Assessment of hydrochemical and temperature measurements
H. Bishop (GSI), 20 September 2021
Cork City is in the southwest of Ireland in the province of Munster. The city centre is located between two river channels that meet downstream at the eastern end of the city, and flow into the marine Lough Mahon and Cork Harbour. Cork was chosen as pilot area for MUSE because it has a long history of ground source heat use, productive aquifers beneath the city centre, and shallow groundwater temperatures in the vicinity that are elevated with respect to the national average.
The aim of the field measurements was to assess spatial and temporal variation in groundwater temperature and chemistry. The data were collected over the course of three field trips in April 2019, November 2019, and August 2020 (Figure 1).
Boreholes with multiple temperature readings display seasonal variation in shallow groundwater temperature roughly 0 – 10 m below ground level, with groundwater temperatures varying from 4 to 10 °C between the coolest and warmest readings. This indicates that the influence of seasonal variation in air temperature is limited to shallow depths. The influence of seasonal air temperature diminishes at a much shallower depth in some boreholes (Figure 2 – Borehole 4) than others (Figure 2 – Borehole 13). The depth of the borehole influences the degree to which this can be determined. Some boreholes are too shallow to determine the depth that groundwater temperatures stabilise (Figure 2 – Boreholes 9 and 12).
Hydrochemical analysis of major ions shows little-to-no seasonal variation observed at each borehole (Figure 3). Boreholes (BH) closer to the mouth of the River Lee (BHs 5 to 10) plot as NaCl-type, indicating a marine influence on the groundwater. Boreholes located west of the city centre (west of Mercy University Hospital) and in the south of the city, around Tramore Valley Park, plot as MgHCO3-type, which relates to the karst limestone aquifer which underlies the gravels and much of the pilot area.
Field measurements have provided a foundation of quantitative data and have helped identify areas which need further research. Going forward, we aim to install new monitoring points to improve spatial resolution of data and deploy data loggers to better observe temporal variation.
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