Bay Best Practices: What's Working to Clean Up The Chesapeake
By Jeff Moeller and Stephanie Costello of the Water Environment Research Foundation
This is the second article in a two-part series on nutrient and sediment reduction in the Chesapeake Bay and findings from the International Stormwater BMP Database. Read part one here.
Nutrients and sediment are part of every waterbody — necessary for keeping a healthy ecosystem naturally in balance. But sometimes everyday activities can create too much of a good thing, elevating levels and throwing off this equilibrium. Excess fertilizer from your lawn or debris that’s being stirred up at the construction site down the block; they can all add up when they make their way to the nearest stream.
The cumulative impact of various individual nutrient and sediment contributions can have devastating effects on waterbodies, and the Chesapeake Bay is case in point. Excess amounts of nutrients and sediment in the Bay region have lead to an environment of harmful algal blooms, low dissolved oxygen, reduced water clarity, and declining aquatic life — and an estimated 15 percent of the phosphorus, 8 percent of nitrogen, and 16 percent of the sediment that is reaching the Bay today comes from urban and suburban stormwater (Bay Watershed Model 2009 Scenario).
In order to meet EPA’s goal of drastically reducing pollutant levels in the coming years, neighboring jurisdictions will need to determine which practices are most effective at managing stormwater, so they can ultimately cut down on these contaminants. The upside is that many control and treatment practices are already being used in locations around the Bay, which means there’s a good sampling of tested methods. But given that every site has unique characteristics, it’s now a matter of determining which practice, or combinations of practices, will do the best job at a particular site.
Image credit: "Chesapeake Bay Bridge," © 2012 Bob Mical used under an Attribution-NonCommercial-ShareAlike 2.0 Generic license: http://creativecommons.org/licenses/by-nc-sa/2.0/deed.en
The Water Environment Research Foundation, with support from the National Fish and Wildlife Foundation, is making it easier to sort through the options with the release of a report detailing how various techniques to control runoff in the region are stacking up. The report, BMP Performance Summary: Chesapeake Bay and Related Areas, prepared by Geosyntec Consultants and Wright Water Engineers, is based on information contained in the International Stormwater BMP Database, a publicly–available website that houses the world’s largest collection of data on best management practices (BMPs) for stormwater. In all, data from nearly 70 Bay-related sites were analyzed, resulting not only in the performance summary report, but also a dedicated Chesapeake Bay web portal with resources and guidance.
So what is working for the Bay? That depends on the pollutant as well as the location, but the report draws conclusions and offers recommendations that can help many stormwater managers select the right techniques to manage and treat runoff for given conditions.
For example, the report points out that phosphorus has a tendency to attach to soil particles and other organic matter, which makes it a good candidate for BMPs with processes that remove particulates, such as sedimentation and filtration. As shown in Table 1, wetland basins and retention ponds, both BMPs containing permanent pools, proved to be particularly good options, and media filters also reduced levels substantially. On the other hand, some vegetated BMPs, like grass strips, might actually leach phosphorus from soil or planting material and resuspend captured particulates.
When it comes to nitrogen, the treatment process is more complicated. Nitrogen in runoff comes in several forms — there’s organic nitrogen and nitrate, and sometimes ammonia can be present. While BMPs with permanent pools appear to be able to reduce nitrate levels, they may actually increase organic nitrogen, and the opposite appears to be true for biofilters and media filters. Based on the Chesapeake Bay region available data, bioretention is the only BMP that resulted in clear statistically significant removal levels; however, most BMPs did reduce concentrations to some degree as evidenced in Table 2. Based on the complexity of the nitrogen cycle, the most effective approach might be a multi-unit process. For example, a BMP designed to reduce total nitrogen might include a permanent pool followed by a vegetated swale.
And what about sediment? It looks like there are a number of options for reducing total suspended solids (TSS). As shown in Table 3, most BMPs were able to bring TSS concentrations down below 20 mg/L, but filtration-based BMPs, like bioretention and media filters, were the most effective, with median effluent concentrations around 10 mg/L, followed by BMPs with permanent pools, such as retention ponds and wetland basins.
While much of the discussion surrounding BMPs often focuses on performance in terms of pollutant concentrations, the report also highlights the fact that volume reduction can be equally important. Whether you’re talking about phosphorus, nitrogen, or sediment loads, they can all be reduced by decreasing the amount of surface runoff. And the report found that many BMPs were capable of doing just that. For example, even in cases where BMP effluent concentrations aren’t significantly reduced, cutting down the volume could help decrease the overall loads that are reaching the Bay.
Another bonus of the analysis report was the finding that BMP performance in the Chesapeake Bay watershed is generally similar to those in other parts of the country. This is good news given that EPA recently tasked Bay jurisdictions with developing detailed plans to reduce pollutant loads. So, once they get the right formula for cleaning up the Bay, the lessons learned could be helpful to other waterbodies that are facing similar problems.