Synoptic Sampling to Determine Distributed Groundwater‐Surface Water Nitrate Loading and Removal Potential Along a Lowland River

Abstract

AbstractDelineating pollutant reactive transport pathways that connect local land use patterns to surface water is an important goal. This work illustrates high‐resolution river mapping of salinity or specific conductance (SC) and nitrate ( ) as a potential part of achieving this goal. We observed longitudinal river SC and nitrate distributions using high‐resolution synoptic in situ sensing along the lower Merced River (38 river km) in Central California (USA) from 2010 to 2012. We calibrated a distributed groundwater‐surface water (GW‐SW) discharge model for a conservative solute using 13 synoptic SC sampling events at flows ranging from 1.3 to 31.6 m3 s−1. Nitrogen loads ranged from 0.3 to 1.6 kg N d−1 and were greater following an extended high flow period during a wet winter. Applying the distributed GW‐SW discharge estimates to a simplistic reactive nitrate transport model, the model reproduced observed river nitrate distribution well (RRMSE = 5–21%), with dimensionless watershed‐averaged nitrate removal (kt) ranging from 0 to 0.43. Estimates were uncertain due to GW nitrate data variability, but the resulting range was consistent with prior removal estimates. At the segment scale, estimated GW‐SW nitrate loading ranged from 0 to 17 g s−1 km−1. Local loading peaked near the middle of the study reach, a location that coincides with a shallow clay lens and with confined animal feed operations in close proximity to the river. Overall, the results demonstrate the potential for high‐resolution synoptic monitoring to support GW‐SW modeling efforts aimed at understanding and managing nonpoint source pollution.