APPENDIX D - DRASTIC MAPPING IN E. BARNSTABLE AND W. YARMOUTH 



Cape Cod Aquifer Management Project Final Report Page D-2 



plain portion of Zone I. The northern third of the study area consists of 

 Sandwich Moraine deposits of silt, sand and gravel (Oldale, 1974). Elev- 

 ations range from 40 to 150 feet above sea level and depths to ground 

 water vary from zero to over 125 feet. Because of changes in aquifer 

 storage caused by variations in recharge, evapo- transpiration and pumping, 

 the elevation of the water table in Zone I may fluctuate as much as eight 

 feet. 



Procedure 



The successful application of DRASTIC is based on the availability of 

 accurate information about the hydrogeology , topography and climate of an 

 area. For Zone I, information was gathered from local water table, topo- 

 graphic and geologic maps, driller's logs, aquifer test data and climato- 

 logical records . 



The table following this report shows the factors, ranges, ratings and 

 weights used to calculate total DRASTIC numbers at locations in the out- 

 wash and moraine settings. Individual factor numbers in the right hand 

 column for each setting were held constant except for depth to water, 

 which varied widely over the area (see Heath and Mascoop , 1987, this vol- 

 ume). Depth to water information was obtained by superimposing contours 

 of water-table and topographic elevations and calculating their difference 

 in feet. Depths to water observed in wells were also used. These methods 

 produced 648 data points within Zone I for which DRASTIC numbers could be 

 derived. The final step of contouring these point values resulted in the 

 DRASTIC map sho-.'m in the accompanying figure. Higher values denote areas 

 of increased vulnerability to contamination. 



Results 



In a national context, the values of relative vulnerability derived by the 

 DRASTIC system may range from a low of 23 for a dry, mountainous setting 

 of shale or clay to a high of 226 for a humid, karst-limestone environ- 

 ment. The derived values of 140 to 185 for the Sandwich Moraine setting 

 and 185 to 210 for the Barnstable Outwash Plain setting in Zone 1 indicate 

 moderate to high vulnerability to contamination. Other areas of Cape Cod 

 that have similar hydrogeologic and climatic conditions are also likely to 

 share these values. The results of this study show that low- lying areas 

 and areas next to surface water bodies are most at risk where the depth to 

 water is less than three feet. The areas that are relatively least vulner- 

 able, with DRASTIC scores of 140 to 159, are the highest portions of the 

 Sandwich Moraine along State Route 6. 



The DRASTIC system of assessing soil and aquifer vulnerability requires 

 knowledge regarding seven factors which affect contaminant mobility. In 

 this application at a 5.7 square mile area in Barnstable and Yarmouth, the 

 factor having the most information to support it was general topography, 



