Chapter 29: Nuclear Energy: Applications – Environment – The Quantification of Submarine Groundwater Discharge (SGD) – Edited by Dr. Mir F. Ali
Like surface water, groundwater also flows towards the sea, in both rivers and aquifers, and discharges submarine groundwater into the sea which is known as submarine groundwater discharge (SGD). Estimates of this SGD vary significantly between 6 percent and 100 percent of freshwater inputs into coastal waters, largely due to the regional and temporal variability of SGD. It is important to know that when aquifers intersect with the shoreline they release fresh water to the ocean. Here is a reality, the impact of river water and sewage on the coastal sea water quality has been well recognized, however, the input of chemicals and nutrients to the sea in the form of SGD is ignored in various parts of the world.
More recently, SGD has received considerable attention in the area of coastal management due to its potential as a freshwater resource in areas with water shortages. Additionally, if SGD is composed of brackish water it may be utilized in desalination plants. On the other hand, SGD may also contain high levels of pollutants (nutrients, metals, pesticides) thus affecting coastal ecosystems. This can result in outbreaks of harmful algal blooms and the contamination of coastal regions. As a management tool, knowledge of the volume of submarine groundwater discharge helps to prevent overexploitation of coastal aquifers and prevent salt water intrusion. Here is a graphical representation of SGD:
It is rather obvious that fresh SGD will occur wherever an aquifer is hydraulically connected with the sea and the water table is above sea level. The driving force behind this process is the hydraulic gradient from the upland region of a watershed to the surface water discharge location at the coast. At the land–sea boundary, seawater flows into aquifer sediments under the force of gravity and thus a complex dynamic flow regime is established as fresh and saline groundwater of different densities interact. Several forcing mechanisms, such as waves, tides, dispersive circulation, and changes in upland recharge, affect the rate of fluid flow for both fresh and saline groundwater and are ultimately important in controlling the submarine discharge of both fluids.
Exchange between SGD and overlying surface waters has become increasingly important due to potential impacts resulting from anthropogenic land uses. The most general definition of groundwater is water in the saturated zone of geologic material. Water in the pores of submerged sediments or rock is, therefore, “Groundwater” since the geological material below the seafloor will be saturated. The latest generalized definition of SGD that has been modified from an earlier version which describe that:
- The flow of water through continental margins from the seabed to the coastal ocean, with scale lengths of meters to kilometers, regardless of fluid composition or driving force.
Groundwater discharge originates inland and carries with it contaminants or nutrients, dissolved or colloidal, that have the potential to impact the chemical budget of surface water ecosystems. This impact, both chemical and physical may be heightened in smaller bodies of water such as embayments or lagoons due to their limited volume and restricted fluid exchange with the open ocean. In addition to freshwater inputs, groundwater discharge occurs as saltwater re-circulation induced by tides.
In general, the driving forces for groundwater flow and its eventual seepage in the coastal zone include hydraulic gradients, buoyant forces and biological forces. As mentioned before, SGD occurrence is as a result of a hydraulic connection and a positive pressure gradient between shallow or deep coastal aquifers and the sea. It occurs mainly as diffuse seepage along the shoreline, offshore seepage or spring discharge. Near-shore diffuse seepage is typical for shallow, unconsolidated, coarse-grained aquifers or in the sand, clay and limestone mixtures of shallow aquifers in Florida. The magnitude of this type of seepage generally decreases with increasing water depth and distance from the coast. Typically coastal aquifers are classified depending on their hydrology, either as shallow – local flow systems, or deep – intermediate or regional flow systems. Shallow aquifers are characterized by high rates of recharge (1–30 cm yr?1) and high rates of groundwater flow (1–100 m yr?1). Rates of recharge and water levels respond rapidly to individual precipitation events. Deep aquifers on the other hand, are much less connected with the surface and are associated with lower rates of recharge (0.01–1 cm yr?1) with concomitant low flow velocities (0.1–1 m yr?1) in general.
SGD can vary widely over time and space. Short period water waves stir or agitate pore water without, necessarily, producing any net flow. This has been referred to as ‘wave pumping” or “wave stirring”. If the density of the ocean water increases above that of pore water for any reason, pore water can float out of the sediment by gravitational convection in an exchange with denser seawater, without net discharge. The process has been referred to as “Floating” or “Salt Fingering”. SGD has also been referred to as “Flushing”. This generally involves a continuous replacement of pore water involving a discharge driven by the hydraulic gradients on shore or pressure gradients in the coastal ocean. Although groundwater flows downhill, it must however follow a tortuous path through small pores in the aquifer material, typically slowing its flow to a crawl. Unlike surface waters that move at noticeably rapid rates, groundwater may only move a few inches in a day. Eventually, it flows into the coastal ocean through seeps and springs, effectively completing the continuous water cycle between land, ocean, and atmosphere.
Until the mid-1990s, studies on SGD did not receive widespread attention, because it was generally thought that SGD rates were not large enough to be a direct influence on ocean water budgets. This omission may in part be due to the inherent difficulty in identifying sites and quantifying rates of SGD, because most SGD occurs as diffusive flow, rather than discrete spring flow. This is in sharp contrast to studies of river discharge or river chemistry, which are obviously more easily sampled and quantified. However, there is a growing recognition that the submarine discharge of fresh, brackish, and marine ground water into coastal oceans is just as important as river discharge in some areas of the coastal ocean.
Although SGD is a potentially important source of nutrients and other dissolved components to the coastal ocean, it has so far remained poorly quantified. An accurate insight into the magnitude and controls of these components, especially the nutrients (nitrogen, N and phosphorus, P) fluxes associated with SGD is necessary in order to understand the functioning of the coastal ocean and how it responds to anthropogenic and natural perturbations. Apart from the amounts of N and P entering the coastal ocean through SGD, it is also important to evaluate the potential effect of SGD on the ratio of N and P in coastal waters, because this ratio determines which nutrient is limiting phytoplankton growth. For instance, the flux ratio of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in rivers has been found to be variable within a range of at least two orders of magnitude.
Radium and radon measurement techniques have been developed to detect and quantify SGD in coastal regions; both radionuclides are enriched in SGD relative to sea water. Sources of SGD can be detected by measuring the spatial distribution of radium and radon in coastal waters. Temporal changes in their concentrations — mainly a result of mixing between SGD and seawater driven by the tides — allow for the volume of SGD to be determined. Moreover, the determination of four radium isotopes (223radium, 224radium, 226radium, and 228radium) helps to understand the dispersion and mixing time scales of SGD in coastal waters. Given the ease in the application of radon and radium as tracers of SGD, their use is expected to increase in coastal areas under environmental pressure.
Over the past few years, several studies used natural radium isotopes and radon (222Rn) to assess groundwater discharge into the ocean. Ideally, in order to provide a detectable signal, a groundwater tracer should be greatly enriched in the discharging groundwater relative to coastal marine waters, conservative, and easy to measure. Radium isotopes and radon have been shown to meet these criteria fairly well and other natural tracer possibilities exist which may be exploited for groundwater discharge studies. In applying geochemical tracing techniques, several criteria must be assessed or defined, including boundary conditions (i.e., area, volume), water and constituent sources and sinks, residence times of the surface water body, and concentrations of the tracer. Sources may include ocean water, river water, groundwater, precipitation, in situ production, horizontal water column transport, sediment re-suspension, or sediment diffusion. Sinks may include in situ decay or consumption, horizontal water column transport, horizontal or vertical eddy diffusivity, and atmospheric evasion. Through simple mass balances or box models incorporating both sediment advection and water column transport, the geochemical approach can be quite useful in assessing SGD.
Radium isotopes are enriched in groundwater relative to surface waters, especially where salt water is coming into contact with surfaces formally bathed only in fresh waters. It was demonstrated by Moore that waters over the continental shelf off the coast of the southeastern USA were enriched in 226Ra with respect to open ocean values.
The radium concentrations also showed a distinct gradient being highest in the near-shore waters. By using an estimate of the residence time of these waters on the shelf and assuming steady-state conditions, one can calculate the offshore flux of the excess 226Ra. If this flux is supported by SGD along the coast, then the SGD can be estimated by dividing the radium flux by the estimated 226Ra activity of the groundwater. A convenient enhancement to this approach is that one may use the short-lived radium isotopes, 223Ra and 224Ra, to assess the water residence time.
The following illustrates Moore’s general strategy to determine the importance of oceanic exchange with coastal aquifers:
- Identify tracers derived from coastal aquifers that are not recycled in the coastal ocean; map their distribution and evaluate other sources;
- Determine the exchange rate of the coastal ocean with the open ocean;
- Calculate the tracer flux from the coastal ocean to the open ocean, hence the tracer flux from the aquifer to the coastal ocean;
- Measure the average tracer concentration in the coastal aquifer to calculate fluid flux; and
- Use the concentrations of other components (nutrients, carbon, metals) in the aquifer or their ratios to the tracer to estimate their fluxes.
Hwang developed a geochemical model for local-scale estimation of SGD. If the system under study is steady state, than radium additions are balanced by losses. Additions include radium fluxes from sediment, river, and groundwater; losses are due to mixing and, in the case of 223Ra and 224Ra, radioactive decay. Using a mass balance approach on a larger scale with the long-lived isotopes 226Ra and 228Ra, Kim determined that SGD-derived silicate fluxes to the Yellow Sea were on the same order of magnitude as the Si flux from the Yangtze River, the fifth largest river in the world.
A steady-state mass balance approach may also be used for 222Rn with the exception that atmospheric evasion must also be taken into account . The main principle of using continuous time-series radon measurements to decipher rates of groundwater seepage is that if we can monitor the inventory of 222Rn over time, making allowances for losses due to atmospheric evasion and mixing with lower concentration waters offshore, any changes observed can be converted to fluxes by a mass balance approach. Although changing radon concentrations in coastal waters could be in response to a number of processes (sediment resuspension, long-shore currents, etc.), advective transport of groundwater (pore water) through sediment of Rn-rich solutions is often the dominant process. Thus, if one can measure or estimate the radon concentration in the advecting fluids, the 222Rn fluxes may be easily converted to water fluxes.
Although radon and radium isotopes have proven very useful for assessment of groundwater discharges, they both clearly have some limitations. Radium isotopes, for example, may not be enriched in fresh water discharges such as from submarine springs. Radon is subject to exchange with the atmosphere which may be difficult to model under some circumstances (e.g., sudden large changes in wind speeds, waves breaking along a shoreline). The best solution may be to use a combination of tracers to avoid these pitfalls.
New and improved technologies have assisted the development of approaches based on radium isotopes and radon. The measurement of the short-lived radium isotopes 223Ra and 224Ra, for example, used to be very tedious and time-consuming until the development of the Mn-fiber and delayed coincidence counter approach. Now it is routine to process a sample (often 100–200 L because of very low environmental activities) through an Mn-fiber adsorber, measure the short-lived isotopes the same day by the delayed coincidence approach, and then measure the long-lived isotopes (226Ra and 228Ra) at a later date by gamma spectrometry. Burnett developed a continuous radon monitor that allows much easier and unattended analysis of radon in coastal ocean waters. The system analyses 222Rn from a constant stream of water delivered by a submersible pump to an air–water exchanger where radon in the water phase equilibrates with radon in a closed air loop. The air stream is fed to a commercial radon-in-air monitor to determine the activity of 222Rn. More recently, an automated multi-detector system has been developed that can be used in a continuous survey mode to map radon activities in the coastal zone. By running as many as six detectors in parallel, one may obtain as many as 12 readings per hour for typical coastal ocean waters with a precision of better than 10–15 percent.
Another approach consists of application of in situ gamma-ray spectrometry techniques that have been recognized as a powerful tool for analysis of gamma-ray emitters in sea-bed sediments, as well as for continuous analysis of gamma-ray emitters (e.g., 137Cs, 40K, 238U and 232Th decay products) in seawater. In situ gamma-ray spectrometers have been applied for continuous stationary and spatial monitoring of radon (as well as thoron, i.e., 220Rn) decay products in seawater, together with salinity, temperature and tide measurements, as possible indicators of SGD in coastal waters of SE Sicily and at the Ubatuba area of Brazil.
Methane (CH4) is another useful geochemical tracer that can be used to detect SGD. Both 222Rn and CH4 were measured along the Juan de Fuca Ridge as a means of estimating heat and chemical fluxes from the hydrothermal vents of that area. Both 222Rn and CH4 were used to evaluate SGD in studies performed in a coastal area of the northeastern Gulf of Mexico. Tracer (222Rn and CH4) inventories in the water column and seepage rates measured using a transect of seepage meters were evaluated over several months within a shallow water location. The linear relationships between tracer inventories and measured seepage fluxes were statistically significant. These investigators found that inventories of 222Rn and CH4 in the coastal waters varied directly with groundwater seepage rates and had a positive relationship (95 percent C.L.). In addition, water samples collected near a submarine spring in the same area displayed radon and methane concentrations inversely related to salinity and considerably greater than those found in surrounding waters. In a related study, Bugna demonstrated that groundwater discharge was an important source for CH4 budgets on the inner continental shelf of the same region. In another example, Tsunogai found methane-rich plumes in the Suruga Trough (Japan) and postulated that the plume was supplied from continuous cold fluid seepage in that area. Another technological advance, the ‘METS’ sensor (Capsum Technologies GmbH, Trittau, Germany), can now automatically and continuously measure methane at environmental levels in natural waters.
Several other natural radioactive (3H, 14C, U isotopes, etc.) and stable (2H, 3He, 4He, 13C, 15N, 18O, 87Sr, 86Sr, etc.) isotopes and some anthropogenic atmospheric gases (e.g., CFCs) have been used for conducting SGD investigations, tracing water masses, and calculating the age of groundwater. Uranium may be removed to anoxic sediments during submarine groundwater recharge (SGR). Moore and Shaw used deficiencies of uranium concentration (relative to expected concentrations based on the U/salinity ratio in sea water) to estimate SGR in several southeast US estuaries. Stable isotope data can help to evaluate groundwater–seawater mixing ratios, important for the estimation of the SGD in coastal areas. Seawater and the fresh groundwater end members often have specific signatures due to different tracers/isotopes. Under good circumstances, such differences between end members would allow calculation of the percent groundwater contribution. This may be especially useful when mixing is occurring between more than two end members including saline groundwater.
Besides the mixing ratio calculations, each tracer can be used for interpretation of various groundwater characteristics. In mixed waters, the selection of the related fresh groundwater end member is an important issue that may be addressed via use of stable isotopes. For example, oxygen and hydrogen isotopes generally carry valuable information about recharge conditions. Such information may include recharge elevation, temperature, and degree of evaporation.
Other variables that change the characteristics of the groundwater component in the mixture are the hydrodynamic properties of the aquifer because of change in length of flow paths, groundwater velocity, and flow conditions (e.g., diffuse or conduit flow). Such hydrodynamic characteristics of the aquifer are important for the chemically reactive (e.g. 13C) and radioactive (e.g. 3H) tracers/isotopes. Such processes have to be taken into account in the interpretation of water mixture calculations.
For evaluating freshwater fluxes, salinity anomalies are useful for estimation of SGD. However, to assess brackish and saline fluxes, which in many cases have more impact on the coastal environment; isotopes have an added advantage over chemical techniques. Various aspects of coastal hydrology could be addressed by investigations using a combination of stable, long-lived, and short-lived isotopes along with other complementary techniques.
- IAEA Nuclear Technology Review 2010: http://www.iaea.org/Publications/Reports/ntr2010.pdf
- Intercomparison of Submarine Groundwater Discharge Estimates from a Sandy Unconfined Aquifer: http://www.whoi.edu/science/MCG/groundwater/pubs/PDF/Nov06/ATIR_Mulligan.pdf
- Submarine Groundwater Discharge – Its Importance and Quantification: http://www.scitopics.com/Submarine_Groundwater_Discharge_Its_Importance_and_Quantification.html
- USGS – Submarine Groundwater Discharge – An Introduction: http://walrus.wr.usgs.gov/sgd/
- IAEA – Nuclear and ISOTOPIC Techniques for the Characterization of Submarine Groundwater Discharge in Coastal Zones: http://www-pub.iaea.org/MTCD/publications/PDF/te_1595_web.pdf