Effect of Bedrock Types on the Chemical Composition of Ground Water

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Published on International Journal of Biology, Physics & Mathematics
Publication Date: May 11, 2019

Omopariola, O. A. & Adeniyi, I. F.
Institute of Ecology and Environmental Science, Faculty of Science, Obafemi Awolowo University
Department of Zoology, Faculty of Science, Obafemi Awolowo University
Ile-Ife, Nigeria

Journal Full Text PDF: Effect of Bedrock Types on the Chemical Composition of Ground Water (Study in Ile – Ile Southwestern Nigeria).

Abstract
This study assessed the effect of different underlying bedrock types on the chemical quality of groundwater obtained from forty (40) hand dug wells in Ile- Ife area of South Western Nigeria using appropriate descriptive and inferential statistical tests. The one way analysis of variance tests between the four different bedrock types (Granite Gneiss, pegmatite, pegmatite schist and undifferentiated schist) showed statistically significant difference (p ≤ 0.05) in nine of the twenty four parameters considered. The higher concentration of Ca2+, Na+ and K+ as well as high nitrate value (11.5± 7.0) mgl-1 are as a result of possible contribution from mineralogical constituents of the rock types which are mainly feldspar and muscovite minerals while the high nitrate value in pegmatite schist may be due to proximity to sewage.

Keywords: Bedrock type, chemical quality, groundwater, mineralogical, Ile- Ife, South Western & Nigeria.

1. INTRODUCTION
Water which for its numerous usefulness, is often referred to as the source of life, is the most abundant liquid on earth. It is perhaps the most important resource by which the earth is endowed being the only planet in our solar system that has enough water to form ocean and seas. However, only a very small fraction(<0.1%) of the water on the planet is directly accessible to man as surface freshwater, notably rivers and lakes while the bulk of it as marine, icebergs and ground water are not easily accessible. Ground water can be harnessed through wells and boreholes and has been used extensively in many parts of the world. In terms of quality, ground water is the most reliable as it is usually low in anthropogenic pollution even in areas with shallow aquifer. However, ground water can be contaminated through mineralization of the rock composition of the terrain thereby making it unsafe for consumption. Inspite of this, ground water remains the main source of domestic water supplies in many part of the world especially in the rural and suburban parts of Nigeria where municipal pipe borne water supply are lacking or are not dependable.
The main objective of this study was to assess the effect of different underlying bedrocks on the chemical quality of ground water from hand dug wells in Ile-Ife area of southwest Nigeria. It is interesting to note that clean, safe and portable pipe borne water supply over the years has become inadequate for human consumption and as a result, as led to many households providing hand dug wells for their individual use and sometimes the public at large. Previous work relating to the present work investigated well water quality and its suitability for human consumption Adediji, A. and Ajibade, L.T. (2005). Only very few chemical parameters were investigated. In addition, Osunbitan et al. (2005) considered groundwater exploitation and highlighted the yield and quality requirement for irrigation demand for crops grown on a small scale. Furthermore, the preponderance of K+ Na+ and Ca2+ in preference to Mg2+ was reported to be the effect of soil characteristics on water quality Abo (2004). Ako et al., (1990), Etu Efeotor 1998, have also demonstrated the influence of underlying geology on the chemical quality of groundwater. Most of these works focused on suitability of groundwater quality for drinking and other use such as irrigation with little relationship established between the chemical parameter and the bedrock geology. However, this present work considered the contributions of the different bedrock geology to water quality and the order of suitability for drinking established. This necessitated the comparism of results with the WHO standards for drinking water. Also principal cluster analysis revealed the relationship of the different bedrock geology based on the water quality parameters better than the analysis of variance. Twenty four common physico-chemical parameters were considered in accessing the water characterization and were related with the underlying bedrock types of the area

2. LOCATION AND PHYSICAL SETTINGS
Ile- Ife conurbation is located approximately between latitudes 70 28.00’Nand 70 30, 03’N Longitude 40 32’E and 40 34.23’E on an elevation range of 260-300m above sea level. It covers an approximate area of 15.2km2 and falls within the Basement complex of Nigeria (Fig1). Ile-Ife is an ancient Yoruba city and home to the Obafemi Awolowo University. It is about 81km from Ibadan the capital of Oyo State and approximately 65km Southeast of Osogbo the Osun State capital. Ile- Ife serves as a major collection points for cocoa grain in the surrounding area for local markets. The inhabitants are primarily town dwelling farmers, the climate of the area is moist monsoon (according to Papadakis 1985).Temperatures are generally high, ranging from 200C to 320C with an annual mean of about 260C.
The rocks of the area are of Precambrian age and lies within the Basement complex of Nigeria. These rock types are Gneiss, Pegmatite, Pegmatite schist and undifferentiated schist (Adepoju 1981 cited Ako et al,1990) and are distributed as shown in fig 1. The rock outcrops are very few and can be seen at the university campus and some parts of Modakeke. The other parts are characterized mainly by laterite and lateritic sandstone.
Figure 1: Map of study Area

3. METHODOLOGY
A total of forty hand dug wells sampling stations covering the four major rock types in Ile-Ife areas were selected for this study. The sampling stations which were randomly selected comprised 10 stations each from the four rock types found in Ile –Ife. A total of eighty water samples were collected from ten (10) randomly selected wells in each area representing the four different bedrock types over the two seasons of the annual cycle. Most of the wells were unimproved with very few improved ones covered with wooden planks or sizeable metal plate. In order to avoid contamination from the use of a metallic container, a clean locally made rubber container was used to draw water from the well. Samples were stored in clean plastic container and adequately labeled for analysis.

3.1 CHEMICAL ANALYSIS OF SAMPLES
The samples were analyzed for sixteen chemical parameters. These samples were not subjected to filtration because they appear clean and clear but for the determination of total dissolved solid; a millipore filter was used. The samples were analyzed for chemical properties about a week after the last set of samples were collected. These are hydrogen ion concentration (pH), conductivity, Alkalinity, Acidity, TDS, Major ions- Ca2+, Mg2+, Na+, K+, Cl-, HCO3-, SO42-, Nutrient compounds -NO3-, PO43-, SiO2 and OM. The hydrogen ion concentration (pH) was determined using a lovibond pH comparator using the right indicator solution. Conductivity was measured using conductivity meter. Total dissolved solid was determined gravimetrically after sample was oven dried to constant weight. Calcium and magnesium were determined titrimetrically using di-sodium EDTA with Calcon and Eriochrome- T black as indicators respectively according to Golterman et al, (1978). Sodium and Potassium were determined by emission photometry using flame analyzer. Chloride was determined titrimetrically using Mecury II Nitrate method using diphenylcarbazone indictor according to standard method by Ademoroti(1996) while Sulphate was measured turbidimetrically using barium chloride procedure. Silica was measured using ammonium mollybdate method while phosphate was determined using Tin II Chloride colorimeric method. Nitrate was determined colorimetrically using Brucine method. Hydrogen carbonate was determined titimetrically using mixed indicator. Organic matter was determined using wet digestion method with potassium di chromate as the oxidant. Hardness was determined by the addition of calcium hardness and magnesium hardness expressed in mg/lCaCO3

4. RESULTS
4.1 CHEMICAL CHARACTERISTICS OF WATER QUALITY
Table 1 gives the chemical characteristics of water quality. The pH values ranged from 6.10 to 7.50 with virtually all the values occurring within the Bromothymol Blue range (pH 6.0 to 7.6) with an overall mean ± s.d pH value of 6.82 ± 0.49. All the samples from the granite gneiss area occurred within the pH range of 6.1 to 7.2, with a mean ± s.d. values of 6.6 ± 0.3. The samples varied from slightly acidic to slightly alkaline. Samples from pegmatite area varied from moderately acidic to slightly alkaline with a mean ± s.d. pH values of 7.0 ± 0.3. The samples from the pegmatite schist area varied from moderately acidic to moderately alkaline while those in the undifferentiated schist area varied from slightly acidic to moderately alkaline with a mean ± s.d. pH values of 6.8 ± 0.3. The pH values showed a highly significant difference (p < 0.01) between the different rock types. The values of the alkalinity and acidity were generally higher in pegmatite schist and pegmatite respectively, while the values of conductivity and TDS were higher in the undifferentiated schist. The mean conductivity values (893.0 ± 236.7Ѕcm-1) in the pegmatite (shallowest) area was higher than the mean conductivity value (784.42 ± 609.5Ѕcm-1) in the undifferentiated schist area (area with deepest wells). The samples from granite gneiss area had the lowest mean conductivity value of 705.8 ± 373.3Ѕcm-1(Table1). In general, conductivity values were observed to increase directly with pH values, showing a significant difference (p < 0.05) between the different rock types. Samples from the undifferentiated schist area were characterized by the highest conductivity values. Overall, alkalinity and acidity values ranged from 24.0mgCaCO3l-1 to 424.0mgCaCO3l -1 and 0.0mgCaCO3l-1 to 80.0mgCaCO3l-1 respectively. Granite gneiss area had the lowest mean ± s.d value of alkalinity (93.8 ± 49.2 mgCaCO3l-1) and highest mean ± s.d value (178.8 ± 119.1 mgCaCO3l-1), in pegmatite area.

Table 1: Mean and range values of the general chemical water quality parameters from different bedrock areas of Ile-Ife.

Alkalinity showed a significant difference across the different rock types (Table1.). Total dissolved solids ranged from 80mgl-1 to 1130.0mgl-1. It was highest in waters from the pegmatite schist area with mean ± s.d value of 669.5 ± 291.8mgl-1 and lowest in waters from the granite gneiss area with mean ± s.d value of 437.0 ± 21.0 mgl-1. Calcium and Magnesium ranged from 2.4mgl-1 to 110.2mgl-1 and 0.0 to 17.1mgl-1 respectively. Sodium ranged from 3.5mgl-1 to 136.2mgl-1, and potassium ranged from 0.0mgl-1 to 196.8mgl-1. Hardness ranged from 8.4 mgCaCO3l-1 to 317.0 mgCaCO3l-1 with an overall mean ± s.d value of 151.0 ± 8.6mgCaCO3l-1. Chloride ranged from 5.0mgl-1 to 372.0mgl-1, while HCO3- and SO42-mgl-1 ranged from 38.4mgl-1 to 508.8mgl-1 and 1.4mgl-1 to 167.1mgl-1 respectively (Table 2). The ionic dominance (in milliequivalent values) for the different bedrock types was Ca2+˃ K+˃ Na+˃ Mg2+ in the granite gneiss area, K+˃ Ca2+ ˃Na+ ˃ Mg2+ in the pegmatite area while in the pegmatite schist and undifferentiated schist area the trend was Ca2+˃ K+˃ Na+ ˃ Mg2+ and Ca2+˃ Na+ ˃ K+˃ Mg2+ respectively. Calcium and Potassium were higher in the pegmatite and pegmatite schist area compared to granite gneiss and undifferentiated schist area, while Magnesium and Sodium were higher in pegmatite schist and undifferentiated schist area relative to granite gneiss and pegmatite. Sulphate was highest in the pegmatite schist area.
The anionic dominance (in milliequivalent) for the different bedrock areas was HCO3- ˃ Cl- ˃ SO42. The ionic balance for the major ions investigated is 11.97%.

Table 2: Mean and Range values of major ions in well water from different bedrock areas of Ile-Ife.

Nitrate ranged from 0.0mgl-1 to 22.7mgl-1, Phosphate ranged from 0.0mgl-1 to 3.8mgl-1, while Silica and Organic matter ranged from 0-1mgl-1 to 9.4mgl-1 and 0.2mgl-1 to 7.0mgl-1 respectively. The mean ± s.d value of nitrate (11.5 ± 7.03mgl-1) was higher in the pegmatite schist and lower in the granite gneiss (Table 3) with an overall mean value of (6.5 ± 6.1mgl-1). Between the different bedrock areas, nitrate showed a very highly significant difference (p < 0.001). The Phosphate mean values were lower in the undifferentiated schist (0.2±0.3 mgl-1) and higher in the pegmatite (0.9±1.0 mgl-1).

Table 3: Mean and range values of nutrient compounds and organic matter of water quality parameter from different bedrock areas of Ile-Ife.

Fig. 2 Cluster Analysis showing relationship between the different bedrock areas of Ile-Ife based on water quality parameters and well characteristics

5. DISCUSSION
5.1 Chemical Characteristics and Contribution of Chemical Composition of Bedrock Areas to Water Quality
The one way analysis of variance test between the four different bedrock types showed statistically significant difference (p ≤ 0.05) in seven (7) of the sixteen (16) parameters considered. These seven (7) parameters include acidity, alkalinity, pH, conductivity, nitrate, silica, and organic matter.
The study area (Ile-Ife) lies within the basement complex of Nigeria and the rock types are crystalline in nature. They are mainly of igneous and metamorphic origins which are siliceous in nature. Therefore, the quality of groundwater is to a large extent dependent on the host bedrock type. This is evident in the cluster relationship of the four different rock types found in Ile- Ife (Fig.2). It is observed that GG and SP form a cluster while the pegmatite and undifferentiated schist which are further apart were established as the shallowest and deepest wells respectively (Table 2). It was also observed that the undifferentiated schist area had low concentration (mgl-l) of K+, PO43-, organic matter, and conductivity. On the other hand, pegmatite was abundant in ions (Ca2+, K+, Cl-, HCO3- and SO42+), acidity, alkalinity, conductivity and TDS. Pegmatite schist formed a cluster with granite gneiss while undifferentiated schist formed a cluster with pegmatite, and lastly pegmatite schist, granite gneiss formed a cluster with pegmatite. This agrees with earlier works of Ajayi and Adegoke-Anthony(1988) and Etu-Efeotor (1998) which demonstrated the influence of underlying geology on the chemical quality of groundwater.