4.1. Chemical and microbial properties of water as influenced by sand dam age and location of scooping holes
Generally, the pH of water abstracted from scooping holes in all the age groups of the sand dams was high and exceeded the maximum recommended levels by World Health Organization of 8.5 [
29]. The pH values observed in our study were higher than what has been reported in other semi-arid environments in Kenya such as Kitui [
18] and Makueni [
19]. In both regions, Ndekezi et al. [
18] and Ndunge et al. [
19] reported the water obtained from scooping holes to have an average pH of 7.6 which was almost 1 pH unit lower than that observed in our study. In our study, the high pH can be attributed to schist rocks which have been observed in the study area (Fig. 1). Schist contain appreciable amounts of carbonates and hydrogen carbonates of calcium, magnesium and sodium which can contributes to the high pH [30, 31]. Water with high pH and rich in calcium bicarbonate has been reported to reduce bone resorption, a critical process involved in transfer of calcium from bone tissue to the blood [
32]. Alkaline water also favors copper-pitting corrosion through cathodic reaction [
33] while reducing efficiency of water treatment through favoring formation of scum [5, 34]. Lack of significant influence of age of sand dams on water pH reported in this study, confirms the influence of rock as the key determinants of pH.
Nitrates and phosphates contents were low in scooping holes across all the age groups of the sand dams compared to the recommended levels by World Health Organization and Kenya Bureau of Standards of 10 and 2.2 mg L
-1, respectively [29, 35]. The contents reported in this study was similar to those obtained by Ndunge et al.[
19], where nitrates ranged between 0.2 and 2.6 mg L
-1 while phosphates was reported to be below 0.2 mg L
-1. The major source of these two nutrients in water is through sediments eroded from agricultural farms or from sewage effluents [
36]. Nonetheless, the low contents could be attributed to possible pollution from fecal materials mainly from livestock and organic materials retained within the sand dam. In addition, formation of CaPO
4 which precipitates out of the water as insoluble solids could further reduce the contents of phosphates. The nitrates also decreased with the age of sand dam, which could be an indication that the stabilization of the dam could have been contributing to better quality water. This could be attributed to N uptake by the vegetation regeneration in the sand dams over time. Regeneration of vegetation around the sand dams could reduce the nitrate content. For instance, water extracted from scooping holes adjacent to sand dam walls had higher nitrate content than that from scooping holes away from the dam wall. The mode of sand dam natural stabilization usually starts from the far end and the sides towards the center of the sand dam. Thus, the vegetation growth around the sand dam could contribute to the observed spatial differences in nitrate content. Besides, the higher amounts of sediments deposited adjacent to the sand dam wall during the initial stages of sand dam filling, which may lead to accumulation of more organic materials near the dam wall than away.
Generally, Fe contents in water abstracted from scooping holes across all age groups of sand dams exceeded WHO recommendations of 0.3 mg L
-1 [
29]. High content of Fe in drinking water could lead to conditions such as fibrosis [37, 38]. However, pathogenesis of Fe-induced fibrosis is not well understood nor has the critical Fe content in drinking water that could damage cells been established. In addition, high Fe content in drinking water reduces the aesthetic water quality by affecting its taste, smell and formation of microfilms in pipes and containers [
5]. Similar amounts of Fe to those reported in this study were reported in Kitui [
18] and in Makueni County [
19]. In Kitui, Ndekezi et al. [
18] reported an average Fe content of 1.9 mg L
-1 from water samples obtained from scooping holes. This was 0.6 mg L
-1 lower than what was reported in this study. Ndunge et al. [
19] on the other hand, reported an average Fe content of 1.5 mg L
-1. In our study, Fe content decreased with sand dam age, an indication of stabilizing of the sand dams and the improving water quality with age.
Water from scooping holes across all age groups of sand dams had TDS within the acceptable range of <1000 mg/L based on WHO standards [
29]. The values we obtained from our study were also lower than what has been reported in other similar studies. For example, in their study, Ndekezi et al. [
18] reported an average TDS value of about 500 mg L
-1 which was 400 mg L
-1 higher than the highest average readings obtained in the current study. In another study within the same region, Kitheka [
17] reported TDS values of up to 3,320 mg L
-1 for the water scooped from sand dams in river Thua in Kitui County. However, the author did not separate the TDS readings of dry season from wet season, despite alluding to influence of seasons in that study. Thus the high TDS in that study could have been as a result of taking readings during extremely dry periods since the dissolved inorganic and organic substances tend to increase during the dry season as more water is lost through evapotranspiration [39, 40]. The TDS values also decreased with age of the sand dam, an indication of sand dam stabilization. Reduction in TDS in the water with stabilization of the sand dams could be of significance in protecting the community in the study area since TDS levels greater than 1000 mg L
-1 could contribute to cardiovascular diseases, diarrhea, and abdominal pain [
4].
Salinity levels were low in our study area, being less than 1% in all the sand dams. This was more than 10 times lower than what was reported by Kitheka [
17] in Kitui County. Water hardness levels were within the allowable range of 100 – 300 mg L
-1 based on WHO standards [
29]. Both salinity and hardness of the water reduced with age of the sand dam.
Based on thermotolerant coliforms, the water abstracted from all the sand dams was not fit for human consumption without any form of treatment since no coliforms should be traced according to KEBS standards [
35]. The presence of coliforms could be attributed to contamination with faecal matter from both animals and humans, since
Escherichia coli was found in all the water samples. During collection of water samples, cattle and goats were observed drinking water from the old scooping holes that were made by the community members. Besides, the open nature of sand dams could exacerbate contamination of the water with faecal materials as noted by Quinn et al. [
15]. Several other studies have reported similarly high thermotolerant coliforms in water abstracted from scooping holes within semi-arid regions [15, 16, 19]. For example, Quinn et al. [
15] observed TTC in majority of water samples abstracted from scooping holes, from Machakos and Makueni Counties, with approximately 20% of the samples containing more than 1000 CFU 100 ml
-1. In a more comprehensive study that covered three counties of Eastern Kenya (Machakos, Kitui and Makueni) across the rainy and dry seasons, Neufeld et al. [
16] reported high TTC with an average of 800 CFU 100 ml
-1 in water samples obtained from scooping holes. Presence of high faecal coliforms pose a health risk in case of outbreak of diseases such as dysentery, diarrhea, hepatitis A, trachoma and skin infections [
25]. Though sand dam age did not have significant effect on TTC, location of scooping holes had significant effect on TTC t age 1-9 years indicating possible peak of decomposition of trapped organic materials and hence proliferation of coliforms. Stabilization of sand dams though encroaching vegetation could attract predators such as protozoa and bacteriophages which have been shown to prey on gram negative bacteria such as E. coli as suggested by McCambridge and McMeekin [
41] and Hobley et al. [
42].