Paper No. 338-5
Presentation Time: 2:15 PM
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.