A comprehensive review of heavy metal contamination in selected boreholes and their impact on water quality management in Kenya

The primary water source among many communities worldwide is groundwater, usually prone to potentially toxic heavy metal contamination. Accordingly, this study comprehensively examines the concentrations of potentially toxic heavy metals in borehole water samples from various parts of Kenya. The review classifies counties into the eight regions—Coastal, Nyanza, Eastern, Western, Central, Rift Valley, North Eastern, and Nairobi. The study uses the traditional review approach to investigate the extent of heavy metal contamination in selected boreholes across Kenya, analysing data to identify contamination patterns, sources, and associated health risks. The findings indicate that concentrations of heavy metals such as lead (Pb), cadmium (Cd), arsenic (As), and chromium (Cr) exceeded the World Health Organization (WHO) permissible limits, particularly in boreholes located near industrial areas, mining zones, and urban settlements. For instance, lead concentration in boreholes within Nairobi’s industrial area, Langata/Karen, and Pangani were 0.15 mg/L, 0.42 mg/L, and 2.25–2.75 mg/L, respectively, representing, 15, 40, and 200 times higher than the WHO limit of 0.01 mg/L. Chromium with concentrations of 0.52 mg/L in Nairobi (Embakasi) and 0.422–0.569 mg/L in Kwale, is more than ten times the WHO permissible concentration. High exposure to such concentrations increases the risk of carcinogenic effects. The concentration of cadmium was found to be 0.08 mg/L, which exceeds the acceptable WHO limit of 0.003 mg/L by apprimately 25 times. Increased exposure to high quantities of cadmium increases the risk of tubular dysfunction. In one study, 0.061 mg/L of mercury was recorded in Kakamega County, which is 10 times higher than that recommended by the WHO, indicating a high etiological risk. Seasonal variations indicate mixed trends for the studies that depict temporal and seasonal changes in concentrations of heavy metals. The concentrations of arsenic, cadmium, chromium, nickel, and zinc were found to be higher during the dry seasons in Nairobi and lower during wet seasons possibly due to dilution effects. This work identifies critical gaps, including limited longitudinal studies, incomplete geographic coverage, and insufficient research for effective remediation strategies. Multiple studies conducted in the Rift Valley, Eastern, Western and North-Eastern regions do not provide sufficient information on seasonal variations of heavy metals in borehole water. Therefore, this review proposes a robust, nationwide monitoring and evaluation system to better understand the scope of heavy metal contamination and its health implications in borehole water management. Addressing these gaps will support evidence-based policy to enhance groundwater quality and safety through interventions by public health authorities. The study provides significant insights to inform the enforcement of available policies such as the Water Act of 2016, which provides mandatory regular monitoring and geo-referencing of boreholes in all counties in Kenya, and aligns with the United Nations Sustainable Development Goal #6 on clean water and sanitation.