Multiple Tracers of Shallow Ground-Water Flow and Recharge in Hilly Loess

Vertical profiles of tritium and nitrate pore-water concentrations were determined to {approximately}8 m depth across two loess hillslopes. Mean recharge fluxes, estimated from chloride mass balance, are 5--10 times larger at the mid- and toe-slope positions than at the top-slope; the magnitudes of the values compare reasonably with results from other methods. The tritium and nitrate profiles exhibit multiple peaks which indicate that piston flow is not the sole flow process in this system. Results of a simple 1-D mixing model suggest that infiltration-exfiltration cycles in zones of plant root activity explain the shallow tritium peaks. Deeper peaks result from preferential vertical and/or lateral flow. The importance of these dispersive processes is underlined by great dilution of observed tritium concentrations, relative to expected concentrations assuming piston flow. In this setting, chloride is useful as a recharge estimator while tritium and nitrate serve as tracers of landscape-scale water movement.