High-frequency NO<inf>3</inf><sup>-</sup> isotope (δ<sup>15</sup>N, δ<sup>18</sup>O) patterns in groundwater recharge reveal that short-term changes in land use and precipitation influence nitrate contamination trends
Citation
Suchy, M., Wassenaar, L.I., Graham, G., Zebarth, B. (2018). High-frequency NO3- isotope (δ15N, δ18O) patterns in groundwater recharge reveal that short-term changes in land use and precipitation influence nitrate contamination trends. Hydrology and Earth System Sciences, [online] 22(8), 4267-4279. http://dx.doi.org/10.5194/hess-22-4267-2018
Plain language summary
The Abbotsford-Sumas aquifer is an important aquifer which straddles the Canada-US border. Groundwater contamination by nitrate in this aquifer has been attributed primarily to intensive agricultural production, and particularly to poultry and small fruit production. The nitrogen and oxygen isotopes of nitrate were used to help identify the source of nitrate in shallow (i.e., young) groundwater in various wells located in the aquifer in order to help better focus remediation efforts. Three distinctive patterns were observed in the wells. First, some wells had nitrate that originated primarily from use of synthetic fertilizers. Second, some wells showed important changes in the nitrate source over time which reflected recent changes in land use. Third, some wells had nitrate that was derived mostly from a combination of synthetic fertilizer and poultry manure. A key finding was that the source(s) of nitrate in young groundwater in this aquifer can be quickly influenced by short-term changes in field management practices and precipitation patterns.
Abstract
Poultry manure is the primary cause of nitrate (NO3-) exceedances in the transboundary Abbotsford-Sumas aquifer (ASA; Canada-USA) based on synoptic surveys two decades apart, but questions remained about seasonal and spatial aspects of agricultural nitrate fluxes to the aquifer to help better focus remediation efforts. We conducted over 700 monthly δ15N and δ18O of nitrate assays, focusing on shallow groundwater (<5 years old) over a 5-year period to gain new insight on spatiooral sources and controls of groundwater nitrate contamination. NO3- concentrations in these wells ranged from 1.3 to 99mgNL-1 (n Combining double low line 1041) with a mean of 16.2±0.4mgNL-1. The high-frequency 15N and 18O isotope data allowed us to identify three distinctive NO3- source patterns: (i) primarily from synthetic fertilizer, (ii) dynamic changes in nitrate due to changes in land use, and (iii) from a mix of poultry manure and fertilizer. A key finding was that the source(s) of nitrate in recharge could be quickly influenced by short-term near-field management practices and stochastic precipitation events, which ultimately impact long-term nitrate contamination trends. Overall, the isotope data affirmed a subtle decadal-scale shift in agricultural practices from manure increasingly towards fertilizer nitrate sources; nevertheless, poultry-derived N remains a predominant source of nitrate contamination. Because the aquifer does not generally support denitrification, remediation of the Abbotsford-Sumas aquifer is possible only if agricultural N sources are seriously curtailed, a difficult proposition due to longstanding high-value intensive poultry and raspberry and blueberry operations over the aquifer.