Analysis and comparison of wellhead protection areas delimitation methods applying a stochastic MODFLOW model as a reference.

Abstract

Sustainable use of groundwater while maintaining economic and social development is a major challenge, and the implementation of wellhead protection areas (WHPA) for public supply wells has been applied as an instrument to overcome it. This study analyzes the WHPA delineation methods: calculated fixed radius (CFR) and two solutions of the WhAEM software (USEPA, 2018), one analytical and one semi-analytical. We compare their results with WHPAs generated by a stochastic three-dimensional MODFLOW-MODPATH model in two scenarios: eight pumping wells operating simultaneously and a single well pumping, both at the same public drinking water supply wellfield located on a coastal plain in Jaguaruna County, south Brazil. For the specific hydrogeological settings, all methods produced satisfactory results when delineating a 50-day time-of-travel (TOT) WHPA for a single well. However, as TOT increases, uncertainties are introduced, and the precision of the results is reduced. Multiple well pumping simultaneously presented similar issues regarding uncertainties caused by three-dimensional flow complexities resulting from well interferences. Despite being the simplest method applied in terms of hydrogeological data needs, the CFR method demonstrated reliability in its results. Additionally, we present an analysis comparing the dimensions of the capture zone with the 10- and 20-year TOT WHPAs, indicating that managing the entire capture zone is the best way to protect groundwater against conservative contaminants. Finally, we compare WHPA generated by a stochastic and a deterministic model to understand how uncertainties can affect model results.