Hydrogeochemical And Isotopic Studies Of Groundwater In Coastal Aquifers Of Ghana: Case Study In The Central Region

ABSTRACT

The major setback in the exploitation of groundwater in the Central Region of Ghana is

poor water qualitydue to high salinity. The source of salinity has not been adequately

addressed. The focus of this study was, therefore,to understandthe hydrogeochemical

processes occurring in the study area in order to determine the origin of salinity and

groundwater residence time. The methodology involved desk study, fieldwork, and

laboratory work. The desk study comprised review of literature, compilation of existing

borehole data, topographical and geological maps. The fieldwork involved collection of

rainwater, surface water, and groundwater and soil samples. One hundred and thirty

seven (137) rainfall events were obtained from Saltpond and Twifo Prasso

Meteorological Stations and Six (6) surface water samples from Ochi Narkwa and

Ayensu Rivers. Seventy-eight (78) groundwater samples were collected. Thirty five soil

samples from four profiles were collected for measurement of Cl- in the soil zone.

Physico-chemical parameters such as pH, temperature, electrical conductivity (EC), total

dissolved solids (TDS), salinity, redox potential and alkalinity were measured in the field.

In the laboratory,Ca2+, Mg2+, Na+, K+, Cl-, SO4

2-, NO3

-, F-, Br-, PO4

3-, δ18O, δ2H, δ13C,3H

and14C were measured.Data obtained were evaluated using bivariate plots, statistical and

graphical methods. The rainwater chemistry in the study area showed the dominant anion

as Cl- ranging between1.07 mg/L and 22.32 mg/L at the coast and 0.48 mg/L to 8.28

mg/L at 90 km from the coast. Higher Cl- content occurred at the coast suggesting the

ocean as a major contributor of Cl- in rainwater.Low TDS between 69.90 mg/L and 93.00

mg/L occurred inthe surface waters showing generally low concentrations of major ions

with dominant hydrochemical facies as Na-Cl. In the shallow groundwater, cations

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occurred in the orderNa+> Ca2+> Mg2+> K+ and the anions in the order Cl-> HCO3

->

SO4

2-. The major hydrochemical facies was Na-Cl. In deep groundwater, the major

cations occurred in the order Na+> Ca2+> Mg2+> K+ and the major anions in the order Cl-

> HCO3

-> SO4

2-similar to that of shallow groundwater. This suggested that the deep

groundwater chemistry were developed from the unsaturated zone. The hydrochemical

facies identified were Ca-Mg-HCO3, Na-Cl, Ca-Mg-Cl-SO4 and non-dominant water

types. Stable isotope composition of rainwater, shallow groundwater and deep

groundwater showed the mechanism of recharge to the aquifers was direct infiltration of

local rainfall of mean isotopic composition δ18O = -3.8 ‰ V-SMOW and δ2H = -18 ‰

V-SMOW. It was established through δ13C - Cl- relationship that the groundwaters may

not be intruded by seawater water. Tritium in the groundwaters ranged from 0.05 ± 0.07

to 4.75 ± 0.16 TU.Eighty-five percent (85%) of the samplessuggestedmodern recharge or

young waters with tritium values ranging between 1.07 ± 0.25 TU and 4.75 ± 0.16 TU.

Fifteen percent (15%) of the samples constituted old waters and covered boreholes CR2-

50 at Ekumfi Asokwa, CR4-05 at Sefara Kokodo, CR4-FZ-22 and CR4-FZ-08 at Ayeldu

with tritium values ranging between 0.05 ± 0.07 TUand 0.67 ± 0.22 TU. 14C content of

the groundwaters ranged between 9.50 pMC and 113.56 pMC. Most of the waters were

of modern recharge except borehole CR2-50 at Ekumfi Asokwa which is older.The

estimated ‗age‘ or residence time of this older water was 19,459 years before present

(BP) based on Akiti‘s model.Spatial distribution of 3H and 14C in the study area showed a

localised system of flow suggesting discontinuous aquifer systems in the study area.

Groundwater salinization in the coastal zone of the Central Region may be caused largely

by halite dissolution and to a minor extent silicate weathering. Study of Cl- profiles in the

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soil zone, revealed occurrence of NaCl in lenses, hence the existence of salt crusts at

depths, between 80 and 120 cm which support halite dissolution in the study area.The

Na/Cl (0.36–5.18), Br/Cl (0.0054–2.08), SO4

2-/Cl-(0.02–4.09), and Ca/SO4(0.35–10.84)

molar ratios suggest that seawater intrusion plays a minimal role in controlling the

groundwater chemistry in the study area. In conclusion, the origin of salinity was halite

dissolution. Most of the groundwaters were of modern recharge except borehole CR2-50

at Asokwa which is older. The residence time of the older water was 19,459 years

BP.Exploitation of groundwater resource of modern recharge is therefore sustainable but

susceptible to contamination because it is easily replenished. It is recommended that the

recharge areas should be protected by enacting laws that will control anthropogenic

activities in these areas. Since the old groundwater encountered in the area is liable to

depletion, groundwater abstraction must be regulated to prevent over abstraction that

would result in depletion and possible collapse of the aquifer. Future studies into origin

of salinity should employ 32S and 34S, 11B and 86Sr/87Sr. Newly-developed dating methods

for young waters such as Chlorofluorocarbons (CFC) should be considered to

quantitatively determine the ages of the young waters.