Scientists Test Technology to Clean Up Washington DC River
By Rosanne Skirble
The Anacostia is a sick river. Signs along its banks warn people not to eat the fish or to swim in the water. Fecal bacteria from combined sewage and storm water overflow pollute the waterway. Its riverbed sediments are contaminated with high levels of PCB, a cancer-causing chemical used for decades to make plastics, paints and lubricants. PCBs are dangerous because they stick to fatty tissue in organisms and move up the food chain into fish and humans. PCBs harm the immune, reproductive, endocrine systems.
Although they've been banned in the United States since 1978, disposal has proved to be difficult and costly.
"Dredging is a kind of main option". "If [the] sediment has PCBs in it, you use a backhoe and scoop them out and you either take them to a landfill or treat them on site." That is Greg Lowry, a professor of civil and environmental engineering at Carnegie Mellon University in Pittsburgh.
He says dredging has its limits.
"In re-suspension of sediments you are basically distributing PCBs," he said. "It is very expensive to actually treat the material. If it is high concentrations of PCBs you need to incinerate them. So you burn them. And when you burn PCBs you make [extremely toxic] dioxins and dioxins can be released. Or, what is mostly done is those sediments are moved out of the river and into a landfill somewhere. So you haven't actually destroyed the PCBs. You have just moved them out of the river and into what is currently considered a safe repository for PCBs."
Greg Lowry offers another tool for river remediation: - a sediment-trapping mat that absorbs the dangerous PCBs.
"So the idea is to put a cap down," he continued. "Stop PCB transport from those sediments into the overlying water column."
Six months ago crane operators lowered what looked like a large roll of carpet into a stretch of the Anacostia River. Greg Lowry says the 929-square-meter (10,000-square-foot) sediment-capping mat sunk slowly into place across from the Washington Naval Yard. It is made of geo-fabric mesh like that used in landfill liners embedded with absorbent particles.
"We've used coke as a sorbent, at very low cost," he explained. "And, then we are looking at activated carbon, which is a higher cost and it an extremely effective sorbent. Activated carbon is used in filters for water filtration. It is a technology that has been around for decades and really just applying it in (a kind of) a new sort of way - actually putting low density, carbon nation sorbent material into the bottom of a river as something that people hadn't thought of before."
Greg Lowry says that while initially invasive, the mat - covered by a thin layer of sand - remains in place and allows for the ecosystem to recover quickly.
"From upstream you are getting clean sediment and clean sediment is depositing and you get re-colonization of the same benthic organisms [those that live in or on the bottom of the river] that were there before," he continued. "And you won't cap 20 miles of the river all at once. You cap in stages and you allow the re-colonization."
Greg Lowry says the cap is a means to buy time while the PCBs break down naturally in the environment. He says the goal is to engineer a system that offers those with a stake in the Anacostia's future a safer and cheaper way than dredging to clean up this important American river.
I’m Rosanne Skirble.

注释：
Anacostia 阿那卡斯提亚河
fecal 排泄物的
bacteria 细菌
sewage 污水
riverbed 河床
sediment 沉积物
PCB 聚氯联二苯（一种环境污染物）
plastic 塑料制品
paint 油漆，颜料
lubricant 润滑剂
fatty tissue 脂肪组织
backhoe 锄耕机
scoop 挖
incinerate 把……烧成灰
sorbent 吸附剂
filter 过滤器