APPENDIX E: HYDROGEOLOGIC CONSIDERATIONS OF ZONE OF CONTRIBUTION METHODS 

 Cape Cod Aquifer Management Project Final Report Page E-8 



hat are averaged over a large and representative volume. Despite their 

 cost, aquifer tests remain the best method for estimating this important 

 aquifer property. 



Most discussions of hydraulic conductivity (K) assume that the geologic 

 materials which store and transmit water are homogeneous and isotropic, 

 implying that the value of K is the same in all directions. However, 

 anisotropy (the condition in which all significant aquifer properties are 

 dependent of direction) is generally the rule in undisturbed, unconsolidat- 

 ed glacial materials. Anisotropy is influenced by the material's environ- 

 ment of deposition, particle size and shape. For example, Palmer (1977), 

 while studying the hydrogeology of glacial outwash deposits in Falmouth, 

 found that hydraulic conductivities in the north south direction of deposi- 

 tion were higher than those which were perpendicular (or east-west) to the 

 direction of stream deposition. Combining a flownet analysis with 

 water-table and saturated thickness maps, he estimated that hydraulic 

 conductivities parallel to the direction of deposition ranged from 140 to 

 167 feet per day, and the lower transverse values ranged from 62 to 81 

 feet per day. Such differences are probably due to the linearity of 

 coarse-grained channel deposits laid down by braided streams. 



Horizontal layers with relatively low hydraulic conductivity will tend 

 to retard vertical flow (Todd, 1980). In Barnstable, dense, fine-grained 

 till and deposits of glaciolacustine (lakebed) silt and clay are commonly 

 present in beds of sand and gravel. These confining layers control the 

 rate at which recharge moves into the aquifer and vertically toward the 

 well screen during pumping. Extensive deposits of till or glaciolucustine 

 clay can isolate buried aquifers from zones of near-surface, groundwater 

 flow (Freeze and Cherry, 1979). At Barnstable Fire District's well number 

 3, for example, a 7-foot thick layer of firm blue clay at an elevation of 

 20 feet below sea level separates upper and lower aquifers consisting of 

 fine to coarse sand and gravel. The well draws water from the lower 

 aquifer at a rate of over 700 gpm. Available well logs for Barnstable 

 Water Company wells ST and SI show that clay layers of varying thickness 

 were penetrated during drilling. These layers, if sufficiently extensive, 

 would tend to restrict contaminant migration to a relatively shallow flow 

 path beneath the ground surface. 



Conclusions 



1. Municipal planners should make a comprehensive review of all existing 

 information regarding the occurrence, movement and quality of ground 

 water in Barnstable (and adjacent areas of neighboring towns). Such a 

 review will guide the subsequent collection of new data to protect 

 public water supplies. To assist in this review process, the following 

 table, using the town of Barnstable as an example, table summarizes 

 governmental sources and types of information available to town plan- 

 ners. Additional information may be available from geotechnical engi- 

 neering companies which have performed work in Barnstable under a 

 contractual basis. 



