CHAPTER 3- LITERATURE REVIEW
3.4 Analysis and modelling of groundwater-surface water interactions
29 the heat sources and sinks of the water body and air and isolates the energy required for the evaporating process (Shaw, 1994).
3.4 Analysis and modelling of groundwater-surface water interactions
30 thinking, and therefore measurement of flows in rivers, groundwater levels, water chemistry and rainfall must form the basis of further research into this complex issue (Kelbe and Germishuyse, 2010).
Groundwater abstraction could potentially impact effluent streams where groundwater is discharged into the river. However, where the water table is positioned below the base of the river (influent or detached streams), groundwater abstraction is unlikely to have any impact on flow in the river. Similarly, construction of a dam or abstraction of large volumes of surface water directly from a river is unlikely to impact aquifers directly adjacent to effluent streams, but may be of importance in the case of influent streams.
Because of this understanding, and given the relatively small area of South Africa drained by perennial rivers, the simplistic assumption that the use of groundwater will result in a corresponding reduction in spring flow and surface water resources (Basson et al., 1997).
Abstracting groundwater from a borehole causes the water table to drop, thereby inducing groundwater flow toward the pumped borehole. This results in a cone of depression forming around the pumped borehole (Fetter, 2001). The depth and extent of the cone of depression is dependent on the rate and duration of abstraction and prevailing geohydrological properties of the aquifer. Should the cone of depression around the pumped borehole reach a surface water body (river, lake, wetland or estuary), then localised hydraulic gradients can change and flow induced from the surface water body into the subsurface. The extent of losses from the surface water body will be dependent on localised hydraulic gradients, hydraulic properties of the subsurface and channel bed and the length of intersection of the cone of depression. The effect of pumping a single borehole will generally remain at a local scale. However, large-scale abstraction from a well field much like the ones in the Kwangwanase and Inkanyezini in Manguzi within the study area or multitude of boreholes could significantly reduce flow in a surface water body on a regional scale. The effect of pumping may only be realised years after pumping started, depending on the rate, volume and duration of groundwater abstracted and the distance between the surface water body and the abstraction points.
31 3.4.1 Determination of groundwater - surface water interaction using natural environmental tracers
Groundwater movement through the catchments can be traced by naturally occurring dissolved chemical constituents, isotopes and physical properties of water. Useful environmental tracers include common physical parameters such as EC, dissolved chemical constituents or their relative abundance in relation to each other, stable isotopes (18O, 2H), radioactive isotopes (3H, 222Rn), and water temperature (Mazor, 1991). These parameters can be used to identify the source of water, rate of movement and the age of water.Tritium (3H) is a useful indicator of the time water has spent in the subsurface.
Nuclear bomb test during the 1950s and 1960s released high concentrations of 3H into the atmosphere and induced high concentrations of 3H in the atmosphere and eventually in precipitation. Groundwater recharged during the bomb testing period can be identified by increased 3H concentrations in groundwater even today giving a tool of dating groundwater. However, due to radioactive decay, the concentration is reduced, therefore lessening the usefulness of this technique.
Naturally the production of tritium introduces 5 Tritium units (TU) to surface water and precipitation. Anthropogenic activities then increased the amount of tritium to the atmosphere in 1952 in the northern hemisphere through bomb tests (Mazor, 1991).
Nevertheless, tritium in groundwater is not significantly affected by chemical processes (Drever, 1997). Tritium concentration tends to vary with seasonal variation, location and mixing of water, so it becomesdifficult to accurately estimate when the groundwater was recharged. However, the amount of TU in water can roughly give an estimate. The following is an interpretation of tritium in water, given by Mazor (1991):
Water with zero tritium (<0.5 TU) has a pre-1952 age.
Water with tritium concentration >10 TU has a post 1952 age.
Water with tritium concentration between 0.5 and 10 TU has a mixture of pre- 1952 and post 1952 water.
32 Elevated concentrations of 3H have been detected in leachate generated by landfills, resulting in 3H becoming a useful tracer for detecting groundwater contamination by waste disposal sites. Chlorofluorocarbons (CFCs) can be used to date groundwater less than 50 years old (Parsons, 1994). These approaches have been used successfully by Richey et al. (1998), Saayman et al. (2002) and others, but are dependent on the collection and analysis of sufficient samples before, during and after rainfall events. Undertaking simple EC profile along the length of a river during various stages of river flow could be a potentially powerful tool to identify zones where groundwater discharges into a river.
This, together with sampling of surface and groundwater, could aid in a better understanding of surface - groundwater interaction (Parsons, 1994).
During the current research, environmental isotopes are used to understand hydrological processes within the study area. The two most useful naturally occurring stable isotope tracers used to trace the interaction between groundwater and surface water are δ2H and δ18O. They are used to distinguish rainwater flow from pre-event flow. This is because rainwater has different isotopic signatures from the water that is already in the catchment (Kendall and Caldwell, 1998). δ18O and δ2H are generally depleted in groundwater compared to surface water because of evaporation in the latter (Coplen et al., 2000).
Craig (1961) measured the isotopic composition of the rivers, lakes and precipitation around the world and established the Global Meteoric Water Line (GMWL). The amount of isotopes the sample water is composed of is expressed in comparison to the amount of isotopes in the standard which is known as the standard mean ocean water (SMOW).
Water with less δ2H and 18O than SMOW has negative δ2H and δ18O, while water with more deuterium and δ18O than SMOW has positive δ2H and δ18O (Mazor, 1991).