Internationally, groundwater discharge is recognised as a significant source and pathway of nutrients and diffuse pollution into lakes.
Identifying 1) exactly where groundwater discharge is occurring 2) whether the associated inputs pose a pollution threat and 3) which lakes across Ireland are potentially at risk; is an extremely difficult and challenging task. This is because groundwater discharges are highly variable both temporally and spatially and originate from essentially invisible sources. A recently funded EPA-STRIVE project (CONNECT) seeks to localise and assess groundwater inputs using lake surface temperature patterns derived from satellite images (Fig. 1).
Fig. 1. Lake surface temperature patterns across Lough Mask, Co. Mayo, Republic of Ireland, generated from a Landsat ETM+ thermal infrared image acquired June 8th 2007 (http://glovis.usgs.gov/). Illustration details groundwater vulnerability to pollution mapped by the GSI (http://www.dcenr.gov.ie)
Groundwater is water held beneath the earth’s surface in pore spaces of rocks and an aquifer is any such body of rock that can yield a useable amount of water. Lakes receive groundwater via inflow through their bed as groundwater discharge (Fig. 2) and will lose groundwater by seepage to the aquifer. While the contribution of groundwater inputs to overall lake water budgets may be small, contaminants from a variety of sources at and below the surface may also be transported including chemicals from contaminated land areas and nutrients from agriculture or domestic sewage systems for example.
Fig. 2. Schematic of groundwater inflow to lakes – precipitation percolates down to the water table and through the aquifer, discharging as groundwater flow into a lake
The use of heat as a tracer for groundwater discharge was recognised in the 1900s and remote sensing methods (Please refer to our previous blog ‘Remote Sensing only Less Remote‘) for groundwater detection are applicable where discernible temperature differences exist between surface water bodies and discharging groundwater. Groundwater maintains an almost constant year round temperature and in Ireland this can be exploited during summer months where groundwater inputs manifest as cold water signals in lake surface temperature patterns generated from satellite thermal images (USGS Landsat ETM+ satellite).
In the example provided, cold water inputs clearly emanate from northern and eastern lake margins of Lough Mask. Moreover, the aquifers surrounding the lake to the north and east have been classified as extremely vulnerable, which means that the potential for transmission of pollution via groundwater pathways into Lough Mask is high.
Although satellite remote sensing has been used successfully in the example given to map thermal anomalies associated with groundwater inputs across a lake, the results must be verified in the field using ancillary datasets. As part of CONNECT, groundwater inputs are verified by measuring surface water radon activities.
The distinctive and measurable difference that exists between radon (222Rn) concentrations in groundwater relative to surface water is the fundamental basis for using radon as an environmental tracer for groundwater discharge into lakes. Radon is a non-reactive noble gas which is produced through the decay of uranium, a naturally occurring mineral in rocks and soil. Concentrations of radon are several orders of magnitude higher in groundwater compared to surface water bodies because of its constant contact with rocks (i.e. radon emanating material). The short half-life of radon (3.82 days) causes an extremely short radon diffusion length which normally results in a very sharp concentration gradient at the surface-groundwater interface. Furthermore, radon is conservative, does not alter in transit from the aquifer and can be measured in very low concentrations.
Surface radon activity across Lough Mask was measured in July 2012 (Fig. 3) and a comparison with surface water temperature patterns reveals a striking correlation. It is clear that the radon hotspots (indicating groundwater sources) are located in the areas of Lough Mask where the temperatures are coldest.
Fig. 3. Surface radon activity (left) across Lough Mask illustrated in blue (low activity) through red (high activity), measured during a survey in July 2012 validates the presence of groundwater and localisation of groundwater inputs along the northern and eastern shorelines.
This work demonstrates the suitability of a remote sensing approach as a comprehensive and cost-effective preliminary assessment tool for the identification and localisation of groundwater discharge into lakes.