Surface water dischargers throughout the country are required to comply with the limitations set forth in their discharge permit. In many states, this includes thermal limitations which were implemented to protect fish and other aquatic life from mortality, immobilization, loss of equilibrium, impaired growth, adverse reproductive effects, and other sub-lethal effects.
In Wisconsin, NR 102 Subchapter II, which was finalized in September 2010, instituted water quality standards for temperature for the first time. For the next two years, discharge permit reissuances slowed considerably as the Wisconsin Department of Natural Resources (WDNR) educated staff and permittees on new water quality standards including thermal limits and the new phosphorus regulations (NR 217, November 2010).
Permit reissuances picked back up in 2012, with many permits including compliance schedules that allowed permittees up to 4 years to achieve compliance with the new thermal limits. As a result, some dischargers have had their final temperature limits in effect since 2016 or 2017, with many more compliance schedules coming due in the next few years.
Thermal Criteria Background
The thermal limits that are included in WPDES permits are based on water quality criteria defined in NR 102 and vary by the classification of the effluent receiving stream. Although some large waterways in Wisconsin (such as the Mississippi River and Rock River) have specific criteria listed, most waterways throughout the state are classified as non-Specific Waters and fall into one of the following categories:
- Limited Aquatic Life Waterways
- Limited Forage Fish Waterways
- Default Warmwater streams (both large and small)
- Cold Water Streams
Limited Aquatic Life Waterways have the least restrictive temperature limits, requiring discharge to remain below 86 degrees Fahrenheit year-round. All other classifications have criteria which may vary monthly.
In-stream criteria are translated into effluent limits using a formula which includes variables for ambient temperature, low flow of the stream, and effluent flow rate. It’s worth noting that this calculation does not take into account the temperature of the historic discharge, only the flow. Historic temperatures are used to determine reasonable potential to exceed the calculated limit. If there is reasonable potential, the limit is included in the permit. Otherwise, it is not. Reasonable potential exists if the P99 (99th percentile of historic data) is greater than or equal to the calculated limit.
Options for Compliance
Several options exist for complying with thermal limits. Construction is often the most obvious option for complying with any parameter. However, capital and operating costs, ambient weather conditions, electricity use, toxicity issues, and/or technology limitations may not make construction the most favorable approach for thermal compliance. Other options permittees can consider to either supplement constructed treatment technology, or to avoid the need for new construction entirely include the following:
 
Site-Specific Low Flow Determination
Stream flow is used to calculate temperature limits. Often, however, the stream flow used by WDNR to calculate limits is an assumed value, or a value from a nearby stream. Because the flow can have a significant impact on the final limits, it may be wise for permittees to pursue site-specific low flow determinations.
Receiving site-specific low flow data is typically a quick turnaround from USGS with data in hand within a few weeks. Once in hand, this site-specific data can be used to more accurately calculate effluent temperature limits. Often, this site-specific low flow determination is the first step of many in ultimately complying with thermal effluent limits.
Instream Mixing Study
If the site-specific low flow determination showed that there is no flow, an instream mixing study is not an option. In that case, alternative effluent limits (discussed further below) or construction may be the permittees only options. If, however, there is base flow in the stream, a mixing study may be a good option for relief on thermal limits.
When determining thermal limits for permits, WDNR assumes that the effluent plume mixes with 25% of the stream. However, the completion of an instream mixing study can help permittees demonstrate that their effluent mixes with up to 100% of the stream, resulting in calculated limits which are significantly less stringent. Mixing studies can be approved for any percentage of the stream, but have typically been approved for 50%, 75% or 100%.
Mixing studies should be completed during low flow conditions, typically in the Fall. While there is some planning before the study and data analysis following the study, the field work itself can be completed in a day. To complete a mixing study, a biodegradeable dye is added to the effluent and photos or video are taken to show that the colored effluent mixes across the width of the stream. For a more accurate mixing study and to ensure that mixing isn’t only happening at the surface, WDNR recommends taking temperature and conductivity cross-sections of the stream as well. WDNR’s rule of thumb is that complete mixing must be achieved within two stream widths downstream of the discharge. When developing a plan for a mixing study, it would be wise to place cross sections at 0.5, 1, 1.5, 2, and 2.5 stream widths downstream. Submitting the mixing study plan to WDNR, though not required, is recommended prior to the completion of the study. This ensures that the correct data is gathered when in the field. Permittees can pursue these studies on their own or with the help of an experience consultant.
Alternative Effluent Limitations
When low flow determinations and instream mixing studies haven’t been sufficient to prevent the need for construction to comply with thermal effluent limits, permittees may choose to pursue a biological study to support Alternative Effluent Limits (AELs). The purpose of the study is to prove that calculated limits are more stringent than necessary to ensure the protection and propagation of the balanced indigenous community of shellfish, fish, and wildlife in and on the body of water into which the discharge is made. This is done by continuously monitoring instream temperatures during all months for which AELs will be requested and completing macroinvertebrate sampling in either the Spring or Fall, and fish sampling during propagation in the Spring and during the Summer to determine the resident population. The complete study takes a year to complete the field work. If permittees choose to pursue AELs, beginning the conversation with WDNR approximately 2-2.5 years prior to permit reissuance is recommended so there is time for study plan development, field work, and data analysis to submit the final request. This timeline can be pared down if necessary, but that can add undue stress to the permittee, the consultant, and WDNR.
Historically, Alternative Effluent Limits (AELs) have been used by dischargers on large bodies of water like Lake Michigan. For dischargers to smaller waterways, there have been a few successful AEL requests in the last few years. When successful, these studies have resulted in thermal limits that are 10-20 degrees less stringent than originally proposed by WDNR.
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With unrivaled experience in the wastewater industry, The Probst Group can help you meet your thermal limits. For more information, or to discuss your options for thermal compliance, please contact us at 262.264.5665 or info@probstgroup.com
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