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SUMMARY 



This document reviews the physical oceanographic and geological aspects of the A. D. 

 Little Work Plan in suppon of the ESTM. Although little detail was provided about the physical 

 oceanographic and geological aspects of the study, these elements arc critical to an impact 

 assessment, and must be described fully and clearly in the Work Plan. In general, the rationale for 

 using the different mix of numerical models proposed by A.D. Little was not clear, and the 

 applicability of the various models (USGS, TEA, ELA, ADDAMS) in different mixes and to 

 address different questions, is not described. 



One specific area of concern is the reliance on the USGS physical oceanographic model of 

 circulation within the Bays system. The report describing modeling results will not be published 

 for months. Since the ESTM rebes heavily on the model results (p. 10-14), clearly the water 

 quality and circulation models must be available prior to completion of the ESTM. The TEA/ELA 

 model is not as comprehensive as the USGS/HYDROQUAL model, and therefore better results 

 would be expected from the latter model, which is progressing steadily forward. 



A further concern regards the transport paths and depocenteis for toxics contained within 

 the effluent. Though the USGS model is the best available description of water motion in the bay, 

 and thence of panicles moving passively with that water, toxics and other effluent material may 

 move much differently due to the effects of sedimentation. Sediments commonly strip the water of 

 various toxic chemicals, and carry them to the bottom rapidly. If this sedimentation occurs near the 

 outfall pipe, there is a potential for deposition and accumulation of toxic material in depocenters 

 throughout the Bay. Such accumulations could affect local benthic fauna, and also could be 

 transported through the food chain to the endangered species of concem. The Work Plan does not 

 address these issues for the outfall tunnel, though it does include some dredge material review in 

 its ADDAMS numerical model for the Third Harbor Tunnel Project, Boston Harbor Improvement 

 Project dredging activities, and Continued use of the Massachusetts Bay Dispwsal Site for dredged 

 materials disposal 



PHYSICAL MODEL REVIEW: 



The A. D. Little Work Plan proposes to use the ADDAMS numerical model to consider the 

 effects of three projects occurring in the same time frame as the MWRA outfall tunnel project: the 

 Third Harbor Tunnel Project, Boston Harbor Improvement I^roject dredging activities, and 

 Continued use of the Massachusens Bay Disposal Site for dredged materials disposal. In 

 addition, the Work Plan proposes to use the USGS model outputs (presumably the velocity data) 

 to derive a second set of water column concentrations. Since the USGS circulation model does not 

 appear to have a sedimentation component, the intent of this exercise as applied to dredged 

 sediments is unclear. Sediments are not passive particles, a notable distinction being their ability to 

 move vertically through the water column under the combined action of turbulence and gravity. 

 Since the USGS model appwars not to include a sedimentation component, the resulting questions 

 (1 and 2 on bottom of p. 11 of the Work Plan) cannot be addressed as proposed using the USGS 

 model. 



The A. D. Little Work Plan also proposes to use the hydrodynamic model output from the 

 USGS to suppon the ESTM for the following purposes: 



