Regulating A Water Quality Control Volume
Filed in Stormwater Management |
We have posted before regarding an approach to integrating water quantity regulation with water quality regulation. We have reviewed other programs nationally and surveyed the literature and trends in this regard, and believe that designating a “water quality control volume” would meet the goal of integrating the Counties strong water quantity regulatory program with water quality.
Many studies already have identified that pollutants accumulate on impervious surfaces, such as parking lots, roofs, driveways and roads. Drainage from these surfaces is almost always “improved” by addition of storm inlets and storm sewers. Unlike pervious surfaces, impervious surfaces begin to discharge stormwater in much smaller events, contributing to annual Runoff volume significantly as well as washing the accumulated pollutants into the water column in what we generally have called the “first flush”. As we have been discussing, the impervious surfaces of the developed site are a “marker” for stormwater quantity and quality, and in concept then, whatever is the “right” water quality control volume should be applied to the impervious area of a site.
Designing for water quantity in terms of flood damages, we typically target much larger storms. Our small release rates tend to mimic the effect of measures which are explicit for “runoff volume control”, but do not capture and treat the first flush.
Discussing this issue at the Ordinance Update Steering Committee, we agreed that a re-focusing and change in emphasis was needed to better meet these dual quantity and quality challenges. The current ordinance demands a high level of stormwater infrastructure to meet it already, and it is not really necessary to think of adding new costs to meet these goals. Instead, we would redistribute the emphasis to fully acknowledge the “water quality control volume” as a high priority, with a corresponding de-emphasis on the total volume of detention storage required.
How much is “enough” for a water quality control volume? Again, we can turn to some analysis of rainfall statistics that compare total rainfall in each storm to the total numver of “rainfall events” in a year. We find the following:
- 70% of all rainfall events in a year have a total rainfall depth of 0.54 inches or less
- 85% of all rainfall events in a year have a total rainfall depth of 0.85 inches or less
- 90% of all rainfall events in a year have a total rainfall depth of 1.06 inches or less
In the above analysis, “events” are separated from each other by a minimum of 3-hours. How would this be applied? The above list thought of another way says, I need to design to capture and treat 1.06 inches of rainfall, applied over the impervious area of a site (assuming it is basically all runoff) if I wish to treat 90% of all the storms that occur. We are ignoring the pervious areas in these “small events, which is a simplification but is balanced by the differences in impervious surfaces runoff characteristics when not all of the drainage from impervious surfaces is “improved”.
So if a site came in that was 10 acres in size and in its developed state it would be 50% impervious, and we wanted to capture 90% of all the events that occur in a year, a simplified approach would be to apply 1.06” rainfall depth to 50% of 10acres, for a total “water quality control volume” of 0.42 acre-feet that would need to be intercepted, held and treated by the use of BMP’s. Corresponding to that calculated volume would be the simplified approach of “subtracting” that volume from any calculated site runoff storage (detention) volume requirement that applies.
So here are some questions for feedback.
- Is this a good idea?
- What percentage of storms, or what rainfall depth, should be used to define the water quality volume? 85% of events-0.85” of rain? More? Less?
- How should it be applied?
Tuesday, March 2nd 2010 at 11:13 am |
Overall I support a water quality treatment volume (WQTV) requirement. However, I believe it is more complex than just requiring a WQTV. For instance, treating 70% of all storm events may be a practicable solution for TSS, but it may not be for phosphorous. It may take treating 90% to be effective for phosphorous. This leads me to several questions. Do we see an appreciable increase in the pounds of a pollutant removed in a year with an increase in the percentage of rainfall events treated? Does it vary depending on the pollutant? What pollutants are we trying to target?
Furthermore, we can design an extended dry detention basin to meet the WQTV requirement and achieve a 20% removal efficiency for phosphorous. We could also design a wet detention basin to meet the same WQTV requirement, but in this case we achieve a removal efficiency of 51%. In both cases we met the WQTV requirement, but in the later case we removed over twice as much of the pollutant. I would recommend that the BMP or BMPs chosen to meet the WQTV requirement be required to meet a certain amount of reduction for the pollutants that we want to target.
The pollutants targeted and the amount of reduction can be based on the pollutants identified in IEPA’s impending TMDLs. I would also propose setting the percentage of storm events to be treated at a level which would achieve the required amount of reduction needed to meet the TMDLs for the targeted pollutants. For example, if treating 85% of the storm events would allow us to reduce the amount of TSS and phosphorous to meet TMDL requirements, then we should consider setting it at this level. Although IEPA is just now beginning to develop TMDLs, we can choose an interim representative TMDL to help us determine the percentage of storm events to treat. I know this probably won’t give us an exact answer as I illustrated, but it may give us some direction.
Lastly, we may not have to subtract the WQTV from the runoff storage volume required. In many cases the WQTV can be designed into the detention basin. Design of sediment removal fore bays, weirs, berms, wetland cells, micro pools, marshes and control structures can all be incorporated in the detention volume while at the same time meeting the WQTV requirement. It’s just a matter of reconfiguring the basin to provide these treatment features.
Tuesday, March 2nd 2010 at 1:02 pm |
I dont think that new development is going to be the vehicle alone by which we can meet TMDLs. I agree that we can incorporate a Water Quality Control volume into the design of a detention basin, what I am suggesting though is that the WQCV be subtracted from the calculation of the required volume of done more locally to where the runoff occurs than the basin.
Monday, April 5th 2010 at 8:14 am |
While user StormBlog9 presents many good points, I would caution that the Steering Committee not rely on impending or approved TMDL reports as the basis for identifying pollutants parameters to reduce. One reason for this avoidance is highlighted through the DuPage River/Salt Creek Watershed TMDL Stage 1 Report (January 2008). That report proposes assigning waste load allocations (WLA) for the manganese and silver parameters when the data indicate that there are only two water quality standard excursions in 45 (4%) observations ranging over the past twenty-five years for manganese and 1 of 18 (6%) and 4 of 195 (2%) over the past thirty years. Nearly none of these excursions, and sparse monitoring performed for these parameters, have occurred within the past five years. Requiring development and redevelopment projects to address pollutant issues that are likely not related to their projects perpetuates the issue that has plagued TMDLs for the past decade; specifically, forcing facilities to reduce pollutant loads whether or not they are the source of pollution because they are the only entity that can be regulated. Finally, phosphorus will not be included in the TMDL report for DuPage County, as there is not a water quality criterion for phosphorus in free-flowing streams. It is important that the revised ordinance maintain focus on the pollutant categories for each type of development project, as included in the most recent ordinance revision.
Friday, April 23rd 2010 at 2:23 pm |
I stand corrected. After more careful consideration I would have to agree with StormBlog44 that we must be very careful to make storm water quality requirements that will actually result in a tangible benefit. We need to see more fishable and swimable streams which is the goal of the Clean Water Act. Improving the water chemistry will not improve the aquatic biology. The DuPage River/Salt Creek Workgroup’s monitoring and analysis clearly shows this to be true. There are, however, some instances where improving the water chemistry is needed where it directly affects the ability to use the water as a drinking source and where it make certain polutants make it unsafe for physical contact. Therefore, we should be careful when drafting the requirements to ensure engineers are chosing the most appropriate BMP for their particular polutant(s) from their development and that the BMP is one that achieves the best result possible given the constraints of the project.
Tuesday, March 9th 2010 at 9:57 am |
I don’t think anything we do right now is going to answer the phosphorous issue. It is an issue unto itself and needs an agressive campaign, al. la. Lake Cty. If we “go there” I think it will make the IDDE discussion (outlaw cheerleaders washing cars) pale by comparison. I think a WQCV approach will go a very long way to addressing most of the pollutants of concern. I’m not suggesting we ignore phosphorous, just that, if we attempt to address it in the manner necessary, the whole ordinance re-write could be derailed. I think we should address it in these terms: “We were charged with re-writing the ordinance and we realize that the phosphorous issue is too great to be handled by our task alone, that the County should address it comprehensively and immediately.”
Tuesday, June 22nd 2010 at 1:38 pm |
From a policy standpoint, this adds another layer of regulation, which increases the cost of compliance. There needs to be a reasonable limit to development costs. If a water quality requirement is added, then reduce or eliminate another requirement within the stormwater ordinance.