## 1 Introduction  
The Scottish explorer Dr [PERSON] visited this study area in April 1859 [1]. His expeditions were the first to map the extent of Lake Chilwa, then observed to approach the foothills of Mount Mulanje perceived some 30 km further south than typically found today [2]. [PERSON] describes the lake as having no outlet and its waters as bitter, but drinkable [1]. Paleo-hydroology studies suggest that 200 years ago, the lake level was possibly 9 m higher than today [3; 4], approaching that of lake enclosure during the Holocene, when levels were much higher, with even higher levels under open lake conditions in the Pleistocene [3; 5; 6; 7]. Lake Chilwa lies within a northeast-southwest-trending tectonic depression in Southern Malawi, now a semi-arid lowland savannah east of the main Rift Valley. The lake is endorheic, moderately saline and shallow, at just 1 to 5 m depth. With a lake area of 683 km\({}^{2}\) and a considerable additional area of wetlands, it forms Malawi's second largest lake after Lake Malawi. It is among Africa's larger lakes of the 670 recorded [8]. Designated a Ramsar Convention wetland of international importance, Lake Chilwa has long been a productive centre of biodiversity, integral to local livelihoods, food security and the Malawian economy to which it annually contributes US$ 21 million, mostly from its fishery [8; 9; 10; 11]. Chilwa Basin groundwater resources have only received modest research through surface water hydrograph or water balance difference [12; 10; 13] alongside limited studies on groundwater quality and pollution sources [14; 15; 16; 17]. [PERSON] and [PERSON] [18] recently examined hydrogeochemical properties of the basin controlling groundwater and surface water quality, including some sampling in our study area. They conclude recommending an urgent need \"to map fresh groundwater aquifers for domestic, agricultural and industrial use\", a key task that our research addresses. Exploitation of groundwater has been pivotal to rural African communities to help meet Millennium and now Sustainable Development Goals (M/SDGs) [13; 19; 20]. Malawi now hosts over 120,000 community water points [21], with over 9000 points in the Chilwa Basin, of which over 5000 are groundwater sourced. This contrasts with c. 400 Basin boreholes operational towards the late 1990s when groundwater extraction was estimated at a fraction of a percent of recharge and the implication drawn was that large stocks of groundwater were unused and potentially available for community water supply [12]. National estimates a little prior (c. 1990) indicate very low groundwater extraction across Malawi of less than 1 mm/yr recharge equivalent [13]. Shallow African, particularly endorheic, lakes such as Chilwa have long been recognised as dynamic and able to recover from large changes in surface area due to drought and flood extremes. The communities dependent on these lakes prove resilient to the dynamic prosperity arising [22]. There is, however, mounting concern over lake recovery resilience [8; 23]. Chilwa has been increasingly threatened by encroachment of subsistence agriculture, upland deforestation, and vulnerability to climate change [8; 9; 10; 22; 24]. A recent quote from a community leader captures this concern: \"the lake is drying due to persistent drought in southern Africa, massive deforestation in the catchment, farming too close to rivers and diversion of the lakes inflets for irrigation\" [23]. The challenge of providing sustainable water supply and meeting growing population demand, whilst ensuring the protection of ecosystem services, fishery and agricultural livelihoods is not insignificant. Our aim is to develop a detailed understanding and conceptualisation of groundwater quality constraints upon community water supply provision. This ambition is set within the wider Malawian/African context of water quality data often being sparse, but becoming increasingly available [25; 26; 27; 28].