High-frequency NO<sub>3</sub><sup>−</sup> isotope (<i>δ</i><sup>15</sup>N, <i>δ</i><sup>18</sup>O) patterns in groundwater recharge reveal that short-term changes in land use and precipitation influence nitrate contamination trends

Abstract

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 spatio-temporal sources and controls of groundwater nitrate contamination. NO3- concentrations in these wells ranged from 1.3 to 99 mg N L−1 (n=1041) with a mean of 16.2±0.4 mg N L−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.

Animal‐Based Organic Amendments and Their Potential for Excessive Nitrogen Leaching and Phosphorus Loading

“…Separated dairy manure solids with high fiber contents are now frequently used as dairy bedding directly or after a so-called 'composting' process (Husfeldt et al, 2012) but can also be used as a relatively recalcitrant soil amendment (Shi and Norton, 2000;Van Kessel and Reeves, 2002;Gale et al, 2006). The release of plant available N from dairy solids and composts reported in these field and laboratory studies is often less than 20% of total N. Nitrate (NO 3 -) contamination in the unconfined transboundary Abbotsford-Sumas aquifer in south-coastal British Columbia has been frequently reported (Wassenaar et al, 2006;Suchy et al, 2018). The overlying land use is intensive production of agricultural crops, particularly berries, which often receive high rates of manure from essentially landless neighboring poultry operations as a soil amendment to enhance organic matter concentration (Zebarth et al, 2015).…”

“…Nitrate (NO 3 – ) contamination in the unconfined transboundary Abbotsford‐Sumas aquifer in south‐coastal British Columbia has been frequently reported (Wassenaar et al, 2006; Suchy et al, 2018). The overlying land use is intensive production of agricultural crops, particularly berries, which often receive high rates of manure from essentially landless neighboring poultry operations as a soil amendment to enhance organic matter concentration (Zebarth et al, 2015).…”

“…Between 1993 and 2004, NO 3 – concentrations in the aquifer, as measured in monitoring wells, increased on average by 14 mg NO 3 – L –1 , exceeding Canadian and international drinking water guidelines by more than 15% (Zebarth et al, 2015). Moreover, water quality did not improve in the following decade (Suchy et al, 2018). High precipitation (1508 mm) and well‐drained soils facilitate leaching of NO 3 – into the aquifer, especially in the wet falls that typically follow dry summers.…”

Decadal Delays in Groundwater Recovery from Nitrate Contamination Caused by Low O2 Reduction Rates

“…According to the status report 2012 of the European Environmental Agency, approximately 25% of all aquifers across Europe are in a poor chemical status, for which mainly nitrate contamination is responsible (Werner & O'Doherty, ). Furthermore, 33% of all groundwater bodies are affected by diffuse pollution from agriculture, and contrary to expectations and major efforts to reduce nitrate inputs into aquifers through changes in land use, land management, and other measures (Suchy et al, ), some EU countries have not met the objectives of the European Water Framework Directive to not further deteriorate and improve the chemical water quality status by 2015 (European Commision, ; European Community, ; Voulvoulis et al, ). A lack of timely response to such measures in the level of nitrate contamination in groundwater has puzzled stakeholders and has prompted the EU to delay its aspiration for good qualitative status for all EU water bodies by more than a decade, from 2015 to 2027 (European Commision, ).…”

Integrated Crop-Livestock Systems: Lessons From New York, British Columbia, and the South-Eastern United States

“…Diverting N from poultry manure away from these sandy soils should reduce nitrate loading and improve aquifer water quality [46] . An alternative use for the N-rich poultry manure could be for grass crops on nearby dairy farms that typically receive 100-200 kg N ha −1 as mineral fertilizer each year.…”