Aspects of the hydrogeochemistry of the Karoo sequence in the Great Fish River basin, Eastern Cape Province, with special reference to the groundwater quality
Tordiffe, Eric Arthur Wolferstan
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The aim of the study was to examine some of the major aspects responsible for the chemical quality of the groundwater in the Great Fish River Basin and its influence on the irrigation water. Approximately 18 000 ha of land are at present irrigated from several weirs down the river. The section of the Great Fish River Basin under discussion comprises an area of approximately 25 000 km² located between longitudes 25°E to 27°E and latitudes 31° 15'S to 33° 15'S. This area is divided into the following geomorphologic provinces: The Marginal Region (lower than 760m), the Great Escarpment (750 - 1070m), the Headbas (1070 - 1370m) and the Interior Plateau (higher than 1370m). Each of these provinces play an important part in controlling the movement and the chemical quality of groundwater in the area. Most of the annual precipitation (350 - 450 mm) occurs between February and March when evapotranspiration is at its highest. Runoff from the entire basin amounts to only 3 percent of the annual precipitation. The rest of the water either evaporates immediately because of the semi-arid climatic conditions, or is temporarily stored in the soil before it is lost to the atmosphere by means of evapotranspiration. It is also pointed out that apart from periods of extreme precipitation, the monthly evapotranspiration always exceeds the monthly precipitation. Such semi-arid climatic conditions, as well as the nature of the soils in the area prohibit a fast infiltration of meteoric water and it is therefore doubted whether as much as 5 percent of the annual precipitation over reaches the groundwater table. The area under discussion is underlain by sedimentary rocks of the Karoo Sequence beginning with the glacial deposits of the Dwyka Tillite Formation (680 m) at the bottom, followed by the marine deposits of the Ecca Group (2 340 m), the transitional deposits of the Koonap Formation (980 m) and the fluvial deposits of the Beaufort Group (4 540 m). Because the Koonap Formation represents the transition between the marine (deltaic) deposits of the Ecca Group and the fluvial deposits of the Beaufort Group, it is regarded as a separate formation not belonging to either group. The Beaufort Group on account of the environment in which the sediments were deposited, is subdivided into the Adelaide Subgroup (reducing environment) and the Tarkastad Subgroup (oxidizing environment). Red mudstone is regarded as indicative of an oxidizing environment and is present only in patches in the Middleton Formation, which forms the lower part of the Adelaide Subgroup. No red mudstone is present in the Balfour Formation, which forms the top half of this subgroup, but becomes very prominent in the Katberg and Burgersdorp Formations of the Tarkastad Subgroup. The Balfour Formation, on lithologic grounds, is subdivided into the Oudeberg Sandstone Member (180 m), The Daggaboersnek member (1 200 m), the Barberskrans Sandstone Member (190 mA) and the Elandsberg Member (320 m). It is suggested that the arenaccous units of the Beaufort Group, i.e. the Oudeberg Sandstone Member, the Barberskans Sandstone Member and the Katberg Formation represent periods of major tectonic activity in the provenance which was located to the south-east. During such activity vast amounts of coarse-grained material were transported and deposited at a relatively fast rate. Owing to the semi-arid climatic conditions, which prevail in the area, the soils tend to be rather alkaline with a high clay content and the poor development of an A-horizon. Calcrete or caliche occurs at or near the surface of most of the soils. Dolorite has intruded the sedimentary strata as concordant and conical sills, as well as near-vertical dykes. The dykes in the south of the area have an orientation of approximately 290°, coinciding with the Cape Fold Belt, whilst farther north, a prominent northerly trend with a weaker easterly trend is observed. In the extreme north, where the sedimentary strata is at its thickest, an almost random orientation is present. Various types of dolerite are encountered in the area and of particular interest is the occurrence of quartz dolorite which has intruded a sill of normal dolorite near Speelmanskop. This leucocratic body is probably the result of magmatic differentiation lower down in the cust, whilst limited differentiation whithin the body itself, both from floor to roof and in an "up-dip" direction, must have occurred. The intrusion of the dolerite is of particular importace because of the fracture zones it causes in the adjacent sedimentary rocks. Such zones are normally open to circulating groundwater. Where the dolerite itself is not fractured it may act as an impervious barrier when crossing the regional flow path of the groundwater. In such cases groundwater compartments are developed. Weathering of the provenance and of the various rock-types in the area, diagenetic processes which proceeded the deposition of the sediments in the Karoo Basin and the adsorption and ion exchange during the interaction of the surface and groundwater with the surrounding rocks, are considered to be the main geochemical factors responsible for the changes in the chemical quality of the groundwater in the area. During the chemical weathering of the rock-forming minerals cations such as Na+, K+, Mg++ are released to solution in the groundwater, whilst compounds such as SiO2O3 regroup to form residual clay minerals such as montmorillonite. Weathering of the sedimentary rocks is, however, limited because of the fact that the primary minerals which constitute such rocks have already withstood at least one cycle of weathering in the provenance. In areas where leaching is vigorous, K+ is, however, removed from illite in the mudstone with the result that this clay mineral adopts swelling features similar to montmorillonite, thus causing the rock to crumble. Dolerite in turn, because of its igneous origin, is more prone to chemical weathering. As a result of compaction the porosity and permeability of the sediments in the Karoo Basin was reduced to extremely low values. The chemistry of the interstitial waters was also altered by this process because of the diagenetic alteration of montmorillonite to illite during which K+ is removed from the water, whilst SiO2, H2O, Na+, Ca++, Mg++ and Fe++ are added to the water. During the compaction process, C1- was accumulated in the remaining water in the lower strata as result of ultra-filtration as the formation water was squeezed through clay-rich mudstone layers. Because of its small ionic radius and high electrical charge, Ca++ is adsorbed by the clay minerals in the mudrock of the area to a far greater extent than any of the other cations present. The maximum concentration of adsorbed Ca++ is observed in the Oudeberg Sandstone Member, which suggests that this unit represents a geochemical marker. A gradual increase in the CEC of the mudrock from the lower strata to this unit is furthermore observed. Sodium concentrations increase toward the south of the study area, therefore suggesting an influence of the palaeomarine environment on the adsorbed cations. Groundwater in the Great Fish River Basin is restricted mainly to joints in the sedimentary rocks and to fracture zones caused by the intrusion of dolerite. The water levels in most of the bore-holes therefore represent a pressure or piezometric surface rather than an actual water table. Such levels, however, regionally represent a surface which closely resembles the surface topography, whilst the flow of groundwater is down the regional slope and the rivers act as effluent drainage canals for the groundwater. Although the groundwater is recharged in the higher lying areas by circulating meteoric water, there appears to be no direct relationship between the seasonal precipitation and the groundwater levels. As far as the origin of the major ions in the groundwater is concerned, the cations are derived mainly from the weathering of primary rock-forming minerals, whilst the anions accumulate from non-lithologic sources. Generally, the groundwater in the areas of recharge, i.e. the higher lying areas, has a pronounced Ca++ and HCO-3-character, whilst in the stagnant low-lying areas Na+ and C1- are the predominant ions. In between the two extremes, groundwater with a prominent Mg++ and SC=4-character is encountered. This trend corresponds well with the normal metamorphism of natural waters and appears to be controlled largely by the topography of the area. Groundwater with a distinctly high Na+ and C1- -concentration also has a high salinity concentration. The pH in turn is highest in the areas of high Ca++ and HCO-3-concentrations and lowest in the areas of high Na+ and C1- -concentrations. All the water of the area is, however, oversaturated in relation to CaCO3 and, where conditions are suitable, calcrete is precipitated. Chloride is the dominant anion in the lower strata of the Karoo Sequence and is attributed mainly to the retention of this ion during the migration of the formation waters through the argillaceous material. High salinities, as a result of high Na+ and C1- -concentrations, prevail in the groundwater up to the Daggaboersnek Member. From the Barberskrans Sandstone Member upward, the concentration of these ions decrease sharply. The cation percentages in the groundwater of the upper strata, however very considerably, thus indicating the influence of chemical weathering. There is more SO=4 in the groundwater of the lower strata, which was deposited under reducing conditions, than in the upper strata, which was deposited under oxidizing conditions. This is attributed to the formation of pyrite under reducing conditions, which can later oxidize to release SO=4 to the water. During the periods of extreme precipitation a considerable amount of meteoric water infiltrates down to the groundwater level, dissolving precipitated salts on its way down. This naturally causes an increase in the salinity of the groundwater and is a result of an increase in Na+ and C1-. Seepage water in the Great Fish River contains Na+ as the main cation and increases gradually in concentration farther downstream. To the north of Cradock HCO-3 is the dominant anion but it decreases rapidly farther downstream, with a concurrent sharp increase in the C1- -concentration. The increase in the Na= and C1- -concentration coincides with an increase in the total salt load farther downstream. A similar trend is observed in the change in groundwater quality down the Great Fish River. This is conclusive proof of the influence of groundwater on the seepage water in the river. The groundwater compartments caused by dolerite intrusions also have a marked influence on the quality of the seepage water. During a single irrigation lead from Grassridge Dam the initial irrigation water reaching the consecutive weirs along the river possessed an extremely high salinity load as a result of the solution of precipitated salts in the river bed as well as the flushing of saline water from stagnant pools. The duration of the saline head increased at each consecutive weir downstream. Such conditions present a serious threat to the irrigable land along the Great Fish River and therefore measures will have to be taken to either prevent such contamination of the irrigation water or to limit the application of such contaminated water by allowing the saline head to pass the various weirs.