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

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.

Assessment of vulnerability and control measures to protect the Salbarua ecosystem from hypothetical spill sites

Population pressure, urbanization, and industrial developments, among other factors, have resulted in severe degradation of environmental resources such as wetlands. Thus, a groundwater model (MODFLOW) was integrated with a particle tracking MODPATH model to simulate the hydrodynamic flow head field and to analyze the vulnerability of the Salburua ecosystem and propose control measures to protect the riparian area. The simulations show that pathways of particle tracking originating at potential contaminant sources will tend to migrate downwards towards the sensitive ecosystem, which suggests that the quality of the hydrological ecosystem is likely to deteriorate in the future. Variation in exit points of particles indicates that the time-related capture areas are affected by changes of the hydraulic gradients. Two control measures of potential sources of pollutants in the vicinity of the Salbarua ecosystem were analyzed. The study results suggest that the travel time-related capture zone with a funnel-and-gate system is much smaller than without the control alternative, which indicates that the gate configuration has an effect on capture zone size and shape and on the residence time with a better attenuation performance. It is also shown that a leakage-proof barrier is less effective for point-source containment, assuming that hydraulic control performance and cost-efficiency are the criteria for pollution control effectiveness. Instead, a program of monitoring wells would effectively characterize water quality in the aquifer and provide a decision support system. This approach may be used in helping water managers to develop more physically based and quantitative protection strategies.