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An industrial user was issued an NPDES permit that required a thermal impact study be conducted in order to evaluate the effects of the thermal discharge from the industrial user on the receiving water. The thermal impact of the discharge was to be evaluated based on the size and temperature gradient produced by the thermal plume throughout the year. The temperature gradient and plume size information would in turn be used to evaluate the effect of the temperature changes on the fish communities in the receiving water.

The thermal discharge from the facility comes from various non-contact, single pass cooling water streams. The facility's discharge enters a 4,000-foot underground pipe which then enters the river through a multi-port diffuser located on the river bed. The facilities operations are seasonal, with the peak-operating season being during the fall and winter. During the peak season, the temperature of the thermal discharge is likely to be highest and the temperature in the river at its lowest. It is during the peak season that the industrial user would have the maximum impact on the river.

The river is quite large, around 700 feet wide and 12.5 feet deep, and has a flow rate between 2,467 cubic feet per second (August) and 14,090 cubic feet per second (March) (U. S. Geological Survey, 2000). The combination of the flow rate in the river and the multi-port diffuser being on the bottom of the river made it difficult to obtain temperature measurements in the river. Therefore, in lieu of field measurements, the thermal plume and gradients would be mathematically modeled.

The Cornell Mixing Zone Expert System (CORMIX) was used to model the thermal impact on the river. CORMIX is an U.S. EPA-approved methodology for simulation of mixing behavior that covers a majority of common discharge and environmental conditions. For the purposes of this study, CORMIX was used to determine the point down river where the thermal discharge decreased to a temperature where it would be in compliance with the set forth water quality standard. At this point, the size of the thermal plume would be calculated.

Information regarding the thermal discharge and the river is needed to accurately model the thermal plume. This information was obtained from the industrial user, state (Michigan Department of Environmental Quality, 2001) and several federal governmental agencies (U.S. Geological Survey, 2000; U.S. Department of Commerce, 1996). The CORMIX program was utilized to determine the thermal plume size at a range of discharge flows/temperatures and river flows/temperatures. Table 1 shows a summary of the CORMIX simulations performed during the times of the greatest impact of the industrial discharge on the river.

The CORMIX simulations showed the multi-port diffuser was very effective at dissipating the heat from the thermal discharge. The diffuser ports cause the thermal discharge to be ‘bubbled’ into the river creating a large surface area for the heat to dissipate. The CORMIX simulations showed the plume produced by the diffuser was almost two-dimensional. This was because the diffuser ports are arranged along a 50-foot pipe and the discharge flow rate forces the flow out vertically, with very little horizontal mixing. The effective heat dissipation of the diffuser in combination with the high flow rate of the river yields a relatively small plume volume of elevated temperature in the river.

Using the thermal plume sizes and temperature gradients obtained from the CORMIX simulations; a literature search was conducted to assess any impacts the thermal plume and elevated temperatures may have on fish in the river. The research obtained during the literature search showed that the elevated temperatures in the river produced by the thermal discharge are not elevated enough to cause any stress or possible mortality for any of the species native to the river (Clapp et al., 1997; Holmes et al., 1994; Hosanson, 1977; Madenjian et al., 1986).

The results of this study have been submitted and approved by the NPDES control authority.

The use of the CORMIX model allowed for simulation of many scenarios that would be impossible to simulate in the field. This approach allowed more data to be gathered and studied than would have been possible without the CORMIX model, saving time and resources in the process.
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Document Type: Research Article

Publication date: 2003-01-01

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