APPENDIX F: EVALUATION OF APPROACHES TO DETERMINE RECHARGE AREAS 



Cape Cod Aqifer Management Project Final Report Page F-3 



Review of Existing Zone II Delineations 



The Committee has reviewed the methods used by SEA Consultants and 

 CCPEDC to estimate Zone II (Zone of Contribution) about the public-supply 

 wells in Barnstable and has concluded that both approaches yield reason- 

 able delineations of the zones. The methods employed are dependent upon 

 analytical models which use the groundwater flow equation or some deriva- 

 tive of it to calculate a groundwater divide (stagnation point). Both 

 methods then use a mass balance approach to circumscribe an area of ground- 

 water recharge or capture which would yield, on average, a quantity of 

 water equal to the assumed withdrawal from the well, and which is bounded 

 on the downgradient side by the stagnation point. The demonstrated appli- 

 cations by SEA and CCPEDC result in similar delineations, but are diffi- 

 cult to compare in detail because different input data were used for re- 

 charge rate, aquifer transmissivity , withdrawal rate, and initial water 

 table conditions. It is concluded that either method can yield an approxi- 

 mate delineation of the zone of contribution, but that they cannot be 

 precise It is observed that the analytical methods used for these delinea- 

 tions are based on simplifying assumptions which do not accurately repre- 

 sent nature with its variations and heterogeneity. Therefore, the delinea- 

 tions, while approaching average conditions, can not be expected to accu- 

 rately reflect the effects of the variations in the real world and there- 

 fore must be imprecise on point by point comparison with the field data. 

 It is further concluded that the input data are subject to judgemental 

 variation and perhaps manipulation which can seriously alter the resultant 

 delineations. The most sensitive of these factors are: recharge rate, 

 withdrawal rate, and initial water table conditions. There is a definite 

 need to establish standard criteria for assigning values to these factors 

 and for assigning aquifer transmissivity as well. 



Conclusions 



Analytical techniques such as those used by SEA Consultants and CCPEDC 

 are useful for preparing initial, simplified estimates of impacts of pump- 

 ing; however, they are incapable of simulating complex aquifer condi- 

 tions. The analytical techniques do not account for multiple withdrawal 

 are recharge points, boundary conditions, spatial and directional varia- 

 tion of aquifer properties and recharge, militarily aquifer systems, and 

 partially-penetrating wells. Numerical models, however, can integrate 

 these variables yielding a higher confidence level in model predictions. 



Recommendations 



1. A demonstration of three-dimensional groundwater modeling is recommend- 

 ed. Ideally, the demonstration would include conditions where the 

 advantages and disadvantages of the modeling approach could be defined 

 and compared with those of the analytical approaches. Opportunities 

 for model verification with past and future water-level data should be 

 utilized. The models should be applied to areas with complex boundary 

 conditions, multiple aquifer systems, multiple withdrawal points, and 

 areally variable recharge, variable aquifer thickness, partial penetra- 

 tion, and changes in aquifer storage. Additional analyses could in- 



