## 3 Results
### Groundwater Quality Compliance with Regulatory Standards
Although microbiological contamination of groundwater sources is much less of a problem than for surface water sources, it can still prove problematic in Malawi arising from flood event or sanitation sources [58; 62; 63]. Fluoride and arsenic identified by the UN's Joint Monitoring Programme as chemical contaminants of concern for'safely managed drinking water' [64] were not analysed in our survey, but data are shown from our previously published national-scale assessments [65; 66]. Observed basin fluoride (Figure S14) reveals only the Mpyupyu Hot Spring exceeds the current Malawi drinking water 6 mg/L (former WHO) guideline with all but one sample in the northern and central Basin below the current \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & \multicolumn{2}{c}{**Requlation Applied**} & \multicolumn{2}{c}{**Other Malawi and WHO Criteria for Comparison**} \\ \hline  & & **Malawi Standard** & **Malawi Standard** & & **WHO (2017)** & \\  & & **MS733-2005** & **MS 214:2013** & & **Health-Based** & \\  & & **MPL** & **Drinking Water** & **Guideline** & **Asesthetic or Taste** \\  & & **Water from** & **Specification-Upper** & **Value** & **Threshold Estimate** \\  & & **Boreholes** & **Limit** & **(\% Exceeding)** & **(\% Exceeding)** & **(\% Exceeding)** \\  & & **(\% Exceeding)** & & & & \\ \hline EC \({}^{2}\) (μS/cm) & 275 & 3500 (3.6\%) & 1500 (23\%) & N/A & \\ TDS (mg/L) & 275 & 2000 (4.0\%) & 500 (55\%) & N/A & 1000 \({}^{3}\) (22\%) \\ Cl\({}^{+}\) (mg/L) & 74 & 750 (1.4\%) & 200 (6.8\%) & N/A & 250 (4.0\%) \\ SO\({}_{4}^{2-}\) (mg/L) & 74 & 800 (-) & 200 (1.4\%) & N/A & 250 (1.4\%) \\ NO\({}_{4}\) (mg/L) & 74 & 45 (-) & 45 (-) & 50 (\(\cdot\)) & \\ Na \({}^{+}\) (mg/L) & 74 & 500 (1.4\%) & 50 (46\%) & N/A & 200 (6.8\%) \\ Ca\({}^{2+}\) (mg/L) & 74 & 250 (9.5\%) & 150 (18\%) & N/A & 300 (6.8\%) \\ Mg\({}^{2+}\) (mg/L) & 74 & 200 (-) & 70 (18\%) & N/A & 500 (\(\cdot\)) \\ Fe\({}^{2+}\) (mg/L) & 74 & 3(\(\cdot\)) & 0.2 (62\%) & N/A & 0.3 (55\%) \\ \hline \hline \end{tabular} \({}^{1}\) Maximum Permissible Level (MPL); \({}^{2}\) electrical conductivity at 25 \({}^{5}\)C; \({}^{3}\) unpalatable water ‘brackish’ threshold; N/A denotes ‘not applicable’ as standard not defined for parameter.  
\end{table} 
Table 1: Phalombe District 2018 groundwater quality survey summary data compared to water quality standard criteria with percentages of samples exceeding in parentheses. WHO 1.5 mg/L guideline. In the southern basin, including Phalombe District, a mix of concentrations occur below 1.5 mg/L and between 1.5 and 4 mg/L. These observations align with our national expectations of predicted risk (Figure S15). Fluoride <1.5 mg/L is anticipated in supplies drawing from basin/lacustrine sediments, and where increased probably due to thinning sediments and boreholes penetrating weathered granite or spenite beneath. Increased 1.5-4 mg/L fluoride may be expected in supplies drilled into alkaline igneous intrusions (granite, syenite, carbonatite). Hot springs may display fluoride >6 mg/L, but their lateral influence as shown for Mpyupyu is restricted with low fluoride found in surrounding boreholes (Figure S14). The lack of major faulting suggests that hot springs found are using the vertical boundaries between the alkaline intrusions and country (host) rock as vertical conduits to transport deep-seated groundwater upwards to the surface, or else the steep geothermal gradient present in the thin crust Rift Valley resulting from shallow heating from active rifting is causing convective vertical transport of meteoric water/shallow groundwater via intrusion-country rock boundaries acting as vertical conduits. The latter is more probable, both Mpyupyu and Chis Island hot springs are adjacent to alkaline intrusions (Figure S14). Our national survey of arsenic found widespread low concentrations with only a few above the WHO 10 ug/L guideline that were usually associated with hot spring/geothermal groundwater, often with elevated fluoride [66].