Harmonic Mean Flow - the Camel's Nose Appears Under the Tent Flap

NPDES permits issued under the Clean Water Act have limits that are set to protect water quality in the river into which pollutants are discharged.  The State's water quality standards are generally expressed as ratios, in the form of milligrams or micrograms per liter.  In order to comply with those standards most dischargers use a mixing zone, where concentrations of a pollutant (e.g., aluminum) in wastewater that are higher than allowed under the standards are allowed to mix with the receiving river, and are diluted. In order to determine how much dilution is available one has to know the flow of the river, the existing load of the pollutant in the river, and the pollutant load and flow of the discharge. The flow of the river used to calculate limits is referred to as the "critical design flow."

In issuing NPDES permits the West Virginia DEP usually uses a very conservative critical design flow referred to as the 7Q10, or the lowest 7 day consecutive flow that recurs at least once every 10 years.  It's a pretty low flow, meaning that the dilution available is low, and that means NPDES permit discharge limits are lower as well, because there's less water to mix with.  Even though the flow in the river, and therefore the amount of water available for mixing, is higher 99% of the time, the low 7Q10 flow is used to set limits  in order to protect water quality even during drought periods.

Using the 7Q10 flow for permitting makes sense if you're trying to protect aquatic life, because fish and bugs are present during low flow conditions.  But water quality standards are also set to protect drinking water supplies, and here is where the 7Q10 doesn't make sense. Human health water quality criteria for carcinogens are set at very low levels because the assumption is that the criterion for, say, benzene, must protect someone who drinks a liter of water  per day from that river for 70 years.  Since the low and high levels of the stream, and it's diluent capacity, tend to average out over that 70 years, a different flow can be used to set limits for carcinogens.  The critical design flow that should be used to calculate permit limits in that circumstance is often referred to as the harmonic mean flow, which roughly approximates the average flow of the river.

So a  low flow is used to set permit limits that  protect fish from acute effects of drought, and an average flow is used to set limits for carcinogens, which involve long term average exposures.  Or, at least, the harmonic mean flow should be used for setting permit limits for carcinogens.  Instead, for reasons I won't go into, but have everything to do with politics, and bear no relation to science, the 7Q10 is used to calculate limits for aquatic life and (overprotective) limits for human carcinogens.

All of this is background to the fact that, very quietly during the 2011 Legislative session, a change was made to the water quality standards to allow use of the harmonic mean flow to set permit limits in a certain place on the Ohio River. (See notice below.) My hat goes off to the accomplished lobbyist who put that change in the standards.  I hope that it is applied statewide in the years to come. 

The West Virginia Department of Environmental Protection has scheduled a public hearing on 2011 state legislative revisions made to Rule 47CSR2, “Requirements Governing Water Quality Standards.”

The hearing is scheduled for 6 p.m. on Tuesday, May 3, at DEP headquarters, 601 57th St., S.E., Charleston, in the Coopers Rock Training Room. In addition to oral comments provided at the hearing, the agency will accept written comments at any time up to the conclusion of the public hearing. No comments will be accepted after that time. 

Revisions made during the past legislative session added language to Rule 47CSR2 that addresses the critical design flow for determining effluent limits for carcinogens on the Ohio River between mile points 68.0 and 70.0.