Wellhead protection areas

One of the main approaches employed for the protection of subsurface water sources for human consumption is the definition of well head protection areas (WHPAs) around the abstraction points, where land-use restriction and safeguard measures must be adopted to ensure the preservation of water quality (static protection). To ensure the protection of drinking water abstractions also from contamination incidents occurring outside the protection zones, WHPAs are usually coupled with dynamic protection systems. As an example, a monitoring network is usually created along their perimeter to periodically assess the quality of the groundwater abstracted from the wells for human consumption.

WHPAs can be defined via geometric methods (i.e. defining the area arbitrarily, such as by drawing a circle of set radius around the pumping well) or via the travel time approach. The latter method takes into account the aquifer type and its hydrodynamic parameters, in particular using the groundwater flow velocity to delineate protection areas defined by the time it takes a contaminant to reach the drinking water extraction well.

WHPAs are usually determined by the calculation of the well capture zone and isochrones along the flow pathlines, using deterministic or probabilistic methods. Both of them are studied and applied by the Groundwater Engineering Group. Deterministic methods consist in the calculation of flow paths in the aquifer (backward particle tracking), taking into account only advective phenomena, while the probable capture zones are studied using a backward probability model based on the adjoint of the classical transport equation of solutes in groundwater.

Concerning deterministic capture zones, the GW Group developed the APA model, a methodology for the automatic delineation of time-related protection areas. The algorithm is an improvement of the standard backward particle tracking and uses non-equally spaced particles around the pumping wells and includes a perimetration algorithm for automatic capture zone encirclement (Tosco et al, 2008).

Although the APA algorithm was initially developed as a tool for a rapid, accurate, and automatic encirclement of wellhead protection areas, the GW Group also applied it for the first time to Pump & Treat systems for the hydraulic control of a contaminated plume in a confined aquifer, demonstrating its efficacy in capture zone delineation. This approach allows to design more efficient P&T systems because the well capture area is able to ensure the containment of both the advective and dispersive flow of the contaminant.