## 3 Results
### Water Quality Controls
#### 3.3.2 Paleo-Lake Controls on Groundwater TDS  
Good drinking water quality groundwater \(<\)600 mg/L TDS predominantly occurs in the mid-to-upland basin in southern Phalombe District (Figure 9). It may discharge as good-quality freshwater baseflow to upper stream reaches. Groundwater at 600-1000 mg/L TDS occurs at mid to lower elevations and is still regarded as less ideal but acceptable quality. Groundwater generally of higher TDS, becoming brackish increasingly occurs at lower basin elevations. A key observation is that almost all groundwater points located outside the southern limit of the high-level lacustrine deposit (paleo lake level) have a TDS \(<\) 1000 mg/L, with the great majority having a TDS \(<\) 600 mg/L (Figure 9). Groundwater there will be drawn from either the colluvium where present or underlying fractured/weathered basement.  
Figure 8: Observed hydrochemical interquartile plots for Phalombe District: (**a**) river water: (**b**) groundwater; and (**c**) Lake Chilwa. EC data are shown for site measurements for both the entire datasets and subset of samples analysed for major ions. TDS data are shown for all on-site measurements and a calculated TDS summed from laboratory ion concentrations analysed (smaller n number that also applies to the individual ion datasets).  
Hence the high-level deposit line effectively demarks low-TDS, good-quality water to its south, transgressing into usually poorer quality, higher-TDS groundwater to its north and closer to the lake. Boreholes within the central to eastern area of the high-level and terrace increase in TDS in the general direction of groundwater flow towards the lake. Boreholes of highest salinity (\(>\)1500-4300 mg/L TDS) track close to the 652 m elevation contour and terrace 5 deposit line (Figure 9). Closer to the lake salinity declines in boreholes penetrating lower numbered terrace deposits but still remains significant at 1000-1500 mg/L TDS.  
River infiltration of low-TDS surface water to groundwater is postulated to cause some local exception to these TDS spatial trends and account for good-quality low-TDS groundwater observed close to some mid-to-lower river reaches crossing the high-level and terrace deposits. In particular, \(<\)600 mg/L TDS groundwater near reach area [a] marked on Figure 9 around terraces 2 to 4 on the lower Phalombe River, perhaps the more localised area of low-TDS groundwater near reach [b] where the Sombani River crosses terraces 4 and 5, and possibly the low to moderate TDS groundwater near  
Figure 9: Phalombe District observed groundwater total dissolved solids (TDS) shown relative to a backdrop of superficial deposits and satellite ground image. Areas of possible river-reach influent seepage of low-TDS water to groundwater are labelled [a] to [c].  
reach [c]. Supporting evidences include: relative groundwater-river levels showing influent gradients (Figure 6); visible incising of some reaches, e.g., around reach [a], carving down through terrace sediments towards the water table; low river TDS (Figure S7); and, boreholes around reach [a] are generally shallower than found regionally (per mWater data shown in Figure S8) with their reduced depths potentially deliberately targeting this shallow, low TDS groundwater resource.