Depth-discrete Geochemical Profiling in Groundwater Using an Innovative In Situ Approach

Abstract:

The presence of nitrate in groundwater is often associated with agricultural activity. Leaching below the root zone to aquifers from agricultural areas is a critical problem in many jurisdictions where concentrations are above drinking water guidelines. Traditionally, nitrate and other water quality parameters are collected using purge and sample techniques. Often this “snapshot” data both disrupts the natural subsurface flow system and is not detailed enough to determine critical water quality or quantity conditions. In this study, depth-discrete, continuous and in situ monitoring techniques are developed. While nitrate is the focus, parameters including temperature, dissolved oxygen (DO), turbidity, redox potential (ORP) and electrical conductivity (EC), are also monitored. Research sites examine a range of hydrogeological conditions from supply wells located in shallow, unconfined sandy aquifers (Norfolk County, Ontario, Canada) to fractured sedimentary bedrock aquifers (Guelph, Ontario) impacted by agricultural activity. The innovative groundwater quality sampling method uses the Submersible Ultraviolet Nitrate Analyzer (SUNA^TM) as well as the robust YSI EXO2 Water Quality Sonde^TM. Depth-discrete well profiling is used to evaluate vertical stratification of nitrate and field parameters along the entire borehole with a focus on the screened interval. The high resolution datasets show zones of changing water quality corresponding to different formations. In open bedrock boreholes in Guelph, distinct intervals were identified at different depths for pH, EC, DO and ORP. In the shallower wells in Norfolk County, increases in DO and EC along the screened interval suggest the presence of fresh groundwater representative of the aquifer, with potential implications for in situ long-term monitoring of groundwater parameters. Detailed profiles of DO and ORP at both sites can be combined with nitrate profile data to determine potential zones of denitrification. Water quality monitoring costs are reduced relative to traditional purge and sample methods using this new technique. The developed in situ monitoring method and equipment configurations can be applied by water managers in agricultural regions to gain better insight into subsurface flow and contaminant transport processes.