APPENDIX E: HYDROGEOLOGIC CONSIDERATIONS OF ZONE OF CONTRIBUTION METHODS 

 Cape Cod Aquifer Management Project Final Report Page E-5 



Horsley (1983) , using the CCPEDC analytical method to determine the zone 

 of contribution on Cape Cod, used a recharge estimate of 13 inches per 

 year based on a study of tritium levels in a glacial drift aquifer (Offer 

 and Larson, 1982). This value was applied to the Cape as a whole. 

 Artificial recharge from septic and waste -water treatment systems was not 

 considered. 



The following table summarizes recharge estimates determined from previous 

 investigations on Cape Cod: 



Hydraulic Gradient 



The hydraulic gradient of the water table is defined as the change in 

 static head per unit of distance in a given direction (Lohman and others, 

 1972) and can be determined from a map of water-table elevations. It is 

 an important measurement in calculating a well's zone of contribution. 

 This dimensionless factor not only governs ground-water flow direction 

 but, combined with estimates of aquifer transmissivity and well discharge, 

 helps to define a zone's downgradient and lateral boundaries. 



The most comprehensive attempt at mapping water-table elevations on Cape 

 Cod to date is that of LeBlanc and Guswa (1977). Drawing 10 foot contours 

 based on well observations obtained May 23-27, 1976, they identified six 

 freshwater lenses on the Cape Cod peninsula: inner Cape Cod (Cape Cod 

 Canal to the Bass River), middle Cape Cod (Bass River to Orleans), and 

 four smaller lenses on outer Cape Cod (Eastham to Provincetown) . Ground 

 water within these lenses moves from points of higher to lower hydraulic 

 head near the shoreline which represents a lateral boundary where ground 

 water is discharged into the sea (Ryan, 1980). 



Water-table elevations in Barnstable range from over 60 feet above sea 

 level at the Sandwich -Barnstable town line and Bottom Road to sea level 

 along Cape Cod Bay to the north and Nantucket Sound to the south. Natural 

 groundwater flow directions in the vicinity of public -supply wells are 

 predominantly toward the southeast, but locally may flow toward the north- 

 east, south or southwest in eastern Barnstable or Osterville. Groundwater 

 elevations are particularly affected by the irregular southern coastline 

 of Barnstable, due to the large number of estuaries and tidal inlets. 



