“…Whereas pollutant sources contributing to surface run-off losses are often visible, those contributing to groundwater losses are hidden and difficult to measure directly (Schilling & Wolter, 2001). Assessing the groundwater contribution to NPS loads at a watershed scale is typically conducted using basinwide stream sampling during baseflow (BF; e.g., Aquilina et al, 2012;Schilling & Wolter, 2001), groundwater vulnerability indicators (e.g., Aller, Bennett, Lehr, Petty, & Hackett, 1987;Pavlis, Cummins, & McDonnell, 2010), statistical modelling (e.g., Nolan, Hitt, & Ruddy, 2002;Schilling & Zhang, 2004), or watershed models (Almasri & Kaluarachchi, 2007;Hu & Tian, 2007;Kourakos, Klein, Cortis, & Harter, 2012). In all cases, it is important to match the appropriate method to the scale of investigation.…”