OS54B-01 INVITED
Subterranean Groundwater Discharge - A Transport Mechanism Linking Land and Water – What do we Know and What Should we Strive to Know
At any land-water interface water is exchanged between subterranean aquifers and surficial waters bodies such as lakes, rivers, seas and the coastal ocean. Various physical forcing mechanisms (density and pressure gradients, tides, waves, currents) induce groundwater-surface water mixing that result in a constant exchange of water (Moore 1990, Church, 1996). This exchange represents a transport mechanism that introduces land based components including nutrients, trace metals, bacteria and organic pollutants into surface waters and impacts groundwater and surface water biogeochemistry and ecosystems. Subterranean groundwater discharge into aquatic systems has been recognized as a significant source of freshwater, nutrients, bacteria, and other dissolved constituents (Ba, CH4, Fe, Mn, U, As, Cu, Hg) to various water bodies (Valiela et al., 1990; Cable et al., 1996; Moore, 1997; Shaw et al., 1998; Corbett et al., 1999, Krest et al., 2000; Kelley and Moran, 2002; Boehm et al., 2004; Charette and Buesseler, 2004; Charette et al., 2005; Charette and Sholkovitz, 2006; Paytan et al., 2006; Bone et al., 2006, 2007; Swarzenski and Baskaran, 2007; Bone et al., 2007). What are the sources of these constituents? What controlled their loading? How would these land–based inputs impact aquatic ecosystems? Can this data be used for designing best management plans associated with efforts of water quality preservation and wetlands reclamation and reconstruction? The current state of knowledge will be briefly summarized, some new data presented and future directions for research suggested.
OS54B-02 INVITED
Mapping Submarine Groundwater Discharge – how to investigate spatial discharge variability on coastal and beach scales
Submarine groundwater discharge (SGD) is now increasingly recognized as an important component in the water balance, water quality and ecology of the coastal zone. A multitude of methods are currently employed to study SGD, ranging from point flux measurements with seepage meters to methods integrating over various spatial and temporal scales such as hydrological models, geophysical techniques or surface water tracer approaches. From studies in a large variety of hydrogeological settings, researchers in this field have come to expect that SGD is rarely uniformly distributed. Here we discuss the application of: (a) the mapping of subsurface electrical conductivity in a discharge zone on a beach; and (b) the large-scale mapping of radon in coastal surface water to improving our understanding of SGD and its spatial variability. On a beach scale, as part of intercomparison studies of a UNESCO/IAEA working group, mapping of subsurface electrical conductivity in a beach face have elucidated the non-uniform distribution of SGD associated with rock fractures, volcanic settings and man-made structures (e.g., piers, jetties). Variations in direct point measurements of SGD flux with seepage meters were linked to the subsurface conductivity distribution. We demonstrate how the combination of these two techniques may complement one another to better constrain SGD measurements. On kilometer to hundred kilometer scales, the spatial distribution and regional importance of SGD can be investigated by mapping relevant tracers in the coastal ocean. The radon isotope Rn-222 is a commonly used tracer for SGD investigations due to its significant enrichment in groundwater, and continuous mapping of this tracer, in combination with ocean water salinity, can be used to efficiently infer locations of SGD along a coastline on large scales. We use a surface-towed, continuously recording multi-detector setup installed on a moving vessel. This tool was used in various coastal environments, e.g. in Florida, Brazil, Mauritius and Australia's Great Barrier Reef lagoon. From shore-parallel transects along the Central Great Barrier Reef coastline, numerous processes and locations of SGD were identified, including terrestrially-derived fresh SGD and the recirculation of seawater in mangrove forests, as well as riverine sources. From variations in the inverse relationship of the two tracers radon and salinity, some aspects of regional freshwater input into the lagoon during the tropical wet season could be assessed. Such surveys on coastal scales can be a useful tool to obtain an overview of locations and processes of SGD on an unknown coastline.
OS54B-03 INVITED
Global Land-Ocean Linkage: Direct Inputs of Water and Associated Nutrients to Coastal Zones via Submarine Groundwater Discharge (SGD)
Direct discharge of freshwater and associated dissolved nutrients via Submarine Groundwater Discharge (SGD) may have potentially important impacts on coastal water bodies, such as increased eutrophication or hypoxia. Yet, at the global scale, SGD has received little attention compared to efforts made to estimate other pathways of nutrients to the ocean such as riverine inputs. Most studies on the nutrient flux to the coastal zone by SGD have focused on local to regional scales, with the major part of the research being carried out in the Northern hemisphere (U.S. and Europe), and concentrating on areas of high total SGD including recycled fluxes from the saltwater / freshwater mixing zone. While at local scales, the effects of this recycling in the 'subterranean estuary' are important to understand short-term changes in nutrient availability, at the global scale, quantification of the yet poorly constrained net fluxes of freshwater and nutrients discharged via this transport path to the oceans is crucial. Here, we present the first steps towards spatially-explicit estimates of nutrient inputs to the coastal zone via freshwater SGD at the global scale, using baseflow estimates from a global hydrological model, combined with assessments of nutrient concentrations in coastal groundwater bodies.
OS54B-04 INVITED
Use of Underwater Resistivity to Map Chloride in Lakebed Sediments Beneath Mirror Lake, NH
Electrical resistivity tomography was used to image a road salt plume discharging from groundwater beneath Mirror Lake, NH. Two types of surveys were conducted to map the extent of the plume and identify potential zones of groundwater seepage. In water deeper than ~2 meters, the electrode cable was placed on the bottom of the lake with the assistance of SCUBA divers. In shallower water, a floating cable was used to avoid debris on the bottom and the water depth profile was used to constrain the data inversion. The plume was observed as a low-resistivity target of around 100 ohm-m, in contrast to lakebed sediments, which ranged from 800 to 3000 ohm-m. The corresponding concentrations of road salt in water collected from temporary piezometers installed in shallow lakebed sediments were only 10-60 mg/L chloride, indicating the sensitivity of the technique to contrasts in geochemistry even at low concentrations. The lake chloride concentration was 3 mg/L. The survey showed the plume was 10 to 20 m wide and extended all the way to low permeability organic sediments 40 m out from shore. The resistivity survey also shows bedrock knobs that bound the plume on the western shore and near the organic sediments. Groundwater seepage was measured at the edge of the organic sediments and used to estimate the relative chloride contributions to the lake via groundwater and surface water. Mirror Lake has experienced increasing chloride concentrations since the construction of a freeway in the watershed in the late 1960's. A berm designed to protect the watershed from road salt was reconstructed after the increase in lake concentrations was reported in the late 1990's. Chloride concentrations have risen less sharply since the improved berm was placed, but the extent of the plume suggests the contribution from groundwater will continue for many years.
OS54B-05 INVITED
Tracing and Quantifying Groundwater Inflow Into Lakes Using Rn-222 and Other Tracers
Ground and surface water possess totally different characteristics with regard to the radon-isotope 222Rn. In groundwater, the activity concentration is in most cases between 1 and 100 kBq m-3. In contrast, the concentrations are well below 0.1 kBq m-3 in lakes and the ocean. Based on these large concentration differences it is possible to trace even small groundwater inflow into surface waters. In some cases, monitoring of the horizontal distribution of radon in lakes is feasible, revealing the concentration gradient due to radioactive decay and mixing between zones of groundwater entry and the open lake. However, essential requirements are sufficiently precise measurements as well as a very low detection limit. Using large-volume water samples (12l) we developed a simple system for radon measurements down to 3 Bq m-3. The water samples, taken in the field, are analyzed in the laboratory by equilibration with a closed and initially radon-free gas loop and measured with an alpha-spectrometer. This method was applied to a small dredging lake (200 x 850 m) in the Rhine Valley (Germany) and successfully enabled the identification of areas with enhanced groundwater inflow. A quantification of the groundwater input based on the radon balance agrees with former studies. Repeated measurements in 2005, 2007 and 2008 proved the detected profile to be robust and showed the radon measurements to be an ideal tool for the investigation of the groundwater-lake interaction. Recent methodical developments intend to raise the precision as well as to lower the detection limit down to substantially smaller values. A new gas extraction and measurement system is based on three simple steps: water degassing through membrane contactors, concentration of Rn on a charcoal trap, and finally the measurement by alpha-spectrometry in a small gas loop after desorption from the charcoal. As a first application, water samples from the same dredging lake were analyzed, yielding the expected Rn profile. In addition to Rn, we use SF6 measurements to study groundwater-lake interaction. E.g., in two small stratified mining lakes in East Germany, the reduction of an SF6 spike by groundwater with low SF6 values was used to quantify the groundwater inflow.
OS54B-06 INVITED
Submarine groundwater discharge of rare earth elements: Evidence of an important trace element flux to coastal waters
Johannesson and Burdige [2007, EPSL 253, 129] suggested that submarine groundwater discharge (SGD) represents a substantial, unrecognized source of Nd to the oceans. Based on a globally averaged terrestrial SGD flux equal to 6 percent of the global river discharge and mean groundwater Nd concentrations obtained from the literature, we estimated a global SGD Nd flux that was within a factor of 2 of the previously proposed missing global Nd flux. To test our hypothesis that SGD is an important source of Nd to the oceans, rare earth element (REE) concentrations were measured in SGD samples collected beneath a coastal lagoon on the Florida Atlantic coast (Indian River Lagoon). Shale (PAAS)-normalized REE patterns for all SGD samples exhibit substantial enrichments in the heavy REEs (HREE) compared to the light REEs (LREE) as shown by their PAAS-normalized Yb/Nd ratios, which range from 5 to 73 (mean = 16). SGD from piezometers located 10 m and 22.5 m from shore exhibit PAAS-normalized REE plots that are most similar to the patterns of the overlying lagoon (surface) water. For example, mean PAAS-normalized Yb/Nd ratios for groundwaters sampled from the 10 m and 22.5 m piezometers are 6.7 and 8.3, which compare well with the PAAS- normalized Yb/Nd ratio of water column samples (8.7). In contrast, the mean PAAS-normalized Yb/Nd ratio of terrestrial-derived groundwater from the piezometer at the shoreline is 41. Neodymium concentrations of the SGD samples range from 230 to 2400 pmol/kg (mean = 507 pmol/kg), and thus are substantially higher than reported for open ocean seawater (typical Nd = 20 pmol/kg). Based on SGD fluxes previously determined with seepage meters, porewater Cl concentrations, and Rn-222 deficiencies of porewaters [Martin et al., 2007, Water Resour. Res. 43, W0544, doi: 10.1029/2006WR005266], we estimate daily inputs of Nd to the Indian River Lagoon of 50 to 2100 umoles for the terrestrial-derived component of SGD, and 171 mmoles for the marine component of SGD (81 to 3400 times greater). Residence times of Nd in the portion of the lagoon studied are estimated to range from 6 to more than 250 years based on the terrestrial-derived SGD flux of Nd, compared to 26 days using the marine-derived SGD flux of Nd. The substantially shorter residence time determined using the marine-derived SGD component compares well with the estimated flushing time for this portion of the estuary (~3 weeks). The similarity between SGD and lagoon water Nd concentrations and PAAS-normalized REE patterns, in conjunction with the larger, marine-derived SGD flux of Nd, strongly suggests that recirculation of lagoon water and subsequent SGD exerts the principal control on Nd concentrations in the lagoon. The elevated Nd concentration for deep groundwater (186 cmbsf) located 22.5 m from shore also agrees well with another study that reported recirculated, marine SGD as a source of REEs to coastal waters [Duncan and Shaw, 2003, Aquatic Geochem. 9, 233]. Thus, our observations demonstrate the importance of recirculated, marine SGD to these lagoon surface waters, and further support our hypothesis that SGD contributes substantial fluxes of Nd to the coastal oceans.
OS54B-07 INVITED
Submarine Groundwater Discharge into Tolo Harbor, Hong Kong, China
Tolo Harbor is an elongate and semi-enclosed bay in igneous rock areas in northeastern Hong Kong. It has
an area of about 50 km2 and the groundwater catchment behind the harbor has an area of 160 km2, which is
well-defined by ridges that reach a maximum elevation of 957 m above sea level. Over the last two decades,
about half of the algal blooms reported in Hong Kong waters occurred in the harbor. Rivers and sewage are
recognized as two key sources of nutrients. It is speculated that this harbor may have relatively high
submarine groundwater discharge (SGD) due to its special topographical and hydrogeological setting and
that the SGD may be another source of nutrients to the harbor.
A research project is conduced to quantify the SGD into Tolo Harbor and to estimate the nutrient flux into the
harbor through this pathway. The geochemical tracers of radon (222Rn) and radium (223Ra, 224Ra,
226Ra, and 228Ra) in groundwater and seawater are measured over the harbor and a seepage meter is
deployed for direct and continuous SGD measurement for 72 hours. The study shows that the geochemical
tracers fluctuate temporally in anti-phase with tidal height and that there is general trend for the geochemical
tracers to decrease with distance offshore. Three sites with relatively high SGD are identified. The residence
time estimated from 224Ra is around 30 days, which correlates well with previous studies. The flux of SGD to
the harbor is estimated by three different approaches including radium and radon budget analyses and
seepage meter. Finally, nutrient flux to the harbor through SGD is estimated, which shows that the nutrient
loading through this pathway is significant. It is suggested that current practice for the management of algal
blooms in Hong Kong, in which nutrient loading through SGD is ignored, should be reviewed and the control
measures of groundwater contamination are obviously required.
http://hydro.geo.ua.edu/jiao/publ.htm
OS54B-08 INVITED
Potential Impacts of Climate Change on Water Resources in the Great Lakes Region
Climate-driven changes in temperature and precipitation are projected to affect many aspects of water resources. Here, we provide examples of several case studies demonstrating the potential effects of climate change on soil moisture, river and stream flow, and lake levels in the Great Lakes and the Midwest. Using the Variable Infiltration Capacity (VIC) large-scale hydrology model and the NOAA/GLERL Great Lakes basin model driven by gridded historical meteorology and statistically downscaled future global climate model projections, simulations of future changes under IPCC emissions scenarios were conducted. Runoff and baseflow are routed to produce streamflow and inputs to the Great Lakes, which are analyzed to identify potential future trends in streamflow characteristics and lake inputs. Additionally, the VIC model with lake and wetland algorithm was also run to study trends in lake ice phenology for Minnesota, Wisconsin and Michigan. In general, we find that warming has been reducing the duration of ice cover on regional lakes in the last 50 years, and based on the future projections many lakes, especially in the southern parts of these states will be essentially ice-free by the end of the century. Under lower emissions over most of this century, competing effects suggest no net changes in Great Lake levels. Under higher emissions, however, the effects of greater temperature increases are projected to begin to dominate over the longer term, leading to lake level decreases of one to three feet before the end of the century.