Presentations 2016

Groundwater Nitrate Attenuation and Changes in Groundwater Quality Across a California Delta Floodplain

University of California, Davis

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Groundwater is an essential resource to California residents, agriculture, and industry. Ongoing drought conditions limit aboveground water resources, causing groundwater to become increasingly vital. This study addressed groundwater quality as it relates to a delta floodplain in California’s central valley. The objectives were to evaluate the role of riparian buffer zones as well as biogeochemical processes in the mitigation of groundwater nitrate (NO3-) and to estimate the spatial, temporal and restoration related changes seen in groundwater quality. We hypothesized that NO3- would decrease in groundwater (mitigation) after restoration of the floodplain and the wetland riparian ecosystem. The study site was located on the Cosumnes River floodplain approximately 30 miles south of Sacramento. The area has a history of agricultural production, but recently underwent restoration to become a preserve. We used 12 groundwater-monitoring wells located in areas of the floodplain representing various physical hydrological parameters and depths to groundwater. Groundwater and river water quality samples were taken bimonthly to monthly with concurrent measurements of groundwater levels. The groundwater quality samples were analyzed for NO3- and ammonium (NH4+) as well as d15N and d18O of NO3-. Major cation and anion analysis was undertaken to evaluate hydrochemical facies and flow patterns within the aquifer, and pH, DO, and EC were recorded for part of the study to better understand environmental variables that might drive nitrogen transformations. Sediment textures were characterized at each well and soil C and N profiles were available for some locations from a concurrent study. Comparison of all these factors revealed tremendous spatial variability in the floodplain, with some temporal variability. Taking into account the ongoing drought (absence of seasonal flooding) it was difficult to discern any changes related to restoration efforts. Nitrate concentrations in riparian areas were significantly reduced compared to those elsewhere on the floodplain, though it was difficult to assess if this was due to low nitrate river inputs or riparian uptake. Isotopic analysis provided evidence of denitrification occurring in at least one well in a riparian area. We hypothesize that as restoration continues to improve riparian habitat, nitrate removal in the floodplain will continue providing a valuable ecosystem service for this type of restoration.