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.