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Thermal Rules Frequently Asked Questions

The Department is expected to review discharges for antidegradation whenever limits increase, whether they're water quality-based or technology-based. Increases aren't prohibited under NR 207, but the Department is required to evaluate whether the increase is needed and whether it's socially and economically justifiable, even when it increases based on new criteria.

Right now, discharges of toxic substances are exempt from antidegradation review when limits increase due to relaxed water quality criteria, but that only applies to toxic substances under the current rule in NR 207.03. This does not currently apply to thermal discharge. Therefore, thermal discharges that contain toxic or other substance additives are subject to antidegradation. If discharge water only receives heat addition, non-contact cooling water, this discharge is not subject to antidegradation regulation. As of the effective date of the thermal rule changes (October 1, 2010), discharges of non-contact cooling water to waters designated as Great Lakes waters, fish and aquatic life waters, or waters listed in Tables 1 through 5 in ch. NR 104 (see NOTE below) are exempt from antidegradation pursuant to s. NR 207.03(8) except in the following cases.

  • First, the discharge shall contain no additives other than those necessary to provide a safe drinking water supply or those similar in type and amount to substances typically added to a public drinking water supply.
  • Second, the discharge must comply with the thermal criteria in ch. NR 102.
  • Third, the Department must have determined that the discharge does not contain concentrations of substances, other than additives specified above, which will exceed water quality criteria established in ch. NR 105, or if such substances are present at levels exceeding ch. NR 105 criteria then the source of the water supply for the permitted facility must be the same water body to which the discharge occurs.
  • Finally, the discharge may not contain groundwater which is withdrawn from a location due to noncompliance with standards contained in ch. NR 140.

If the discharge of non-contact cooling water is to a water body designated as an outstanding or exceptional resource water, new or increased discharges are subject to antidegradation.

NOTE: When ch. NR 207 was revised in 1997, paragraphs within s. NR 207.03(8) were, unfortunately, not renumbered at the time. The water bodies intended to be covered under the above language were spelled out here, rather than incorporated by code section references which are no longer correct.

The maximum surface areas allowed for on-shore and off-shore discharges are different in the rule. For discharges to inland lakes or impoundments, on-shore discharges allow a maximum mixing zone area of 15,708 square feet, while off-shore discharges allow a maximum mixing zone area of 31,416 square feet, or twice the shoreline area. Please note this does NOT mean the limits differ by a factor of 2, as the surface area is only one factor or element in the calculation. These areas are based on the assumption of a full or half-circle with a radius of 100 feet. If the discharge is 100 or more feet off-shore, the maximum mixing zone is based on a full circle with a 100-foot radius or 200-foot diameter, while a semi-circle of the same radius is assumed for a discharge along the shore. Theoretically, if a discharge outfall is less than 100 feet off shore, the dimensions of the maximum mixing zone should be calculated based on the area of a figure extending from the shoreline too 100 feet away from the outfall, so it would be a site-specific calculation. Otherwise, any outfall 100 or more feet from the shore would be evaluated as an off-shore discharge.

For discharges to the Great Lakes, the difference is much less significant. There, the area definitions are based on dimensions specified in s. NR 102.07(1)(b). A shoreline discharge to Lake Michigan or Superior considers a maximum mixing zone based on a rectangle extending 1,250 feet along the shoreline in either direction from the outfall and 1,250 out into the lake. The result is a rectangle measuring 2,500 feet by 1,250 feet, or an area of 3,125,000 square feet. Off-shore discharges are considered to have an effluent plume based on a circle with a 1,000 foot radius, which equals an area of 3,141,593 square feet. Presumably, a discharge 1,000 feet or more from the shoreline would be considered off-shore. In reality, though, the difference between these two areas is only about ½ of 1%. Unlike the inland lakes or impoundments where the percentage difference is relatively much greater, in the Great Lakes the difference between these two areas is small enough that the distance from the shoreline will have a minimal impact on the calculated limit. Either mixing zone area could be used in the calculation of limits, with the results being almost exactly the same.

What specifically is required of the demonstration referenced in NR 106.53 (c) to obtain an alternate receiving water flow rate (Qs)?

In most cases, the Qs value used in the determination of thermal effluent limitations will be the same as the value used to calculate effluent limitations for toxic substances. Most often, that Qs is equal to1/4 of the Q7,10 – the estimated average 7-day low flow which occurs once in 10 years. Alternatively, ¼ of the 4-day, 3 year biologically-based low flow may be used if available. As stated in NR 106.53 (c), alternate Qs values may also be used if the appropriate data are available. Some permittees may request use of monthly or seasonal low flow values in the development of effluent limitations. In order to use these alternative values, permittees or a group of permittees must contact the USGS to obtain estimates of low flows or low flow updates. USGS will provide these types of estimates for a fee. At this time, the USGS contact for low flow estimates is hydrologist Rob Waschbusch (see contact information below). Alternative low flow values derived by USGS must be provided directly to the Department on USGS Letterhead and can be sent to Mr. James Schmidt – WT/3, Bureau of Watershed Management, 101 S. Webster Street, Madison, Wisconsin 53702.

Rob Waschbusch, Hydrologist
U.S. Geological Survey
Wisconsin Water Science Center
8505 Research Way
Middleton, Wisconsin 53562
Phone: 608.821.3868

Why is it beneficial to collect temperature measurements more than once per week, as required?

It is recommended that dischargers collect samples 12 times per month, instead of the required 4 samples per month, in order to provide sufficient data to quantify and use a P99 calculation.

What is meant by a "single highest discrete temperature"?

The single highest discrete temperature is the highest value recorded during a 24-hour data array.

What is 4Q3 and how do you use it?

4Q3 is defined as a 4-day average low flow found once in three years. That is similar in concept to a 7Q10, the 7-day average low flow found once in 10 years. As for how it’s used, the simple answer is "never." It is stated in s. NR 106.53(1)(a) that for thermal limit calculations that if sufficient information is available to calculate a biologically based receiving water design flow, 1/4 of the 4-day, 3-year biological flow is used. The important thing to note from this is that the 4-day, 3-year biologically-based low flow is, according to USGS and EPA, NOT necessarily the same as the 4Q3. According to USGS, their belief is that the biologically based low flow represents the flow at which negative consequences on instream biology would begin to be observed; from there USGS could determine a return period based on that flow to see if it corresponds to a 4-day, 3-year condition. The 4B3 should not be assumed to be statistically based, the way 7Q10 corresponds to a specific return period or frequency of occurrence.

In reality, though, EPA assumes the 4B3 and 7Q10 are similar to each other based on a 1986 document (Technical Guidance Manual for Performing Waste Load Allocations, Book VI, Design Conditions: Chapter 1—Stream Design Flow for Steady-state Modeling. August 1986), written as a prelude to their development of the Technical Support Document for Water Quality-Based Toxics Control. In Wisconsin, 4Q3’s have been calculated rarely, and 4B3’s not at all, and where the 4Q3 estimate has differed from 7Q10 has typically been where flow regulation has occurred such as a dam. Where such regulation does not occur, the two flows (4-day, 3-year and 7-day, 10-year) should be similar enough to exert minimal impact on calculated effluent limits. Where dams restrict all or a part of a river’s flow, rather than use any low flow estimate, we suggest finding out if there is a minimum flow requirement for the operation of the dam from an agency such as FERC. That would supercede any low flow estimate and is allowed under s. NR 106.53(1)(b).

NOTE: The same discussion applies to the use of low flows for the calculation of limits for toxic substances. The 4-day, 3-year biologically-based flow is also referenced in ss. NR 106.06(4)(c)5 and 6 for toxic substances, as well as s. NR 106.32(3)(b)1 specifically for ammonia, with the general reference to flow restrictions following those subdivisions.

What happens if I don’t have enough data to perform a reasonable potential analysis?

The reasonable potential analysis determines if a discharge is likely to exceed the calculated WQBEL. Without sufficient data, the Department will not be able to make an adequate reasonable potential analysis. In this case, the permit drafter will put the calculated WQBEL into the permit subject to drop. Once sufficient thermal data has been collected, the permittee can request for the thermal limit to be dropped from the permit based on the collected data. For more information see NR 106.56(12)

NOTE: If sufficient data has been collect to warrant a thermal limit be dropped from a permit and the "limit subject to drop" language has been included in the permit, the permit change will not be re-public noticed.

When is it required to monitor using continuous methods? What are the minimum temperature monitoring requirements?

There is no requirement to use continuous monitoring unless specified by the discharge permit. If unspecified, a discharger may collect data using continuous methods or non-continuous methods. Whether you choose to (or the permit requires you to) sample using continuous or non-continuous methods, data should only be collected during the active discharge. Therefore, the minimum requirement for non-continuous sample methods is to collect and record temperature at 6 evenly spaced intervals during the active discharge covering the 24-hour period; the minimum requirement for continuous sample methods is to collect and record temperature at intervals not more than 15 minutes during an active discharge in any 24-hour period (NR 218.04(13)).

For example: if a company discharges for 9 hours during a 24-hour period, they could sample every hour and a half using non-continuous methods or sample every 15 minutes (or less) during the 9 hour period using continuous methods. However, further specification may be written into the permit with regards to temperature monitoring.

Regardless of the collection method, the objective of the sampling is to determine the highest maximum value for any particular day that is associated with an active discharge period.

Do permits issued before October 1st but modified after October 1st for reasons other than thermal need to be evaluated for thermal limits at the same time as the modification?

If a permit is issued prior to October 1st, but gets modified after October 1st, that permit does not need to be evaluated for thermal limits if the modification is unrelated to thermal. In this case, the thermal limits would be evaluated, as typical, at the time of permit reissuance. It is advantageous, however, for facilities to begin thermal monitoring prior to permit reissuance.

Last revised: Tuesday May 15 2012