SNOQUALMIE, Wash.—Out in the Snoqualmie forests of eastern King County, on the west side of the Cascades, the ground is black and mucky. It’s an unnatural scene with a toilet-bowl stench.

And it comes from tons upon tons— about 120,000 tons a year—of sewage sludge that King County, home of Seattle and more than 2 million people, disposes of annually in these forests and on farms and rangelands of central and eastern Washington.

This sludge, known euphemistically as “biosolids,” consists of semi-liquid waste obtained from the processing of municipal sewage. The goal of this process is to obtain clean water to release into the environment, and thus, the cleaner the water, the more toxins and contaminants are retained in the sludge. This sludge used to be dumped into the ocean, but because of its toxicity this process is banned. Instead we now spread it in our forests and on our agricultural lands.

The spreading of this sludge carries considerable risk and should be stopped. As scientists, we are increasingly concerned with this reckless practice.

An unimaginably large number of chemical and biological contaminants exist in sewage sludge, and many persist for decades after disposal. Scientific investigations have identified only a tiny fraction of the total contaminant load. We cannot describe with any confidence the true range of contaminant risk from the sludge. Call it an “unknown unknown.”

But we get an idea of the potential synergistic interactions from the contaminants that we do know.

To illustrate the difficulties, take just one group of persistent, bioaccumulative, and toxic compounds known to be in sewage sludge at high concentrations: brominated flame retardants. Perhaps the most well known sub-class of the brominated flame retardants are called polybrominated diphenyl ethers (PBDEs). There are 209 different PBDEs, each of which has a unique toxicology and environmental fate.

Our initial analyses—not yet published—revealed high concentrations of PBDEs in the soil and water of forests “treated” with sludge.
PBDEs have been studied around the world for several decades, yet we still have a poor understanding of the true risks from their release into the environment. We have, for instance, no definite information on how PBDEs affect human health. But research on laboratory animals suggests an array of adverse health impacts, including thyroid hormone disruption, delayed puberty, learning impairments and behavioral changes.

Not surprisingly, we also find polychlorinated biphenyls, or PCBs, compounds the U.S. Environmental Protection Agency banned in 1979 because of their lengthy residence time in the environment and their link to cancer and birth defects in lab studies. The PCB family also has 209 different members, all with varying degrees of toxicity. There are another 210 chlorinated dioxins and dioxin-like “congeners,” also found in some sludges.

These are just the contaminants we have identified. To refer to our current knowledge as the tip of the iceberg would grossly overestimate what we actually know.

Meanwhile the sludging continues. The sludge is hauled by tandem dump trucks into the forest via old logging roads, where it is dumped into skids and then loaded into spreaders.