Groundwater Dye Tracing in Lusaka, Zambia: Delineation of Contribution Areas for Drinking Water Sources

Lusaka, Zambia is a fast-growing city with more than 2 million people straining the karst groundwater which supplies 50% of its population. A well-developed epikarst underlying much of the city is often exposed by mining, increasing groundwater vulnerability. In densely populated areas, without proper waste or sewage disposal, drinking-water quality is severely degraded. To sustainably manage groundwater quantity and quality it is important to know groundwater recharge areas, flow paths, and travel times. Groundwater dye tracing is a commonly applied tool for such investigations but has not previously been used in Zambia.

Investigations focused on the community of Muloni, an unplanned development on the Cheta Limetsone west of Lusaka. Adjacent to Muloni is a commercial quarry where groundwater is intensively extracted. Muloni residents derive water from springs, boreholes, and hand-dug wells. Dye-tracing goals included delineation of source-water areas, quantification of groundwater travel times, and assessment of the possible effects of pumping at the quarries on drinking-water sources. Water quality assessments, based mainly on E. coli concentrations, were used to select the initial sites for the dye-trace injections.

Water quality results suggested separate recharge areas for some of the deep boreholes and the quarry (15 m below the village) which were relatively free of E. coli. The springs, shallow wells, and some boreholes seemed directly plumbed into the epikarst with E. coli concentrations in excess of 300 CFU/100 mL. Pumping from the quarries has resulted in no apparent reduction in spring flow.

Charcoal dye receptors were placed at springs, quarries, and shallow wells. Five liters of approximately 400 ppb Rhodamine WT and Fluorescein were injected into two different latrines upgradient of springs and wells. Receptors were collected and analyzed weekly over about three months, but no breakthrough was observed. Dye tracing experiments continue with higher dye concentrations at additional injection points.