Investigating Hydraulically Active Fractures in A Shallow Meta-Sedimentary Bedrock Aquifer Using Single Well Tracer Testing, Mount Stewart, Northern Ireland

Knowledge of groundwater flow/mass transport, in poorly productive aquifers which underlie over 65% of the island of Ireland, is necessary for effective management of catchment water quality and aquatic ecology. This research focuses on a fractured low-grade Ordovician/Silurian greywacke sequence which underlies approximately 25% of the northern half of Ireland. Knowledge of the unit’s hydrogeological properties remain largely restricted to localised single well open hole “transmissivity” values. Current hydrogeological conceptual models of the Greywacke view the bulk of groundwater flowing through fractures in an otherwise impermeable bedrock mass.
Core analysis permits fracture characterisation, although not all identified fractures may be involved in groundwater flow. Traditional in-situ hydraulic characterisation relies on cumbersome techniques such as packer testing or geophysical borehole logging (e.g. flowmeters). Queen’s University Belfast is currently carrying out hydraulic characterization of 16 boreholes at its Greywacke Hydrogeological Research Site at Mount Stewart, Northern Ireland.
Development of dye dilution methods, using a recently-developed downhole fluorometer, provided a portable, user-friendly, and inexpensive means of detecting hydraulically active intervals in open boreholes. Measurements in a 55m deep hole, three days following fluorescent dye injection, demonstrated the ability of the technique to detect two discrete hydraulically active intervals corresponding to zones identified by caliper and heat-pulse flowmeter logs. High resolution acoustic televiewer logs revealed the zones to correspond to two steeply dipping fractured intervals. Results suggest the rock can have effective porosities of the order of 0.1%.
Study findings demonstrate dye dilution’s utility in characterizing groundwater flow in fractured aquifers. Tests on remaining holes will be completed at different times following injection to identify less permeable fractures and develop an improved understanding of the structural controls on groundwater flow in the uppermost metres of competent bedrock.

A comparison of nitogen fate and transport in catchments underlain by high and low permeability aquifers using chemical and isotopic tools.

Variability in nitrogen fate and transport in different catchments types is often not considered. This research considers the importance of such nitrogen processes within groundwater pathways in two agricultural catchments in Ireland; a well drained catchment, underlain by karstified Carboniferous limestone, and a poorly drained catchment, underlain by Silurian greywacke.
Depth specific low-flow groundwater sampling was used to evaluate the hydrochemical stratification in groundwater. Groundwater samples, as well as surface water samples, along river courses were analysed for nitrogen species (NO3, NH4 and NO2) and nitrate isotopes (d15N and d18O) as well as field parameters and major ions
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The dominant nitrate (NO3) groundwater pathway in the poorly drained greywacke catchment is through the shallow weathered bedrock, as indicated by transmissivity values and the ionic and isotopic signatures, and a clear reduction in NO3 concentration is observed with depth. A similar chloride trend would suggest dilution is a major factor, however d15N and d18O isotopic values producing an enrichment ratio of 1.8 indicate that denitrification is also an important process involved in the fate of the NO3 within the groundwater flow system. This consistent trend with depth is in contrast to the stratification pattern observed in the karstified catchment. NO3 was not detected in the shallow groundwater pathway; the dominant groundwater pathway is in the deeper groundwater where there is little change in the nitrate isotope values with depth (d15N values range between 4.1 and 4.6 ‰). This deeper groundwater contributes the dominant proportion of the river flow through a number of springs. As a result, the deeper groundwater, springs and river have a similar ionic signature and NO3 concentration range (23 ± 3 mg/l). Despite this pattern, the NO3 isotopes show a distinct difference in isotopic values between the deeper groundwater in the diffuse karst and the springs indicating some denitrification is occurring during groundwater discharge into the river. Furthermore the isotopes give an indication of the variability of the spatial extent of the springs and the complexities of the fissures through which they are fed. The results of this study clearly show the importance of the geology in the fate and transport of NO3 in agricultural catchments.

The influence of bedrock hydrogeology on catchment-scale nitrate fate and transport in fractured aquifers

Characterising catchment scale biogeochemical processes controlling nitrate fate in groundwater constitutes a fundamental consideration when applying programmes of measures to reduce risks posed by diffuse agricultural pollutants to water quality. Combining hydrochemical analyses with nitrate isotopic data and physical hydrogeological measurements permitted characterisation of biogeochemical processes influencing nitrogen fate and transport in the groundwater in two fractured bedrock aquifers with contrasting hydrogeology but comparable nutrient loads. Hydrochemical and isotopic analyses of groundwater samples collected from moderately fractured, diffusely karstified limestone indicated nitrification controlled dissolved nitrogen fate and delivery to aquatic receptors. By contrast nitrate concentrations in groundwater were considerably lower in a low transmissivity highly lithified sandstone and pyrite-bearing shale unit with patchy subsoil cover. Geophysical and hydrochemical investigations showed shallower intervals contained hydraulically active fractures where denitrification was reflected through lower nitrogen levels and an isotopic enrichment ratio of 1.7 between δ15N and δ18O. Study findings highlight the influence of bedrock hydrogeological conditions on aqueous nitrogen mobility. Investigation results demonstrate that bedrock conditions need to be considered when implementing catchment management plans to reduce the impact of agricultural practices on the quality of groundwater and baseflow in receiving rivers.