By Seth Brown, stormwater program and policy manager, Water Environment Federation
On Jan. 3, a Federal Court in Virginia ruled that runoff and other “nonpollutants” could not be used as surrogates for pollutants to meet a total maximum daily load (TMDL). However, there is significant misinformation or misunderstanding of the case. For example, referring to the TMDL as attempting to “regulate water” is a common misconception.
TMDLs employing flow or impervious cover as a surrogate are known as “flow-based TMDLs” and have been used in several other cases, especially in the Northeast. This article clarifies the facts and nuances of this case in order to provide proper context for larger issues and potential implications that go beyond this ruling.
Ruling and results
The flow surrogate in Accotink Creek’s TMDL was based on sediment rating curves, which helped determine flows that could be generated within the watershed while still meeting the creek’s water quality standard. According to the analysis, the existing unit-area flow rate would have to be reduced by 48%, from 92.5 m3/ha•d (1327.7 ft3/ac•d) to 47.7 m3/ha•d (681.8 ft3/ac•d; see the technical notes at the end of this article for more details on how the TMDL was calculated).
Reducing flows by 10% to 20% in an urbanized watershed is a technically significant challenge, but requiring a reduction in runoff of nearly 50% may be unrealistic. Natural features, such as poorly draining soils and steep slopes in the watershed, present serious technical limitations.
Background on Accotink watershed
The TMDL at issue is for Accotink Creek (Fairfax, Va.). Much of the watershed was developed in the 1950s and 1960s, long before the impacts of stormwater runoff were well understood and regulated. It is a typical urbanized watershed in the Mid-Atlantic region with nearly 25% total impervious cover. This is especially significant for a watershed that is 124 km2 (47.9 mi2) in area. It is notable that more than 70% of the watershed’s soils are D-type soils and 10% are type C. A combination of significant urbanization, steep slopes, and poor-draining soils leads to high amounts of stress on streambanks and streambeds throughout the system, which is exacerbated by a lack of stormwater management throughout.
Fairfax County, which holds the Municipal Separate Storm Sewer System program (MS4) permit for the Accotink Creek watershed, has been moving forward aggressively in its development of an innovative and well-funded stormwater program aimed at reducing runoff volumes, as well as protecting and restoring streams in Accotink Creek and beyond. I believe Fairfax County is not against doing all it can to address the impacts of uncontrolled stormwater runoff. However, the county’s involvement in the case against the U.S. Environmental Protection Agency (EPA) may be more about how to control and manage runoff rather than if it should do so. Perhaps stormwater runoff is best regulated under the National Pollutant Discharge Elimination (NPDES) program, as it respects the complex programmatic nature of stormwater management.
TMDL versus NPDES
The TMDL program works upstream to determine the acceptable daily pollutant load a stream can receive while still maintaining water quality standards. The program does so with little consideration of the practical ability to implement the TMDL.
To contrast, the NPDES MS4 has traditionally been more flexible and pragmatic in the management of runoff. This is evidenced by the recognition of nonquantitative aspects of stormwater management, such as public engagement, participation, and outreach. The MS4 program relies on local government’s knowledge of how to best manage stormwater in its jurisdiction. The program is also iterative and, therefore, adaptable, which allows for incorporation of new technologies and approaches as they arise.
Wrong tool for the wrong time
While this legal question is not likely to disappear anytime soon, a bigger question looms about whether the Clean Water Act is out of date. This legislation has provided many benefits since its inception in 1972. However, the growing list of legal questions at various levels and across numerous programs indicates that a new Clean Water Act is needed to tackle the water quality problems of today and in the future (see the Water Environment Federation’s Clean Water Act Modernization position statement). This should compel stormwater professionals to call on Congress to address one of the most significant and aggressively growing water quality problems in this country.
It should be noted that the soil type distribution for the unimpaired watersheds are significantly different than the Accotink Creek watershed. For instance, while over 70% of Accotink Creek’s soils are Type D (poorly-draining), the soils in Buffalo and Catoctin Creek watersheds (the unimpaired watersheds) are 53% and 70% Type B soils (well-draining soils), respectively. This differentiation in dominant soil types between the subject watershed and the unimpaired reference watersheds points to a related divergence in the runoff-sediment relationship as well, which may call in question the resulting allowable runoff unit volume for Accotink Creek.
The analysis in the Accotink ruling identifies what I consider to be the key path to a solution by stating (see page 8, under Chevron Step Two): “If sediment level is truly ‘a function of’ the amount of stormwater runoff, as EPA claims, then the TMDL could just as easily be expressed in terms of sediment load.” Pretty simple, in my opinion. If, however, the so-called “function” that relates sediment level to stormwater quantity is not well known, the TMDL approach allows such uncertainty to be included in the margin of safety.
John,
Good point regarding the margin of safety in the TMDL development. One thing to consider is just how uncertain the flow-sediment relationship is actually. Sediment transport theory is very complex, and it is not uncommon for results to be off by an order of magnitude – this is a significant difference. But focusing specifically on the Accotink case, the two unimpaired reference watersheds used have much better draining soils (70% B soils) when compared to Accotink (70% D soils), so the inherent difference between Accotink and the unimpaired watersheds goes beyond just uncertainty, in my opinion.
Seth, Based on your article it seems like the problem in Accotink is the erosion in the streams. The erosion is more like be the result of high flows (or even extremely high). Most of the flow control (also WQ ) structures tend to control the initial flush and not very high peaks. TMDL implementation that reduces 50% total runoff will probably have no effect on the real problem, unless it is the reduction of peak flow. If it is the peak flow, how are they expecting to reduce the peak flow from a 100 year or higher rainfall event?
Sabu,
Based upon fluvial geomorphic theory, the “channel-forming” discharge (often referred to as the bankfull or “effective” discharge) is that flow which – which looking over a long temporal horizine – carries enough erosive power yet occurs frequently enough to move the most sediment in the stream. Intuition may tell you that it is large, flooding (100-year) events that move the most sediment in a stream, but in most cases, it is a much higher-frequency event, such as the 1.5-year storm, that actually reflects the synergy between stream power and flow frequency. It should be noted that in an urban context, the channel forming flow frequency has been known to be as high as 1-year or even a 6-month event. Added to this flow dimension is the duration of flow – when flow in a channel exceeds the scouring velocity for a long period of time, the marginal amount of erosion which occurs in the channel increases over a situation where the duration of scouring is very short.
With all of this in mind, a reduction of the unit volume of discharge within a watershed for the 1-year event would indeed reduce the amount of erosion within the stream.