Abstracts

271 - Field evaluation of nitrate transport and matrix storage
processes in Prince Edward Island’s fractured sandstone aquifer
Amanda Malenica, Steven Chapman, Beth Parker, & John Cherry
G360 Centre for Applied Groundwater Research, University of Guelph, Ontario, Canada
Mark Grimmett & Yefang Jiang
Science and Technology Branch, Agriculture and Agri-Food Canada, Charlottetown, Prince
Edward Island, Canada
Cathy Ryan
Department of Geoscience, University of Calgary, Calgary, Alberta, Canada
The province of Prince Edward Island (PEI) is unique in Canada because 100% of the
population is reliant on groundwater for drinking water supply. Of PEI’s total land area,
42% is cleared for agricultural use, 15% of which was planted with potatoes in 2014 (PEI
DAF, 2014). Nitrate concentrations have increased steadily in municipal and domestic
wells and in surface water since the mid-1980s, as have the production of potatoes. Ecological
impacts are detrimental to estuarine biota and evidenced by visible eutrophication
and anoxic events. Although beneficial management practices (BMPs) have been identified
and implemented, groundwater and surface water nitrate concentrations have not decreased
substantially. The accumulation of nitrate mass diffusing into the porous rock matrix
over time from fractures that dominate groundwater flow could be impeding the rate
of nitrate concentration decrease. The relatively low-K rock matrix between the fractures
constitutes the storage capacity of a dual-domain flow and transport system. Improvements
in groundwater quality resulting from reduced nitrate loading are likely tempered
by back-diffusion of the nitrate stored in the matrix where diffusion back to the fractures
could delay down-gradient improvements in water quality.
The objective of this study was to characterize the nitrate distribution at two different
study sites within a discrete fractured network (DFN) context, including quantitation of
nitrate concentrations in the rock matrix. Continuous cores, up to 45 m deep, were logged
for lithology, discrete fractures and advective pathways. Discrete depth samples were collected
adjacent to fractures and at varying distances into the matrix between fractures to
quantify the immobile nitrate porewater concentrations and evaluate the magnitude of
attenuation by diffusion and redox controlled reactions. Results showed local peaks in
matrix nitrate concentrations, as high as 36 mg/L-N, primarily in the partially saturated
overburden and upper saturated fractured bedrock. Nitrate concentrations did not completely
attenuate with depth. This suggests the denitrification process is not strong, likely
due to lack of electron donors and oxygenated conditions. Multi-level monitoring systems
(MLS) were designed to facilitate high–resolution spatial and temporal measurements of
mobile groundwater in fractures (hydraulic head, nitrate and other major ions and isotopes).
Results demonstrate the influence of various mechanisms, including matrix diffusion
and redox conditions, on the movement and attenuation of nitrate. Understanding
these controlling processes is critical to informing conceptual models of flow and transport
in sedimentary bedrock aquifers and understanding efficacy of BMP implementation and
for development of sustainable agricultural practices.
IAH-CNC 2015 WATERLOO CONFERENCE
147