Influence of a carbon amendment on nutrient and metal concentrations at a managed aquifer recharge site

Managed aquifer recharge (MAR) can increase groundwater supply and improve water quality during infiltration of surface water. Subsurface biogeochemistry can be modified with a carbon amendment, such as wood chips, which can help to stimulate microbial processes. Carbon-rich soil amendments have been shown to increase denitrification and reduce N loading, but these same amendments may create conditions favorable for mobilizing redox sensitive elements, including trace metals. To better understand the influence of carbon amendments on the leaching of trace metals during infiltration, we sampled fluids from a MAR field site in central coastal California. Wood chips were mixed in to the upper 40-50 cm of soil across a ~300 m² area in a MAR infiltration basin prior to the start of the rainy season. Water samples were collected in this system from: 1) the subsurface using piezometers installed near the base of the soil amendment layer, 2) the subsurface at an adjacent location where the native soil was not modified, and 3) an inflow culvert through which runoff enters the basin. Water samples were taken during 6 rain events and analyzed for nutrients, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and select dissolved metals (Fe, Mn, and As). Preliminary results show that water infiltrating through the carbon amendment had a reduction in nitrate (2.7 mg/L) and an increase in As (17 ug/L), Mn (3400 ug/L), and DOC (70 mg/L) concentrations. Infiltrating water is likely to pass through a thick vadose zone at this site before reaching the primary aquifer, and additional reactions and processing may occur. We are completing laboratory microcosm and leaching experiments to understand the influence of soil carbon amendments on transport and fate of trace metals during MAR, and will continue and expand monitoring of the field site to better understand impacts on groundwater quality over time.