Comparing isotopic groundwater age measurements with simulated groundwater ages: example of the Abbotsford-Sumas Aquifer (USA and Canada) and application

• Published in: Water and environment journal : WEJ 2012-03, Vol.26 (1), p.30-37
• Main Authors: Chesnaux, R., Allen, D. M., Simpson, M. W. M.
• Format: Article
• Language: English
• Subjects: groundwater; isotopic age dating; numerical modelling; particle tracking
• ISSN: 1747-6585; 1747-6593
• DOI: 10.1111/j.1747-6593.2011.00260.x

Elevated nitrate concentrations have been documented since the 1970s within the trans‐national Abbotsford–Sumas aquifer, situated in the central Fraser Valley of southern British Columbia, Canada, and northern Washington State, United States. Nitrate concentrations in excess of 10 mg/L NO3–N are commonly observed in the monitoring wells. A groundwater model is used in this study to conduct a particle tracking analysis to estimate groundwater travel times and compare these with groundwater ages determined from 3H/3He concentrations measured in several monitoring wells. Groundwater ages estimated from particle tracking show excellent agreement with measured ages (slope=1), although there is scatter (R2=0.76), reflecting the complexity of the travel pathways and ambiguities in groundwater ages because of mixing. An assessment of the efficacy of recommended nutrient management practices, which were implemented in 1992 to reduce nitrogen loading, suggests that wells shallower than about 20 m should be monitoring a decrease in nitrate concentration. However, persistent elevated nitrate concentrations even at shallow depths point to the lingering effects of a remnant manure–nitrate signature in combination with continued high loading.