Regional scale assessment of Submarine Groundwater Discharge in Ireland combining medium resolution satellite imagery and geochemical tracing techniques
Abstract
This paper sets the foundation for the use of freely available Landsat Enhanced Thematic Mapper (ETM +) thermal infrared (TIR) imagery in a regional scale assessment of submarine groundwater discharge (SGD) to coastal waters. A comprehensive, tiered, three-step approach is proposed as the most effective and affordable means to determine the spatial extent and scale of SGD from coastal aquifers to the coastal margin. As the preliminary step, Sea Surface Temperature (SST) values derived from Landsat ETM + TIR are used to successfully detect plumes of colder water eventually associated with SGD in close proximity to the shoreline. Subsequently, potential sites of SGD are linked to geological features on land acting as possible sources, by combining within a Geographical Information System (GIS), mapped temperature anomalies with ancillary on-shore spatial datasets describing bedrock geology including aquifer fault lines. Finally, nearshore surveys mapping the activity of 222Rn (radon) and salinity are carried out to verify the presence of SGD and provide a qualitative assessment of fresh groundwater inputs to the coastal zone.Practical application of the complete approach in the context of coastal zone management is illustrated through a case-study of the Republic of Ireland. As part of this study, over 30 previously unidentified links between aquifers on land and the sea are shown along the Irish coast, hence illustrating the tight coupling between coastal waters and groundwater inputs at an unprecedented spatial scale. The study demonstrates the potential of the combined applications of remote sensing methods and geochemical tracing techniques for a cost-effective regional-scale assessment of groundwater discharge to coastal waters.
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- Ground‐based TIR imagery can be used to detect small‐scale groundwater discharge springs that are not readily apparent to the naked eye, even in porous systems where groundwater seepage is expected to be primarily diffuse (Röper, Greskowiak, & Massmann, 2014). A common approach in TIR studies of groundwater discharge is to couple the remote sensing measurements with in situ ground‐truthing data of temperature, salinity, and 222 Rn (Kelly et al., 2013; Lee et al., 2016; Mejias et al., 2012; Tamborski et al., 2015; Wilson & Rocha, 2012). 222 Rn (radon‐222) is a useful tracer of SGD because it is several orders of magnitude greater in groundwater compared to open surface waters and is an inert, nonreactive noble gas (Swarzenski, 2007).
[Show abstract] [Hide abstract] ABSTRACT: Fluxes of submarine groundwater discharge (SGD) were investigated into two tidal rivers on the north and south shore of Long Island, NY during July 2015. Ground-based handheld TIR imagery, combined with direct push-point piezometer sampling, documented spatially heterogeneous small-scale intertidal seepage zones. Pore waters were relatively fresh and enriched in nitrogen (N) within these small-scale seeps. Pore waters sampled just 20 cm away, outside the boundary of the ground-based TIR-located seepage zone, were more saline and lower in N. These ground-based TIR-identified seeps geochemically represented the terrestrial fresh groundwater endmember, whereas N in pore waters sampled outside of the TIR-identified seeps were derived from the remineralization of organic matter introduced into the sediment by tidal seawater infiltration. A 222Rn time-series was used to quantify fresh SGD associated N fluxes using the N endmembers sampled from the ground-based TIR pore water profiles. N fluxes were up-scaled to groundwater seepage zones identified from high-resolution airborne TIR imagery using the two-dimensional size of the airborne TIR surface water anomalies, relative to the N flux from the time-series sampling location. Results suggest that the N load from the north-shore tidal river to Long Island Sound is underrepresented by at least 1.6 – 3.6 %, while the N load from SGD to a south-shore tidal river may be up to 9 % higher than previous estimates. These results demonstrate the importance of SGD in supplying nutrients to the lower reaches of tidal rivers, and suggests that N loads in other tidal river environments may be underestimated if SGD is not accounted for.- The images used were mostly cloud free with a flyover time between 11:00 and 11:45 GMT (local time). As our focus was on the water temperature in the bay, land pixels were removed from the field of analysis, leaving only pixels that denoted water during high tide [15]. The processed images cover Kinvara Bay entirely and were subsequently used to create sea surface temperature and temperature anomaly maps as a function of time over a typical year.
Combining Remote Temperature Sensing with in-Situ Sensing to Track Marine/Freshwater Mixing Dynamics
[Show abstract] [Hide abstract] ABSTRACT: The ability to track the dynamics of processes in natural water bodies on a global scale, and at a resolution that enables highly localised behaviour to be visualized, is an ideal scenario for understanding how local events can influence the global environment. While advances in in-situ chem/bio-sensing continue to be reported, costs and reliability issues still inhibit the implementation of large-scale deployments. In contrast, physical parameters like surface temperature can be tracked on a global scale using satellite remote sensing, and locally at high resolution via flyovers and drones using multi-spectral imaging. In this study, we show how a much more complete picture of submarine and intertidal groundwater discharge patterns in Kinvara Bay, Galway can be achieved using a fusion of data collected from the Earth Observation satellite (Landsat 8), small aircraft and in-situ sensors. Over the course of the four-day field campaign, over 65,000 in-situ temperatures, salinity and nutrient measurements were collected in parallel with high-resolution thermal imaging from aircraft flyovers. The processed in-situ data show highly correlated patterns between temperature and salinity at the southern end of the bay where freshwater springs can be identified at low tide. Salinity values range from 1 to 2 ppt at the southern end of the bay to 30 ppt at the mouth of the bay, indicating the presence of a freshwater wedge. The data clearly show that temperature differences can be used to track the dynamics of freshwater and seawater mixing in the inner bay region. This outcome suggests that combining the tremendous spatial density and wide geographical reach of remote temperature sensing (using drones, flyovers and satellites) with ground-truthing via appropriately located in-situ sensors (temperature, salinity, chemical, and biological) can produce a much more complete and accurate picture of the water dynamics than each modality used in isolation.- The lack of basic hydrogeological information is common in many remote regions requiring large spatial scale investigation. Groundwater discharge qualitative assessments using radon have been successfully applied in freshwater ecosystems (Schubert et al., 2006; Wilson and Rocha, 2014) and coastal zones (Dulaiova et al., 2010; Macklin et al., 2014; Maher et al., 2015; Stieglitz et al., 2010; Wilson and Rocha, 2012). Major ion geochemistry has often been studied in riverine settings to explain spatial and temporal variations controlling the ecosystem and the role of groundwater and surface water interactions (Guggenmos et al., 2011; Gurumurthy et al., 2015; Sheldon and Fellows, 2010; Varol et al., 2013; Xiao et al., 2015; Yuan et al., 2011).
[Show abstract] [Hide abstract] ABSTRACT: Coal seam gas (CSG, or coal bed methane) mining is rapidly growing, with poorly understood impacts on groundwater and surface water systems. Here, we use chemical tracers to investigate groundwater-surface water connectivity in an Australian river system (Richmond River Catchment, New South Wales) prior to CSG extraction but after ~ 50 exploratory CSG wells were drilled. We performed four surveys of 29 interconnected creek and river sites, over contrasting hydrological conditions. Radon was used to determine if a surface water segment was gaining groundwater. Radon observations over four seasons revealed that 28 out of 77 surface water segments were clearly gaining groundwater, 5 were possibly gaining groundwater and 44 were undetermined. This is equivalent to gaining segments in 333 km (39%) of surface water from the 864 km being investigated. High spatial and temporal variability in groundwater gaining segments was found. Na/Cl ratios were used to determine the fraction of groundwater in surface water. Overall, the groundwater contribution in surface waters was 14 to 24% higher in post flood conditions than during the other three surveys of baseflow and moderate flow conditions. The results serve as a regional baseline assessment of river water chemistry and groundwater-surface water connectivity prior to the planned development of CSG fields. Our geochemical tracer approach allows for a quick qualitative assessment of groundwater-surface water connectivity in poorly gauged river systems and can define priority locations where groundwater extraction for CSG mining should be carefully managed.- Historically, GIS has many times been applied to integration, processing and dissemination of marine data, including satellite images [15]. In recent years, GIS has been applied to analysis of Global Navigation Satellite System (GNSS) data for the purpose of urban traffic monitoring [13], remote sensing of air and land temperatures [27], temperature monitoring of water balance in India [11] as well as estimating submarine groundwater discharge [28]. However, most of the aforementioned research has been conducted manually, and thus the proposed applications did not form operative systems [16].
[Show abstract] [Hide abstract] ABSTRACT: Estimation of surface temperature using multispectral imagery retrieved from satellite sensors constitutes several problems in terms of accuracy, accessibility, quality and evaluation. In order to obtain accurate results, currently utilized methods rely on removing atmospheric fluctuations in separate spectral windows, applying atmospheric corrections or utilizing additional information related to atmosphere or surface characteristics like atmospheric water vapour content, surface effective emissivity correction or transmittance correction. Obtaining accurate results of estimation is particularly critical for regions with fairly non-uniform distribution of surface effective emissivity and surface characteristics such as coastal zone areas. The paper presents the relationship between retrieved land surface temperature, air temperature, sea surface temperature and vegetation indices (VI) calculated based on remote observations in the coastal zone area. An indirect comparison method between remotely estimated surface temperature and air temperature using LST/VI feature space characteristics in an operational Geographic Information System is also presented.- The temperature contrast between groundwater and seawater significantly decreased during October because of the seasonal variation in the air temperature of Jeju Island. To facilitate the comparison of the SST values between different measurement times, we normalized the SST maps and generated temperature anomaly maps based on Equation 5 (Figure 6; Wilson and Rocha, 2012). At both sites, robust relations between SGD plumes and tidal stage were observed (Figure 6).
[Show abstract] [Hide abstract] ABSTRACT: Submarine groundwater discharge (SGD) is a global phenomenon that carries large volumes of groundwater and dissolved chemical species such as nutrient, metals, and organic compounds to coastal zones. We report the influence of SGD on the coastal waters of Jeju Island, Korea, using high-resolution aerial thermal infrared (TIR) mapping techniques and field investigations. An aircraft-based system was implemented using a cost-effective TIR camera for aerial TIR mapping. Ground-based calibrations and system integration with GPS/IMU (global positioning system/inertial measurement unit) were performed for the aerial systems. The aerial surveys showed distinct low-temperature signatures of SGD along the coasts of Jeju Island, revealing large groundwater inputs from the coastal aquifers to the ocean. Multiple aerial surveys over a range of seasons and tidal stages revealed that SGD rates dynamically affect the sea surface temperature (SST) of the coastal zone. The in-situ measurements supported that SGD has a substantial influence on the coastal water chemistry as well as SST. Our observations highlight the extent to which aerial-based TIR mapping can serve as a powerful tool for studying SGD and other coastal processes.- A total of 4 suitable Landsat thermal images (Table 1) of Lough Mask were obtained for processing from the USGS GLOVIS public access online facility (USGS, 2013). The imagery was acquired during summer months when in Ireland maximum temperature differences between groundwater and surface water exist and can be detected using satellite remote sensing (Wilson and Rocha, 2012). All of the images obtained were largely cloud free with a scene centre flyover time between 11:15 h and 11:30 h GMT (local time) and land pixels in each scene were masked based on a threshold of Landsat ETM+ band 5 (near infrared) image values.
[Show abstract] [Hide abstract] ABSTRACT: The combination of thermal imagery and geochemical tracing has been demonstrated as an affordable and effective technique to identify potential groundwater discharge sites in coastal areas on a regional scale. In this paper, a combined multi-tracer approach is evaluated in its applicability to lakes and verified as an appropriate and powerful means to localise and assess groundwater-lake interactions, demonstrated through a case study of Lough Mask in the west of Ireland. Surface water temperature patterns generated from Landsat 7 Thermal Infrared (TIR) images were used to locate groundwater inputs captured as anomalous cold plumes visibly emanating from shallow lake margins during summer months. Radon-222 was used to confirm the presence of groundwater and to detect localised seepage points or groundwater " hotspots ". Conductivity was used as a secondary tracer in support of radon to identify areas of active groundwater inflow. Radon results show that groundwater enters the lake through carboniferous limestones adjacent to the north and east of the lake and no groundwater inflows were observed from the west characterised by Ordivician sandstones and mixed volcanics. The observed strong anti-correlation between mapped radon and satellite derived temperature values implies that decreases in surface water temperatures are associated with increases in radon activity and hence groundwater inputs to the lake. Moreover the spatial pattern of mapped temperature anomaly displays a positive correlation to the mapped radon and conductivity anomalies which further suggests that the tracers are inextricably linked and support a common groundwater source. The study demonstrates the suitability of a multi tracer approach as a comprehensive and cost-effective preliminary screening tool for groundwater-lake interactions with the potential for application elsewhere. This information is important and can be used in support of national water policy and legislation by helping to identify for example, lakes at risk of failure to comply with Water Framework Directive (2000/60/EC) water quality objectives particularly where mapped inputs are linked to groundwater bodies classified as less than good status as per the requirements of the Groundwater Directive (2006/118/EC). Evaluating the potential occurrence and understanding where groundwater discharge occurs is the first step towards more in-depth geochemical surveys that seek to clarify the role played by groundwater in lacustrine biogeochemical budgets.
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