The human health criterion for mercury has been replaced by a criterion for methyl mercury and it is expressed as mg/kg. This means that the criterion is a concentration of methyl mercury in the tissue of a fish specimen. Methyl mercury criterion expressed as a concentration in fish tissue is useful for determining if a water body is impaired, but how will it be used in establishing permit limits? Will the limits be based upon total mercury or methyl mercury? If total mercury, how will an appropriate limit be derived?
I understand that EPA has a methodology for deriving an in-stream concentration or “reference ambient concentration” or RAC from essentially the same data used to derive the fish tissue criterion. This methodology is described in Chapter 2 of U.S. E.P.A.’s Technical Support Document For Water Quality-based Toxics Control, March 1991. We recommend that Table 40 include both a criterion in terms of mg/kg in fish tissue for listing purposes and one for in-stream concentration in terms of ug/l for use in the Department’s “Reasonable Potential Analysis” or RPA spreadsheet that is used to calculate effluent limits.
The methodology described above has a number of variables many of which would vary from stream to stream and from fish species to fish species. The criterion set forth in Table 40 could cite different fish species or could be based upon the more sensitive species. If a discharger is dissatisfied with the variables used to define the in-stream criterion, it could develop more site specific numbers if it so chose and provide them to the Department for consideration..
I make the above suggestions for two basic reasons:
(a) With an in-stream criterion, a discharger can use DEQ’s RPA spreadsheet to calculate potential effluent limits and better judge the impact of the proposed criteria.
(b) We believe that DEQ permitting staff already has extreme difficulties meeting their permit drafting commitments. This is because of the high complexity associated with determining water quality based effluent limits and meeting other requirements. Including an in-stream criterion in Table 40 would reduce the time staff would need to draft permits.
Frankly, I suspect that DEQ is anxious to put this rule-making process behind it and is likely to want to kick this can down the road to a later date in some IMD. I think it should be done now because it may turn out much more difficult than DEQ anticipates. Not to pour salt in an open wound, but the difficulty DEQ has had developing an IMD on implementing the temperature standard should provide incentive to address this issue now and not later.
Thanks.
Dick Nichols, P.E.
Senior Water Quality Engineer
521 SW 6th Street, Suite 100
Redmond, Oregon 97756
541 504-9960 x 225
541 504-9961 fax
dnichols@newtonconsultants.co