The Role of Riparian Buffer Zones in the Mitigation of Groundwater Nitrate in the Cosumnes River Floodplain.

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 addresses two major issues concerning California groundwater and how both issues relate to restoration of a natural floodplain: 1) depletion of groundwater resources and 2) nitrate contamination.  The objectives of this study are to quantify the role of riparian buffer zones in the mitigation of groundwater nitrate and estimate groundwater recharge of a restored floodplain. We hypothesize that recharge rates and nitrate mitigation will increase after restoration of the floodplain and the wetland riparian ecosystem.  The study site is located on the Cosumnes River floodplain approximately 30 miles south of Sacramento.  The area has a history of agricultural production, but is currently undergoing restoration to become a preserve.  We are utilizing 12 groundwater monitoring wells located in diverse areas of the floodplain representing various ecosystems and management practices.  Groundwater and river samples are taken bimonthly with concurrent and ongoing measurement of groundwater levels. These samples are measured for nitrate and ammonium as well as ∂¹⁵N and ∂¹⁸O.  Recharge rates will be determined from solute tracer techniques utilizing the isotopic analysis.  Because the area has a complex stratigraphy and hydrogeology, it is essential to account for the heterogeneity of the stratigraphy and the complex hydrologic flow paths as well as the amount of recharge occurring in order to fully understand the process and overall mass flux of nitrate involved in the attenuation. Hydrologic modeling and sediment well log data partially account for this complexity, but isotopic data will provide missing information about nitrate sources and flow paths.  Regardless, preliminary nitrate concentration data support the hypothesis that there is increased nitrate attenuation in undisturbed and restored riparian buffer zones compared to uncultivated agricultural fields.