Determining the area where a well withdraws water from is a challenging task. Water flows through the ground in complex ways, depending on surface topography, the underlying geology, and other factors.
In Pennsylvania DEP’s Source Water Protection Program, a computer model of the study area is used to identify the sources of the public water system’s drinking water. The model is called the Groundwater Modeling System (GMS), which was developed by the United States Department of Defense. GMS is widely used and well-accepted among geologists and hydrologists, and is commonly used to help manage groundwater resources. The model incorporates a significant amount of information about the study area – topography, geology, slopes, soils, infiltration rates, and water table elevations. In addition, data from previous studies, PADEP aquifer testing, and field measurements is also often utilized.
Rigorous delineation must be performed by or under the supervision of a Registered Professional Geologist. A detailed description of the local hydrogeologic setting and some form of a conceptual ground water flow model must be provided, along with relevant hydrogeologic data with sources/references, supporting calculations, and any other information necessary for the reviewer to reproduce the steps involved in delineating the Wellhead protection area.
The level of delineation depends upon the source water protection management option that will be used: for example, a regulatory approach (overlay zoning or ordinance) to a Wellhead protection area may require a rigorous delineation in order to be legally defensible, while an educational program may only need a 1/2 mile radius delineation to identify the target audience for the education and outreach effort.
Three Zones For Groundwater Sources – Zone I, Zone II and Zone III
Zone I is the protective zone immediately surrounding a well, spring or infiltration gallery. It is the smallest zone, closest to the well. and the most critical in terms of protection required.
Zone I is required to be a 100 to 400 foot radius, depending on the specific source and aquifer characteristics. Zone I area may be determined using the graphs or calculations in PADEP’s Compliance Assistance Document entitled “Recommended Zone I Wellhead Protection Area Delineation Methodology” (DEP, 2005b).
For all wells permitted by PADEP after October 9, 1995, the water supplier is required to own or substantially control the Zone I wellhead protection area to prohibit activities within the zone that may have a potential adverse impact on source quality or quantity. For wells permitted before the October 1995 Zone I requirement, an acceptable Zone I would also be the permitted isolation distance, which is typically a 100 foot radius.
Zone II encompasses the area of the aquifer through which water is diverted to a well or flows to a spring or infiltration gallery. The land that contributes groundwater to a pumping well is referred to as the capture zone, or the zone of diversion. Zone II is the surface representation of the capture zone. Zone II must be a one-half mile radius unless a more detailed delineation is approved. The Zone II delineations represent the volume of water entering the sources in a 10-year time of travel.
This means that groundwater that resides below the area identified as Zone II has a high probability of reaching the corresponding source in ten or fewer years.
Zone III lies beyond Zone II and is the area that contributes significant surface water and ground water to Zone I and Zone II. Zone III, or the zone of contribution, is the portion of the watershed that can contribute water to the capture zone. In some settings, a Zone III Wellhead protection area may not be required.