A small rain garden in the yard or a big stormwater pond at a commercial site may both be doing double duty when it comes to making the Bay and its tributaries safer for fish and other aquatic life.

Stormwater controls aimed at reducing nutrient and sediment pollution in the Bay can also curtail the flow of toxic pollutants into urban rivers, often some of the region’s most contaminated waterways.

Controls already in place may be reducing toxic runoff from cities by 25 percent, and that reduction could increase to 40 percent by 2025 as cities and counties install more stormwater controls to meet Bay nutrient and sediment goals, according to estimates made by the Chesapeake Stormwater Network.

“The good news is that the new investments we’re making, and the long history of stormwater requirements, does have this supplemental benefit,” said Tom Schueler, executive director of the nonprofit organization.

Schueler and colleague Anna Youngk recently completed a pair of reports for the Bay Program examining the effectiveness of best management practices, or BMPs, to control toxic pollution in urban and agricultural areas. The authors reviewed hundreds of scientific papers and reports to evaluate whether some of the BMPs used to meet Chesapeake nutrient and sediment reduction goals could provide a dual benefit by also reducing toxic pollutants.

While only a handful of the studies addressed the toxics issue, they did indicate that stormwater controls can help reduce contamination in urban areas, where the usage of heavy metals and chemicals often referred to only by their acronyms — such as PCBs, DDT and PAHs — have left a legacy of contamination and fish consumption warnings in many places. Schueler said that many of those contaminants tend to bind to sediment particles and are readily trapped by stormwater control ponds, where they settle to the bottom.

But the picture is murkier with agricultural best management practices, where the toxic issues are often different, and there were fewer studies to draw upon. Further, the most widely used practice — conservation tillage — triggered an increase in the use of some herbicides over the years.

“On the urban side, the evidence is a lot more convincing,” Schueler said. “But that is because most of the urban toxic contaminants behave like sediment in stormwater BMPs, which are reasonably good at removing sediment even though they are not always that great at removing nitrogen and phosphorus.”

Stormwater ponds and other runoff controls have already reduced toxic pollution by a quarter, and will provide greater benefits as older urban areas built before modern regulations are retrofitted with ponds and other controls, the reports said. Prioritizing them in “hotspots” with a history of industrial, commercial or transportation land uses will likely produce the greatest toxic reductions, the reports added.

While stormwater ponds benefit streams by trapping sediment particles and the contaminants that attach to them, the reports cautioned that these ponds could potentially build up toxins to the point where they would need some type of cleanup themselves.

“Those ponds are not necessarily a bad place, but what do we do with them 20 years from now?” Schueler asked. “There is no question there is a witch’s brew of toxins in the sediments.”

Further, while aquatic life in rivers may be protected, the ponds may attract amphibians and other wildlife which, in turn, can be exposed to higher concentrations of toxins.

“There is probably some toxicity to aquatic life that are in the ponds, but it is pretty well unstudied at this point,” Schueler said. “We are getting into places where some of the science has never been performed, which astonishes me.”

The papers did suggest that low-impact development practices being heavily promoted also effectively trap many toxic pollutants. Those practices, such as biofilters and swales, slow runoff and promote infiltration, and some research indicates that soil microbes and vegetation present in those practices also help break down those contaminants, the papers said, reducing the risk of bioaccumulation in wildlife. Also, because they do not create aquatic habitat such as ponds, they are less likely to attract frogs, turtles, birds and other wildlife that could be exposed to chemicals, Schueler said.

On farmland, a limited amount of research indicates that some runoff BMPs, such as vegetated buffers and constructed wetlands, can help remove some chemicals.

But the agricultural picture is complicated because the most widely used BMP is conservation tillage, which reduces or eliminates the plowing of croplands to limit erosion. But reduced tillage, the papers note, has increased herbicide use over the years to control weeds, which are no longer being plowed under. Farmers have switched to herbicides thought to have less environmental impact, but there’s recent evidence that weeds are gaining resistance, causing a switch to more toxic chemicals, the papers said.

Tracking the amount and types of herbicides applied, and their impact on waterways was difficult, Schueler said, because of a lack of data. Also, many herbicides rapidly break down into other chemicals, and there is even less information — and monitoring — on them. “ The degradates are often as persistent or as toxic as the parent compounds,” he said.

Some studies have shown that ponds, wetlands and vegetated buffers can help remove herbicides from runoff before it reaches streams, though the papers found only a handful of studies, most of which were done in other areas of the country.

The papers also examined hormones such as estrogen and testosterone, which are found in animal wastes and biosolids, the solid residue from treating human waste. Those hormones can be effectively removed by BMPs such as buffers and wetlands, the reports said, but they also noted that many livestock producers have stopped giving them to their animals in response to concerns from consumers, restaurants and retailers.

Some concerns remain, though. Greg Allen, of the EPA Bay Program office and co-chair of the program’s Toxic Contaminants Workgroup, which had requested the reports, cautioned that while fewer hormones are being added to feed, they may still pose a threat to aquatic life, especially in areas with large and sometimes growing concentrations of livestock.

“There’s probably been net reduction and that’s a good thing for a lot of reasons, but it doesn’t mean that biogenic hormone loading is stopping,” Allen said. “Some of those biogenic hormones are produced naturally by the animals, no matter what they are fed.”

The papers also examined antibiotics that can be found in animal wastes and biosolids from municipal wastewater treatment plants, but found little research indicating whether those drugs are removed by BMPs. Even so, the papers said there is evidence that consumer pressure is contributing to the reduced use of antibiotics in agriculture, as is the case with hormones.

One of the hopes of the workgroup was that by identifying additional benefits for nutrient and sediment BMPs, state and local officials could use that information when writing new Bay cleanup plans in 2018. Those plans will guide actions through 2025, the deadline by which states must have put in place all of the measures needed to meet nutrient and sediment pollution limits set by the Chesapeake Bay Total Maximum Daily Load.

“I would hope that this information better focuses on areas where you could get co-benefits from different actions, and in particular where we could take advantage of the nutrient and sediment BMPs that will be implemented over the next decade, and try to get a reduction in toxic contaminants,” said Scott Phillips, Chesapeake Bay coordinator with the U.S. Geological Survey and co-chair of the toxics work group.

Copies of the reports can be found here.