Solid wastes are being produced everyday by residential, commercial and agricultural sources as a direct consequences of human activities. In an attempt to dispose of these large volume of daily wastes, man has carelessly polluted the environment especially surface, groundwater, soil and air through leachate and landfill gases. Pollution of groundwater is a major threat posed by leachate which is formed by anaerobic decomposition of waste and may infiltrate and join the aquifer (Tesfaye, 2007). According to Freeze and Cherry (1979), water table mounding and gravity causes leachate to move through the subsurface soil to the bottom and sideway until it reaches the groundwater zone thereby polluting the groundwater. With the inconsistent variation of groundwater table soil condition and contamination by leachate plume through percolation, infiltration and seepage, groundwater quality determination assumes greater significance in the field of water quality management (Mohan et al., 1998). At Ibadan, the capital of Oyo State in southwestern part of Nigeria, there is scarcity of pipe borne water due to non-availability and inadequate presence of laid down pipe in most parts of the city. Consequently, groundwater from hand-dug wells serves as an alternative and major source of water supply for domestic purposes. Siting of dump site within the vicinity of residential areas can contaminate groundwater quality of wells bordering the landfill. The use of polluted groundwater for drinking and consumption purposes can cause major health problems. According to WHO, about 80% of all diseases in human beings are caused by water (Ramakrishnaiah et al., 2009). Therefore, a periodic assessment of groundwater quality is necessary in order to ascertain the quality of water to be used for human consumption purpose as well as to provide an overall scenario about the sources of groundwater contamination, thereby open an avenue for better planning to achieve sustainable management of groundwater.
The Groundwater Quality Index (GWQI) indicates the overall quality of waters in terms of a single value at a certain location and time, based on several water quality parameters (Saeedi et al., 2010). It is a mathematical equation used to transform large number of water quality data into a single number (Stambuk-Giljanovic, 1999).
It is also one of the most effective ways of communicating the information on water quality trends to the general public and policy makers in water quality management. It is associated with the need to provide a general means of comparing and ranking various bodies of water throughout a particular region (Armah et al., 2012). Moreover, GWQI assessment is important in assessing the spread of water-borne diseases as several epidemiological studies advocated that greater percentage of human diseases in the world are due to poor quality of drinking water.
Several researchers have evaluated groundwater quality using indexing method. Sayed and Gupta (2013) investigated the quality of groundwater samples from hand-pump and bore wells in Beed City of Maharashtra India. The quality of groundwater in Tarkwa Gold Mining area in Ghana was assessed using GWQI method by Armah et al. (2012) while Gupta and Roy (2012) evaluated spatial and seasonal variations in groundwater quality at Kolar Gold Fields, India, Rao and Nageswararao (2013) used the method of GWQI to assess the quality of groundwater at Greater Visakhapatnam city using water quality index method.
In this study, groundwater samples from hand-dug wells within the vicinity of landfill were surveyed to analyze physico-chemical characteristics of water for the assessment of safe drinking water source and seasonal variation of GWQI for hand-dug wells around dump site to ascertain their suitability for drinking and consumption purposes.
MATERIALS AND METHODS
Study area and its local geology: Ibadan is located approximately within the squares of longitude 30 351-40 101 7 east of the Greenwich meridian and latitude 70 201-70 401 north of the equator. In this locality, wastes are dumped indiscriminately on open grounds in so many places. There are several collection points from which refuses are cleared by government trucks at regular intervals and deposited at the central landfill sites managed by the government. The city generates about 1,618,293 kg of solid waste daily. There are four designated dump sites (open landfill) in Ibadan namely: Aba-Eku, Ajakanga, Awotan and Lapite. For this study, the study area is Ajakanga landfill in southwestern part of Ibadan. Ajakanga landfill lies between longitude of 30 50 187-30 50 696 E and longitude 70 18 021-70 18 979 N. It was opened in 1998 and still in operation till date. The study area falls within the humid and sub humid tropical climate of southwestern Nigeria with a mean annual rainfall of about 1230 mm and mean maximum temperature of 32°C.
The geology of the area is a basement complex formation of southwestern Nigeria and are mainly the metamorphic rocks of precambrian age with few intrusions of granites and porphyries of Jurrasic age. The dominant rock types are quartzite of metasedimentary series, banded gneiss, augen gneisses and migmatites which constitute the gneiss-migmatite complex. Other minor rock types include pegmatite, quartz, aplites, anphibolites and xenolith (Okunlola et al., 2009). Banded gneiss constitutes over 75% of the rocks in and around Ibadan while augen gneisses and quartzites share the remaining in about equal percentages (Okunlola et al., 2009), as shown in Fig. 1.
Collection of samples: Ten water samples were collected from hand-dug wells bordering Ajakanga landfill in the month of March and August, 2013 inside 2 L polyethylene bottles. The bottles were washed thoroughly with dilute nitric acid and then rinsed with water. Prior to sampling, sampling bottles were rinsed thoroughly with groundwater to be analyzed before sampling process. The samples were collected in different seasons, dry season in March, 2013 and wet season in August, 2013. Preservation of water samples and analyses were carried out as per standard methods of APHA (1998). Parameters such as pH, TDS and EC were measured in situ with the aid of multipurpose conductivity meter. The depth of the well, depth to static water level and geographic coordinates of the sampling points were also taken on the field during both seasons (Table 1). The sampling locations and dump site are depicted in Fig. 2. Sodium and potassium were determined with flame photometric method, calcium and magnessium concentration were analyzed using absorption mode of Atomic Absorption Spectrometric (AAS) method. Sulphate and nitrate were analyzed by turbidimetric and UV spectrophotometric method, respectively, chloride, carbonate and bicarbonate by titration method while total hardness was determined by Ethylene Diamine Tetra Acetic Acid (EDTA) titration method using Eriochrome black-T as an indicator. The obtained chemical parameters were used for the computation of GWQI from the point of view of assessing suitability for drinking and human consumption purposes during both seasons.
Groundwater quality index: For GWQI analysis, 11 parameters consisting of pH, TDS, TH, HCO3¯, Cl¯, NO3¯, SO42¯, Na+, K+, Ca2+ and Mg2+ in each sample were assigned a weight (wi) according to their relative importance in the overall water quality for drinking purpose. Nitrate was assigned maximum weight of 5 due to its major importance in water quality assessment. The weight of other parameters varied from 2-5 depending on their significant importance in water quality determination. The relative weight of chemical parameters is shown in Table 2.
In the second step, the relative weight (wi) is calculated using the equation:
where, Wi is the relative weight, wi is the weight of each parameter and n is the number of parameters. In the third step, the quality rating scare (qi) was calculate by using:
|Fig. 2:|| Map of the study area showing water samples locations
|Table 1:|| Well parameters for Ajakanga water samples (dry and wet season)
where, qi is the quality rating, Ci is the concentration of selected parameter in mg L-1 and Si is the WHO drinking water standard (WHO, 2007).
|Table 2:|| Relative weight of chemical parameters
|Table 3:|| Water quality index scale
For calculating the GWQI, the sub index SIi is first determined for each parameter which is then used to determine the GWQI using the Eq. 3:
Based on GWQI value, quality of water was assessment using the water quality index scale (Mishra and Patel, 2001; Sindhu and Sharma, 2007). This is shown in Table 3.
RESULTS AND DISCUSSION
The concentration of water quality parameters have been compared with the drinking standard prescribed by Nigerian Standard for Drinking Water Quality (NSDWQ) and World Health Organization (WHO, 2007) for both sampling periods and the percentage compliance as shown in Table 4.
Water quality parameters around Ajakanga landfill: The pH values of water samples during dry and wet season sampling periods ranged from 6.97-7.81 and 6.71-7.33, respectively. The result did not vary significantly in both seasons. All pH values for the two seasons lie within the permissible limit. The TDS concentrations for both dry and wet seasons varied from 88-299 mg L-1 and 95-351 mg L-1, respectively. Seasonal changes showed the highest (299 mg L-1) value at S1 (90 m to the gate of Ajakanga landfill) during dry season and highest (351 mg L-1) at S10 (well inside Garden Farm) in wet season.
Comparison of water quality parameters with drinking water standard for dry and wet season
Dumping activities might have caused high value of TDS in well 1 while agricultural runoff and animal husbandry practice might have caused high value in well 10. The observed values are within the permissible limit. Electrical conductivity measures the amount of dissolved ions in a solution. The EC value showed highest value of 598 mS cm-1 at well 1 in dry and 705 mS cm-1 at well 10 during wet season. All EC values in both season lie within the standard limit of WHO and NSDWQ.
The average concentration of Total Hardness (TH) varies form 46-406 and 116-432 mg L-1 during dry and wet sampling periods, respectively. Highest value of TH (406 mg L-1) was observed in well 10 during dry and 432 mg L-1 during wet season in well 1 (about 90 m to the landfill). Based on Sawyer and McCarty (1967) classification for total hardness, 20% of water samples revealed Soft class, 40% showed Hard class, 30% revealed Moderate class while 10% falls under Very hard (as shown in well 10) during dry season. During wet season sampling period, none of the samples fall under Soft class of hardness, 10% revealed Moderate class, 60% indicated Hard class while 30% showed Very hard class. At all sampling locations, total hardness was higher in wet season than in dry season.
The chloride concentration of water samples during dry and wet seasons ranged from 16-113 and 10-53 mg L-1, respectively. The observed values for chloride in both seasons were within the permissible limit. Nitrate concentration in groundwater and surface water is normally low, ranging from 1.54-15.9 mg L-1 in dry season and 0-3.9 mg L-1 during wet season. The low concentration of nitrate value for the study area during both sampling periods were found to be within the limit of 50 mg L-1 specified by WHO. Seasonal variations of bicarbonate in groundwater showed higher value of 586 mg L-1 at well 14 during dry season and 610 mg L-1 at well 20. All bicarbonate values for both seasons at all sampling locations lie within the specified standard limits. Sodium concentrations in groundwater ranged from 12-30 and 11-24 mg L-1 during dry and wet seasons period. High value of 30 mg L-1 was observed in well 1 during dry season while well 6 and 10 have highest value of 24 mg L-1 during wet season. There is no significant seasonal variation of potassium. The lowest and highest concentration value in both seasons are the same. The lowest (1 mg L-1) concentration was found at well 3 in dry and the highest (6 mg L-1) at well 4 during wet season. The low concentration of K+ in groundwater may be due to the fact that most potassium bearing minerals are resistant to decomposition by weathering process and fixation in the formation of clay minerals (Scheytt, 1997).
The calcium concentrations during both sampling periods ranged from 1.32-49.2 mg L-1 and 2.01-173.4 mg L-1, respectively. At most of the locations, calcium values were higher in wet than dry season. Highest values of 49.2 and 173.4 mg L-1 in both dry and wet seasons were observed in well 10. The magnesium concentration value ranged from 1.12-14.23 and 3.29-49.32 mg L-1 during dry and wet seasons, respectively with well 10 having highest value in both seasons. The average concentration of calcium in all analyzed water samples lie within the specified limit of WHO and NSDWQ.
The computed GWQI values for 10 sampling locations in dry and wet season are given in Table 5. The minimum and maximum values of GWQI indicate the range of water quality of sampling locations in both seasons. In dry season, 50% of water samples belong to Excellent class while the remaining 50% belong to Good class. In wet season, the range of GWQI showed that 70% of analyzed groundwater samples belong to Excellent class while 30% belong to Good class. The dilution properties due to rain might be the reasons for improved water quality in wet season. It was observed that even at the same sampling location, the quality of water varied for some sampling locations.
At location S1, S2 and S10, the water quality is Good in both dry and wet seasons. However, at well 4 ( S4) it is Good in dry season but Excellent in wet season. Similarly at S6, the water quality was Good in dry season but Excellent in wet season.
The GWQI values of groundwater samples valued from 16.8-38.4 and 11.4-48.9 during dry and wet seasons, respectively. The status of water samples during dry and wet season sampling periods based on Mishra and Patel (2001) are presented in Table 5.
The degree of a linear association between any two parameters as measured by Pearson correlation coefficient for both seasons are presented in Table 6 and 7 for dry and wet season, respectively. It was observed that there is very strong association between EC and TDS, carbonate and bicarbonate for both seasons.
|Table 5:|| GWQI of sampling locations in dry and wet season
|Table 6:|| Correlation coefficient of Ajakanga water samples parameters during dry season
|***Correlation is significant at the 0.05 and 0.01 level (2-tailed), respectively|
|Table 7:|| Correlation coefficient of Ajakanga water samples parameters during wet season
|***Correlation is significant at the 0.05 and 0.01 level (2-tailed), respectively|
This buttress the fact that EC depends largely on the quality of the dissolved salts present in the sample.
The result of physicochemical parameters of groundwater from hand-dug wells at ten different sampling locations showed that most groundwater samples fall within the standard limit by WHO and NSDWQ. Effect of leachate and agricultural runoff might caused higher concentration of some parameters in wells 1 and 10, respectively. Assessment of GWQI values show their fitness for drinking and consumption purposes as GWQI values during both seasons fall below 100. Water quality Index showed more Excellent status in wet season than in dry season.
The highest value of GWQI during both sampling periods were observed at well 10 which might be due to agricultural runoff, leaching of fertilizers and low depth of the well. The high value of GWQI at well 10 has been found to be mainly due to higher concentration values of TH, HCO3¯, Ca2+ Mg2+ and TDS.
Based on the results of physicochemical parameters analysis of water samples, the groundwater can be used for drinking and consumption purposes. The analysis of GWQI concludes that the groundwater of the study area fall within the Excellent and Good category, thus fit to domestic purpose.