GROUNDWATER CHARACTERIZATION IN PARTS OF THE LOWER BENUE TROUGH, NIGERIA

ABSTRACT

Groundwater constitutes the main sources of water supply in part of lower Benue Trough (mainly Gboko area). Sustainable development and management of the resources require an understanding of its holistic characteristics. In this study, an attempt has been made to study the groundwater quantitative potential distributions, its geochemical evolution, geochemical characteristics, and quality. The methods of investigation employed included pumping tests, static water level measurement, groundwater and subsurface soil samplings. The results of the pumping tests carried out within Gboko metropolis showed that the Gboko Central has the lowest groundwater potential with transmissivity value of  8.208m²/day while Gboko Low Cost area has the highest groundwater potential with transmissivity value of 281.52m²/day. The variation in the groundwater potential is attributed to varying degree of weathering. Nitrate concentration in the studied groundwater ranged from 0mg/l (below detection limit of 0.02) to 156.85mg/l and its concentration has a negative correlation (correlation coefficient of -0.0997) with water well depth. Nitrate was also found to pollute the groundwater of the area with its contamination observed to result from both agricultural practices and wastes, and also from domestic wastes. The groundwater from the urban land use (especially the poor income earners with poor environmental sanitation) have more dissolved nitrate than those from the rural agricultural land use. The concentration of each heavy metal tested in groundwater of the area varied spatially throughout the area. Co, Fe, Pb, Ba and Mn were found to be above the stipulated WHO 2008 guideline values. The sources of heavy metals in the groundwater were found to be both geogenic origin (weathering of host rocks) and anthropogenic (agricultural wastes and practices, domestic wastes and urbanization effects). The groundwater of the area is not generally potable as 84% of the groundwater samples tested was found to have heavy metal content. Principal component analysis revealed 12 controlling processes that are affecting the hydrochemical characteristics of the groundwater. Some of these processes include weathering of host rocks REE minerals, geogenic and anthropogenic contamination, uranyl complex factor, alkaline earth metal and heavy metal factor, redox potential factor, mixed or diverse contamination sources, metallurgical material wastes factor, and arsenic contamination factor. Cluster analysis grouped the groundwater samples of the area into 16 groups with various groups falling under high or low pollution loading. The results of the rare earth elements (REEs) showed that the groundwater of the area is characterised with predominance of light rare earth elements (LREEs) formed under an oxidising condition over heavy rare earth elements (HREEs) formed under a reducing condition. Also, the groundwater of the area all showed positive Ce and Eu anomalies. The positive Ce anomaly may be attributed to oxidation of Ce³⁺ to Ce⁴⁺ and incorporation of Ce into Mn and Fe oxyhydroxides phases while positive Eu may be attributed to weathering of zircon rich minerals. The δ¹⁸O composition of the groundwater samples of the study area ranged from -3.14 ^o/_oo to -28.45 ^o/oo, δ²H ranged from -28.45^o/_oo to -14.53^o/_oo,and the groundwater showed a general depletion in both δ¹⁸O and δ²H. Also, 85% of the groundwater of the area was formed or recharged under a cooler climate than the present day. The aquifer vulnerability GOD model classified the aquifers of the area into three zones namely, the high vulnerability, the moderate vulnerability and the low vulnerability zones. The moderate and low vulnerability zones predominate in the area. The results of the subsurface soil samples analysis revealed that the soils in the area have been polluted by As, Cd, Ag, Au, Mn, Mo, Ni, Pb, Co, Cr, Cu, Hg, Sb and V. It was observed that both geogenic sources and anthropogenic activities mainly agricultural activities and urbanization are contributing to the contamination and pollution of the soils.

CHAPTER ONE

INTRODUCTION

• GENERAL INFORMATION

Water resources, irregularly distributed in space and time, are under pressure due to the combination of both naturally occurring conditions and anthropogenic actions. The need to develop more sustainable practices for the management and efficient use of water resources as well as the need to protect the environmental ecosystems where these resources are located, has led to major shifts in awareness and public concern over the years. Economic convenience and political criteria continue to govern water resources development decisions at most local, regional, national and international levels, without taking into account environmental issues.

Population growth and rapid economic development have accelerated freshwater withdrawals. Water use shows high variability globally, both within continents and across users.

The consumptive uses of freshwater from agriculture, industry and domestic sectors place the greatest pressures on natural systems, both in quantity and quality. In many countries of the world, agriculture is by far the main user of water.  According to Food and Agriculture Organization, FAO (2013) irrigated agriculture accounts for about 70% of water withdrawals from available sources followed by industry (19%) and domestic water supply (12%). The annual freshwater withdrawals by regions for various uses are presented in Table 1.About 92% of global water uses (agriculture, industrial and municipal supply) are met by withdrawals from renewable sources, either surface water or groundwater. In many areas of scarce freshwater resources, treated brackish water and wastewater are often used to meet demand. Again, according to FAO (2013) around 20% of the total water use demand globally is from groundwater sources, and about 74% is from surface water. Groundwater is the major source for drinking water, while surface water is for irrigation, energy and industry.

The importance of groundwater is gaining recognition because this resource constitutes the predominant reservoir and strategic reserves of freshwater storage in the world. It represents 30% of the freshwater resources, and as much as 96 percent of the fraction in liquid state (WWAP, 2006; Shiklomanov and Rodda, 2003). Groundwater supplies more than 1.5 billon urban dwellers with water and it is extensively used for rural water supply. Groundwater is the predominant source (about 55%) for public water supply

Table 1: Annual freshwater withdrawal by region

*Sub Saharan Africa; sources: UNDP et al 2000; World Bank, 2000; FAO, 1995

due to its generally higher quality than surface water (EEA, 2009). In some locations it provides a more reliable supply than surface water in the dry seasons because of its large storage capacity. Moreover, groundwater resources are cheap to obtain because of their widespread occurrence and they generally present good natural quality (Zaporezec, 2002). Groundwater is less vulnerable to anthropogenic impacts than surface water bodies, because it is naturally protected by the soil and the confining strata. However, as a result of large storage and long residence times when aquifers become polluted, contamination is persistent and difficult to reverse (Clarke et al., 1996).

Throughout the world, most countries’ practices of urban and industrial development, agriculture and mining activities have caused groundwater contamination. Water quality is influenced by both direct point sources and non-point (diffuse) pollution. Diffuse pollution from farming activities and point source pollution from sewage treatment and industrial discharge are the principal contaminant sources. Concerning agriculture, the key pollutants include pesticides and organic fertilizers. The contaminants most commonly associated with organic fertilizers are oxygen-demanding substances, ammonia, nutrients (particularly nitrogen and phosphorus), sediments, pathogens and odorous compounds. Organic fertilizer is also a potential source of salts and trace metals, and to a lesser extent, antibiotics, pesticides and hormones.

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