06-096 C.M.R. ch. 530, § 4 - Water Quality Based Limit Derivation

Water quality based limits must be developed by using the following procedures.

A. Calculation of dilution factors. A simple dilution model using design stream flows specified in this section must be used to determine allowable effluent limits unless there is information that makes another model approved by the Department more appropriate. All substances are assumed to be persistent in the environment. With a non-continuous discharge (such as a lagoon which can be impounded or a continuous discharge prohibited from discharging under specified conditions), the dilution factors can be based on a guaranteed minimum stream flow or tidal stage below which a discharge will not occur. The discharger must submit a request for a license modification that reflects a different minimum stream flow. If the Department approves an alternate stream flow, the license must include a monitoring and reporting requirement, and must include an accurate means of measuring stream flow that is calibrated annually.

(1) Dilution factors (DF) for freshwater discharges are calculated using the following models:

(a) If the entire water supply that ultimately makes up the effluent flow (Qe) is taken from the receiving water upstream of the location from which the stream design flow (Qr) is calculated or measured, then:

DF = Qr/Qe

(b) If the water supply is comprised of water taken from the receiving water (Qw) and/or from another location (Qo), then:

DF = (Qr + Qo)/(Qw + Qo)

(2) For estuaries where tidal flow is dominant and marine discharges, dilution factors are calculated as follows. These methods may be supplemented with additional information such as current studies or dye studies.

(a) For discharges to the ocean, dilution must be calculated as near-field or initial dilution, or that dilution available as the effluent plume rises from the point of discharge to its trapping level, at mean low water level and slack tide for the acute exposure analysis, and at mean tide for the chronic exposure analysis using appropriate models determined by the Department such as MERGE, CORMIX or another predictive model.

(b) For discharges to estuaries, dilution must be calculated using a method such as MERGE, CORMIX or another predictive model determined by the Department to be appropriate for the site conditions.

(c) In the case of discharges to estuaries where tidal flow is dominant and marine waters, the human health criteria must be analyzed using a dilution equal to three times the chronic dilution factor.

B. Stream design flows. For estuaries where tidal flow is not dominant and fresh waters, stream design flows used in the analyses of dilution factors from dilution models must be consistent with the exposure of the population at risk to any and all toxic pollutants. In situations where an island, channel or other physical circumstance divides the flow of a stream, the Department shall use appropriate adjustments or assumptions that best describe that particular situation.

(1) Analyses using numerical acute criteria for aquatic life must be based on 1/14 of the 1Q10 stream design flow to prevent substantial acute toxicity within any mixing zone and to ensure a zone of passage of at least 3/4 of the cross-sectional area of any stream as required by Chapter 581. Where it can be demonstrated that a discharge achieves rapid and complete mixing with the receiving water by way of an efficient diffuser or other effective method, analyses may use a greater proportion of the stream design flow, up to and including all of it, as long as the required zone of passage is maintained. Flows that allow bioaccumulation of compounds to levels that are toxic, carcinogenic, mutagenic or teratogenic are not to be used in setting effluent limits.

(2) Analyses using chronic criteria for aquatic life must be based on 7Q10 stream design flow.

(3) Analyses using human health criteria must be based on harmonic mean or other stream flows consistent with the duration of exposure.

C. Background concentrations. The background concentration of specific chemicals must be included in all calculations using the following procedures. The Department may publish and periodically update a list of default background concentrations for specific pollutants on a regional, watershed or statewide basis. In doing so, the Department shall use data collected from reference sites that are measured at points not significantly affected by point and non-point discharges and best calculated to accurately represent ambient water quality conditions. The Department shall use the same general methods as those in section 4(D) to determine background concentrations. For pollutants not listed by the Department, an assumed concentration of 10% of the applicable water quality criteria must be used in calculations. Alternatively, dischargers may provide the Department with site specific information of the same nature to supplement or replace the foregoing values. Sampling plans and/or other means of developing site-specific information must be approved in writing in advance by the Department, and must be consistent with the concepts described in section 4(D), below.

D. Site-specific adjustments for local conditions. The Department may use available information to evaluate physical and chemical characteristics of a specific receiving water and adjust calculations of the degree to which they influence the relative toxicity of individual pollutants in that situation. The information may include tests conducted by the Department, the discharger or another organization, provided that approved methods are used for sample collection and analysis. Once being accepted by the Department as valid data, this information may be used in place of the assumptions used to develop statewide water quality criteria for the effected pollutants and discharger.

(1) While the Department may require or allow different sampling procedures best designed to provide relevant and meaningful information in specific situations, sampling plans must generally include collection of receiving water samples at appropriate locations at least once a month for a period of a year. Samples for temperature, pH and salinity must be collected at a point directly upstream or up current of the discharge. Samples for hardness or other characteristics must be collected at a point upstream in the receiving water not significantly influenced by point source or non-point source discharges that may affect the characteristic in question.

(2) If a discharger wishes to have its effluent evaluated using water quality criteria expressed as dissolved metal instead of total metal, it may submit to the Department supporting test information. Upon finding the information appropriate and complete, the Department shall use that information to develop a translator value following guidance provided by EPA ("The Metals Translator: Guidance for Calculating a Total Recoverable Permit Limit From a Dissolved Criterion, USEPA publication 823-B-96-007, June 1996, EPA, Office of Water, Washington DC,). The testing done must include samples of the effluent and receiving water mixed in proportion to the discharger's dilution factor and analyzed for total and dissolved metal, and samples designed to evaluate the impact of loadings of total metal to receiving water system.

In evaluating information submitted, the Department shall use the combination of chemical characteristics, stream flows and times of the year that best represent the most critical or sensitive conditions for assessment of toxic impacts by specific pollutants.

E. Water quality reserve. In allocating assimilative capacity for toxic pollutants, the Department shall hold a portion of the total capacity in an unallocated reserve to allow for new or changed discharges and non-point source contributions. The unallocated reserve must be reviewed and restored as necessary at intervals of not more than five years. The water quality reserve must be not less than 15% of the total assimilative quantity. The Department may increase this amount where it has information that significant non-point sources of a pollutant are present in a watershed. The Department may allocate quantities held in water quality reserve to new or changed dischargers according to the principles of the State's anti-degradation policy described in 38 MRSA, section464(4)(F). Notwithstanding the above, for the purpose of calculating waste discharge license limits for toxic substances, the department may use any unallocated assimilative capacity that the Department has set aside for future growth if the use of that unallocated assimilative capacity would avoid an exceedence of applicable ambient water quality criteria or a determination by the Department of a reasonable potential to exceed applicable water quality criteria.

F. Effect of multiple discharges. Where there is more than one discharge into the same fresh or estuarine receiving water or watershed, the Department shall consider the cumulative effects of those discharges when determining the need for and establishment of the level of effluent limits. The Department shall calculate the total allowable discharge quantity for specific pollutants, less the water quality reserve and background concentration, necessary to achieve or maintain water quality criteria at all points of discharge, and in the entire watershed. The total allowable discharge quantity for pollutants must be allocated consistent with the following principles.

(1) Watershed-wide determinations of the total allowable discharge quantity, allocations to dischargers and the water quality reserve amount must be updated at intervals of not more than five years. The results of updated allocations will be made available to interested parties for comment.

(2) Evaluations must be done for individual pollutants of concern in each watershed or segment to assure that water quality criteria are met at all points in the watershed and, if appropriate, within tributaries of a larger river.

(3) The total assimilative capacity, less the water quality reserve and background concentration, may be allocated among the discharges according to the past discharge quantities for each as a percentage of the total quantity of discharges, or another comparable method appropriate for a specific situation and pollutant. If there is a need to reduce past discharge quantities, the Department may exclude de minimus discharges and for remaining discharges establish necessary reductions and effluent limits with proportional consideration of such factors as each facility's relative size and concentration of pollutants. Past discharges of pollutants must be determined using the average concentration discharged during the past five years and the facility's licensed flow. Where insufficient discharge data exist to accurately characterize a discharge, or for new discharges, the Department shall use an effluent concentration representative of similar discharges. For effluent concentrations reported as non-detectable, the concentration must be taken as one-half of the detection level specified by the Department.

(4) When calculating past discharge loads for use in this section, the Department may exclude data that represent improper control of pollutants, are non-representative of a discharge or violate the established effluent limits or water quality limits as calculated on an individual discharge basis for the facility.

(5) The amount of allowable discharge quantity may be no more than the past discharge quantity calculated using the statistical approach referred to in section 3(E), but in no event may allocations cause the water quality reserve amount to fall below the minimum referred to in 4(E). Any difference between the total allowable discharge quantity and that allocated to existing dischargers must be added to the reserve.

(6) The total allowable discharge quantity must be set to assure compliance with water quality criteria in downstream estuarine and marine waters.

(7) In conducting evaluations, the Department shall use the same background concentrations and other receiving water characteristics for all discharges in an affected segment. (Adjustments made pursuant to sections 4(C) or 4(D) may be developed in a common effort and applied to all dischargers in the affected segment.)

Notes

06-096 C.M.R. ch. 530, § 4