APPENDIX E: HYDROGEOLOGIC CONSIDERATIONS OF ZONE OF CONTRIBUTION METHODS 

 Cape Cod Aquifer Management Project Final Report Page E-7 



The change in storage produced by the filling or draining of aquifer pore 

 space is dependent upon the rate of change of water-table fluctuations, 

 particle size, sorting, time and other factors. Therefore, the values 

 shown in the table above are only an approximate measure of the relation 

 between storage and head in unconfined aquifers (Lohman and others 1972) . 

 More consistent estimates of Sy at any one location can be determined by 

 aquifer pump tests and drawdown measurements at observation wells. Most 

 Sy values determined at wells in Barnstable range from 0.20 to 0.29, which 

 are consistent with the sand and gravel materials in which they are 

 screened. 



Both CCPEDC and SEA selected uniform values of Sy which were not directly 

 determined from public -supply-well testing, but from secondary sources. 

 According to Horsley (1983), an Sy of 0.25 was taken from Todd (1959, 

 Table 2.2) which summarized data attributed to Poland and others (1949) 

 from their work in California's Sacramento Valley. SEA chose to use the 

 uniform value of 0.20 for its modified numerical model of Barnstable. It 

 is identical to that used by Guswa and LeBlanc (1981) in their digital 

 model of the Cape Cod aquifer. 



The rate at which water is transmitted through a unit width of an aquifer 

 under a unit hydraulic gradient is defined as transraissivity (Lohman and 

 others, 1972). This property can be visualized as the rate water will 

 move through a vertical strip of the aquifer one foot wide and extending 

 through its saturated thickness under a hydraulic gradient of 100 percent. 

 This rate is commonly measured in terms of square feet per day (ft /d) 

 or gallons per day per foot (gpd/ft) . An aquifer whose transmissivity is 

 less than about 150 ft /d may supply only enough water for small 

 diameter domestic wells. At localities where the transmissivity is 

 greater than about 1000 ft /d, sufficient water for municipal, 

 industrial or irrigation wells is usually available. 



In an unconfined aquifer, such as that which provides water to Barnsta- 

 ble's public and private wells, transmissivity is the product of the aqui- 

 fer's horizontal hydraulic conductivity and its saturated thickness (the 

 vertical distance between the water table and a relatively impermeable 

 layer such as thick clay or bedrock). Therefore, an aquifer's ability to 

 transmit water will change in direct proportion to any change in saturated 

 thickness due to natural or man-made water-table fluctuations (see table 

 9). Values of transmissivity determined by aquifer tests (or computer 

 models) represent estimates based on saturated thicknesses used for a 

 particular time or observation. They may not represent average values. 

 Multiplying the hydraulic conductivity (obtained by an aquifer test) by 

 the minimum known saturated thickness (based on observed water-table eleva- 

 tions) will yield a conservative value of transmissivity at a well site. 



Because transmissivity indicates how much water moves through an aquifer 

 it is important for predicting the drawdown of a well at various distances 

 from a pumped well, the drawdown in a well at any time after pumping be- 

 gins, and the downgradient and lateral boundaries of a well's zone of 

 contribution. Aquifer tests provide insitu measurements of transmissivity 



