migration—focused fl ow beneath seasonally ponded conditions that are common in topographic depressions adjacent to leaky irrigation wells in agricultural land use settings (Fig. 1). Leaky irrigation wells refers to all unwanted or accidental discharge of groundwater at the land surface from irrigation pipes and well-head plumbing during the growing season when irrigation wells are used. Previous investigations of fi eld-scale depression-focused recharge have shown rapid mobilization of chemicals to the water table and little or no chemical degradation (Derby and Knighton, 2001). Th is study was motivated by a discrepancy between observed groundwater chemistry and estimated water and chemical fl uxes in the vadose zone above the High Plains aquifer (McMahon et al., 2006). Elevated concentrations of agrichemicals (NO3 and pesticides) have been measured in recently recharged groundwater (<50 yr in age based on 3H) of the High Plains aquifer (Bruce et al., 2003; Gurdak and Qi, 2006; Stanton and Fahlquist, 2006; Gurdak et al., 2007b). Calculated advective chemical transit times of 49 to 10,500 yr from measured 3H and Cl− profi les in the relatively thick (15–60-m) vadose zones beneath some natural rangeland and irrigated agriculture settings, however, are not consistent with measured concentrations of agrichemicals in groundwater (McMahon et al., 2006; Gurdak et al., 2007a). Th e calculated advective chemical transit times generally far exceed the time since the beginning of widespread irrigated agricultural on the High Plains approximately 50 to 60 yr ago. For this study, we hypothesized that focused fl ow beneath seasonal ponds adjacent to leaky irrigation wells and in agricultural fi elds (Fig. 1) enables vertical preferential pathways through Suscep bility to Enhanced Chemical Migra on from Depression-Focused Preferen al Flow, High Plains Aquifer