## Materials and Methods
### Study Setting
#### 2.1.2 Geology and Hydrogeology  
The Basin is underlain by the Precambrian-Lower Palaeozoic Malawi Basement Complex of metamorphic and igneous rocks, exposed in Shire Highlands to the west, and dipping below the lacustrine and alluvial deposits of the Chilwa-Phalombe Plain [5,31] (Figure 2a) (with geological detail of the Phalombe District shown in Figure S2). The Shire Highlands Basement of high-grade metamorphic rocks comprising predominantly charnokitic granulites of quartz and feldspar are replaced around the Mulanje Massif in the south by coarse-grained biotite gneiss rock derived from granite that weathers to sandy soils [5,31,32]. The Basement is intruded by Upper Jurassic to Lower Cretaceous alkaline igneous rocks comprising intrusions, volcanic vents and radial dykes [31]. Intrusions in the Phalombe District typically comprise granites and syenites and form the prominent Mulanje Massif that rises precipitously to over 3000 m asl (metres above sea level) in the south. Distinctive volcanic vents occur to the immediate south of Lake Chilwa at Tundulu (Figure 2a) as a complex of alkaline silicate rocks, carbonatites rich in sodium and calcium carbonates [5]. Alkaline dyke swarms are widespread, particularly to the north east of the Mulanje Massif [31]. Hot springs at Mypupyu to the west of Lake Chilwa and off Chisi (Nchisi) Island in Lake Chilwa represent the final remnants of this period of igneous activity [5] and have been postulated to contribute to lake chloride and salinity [14]. Superficial deposits resulting from weathering of the surrounding highland and underlying Basement rock extensively cover the Basin within a post Cretaceous tectonic depression [5,33] (Figure 2a). A complex mix of Quaternary fluvial (alluvial) and lacustrine deposits exist from spatial/temporal influences of open and closed Lake Chilwa paleo-environments. Colluvium foothills surround the Massif to c. 700 m asl incised by small stream gorges. The Phalombe Plain of predominantly lacustrine deposits slopes gradually northwards, merging with the Lake Chilwa depression. The apparently flat plain comprises several terrace levels associated with successive changes in the lake level. Island-like hills occasionally rise above the plain and colluvial pediments provide good agricultural land around isolated inselbergs. Otherwise, lowland plains comprise treeless lacustrine flats with some dambo wetland present [31] that now mostly tend to be seasonal in occurrence. Distribution of aquifer types and their potential yield (Figure 2b) reveal the significance of Quaternary superficial deposits (alluvium, colluvium, etc.) in the Basin lowlands, changing to fractured and weathered basement aquifers towards mid-Basin elevations and higher ground. Superficial deposits typically represent Malawi's most productive aquifers although possible yields of 151/s will be restricted to the more permeable fluvial sediment sand-gravel lithologies deposited in high-energy environments, e.g., thick buried paleo-channels or alluvial fans associated with former river courses [13,34]. Lacustrine deposits of low-energy deposition that comprise predominantly finer-grained lithologies and more common across the Basin generally may be of much more modest aquifer potential. Layered aquifer systems may be expected with individual layers tending to an aquitard nature where very fine-grained silt/clay lacustrine deposits are thick and laterally extensive. Better aquifer units for water supply in the Chilwa Basin are most likely associated with former riverine environments where fluvial deposit accumulations are more extensive. The Chilwa system location perched on the eastern edge of the Rift Valley with associated potential for increased clayey lithologies renders it distinct from other alluvial areas in Malawi located in the Rift Valley floor [13]. Fractured/weathered basement rock where near surface may serve as aquifers. Yields vary locally and are often approximately 1 l/s or lower [13,34,35] (Figure 2b).