GROUNDWATER UNDER THREAT FROM DIFFUSE CONTAMINANTS: IMPROVING ON-SITE SANITATION, AGRICULTURE AND WATER SUPPLY PRACTICES Validating a continental-scale groundwater diffuse pollution model using regional datasets Issoufou Ouedraogo 1 & Pierre Defourny 1 & Marnik Vanclooster 1 Received: 23 June 2017 /Accepted: 1 December 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017 Abstract In this study, we assess the validity of an African-scale groundwater pollution model for nitrates. In a previous study, we identified a statistical continental-scale groundwater pollution model for nitrate. The model was identified using a pan-African meta-analysis of available nitrate groundwater pollution studies. The model was implemented in both Random Forest (RF) and multiple regression formats. For both approaches, we collected as predictors a comprehensive GIS database of 13 spatial attributes, related to land use, soil type, hydrogeology, topography, climatology, region typology, nitrogen fertiliser application rate, and population density. In this paper, we validate the continental-scale model of groundwater contamination by using a nitrate measurement dataset from three African countries. We discuss the issue of data availability, and quality and scale issues, as challenges in validation. Notwithstanding that the modelling procedure exhibited very good success using a continental-scale dataset (e.g. R 2 = 0.97 in the RF format using a cross-validation approach), the continental-scale model could not be used without recalibration to predict nitrate pollution at the country scale using regional data. In addition, when recalibrating the model using country-scale datasets, the order of model exploratory factors changes. This suggests that the structure and the parameters of a statistical spatially distributed groundwater degradation model for the African continent are strongly scale dependent. Keywords Groundwater nitrate . Random Forest (RF) . Validation . Scale issue . Country . Africa Introduction Throughout the world, groundwater is an important source of fresh water, used by industry, agriculture and domestic users. However, worldwide, groundwater systems are experiencing in- creasing threat from and risk of pollution from agricultural activ- ities, urbanisation and industrial development (Foster et al. 2003; Aljazzar 2010; Charrière and Aumond 2016; Constant et al. 2016). According to Gurdak (2014), all groundwater resources are vulnerable to nonpoint source (NPS) contamination. Diffuse NPS pollution from farming activities and point source pollution from sewage treatment and industrial discharge are the principal contaminant sources (Boy-Roura 2013). One of the most com- mon and persistent problems of groundwater pollution is associ- ated with diffuse pollution generated through the intensification of agricultural activities over the last decades, with increased use of chemical fertilisers and higher concentrations of animal excre- ment in smaller areas (Boy-Roura 2013). Agricultural land use leads to elevated concentrations of nutrients. According to Haller et al. (2013), on a global scale agricultural land use represents the largest diffuse pollution threat to groundwater quality. Elevated concentrations of nutrients (especially nitrogen and phosphorus) can cause a variety of problems, including degradation of eco- systems (for example, eutrophication of water bodies), and hu- man health issues. Nitrate is the most ubiquitous nonpoint (NPS) contaminant of groundwater resources worldwide (Spalding and Exner 1993). Nitrate ingestion has been linked to methemoglo- binemia, adverse reproductive outcomes, and specific cancers (Ward et al. 2005). In Africa, groundwater is a crucial natural resource supporting the development of the continent, but it is also subject to many pressures. Two main threats are overexploitation and Responsible editor: Kenneth Mei Yee Leung * Issoufou Ouedraogo issoufou.ouedraogo@uclouvain.be 1 Earth and Life Institute, Université catholique de Louvain, Croix du Sud 2, Box 2, 1348 Louvain-la-Neuve, Belgium Environmental Science and Pollution Research https://doi.org/10.1007/s11356-017-0899-9