If you are modeling water quality in a river that has inflow from a groundwater source, enter the concentration of each constituent in that inflow from groundwater to surface water. Note that the quality of water flowing into groundwater will not affect the quality of outflow specified here--groundwater quality is not modeled in WEAP due to the inherent complexities of such a system.
Entered on: Data View, Branch: Supply and Resources \ Groundwater, Category: Water Quality, Tab: <Constituent> Concentration, Temperature