## 4 Results  
### Hydrochemical Observations  
Groundwater in the study area is exclusively Ca-Mg-HCO\({}_{3}\) type. Hydrochemical data (HCO\({}_{3}\)\({}^{-}\), Ca\({}^{2+}\), Na\({}^{+}\), Mg\({}^{2+}\), F\({}^{-}\) and pH) were plotted with TDS alongside a TDS map (Figure 4). Groundwater is least mineralised around the Nkhoma and Chilenje Hills. Two natural springs were sampled at the base of the southeast slope of Chilenje Hill. These springs contained the least mineralised groundwater. Increasing fluoride concentrations follow a southeast-northwest trend, with the highest fluoride at locations with the highest HCO\({}_{3}\)\({}^{-}\), Na\({}^{+}\) and Mg\({}^{2+}\). Low TDS samples from Augen gneiss display notably higher (an order of magnitude) Na\({}^{+}\), Mg\({}^{2+}\) and F\({}^{-}\) signatures than low TDS samples from charnockitic gneiss and granulite. Concentrations then increase with TDS following a similar trend in both lithologies. Geochemical data can be viewed in Table S2 in the supplementary materials.  
Correlation is not observed between fluoride concentrations and temperature data that span 21-27 \({}^{\circ}\)C (groundwater temperatures in Malawi not influenced by geothermal sources are typically around 20-30 \({}^{\circ}\)C [7]). Temperatures appear to increase northwest with elevation decline (Figure 1) corresponding to where the water table may be shallow and more vulnerable to surface temperature fluctuations.  
Fluoride was plotted with Ca\({}^{2+}\) alongside fluorite (CaF\({}_{2}\)) equilibrium (Figure 5). For water with TDS above 200 mg/L, a few water samples approach fluorite saturation but this may be a common ion effect with calcite. Low TDS samples (least mineralised-recent recharge) all plot well below equilibrium  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Risk Exposure** & \multicolumn{2}{c}{**Values for Age Groups**} & \multicolumn{1}{c}{**Unit**} \\
**Factors** & \multicolumn{2}{c}{**Values for Age Groups**} & \multicolumn{1}{c}{**Unit**} \\ \hline  & **Adults (>19 Years Old)** & **Children (12 Years Old)** & **Children (6 Years Old)** & \\ \hline C & & & (mg/L) \\ DI & 2 & 1.7 & 1 & (l/day) \\ F & 365 & 365 & 365 & (days/year) \\ ED & 19 & 12 & 6 & (years) \\ BW & 70 & 40 & 15 & (kg) \\ AT & 6935 & 4380 & 2190 & (days) \\ RJD & 0.06 & 0.06 & 0.06 & (mg/kg/day) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Values used to calculate the hazard quotient (HQ) for the study area water points per age group (adapted from: [38; 39]). ‘C’ values are not included as they represent individual fluoride concentrations for each water point and vary from water point to water point.  
Figure 4: Total dissolved solids (TDS) map of the study area with log plots of relevant hydrochemical concentrations vs. TDS. Map shows TDS values increasing northwest in the direction of decreasing altitude. Graphs show increasing trends in relevant ions with increasing TDS (as seen on the map).  
displaying both low Ca\({}^{2+}\) and low fluoride (Figure 5); all are located near the foot of the Nkhoma and Chilenje Hills (Figure 1).  
A Gibbs diagram plot confirms rock weathering is the dominant process controlling groundwater composition (Figure 6). Just a slight incline towards evaporation suggests minimal evaporation influence on shallow groundwater geochemistry in this system.