AGRICULTURE-DERIVED NITRATE SOURCE CHARACTERIZATION IN AN UNCONFINED AQUIFER USING HIGH FREQUENCY NITRATE ISOTOPE SAMPLING

The Abbotsford-Sumas aquifer (ASA) is a shallow unconfined and intrinsically vulnerable aquifer in southwestern British Columbia, with groundwater flow patterns generally from north to south across the Canada-US border. Predominant agricultural activities, overlying large portions of this aquifer include intensive berry (raspberry and blueberry) and poultry production. The application of synthetic and/or poultry-manure fertilizers to berry fields has been attributed to elevated groundwater-nitrate concentrations for several decades, with frequent and sustained exceedance of drinking water criteria in large areas of the aquifer. This study examined the natural abundance of stable isotopes δ ¹⁵N and δ ¹⁸O of nitrate in monitoring wells with young groundwater (< 5 yrs. ³H/³He age), through analysis of monthly sampling to quantify temporal patterns and gain further insight on sources and controls of groundwater nitrate contamination. Observed groundwater nitrate concentrations ranged from 1.3 to 99 mg/L-N (n=1096), with a mean of 16.5 mg/L-N, exceeding the drinking water MAC of 10 mg/L-N. Monitoring sites (n=20) had 4 distinct δ¹⁵N-NO₃ ranges: (i) 3-8‰ (20%), (ii) 6-10‰ (50%), (iii) 9-16‰ (15%), and (iv) full range 2-16‰, with δ ¹⁸O enrichment (15%), indicating influence of specific near-field management practices and occurrence of transformation processes. In addition, 4 distinct δ¹⁵N trends were observed: (a) stable or unchanged (20%), (b) enriching (35%), (c) depleting (10%), or (d) variable or showing seasonality (35%), illustrating diversity in nitrate isotope patterns despite the relatively local scale of study area. Results indicate that both poultry manure (δ¹⁵N range: +7.9 to +11 ‰) and synthetic fertilizer (δ¹⁵N range: -2.8 to +0.3 ‰) are sources of elevated groundwater nitrate; however, when compared to previous mean nitrate isotope results from aquifer (Wassenaar et al., Env. Science and Tech. v.40(15), 2006) +10.1±9.0 ‰ (n=70) and (Wassenaar, Applied Geochemistry, v.10(4), 1995) +10.2±4.0 ‰ (n=56), the mean ASA groundwater nitrate isotope signature appears to have shifted further to a more depleted NH₄ and Urea based fertilizer source, with a mean δ ¹⁵N of +7.9±3.0 ‰ (n=762), although it is unclear if this represents a change in land-use practice or a result of higher frequency sampling.