Quantifying Surface Water, Porewater, and Groundwater Interactions Using Tracers: Tracer Fluxes, Water Fluxes, and End-member Concentrations

Tracer approaches to estimate both porewater exchange (the cycling of water between surface water and sediments, with zero net water flux) and groundwater inflow (the net flow of terrestrially derived groundwater into surface water) are commonly based on solute mass balances. However, this requires appropriate characterization of tracer end-member concentrations in exchanging or discharging water. Where either porewater exchange or groundwater inflow to surface water occur in isolation, then the water flux is easily estimated from the net tracer flux if the end-member is appropriately chosen. However, in most natural systems porewater exchange and groundwater inflow will occur concurrently. Our analysis shows that if groundwater inflow (Q_g) and porewater exchange (Q_p) mix completely before discharging to surface water, then the combined water flux (Q_g + Q_p) can be approximated by dividing the combined tracer flux by the difference between the porewater and surface water concentrations, (c_p – c). If Q_g and Q_p do not mix prior to discharge, then (Q_g + Q_p) can only be constrained by minimum and maximum values. The minimum value is obtained by dividing the net tracer flux by the groundwater concentration, and the maximum is obtained by dividing by (c_p – c). Dividing by the groundwater concentration gives a maximum value for Q_g. If porewater exchange and groundwater outflow occur concurrently, then dividing the net tracer flux by (c_p – c) will provide a minimum value for Q_p. Use of multiple tracers, and spatial and temporal replication should provide a more complete picture of exchange processes and the extent of subsurface mixing.