## Materials and Methods
### Phalombe District Water Quality Survey  
A water quality survey was undertaken to assess (handpump) groundwater quality across the Phalombe District supported by targeted surface water sampling and collation of water-level data [43,44]. Rationale for selection of the Phalombe District is provided by the mWater data, which highlighted that groundwater-sourced water points for which communities report 'poor water quality' were predominantly located in that district (Figure S3). The survey question defines poor water quality in a local community context as \"salty, bad taste, colour, or other contamination\", therefore the specific quality problem is usually not apparent. Further, historical information of poor water quality may, or may not be based on a laboratory analysis. Figure S3 hence provides a qualitative indicator of groundwater quality issues that appear predominantly within the Basin superficial aquifers (comparing to Figure 2b). Survey design within the District was developed by Phalombe District Water Office staff who identified 98 boreholes providing spatial coverage with reliable supporting data, supplemented by 34 African Development Bank (AfDB) boreholes drilled in 2017. The latter allowed acquisition of 2017 water table data. Together, these formed the priority target of the sampling campaign supported by a Government extension worker (Area Water Monitoring Assistant or Area Mechanic) who provided local knowledge of villages and borehole locations. Additionally, each field mission included visits to every water point visible from roads travelled between targeted sites. A total of 335 locations (incl. groundwater and surface water) were visited and field parameter reconnaissance surveys conducted at each site during the May/June 2017 dry season. Reconnaissance surveys and the desk-study data resulted in selection of 92 sites for laboratory hydro-chemical analysis. Duplicate acidified (for metals) and non-acidified samples were submitted to the laboratory for eight of these sites for quality assurance-quality control, giving a total of 100 sample sets analysed. Paired t-tests for each measured analytes of duplicates returned a p value of \(>\)0.05, confirming that there was no significant difference between the eight sample pairs tested at a 95% confidence level. Samples with ion balance errors of greater than 5% were discarded causing a minor reduction, from 92 to 85, of samples being taken forward for data analysis. The total numbers of sites reconnaissance sampled and samples obtained with acceptable ion balances (shown in parentheses) were: for groundwater, 296 (73) supply boreholes fitted with Afridev hand-pumps, 4 (1) boreholes fitted with solar pumps, 6 shallow wells, 2 (1) gravity-fed taps; and for surface water, 23 (7) river locations, 1 (1) Mpoto Lagoon site, and 3 (2) Lake Chilwa sites. Reconnaissance field-survey hand-held probe measurements of pH, temperature, electrical conductivity (EC), total dissolved solids (TDS) and oxidation and reduction potential (ORP) were made at all sites. Probes were calibrated daily with the exception of ORP, which was calibrated in Scotland prior to field work (and hence ORP data are treated with caution). Hydrochemical samples were collected using a distilled water-rinsed pair of acidified and non-acidified polyethylene bottles, stored at 4 \({}^{\circ}\)C during transportation and holding at Malawi Southern Region Water Development Office Laboratory and filtered through 0.45 \(\upmu\)m pore size Whatman filters prior to laboratory analysis. A standard titration method was used to determine carbonate as CO32- and bicarbonate as HCO32- (standard hydrochloric acid method), magnesium as Mg2+ and calcium as Ca2+ (standard EDTA method) and chloride as Cl- (standard silver nitrate method).