Managed Aquifer Recharge as Tool for Sustainable Management of Groundwater Quantity and Quality in Agricultural Basins
University of California, Santa Cruz
Managed aquifer recharge (MAR) is used to augment water supplies and limit the adverse impacts of groundwater overdraft in many basins. As groundwater demands continue to increase and MAR projects become more common, it is increasingly difficult to secure pristine sources for MAR throughout the year. Instead, resource managers must explore options for using MAR sources such as stormwater runoff, treated wastewater, and supplies influenced by agricultural activity. Many such sources are impaired by elevated nutrient concentrations and thus there is a need to understand the conditions under which the quality of managed recharge can be improved. In this study, we quantify the rates and controls on denitrification during MAR with an emphasis on the load reduction that occurs during infiltration. Our study site includes a 3 hectare percolation pond located in central coastal California into which excess flows are diverted from a nearby wetland during the rainy season. Recharged water passes through a 20 to 30 m thick vadose zone below the pond and enters a perched aquifer from which it is recovered and distributed to local growers. The concentration of nitrate in diverted wetland water is generally between 10 µmol and 100 µmol but values over 1000 µmol have been observed. Major element and stable isotopic analyses of infiltrating water and groundwater in combination with point measurements of infiltration indicate considerable spatial and temporal variability in hydrologic and geochemical conditions beneath the recharge pond. Nitrate concentrations are consistently reduced by 50-90 percent during infiltration. Nitrate isotopic analyses suggest that most of this reduction in nitrate concentration results from denitrification. The average rate of denitrification during infiltration was 30 µmol N/L/d, although rates as high as 200 µmol N/L/d were observed. While denitrification rates were highest at high infiltration rates, denitrification was most efficient at reducing nitrate concentration when the rate of infiltration was between 0.2 m/d and 0.5 m/d. At most sample locations denitrification was not observed when the rate of infiltration exceeded 0.75 m/d. Despite high concentrations of dissolved organic carbon (DOC) in the recharge water, modest reductions in DOC during infiltration suggest that solid phase materials may also supply electron donors for denitrification in this system. Integration of physical and chemical data indicates that denitrification may result in a 50percent reduction in the nitrate load delivered from the pond to the underlying aquifer. Collectively, these data suggest that denitrification during managed aquifer recharge may be an effective tool for achieving regional nutrient load reduction goals.