Nutrient delivery by groundwater discharge to headwater streams IN agricultural catchments

Nutrient concentrations in riparian groundwater are variable from site to site, making it challenging to use these data to quantify nutrient fluxes contributed by groundwater discharge to headwater streams. Instead, we define event-excluded baseflow (BFEE), and use stream discharge data and stream samples collected during BFEE to estimate groundwater discharge and loading of nutrients to the streams. Based on this method, in two study catchments (Thames River basin, Ontario, Canada) groundwater discharge during BFEE contributed about 30% of the total streamflow and 20%-30% of the total loading of soluble reactive phosphorus (SRP) and nitrate. Previous estimates of baseflow using hydrographic separation techniques indicate that groundwater discharge likely contributes even larger fractions, similar to 50%-60% of flow in these catchments. We infer that groundwater likely contributes similar to 40% of the annual SRP load and similar to 50%-60% of the nitrate-N load. These results indicate that groundwater discharge plays an important role in the nutrient loading to headwater streams. This reinforces the findings of earlier studies that have inferred a dominant influence of legacy reservoirs of nutrients in the subsurface of similar agricultural catchments on the loads of these nutrients in headwater streams.