Assessing the influence of various soil carbon amendments on nutrient and trace metal loading during infiltration for managed recharge
Abstract
Managed aquifer recharge (MAR) can increase groundwater supply and improve water quality during infiltration of surface water. Carbon-rich soil amendments can increase denitrification and reduce nitrogen (N) loading during infiltration for MAR but may create conditions favorable for mobilizing redox-sensitive elements, including trace metals. To elucidate these issues, we sampled fluids from a newly constructed MAR field site in central coastal California. The field site collects runoff from a 1,300-ac drainage area into a 1-ac sedimentation basin and a 4-ac infiltration basin. Two acres at the base of the infiltration basin were modified by tilling 30 cm of carbon amendments into the top 45-60 cm of native sediment. Selected carbon amendment treatments were: biochar, woodchips, a 50/50 mix of biochar/woodchips, a 75/25 mix of woodchips/alfalfa, and crushed almond shells. Pressure gauges and a sonic-sensor system were used to measure system inflows, and thermal probes were used to measure infiltration rates using heat as a tracer. Subsurface water samples were collected during rain events from a shallow water well on the property and at two depths (35 and 55-cm) within the treatment areas and at an adjacent control site. Surface water samples were collected from the inflow, and at the north and south ends of the infiltration basin. Water samples were collected for nutrients (NO3-, NO2, NH4+, SiO44-, PO43-), dissolved organic carbon, dissolved inorganic carbon, and trace metal concentrations (As, Fe, Mn, Cu, Pb, U).
Here we present the N (NO3-, NO2-, NH4+) data and focus on the comparisons of differences in N concentration and speciation between water flowing into the basin and waters at depth which provides insight into N processing. We found that the biochar/woodchips mix, woodchips, and almond shells were a net sink of N, whereas the biochar and woodchips/alfalfa treatments were a net source of N. These differences in N dynamics suggest that the individual amendments may have distinct effects on microbial processing of carbon, which drives changes in redox state from O2 consumption. Additional water analyses are underway, and we expect trace metal concentrations may be similarly changing due to the close connection of redox state on N and trace metal biogeochemistry.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMH112.0010S
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES;
- 1830 Groundwater/surface water interaction;
- HYDROLOGY;
- 1871 Surface water quality;
- HYDROLOGY