Public water supply wellfields provide water for a variety of beneficial uses in homes, businesses and industries, and for farmland irrigation in many communities. To protect those water supplies, water purveyors often perform analyses to determine the area over which water enters and flows though the aquifer supplying a well or wellfield, so that measures can be taken to prevent and/or minimize the potential contamination of groundwater in those areas.
“Zone II is that area of an aquifer which contributes water to a well under the most severe pumping conditions that can be realistically anticipated (180 days of pumping at safe yield with no recharge from precipitation). “
Typically, one of the most common methods utilized to delineate Zone II Areas (and groundwater protection areas in general) is a groundwater flow model. AnAqSim is particularly well-suited for this type of analysis because it has a number of special features and tools that make it easy to delineate pumping well Zones of Contribution in general, and Zone II Areas in particular, including the following:
- AnAqSim’s ability to automatically calculate recharge or leakage over an area (this is useful in confirming that the model area determined from pathline tracing to be contributing recharge to the well matches the specified withdrawal rate at the well; this area can also be used as a basis for pathline tracing for the Zone II);
- the ability to capture a snapshot of heads for a selected time step in a transient aquifer simulation (e.g. “freeze” calculated heads on the 180th day following termination of recharge as is required for a Zone II analysis), and then perform pathline tracing;
- easy setup of a grid of pathline starting locations over some area of the aquifer surface for forward tracing toward the pumping well; and
- constraining the display of pathlines, so that only those pathlines captured by selected model features (e.g. wells for which wellhead protection areas are to be delineated) are shown on the plot.
Although the example is focused on the delineation of a Zone II area in Massachusetts, the techniques used in delineating the Zone of Contribution (ZOC) and Area Contributing Recharge (ACR) for the well can also be applied for delineating groundwater protection zones under other regulatory programs.
The example, which comes with a detailed guide on model set up and Zone II delineation with AnAqSim(and all associated modeling files), can be downloaded for free HERE.