title: High resolution synoptic salinity mapping to identify groundwater--surface water discharges in lowland rivers.
abstract: Quantifying distributed lateral groundwater contributions to surface water (GW-SW discharges) is a key aspect of tracking nonpoint-source pollution (NPSP) within a watershed. In this study, we characterized distributed GW-SW discharges and associated salt loading using elevated GW specific conductance (SC) as a tracer along a 38 km reach of the Lower Merced River in Central California. High resolution longitudinal surveys for multiple flows (1.3-150 m(3) s(-1)) revealed river SC gradients that mainly decreased with increasing flow, suggesting a dilution effect and/or reduced GW-SW discharges due to hydraulic gradient reductions. However, exceptions occurred (gradients increasing with increasing flow), pointing to complex spatiotemporal influences on GW-SW dynamics. The surveys revealed detailed variability in salinity gradients, from which we estimated distributed GW-SW discharge and salt loading using a simple mixing model. Modeled cumulative GW discharges for two surveys unaffected by ungauged SW discharges were comparable in magnitude to differential gauging-based discharge estimates and prior GW-SW studies along the same river reach. Ungauged lateral inlets and sparse GW data limited the study, and argue for enhancing monitoring efforts. Our approach provides a rapid and economical method for characterizing NPSP for gaining rivers in the context of integrated watershed modeling and management.
authors.affiliation: School of Engineering and Sierra Nevada Research Institute, University of California, Merced, California 95343, United States.
authors.forename: Henry
authors.initials: H
authors.lastname: Pai
authors.affiliation: School of Engineering and Sierra Nevada Research Institute, University of California, Merced, California 95343, United States.
authors.forename: Sandra R
authors.initials: SR
authors.lastname: Villamizar
authors.affiliation: School of Engineering and Sierra Nevada Research Institute, University of California, Merced, California 95343, United States.
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Harmon
mesh_terms: D002140:California; D004784:Environmental Monitoring / Q000379:methods*; D060587:Groundwater / Q000737:chemistry*; D045483:Rivers / Q000737:chemistry*; D054712:Salinity; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014874:Water Pollutants, Chemical