The hydrological cycle describes the continuous movement of water above, on, and below the surface of the earth. The water on the Earth’s surface – surface water – occurs as streams, lakes, rivers as well as bays and wetlands. The water below the surface of the Earth primarily is ground water, but it also includes soil water (Sphocleous, 2000). Interactions between groundwater and surface water play a critical role in the functioning of riparian ecosystems. These interactions can have significant implications for both water quantity and quality. Identifying potential exchange of water between the aquifer and stream channel has therefore been investigated by many researchers using a variety of methods (USGS – Ground Water Information, 2008).
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Assessing groundwater-surface water interactions is often complex and difficult. There are many factors which influence groundwater-surface water interactions such as river bed characteristics, geology, geomorphology and climate. In general a number of methods have been used to ascertain the nature of groundwater surface water interactions across different catchments. These methods include several tracers used to identify the exchange of surface and groundwater, such as heat, ion chemistry, isotopes and viruses. Potential surface aquifer interactions have also been quantified using remote sensing and models (USGS, 2008; Kalbus et al, 2006).
The purpose of this essay is to review the various techniques used to determine groundwater and surface water interactions and their importance whilst encompassing significant case studies from around the world and within Australia.
Surface water and groundwater (GW-SW) have long been considered separate entities, and have been investigated individually. Although chemical, biological and physical properties of surface water and groundwater are indeed different, they are not isolated components of the hydrologic system, but instead interact in a variety of physiographic and climatic landscapes. Therefore development or contamination of one commonly affects the other (Kalbus et al, 2006). To understand GW-SW interactions, it is necessary to understand the effects of the hydrogeological environment on GW flow systems, that is the effects of topography, geology, and climate as these factors are the major influences on the type of techniques use to determine GW-SW interactions (see figures 1, 2 & 3) (USGS, 2008).
Figure 1. Groundwater seepage into surface water Figure 2. Subaqueous springs resulting from ground
water flow through highly permeable sediments (USGS, 2008) (USGS, 2008)
Figure 3. Ground-water flow paths vary greatly in length, depth and travel time from points of recharge to points of discharge in the ground-water system (USGS, 2008)
Many studies of GW-SW interactions involve the use of more than one technique in attempting to determine nature of exchanges. Environmental tracers are naturally occurring dissolved constituents, or physical properties of water that can be used to track water movement through water sheds. Often tracers such as chlorofluorocarbons (CFC’s), conservative and non-conservative ions, stable and radio-isotopes can be coupled with piezometric monitoring and computer modelling to aid in determining the movement and character of GW or SW (Hohener et al, 2003).
CFC’s are synthetic halogenated volatile organic compounds that have been manufactured since 1930 and can be detected analytically in water in small concentrations. Previous review articles have occasionally summarised the use of CFC’s as tracers for dating pristine groundwater as a failure due to local CFC contamination in excess of the equilibrium with modern air. However, CFCs do provide hydrogeological tracers and dating tools for young groundwater on a time-scale of 50 years (Hohener et al, 2003).
Since the mid 1970’s, CFCs have been used routinely by hydrologists and various disciplines, for dating and tracing water masses. Using gas chromatographs and electron capture detectors, analytical methods for CFCs in water with detection limits for some particular CFC’s, have been developed. Generally, the presence of detectable concentrations of CFCs in groundwater indicates recharge after the late 1940’s, or mixing of older water with younger water. Groundwater samples with CFC concentrations between the analytical detection limit and the equilibrium with atmospheric concentrations at recharge temperature can potentially be used for age-dating. The use of CFCs dating techniques allows hydrologists and scientists alike to determine groundwater recharge and mixing aiding in detecting GW-SW interactions (Hohener et al, 2003; Schilling et al, 2010).
Researchers utilize a wide variety of conservative and non-conservative tracers for hydrological studies. In addition, stable isotopes of oxygen and hydrogen, which are part of the water molecule, are used to determine the mixing of waters from different sources (USGS, 2008; Rodgers et al, 2004). This is successful because of the differences in the isotopic composition of precipitation among recharge areas, the changes in the isotoic composition of shallow subsurface water caused by evaporation and temporal variability in the isotopic composition of precipitation relative to groundwater. For example, 87Sr/86Sr ratios can be used to distinguish between groundwater discharge and surface mixing. Strontium isotopes used in combination with more conventional tracers such as deuterium and 18O have helped to establish the sources of differing groundwater types entering lakes (Rodgers et al, 2004).
Radioactive isotopes are useful indicators for the amount of time that water has spent in the groundwater system. Deuterium and 18O have been used together with both radioactive tracers (3H/3He) and other non-conventional tracers like rare earth elements (REEs) to determine groundwater inflow and outflow from large lakes such as East African Rift Valley lakes (Ojiambo et al., in review). Lyons et al. (1998) also have used sources of both radioactive (36Cl) and non-radioactive (37Cl) tracers to ascertain sources of solutes for Antarctic lake systems (Lyons et al, 1998).
Another useful indicator is 222Radon which is a chemically inert radioactive gas that has a half-life of only 3-4 days. It is produced naturally in groundwater as a product of the radioactive decay of 226radium in uranium-bearing rocks and sediments (Lyons et al, 1998). Several studies have documented that radon can be used to identify locations of significant groundwater input into a stream, such as from springs. In France a study was conducted where radon was used to determine stream-water loss to groundwater as a result of ground-water withdrawals (USGS, 2008).
As shown in figure 4, sharp changes in chemical concentrations were detected over short distances as water from the Lot River in France moved into its contiguous alluvial aquifer in response to pumping from a well. An environmental tracer was used to determine the extent of mixing of surface water with ground water, and radon was used to determine the inflow rate of stream water. Then the rate at which dissolved metals reacted to form solid phase during movement of stream water toward the pumping well could be calculated (USGS, 2008).
Conservative and non-conservative ions as tracers can also be used to parameterize groundwater models as well as to calculate the age and recharge location of ground waters. This can be done by directly introducing 3H in a groundwater system to determine groundwater flow paths which assists in the model parameterization coupled with the use of deuterium, 18O, 3H/3He ratios, and the recently developed 4He in-growth technique to guide parameterization of a groundwater model of a regional aquifer (Sophocleous, 2000). Familiarity with the use and limitations of numerous conservative and non-conservative tracers to ground water and surface water environments is an important component with potential applications of these techniques, GW-SW interactions can be inferred (Sphocleous, 2000; Schilling et al, 2010).
In Australia, the transport of saline groundwater from local and regional aquifers to the lower River Murray is thought to be influenced by lagoons and wetlands present in adjacent floodplains. In the study by Banks et al, (2009), interactions between a saline lagoon and semi-confined aquifer at a floodplain on the River Murray were studied using hydrogeological techniques and environmental tracers (Cl-, Î´2H and Î´18O) (Banks et al, 2009).
The results showed using piezometric surface monitoring that the lagoon acted as a flow-through system intercepting local and regional groundwater flow. The mass balance was determined using chloride, and showed that approximately 70% of the lagoons winter volume was lost due to evaporation. Next a stable isotope mass balance was used to estimate leakage from the lagoon to the underlying aquifer. This showed that approximately 0-38% of the total groundwater inflow into the lagoon was lost to leakage, as opposed to 62-100% groundwater inflow which was lost to evaporation (Banks et al, 2009).
Through the use of piezometric surface monitoring and tracers, Banks et al, (2009), were able to determine GW-SW interactions. This allowed them to conclude that the floodplain wetland behaved as groundwater flow-through systems, intercepting groundwater discharge, concentrating it and eventually recharging more saline water to the floodplain aquifer. Being able to trace, determine and understand GW-SW interactions such as those presented here, ultimately benefits effective management of salinity in Australia (Banks et al, 2005).
Further studies of the Murray River and the Murray Basin have concluded that salinity could also be contributed to by flow regulation and water diversion for irrigation as this could considerably impact the exchange of surface water between the Murray River and its floodplains (Allison et al, 1990; Lamontage et al, 2005). Through use of piezometric surface monitoring and environmental tracers (Cl-, Î´2H and Î´18O), Lamontagne et al, was able to conclude that Murray River was losing under low flow conditions. Environmental tracer data suggested that the origin of groundwater is principally bank recharge in the riparian zone and a combination of diffuse rainfall recharge elsewhere on the flood plain. This information was critical in deciphering that bank discharge occurred during some flood recession periods and understanding that the way in which the water table responded to changes in river level was a function of the type of stream bank present (Lamontage et al, 2005).
In the Western Murray basin, the clearing of native vegetation in a semi-arid region of southern Australia is thought to have lead to increases in Groundwater recharge. Unsaturated zone chloride and matric suction profile estimates suggest there is a significant time delay in aquifer response to pre and post clearing recharge (Allison et al, 1990). Predictions of the time delay lag in aquifer response have been verified using bore hydrographs. The results show that in some areas of light soil and shallow water table the water is now rising, however in other areas of heavy soil the water is not yet beginning to rise. The effects of increased recharge on the salinity of the River Murray, a major water resource, have been predicted that the salinity of the river will increase about 1µS cm -1 year -1 over the next 50 years. These results show the crucial role hydrological analysis and environmental tracers play in major resource management throughout Australia and potentially the world (Allison et al, 1990).
Groundwater and Surface water are not isolated components of the hydrological system and therefore should not be studied or managed as such. There are many factors which influence and control both GW and SW flow paths and interactions within the hydrological cycle. Through use of monitoring systems, modelling, and environmental tracers a better understanding of the complex interactions between GW-SW can be gained. Although further study is needed and techniques can be improved upon, it is through a better understand of the hydrological cycle and its complex interactions that more appropriate management plans can be made to ensure the resource is available to all in the future.
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