GROUNDWATER RESPONSE UNITS: A NEW APPROACH TO EVALUATE NONPOINT SOURCE POLLUTANT LOADING AT A WATERSHED SCALE

Groundwater quality is threatened by agricultural nonpoint source pollution but assessing groundwater conditions at a watershed scale is difficult because monitoring wells cannot be installed everywhere. Unlike surface water which flows to a pour point of a basin, groundwater monitoring typically involves the use a small number of wells to characterize conditions over large geographical areas. Recently, we have been developing a new approach to evaluate groundwater quantify and quality at a watershed-scale. Borrowing the Hydrologic Response Unit concept from SWAT, we evaluate representative "groundwater response units" or GRUs in watersheds based on representative recharge, land use and landscape position characteristics. Recharge at individual wells is estimated using the water table fluctuation method and compared to baseflow discharge from the watershed. Representative GRU responses are then summed to quantify groundwater conditions in a watershed. Results from three case studies are used to demonstrate the GRU approach. In the 275 km2 Clear Creek watershed, 13 monitoring wells were established along downslope transects in representative GRUs. More groundwater recharge was found to occur in the floodplain (404 mm) compared to the uplands or sideslopes (281 and 165 mm, respectively). Row crop areas accounted for 96% of the baseflow nitrate loads, but urban areas were found to contribute proportionally larger orthophosphorus (OP) and Cl loads. In the 52 km2 Walnut Creek watershed, the GRU approach demonstrated that groundwater OP concentrations and mass loading rates were three times higher in floodplains than in uplands. In the watershed of a large glacial lake, we used GRUs to quantify the groundwater contributions to the lake nutrient enrichment. Results from these studies indicates that the GRU methodology can minimize the number of wells needed to characterize groundwater agricultural NPS pollution at a watershed scale.