ABSTRACT. Two areas, Nchalo and Ndakwera, are described in more detail to illustrate

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1 11.1//-1y-, ABSTRACT Non potable groundwater of sodium chloride composition has been encountered by wells and boreholes constructed in the unconsolidated valley fill deposits of the Lower Shire Valley, in particular along the western valley side. Six possible origins of this water are briefly outlined, the relative importance of each being dependant upon the location. The most important process is thought to be due to the occurrence of sedimentary rocks that crop out along the western hills. Weathering and erosion of these rocks has resulted in comparitively fine grained deposits of low permeability and containing abundant soluble material. Alternating upward and downward eluviation, together with cationic exchange and low recharge, results in a poor quality water with chloride as the principal anion. In contrast the eastern side of the valley is comprised of alluvial fan deposits derived from the relatively insoluble adjacent metamorphic rocks of the MalaQi Basement Complex. A4 a peettat ioluble material is less abundant and the deposits/of greater permeability. Recharge to these sediments is at least twice that of the western valley floor. Groundwater is therefore of good quality having bicarbonate as the dominant anion. Two areas, Nchalo and Ndakwera, are described in more detail to illustrate two origins of saline groundwater and the importance of this water briefly discussed.

2 TEXT FOR A.A.S.M. LECTURE ON SALINE GROUNDWATER IN THE LOWER SHIRE VALLEY by R.B. Bradford, B.Sc. M.Sc., Groundwater Division, Geological Survey Department, P.O. Box 27, Zomba. (to be read by Mr. F. Phiri) 1. INTRODUCTION Saline groundwater in the Lower Shire Valley was first recorded in the late 1920 s and early 30ts when wells were constructed for rural supplies by the Geological Survey Department near the lower lying parts of the valley. Some 500 boreholes throughout the valley have now been constructed and saline water has been found to occur at varying depths and in a variety of localities predominantly along the West Bank area from Chikwawa to Tengani. As elsewhere in Malath electrical resistivity measurements are employed to locate groundwater in\,threl-ley,. Although this method can indicate the existence of saline groundwater it has not proved wholly reliable as the interpretation of the results is complicated by the complex succession of the deposits infilling the valley. If however, the caused or causes) and the extent of saline water could be determined by analyses of the groundwater the interpretation of geophysical data would be made easier and would, in addition, help contribute to the development plans for the valley. The preliminary phase of this work included the collection of 146 essentially random borehole samples during 1971 and Most of the boreholes sampled are less than 60 m (180 ft) in depthialong the West Bank. Extended partial analyses were performed in the Geological Survey Laboratory. An additional 83 analyses were available from published reports from wells, thermal springs, rivers and waterholes. Water compositions are variable throughout the valley. The predlm/balet watan Sn typeg awe bicarbonate watle1,1e. Around the valley sides, particularly along the East Bank and near Nsanje, calcium bicarbonate water predominate. Towards the valley centre these waters change to a bicarbonate water with approximately equal proportions of the major cations and then, with the greater time contact with soluble materials together with cationic exchange jto a sodium bicarbonate composition, the most common type of water in the valley. In apparently localized areas sodium chloride waters are foundwhich,although having similar bicarbonate concentrations differ from the bicarbonate waters generally by their high sulphate content and low silica together with an increase in all other ions. 2. POSSIBLE ORIGINS At least six different, although interelated, processes may give rise to poor quality groundwater and the influence of each is dependent upon the locality.

3 2. It is proposed to briefly discuss these possible origins and then illustrate the origin of saline waters that occur at Ndakwera and Nchalo. alieellatli (Soils, climate and vegetation) Origins one and three in the diagram can be linked together under 'vertical mineralized leakage'. This is the result of the effect of the semi-arid climate, with marked wet and dry seasons, on the soluble material in the soils and fine-grained sediments through which recharge:water has passed. Under such climatic conditions soils are inadequately leached and soluble salts, especially sulphates, tend to accumulate. Alternating upward and downward eluviation occurs as a result of the marked wet and dry seasons. Some soluble materials are irreversibly precipitated, such as calcium carbonate. During the wet season, or in a period of higher than average rainfall, soluble salts will be carried downwards to any coarser grained deposits that constitute an aquifer. The water composition will depend upon the base-exchange capacity of the soil, the soil parent rock type and soil permeability, which will be illustrated by the Nchalo example. Towards and beneath the extensive marsh areas of the valley or in other areas of impeded surface and subsurface drainage, saline soils are likely to develop by evaporation o [ soil or shallow groundwater, which are commonly marked by a stunted vegetation. This was suggested as the original cause of saline water. The Halcrow Report in 1954, whilst briefly discussing groundwater, concluded thatg- "the origin of the salt is not clear and there does not seem to be mush connection between the occurrence of eeline soils and salt well-water'. e----- At Mbobo to the south of Sorjin there is a large area of saline soil patches but a well nearby has potable water. Conversely at Alimenda, which is the marsh N.B. of Ngabu and also surrounded by patches of salty ground there is a well with saline water. A borehole in this area passed through a thick sequence of clays underlain by an aquifer of less than (3 m) 10 ft thick and which has water of sodium-chloride composition, a total dissolved solids content of nearly 9000 mg/1 and a high sulphate content. At Sorjin, however, the area is underlain by coarse deposits and most recharge is derived from the streams draining the hill nearby and sonsequently any saline water passing through the soils may be diluted by this recharge An additional factor of importance where thick clays occur, is the positioning of the slotted casing in each borehole. If this is placed against deposits of low permeability, as well as the main aquifer, saline water will be obtained from the former, particularly if the slotted casing is emplaced from the water table downwards. At the water table, soluble salts will most likely occur from eluviation and drillers often report saline water on first striking water. Where the salts are restricted to clays underlain by thick coarser-grained deposits, such as beneath the S.U.O.O.M.A. sugar estate or between Bangula and Tengani, the saline water can usually be effectively sealed off so that a potable supply may be drawn from the underlying deposits. Saline boreholes are commonly associated with dry or low yielding boreholes, such as between Ngabu and Nchalo. In these boreholes water is not usually struck until about 25 m (75 ft) beneath the surface. This would indicate a low recharge

4 3. potential and a sequence of low permeability and/reflects the inability of recharge water to flush soluble salts from the soil and underlying deposits. In contrast along the losing stretches of streams, where most recharge takes ) place, groundwater, is usually encountered at less than '17 m (50 ft) and is of a low total dissolved solids. ORIGINS 2 (Cationic Exchange) With the passage of groundwater through the aquifer, sodium adsorbed onto clay minerals is exchanged for calcium and magnesium ions in the groundwater, as illustrated by the bicarbonate water compositions mentioned earlier. This process is reversible and may be selective; calcium being exchanged more so than magnesium An indication of the extent of this process is the ratio in equivalents per million of sodium to the sum of calcium and magnesium. Values of much greater than one indicate that cationic exchange has played an important role in contributing sodium. As minerals with a high exchange capacity are widespread and abundant) this process is important in altering the composition of the dissolved solids. Calcium may be preferentially precipated which may affect the use of the ratio of sodium to calcium and magnesium. However, as the sodium increases so does the chloride content and almost all the saline groundwaters analysed have a high ratio, particularly at Ndakwera. The same process is occurring in the sedimentary rocks that from the western watershed of the valley and which in part underlie the valley floor. Towards the valley centre in the unconsolidated deposits the increased time-contact and the greater proportion of finer grained material means that cationic exchange is more important in that part of the valley. Saline waters but usually of comparitively low total solids may result in particular from this process. 020IU. (Contribution from sedimentary rocks) Both the Karroo and Cretaceous sedimentary rocks have abundant calcareous cementing material and are high in sodium, potassium, magnesium and iron. They were deposited mainly as coarse-grained torrential or deltaic-type sediments which were derived from the surrounding high-relief crystalline metamorphic rocks. Except for scattered outliers, such as in the Nsanje Hillis, they were deposited within a fault-controlled basin, the western margin of which was formed by the Mwanza and Namalambo Faults. The subsurface extension of the former continues from the eastern edge of the Mwanza Valley to at least Chiromo. Part of the Cretaceous Lupata Series and the Red Beds and the top of the Upper Sandstones of the Karroo sequence are generally fine grained and contain gypsum. Groundwater in these rocks is compardatively deep and liable to become increasingly mineralized with depth due to the increased overburden, restricted outlet and passage through the aquifer unit. Cretaceous rocks in South Africa and Mozambique are known to give saline groundwater-. Contribution of saline water from these rocks was proposed to account for saline waters. The piezometric surface for the sedimentary rocks has not yet/delineated as few boreholes are known to penetrate them. Assuming, however, that the piezometric surface extends into the overlying unconsolidated deposits west of the Mwanza Fault, water will move upwards by leakage Into these deposits and thereby contribute saline water. This may be of more importance towards their

5 outcrop. The presence of saline water in the Karroo was verified by a borehole velem sa at Ndakwera Clinic in 1972 wlailla atamook saline wateraat 168 ft and which rose to 72 ft beneath the surface. No analysis of this water, however, has been made, it being abandoned due to the unpotable water and a low supply. It is intended to briefly discuss saline water thought to occur in the Karroo by the Ndakwera examplep. ORIGINS 4 and 6, (Faults as barriers to g.w. movement) Seismic data indicates that movements still occur along the Mwanza Fault, The Nkombedzi river when it meets this fault north of Tomali changes from a north easterly direction, the 'Cretaceous tilt', to a southeasterly direction parallel, and just to the east of, this major fault. The Shire River similarly changes direction to follow this line joom south of Nchalo, possibly as far as Tengani. Above the Mwanza Fault, in particular, lenses of coarser grained deposits may have been offset against deposits of low permeability thereby impeding groundwater flow and giving rise to localized areas along the fault of saline water. As yet, however, data is insufficient to illustrate this possible origin. The consolidated sedimentary rocks dip at low to moderate angles towards the Mwanza Fault, which forms a zone of dislocation of about one mile in width. Along this fault there arise low magnitude, thermal springs which represent the last stages of Cretaceous hydrothermal activity. Analyses of two thermal springs show them to be of a sodium chloride sulphate composition with a fairly low total solids of less than 1000 mg/1, although the thermal water at the sites probably originator from deeply percolating meteoric water in the metamorphic rocks and not the Karroo. However, the presence of these springs suggests that localized areas of saline water may arise along the Mwanza Fault. During the Lockwood Survey Investigation in the results of a gravity geophysical survey and conductivity readings led to the proposal that there was a link between the underlying structure and high conductivity readings. This was thought to be due to saline waters circulating in the sedimentary rocks being forced to rise towards the surface at the impermeable barrier created by the Mwanza Fault, To account for high conductivity readings to the east of this fault, it was suggested that saline water was either moving downdip along the base of the unconsolidated deposits or via north easterly trending fractures that intersected this fault. Examination of the Nchalo area which lies astride this fault suggests, however, that the saline water is more likely the result of other processes, Having summarized several possible origins that may give rise to the saline groundwater it is now proposed to discuss by two examples the effects of soil compositions at Nchalo and of faults in the sedimentary rocks at Ndakwera, Nchalo Township is some 35 km (22 mis) south of Chikwawa along the main West Bank road to Bangula. It is situated adjacent to the Nkombedzi river and astride the postulated subsurface extension of the Mwanza Fault which was determined in 1971 by magnetic geophysical work. This river divides the western pedisediments from the Mwanza Nkombedzi alluvial fan and to the south of this river the gradient changes from to 1g170 or 200, Little material is brought down by the Nkombedzi, possibly due to the headwaters arising on the fine grained Carbonaceous Shales

6 5. of the Karroo, and along its lower course at Nchalo there is an extensive dambo' area The Makanga river, in part draining the Karroo Red Beds which contain evaporites, joins the Nkombedzi just north of Nchalo. Eleven boreholes in the area surrounding Nchalo have been sampled and five other analyses from wells were available from early Geological Survey reports, As the fault position is approached there is a rapid rise in all ions except bicarbonate,and once the fault line is crossed there is apparently a corresponding drop. However, a borehole at St. Mathew's Parish and two other boreholes drilled in 1973 for the new township supply are not saline even though lying on the fault zone, Thia anomaly would suggest there may be some other cause of the saline groundwater. An increase in the sulphate content from less than 1(epm to as much as 9(6.c.m. at the Nchalo dambo and a change in the ratios of selected ions at the same point suggest that the composition of recharge waters may be important. The boreholes were subsequently grouped according to the soil types. From data concerning the soluble salt content and base exchange capacity of the soils, as given in the Halcrow study report of 1954, a reasonable correlation with the groundwater was shownl the Lupata Drift to the west having good quality waters with low sulphate; the 'grey brown/ soils to the east having a higher total solids, a sodium chloride composition and moderate sulphate; and, in between, the dambo soils, with their impeded drainage, having a high to very high total solids, high sulphate and also of sodium chloride composition. Other points in favour of the influence of the soil and subsurface deposits, which were derivered from the adjacent sedimentary rocks, are that saline water was either encountered when water was struck or within the thick clays in the area; the most saline boreholes go dry for part of year, suggesting a low recharge or a sequence of low permeability; the positioning of screens against less permeable deposits in certain boreholes; and that supplies of potable water could be obtained from the coarse grained aquifers even above the fault zone. In addition, by extrapolation of the top of the Cretaceous, that crops out about 8 kiss (5 mls) to the west, and also on the basis of depths reached by boreholes, the Cretaceous lies at least 83 m (250 ft) below the surface at Nchalo. It is therefore'_ unlikely that any seepage from the rooks before or at the fault itself doesfnot%influence groundwater compositions at the comparitively shallow depths penetrated by the boreholes and a more plausible explanation is the influence of soluble salts, both in the soils and sediments, derived from the sedimentary rocks, which is combined with poor groundwater circulation and recharge, particularly in the dambo soil areas. The second example of the occurrence of saline water is drawn from the Ndakwera Market area, This lies astride the triangle formed between the middle stretches of the Nkombedzi and Mwanza rivers, The area has fertile soils and is being developed as a smallholder agricultural scheme but the presence of poor quality groundwater led to a request to sampling of all the boreholes in the area to delineate its extent. Groundwater compositions varied from a calcium bicarbonate to sodium chloride sulphate composition with total solids ranging from less than 1000 mg/1

7 b. in the former to nearly mg/1 in the latter. The ratio of sodium to calcium plus magnesium was very high for the borehole with the highest total dissolved solids. Examination of the available borehole logs for the most saline boreholes just north of Ndakwera Market showed that some metres (40-70 ft) of sandy clays overlie thin sandy horizons on Karroo sandstones at a shallow depth of less than about 27 m (80 ft), Karroo rocks outcrop less than 3 kms (2 mis) to the north west of Ndakwera. Along the road from a short distance east of Ndakwera bedrock was not encountered at 150 ft. Cross sections of the geology together with the chemistry showed how the groundwater compositions changed very rapidly over short distances, A trilinear plot of the equivalents per million as percentages also demonstrated how certain waters fell into distinct groups according to their location and correlated with the calculated yields. The borehole logs and chemical data suggest that the Mbuzi Fault, which brings Karroo marls against the sandstones, extends north easterly beneath the unconsolidated deposits just north of Ndakwera thereby creating a barrier to groundwater movement in the sandstones which are thought to be penetrated by the boreholes with saline water. The effect of this fault may decrease to the east as the overlying thickness of unconsolidated deposits increases and similarly adjacent to the river channels an adequate groundwater supply can be obtained from gravels recharged by the rivers above the top of the Karroo sediments. To conclude this talk it is appropriate to briefly discuss the importance of delineating the areas in which saline water may be encountered, The availability of potable groundwater supplies from the unconsolidated deposits of the valley floor has enabled the development of the valley. But the presence of saline groundwater locally restricts this development and limits the total groundwater reserves. Although saline groundwater with up to 5000 mg/1 total solids can be used by cattle, and the very highest for possible local salt production, it becomes necessary to import potable water into areas of potential development but having saline water. This has now been carried out at Ndakwera where five recently constructed boreholes in areas of good quality water are pumped from 2 kms (li mls) away into the saline area. Sodium absorption ratios were calculated for seventy random samples collected in January Over 30% have a high to very high sodium hazard and over 80% a high to an exceedingly high salinity hazard and which are therefore unsuitable for irrigation purposes. Groundwaters usually containing more than 300 mg/1 of chloride give unsuitable irrigation water and much of the central part of the valley has such values. Irrigation potential is therefore mainly restricted to the East Bank and to the south of Tengani. In additioni unlees suitable groundwater management is undertakenl overabstraction, which may be occurring in such areas as Nchalo already, may result in increased contribution from saline waters present in the more clayey sequences. Groundwaters with more than 1000 mg/1 and 500 mg/1 of chloride create corrossive conditions that not only reduce the productive life of boreholes so affected but also increase maintenance costs, In each annual borehole programme at least five boreholes are usually abandoned due to saline water which, with the ever increasing costs of borehole

8 7. construction, represents quite a considerable loss in not only capital. A comparitively small outlay to corroborate the suggested origins presented here could have large benefits, On the basis of the present available data further work is justified which should include soil analysis, further areal water sampling combined with depth sampling and in particular subsurface radioactive logging. (Thanks to audience)

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