Biochemical Reduction of Nitrate in the Deep Vadose Zone Under Managed Aquifer Recharge
Abstract
Nitrate contamination of groundwater is a global challenge threatening ecosystem health and access to reliably clean drinking water in both developing and developed nations. In watersheds with extensive agriculture and deep vadose zones, legacy nitrate (NO3-) is particularly pernicious in its ability to continually act as a diffuse source of pollution to streams (when stored at shallow depths) and underlying aquifers. Denitrification, the process by which microorganisms use NO3-as a terminal electron acceptor and sequentially reduce it to NO2-, NO, N2O and finally N2, represents a permanent sink of NO3-. However, little is known about the potential to reduce NO3-in the deep vadose zone or factors that affect it, with most studies assuming denitrification cannot occur at depth due to substrate limitation. This work will focus on an agricultural field site in Modesto, California that is underlain by alluvial sediments and paleosols with a deep water table (~10-30 meters below surface). Cores down to nine meters were collected and dissolved organic carbon, microbially available iron, NO3-, and denitrification potential by the acetylene method were determined. Our analyses show that denitrification occurs down to nine meters, with up to 60% of NO3- being reduced to N2O in the deeper subsurface, with and without the addition of glucose. We further use data from denitrification assays to develop a comprehensive reaction network using TOUGH REACT, and validate the model using N isotopes taken before and after flooding at the field site. The calibrated model is then used to quantify the fate, transport, and cycling of N in the presence of managed aquifer recharge (AgMAR). In particular, we use the reactive transport model to analyze the effects of lithological heterogeneities and surface hydrologic regime on denitrification potential. We conclude that denitrification could increase under managed aquifer recharge on agricultural lands (AgMAR) compared to normal irrigation patterns.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H51G..05W
- Keywords:
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- 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES;
- 1630 Impacts of global change;
- GLOBAL CHANGE;
- 1831 Groundwater quality;
- HYDROLOGY;
- 1871 Surface water quality;
- HYDROLOGY