Source Protection Zones in the vicinity of Scarborough, North Yorkshire. Red dots are public water supply wells, pink areas are considered to be the limit of 50-day travel times within groundwater; green areas represent 400 day travel times and the blue area represents the entire catchment of the wellfield.
Source Protection Zones in the vicinity of Scarborough, North Yorkshire. Red dots are public water supply wells, pink areas are considered to be the limit of 50-day travel times within groundwater; green areas represent 400 day travel times and the blue area represents the entire catchment of the wellfield.
Advection-Dispersion model of tracer breakthrough at a public supply well, showing two distinct components of flow (different flowpaths). Experiment and modelling by A.Foley.
Advection-Dispersion model of tracer breakthrough at a public supply well, showing two distinct components of flow (different flowpaths). Experiment and modelling by A.Foley.

The National Rivers Authority (NRA; now part of the Environment Agency) published its Policy and Practice for the Protection of Groundwater in 1992 (NRA, 1992). This policy was adopted in response to the Water Resources Act of 1991 and was implemented in two ways; 1) Mapping groundwater vulnerability (NRA, 1995a); and 2) Defining groundwater Source Protection Zones (NRA, 1995b). The latter requires definition of a 50 day travel time zone, a 400 day travel time zone, and a zone describing the catchment of the well or source (see Figure to left).

The most commonly used method for delineating the capture zone is by using numerical groundwater models with particle-tracking codes. However, these are frequently based upon easily disproven assumptions regarding the structure of permeability within the aquifer, with tracer tests providing the only means of directly testing the outputs from these models.

Modelling Source Protection Zones is an expensive and time consuming business, especially when models are frequently shown to be erroneous (the catchment SPZs shown in the figure were demonstrated to have travel times of less than three weeks, rather than over 400 days). It is thus probable that constraining the model inputs with actual data on flow velocities, directions and the likely mechanisms attenuating contaminant transport, will result in significant cost savings during SPZ modelling projects.