real time hydrodynamic model is being 

 developed by Camp, Dresser and McKee . 

 This model, like the VIMS model, is 

 also a finite element model. The 

 model will be used to investigate the 

 nature of the hydrodynamics in areas 

 determined to have a dissolved oxygen 

 problem using the VIMS model. If 

 vertical mixing cannot be assumed, 

 then the depth averaged concentration 

 will be vertically profiled using 

 historical data. 



The key indicator used in this 

 approach to define the existence of a 

 eutrophication problem will be dis- 

 solved oxygen concentrations. In 

 order to assess possible biota 

 effects as the result of depressed DO 

 concentrations, an indices approach 

 will be taken. A preliminary report 

 on selected and developed indices for 

 use in the detection, measurement and 

 assessment of estuarine nutrient en- 

 richment was prepared in October of 

 1979 by the Chesapeake Research Con- 

 sortium and will be used for this 

 purpose. 



TOXIC RISK ASSESSMENT 



As a final component of the pro- 

 gram, the toxic chemicals of concern 

 will be subjected to a risk analysis. 

 These chemicals are being identified 

 in the sediments , water and selected 

 biota of the bay and from effluents 

 that enter the bay. Considerable 

 background data are being developed 

 on the sources, loadings to the tidal 

 estuary and transport and fate of 

 toxic chemicals in the bay. This 

 information will be used to develop 

 an exposure assessment model which 

 will estimate the concentration of a 

 toxic chemical at a specified loca- 

 tion and time to the extent possible 

 in the estuary. This model, coupled 

 with information on toxicity of 

 specified compounds to selected 

 organisms and knowledge about the 



distribution and abundance of organ- 

 isms, will be the basis for the risk 

 assessment. 



Three levels of risk assessment 

 are to be used. All chemicals of 

 concern will be evaluated by the 

 automated methods which constitutes 

 level one. Lists of toxic chemicals 

 will be compared to the chemicals of 

 concern. Chemical Abstract System 

 (CAS) numbers will be used in the 

 searches. The lists of chemicals 

 will include the proposed and estab- 

 lished water quality criteria chem- 

 icals and other lists of chemicals 

 from various resources which include 

 toxicity data that can be easily 

 stored on a computer system. Only 

 select chemicals will be evaluated by 

 the second level of risk assessment. 

 The second level involves a thorough 

 literature search and review for 

 select chemicals of concern. When 

 data are still deficient and/or there 

 is reason for extra concern, then the 

 third level of assessment will be in- 

 cluded. The third level involves a 

 structure-activity relationship ap- 

 proach. The structure-activity rela- 

 tionship approach invlolves the 

 generation of analogs and/or metab- 

 olites followed by a literature 

 search and review of these compounds. 

 Principles of chemistry and toxi- 

 cology are used to evaluate the 

 literature data and to estimate the 

 hazard of the chemical of concern. 



SUMMARY 



The bay-wide methodology as 

 presented here characterizes the 

 system as to what is coming into the 

 system, what is currently in the 

 system, how it moves about within 

 the system and an approach to assess 

 its impacts. In the toxics as well 

 as the nutrient program areas, the 

 framework is to identify the present 



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