CHAPTER 4 - THE TECHNICAL BASIS FOR GROUNDWATER PROTECTION 



Cape Cod Aquifer Management Project Final Report Page 23 



4.4 DRASTIC Analysis of Aquifer Vulnerability 



The DRASTIC methodology is a model for assessing the \ailnerability of 

 contamination to an aquifer (described in Appendix D) and is based on an 

 approach that addresses regional hydrogeological factors. Appendix D, 

 "DRASTIC Mapping of Aquifer Vulnerability in Eastern Barnstable and 

 Western Yarmouth, Cape Cod, Massachusetts," by Heath describes the 

 methodology and the analysis that was performed in one of Barnstable's 

 wellhead-protection areas, utilizing the water-table information generated 

 by CCAMP. This effort assessed the variations in the vulnerability of 

 groundwater to contamination in a 3650 acre zone of contribution in 

 eastern Barnstable and western Yarmouth. 



4 . 5 Identification of Zone II. the Wellhead-Protection Area 



For the protection of public-water supplies, the Aquifer Assessment 

 Group focused on the delineation and protection of the recharge areas for 

 public -supply wells. These recharge areas, referred to as the zone of 

 contribution (ZOC) or wellhead-protection area, are described in Massachu- 

 setts by Zones I, II and III (Figure 4.2). Zone I is the traditional 400 

 foot radius that a water supplier must own or control as required by DEQE 

 Drinking Water Regulations (310 CMR 22.00) to protect groundwater from 

 microbiological contamination. The primary recharge area to a well. Zone 

 II is defined as the area that "recharges a well under the most severe 

 recharge and pumping conditions that can be realistically anticipated. It 

 is bounded by the groundwater divides which result from pumping the well 

 and by the contact of the edge of the aquifer with less permeable 

 materials such as till and bedrock." Zone III is "that land area beyond 

 the area of Zone II from which surface water and groundwater drain into 

 Zone II" (310 CMR 24.00). Zones I, II and III are two-dimensional map 

 representations of a three-dimensional subsurface volume. The two- and 

 three -dimentional areas of these zones in a typical New England Valley 

 aquifer are depicted in Figures 4.2. 



The delineation of a wellhead-protection area forms the basis for a 

 comprehensive groundwater protection program. As Zone II represents that 

 land area that provides the primary groundwater recharge to a public sup- 

 ply well, any contaminants that infiltrate the soil and are not immobi- 

 lized or attenuated in the soil will move down into the aquifer and travel 

 through the aquifer towards the well. For this reason, protection of the 

 well's water quality must involve the proper management of all land uses 

 in the wellhead-protection area. Figure 4.5 depicts typical land uses in 

 a wellhead-protection area. 



4. 6 Approaches to Zone II Determinations in the Project Area 



The Aquifer Assessment Committee was charged with the evaluation of 

 existing methods to delineate wellhead-protection areas and the determina- 

 tion of alternative delineation approaches that would be appropriate for 

 the pilot area. The Committee reviewed the methods used by SEA 

 Consultants and CCPEDC to estimate wellhead-protection areas for the 



