## 2 Study Area  
### Setting  
Malawi is situated at the southern periphery of the EARS (Figure 1). In central and northern Malawi, it features as a deep freshwater lake, Lake Malawi [24]. In southern Malawi, the valley floor is exposed at the surface as a series of sedimentary basins [7]. The traditional authority (TA) Mazengera study area is in the district of Lilongwe, central Malawi (Figure 1). It lies on the eastern edge of the Kasungu-Lilongwe Plain, an elevated plateau composed of Lower Palaeozoic-Precambrian basement rock and colluvium of around 6000 km\({}^{2}\) with elevations varying from 1000-1800 m above sea-level (masl). Drainage in the study area generally flows northwest from the Nkhoma and Chilenje hills towards the Lilongwe River, and southeast towards the Linthipe River (Figure 1). Figure 1: Location of study area within traditional authority (TA) Mazengera alongside [PERSON] et al. [13] study area at Nathenje area. The figure depicts Malawi’s location within the east African rift system, marks the locations of all known hot springs and provides 3-D visualisation of the study area topography and drainage (2\(\times\) vertical exaggeration), above a 3-D visualisation of groundwater surface indicating the main recharge and discharge points (5\(\times\) vertical exaggeration). Climate in the region is sub-tropical/semi-arid with distinct wet (November-April) and dry (May-October) seasons [25]. Monthly rainfall at the peak of the wet season (January) can reach an average of 222 mm, but falls to \(<\)1 mm at the dry season height (August). Mean annual precipitation averages 883 mm [26]. This type of climate is conducive to shallow groundwater fluoride enrichment that is found in a similar environment in northeast Sri Lanka. There, the semi-arid climate appears the main factor controlling enrichment due to a number of processes including low precipitation (recharge) and evaporative concentration [15]. Under semi-arid conditions, evapotranspiration at the surface where the water table is shallow may increase fluoride concentrations by a factor of 10-100 [3]. Fluoride occurrence in the study area had not been well characterised prior to this study. The Malawi Government internally released a 'Hydrogeology Atlas (2018)' containing six data points displayed simply as \(<\)6 mg/L fluoride [27]. Only one study has conducted fluoride research nearby [13] (Figure 1) which documented fluoride occurrence in the groundwater of Nathenje area, about 20 km southwest of our study area (Figure 1). They sampled 176 boreholes over two years (2001-2002) during both wet and dry seasons. Basement geology comprised an identical range of gneiss lithologies to our study area, however, their western section has considerable colluvium (superficial sediment) which is mostly absent in our case. Over 50% of their samples had fluoride concentrations exceeding the WHO 1.5 mg/L drinking water standard. Concentrations were slightly higher during the dry season, but with spatial trends of highs and lows in concentration comparable across the seasons. Concentrations increased east-northeast leading them to hypothesise high fluoride may be anticipated out with their study area in a geological trend in that direction, towards TA Mazengera, but do not elaborate on specific lithologies. They concluded that in-situ weathering of fluoride-bearing minerals in the basement rock (biotite and hornblende in weathered gneiss) is the probable contributing source of fluoride to groundwater. Highest groundwater fluoride is typically associated with the rift valley hydrothermal processes and alkaline igneous intrusions [7; 28]. Low Ca\({}^{2+}\) groundwaters were identified as target waters for elevated fluoride (Na-HCO\({}_{3}\), Na-Cl-types) and the breakdown of biotite and hornblende (constituent phases of the basement gneiss of Malawi) is a probable source of fluoride, as fluoride can replace OH\({}^{-}\) groups in the crystal lattices of those minerals [13; 28].