Presentation Abstract

Dischargers to streams in Wisconsin and other states are typically only allowed to use 25 percent of the seasonal low flow for chronic toxicity mixing zones. In Wisconsin, 25 percent is used as a default percentage unless the Department of Natural Resources (DNR) determines through their own observations that the discharged effluent mixes rapidly and completely with the stream within a short distance of the outfall, as is often the case with very small streams, or if there is an obvious zone of passage available for fish and other mobile aquatic life. If this determination is made, the DNR may allow 100 percent of the low flow to be used. The DNR may also allow a permittee to perform a mixing zone study to determine the geometry and volume of the mixing zone. The chronic toxicity mixing zone volume is important because it is used to determine the instream waste concentration (IWC) for whole effluent toxicity tests, and may also be used for effluent limit calculations related to toxic compounds such as chloride, ammonia, and metals.

This presentation will include a discussion of the reasons for performing mixing zone studies, work plan preparation, field procedures, calculations, and results of mixing zone studies. Several scenarios will be presented to help the audience understand when such studies are warranted. There may be situations when mixing zone studies are not warranted, and these will be discussed as well.

The objectives of a mixing zone study may include one or more of the following: (1) determine the approximate geometry of the effluent plume, (2) determine at what location the effluent mixes with the river and whether this location indicates mixing is rapid, or (3) determine whether the effluent plume hugs the bank until it completely mixes with the stream, such that free passage is provided for aquatic life.

Mixing zone studies are conducted in accordance with an agency-approved work plan. For streams, the fieldwork should be performed during a period of low flow. Typically, a state- or USEPA-approved dye is injected into the effluent to provide a visual indication of the plume geometry and size. The concentration of dye in the effluent and receiving water can also be measured using fluorometric or other methods. Field parameters such as conductivity and temperature are typically measured during the study. If a real-time USGS gauging station is not available for the stream reach, the stream velocity and flow rate may be determined in the field using methods that will be presented.

The field data are evaluated to help determine the plume geometry and the amount of mixing occurring in the receiving water. In streams, this may include comparing the calculated theoretical complete mixed concentration of a conservative parameter to the actual field results for that parameter. The geometry and size of the effluent plume are then compared to state guidelines to determine if the plume is rapidly mixed or allows a zone of passage.

The results of a mixing zone study may be used by the regulatory agency to recalculate the allowable IWC and, potentially, effluent limitations for toxic compounds such as chloride, ammonia, or metals. Depending on the seasonal low flow in the receiving stream (i.e., the Q7,10), changing the mixing zone from 25 percent of the low flow to 100 percent can represent significant cost savings to the permittee. At a minimum, it may result in fewer chronic whole effluent toxicity test failures. In some cases, it may result in removal of a parameter from a discharge permit. The ultimate outcome could include reduced monitoring and treatment costs.