Impact of Gas-Flaring on the Quality of Rain Water, Groundwater and Surface Water in Parts of Eastern Niger Delta, Nigeria

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Plate 2. Cluster of zinc roofs with various degree of rusting/caving-in

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Plate 3. Death of aquitic animals in the rivers in the area due to acid-rain caused by gas flaring

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2.2. Physiography, Geology and Hydrogeology of the area

The area is a low land and characterized by two distinct seasons: a dry season (November to March) and rainy season (April to October). The study area is underlain by the coastal plain sand of Miocene age (Figure 1). The study area is underlain by unconfined aquifer of regional extent. Recharge into the unconfined aquifer is by direct infiltration of rainwater. The formation is made up of friable, fine to coarse grained sands with minor clay intercalations (Onyeagocha, 1980). It consists of water-bearing, porous and highly permeable continental sands. Records from drilled boreholes indicate shallow groundwater table while pumping test analyses show high hydraulic characteristics in terms of yield, transmissivity and storativity. Petrographic study on several thin sections (Onyeagocha, 1980) shows that quartz makes up more than 96% of all the sand grains. The general thickness of the coastal plain sands is variable and ranged from 180 m to 2000 m (Avbovbo, 1978). The annual groundwater recharge ranged between 20% and 30% of the annual rainfall of about 2500 mm (Uma and Egboka, 1987).

Figure 1. Geology map of the study area

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Figure 2. Digital terrain model of the study area overlapped with contour and flow lines

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2.3. Sampling and Laboratory Analysis

Altogether 38 samples including 20 groundwater samples, 10 rain water samples and 8 surface water samples were collected and analyzed. All the samples were preserved by refrigeration and analyzed within 24 hours of collection. The water sampling and analyses were carried out in accordance with American Public Health Association (APHA, 1995), standard for sampling and analyzing water and waste water. The physical parameters such as pH and conductivity were determined on the field using a Gallenkamp pH meter and Hach conductivity meter WPA 400 digital model respectively. The coordinates of each samples location was taken using the global positioning system and the data was used to generate the digital terrain model of the area overlap with the contour map and groundwater flow direction (Figure 2). The heavy metal concentrations were analyzed using atomic absorption spectrophotometer (Model: YOUNGLI AAS 8010).

3. Results and Discussion

The mean concentrations of some of the parameters analyzed are shown in Figure 3 and Figure 4 while the scatter plot and correlation graph of the different water sources analyzed are illustrated in Figure 5 and Figure 6 respectively. The Conceptual Model of the environmental and socio-economic impact of acid-rain is illustrated in Figure 7. The effect of gas flaring on the quality of the water sources was studied using hydrochemical data from water samples in the vicinity of the gas flaring stations. The concentration of the major anions (bicarbonate, sulphate and nitrate), temperature, conductivity, heavy metal content (HM) and total hydrocarbon content (TH) increase appreciable in the vicinity of the gas flaring stations and consistently decreased away from the gas flaring points (Figure 3 and Figure 4). Their enrichment in the water sources may be attributed to the gas flaring activity in the area (Uma, 1989; Amadi et al., 2012). The rain water samples show concentration and variation trends similar to those of surface water and groundwater (Figure 3). The concentrations of pH, moisture content (MC) and bacteria count (BC) were lowest at vicinity of the gas flaring stations but increases away from the gas flaring points. The increased acidity of the rain water in the gas flaring stations may be responsible for the low pH (5.2) as control samples taken far away the gas flaring stations have higher pH (6.7). Similarly, the low concentration of MC and BC can be attributed to the intense heat emanating from the gas flaring station. Such heat (elevated temperature) may be capable of drying up the soil moisture and kill the bacteria in the soil/water as bacteria cannot withstand high temperature (Figure 4).

Figure 3. Graph of water sources and their concentrations

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Figure 4. Graph of concentrations versus distance (km) to gas flaring station

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Figure 5. Scatter plot of water sources versus concentrations in mg/l

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The similarity in distribution pattern of the geochemical constituents from the different water sources (Figure 5) in the vicinity of the gas flaring stations is a clear indication that that the enrichment of these ions are associated with the gas flaring activity. A comparison of the distribution pattern (Figure 6) and the groundwater flow direction (Figure 2) revealed a striking similarity. The gas flaring station overlies a groundwater recharge zone and the direction of the decreasing ionic concentration is parallel to the direction of groundwater flow away to the discharge zone (Amadi et al., 2014). This relationship is an indication that the movements of contaminants (TH, HM, SO₄^2-, HCO₃^- and NO₃^-) ions are likely assisted by advection, which is the contaminant transport by the bulk volume of the migrating groundwater. Surface water sampled at the discharge area of the groundwater flow from the gas flaring stations also showed evidence of geochemical enrichment. The results are consistent with the concentration of the ions in the rain water and groundwater samples from the locations.

Figure 6. Correlation analyses of water sources versus concentrations in mg/l

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Figure 7. Conceptual Model of the Environmental and Socio-economic impact of Acid-rain (Amadi, 2013)

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