APPENDIX E: HYDROGEOLOGIC CONSIDERATIONS OF ZONE OF CONTRIBUTION METHODS 

 Cape Cod Aquifer Management Project Final Report Page E-4 



in Tables 4 and 5, indicate that average annual ET is relatively constant 

 for any station over a number of years, varying less than 2 inches over 

 the observation period. The least amount of ET was found at the Chatham 

 station (24.7"), while the highest calculated ET was found at the Woods 

 Hole station (25.93"). Based upon the small difference in calculated ET 

 between widely-spaced stations on Cape Cod, Palmer concluded that the 

 amount of precipitation is the principal factor affecting the amount of 

 natural recharge at any one locality. 



Palmer also calculated the potential evapotranspiration from the Long Pond 

 pumping station in Falmouth from 1960 to 1976 (Table 6) and subtracted 

 these values from observed precipitation collected at Hatchville to give 

 estimated recharge values (Table 7). The data indicate a mean loss of 

 recharge over June, July and August with the greatest deficit occurring in 

 July (-2.29 inches). The highest recharge estimates occurred from Novem- 

 ber to March with the highest value in December (4.47 inches). Annual 

 recharge during the drought year 1965 was nearly nil when ET nearly 

 equaled precipitation in the Falmouth area. It reached a high level of 

 nearly 48 inches in 1972, which had a relatively cool summer and wet au- 

 tumn. While the data presented by Palmer are only estimates at one locali- 

 ty, they indicate that annual recharge can vary considerably from one year 

 to the next. 



Strahler (1972) also used the Thornthwaite method to determine monthly 

 potential ET and groundwater recharge based on temperature and precipita- 

 tion observed at Hyannis and Provincetown from 1931 to 1952. His calcula- 

 tions indicate a mean annual recharge to be about 18.3 inches at Hyannis 

 and about 17 inches at Provincetown (see Table 8 and Figure 1) . He esti- 

 mated that for locations on Cape Cod having a higher mean annual precipita- 

 tion than Hyannis (42.8 inches/year), the excess precipitation may be 

 added directly to the ground-water recharge. For example, after superim- 

 posing the precipitation data observed at Falmouth's Hatchville station 

 onto potential ET data determined at Hyannis, Hatchville, which had 4 more 

 inches of precipitation, showed an estimated recharge of 22 inches per 

 year. 



Delaney (1980) estimated ET at Edgartown on Martha's Vineyard to be 23.7 

 inches annually for the years 1947-1977, yielding an average recharge rate 

 of approximately 22.2 inches per year. Additive recharge from septic 

 outflow was not estimated for this study. 



Guswa and LeBlanc (1981) recognized that aquifer recharge on Cape Cod is a 

 combination of natural recharge from precipitation and artificial recharge 

 from sources such as waste-water treatment plants and septic systems. 

 Using a digital three dimensional model to simulate ground water flow in 

 the Cape Cod aquifer they estimated that recharge rates within the study 

 area range from a low of 6 to a high of 22 inches per year. 



SEA Consultants (1985) used values taken from the numerical model devel- 

 oped by Guswa and LeBlanc and added artificial recharge rates from septic 

 tanks as 4.3 inches per year for zones 3,4,5,6,7 and 9, and 9 inches per 

 year for zones 1,2 and 8. 



