APPENDIX E: HYDROGEOLOGIC CONSIDERATIONS OF ZONE OF CONTRIBUTION METHODS 

 Cape Cod Aquifer Management Project Final Report Page E-3 



Available water - (depth of pumping well) - (screen length) - (static 

 water level) - (5 feet) 

 Safety factor =0.75 



Safe yields determined by CCPEDC and SEA for the 31 public-supply wells in 

 Barnstable are available from computation sheets (Table 1) and on Table 2 

 (SEA, Table 8.10, 1985), respectively. Information on safe yields used by 

 SEA resulted from interviews with the water companies. The two sets of 

 well capacity values differ considerably, and agree for only 7 wells, or 

 23 percent of public-supply wells in Barnstable (Table 3). According to 

 CCPEDC, safe-yield capacities range from 290 gpm at Centerville Oster- 

 ville's Craig #11 to 1000 gpm at Barnstable Water Company's Airport well. 

 Values for safe yields published by SEA range from 275 gpm at Cotuit's 

 Electric #3 to 1,400 gpm at Barnstable Water Company's MDL 1 and 2. The 

 use of different sets of values for safe yield for over 75% of the wells 

 makes it difficult to compare zones of contribution because variations in 

 pumping capacities strongly influence the size of recharge areas. 



Recognizing that Barnstable public-supply wells do not continuously pump 

 at their full capacity, CCPEDC multiplied the safe yield by a general 

 reduction factor to more closely approximate average well discharge. A 

 value of 60 percent was used uniformly for all wells to simulate a 

 steady-state condition over an extended period of time. In SEA's method, 

 pumping of public-supply wells within zones 1, 2 and 8 were kept at their 

 rated safe yields to simulate future average day demand. The discharges 

 for wells in the remaining zones were simulated at 60% of their rated safe 

 yield. 



Recharge 



Recharge may be defined as the volume of water which reaches the saturated 

 zone of an aquifer where it is available for extraction. Several investi- 

 gators have estimated recharge rates in inches per year for different 

 areas of Cape Cod and Martha's Vineyard. Palmer (1977) studied aquifer 

 recharge while conducting research at a wastewater experimental site at 

 the Massachusetts Military Reservation, eight miles west of Barnstable 

 township. He described the water balance at this site by the equation: 



Recharge - P - ET - SR 



where P - precipitation, ET - evapotranspiration from surface water 

 bodies and vegetation and SR - surface runoff. Because of the high 

 permeability of surficial deposits for most of Cape Cod, Palmer assumed 

 that there is little or no runoff and the last component could be 

 neglected. Therefore, the above equation reduces to: 



Recharge - P - ET 



Using mean monthly temperature and precipitation data for several climato- 

 logical stations on Cape Cod, he estimated annual evapotranspiration lev- 

 els from 1965 to 1975 using the Thornthwaite calculation, (Thornthwaite 

 and Mather, 1957). The results of these calculations, which are presented 



