Assessment of the presence of metals and quality of water used for irrigation in Kwara State, Nigeria

INTRODUCTION

The importance of water for human and other biological systems cannot be over emphasized (Suresh et al., 1991).Agriculture is the major consumer of water, as irrigation uses more than two-thirds of the World’s available freshwater resources (Shady, 1998). It is often assumed that water supply constitutes the primary limiting constraint to production (Doorenbos & Pruitt, 1976). Pollution is caused when a change in the physical, chemical, or biological condition of the environment harmfully affects the quality of human life, including other animals’ lives and plants (Okoye et al., 2002). Water pollution occurs when unwanted materials, potential to threaten human and other natural system, find their ways into rivers, lakes, walls, streams, boreholes, or even reserved fresh water in homes and industries (Naseem et al., 2010). The pollution of water bodies transport from surface run off and uncontrolled discharge of untreated and partially-treated sewage and domestic wastes is a major source of water pollution (Izonfuo & Bariweni, 2001). The contamination of the environment, food, and water by pesticides has become a considerable concern, leading researchers to investigate their occurrence, distribution, and concentration in several ecosystems (Shankar et al., 2004). Discharge of heavy metals into irrigation water, due to fertilizer and pesticide use by farmers can result in the accumulation of certain heavy metals and other dangerous substances (Ghafoor et al., 1999). The yield of salt sensitive plant is negatively affected by heavy metals accumulation in soils, thus posing serious risks to human health, when consumed (Misra & Mani, 1991; Wenzel & Jackner, 1999).

Toxicity in plants normally results when certain ions are taken up with the soil-water and accumulate in the leaves, during water transpiration to an extent that damages the plant. The usual toxic ions in irrigation water are chloride, sodium, and boron; their damage, depending on time, concentration, crop sensitivity, and crop water use (Eckenfelder, 1980).

Toxic metals are known to have serious health implications, including carcinogenesis-induced tumor promotion, hence the growing consciousness about health risks, associated with environmental chemicals has brought a major shift in global concern towards the prevention of heavy metal accumulation in soil, water, and vegetables (Ahmed et al., 2009). Excessive accumulation of trace elements in agricultural soils through irrigation will affect food quality and safety by constraining important plants growth nutrients like Potassium, Zinc, Phosphorus, and Nitrogen (Gibbs et al., 2006; Sharma et al., 2008). In irrigation pollution studies, water evaluation emphasis is on chemical and physical characteristics (Ayers & Westott, 1994). The quality of irrigation water is generally judged by its total salt concentration or Electrical Conductivity (EC), relative proportion of cations or Sodium Absorption Ratio (SAR), and bicarbonate and boron content of water (Michael, 1999). The quality evaluation of irrigation water for metal presence is, therefore, a critical management tool, necessary to increase agricultural production, decrease pollution, and improve food safety.

MATERIALS AND METHODS

Kwara State lies between latitudes 7°45 N and 9°30 N and longitudes 2°30 E and 6°25 E of Greenwich Meridian (KWSMI, 2002). The State falls under the tropical climate of distinct wet and dry seasons. The former lasts from late March/ April to late October/ early November with a mean annual rainfall of 1318 mm. The latter commences in October through December and ends in late March (Oyegun, 1983). The larger proportion of Kwara State belongs to dry sub-humid, while its north-western fringe belongs to semi-arid climatic regions (Olaniran, 1988). The mainstay of the state’s economy is agriculture and farmers specialize in arable crops such as groundnut, sorghum, cassava, yam, cowpea, maize, yam, and rice (KWADP, 2010).

Thirty locations in Kwara State were randomly selected for the collection of water samples. The locations were from Kwara North, Kwara Central and Kwara South senatorial districts. Figure 1 illustrates the location and positions of the sample collection points.

Fig. 1. Location and positions of sample collection points in Kwara State

The samples were collected from September 2015 to November 2015, from rivers, ponds, and streams that served as irrigation sources for agricultural production. Replicate water samples were collected from five different points in each location, being poured into 2-litre plastic containers as well as 200 cm³ reagent bottles. The bottles were properly cleansed with distilled deionised water, beforehand. Collection was carried out by careful immersion of the sample containers deep inside the water, next to be sealed with tight fitting corks and stoppers, in order to avoid air bubbles. Samples were transported to a refrigerator (4°C) in the laboratory for chemical analysis, according to the standard tests (APHA, AWWA and WPCF, 1975). The sodium adsorption Ratio (SAR) was calculated using the formulae, according to Michael et al. (2008).

The ata collected from each parameter were subject to analysis of variance (ANOVA), via theStatistical Package for Social Sciences (SPSS) version 15.0. The locations within a senatorial district were treated as replicates for that zone. The significant treatment means were separated using the Duncan’s New Multiple Range Test at 5% level of probability.

RESULTS AND DISCUSSION

Table 1 shows results from the analysis of some chemical properties and metal concentration in the water samples. The analyzed parameters were Total Suspended Solids (TSS), Total Dissolved Solids (TDS), Biochemical Oxygen Demand (BOD), Chemical oxygen demand, Sulphate ion concentration, Nitrate ion concentration, and Chloride ion concentration.

Total Suspended Solids (TSS) connote the presence of silt and other dissolved solutes that contribute to the turbidity of a water body and is considered a warning sign for pollution (Wilber and Clarke, 2001). TSS levels in the water samples were between 10.3 mg/L in Kwara North to 90.6 mg/L in Kwara South. One positive effect of the presence of suspended solids in water is that pesticides and metals tend to adsorb to them, thereby reducing toxicity effects. The observed levels were within TSS permissible limit of 30 mg/L, set by Federal Environmental protection Agency (FEPA, 1991). This implies that there was reduced incidence of pathogens in the irrigation waters, which is in agreement with Gray et al. (2000), who are also of the opinion that a TSS value above the permissible limit is an indication of the high presence of pathogenic organisms in the waters.

The Total Dissolved Solids (TDS) value of 70.3 mg/L was significantly the highest in Kwara North, compared to 80.4 mg/L in Kwara South and 30.6 mg/L in Kwara Central. The TDS value is a measure of the amount of mobile charged ions and minerals salts in a given volume of water. An elevated TDS value beyond the optimum level can induce toxicity in plants and possibly increase the osmotic potential of the soil water, the consequence of which is an increase in the amount of energy plants that must be expended to take up water from the soil.The TDS range (30.6-80.4 mg/L), obtained in the study, was far below the permissible range of 450-200 mg/L for irrigation water by FAO (1985). Mustapha (2008) obtained similar high TDS values while assessing the Water Quality of Oyun Reservoir in Offa, Kwara State, Nigeria.    

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen in water samples, being an effective indicator of organic quality of water that determines pollution levels (Clair et al., 2003). Results from the study showed that highest BOD levels were recorded significantly in Kwara South (31.4 mg/L) and Kwara Central (30.9 mg/L), with the lowest value belonging to Kwara North (13.2 mg/L). These values were far above the European Union value of 3.0-6.0 mg/L for safe BOD values for irrigation water (Chapman, 1996), indicating that the waters were highly polluted, possibly by pesticides and other pollutants. This was similar to the results of Adebayo and Usman (2009), who detected BOD values between 10 and 20 mg/L in Asa-River in Ilorin-Nigeria.

The Chemical Oxygen Demand (COD) is the amount of dissolved oxygen, a measure of the content of oxidizable organic as well as inorganic matter of the given water sample. Tepe et al. (2004) linked higher COD values to the possibility of water pollution. The values of the COD in the present study oscillated between 45 mg/L in Kwara South, 36.4 mg/L in Kwara Central, and 23.2 mg/L in Kwara North. They were above the limits of 20 mg/L for water bodies and, therefore, could be classified within the range of polluted waters, according to Chapman (1996). Kolawole et al. (2011), while assessing the water quality in Asa River in Kwara State-Nigeria also found a mean COD value between 5.0-58.4 mg/L. This was attributed, among other things, to be due to poorly-executed agricultural activities near the river banks.

Sulphate ion concentration was significantly the highest in Kwara South (7.0 mg/L) and Kwara Central (6.8 mg/L). Shinde et al. (2011) reported that sulfate ion is a major contributor to salinity in many irrigation waters and though it is rarely a problem, at very high concentrations it is leached into water sources, interfering with uptake of other nutrients in the soil. The concentration of sulphur, detected in this study, was below 250.00 mg/L, i.e. the recommended value set by FAO (2005). This could indicate that the farmer’s use of fertilizers was restricted and that there were no industries around the study area to use sulphates that can pollute the water sources.

Nitrate ion concentration was also significantly the highest in Kwara Central (8.9 mg/L) and Kwara South (8.7 mg/L), while it was the lowest in Kwara North (6.4 mg/L). These values were below 50.00 mg/L, i.e. the maximum limit recommended as standard for good water quality for irrigation by FAO (2005). Nitrogen has been proven to stimulate crop growth and its presence in irrigation water is most frequently in the form of nitrate ions. It is likely that nitrate ions, present in irrigation water, will have the same effect as soil-applied fertilizer nitrogen. It is, therefore, assumed that the irrigated plant optimum fertilizer requirement should be observed to obtain expected crop yield. In a similar study, Ogidiaka et al. (2012) also detected low nitrate ion concentration in surface waters of Ogunpa River at Bodija- Nigeria.

The lowest Chloride ion concentration was recorded in Kwara South (31.2 mg/L), while the highest values belonged to Kwara Central (67.0 mg/L) and Kwara North (65.2 mg/L). Chlorides are the most stable components in water; their concentration, largely unaffected by most natural physicochemical and biochemical processes (Aremu et al., 2011). Water, containing chloride concentration below 150 mg/L, is considered safe and can support healthy plant growth for most crops, provided that proper irrigation management practices are taken (Landon, 1991). The chloride concentration in the water samples analyzed stayed within the safe limits of 250 mg/L for irrigation water set by World Health Organization (2006).

Table 3 shows the result of analysis of the physico-chemical properties of the samples for temperature, pH, alkalinity, turbidity, Conductivity, and total hardness.

The temperature values of the water samples showed a significant difference (P˂ 0.05) between the three senatorial zones. The values recorded were 23.3^°C (Kwara South), 26.1^°C (Kwara Central), and 28.2^°C (Kwara North). These temperature values were within the range suitable for agricultural purposes in Nigeria, set by the Federal Ministry of Environment. Increased temperature normally facilitates the rate at which solutes dissolve, which improves mineral uptake and optimum plant growth (Xu et al., 2014). Brockwell and Gault (1976) had earlier confirmed a significant effect on growth of legumes by irrigation water temperature treatments below 20^°C than above. This implies that irrigation water temperature in the study area was suitable for the optimum growth of arable crops.

The pH of the water samples was significantly the highest in Kwara South (7.2) and had an average value of 6.0 and 6.2 in Kwara Central and Kwara North senatorial zones, respectively. The pH characteristic of irrigation water has a significance influence on soil and plant reactions. According to Arimoro (2007), irrigation water outside the range of 4.5 to 8.5 may contain toxic ions and can cause nutritional imbalance. Bauder et al. (2010) believed that suitable water for irrigation should have a pH range between 6.5 and 8.4. The pH values, recorded in the present study, were within suitable limits for irrigation and agricultural production. The values were similar to those, reported by Morriso et al. (2001), but were lower than the range of 8.94 -10.34, reported by Akpan et al. (2008).

The alkalinity levels in the water samples were not significantly different in the senatorial zones and ranged from 729.3 to 752.1 mg/L. Alkalinity provides information on the natural salts, dissolved in the water samples, being a measure of the water’s capacity to neutralize acids as well as an indicator of the productive potential of the irrigation water (Manahan, 1994). The values, obtained for this study, were far above WHO (2004) recommendation of standard of 80-120 mg/L. The high alkalinity observed in the samples may be due to agricultural and economic activities from overflow of industrial and agricultural wastes. Ogedengbe and Akinbile (2004) attributed high alkalinity levels to increases in the presence of bicarbonates and carbonates from effluents and leachates.

The turbidity value was significantly the highest in Kwara Central (22.1 NTU) and relatively the lowest in Kwara South (14.1 NTU). Turbidity is a measure of the quantity of solid matter, present in the suspended state of the water samples. The turbidity values reported in the study for the districts were far greater than 5.0 NTU, the limit given by WHO (2008), and this according to US EPA (2004), could cause pathogenic disease occurrence. Ololade and Ajayi (2009) in a similar study obtained high turbidity values in water samples obtained from major rivers along the highways in Ondo State, Nigeria, and attributed it to the presence of decaying organic matter. Adefemi et al. (2007) and Wakawa et al. (2008) had also reported very high turbidity values for some rivers in Nigeria as a result of runoff of pesticides into the water sources. The high values obtained in the present study could also be attributed to the same reason.

The highest conductivity value of 1174.7 µs/cm was measured in Kwara North, while the lowest value belonged to Kwara South (1157.8 µs/cm). These values fell below FAO's maximum acceptance level of 1200 μs/cm (1985) for irrigation water. This indicates that the water sources were not expected to have any salinity problems since the electrical conductivity is a parameter that summarizes the total dissolved solids, showing a correlation with higher alkalinity (Sandgren, 1993). The results found in this study, were similar to those, found by Igbinosa and Okoh (2009).

Kwara North presented the highest total hardness of 880 mg/L, followed by Kwara Central (640 mg/L), while the lowest value belonged to Kwara South (320 mg/L). Based on Tyson and Harrison (1990) classification of harness and softness in terms of mg/L (0-60 soft, 60-120 moderately soft, 121-180 moderately hard, and >180 hard), the water samples in this study could be classified as hard. Srinivasamoorthy et al. (2009) believe that hardness in water samples is mostly due to the leaching of Ca and Mg ions into the groundwater. Hardness does not affect plants directly, but has indirect impact on plant growth through the soil. It is assumed, therefore, that hardness observed in the study was caused by the leaching of ions into the water sources, used for irrigation. This assumption is in agreement with Nweke et al. (2013).

CONCLUSION

The study concludes that elements of water pollution varied with the collection site across Kwara State. While some of the analysed parameters, compared favourably with acceptable standards for safe use for irrigation purposes, others showed levels above tolerable limits. The values of water quality indices, particularly the sodium adsorption ratio, were intensely high for locations in the Kwara North senatorial district. It is, therefore, recommended that regular assessment of irrigation water quality should be carried out and non-point source pollution prevented by the regulated use of agro-chemicals and fertilizers by farmers in Kwara State, Nigeria.