CONNECT Project

COmbiNing earth observatioN and gEochemical traCing Techniques (CONNECT) for groundwater detection and evaluation in Ireland

Project overview Team members Project background References Work packages

Project Overview

COmbiNing earth observatioN and gEochemical traCing Techniques (CONNECT) for groundwater detection and evaluation in Ireland was recently awarded funding under the EPA-STRIVE Water Research Programme 2012 (2012-W-MS-13). The project which commenced in April 2013 is led by Dr Jean Wilson (Principal Investigator) hosted within the Biogeochemistry Research Group headed by Professor Carlos Rocha (co-Principal Investigator) at the Centre for the Environment, Trinity College Dublin. The CONNECT project research team also includes Professor Catherine Coxon (Project Participant).

In short, the purpose of CONNECT is to further develop earth observation and geochemical tracing techniques that identify (map), characterize and evaluate the occurrence and impact of groundwater discharge to lakes and coastal waters. The potential for these techniques was previously explored and evaluated by Jean and Carlos as part of an EPA-STRIVE funded Research Fellowship between 2008 and 2012 (2008-FS-W-4-S5), a synthesis and full research report detailing the results from the fellowship will made available from the EPA (http://www.epa.ie/) very shortly. The overall aim of the current project is to examine the connectivity between ground and surface water by combining the results from a national assessment of the potential for groundwater discharge undertaken via remote sensing and GIS techniques with available hydrogeological, geological, geochemical and water quality data that further characterize and assess the potential for and impacts of groundwater discharge.

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CONNECT Team Members

Project Background

Surface water commonly is hydraulically connected to groundwater and forms an integral part of groundwater flow systems. This implies essentially, that surface water and groundwater are the same resource and the two components cannot be considered in isolation across a catchment. An improved understanding of the connection between surface and groundwater is increasingly viewed as a critical prerequisite to effectively managing these resources (Sophocleous, 2002) and water resource managers internationally, have begun to incorporate management strategies that require quantifying flow between surface and groundwater (Danskin, 1998; Jacobs and Holway, 2004; EPA, 2006; Brodie et al., 2007).

Nearly all surface water features interact with groundwater and these interactions may take many forms. For example, surface waters may receive inputs of freshwater and solutes from groundwater and surface water can be a source of groundwater recharge. Such interactions may potentially cause changes to and degradation of, water quality. Fresh groundwater discharge is considered to be less than 10% of the total freshwater flux to the ocean globally; however the inputs of associated nutrients and contaminants may be far more significant because concentrations in groundwater are several orders of magnitude higher than in surface waters (Slomp and Van Cappellen, 2004). Therefore, relatively small groundwater discharge rates can deliver comparatively large quantities of solutes including nutrients to lakes, rivers and coastal areas.

Nutrient supply via groundwater discharge has been linked to eutrophication and suggested as a potential precursor of harmful algal blooms (Anderson, 2009). Despite acknowledgement of the potential impact on water quality and coastal ecosystem functioning, surface-groundwater interactions remain a poorly-understood and often overlooked process when implementing coastal, estuarine and lake monitoring and management programs. For instance, EU directives such as the Water Framework Directive (WFD 2000/60/EC) aimed at improving the quality of the water environment do not acknowledge groundwater discharge as a potential nutrient source for assessment or monitoring. This is because the spatially and temporally heterogeneous nature of groundwater discharge from an essentially invisible source renders locating and quantifying discharge rates an appreciable challenge. Consequently, the quantitative distinction between groundwater discharge and easily gauged surface runoff sources may be impaired when implementing water management policy based on current nutrient monitoring programs for example.

In recognition of both the significance of groundwater discharge as a potential source of contamination and the challenges to locating the contribution of groundwater discharge to the coastal zone, a comprehensive cost-effective remote sensing methodology to facilitate a regional assessment of SGD (submarine groundwater discharge) was developed as part of a STRIVE funded postdoctoral fellowship (2008-FS-W-4-S5). The overarching goal of the fellowship was to identify and characterise coastal locations of SGD through the integration of satellite thermal remote sensing, ancillary geological and hydrogeological data and geochemical tracing (Radon-222, salinity) techniques.

The study identified over 30 potential SGD sites around the coastline of Ireland based on thermal anomalies, the results of which have been published in the leading international peer-reviewed remote sensing journal, Remote Sensing of Environment (Wilson and Rocha, 2012). Clearly discernible cold water plumes emanating from nearshore waters around the coastline of Ireland were captured using freely available Landsat ETM+ thermal imagery. A tiered, three-step approach was proposed as the most effective and affordable means to determine the spatial extent and scale of SGD from coastal aquifers to the coastal margin. Sea Surface Temperature (SST) values derived from Landsat ETM+ TIR were used to successfully detect plumes of colder water eventually associated with SGD in close proximity to the shoreline. Subsequently, potential sites of SGD were linked to geological features on land acting as possible sources by combining within a GIS, mapped temperature anomalies with ancillary on-shore spatial datasets describing bedrock geology including aquifer fault lines. Finally, nearshore surveys mapping the activity of radon and salinity were carried out to verify the presence of SGD and provide a qualitative assessment of fresh groundwater inputs to the coastal zone. Given the demonstrated potential of remote sensing for groundwater discharge identification and mapping at the coast, CONNECT has been funded to expand the technique beyond the confines of the original fellowship programme with the specific aim of applying the analysis tool as part of a national screening of lakes and to continue to monitor and evaluate potential coastal and estuarine groundwater discharges.

Combining Earth Observation and Geochemical tracing for groundwater detection:
An example from Lough Mask, Co. Mayo

• Radon is an excellent tracer of groundwater discharge. It is naturally enriched in groundwater relative to surface water, is conservative (does not alter in transit) and can be detected at very low levels.
• Temperature can be used as a tracer for groundwater by comparing the relatively constant temperature of groundwater with that of surface waters which fluctuate with season. Its application as a tracer is dependent upon whether a detectable difference exists between the temperatures of groundwater and the surface water body into which it discharges.
• The image to the left displays the spatial distribution of near surface radon activity (222Rn) in units Becquerel’s per metres cubed (Bqm-3) measured across Lough, Mask Co. Mayo during a field campaign undertaken in July 2012. Radon measurements have been mapped such that the gradient from dark blue tones through green, yellow and red illustrate an increase in radon activity values. Radon “hotspots” are clearly visible on the map along the north and eastern margins of the lake in contrast to the west and south west where very low radon activities were observed.
• The image on the right is a thermal map of the lake (°C) generated using Landsat 7 ETM+ band 6 imagery acquired June 8th 2007. Despite the time difference between the radon survey and satellite acquisition date, the spatial pattern of radon activity and temperature across the lake is very similar. Zones of cooler surface water temperatures illustrated using dark blue tones are readily apparent and clearly emanate from the north, north east and east lake margins.
• The thermal map reveals potential groundwater sources along the north and eastern shoreline adjacent to fissured and karstic aquifer bedrock types highly conducive to the transmission of water, verified by the radon survey.

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References

ANDERSON, D. M. (2009) Approaches to monitoring, control and management of harmful algal blooms (HABs). Ocean and Coastal Management, 52, 342-347.
BRODIE, R, SUNDARAM, B, TOTTENHAM, R, HOSTETLER, S, AND RANSLEY, T. (2007) An overview of tools for assessing groundwater-surface water connectivity. Bureau of Rural Sciences, Canberra
DANSKIN, W.R. (1998), Evaluation of the hydrologic system and selected water-management alternatives in the Owens Valley, California: U.S. Geological Survey Water-Supply Paper 2370–H, 175 pp.
EPA (2006) Water Framework Directive Monitoring Programme V1.0. EPA, Johnstown Castle, Wexford, Ireland.
JACOBS, K.L. and HOLWAY, J.M. (2004) Managing for sustainability in an arid climate—Lessons learned from 20 years of groundwater management in Arizona, USA. Hydrogeology Journal, 12 (1), 52–65.
SOPHOCLEOUS, M. (2002) Interaction between groundwater and surface water: the state of the science. Hydrogeology Journal, 10, 52-67.
SLOMP, C. P. & VAN CAPPELLEN, P. (2004) Nutrient inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact. Journal of Hydrology, 295, 64-86.
WILSON, J. & ROCHA, C. (2012) Regional scale assessment of Submarine Groundwater Discharge in Ireland combining medium resolution thermal imagery and geochemical tracing techniques. Remote Sensing of Environment, 119, 21-34.

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Summary of CONNECT Work Packages

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