INSIGHTS FROM COMPARISON OF GROUNDWATER AND ROCK CORE NITRATE CONCENTRATIONS IN A SEDIMENTARY BEDROCK AQUIFER

The province of Prince Edward Island (PEI) is unique in Canada because 100% of the population is reliant on groundwater for their drinking water supply. A balance must be found between sustainable agriculture and source water protection. Studies across the island have shown increasing nitrate levels in surface and ground waters associated with agricultural practices, the ecological impacts of which are detrimental to aquatic biota and evidenced by eutrophication of surface waters. A study was conducted on a conventional farm near Souris, PEI. A portion of the field was taken out of production (2011) for a five-year period during which nitrate concentrations in the underlying bedrock aquifer were monitored.

The objective of this study was to characterize the nitrate distribution within a discrete fractured network (DFN) context. Discrete depth samples were collected adjacent to and between fractures from continuous cores up to 60 m deep and underwent both rock matrix and porewater analysis. Rock matrix porosity and permeability were measured; major ion concentrations in the pore water were quantified. Multi-level monitoring systems were designed (with up to 11 ports per well) using corehole and geophysical profiles for spatial and temporal monitoring of groundwater hydraulic head, nitrate and other major ions and isotopes. As part of DFN methodology, multi-level monitoring systems are tools that facilitate high-resolution spatial and temporal measurements of mobile groundwater in fractures. Continuous core logs provide depth discrete information about the relatively immobile porewater in the rock matrix. Results showed that elevated nitrate concentrations persist in the shallow subsurface, suggesting that the denitrification process is not strong; likely due to the lack of electron donors. The site is within 100 m of a discharge zone and upward hydraulic gradients were observed. This likely impedes the downward migration and attenuation of the shallow nitrate.

DFN results demonstrate the influence of mechanisms, such as diffusion and redox, on the movement and retardation of nitrate is critical to informing conceptual models of flow and transport in sedimentary bedrock aquifers and the development of sustainable agricultural practices and source water protection.