Identifying Groundwater Fluoride Source in a Weathered Basement Aquifer in Central Malawi: Human Health and Policy Implications

Abstract

Consumption of groundwater containing fluoride exceeding World Health Organization (WHO) 1.5 mg/L standard leaves people vulnerable to fluorosis: a vulnerability not well characterised in Malawi. To evaluate geogenic fluoride source and concentration, groundwater fluoride and geology was documented in central Malawi where groundwater supplies are mainly sourced from the weathered basement aquifer. Lithological composition was shown as the main control on fluoride occurrence. Augen gneiss of granitic composition posed the greatest geological fluoride risk. The weathered basement aquifer profile was the main factor controlling fluoride distributions. These results and fluoride-lithology statistical analysis allowed the development of a graded map of geological fluoride risk. A direct link to human health risk (dental fluorosis) from geological fluoride was quantified to support science-led policy change for fluoride in rural drinking water in Malawi. Hazard quotient (HQ) values were calculated and assigned to specific water points, depending on user age group; in this case, 74% of children under six were shown to be vulnerable to dental fluorosis. Results are contrary to current standard for fluoride in Malawi groundwater of 6 mg/L, highlighting the need for policy change. Detailed policy recommendations are presented based on the results of this study.

Predicting Groundwater Vulnerability to Geogenic Fluoride Risk: A Screening Method for Malawi and an Opportunity for National Policy Redefinition

“…Seasonal effects on fluoride were excluded due to a negligible effect in Malawi, where minor seasonal variations have comparable spatial trends across the seasons (wet and dry) [18]. Geochemical studies suggest that rock-silicate weathering (of aquifer material) is the dominant source of groundwater fluoride [6,11] and was the focus of this study.…”

“…The collation was achieved and discussed within our earlier published works: In brief summary, fluoride data from groundwater quality analyses across Malawi were compiled, and augmented by field work carried out by the Climate Justice Fund: Water Futures Programme (CJF) for the period 2016-2018 [5]. Data were further augmented by our local study in Central Malawi [6]. Hot spring data were collated for this study (n = 63) from published literature [5,[42][43][44][45][46] and data provided by the Malawi Ministry of Forestry and Natural Resources (formerly Ministry of Agriculture, Irrigation and Water Development).…”

“…Our comprehensive mapping of rural water supplies in Malawi indicate international aid programmes have drilled nearly 60,000 boreholes and wells, most without prior hydrogeological knowledge or assessment of potential geo-hazards, with 24.5% resulting in poor performance and another 14.1% resulting in failure and abandonment [2][3][4]. The proportion suffering from poor water quality is unknown, however issues with iron, salinity, fluoride, and others are widespread [3,[5][6][7][8]. Considering the reliance on groundwater in Malawi, significant focus is needed to mitigate risks of geogenic fluoride and to sustainably assess and manage existing supplies.…”

“…Two distinct source types are evident: shallow weathering of aquifer rocks of diffuse occurrence; and highly localised hydrothermal inputs from hot springs along rift faults. Our approach herein is further underpinned by relationships found in our recent Central Malawi study setting where we mapped geogenic fluoride risk using statistical relationships identified between groundwater fluoride concentrations and aquifer lithology [6]. Zones of generic geogenic fluoride risk could be mapped, based on the statistical likelihood of a lithology producing groundwater with fluoride concentrations exceeding the WHO standard of 1.5 mg/L.…”

“…Zones of generic geogenic fluoride risk could be mapped, based on the statistical likelihood of a lithology producing groundwater with fluoride concentrations exceeding the WHO standard of 1.5 mg/L. Whilst the method developed used fluoride concentration data with corresponding lithological data, we [6] proposed the method could be extended to cover lithologies without corresponding fluoride concentration data where extrapolations on likely fluoride content may be reasonably made based on the wider international geogenic source literature.…”