## 3 Results
### Water Quality Controls
#### 3.3.3 Paleo-Lake Controls on Groundwater Hydrochemical Water Type  
Trends in anion and cation composition are confirmed in the Piper plot (Figure 10a). The plot is similar to the Basin-scale data plot of Missi and Atekwana [18] and plots of Lake Malawi shoreline area groundwater [57]. Sampled cations largely fall within a mixed-type domain with a very linear trend in composition from Ca (and Mg) water to Na (and K) that is attributed to increased cation exchange of Ca in recent recharge groundwater with sodium from clay-based lithologies progressively encountered along flowlines. Anions exhibit a predominant HCO\({}_{3}\)-type water with a trend to increased Cl content. Data confirm anticipated occurrence of young recent recharge waters associated with the basement rock-colluvium-superficial deposits and the influence of acidic conditions from soil CO\({}_{2}\) and rock mineralogy that promote dissolution of Ca (and Mg) carbonates leading to the predominant Ca-HCO\({}_{3}\) water type at 46% of sites. Increased groundwater residence times and influence by clay lithologies, especially where high-level and terrace lacustrine deposits occur that increase cation exchange opportunity and Na exchange for Ca and greater Ca-Na-HCO\({}_{3}\)-type waters (38% of sites) and increased sodium in groundwater (Figure S9). Exchange may be associated with montmorillonites formed from alteration of silicates [18; 57]. Evaporitic enrichment (including mixing with evaporated water) is highly likely and drives increased sodium and chloride resulting in occurrence of Ca-Na-HCO\({}_{3}\)-Cl-type waters accounting for 10% of sites, Na-HCO\({}_{3}\)-Cl at 5% and Ca-HCO\({}_{3}\)-Cl at 1% of sites.  
The spatial distribution of water type confirms the occurrence of young Ca-HCO\({}_{3}\)-type groundwater predominantly in the uplands recent recharge area outside the high-level deposit line (Figure 10b). Ca-Na-HCO\({}_{3}\)-type waters occur within, but also outside the High Deposit line indicating ion exchange is prevalent in both areas affecting both low- and higher-TDS waters. This is attributed to an increasing occurrence of clay lithologies in high-level and terrace deposits closer to the lake, and occurrence in the weathered basement accounting for Ca-Na-HCO\({}_{3}\)-type waters beyond the high-level deposit line. This may likewise account for the lack of obvious relationship of Piper plot trends with TDS. Whilst the water types with minor chloride in some, but not all, samples track along upper terraces 3 to 5, the 631 m closed lake contour (Figure 10b), the salinity of the system appears largely driven by higher concentration occurrences of the Ca-HCO\({}_{3}\) and cation exchanged Ca-Na-HCO\({}_{3}\) water types. Limited groundwater chloride is consistent with low chloride as a constituent of silicate rocks, the lack of evaporite deposits in the Basin, and limited input from rainfall anthropogenic sources such as pit latrines and agriculture [18; 58].  
Predominant hydrochemical signatures of Ca-HCO\({}_{3}\) and Ca-Na-HCO\({}_{3}\)-type groundwaters support a hydrological conceptualisation of young recharge waters evolving through contact with minerals in the Paleo-lake environments. Groundwater lateral flow through weathered, clay-rich basement, sandy deposits containing increased frequencies of lacustrine origin clay layers or rainfall or influent river recharge infiltration through leaky, clay-rich high-level or terrace deposits may account for evolution from Ca-HCO\({}_{3}\) to Ca-Na-HCO\({}_{3}\) groundwater types.