Links between Hydrologic Processes, Water Chemistry, and Microbiology during Rapid Infiltration for Managed Recharge

Abstract:

We present results from a series of plot-scale field tests investigating the role of physical infiltration dynamics and the addition of a carbon-rich permeable reactive barrier (PRB) on nitrogen removal and associated cycling of trace metals during rapid infiltration. Recent studies have shown that denitrification rates and associated N load reduction can be increased during moderate-to-rapid infiltration (< 1 m/day) for managed aquifer recharge (MAR) through use of a carbon-rich PRB. The primary goal for this study is to determine the potential for N removal during rapid infiltration (> 1 m/day) and how the presence of a PRB affects subsurface redox conditions and additional analytes, particularly trace metals. We conducted experiments on a ranch in the Pajaro Valley, central coast California, at a site that is being considered for a new MAR project. Three 1-m^2 plots, each underlain by a 30-cm-thick PRB, were constructed at the field site: one PRB consisted of wood chips, one was a 1:1 mixture of native soil and wood chips, and a control consisted of native soil with no amendment. Infiltration tests lasted 14-16 days, and fluid samples were collected at the surface and shallow subsurface during infiltration and analyzed for N species, dissolved organic carbon, and metals (Fe, Mn, U, and As). Additionally, soil samples were collected before and after infiltration to evaluate grain size, C/N content, and microbial ecology. Plots with carbon-rich PRBs demonstrated increased subsurface DOC concentrations, increased reducing conditions, and increased N load reduction relative to the native soil plot at rapid infiltration rates (~1-4 m/day). N removal was most efficient in the 1:1 mixture plot, potentially because denitrifying microbes in the soil were placed in close proximity to the wood chips. These results, in combination with other studies, indicate that both water quality and quantity can be improved during managed recharge, even during rapid infiltration.