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. Poultry manure is the primary cause of nitrate ( NO 3 - ) exceedancesin the transboundary Abbotsford–Sumas aquifer (ASA; Canada–USA) based on synopticsurveys two decades apart, but questions remained about seasonal and spatialaspects of agricultural nitrate fluxes to the aquifer to help better focusremediation efforts. We conducted over 700 monthly δ 15N and δ 18O of nitrate assays, focusing on shallow groundwater(< 5 years old) over a 5-year period to gain new insight onspatio-temporal sources and controls of groundwater nitrate contamination. NO 3 - 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 . Thehigh-frequency 15N and 18O isotope data allowed us to identifythree distinctive NO 3 - source patterns: (i) primarily from syntheticfertilizer, (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 thesource(s) of nitrate in recharge could be quickly influenced by short-termnear-field management practices and stochastic precipitation events, whichultimately impact long-term nitrate contamination trends. Overall, theisotope data affirmed a subtle decadal-scale shift in agricultural practicesfrom 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, remediationof the Abbotsford–Sumas aquifer is possible only if agricultural N sourcesare seriously curtailed, a difficult proposition due to longstandinghigh-value intensive poultry and raspberry and blueberry operations over theaquifer.

hydrology and earth system sciences

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