## 5 Discussion  
### Geological Fluoride  
Augen gneiss poses the highest potential for elevated groundwater fluoride, ascribed to its granitic-type (alkaline) composition. The augen gneiss is a metamorphosed granite and hosts an abundance of fluoride-bearing minerals such as hornblende, biotite (and accessory apatite) along with characteristic Na-feldspar mega-crystals which are an additional source of sodium (recognised to be conducive to fluoride enrichment). Solubility of fluoride-bearing minerals is generally low. One dimensional reactive-transport equations have shown long residence times are required to produce appreciable concentrations of fluoride in groundwater from silicate rocks, however, higher reactive surface area can significantly increase the rate at which concentrations accumulate [42]; meaning, weathered basement aquifers will have significantly increased fluoride mobilisation potential. Samples represent recent recharge (low mineralisation) and it is likely that increased reactive surface area of the weathered basement aquifer (Saprock, saprolite and fractured genesis) is a key control on the release of fluoride and may account for observed concentrations. Our geological, geochemical and hydrogeological data support this hypothesis. Fluorite solubility is usually expected to control fluoride concentrations in groundwater leading to a proportional relationship between F- and Ca2+ ions in solution. Clay minerals and fine sands are moderate adsorbents for fluoride [3], while clay minerals can also be a source of Na+ for ion exchange with Ca2+, furthering the potential for fluoride enrichment [3; 7]. Water type, therefore, plays an important role in fluoride concentration. Lowest fluoride is expected in recharging water, naturally increasing with groundwater evolution through: Ca-CaCO3 (recharge); Ca-Mg-HCO3; Na-HCO3; Na-Cl (endmember) [3; 28]. While elevated fluoride is expected in geochemically evolved waters, it is commonly found in Ca-Mg-HCO3-type where there are basement lithologies [39]. This corresponds with our study area where the dominant water type is Ca-Mg-HCO3 and basement lithologies dominate. Figure 4 shows that Mg2+ increases northwest. The loss of Ca2+ for Mg2+ may facilitate the increasing fluoride trend seen in that direction as there is less Ca2+ available to precipitate fluorite (CaF2). Groundwater samples from augen gneiss plot closest to equilibrium in Figure 5, indicating that equilibration of fluorite (ultimately controlled by equilibration of calcite) is an active process in those samples. Regional groundwater flow (northwest) is unlikely to be the dominant process producing the hydrochemical trends in that direction (Figure 4). This is due to the shallow, uneven and seasonally isolated nature of the hydrogeological profile (Figure 3) and low hydraulic conductivity values in those aquifers. Local groundwater flow may, however, have increased influence (albeit minimal) during the wet season where there is higher potential for sub-aquifer connectivity via an elevated water table (Figure 3). The surface of unaltered gneiss beneath the study area is locally uneven, with regolith aquifer thickness ranging from 0-60 m below the surface. Elongated gneiss mounds \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Lithology** & **\(n\)** & \multicolumn{3}{c}{**Hazard Quotient (HQ) = > 1**} \\ \hline  & & **Adults** & **Children** & **Children** \\  & & **(>19 years old)** & **(12 years old)** & **(6 years old)** \\ \hline Augen gneiss & 16 & 50.00\% & 68.75\% & 93.75\% \\ Hornblende-biotite gneiss & 3 & 33.33\% & 66.66\% & 100.00\% \\ Charnocktic gneiss and granulite & 20 & 10.00\% & 20.00\% & 55.00\% \\ All & 39 & 28.21\% & 43.59\% & 74.36\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary statistics for HQ values >1 per lithology. Values are percentages of sampled water points with an HQ value >1 for each lithology. striking southwest-northeast often breach the surface exposing the uneven nature of the bedrock. These may be potential barriers to groundwater flow.