Origin of the shallow groundwater system in the southern Voltaian Sedimentary Basin of Ghana: an isotopic approach

Journal of Hydrology 233 (2000) 37±53

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Origin of the shallow groundwater system in the southern Voltaian Sedimentary Basin of Ghana: an isotopic approach S.Y. Acheampong a, J.W. Hess b,* a

Graduate Program of Hydrologic Sciences, University of Nevada, Reno and Division of Hydrologic Sciences, Desert Research Institute, University and Community College, System of Nevada, 2215 Raggio Parkway, Reno, NV 89512, USA b Division of Hydrologic Sciences, Desert Research Institute, University and Community College System of Nevada, 755 E. Flamingo Road, Las Vegas, NV 89119-7363, USA Received 11 June 1999; received in revised form 21 January 2000; accepted 1 March 2000

Abstract The development and management of groundwater resources require an ability to understand and identify the recharge source to the groundwater system. Stable and radioactive isotope data have been used to investigate the source of recharge and the age of the shallow groundwater system within the southern Voltaian Sedimentary Basin (SVSB) of Ghana as part of an on going groundwater development project. The tritium concentrations in the groundwater samples are very low and range from less than 1±7.2 T.U., while measured 14C content ranges from 43 to 108% modern carbon. The tritium concentration of rainfall ranges from less than 1±4 T.U. The d 18O values of groundwater samples range from 24.2 to 22.6½ while the d D values range from 221 to 210½. Stable isotopic data of the groundwater samples lie either on or close to the global meteoric water line (GMWL) on the d D± d 18O plot and indicate that the shallow groundwater in the area is derived from meteoric water that has undergone no signi®cant degree of kinetic evaporation during recharge. No indication of paleo recharge can be inferred from the 18O and 2H composition of the groundwater. Current tritium concentration in the groundwater is generally low, however, it proved useful in the qualitative identi®cation of modern recharge. Radiocarbon-deduced ages range from about 3200 ^ 350 years B.P. to modern and indicate young recharge to the shallow groundwater system. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ghana; Voltaian; Groundwater; Isotope; Deuterium; Oxygen-18

1. Introduction Groundwater plays a vital role in the economy of Ghana and acts as the major source of potable water supply to a majority of the rural population. A number of groundwater development schemes are being undertaken in the country even though the source * Corresponding author. Tel.: 11-702-895-0451; fax: 11-702895-0451. E-mail address: [email protected] (J.W. Hess).

and amount of replenishment to the underlying aquifers in most parts of the country have not been ascertained. As a result, some communities experience shortages and very low well yields shortly after the installation of the water delivery systems (Acheampong et al., 1985). In some instances, the wells are locked up for extended hours during the day for water level recovery to occur and it is not known whether the shallow groundwater system is being mined or is a renewable resource. The growing importance of groundwater supplies in Ghana demands an ability

0022-1694/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0022-169 4(00)00221-3

Fig. 1. Location map of southern Voltaian sedimentary basin showing some of the communities where wells were sampled for this study.

38 S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

to develop and manage this subsystem of the hydrologic cycle judiciously. An understanding and identi®cation of the recharge source is therefore necessary for a viable long-term groundwater development. Due to the paucity of hydrologic data, however, there are dif®culties with conventional hydrological approaches in studying recharge processes in Ghana as a whole, and the study area in particular. It has been hypothesized (Buckley, 1986) that the shallow groundwater system in the SVSB is like a sponge which gets ®lled during the rainy season and is discharged by wells and evapotranspiration. The main purpose of this study was to investigate whether observations of environmental isotope content can improve our understanding of the shallow groundwater system in the SVSB of Ghana, speci®cally, the source and age of the groundwater. The identi®cation of the source of recharge could be useful for the management of the underlying aquifers to meet the increasing demand for new wells in the area, and also help address the environmental issues concerning the effects of water level decline on the local plant communities. 2. Geologic and hydrogeologic setting The Voltaian Sedimentary Basin covers approximately 43% of Ghana. The SVSB covers about 5000 km 2 and lies in south-central Ghana (Fig. 1) between latitudes 6845 0 N and 7815 0 N and longitudes 0845 0 W and 0815 0 E. Rocks of the Voltaian System underlie it, which consist of sandstones, shales, mudstones, conglomerates, limestones and tillites. The sandstones are medium to coarse grained and are sometimes conglomeratic. The sandstones and conglomerates underlie about 80% of the area in the central and northern parts, while the shales and mudstones cover parts of the western and southeastern parts of the study area (Fig. 2). Much of the primary porosity of the bedrock in the basin has been destroyed due to consolidation and cementation (Bannermann, personal communication, 1988). The hydrogeology of the area is not well understood and only limited groundwater exploration programs have been conducted in the basin. Data from existing boreholes and hand-dug wells reveal rather complex hydrogeological conditions. Depth to

39

water varies throughout the basin but is generally shallow, ranging from 3.0 to 41 m. The main aquifers in the basin are the weathered zone aquifer and the fracture zones. As a unit, the weathered zone ranges from about 4 to 20 m thick (Prakla Seismos, 1984). In most villages, the weathered zone aquifer is developed for water supply by hand-dug wells (Acheampong et al., 1985), but most of these wells dry up during the dry season. The fracture zones are developed in the bedrock at depths of 20 m or more below ground surface (Buckley, 1986). These zones are mostly near vertical, narrow and linear. Groundwater ¯ow is fracture-controlled (Bannermann, personal communication, 1988; Prakla Seismos, 1984; Buckley, 1986) and generally occurs under semi con®ned conditions, but con®ned conditions may exist in places with the static water levels (SWL) ranging from 1 to almost 20 m below land surface. Transmissivities vary between 1 and 72 m 2/day (Buckley, 1986; Minor et al., 1995; Acheampong and Hess, 1998). Estimated well yields range from 7 to 700 l/ min. No clear pattern of well yields in relation to rock type can be established, though the highest and some of the lowest yields are associated with the sandstone bedrock aquifer, which covers more than 50% of the study area. Good quality groundwater occurs within the aquifers with ®eld pH values of the samples ranging from 5.39 to 7.63 and the electrical conductivity (EC) values varying from 250 to 2580 mS/cm with over 90% of them being less than 1000 mS/cm (Acheampong, 1996). The annual rainfall in the area ranges from 120 to 160 cm with a mean of about 140 cm per annum (Dickson and Benneh, 1970). The rainfall pattern follows the northward advance (in summer) and southward retreat (in winter) of the inter-tropical convergence zone (ITCZ) which separates the dry desert air from the Sahara from the moisture-bearing, southwest monsoon ¯ow from the Atlantic Ocean (Dickson and Benneh, 1970). From the meteorological records of Kete Krachi, about 96 km east of the study area (Water Resources Research Institute, unpublished data), monthly mean potential evapotranspiration is exceeded by monthly mean rainfall during four months of the year (June, July, September and October). The vegetation is a transition between Semi-Equatorial and Guinea Savannah type and consists of coarse tussock grass

40

S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

Fig. 2. Geologic divisions of the southern Voltaian sedimentary basin (W. B. Apambire, personnel communication; modi®ed by Acheampong, 1996).

of varying heights and large trees such as Triplochiton acleoroxylon and Ceiba pentandra. The soils are regionally moderately well-drained with a pH of approximately 6.0 (Asiamah, 1988). 3. Methods of data collection and analysis Forty-four hand pump wells ranging in depth from 20 to 60 m were sampled twice in January and July/ August 1994 (Fig. 3). In addition, ®ve samples from

Lake Volta and eight rainfall samples at selected sites in villages were collected. Sets of samples were collected after purging the wells to a point where chemical parameters such as EC and pH of the pumped groundwater had stabilized. Most of the wells are being used for domestic water supply and were being pumped during the sampling period, so purging mostly lasted for between seven and ten minutes. Un®ltered groundwater and lake water samples were collected in 30 ml glass bottles with poly-sealed lids for stable isotopic analysis, while

S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

41

Fig. 3. Locations in southern Voltaian sedimentary basin where groundwater samples were collected.

those for tritium analysis were collected in 1-liter glass bottles and preserved for analysis. Fourteen samples were collected for tritium analysis and eight of them (four each from the sandstone and conglomerate aquifers) were sampled for radiocarbon analysis. Precipitation of groundwater samples for 14C analysis was done in the ®eld using the method described by Haynes and Haas (1980). All the sample analyses were performed at the

Desert Research Institute (DRI) Laboratories in Nevada, USA. Enriched tritium analyses were performed at the Water Chemistry Laboratory. Tritium samples were enriched by electrolytic enrichment and analyzed by the liquid scintillation counting method (Thatcher et al., 1977). Tritium concentrations are expressed in tritium units (1 T.U. ˆ 3.24 pCi/ml; 1 tritium atom per 10 18 hydrogen atoms). Radiocarbon analyses were done at the

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S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

Radiocarbon Laboratory using standard benzene synthesis (Pollach and Stipp, 1967) and liquid scintillation techniques. Radiocarbon activities are reported as the percent of the activity of modern carbon (pmc). Stable isotopic analyses (d D, d 18O and d 13C) were done at the environmental Isotope Laboratory using a Finnigan Mat Delta E mass spectrometer. Ratios of stable carbon isotopes were measured on the CO2 evolved in the radiocarbon laboratory from the samples. The guanidine hydrochloride method (Dugan et al., 1985) was used for the preparation of samples for d 18O analysis. Hydrogen gas extraction was done by the Kendall and Coplen (1985) method using zinc as the reducing agent at 4508C. The stable isotopic compositions are reported in the d notation:   Rsam 2 Rstd dˆ 1000 …1† Rstd where R is the isotope ratio (e.g. D/H) and all values are reported in per mil in reference to V-SMOW (Gon®antini, 1978). Positive values show the sample to be enriched in the heavy isotope species, and negative values show depletion in the heavy isotope species relative to the standard. Measurement accuracy is 0.2 and 1.0 units, respectively, for d 18O and d D. The d 13C values are reported in per mil deviations from the Peedee Belemnite standard (the Belemnitella Americana of the Cretaceous Peedee Formation in South Carolina). 4. Analytical results 4.1. Deuterium, oxygen-18 and tritium Stable isotope data of d D and d 18O are plotted in Fig. 4 together with the global meteoric water line (GMWL) (Craig, 1961) and the regression line for Lake Volta samples collected for this study. The stable isotope data are given in Table 1. Replicate results, comparison with chemical analysis, and consideration of internal consistency were used to evaluate the quality of the stable isotope data. The d 18O values of the groundwater samples range from 24.2 to 22.6½ with a mean of 23.0½ and the d D values vary from 221 to 210½ with a mean of 214½. These values are more depleted than those of seawater …dD ˆ 0; d18 O ˆ 0½† and suggest a

meteoric origin for the groundwater. The most depleted groundwater samples …d18 O ˆ 24:2; dD ˆ 221½† occur at Forifori (WVI 721) and Asukese #1 (WVI 713) in the southwest and also in parts of the northwest at Hawanyanso (WVI 686) and Kwabena Amoah (WVI 720) (Fig. 3). The groundwater samples lie on a regression line of equation dD ˆ 6:7…^0:42†d18 O 1 6:3…^1:3† …r ˆ 0:87†: The rainfall samples show a great variation in isotopic composition; the d 18O values range from 27.4 to 11.3½ with a mean value of 23.2½, while the d D values range from 246 to 117½ with an average of 218½. The most isotopically depleted rainfall samples were those collected during the major rainy season. Although the samples do not cover the whole year, they exhibit isotopic behavior quite characteristic of tropical regions (Akiti, 1980; Mathieu and Bariac, 1996). The samples plot on a line of equation dD ˆ 7:02…^0:48†d18 O 1 4:3 …r ˆ 0:98†. The slope (7.02) and the deuterium excess ªdº value of 7.6 (i.e. d D± 8d 18O) are both less than those of the GMWL and suggest that much of the rainfall occurred at a mean humidity of less than 100% (Gon®antini, 1986). Evaporation under a relative humidity of less than 100% will result in a slope of 5 ^ 2 on a d D± d 18O plot. The values also re¯ect the irregular manner in which the rainfall samples were collected. As noted by Mathieu and Bariac (1996), the isotopic compositions at the beginning and end of the rainy season (monsoon) are characterized by lighter rainfall with relatively enriched d values. Rainfall samples collected in the study area at these times and also during the dry season are isotopically enriched, exhibit amount effect (a process whereby the isotopic composition of samples from light rainfall is more enriched than that of samples collected from heavy rainfall), and are characterized by isotopic compositions of evaporating raindrops. Dansgaard (1964) observed that at a given location, heavier rainfall tends to produce more depleted d D and d 18O composition. The amount effect could produce less isotopically depleted rainwater during the dry season as the air moisture is subjected to less Raleigh condensation process. The most enriched samples collected in the study area were the Lake Volta samples, with a mean d 18O of 1.4½, and ranged from 0.6 to 2.2½. The corresponding d D values range from 3 to 14½ with a

S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

43

Fig. 4. d D± d 18O relation of groundwater, rainfall, and Lake Volta samples collected from the southern Voltaian Sedimentary Basin of Ghana in 1994. The global meteoric water line (GMWL) is also shown.

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S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

Table 1 Environmental isotope data for groundwater, rainfall, and Lake Volta water samples in southern Voltaian sedimentary basin of Ghana collected in January and July, 1994 Sample number

Location

Date

d 18O (½)

d D (½)

WVI 699 WVI 702 WVI 744 WVI 742 WVI 753 WVI 720 WVI 724 WVI 686 WVI 729 WVI 711 WVI 706 WVI 709 WVI 704 WVI 683 WVI 773 WVI 772 WVI 764 WVI 750 WVI 769 WVI 766 WVI 752 WVI 757 WVI 749 WVI 756 WVI 748 WVI 732 WVI 726 WVI 727 WVI 708 WVI 740 WVI 721 WVI 747 WVI 402 WVI 736 WVI 738 WVI 401 WVI 718 WVI 713 DAAP 1 DAAP 3 WVI 761 WVI 763 WVI 730 WVI 712 Lake Volta Lake Volta Lake Volta Lake Volta Lake Volta Rain Rain

Bonkrong Alizuma Koranteng-Krachi Koranteng Odumasua Kwabena Amoah Nsuogya Anafo Hwanyanso Praprabaabida Abotan-Domeabra Dunkro Maame Krobo Amariya Asanyanso Adeemra Donkorkrom Abotanso #2 Oframoase Amankwakrom Donkorkrom Samanhyia Bebuso Kyemfre Okyeame Kissi Kwasi Kuma Ayeben Demso Asukese #2 Maame Krobo Atakora Forifori Odumase Tease Mmradan Sekyidrom Tease Takoratwene Asukese #1 Abomosarefo Apeabra Adukrom Duvor Aduonum Abotan Amankwa Tono Ekye Amanfro Obosomano Amankwa Tono Ekye Amanfro Amankwakrom Samanhyia

1/21/94 1/21/94 1/19/94 1/19/94 1/19/94 1/20/94 1/20/94 1/20/94 1/21/94 1/21/94 1/22/94 1/22/94 1/22/94 1/21/94 1/16/94 1/16/94 1/16/94 1/17/94 1/16/94 1/16/94 1/17/94 1/17/94 1/18/94 1/17/94 1/17/94 1/18/94 1/18/94 1/18/94 1/21/94 1/18/94 1/21/94 1/17/94 1/19/94 1/17/94 1/17/94 1/21/94 1/20/94 1/20/94 1/16/94 1/16/94 1/17/94 1/17/94 1/21/94 1/21/94 1/16/94 1/23/94 7/20/94 7/26/94 8/1/94 8/14/94 8/24/94

22.7 22.7 23.6 22.6 23.1 23.2 23.6 23.7 22.7 22.7 22.7 23.4 23.4 22.7 22.9 23.1 22.9 23.1 22.9 23.2 23.6 23.3 22.9 23 22.8 23 22.9 23 23.4 22.9 23.7 22.6 22.6 22.8 23 22.9 22.6 24.2 22.6 22.9 22.9 23.7 22.9 23 0.6 2.2 2 0.8 1.6 27.4 26.8

214 213 219 210 215 219 220 218 213 213 212 215 217 213 213 215 212 213 212 215 217 214 212 214 212 215 212 212 215 213 221 211 212 212 213 213 213 221 210 214 212 218 212 213 3 11 14 4 6 246 244

3

H (T.U.)

,1.4 ,3.1

,0.31 ,0.31 ,0.31 ,3.1 ,3.1

5

,3.1 7.19 ,3.1 ,3.1 4.7 ,3.1

,0.31

S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

45

Table 1 (continued) Sample number

Location

Date

d 18O (½)

d D (½)

Rain Rain Rain Rain Rain Rain

Donkorkrom Forifori Donkorkrom Donkorkrom Maame Krobo Tease

7/20/94 5/11/94 2/3/94 7/26/94 4/5/94 3/4/94

26.7 27 1.3 21 1.2 1

245 246 17 5 3 12

mean of 8½. Due to kinetic isotope fractionation, the lake water samples have a greater relative enrichment of d 18O than deuterium and plot below the GMWL (Fig. 4). Despite the enrichment in stable isotopic content of the Volta Lake samples, their ECs are very low and show the sensitivity of stable isotopes to evaporation in comparison with ECs. Tritium concentrations in the groundwater samples range from less than 1 to 7.2 T.U. (Table 1), and indicate groundwater recharge from modern rainfall. Modern recharge consists of groundwater in equilibration with an atmosphere contaminated by nuclear weapons testing initiated around 1953 (Freeze and Cherry, 1979). Tritium distribution of the groundwater samples does not show a de®nite pattern. The measured tritium concentrations in 1994 rainfall samples collected for this study range from less than 1 to 4 T.U. 4.2. Carbon-14 and d 13C The d 13C values of the dissolved carbon species, 14C activities in percent modern carbon (pmc) and 14Ccalculated ages are given in Table 2. The d 13C isotopic fractionations of all samples were corrected to 225½ with respect to PDB, the postulated mean value of terrestrial wood (Stuiver and Polach, 1977). The d 13C concentrations range from 220.3 to 215.2½. These values fall closer to the range for those of some common plant material (223 to 23½) than those from carbonate minerals (22 to 0½) (Mazor, 1991). The 14C concentrations of the groundwater samples range from 43 to 108% modern carbon (pmc). There is no signi®cant difference in activities between samples collected from sandstone and conglomerate aquifers. Only one out of the eight samples analyzed had a 14C activity of less than 50 pmc. The relatively high 14C activities observed for most of the samples suggest a

3

H (T.U.)

,3.1 4 ,3.1

local young recharge source of water for the area. DoÈrr et al. (1987) observed that 14C content of young shallow groundwaters ranges from 90 pmc to about 50 pmc, depending on local conditions of carbonate dissolution. 5. Water chemistry The groundwater chemistry data are reported by Acheampong (1996). Groundwater in the study area is characterized by Ca 21 and Na 1 ions and mainly HCO2 3 ions. Dissolved inorganic carbon concentration increases with distance along all estimated ¯ow paths. The shallow groundwater is mainly a mixed cation-bicarbonate water and evolves from either Ca±Na±HCO3 or Ca±Na±Mg±HCO3 water to relatively concentrated Ca±Na±HCO3 water (Acheampong, 1996). No rock cuttings from the wells were obtained for analyses. However, petrographic and X-ray diffraction analyses of rock samples collected from outcrops (Acheampong and Hess, 1998), indicate that the mineralogy of the sandstones and conglomerates is similar and is dominated by sodic plagioclase feldspars and quartz with minor chlorite, muscovite, calacite, talc and zeolite. The major minerals in the shale are quartz and plagioclase with minor chlorite and muscovite. The calculated PCO2 values are all signi®cantly higher than atmospheric and suggest that reactions take place largely under partially open and open conditions. Field determination of dissolved oxygen (DO) at the well heads revealed the occurrence oxidizing conditions in the study area without regard to aquifer type or location (Acheampong, 1996). 6. Discussion The possible reactions in the in®ltration zone can be

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S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

written as: CO2 1 H2 O 1 MeSilicate ˆ Me1 1 HCO2 3 1 H4 SiO4 1 clay

…2†

where Me 1 is a metallic ion (Blavoux and Olive, 1981). The CO2 results from mineralization of ground organic matter and/or from plant respiration, which builds up high partial pressures. The carbon dioxide partial pressures (PCO2) calculated by the Wateq4f program (Ball et al., 1987) using alkalinity and ®eld pH range from 10 22.0 to 10 20.48 atmospheres and are given in Table 2. These values drop by an order of magnitude along ¯ow paths from recharge (WVI 402 in Fig. 3) to topographically low areas (WVI 721 in Fig. 3); at the same time, total dissolved solids (TDS) and dissolved inorganic carbon (DIC) concentrations also increase and suggest that CO2 is being rapidly 22 converted to HCO2 during mineral 3 and CO3 weathering. 6.1. Modeled ages Groundwater ages were modeled using the mass balance geochemical code NETPATH (Plummer et al., 1991). In modeling the 14C ages for the groundwater, the in¯uence of the bomb 14C on the initial 14C content was considered because of the presence of tritium in some of the samples. The pre-bomb 14C content of soil CO2 was taken as 95 pmc, as suggested by DoÈrr et al., (1987)for samples with tritium contents less than 3.1 T.U. due to the in¯uence of the relatively slow 14C depleted soil organic reservoir on the CO2 production in the unsaturated zone during the period of groundwater in®ltration. Samples with tritium contents greater than 3.1 T.U. were assigned soil CO2 carbon-14 activity of 100 pmc. The other parameters which were not measured were assigned the following values in the model: activity of the mineral carbonate, 0 pmc; d 13C of mineral carbonate, 0½ and d 13C of soil gas, 225½. The groundwater generally ranges from a mixedcation bicarbonate to sodium bicarbonate. The evolution of Na±HCO3 water from a mixed-cation-HCO3 water can occur in many ways including pure silicate weathering, cation exchange, anaerobic degradation of organic matter, and proton exchange. Silicate dissolution is enhanced in the recharge area

by high PCO2 (10 20.8). The weathering process releases Na 1 and HCO2 3 ions into solution. Considering the evolution of a Ca±Na±HCO3 water at WVI 402 to Na±HCO3 water of WVI 721 along Flow path I, (Fig. 5) a summary reaction that accounts for the observed chemistries is: Ca±Na±HCO3 water 1 6:5Albite 1 6:1CO2 10:1Na-Exchange ˆ 10Kaolinite 1 0:14Calcite 10:05Mg21 =Ca21 Exchange 1 Na±HCO3 water

…3† where the numbers are mass transfer in mmols/kg water. There is an overall decrease in the PCO2 values from 10 20.8 at WVI 402 to 10 21.5 at WVI 721 and suggests that CO2 is being consumed in silicate hydrolysis. In order to get enough CO2 to produce the Na± HCO3 water in WVI 721, the reaction needs to take place in the soil zone. The abundance of vegetation and relatively high PCO2 associated with plant respiration could supply hydrogen ions through the formation and dissociation of carbonic acid for dissolution reactions. The d 13C of soil gas in equilibrium with HCO3 in the water (while forming in the soil zone) would need to be 223½ to get the observed 215.9½ in the HCO3. In such an open system, the initial 14C of the water as it entered the saturated zone would be near 100 pmc. The modeled age of the water at WVI 721 is about 6000 years, if no other reactions took place in the saturated zone. Using the EXCHANGE phase in NETPATH, cation exchange processes were modeled in addition to pure silicate weathering, exchanging Ca 21 and Mg 21 for Na 1 on exchangers in the aquifer. In this model, calcite is dissolved to provide the divalent cations to exchange for Na 1. To get enough CO2 to make all the bicarbonate, the reaction will have to occur in the soil zone. If this occurs, then the isotopic process is similar to the silicate weathering process discussed above since the water is in isotopic equilibrium with soil gas. On the other hand, if the CO2 source is within the aquifer, the calculated d 13C value will be similar to the observed if the CO2 was 225½. The resulting age would be 3200 ^ 350 years because of the dissolution of calcite (assumed 0 pmc).

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47

Fig. 5. Spatial distribution of d D composition of groundwater in southern Voltaian Sedimentary Basin of Ghana.

This age seems more realistic since it incorporates all the processes believed to be occurring within the shallow semi-con®ned groundwater system. All the other samples are modern in age. Since all the groundwater samples are aerobic, the

degradation of organic matter within the aquifer is discounted. In general, it is observed that all mixed cation-bicarbonate and Na±HCO3 waters in the study area could evolve either in the soil zone or within the bedrock aquifer. They may therefore not be really

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S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

Table 2 14 C and d 13C content and age of the shallow groundwater in southern Voltaian Sedimentary Basin of Ghana collected in July/August 1994. The d 13C values were collected to 225 ½ with respect to PDB (Stuiver and Polach, 1977) Sample number

Location

d 13C (½)

14

DAAP 1 WVI 402 WVI 721 WVI 736 WVI 747 WVI 749 WVI 772 WVI 769

Abomosarefo Tease Forifori Mmradan Odumase Kyemfre Donkorkrom Amankwakrom

215.96 215.93 215.85 218.92 220.29 217.27 219.18 215.22

108.83 85.55 43.35 87 103.78 96.77 84.71 66.83

evolutionary but rather independently formed during and after the recharge process. 6.2. Tritium content of rainfall and groundwater The variations in tritium concentrations provide insight into the local recharge mechanism. The measured tritium values in the groundwater samples range from less than 1 to 7.2 T.U. and are quite comparable to values of between 15 and 27 T.U. observed in northeast and southeast Ghana in 1978 by Akiti (1980), which would have decayed to between 5 and 9 T.U. in 1994 based on the tritium half-life of 12.43 years. They are also comparable to a value of 2 T.U. observed by Loehnert (1988) in southwest Nigeria. The generally low tritium content of the groundwater is a re¯ection of the depleted tritium concentration in the atmosphere in the northern hemisphere. More especially, considering the fact that the study area lies only 78N of the equator. The tritium content of present-day rainfall collected for this study varies from less than 1 to 4 T.U. A plot of tritium content versus 14C content of the groundwater samples is shown in Fig. 6. The general lack of correlation is recognizable. However, the presence of above-background tritium values in these groundwater samples indicates that the groundwater is being recharged under modern climatic conditions. The presence of tritium in WVI 747 and WVI 749 and their high 14C activities indicate that these waters are quite recent. Sample WVI 721 has a tritium concentration of less than 3.1 T.U. and a low 14C activity of 43 pmc and suggests a probable mixing of old groundwater and modern rainfall. Because of the low tritium value, the relative dilution my modern

C (pmc)

Log Pco2

14

21.1 20.88 21.56 21.13 20.48 21.31 21.54 21.64

Modern Modern 3200 Modern Modern Modern Modern Modern

C age (years B.P.)

rainfall may be small and so its calculated age of 3200 ^ 350 years B.P. may be considered a reasonable minimum value. The tritium and 14C data suggest the presence of a localized zone of mixing waters in the area but this could not be delineated with the present data. 6.3. Deuterium and oxygen-18 content of rainfall Fig. 7 shows the d D± d 18O relationship of rainfall collected in the southern Sahara (Mali, Chad and northern Nigeria) from the International Atomic Energy Agency (IAEA)/World Meteorological Organization (WMO) rainfall network (Dray et al., 1983), southeast Ghana (Akiti, 1980), and for this study. They lie on a regression line with the equation dD ˆ 7:06d18 O 1 5:5 …r ˆ 0:97†: The rainfall events are all of tropical maritime (monsoon) origin, however, there is a scatter in the isotopic composition at any one place and seasonal variations seem quite apparent. The rainfall samples plotted in Fig. 4 are discrete samples of individual storms of variable duration collected on different days at different locations in the study area from February to August 1994. The mean isotopic composition may thus not be very representative of the mean annual amount-weighted value for rainfall in the area. The two populations of rainfall data in Fig. 4 may be related to the seasonal variations in intensity and duration of rainfall in the area. The most depleted samples were collected from May to August during the rainy season, while the most enriched samples were collected in the dry season from February to April (Table 1). The observed differences in the isotopic compositions of the two groups of rainfall samples may therefore be attributed to the

S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

49

Fig. 6. 3H- 14C relation of groundwater in southern Voltaian Sedimentary Basin of Ghana.

amount and duration of the rainstorms in the different seasons from which the samples were collected, since the latitudinal, altitudinal, and inland distance changes are minimal. Analysis of the rainfall data in Tease (in the study area) (Water Resources Research Institute (WRRI), unpublished data) shows that about 75% of the annual rainfall occurs during the main rainy season from April to October, while about 25% occurs in the dry season (November±March). Using the mean isotopic compositions of the rainfall

data for the two rainy seasons, a mean isotopic composition of 25.13 and 232.3½ in d 18O and d D, respectively, was obtained for the rainfall in the area. These values are within analytical errors of the values of 25.34 and 232.8½, respectively, in d 18O and d D obtained at the interception of the evaporation line and the GMWL (Fig. 4). The rainfall samples are therefore quite representative of the meteoric isotope input into the local hydrological system for groundwater and surface water studies.

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S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

Fig. 7. d D± d 18O relation of monsoonal rainfall samples in parts of West Africa.

6.4. Deuterium and oxygen-18 of groundwater and surface water Stable isotopic concentrations of deuterium and 18O

provide important information on the recharge conditions of groundwater and its relationship with surface water. Their value as natural tracers is that they neither decay with time nor are they removed from

S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

water by exchange processes during movement through most low temperature aquifer materials. Since no geothermal effects have been observed in the SVSB, stable isotopes of oxygen and hydrogen can be regarded conservative. All the groundwater samples lie on or close to the GMWL (Fig. 4) and indicate that the groundwater has not been greatly fractionated by kinetic evaporation. The isotopic composition of the groundwater is an integration of the isotopic variation of the individual rainfall events. The mean isotopic compositions of 23.0½ in d 18O and 214½ in d D, respectively, are therefore reasonable representations of the groundwater in the study area. The mean d 18O value of 23.0½ is similar to the mean value of 22.8½ obtained by Akiti (1980) for groundwater in the Accra Plains of Ghana (about 160 km south of the study area). A regression analysis of the groundwater data shows them to lie on a line of gradient 6.6, (Fig. 4) which is not statistically distinguishable from the gradient (7.02) of the rainfall samples. The similarity between the two gradients indicates that probably most of the recharge water enters the bedrock aquifer through fractures and joints without any signi®cant evaporation. 6.5. Aquifer recharge Stable isotopic compositions of the groundwater in the SVSB have not been de®ned through any form of long term repeated sampling. However, there was no appreciable difference between the isotopic compositions of samples collected in the dry and rainy seasons, respectively. The groundwater samples lie between the rainy season and dry season rainfall samples on the d D± d 18O plot and show that the groundwater is a mixture of local rainfall that occurs during the two seasons. Though the groundwater samples are not clearly evaporated under kinetic conditions, they exhibit some variation in stable isotopic composition and show a slight distribution along the GMWL (Fig. 4). This moderate variation may be related to the recharge process as groundwater retains minor variations of the isotopic composition of the different storms at different sites during recharge. It is also possible that the groundwater was subjected to physical processes before entering the ¯ow system. Evaporative effects in the soil zone on the isotopic composition of groundwater in the SVSB cannot be

51

quanti®ed at this time because of inadequate data on the matrix and fracture ¯ow processes by which groundwater in the area is recharged. In addition, a regular, long-term sampling is needed in order to quantify the seasonal recharge in the study area. The study area is quite small and low-lying with very minimal difference in the longitude and latitude. The major process that may produce any signi®cant difference in the stable isotopic composition of the rainfall is therefore the amount and intensity of the rainfall. Dansgaard (1964) noted that at any given location, heavier rainfall produces more depleted d D and d 18O composition. The mean annual rainfall in the study area is about 1400 mm. However, over the previous 13 years before the samples were taken (1983±1994), Ghana as a whole experienced a number of drought years and the mean annual rainfall in the study area had fallen to 1150 mm. These drought years could have produced the relatively enriched isotopic signature in the shallow groundwater system. For this hypothesis to be valid, however, the groundwater in the area should have been recharged within this time period. This period could not be precisely determined by the 14C and tritium data. However, since most of the groundwater samples lie either close to or along the GMWL (Fig. 4), it can be inferred that the groundwater originated from local rainfall and the variation in its isotopic composition may be the result of natural variations in the regional and local climate. The main surface water body is Lake Volta, which was formed as a result of the damming of River Volta in 1966. The Lake Volta samples have isotopic compositions strongly affected by evaporation. The samples lie on an evaporation line with the equation dD ˆ 5:95d18 O 2 0:96; which intercepts the GMWL approximately at a point with dD ˆ 232:8 and d18 O ˆ 25:34½ (Fig. 4). The interception of the evaporation line with the GMWL identi®es the mean isotopic composition of the parent rainwater (Fontes, 1980; Ferronsky and Polyakov, 1982). Thus, the lake water is recharged from rainfall with mean d D and d 18O values of 232.8 and 25.34½, respectively, and has undergone extensive evaporation. The spatial distribution of the isotopic composition of the groundwater (Fig. 5) does not show any consistent pattern of isotopic enrichment towards Lake Volta and suggest that probably, the lake

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S.Y. Acheampong, J.W. Hess / Journal of Hydrology 233 (2000) 37±53

does not provide any signi®cant recharge to the groundwater system. This is also supported by the groundwater ¯ow direction deduced from the potentiometric map (Fig. 5). Pumping from the shallow groundwater system is therefore not likely to induce any signi®cant recharge from Lake Volta. 7. Conclusion Radiocarbon, tritium, and stable isotopes of oxygen-18 and deuterium measurements of groundwater in the SVSB of Ghana have been made to provide a framework for a better understanding of the source of recharge to the shallow groundwater system. The tritium concentrations, although generally low, are very close to current levels in rainfall and demonstrate that modern recharge occurs. The radiocarbon and tritium data suggest that the groundwater system varies from about 3200 ^ 350 years B.P. to modern in age. The groundwater system in the SVSB is representative of the recharge under modern climatic conditions. The meteoric variations in the stable isotope compositions indicate that the groundwater is not greatly affected by kinetic evaporation. The observed meteoric variation in isotopic composition of the groundwater is due to the natural variations in local and regional climatic conditions. Isotopic evidence indicates that the groundwater system does not derive any signi®cant recharge from the Lake Volta system. Acknowledgements The authors are very grateful to the Conrad N. Hilton Foundation, USA that generously funded the study. We are very grateful to the staff of World Vision International Ghana Rural Water Project and the Water Resources Research Institute (WRRI) of the Council for Scienti®c and Industrial Research (CSIR), Accra, Ghana, for their help in obtaining hydrogeologic data and other logistic support. Our special thanks go to Daniel Gyasi Frempong of the WRRI and Daniel Agyekum Dampare of the National Mobilization Programme of®ce in Donkorkrom for their assistance in the ®eld. The review comments by Robert Gon®antini and an anonymous reviewer helped improve the manuscript signi®cantly. Neil Plummer provided a

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