Raining on Rain Gardens

So many bad assumptions, so little time. A report in Tues.(11/18/08) Coastland Times suggests that the oceans off the Outer Banks are awash in bacteriological contamination and that the best solution is for all of us to manage our residential stormwater on our lots and virutally every element of that statement is just wrong.
The League of Women Voters asked John McCord of UNC-CSI and Jan Deblieu of the NC Coastal Federation to talk to them about stormwater. The paper reports that pollution levels on the Outer Banks rival those of polluted California beaches, except they don't.
UNC-CSI has been testing for bacteria in the storm water drains, not in the ocean. They found high levels of bacteria in the storm drains. They can't correlate that with a threat to swimmers because they don't test in the ocean. The people who test in the ocean don't report the pollution predicted by UNC-CSI's finding, in part because they test on very different schedules. UNC-CSI tests storm drains after a rain event when the expect to find pollution. The State tests on a regular schedule, weekly in the summer time, in the waters people use for recreation. The two provide very different data sets but there are some points you can make.

• Point One: Don't swim in a storm water ditch or culvert, swim in the ocean.
• Point One and a half: Don's swim in the ocean near a storm drain after a rain storm.
  
• Point Two: As the CT notes bacteria don't last long in the ocean.
• Point Three: State tests for bacteria in the ocean don't find high levels of bacteria. You can see the results for 2007 state bacteria tests at the Natural Resources Defense Council site as part of their report on beach pollution (hint go to page 6 for the Dare sites).
• Point Four: The bacteria in the storm drains comes from hard surfaces like roads in the drainage basin and improperly install or failing septic systems. It happens during rain events because the bacteria get washed off the roads and the rising water table reduces the effectivemess of septic systems.

I called Jan Deblieu to talk about what she said. The paper reported that she "described Dare's Outer Banks as 'the land of outfalls'". Jan doesn't recall such a comment and Jan recognized that only about 15 miles of the Outer Banks is actually served by storm drains - ocean or sound. Corolla, Duck, Southern Shores, Kitty Hawk and all of Hatteras and Ocracoke Islands lack this infrastructure. Remember how long it took Kitty Hawk to drain the area between the highways after Hurricane Isabel? Know why? no storm drains. No way to get the storm water back to the ocean.
Back to pollution for a minute. There is no disputing that the storm drains pick up a lot of pollutants after a rain storm. There is no disputing that part of the problem is the draining of large roadways and parking lots in to the the stormwater system. The other major source of pollution is bacteria from septic systems that is captured due to a rise in the water table.
What can be disputed is the extent that managing stormwater on residential sites not part of a storm water drainage basin will affect pollution levels in the ocean or sound. UNC-CSI's Mr. McCord is reported to have suggested using storm water Best Management Practices (BMPs) like cisterms and rain gardens to reduce pollution. The problem is that the hard surfaces that pollute, like roads and commercial parking lots, aren't the target of his rain gardens, single family homes are. Manageing stormwater on on a site means managing where it infiltrates into the ground. In the end it doesn't matter where it infiltrates as long as it does.
An approach that might actually work to reduce pollution is to make sure that septic systems are installed with sufficient space between the drainfield and the water table. The standard separation is 18 inches. Increasing this to 24 inches by adding fill in the drain field (called a mound system) or adding additional filteration for the effluent could substandtially reduce the amount of bacteria that hits a rising water table. It costs between $3,000-$5,000 to fix a septic system. If UNC-CSI really wants to impact pollution why not get behind a stalled Nags Head request to the Clean Water Management Trust Fund for $200,000 to fix some older septic systems the town has identified as potential pollution sources.
DOT and UNC-CSI are planning to install filters at the ocean outfalls. Each filter system costs $1,000,000 and requires another $100,000 in annual maintenance. It might be cost effective to fix the source of pollution rather than filter out the problem. Jan Deblieu reports that the filters could be bypassed by overflow as frequently as twice a month on average. It is exactly these flows that need to be captured. Fixing on site systems would reduce storm water pollution effectively and at lower cost.
It would not however impact pollution coming from paved surfaces what is called first flush pollution. Jan suggested a strategy that I will pass on but have not entirely embraced. She suggests using the drainage system as an infiltration basin and limiting the amount of flow to the ocean or sound until certain thresholds had been reached. This could allow storm water in infiltrate back into the water table and be filtered naturally. This is what happens to stormwater in all the areas not served by a drainage system. Unfortunately what also happens is flooding. From Duck to Hatteras and Ocracoke heavy rains mean flooded roads. It might be possible to take some of the money allocated to cleaning up storm drains and using it to expand the capacity of infiltration systems but that capacity is limited. Until I see an engineered analysis of this strategy it is impossible to support it. I would support an exploration of this strategy.
So why am I writing this now? The decisions to install filters was made while the pollution data was being developed. It started before there was any meaningful data. Now that we actually know what is going on we can better analyse the solution. I have written about the lack of data and the battle over storm water pollution before so I won't revisit that topic.
We need an effective storm water system to handle flooding from major storms and ocean overwash. We need to limit the amount of pollution it causes. I am not convinced that the filtering systems being proposed will have much impact. I am totally convinced that rain gardens and cisterns in residential development won't have any impact on pollution.
Ciao
PS. It was good to talk to Jan before I wrote this. The only problem is that it tempered my anger so I apologize if this rant isn't quite as mean spirited or sarcastic as usual. I try to do better.
PPS. We miss you Monty! (and Paula and Kevin too)