MATRIX DIFFUSION EFFECTS ON NITRATE TRANSPORT AND FATE IN SEDIMENTARY BEDROCK

Many agricultural areas are commonly underlain by fractured sedimentary rock. When overburden is thin (ie. less than 8m), its attenuative capacity is poor. Consequently, underlying bedrock aquifers are vulnerable to contamination from surface. Nitrate concentrations in groundwater can often be linked to the agricultural application of granular and manure fertilizers. Human health concerns regarding nitrate concentrations in drinking water supplies are well documented. A study was conducted in rural Prince Edward Island (PEI), between 2011 and 2014, where farming is a socio-economic staple of the island community. PEI is unique, in that they are wholly reliant on groundwater for their drinking water supply.

This study utilizes a methodology referred to as the Discrete Fracture Network (DFN) approach, which aims to characterize the properties of the fracture network and the rock matrix, in the context of the groundwater flow system and contaminant mass distribution. Field data sets and parameters inform numerical models to evaluate diffusion processes and impacts. In fractured rock, nearly all groundwater flow occurs in the interconnected fracture network, providing the primary contaminant transport pathways. Fractured sedimentary rock units have very low bulk fracture porosity (~10^-3 to 10^-5) and high rock matrix porosity (~5-20%), which is attributable to its large storage capacity for contaminants. Further, negligible flow rates often result from low matrix permeability.

Matrix diffusion is the mechanism for transfer of contaminant mass from fracture-to-matrix. This mechanism retards the front of the contaminant zone, reducing its mobility. Thus, matrix diffusion can be construed as positive, in that it defers the negative impacts to water supply wells and groundwater discharge areas. This deferral, however, can result in the potential for large nitrate storage in the matrix and slow contaminant release, via back diffusion, to groundwater. Thus, regardless of improvements to agricultural practices that reduce nitrate inputs, the existing long-term nitrate storage in the matrix impedes aquifer restoration.