The natural geological environment provides a characteristic groundwater composition from region to region, and geochemical processes need to be taken into account in the context of groundwater protection. The natural baseline needs first to be defined and recognised before plans can
Natural geochemical reactions may impart adverse properties to some groundwaters in terms of trace element deficiency or excess (Edmunds and Smedley 1996). The main constituents of water giving rise in excess to health problems in African countries are F and As. High Fe concentrations are of more local importance, either as a result of reducing conditions or in certain acidic environments, but waters with high Fe present problems of
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Figure 46. Contents of mean annual recharge (in mm) derived from use of chloride balance for the area of northern Senegal.
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UNESCO /NWRC/ACSAD Workshop 5 on Wadi Hydrology “and ‘“Groundwatrr Protection ”Indicators of Groundwater Quality
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Figure 5. The Butana area, Sudan, showing that recharge occurs only via the wadi system and River Nile.
acceptability rather than health. Deficiencies in I may also be a major problem in some countries. High salinity however, as a result of natural processes, is the main limit to potability and health. I
Geochemical processes that are important in controlling and modifying groundwater composition may be summarised; and this helps to identify some parameters which are important in contributing to groundwater protection, as indicators, and for monitoring.
3.1 Acid attenuation and mineral dissolution
The main process of groundwater mineralisation takes place by acid-base reactions in the top few metres of the earth’s crust. Natural rainfall acidity is around pH5 and although atmospheric CO, concentrations are low, they may increase by up to two orders of magnitude, due to microbiological activity in the soil zone. The concentration of CO, is of fundamental importance in determining the extent of reaction, as well as reaction pathways in the groundwater flow system. The nature of geochemical reactions also depends upon whether the system remains open or closed with respect to the atmospheric/soil CO, source. Reactions with CO, are the most important in determining groundwater quality in both carbonate and non-carbonate aquifers (Appelo and Postma 1993).
During the hydrolysis of silicate, minerals silica and a secondary mineral are produced as well as HCO,. In carbonate aquifers, saturation with calcite (or other carbonate minerals) is quickly reached; while in non- carbonate terrains the hydrolysis of silicate minerals is relatively slow. The saturation index (SImineral) forms a
useful index of the extent of mineral reaction and different HCO, concentrations at equilibrium reflect different pC0, in the recharge area. Alkalinity (HCO,) is therefore recommended as a first order indicator for monitoring the progressive neutralisation of acidity and the extent of water- rock interaction; pH, which is easily buffered by reactions similar to the above, is not a good indicator of change except below pH 5.5. Most metals will only be mobile under acidic conditions and therefore pH measured in the field is an important indicator of whether heavy metals (from natural or man-made sources) will be mobilised.
3.2 Redox reactions
Under natural conditions, groundwaters undergo redox changes moving along flow lines (Champ et al 1979). In the context of geoindicators over timescales of 50- 100 years, the most significant process is the consumption of oxygen.
The solubility of oxygen in groundwater at the point of recharge (around lo-12 mg/l) reflects the ambient air temperature and pressure. The concentrations of dissolved oxygen (DO) in newly recharged groundwaters may remain high (8-lOmg/l) indicating relatively little loss of DO during residence in the soil or unsaturated zone. Oxygen reacts slowly with organic matter, and/or with Fe*+ released from dissolution of impure carbonates, some silicates or with sulphides. Oxygen may persist, however, for many thousands of years in unreactive sediments. Complete reaction of oxygen is marked by a fall in the redox potential (Eh) by up to 300mV. This provides a sensitive index of aquifer redox status. A decrease in the concentration of oxygen in pumped groundwaters therefore may herald changes in the input conditions. An increase in dissolved
iron (Fe*+) concentrations, as well as Eh, may be useful as secondary indicators.
3.3 AdsorptionJdesorption
Many rocks and minerals in contact with groundwater have the capacity to release ionic constituents in exchange for ions contained in groundwater. This process will modify the groundwater along flow paths and has the net effect of retarding the flow of solutes relative to the water molecule.
As well as giving rise to significant changes in water quality such as water softening, adsorption reactions will also have the net effect in retarding the migration of pollutants.
3.4 Salinity changes
Chloride in shallow groundwater of active recharge systems may be derived mainly from the atmosphere. It remains inert and is conserved during hydrogeochemical reactions.
Thus it is an important indicator of meteoric inputs; under natural conditions, changes in concentration reflect mainly changes in the physical processes (evaporation, transpiration) taking place in the soil and the unsaturated zone and, therefore salinity levels can often be equated with recharge rates as described above. Increased groundwater abstraction rates may lead to increases in salinity from a variety of sources including sources of saline formation water, recent generations of sea water (resulting from climatic change in coastal areas for example) and drawdown of salinity from sebkhats, for which Cl measurement (supported by SEC) is an essential primary indicator. A range of other geochemical parameters are needed to characterise the sources of salinity. The Mg/Ca ratio is the most useful secondary indicator of recent marine intrusion; modern sea water has a high Mg/Ca ratio but ancient marine waters have molar Mg/Ca ratios < 1, resulting from loss of Mg and reequilibration with both calcite and dolomite. Isotopic ratios ( 180 and *H) as well as Br/Cl may also be diagnostic of the source of salinity.