AWWA WKICO66054

Contamination of groundwater (GW) by nitrate from agricultural sources is of particular importance in Prince Edward Island (Canada), where agricultural lands dominate the landscape and GW is the sole source of potable water. In this region and in many others, the transfer of N compounds from agricultural soils to aquifers is a key process, and it is important to assess if values coming from residual soil nitrate estimations, and associated estimates of N loading to aquifers match estimations based on GW concentrations measured at the top of the aquifer. The nitrate "dual isotope" characterization is gaining popularity in the field of hydrogeochemistry, and such study promises to shed important insights into the processes involved in the transport of nitrogen from agricultural soils to GW. The current watershed scale sampling program indicates that nitrate concentrations in the region are relatively stable throughout the year, as are the water isotope characteristics in both surface water (SW) and GW. In contrast, values in dissolved nitrate in both GW and SW display distinct seasonal characteristics. The differing behavior of the water and nitrate oxygen isotopes can be resolved by invoking mixtures of recent recharge water (soil leachate) characterized by high nitrate concentrations and seasonally distinct isotopic characteristics, with deeper and older multi-year GW having lower nitrate concentrations and practically uniform isotopic characteristics. Mixing 25% or less of seasonal recharge with older, less impacted GW accounts for the observed characteristics of GW from domestic wells, implying that the greatest proportion of N flux in GW is at this time restricted to relatively shallow portions of the aquifer. This same rationale also helps explain why currently observed nitrate concentrations in well water in the watershed are below those that would otherwise be expected based on calculated estimates of nitrate loading to the aquifer. Includes 17 references, tables, figures.