Influence of soil carbon amendments on denitrification in linked field and laboratory managed aquifer recharge systems

Managed aquifer recharge (MAR) includes a diverse set of tools and techniques that direct excess surface water into aquifers to benefit human and aquatic systems. MAR systems can also function as quasi-natural laboratories for developing a process-based understanding of carbon, nutrient, and contaminant cycling as a consequence of soil, water, and microbiological interactions. Adding a carbon amendment as part of a permeable reactive barrier (PRB) may improve water quality by inducing denitrification and related processes (e.g., anammox, dissimilatory nitrate reduction to ammonium, and others). This study focuses on an active MAR field site in central coastal California, which receives stormwater runoff from surrounding agricultural fields and rangeland. Through a linked laboratory and field experiment, we assess the influence of carbon-rich soil amendments, including woodmulch and almond shells. In addition to testing the use of these materials in an active infiltration basin used for MAR, we are running parallel laboratory experiments using in-tact soil cores (1 m long) from the MAR basin. Fluids are sampled from the field and lab during infiltration, and analyzed for nitrate, nitrite, ammonium, dissolved organic carbon, dissolved inorganic carbon, and geogenic metals (iron, magnesium, and arsenic). We predict that rates of nutrient cycling can be enhanced and lead to increased denitrification at both the field site and in laboratory tests. More work is being done on analyzing carbon and nitrogen isotopes and carbon budgets of both the fluids and solids. Additional tests are also underway using different amounts of the almond-shell carbon amendment. Varying the amount of the organic carbon added will aid in determining best practices for application to operating MAR systems, with the goal of simultaneously improving water supply and water quality.