Journal of Spatial Hydrology Volume 11 Number 1 Article 5 2011 Studies on Major Ion Chemistry and Hydrogeochemical Processes of Groundwater in Port Harcourt City, Southern Nigeria Follow this and additional works at: https://scholarsarchive.byu.edu/josh BYU ScholarsArchive Citation (2011) "Studies on Major Ion Chemistry and Hydrogeochemical Processes of Groundwater in Port Harcourt City, Southern Nigeria," Journal of Spatial Hydrology: Vol. 11 : No. 1 , Article 5. Available at: https://scholarsarchive.byu.edu/josh/vol11/iss1/5 This Article is brought to you for free and open access by the Journals at BYU ScholarsArchive. It has been accepted for inclusion in Journal of Spatial Hydrology by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Journal of Spatial Hydrology Vol.11, No.1 Spring 2011 Studies on Major Ion Chemistry and Hydrogeochemical Processes of Groundwater in Port Harcourt City, Southern Nigeria H. O Nwankwoala and G.J Udom Abstract Of recent, the rapid deterioration of groundwater quality in Port Harcourt, Southern Nigeria due to unregulated exploitation resulting from increasing growth in the oil and gas production activities has become a major concern. The predisposition of groundwater to pollution and the realization of its serious health and economic consequences demand knowledge of the ambient groundwater quality and of the processes leading to an improved understanding of the groundwater in the area. Groundwater samples were collected from eighteen (18) representative boreholes spread over the Port Harcourt City. This was done to assess and determine the geochemical processes occurring within the aquifer systems using groundwater chemistry and ionic ratios. Properties such as electrical conductivity, pH and major ion concentrations, such as Ca, Mg, Na, K, Cl, HCO3, and SO4, of groundwater were taken into consideration. Concentrations of these cations and ions in the groundwater systems of the area vary spatially and temporally. Abundance of these ions are in the following order: Ca > Mg >Na > K = HCO3 > Cl > SO4 > NO3. Ca - Mg - HCO3 and Ca- Mg- SO4- Cl are the dominant hydrochemical facies of the study area. Results show that ion-exchange processes, carbonate and silicate weathering are responsible mechanisms for the groundwater chemistry of the area. Hydrochemical indices (Mg/Ca, Cl/HCO3 and Cation Exchange Values (CEV) generally indicates low- salt inland waters, with minimal marine influence. The hydrochemical evidence reveals the importance of recent management decisions (reduced exploitation/controlled pumping) in determining the evolution and distribution of groundwater salinity within the aquiferous zones. This framework, as the study observes, will lead to improved understanding of the hydrochemical characteristics of the aquifer systems of the area. Introduction Following huge oil exploration and production activities in Port Harcourt area, there is a continuous increase in demand for fresh groundwater, the major source of urban water supply for domestic and industrial uses. The increased rate of groundwater abstraction rate poses severe pressure on groundwater resources, as almost every home has a well. Also, most of the companies that generate chemical effluents discharge their wastes directly into the sea or creeks, without regard to the effects of the effluents on coastal aquifers and aquatic life. Groundwater chemistry is largely a function of the mineral composition of the aquifer through which it flows. The hydrochemical processes and hydrogeochemistry of the groundwater vary spatially and temporally, depending on the geology and chemical characteristics of the aquifer. Apodaca, et al., (2002), Martinez & Bocanegra, (2002), have inferred that hydrogeochemical processes such as dissolution, precipitation, ion-exchange processes and the residence time along the flow path control the chemical composition of groundwater. Abimbola, et al., (2002); Olatunji, et al., (2001), also established that geology plays a significant role in the chemistry of subsurface water. Moreso, the importance of mineral diagenesis in the geochemical evolution of groundwater has been elucidated by (Wicks, et al., 1995; Back, et al., 1983; Plummer, 1977; Bredehoeft et al., 1983; Hendry & Schwartz, 1990). Studies by ____________________________________________________________________________ Department of Geology, University of Port Harcourt, P.M.B 5323, Choba, Port Harcourt, Nigeria. Nwankwoala and Udom JOSH vol. 11 (34-40) (Goldenberg et al., 1986; Jones et al., 1969; Drever, 1988; and Keller et al., 1991) have shown that when soluble minerals undergo diagenetic reactions, they provide a medium for cation- exchange reactions as well as present a significant influence on the geochemistry of an aquifer system. Previous studies carried out in the area have tended to emphasize only the general water supply problems (Etu- Efeotor & Odigi, 1983; Amajor, 1986). Amadi, et al., (1989) assessed the hydrogeochemistry of groundwaters in parts of the Niger Delta. Etu-Efeotor, (1981); Udom, et al., (1999); Nwankwoala, et al., (2007), acknowledged that the groundwater quality in the area is rapidly deteriorating. Increase in population and rapid urbanization has made groundwater the major source of water supply, hence, it is very essential to understand the hydrogeochemical processes that take place in the aquifer system. In this paper, an attempt is made to evaluate the different water types and hydrogeochemistry of the main source of water supply in the area. This study also provides an opportunity to observe a detailed profile of the hydrogeochemical facies distribution and processes of groundwater, with a view to predicting their water character. The Study Area Port Harcourt City is located within the Niger Delta Basin of Southern Nigeria. The area lies between latitudes 4030’ and 5000’N and longitudes 6045’ and 7030’E (Fig.1). The area is characterized by alternate wet and dry seasons (Iloeje, 1972). The rainy season starts in March and ends in October, with a peak in June and July. The rains are ushered in by the south west rain bearing winds which blow from the Atlantic Ocean into Nigeria. Within the rainy season, there is a short period of little or no rain called the ‘August Break’ which is commonly experienced in the month of August. The dry season begins in November and lasts till March, with a short harmattan in December and early January. It is brought by the dry northeasthern winds which blow across the Sahara desert into Nigeria. Relative humidity values are generally over 80%, and the mean annual temperature is about 280C in the area. Fishing, small scale agriculture and huge oil exploration and exploitation dominate economic activities in the area. Journal of Spatial Hydrology 35 Nwankwoala and Udom JOSH vol. 11 (34-40) Fig. 1: Location Map of Study Area Showing Sampled Points Brief Geology/Hydrogeology of the Area Port Harcourt, one of the coastal cities in the southern part of Nigeria is located within the oil rich Niger Delta Sedimentary Basin. Figures 2 and 3 show the spatial variability of boreholes and water depth in the study area. Generally, the Delta is characterized by three formations, namely Akata (oldest), Agbada and Benin (youngest). These formations consist primarily of regressive Tertiary age sediments. The detailed geology of the Niger Delta formation is given by Reyment (1965), and Short & Stauble, (1967). The basal Akata Formation consists of low density, high pressure, shallow marine to deep water shales (Schield, 1978). The Agbada Formation consists of alternating deltaic (fluvial, coastal, fluviomarine) sands and shale. The Benin Formation (Miocene – Recent) consists of freshwater continental (fluviatile) sands and gravels, with occasional clay layers, and an overall thickness of 2100m thick at the basin centre (Weber & Daukaro, 1975). It is the most prolific aquifer in the study area. Overlying this formation are the Quaternary deposits, 40 – 50m thick (Table 1): an unconfined aquifer sequence comprising rapidly alternating sequences of sand and silt/clay, with the latter becoming increasingly prominent seawards (Etu-Efeotor & Akpokodje, 1990; Ngah, 2002). Journal of Spatial Hydrology 36 Nwankwoala and Udom JOSH vol. 11 (34-40) The thin clay units in the Benin Formation have resulted to a multi-aquifer system in the study area as it is the case in most parts of the Niger Delta where this formation outcrops (Etu-Efeotor, 1981; Edet, 1993; Udom et al., 1997, 1998). Etu-Efeotor (1981) identified two major aquifers in Rivers State from strata logs. The upper one is more prolific and extends to about 80 metres, while the underlying one is less prolific. Studies by Etu-Efeotor & Odigi, (1983), Amadi, (1986), Odigi, (1989), Amadi & Amadi, (1990), Nwankwoala et al., (2008), and other available borehole records from the Rivers State Water Board as well as information from other water agencies, show that borehole depths in the state commonly range from 35 – 90 metres; Static Water Level (SWL), 1 -20metres; transmissivity, 500 – 10,000m2/d; and hydraulic conductivity, 5 – 60m/d. Table 1: Quaternary deposits of the Niger Delta (after Etu-Efeotor & Akpokodje, 1990) Geologic Unit Lithology Age Alluvium Gravels, sand, clay, silt Freshwater, back swamps Sand, clay, silt, gravel Mangrove and some saltwater/ Medium-fine sands, clay and some silt Quaternary Back swamps Active/abandoned beach ridges Sand, clay and some silt