Relation of pathways and transit times of recharge water to nitrate concentrations using stable isotopes

Oxygen and hydrogen stable isotope values of precipitation, irrigation wate r, soil mater, and ground mater were used with soil-moisture contents and w ater levels to estimate transit times and pathways of recharge water in the unsaturated zone of a sand and gravel aquifer, Nitrate-nitrogen (nitrate) concentrations in ground water were also measured to assess their relation to seasonal recharge. Stable isotope values indicated that recharge water u sually had a transit time through the unsaturated zone of several weeks to months. However, wetting fronts usually moved through the unsaturated zone in hours to weeks, The much slower transit of isotopic signals than that of wetting fronts indicates that recharge was predominantly composed of older soil water that was displaced downward by more recent infiltrating water. Comparison of observed and simulated isotopic values from pure-piston flow and mixing-cell water and isotope mass balance models indicates that soil m ater isotopic values were usually highly mixed, Thus, movement of recharge water did not occur following a pure piston-flow displacement model but rat her follows a hydrid model involving displacement of mixed older soil water with new infiltration water. An exception to this model occurred in a topo graphic depression, where movement of mater along preferential flowpaths to the water table occurred within hours to days following spring thaw as res ult of depression-focused infiltration of snow melt. In an adjacent upland area, recharge of snow melt occurred one to two months later. Increases in nitrate concentrations at the mater table during April-May 1993 and 1994 in a topographic lowland within a corn field were related to recharge of wate r that had infiltrated the previous summer and was displaced from the unsat urated zone by spring infiltration. Increases in nitrate concentrations als o occurred during July-August 1994 in response to recharge of water that in filtrated during May-August 1993, These results indicate that the largest g round water nitrate concentrations were associated with recharge of water t hat infiltrated into the soil during May-August, when most nitrogen fertili zer was applied.