Pollutant removal efficiency and reliability of sand dams as a sustainable in situ rainwater harvesting option in dryland environments: a case study from South-Eastern Kenya's drylands

ABSTRACT The graphical abstract illustrates groundwater system fluxes in a sand dam, an in-situ rainwater harvesting system, showing key inflows (recharge from surface runoff) and outflows (evaporation, seepage, water abstraction) and major water uses. Water quality is assessed by comparing influent and effluent concentrations to determine pollutant removal efficiency. Positive removal efficiencies were observed for turbidity, TSS, dissolved oxygen, BOD, COD, ammonia, phosphates, iron, and copper, whereas negative removal efficiencies occurred for electrical conductivity, TDS, total hardness, fluoride, sulphates, nitrates, magnesium, zinc, chromium, and total coliforms. Overall, sand dams demonstrated high effectiveness in removing turbidity-related pollutants but limited reliability for salts, metals, and microorganisms. Drylands sustain billions of people and ecosystems. In Kenya, drylands cover 89% of the country, home to 36% of the population, 70% of the livestock and 90% of the wildlife. These groups are particularly vulnerable to water scarcity, mitigated through sand dams, an in situ rainwater harvesting technology. These function as natural slow sand filters to provide drinking water; yet, their filtration performance remains understudied. This study therefore examined their filtration capacity. Three sand dams were purposively selected and equipped with temporary sand-point wells to sample outflow, while stormwater inflow was sampled to assess removal efficiencies of 23 water quality parameters. A one-sample two-tailed t-test was used to evaluate whether the mean removal efficiency differed significantly from the assumed value. Results showed that 9 of the 22 parameters differed significantly from the hypothesized value of 60% (p < 0.05), whereas 13 did not (p ≥ 0.05). Sand dams effectively reduced suspended solids (85.68%), turbidity (86.94%) and iron (72.2%) but exhibited negative removal efficiencies for microorganisms and metals. Negative efficiencies may reflect environmental and operational complexities rather than treatment system failure. Therefore, water collected from sand dams for drinking should undergo household treatment such as boiling, chlorination or solar disinfection before consumption.