222Rn isotope as a tool for monitoring functionality of water wells

Abstract

In Sub-Saharan Africa, hand pump-fitted water wells (HPWs) are characterized by poor functionality marked by rapid post-construction decline in yield. A substantial number of the HPWs show a low degree of reliability and poor water quality. Monitoring changes in performance is prerequisite to inform preventive maintenance of the HPWs. Borehole performance monitoring often requires a logistically demanding pumping test procedure. Here we demonstrate the applicability of a naturally occurring Radon-222 isotope (²²²Rn) as a complementary tool to monitor post-construction performance of HPWs. We measured ²²²Rn recovery (the ratio of ²²²Rn in the HPWs to that of the aquifer) in 32 HPWs and the host aquifers. Pumping and reliability tests have been conducted on the HPWs before taking the ²²²Rn measurements. The HPWs have been classified into four functionality classes a) high yield and reliable, b) high yield but unreliable, c) low yield and reliable and d) low yield and unreliable. In the first category, there is a high ²²²Rn recovery revealing a quick through flow of groundwater in the wells. This further demonstrates the healthy functioning of boreholes without screen clogging effects and a high permeability of the aquifer material in the vicinity of the well. The fourth category shows the lowest ²²²Rn recovery revealing a slow flow of water in the well owing to low permeability, declining water level, screen clogging, poor initial design and high water stagnation in the boreholes. The substantial difference in ²²²Rn recovery between the four categories reveals the isotopic tracer can be used as a promising independent tool to monitor post-construction changes in the performance of HPWs without the need for dismantling the HPWs for inspection.