The physicochemical parameters obtained from analysis of the water samples collected from the hand-dug wells are presented in Table 1. The well water within the study area had pH values ranging from 7.32 ± 0.07 in well A6 to 7.50 ± 0.09 in Well A2. Water in all the wells was slightly alkaline but had their pH within the WHO recommended range of 6.5–8.5 for drinking water (WHO 2010). At these pHs, the efficiency of dissolving and leaching of toxic metal from the surface through the soil to groundwater is insignificant (Chotpantarat et al. 2014). Therefore, pH had no influence on the levels of metals in the groundwater.
EC values of the groundwater samples are presented in Table 1. EC gives an indication of the amount of total dissolved substitution in water (Yilmaz and Koc 2014). The EC values obtained ranges from 117 ± 3.66 to 421 ± 2.83 µS/cm. The average electrical conductivity of the water sample recorded was 215 ± 116 µS/cm. Well A6 recorded the highest conductivity value and the lowest value was recorded by well A4. The average values of the E.C for all the wells were below the WHO recommended values that range from 1000 to 1500 µS/cm. The highest mean concentration of the EC recorded in the shallow wells within the study area was below the WHO recommended limit of 1500 µS/cm. The relatively low conductivity of the groundwater samples for the entire study area gives a picture of very little solute dissolution generally in the shallow groundwater (Oyem et al. 2014).
WHO does not have any value as the acceptable limit for alkalinity in water for domestic use, because alkalinity does not pose any known health risk to humans. But the USGS states that concentrations above 300 mg/L HCO3 will have some effect on the palatability of the drinking water. Well A1 recorded the highest concentration at 372 ± 2.44 mg/L HCO3, and the lowest concentration of 220 ± 0.25 mg/L HCO3 was recorded at well A3. Wells A1 and A6 had the alkalinity levels above the USGS limit.
Studies have shown that regular consumption of hard water can have a lowering effect on the rate of cardiovascular disease. However, there are some negative effects associated with the used of hard water. These include wastage of soap, staining of washed clothes and scaling of pipes and boilers (Afuye et al. 2015; Sengupta 2013). This study found that the total hardness of water from shallow groundwater wells in this area ranged from 206 ± 4.09 mg/L in well A2 to 921 ± 3.82 mg/L in well A6. WHO recommends total hardness levels in water to be 500 mg/L. This implies that the average concentration of hardness of the sampling area is acceptable for human consumption. No health-based guideline value has been proposed for hardness; however, the degree of hardness in water may affect its acceptability to the consumer in terms of taste and scale deposition.
High chloride levels in water may arise from sources such as runoff containing salts, the use of inorganic fertilizers, landfill leachates and septic tank effluents (Ugboaja 2004; Afuye et al. 2015). Although chloride in water seems not to pose any health hazard to humans, consumption of an excessive amount of it can lead to some health hazard even though the ion is known to be non-cumulative. High chloride levels may also render freshwater unsuitable for agricultural irrigation (Afuye et al. 2015). Concentration of Cl− in the water samples from the study area ranges from 95.36 ± 2.11 to 499.68 ± 3.12 mg/L. Well A6 recorded the highest Cl− concentration which is above the recommended WHO limit of 250 mg/L. All the remaining wells recorded values below the WHO limit for drinking water with well A2 recording the lowest chloride concentration (Table 2) variation in Cl− concentrations among the wells was significant. This is an indication of possible anthropogenic influence in the Cl− concentration in the wells.
Health studies have shown that the addition of F− to water supplies in levels above 0.6 mg/L of F− could lead to a reduction in tooth decay especially in growing children. On the other hand, consumption of water with F− concentration above 1.5 mg/L leads to dental fluorosis and in extreme cases skeletal fluorosis (Kumar and Puri 2012). Concentrations of fluoride recorded vary from 0.06 ± 0.02 to 0.33 ± 0.11 mg/L. Well A5 recorded the highest fluoride concentration and well A1 recorded the lowest. Fluoride levels in all the samples were below WHO recommended concentration of 1.5 mg/L (Kumar and Puri 2012). Thus, the water from the wells does not pose any F−-related health issues on consumers. There was no significant variation in F− concentrations within the sampled wells.
Sulfides of metals that are present mainly in sedimentary rocks can be oxidized to sulfate which can leach and contaminate shallow groundwater. Excess sulfate in drinking water has a laxative effect, especially in combination with other metals such as Mn and Na (Chindo et al. 2013). The use of water for domestic purposes is therefore limited by high sulfate concentrations; hence, a limit of 500 mg/L has been set by the WHO for drinking water. The sulfate levels in water from the shallow wells ranged within the study area ranged from 47.14 ± 2.19 mg/L in well A2 to 856.78 ± 4.11 mg/L in well A6. Sulfate content of all the well water was below the WHO limit set for drinking water except that of well A6.
Contamination of NO3− in groundwater may result from runoff of inorganic fertilizers from farmlands and soluble NO3− compounds from nitrogenous waste products in human and animal excreta that may leach into the wells (Chindo et al. 2013). Nitrate is therefore one of the most common groundwater contaminants in rural or farming areas. High level of NO3−in drinking water can cause methemoglobinemia in infants. Nitrate can also be converted to nitrite after ingestion which can react\ with organic compounds to produce N-nitroso compounds in the stomach. Many of these N-nitroso compounds are carcinogenic to humans (Xu et al. 2015). Concentrations of NO3− in the water samples ranged from 17.20 ± 1.76 in well A6 to 169.95 ± 2.19 mg/L NO3− in well A1. Wells 1A, 2A, 3A and 5A had their mean NO3− concentrations above the WHO recommended value of 50 mg/L for drinking water. The high concentrations of NO3− may be of high potential health risks to human especially infants who use this water from these wells for domestic purposes including drinking. Even though no significant variations were observed in the NO3− concentrations within the water samples from the individual wells, significant variation in NO3− levels was observed among the different wells.
Calcium is one of the essential elements that are beneficial to human health, though is one of the ion that contribute to water hardness. Waters which are rich in calcium possess a high degree of hardness and are very palatable. Adequate intake of calcium helps in strengthening bones and teeth (Sengupta 2013). There is some evidence indicating that the incidence of heart disease is reduced in areas served water with a high degree of hardness. Concentrations of calcium determined in water from the shallow wells within the sample area range from 72.00 ± 1.23 to 308.80 ± 2.11 mg/L. Well A6 recorded the highest value, while well A2 recorded the lowest concentration as presented in Table 2.
Concentrations of magnesium in the water samples ranged from 28.24 ± 2.71 to 118.72 ± 2.11 mg/L, and well A6 recorded the highest concentration and well A2 recorded the lowest. Generally, the concentrations of Na and K in all the water from the wells were low. Levels of these metals in all the wells were below their recommended values by the WHO for drinking water. Statistical analysis showed that there were no significant variations in the concentrations of Na and K within the wells as well as among the different wells considered for this work.
Some heavy metals are useful to the human system in very small quantities. Iron, for example, is made a part of some multivitamin drugs and products. Although iron is a very important dietary requirement in the human system, too much iron in drinking water could cause gastrointestinal upset, nausea, vomiting and constipation in severe cases, iron toxicity could lead to organ damage, coma or even death (Grazuleviciene et al. 2009). Concentrations of Fe recorded in the water samples ranged from BDL to 0.46 mg/L. Two of the wells (A2 and A4) had their Fe concentrations above the WHO recommended. The mean Fe concentrations recorded in the other wells were below the WHO recommended value of 0.3 mg/L.
Zinc is also an essential micronutrient in human beings (Patil and Ahmad 2011). But at very high concentrations, it may cause some toxic effect. Zinc concentrations recorded in the water samples ranged from BDL to 0.047 ± 0.01 mg/L (Table 2). The mean concentration of Zn in all the samples was below the WHO recommended value of 3.0 mg/L.
Long-term exposure to even low concentrations of Cd can cause adverse health effects on the kidneys and lungs (Dutta et al. 2015). In this study, Cd concentration ranges from BDL in well A4 to 0.04 mg/L in well A2. Cadmium levels in all the water samples from the wells were high than the WHO recommended value of 0.005 mg/L except that of well A4. The high levels of Cd in the water samples may be due to anthropogenic activities that include fertilizer use. This is because the main activity in the study area is agriculture. The use of fertilizers is known to increase Cd in the environment (Wu et al. 2010).
Lead exposure is associated with multiple health effects on different organ systems at both elevated and at distinctive levels (Health Canada 2013; Brown and Margolis 2012). At elevated levels, lead can lead to seizures and eventually death. At low levels of lead exposure, there is a large body of evidence to demonstrate that lead is associated with a number of different neurological and developmental outcomes (Health Canada 2013; NTP 2012; US EPA 2013). In this study, well A6 recorded mean Pb level that was higher than the WHO recommended value of 0.01 mg/L Lead (Table 3).
Ingestion of water containing significant amount of metal is toxic and poses risks to human health. Concentration of the heavy metals that were considered in this work (Cd, Pb, Fe and Zn) in the selected wells shows that Cd and Pd toxicities are present in some of the well samples since their concentrations in some of the wells were high than the WHO accepted limit for drinking. Based on the metal metals levels in the water samples from the wells, non-carcinogenic hazard through ingestion and dermal exposures were assessed and the results presented in Table 4. In this study, the HQing ranged from 1.05E − 03 for Zn in well A6 to 2.52 for Cd in well A2. The HQing values for Cd in wells 2, 3 and 5 were observed to be higher than the safety limit of 1, whereas that recorded for Fe, Zn and Pb in all the well water samples were far below the safety limit, this demonstrated that these metals could not pose adverse health effects to the recipients via ingestion of contaminated water. Again the estimated HI values due to ingestion of water from the wells selected in this study ranged from 2.45E−01 in well 4–2.54 in well 2 Table 4. The HI in few of the wells were less than 1, indicating small to no hazard to the local population; however, high non-carcinogenic hazards were recorded in well 2, 3 and 5 indicating a potential hazard to the health of the local residents.
On the other hand, the HQderm values of all the metals detected in the water samples were found to be lower than one (1), indicating that there was no health risk associated with water samples via dermal absorption or skin exposure to the inhabitants (Table 5). Zn in well A1 recorded the least HQderm, while Cd in well A5 recorded the highest. However, the estimated levels of HIderm were also less than the safe limit of 1, indicating that there were no cumulative potential adverse health risks in water samples via dermal absorption. Well A3 and A5 recorded the highest cumulative potential for non-carcinogenic health risks due to dermal exposure as presented in Table 5.
Heavy metal such as Cd and Pb that were considered in this work is known to have the potential to produce carcinogenic risk when one is exposed (De Miguel et al. 2007; Tchounwou et al. 2014). The probable carcinogenic risks posed by these metals through ingestion for the residents within the study area were estimated using Eq. 5. The results showed that CRing for Pb in the water samples was 3.69E−06 in all the well water samples where Pb was detected. The risk of developing cancer from Pb as a result of consuming water from wells A1, A2 and A5 showed no significant differences (p > 0.05). In general, EPA considers excess cancer risks that are below about 1 chance in 1,000,000 (1.0E−06) to be so small as to be negligible. In this study, there is a possibility of 3–4 persons in every 1,000,000 developing cancer as results of lifetime exposure to Pb in water from the wells. Cancer risk computed for Cd ranged from 1.82E−02 in a well A5 to 9.09E−02 in well A1 and A6. These risk values indicate that consumption of water from these wells would result in an excess of 2–9 cancer cases per 100 people. Cadmium emissions have increased dramatically in recent years, reason being that cadmium-containing products are seldom recycled, but ends up at refuse dump together with household waste. Recent data indicate that adverse health effects of cadmium exposure may occur at lower exposure levels than previously anticipated resulting in lung cancer, kidney damage and also bone effect and fractures. Therefore, measures should be taken to reduce Cd exposure in order to minimize the risk of adverse health effects (Järup 2003) (Table 6).