Non-piped groundwater sources supply over 2 billion people in low- and middle-income countries

<title>Abstract</title> <p>The source through which people obtain their drinking water, whether through a household tap, shallow well or a tanker truck, underpins water security, including safety1,2, cost3, and climate resilience4. Yet source-explicit drinking water use remains poorly understood and absent from global assessments5,6; masked by broad classifications, such as the improved/unimproved dichotomy, that obscure profound differences between source types7,8. Using household survey data from 53 countries to train and validate random forest models, we generate the first high-resolution (5 km), source-explicit estimates of drinking water source use across low- and middle-income countries (LMICs). Here we show that over 2 billion people (31% of the LMIC population) use non-piped groundwater, including boreholes, dug wells, and springs, as their primary drinking water source, rivalling piped supplies in scale and exceeding them in sub-Saharan Africa and South Asia. We further identify that bottled and delivered water markets each serve over 450 million people. These non-piped sources are almost entirely absent from the global databases9 and vulnerability indices used to assess water security, leaving the drinking water use of nearly half the LMIC population effectively invisible in global monitoring, with direct consequences for hydrological modelling10,11, climate vulnerability assessment12, and water governance13.</p>