APPENDIX E: HYDROGEOLOGIC CONSIDERATIONS OF ZONE OF CONTRIBUTION METHODS 

 Cape Cod Aquifer Management Project Final Report Page E-6 



Water-level elevations are periodically determined at ten established U.S 

 Geological Survey observation wells (Table 9) located throughout Barnsta- 

 ble. These observations, which extend back as early as January, 1958 at 

 one well (AlW 230), indicate that water-table fluctuations range from a 

 low of 3.53 feet in Osterville to a high of 7.67 feet in northeast Barnsta- 

 ble. Changes in water-table elevations occur as a result of several fac- 

 tors: well discharge, evapotranspiration, precipitation and aquifer re- 

 charge. Because these factors change over time, multiple water level 

 measurements are needed to identify typical groundwater levels for a par- 

 ticular time of year. 



Estimates of hydraulic gradient used by CCPEDC in the vicinity of 

 public-supply wells were determined from the 1976 water-table map of 

 LeBlanc and Guswa. In general, head variations were measured over two to 

 three 10 foot contour intervals in the direction of groundwater flow at 

 the well site. Hydraulic gradients determined by this method range from 

 .0012 at BFD PS3 to .00638 at C/0 wells #1A and #15. This information was 

 incorporated along with estimates of well discharge and transraissivity to 

 calculate the downgradient limit of each zone of contribution. 



Hydraulic Properties 



Specific yield, transmissivity and vertical hydraulic conductivity are the 

 principal hydraulic properties that determine an aquifer's capacity to 

 store, transmit and yield water. The storage term for an unconfined aqui- 

 fer is specific yield, which is generally defined as "the change that 

 occurs in the amount of water in storage per unit area of unconfined aqui- 

 fer as the result of a unit change in head "(Lohman and others 1972). 

 Specific yield (Sy) is equivalent to the ratio of the volume of water that 

 saturated rock or soil will yield by gravity drainage to the volume of 

 rock or soil. The usual range of Sy is 0.01 to 0.30. Several workers 

 have compiled representative values of Sy for various unconsolidated mate- 

 rials. Johnson (1967) determined specific yields for common soils which 

 are found in Barnstable, as well as in other areas of Cape Cod: 



Material Diameter, mm. Specific Yield. Percent 



Coarse Gravel 16.0-32.0 23 



Medium Gravel 8.0-15.0 24 



Fine Gravel A. - 8.0 25 



Coarse Sand 0.5-1.0 27 



Medium Sand 0.25-0.5 28 



Fine Sand 0.125- 0.25 23 



Silt 0.004- 0.062 8 



Clay < 0.004 3 



Till, Predominantly Silt Variable 6 



Till, Predominantly Sand Variable 16 



Till, Predominantly Gravel Variable 15 



