CAPTURE ZONE DELINEATION AND WELL VULNERABILITY MAPPING FOR COMPLEX AQUIFER SYSTEMS

A well capture zone provides information on the time taken for a contaminant to reach a well. The standard approach for the delineation of well capture zones is backward advective particle tracking. For complex aquifer systems, this approach can produce erratic particle tracks that are difficult to interpret in terms of a capture zone outline, particularly in three dimensions, and may raise questions about the credibility of the delineation. An alternative to backward particle tracking is the backward-in-time advective-dispersive transport approach. A key advantage of this approach is its capability to represent local heterogeneities through the dispersion term. The method produces unambiguous capture zones without the need for interpretation, and allows a clear distinction between the surface capture zone and the maximum extent capture zone, which is essential for complex 3D systems.

A step beyond the conventional capture zone approach is the concept of well vulnerability. This approach provides information not only on the time taken for a contaminant to reach a well, but also on the expected concentration at the well, or on the time taken to breach a given drinking water standard. The method integrates the conventional capture zone concept with the aquifer vulnerability concept, taking into account all potentially important protective mechanisms such as dispersion and attenuation. Well vulnerability can be determined either by forward transport modelling, placing a number of hypothetical contaminant sources throughout the capture zone and recording the breakthrough curves at the well, or, much more efficiently, by backward transport modelling, placing a contaminant source at the well and recording the breakthrough curves at a number of detection points within the capture zone. The results are plotted in the form of well vulnerability maps. The concept can be used to quantify the performance of various protective barriers or mechanisms. The approach is demonstrated by application to a complex multi-aquifer system, the Waterloo Moraine aquifer. The question of capture zone validation by means of environmental tracer data is also addressed.