Assessment of microbial and heavy metal contamination in shallow hand-dug wells bordering Ona River, Southwest Nigeria

Ona River is one of the three major rivers draining the city of Ibadan. Groundwater is the major source of drinking water in the metropolis; however, data on quality of shallow aquifers bordering Ona River is relatively scarce. This study aimed to evaluate bacteriological status, heavy metal content, and associated human and ecological health risks in hand-dug wells nearby Ona River. A total of 24 water samples from 12 sampling points were collected for chemical and microbial analyses. Heavy metals and microbial pathogens were analyzed using atomic absorption spectrometry and total plate count methods, respectively. Analyses of microbial and heavy metal (HMs) data showed that shallow hand-dug wells within the vicinity of Ona River were bacteriologically contaminated while most of analyzed heavy metals (except manganese) exceeded the drinking water quality standards. Interpretation of microbial and heavy metal (HMs) data identified predominance of anthropogenic activities as the major source of contamination in drinking water. Further scrutiny of HM data through integrated pollution indices identified two nearby wells ( S 7 and S 8 ) exceed the safe limits and pose considerable risk to inhabitants. In terms of ecological risk index (ER), cadmium exhibited considerable to very high ER in all collected samples while manganese and zinc showed low ER in all analyzed water samples. Potential of non-carcinogenic risk through ingestion pathway in the study area was identified with the order of contributive ratios by HMs as Cd > Pb > Zn > Fe > Mn. The calculated target hazard quotient (THQ) due to ingested HMs for three human population categories exceeds the safe limit in the order of adult