Barges have set up and are getting ready to dredge the bottom of Lake Union just west of Gas Works park. Larry Altose, from the Washington State Department of Ecology, writes about the work on their blog:
The contamination we’re about to remove existed before the present owner purchased the property in 1994. At that time, Northlake Ship Yard and Ecology signed a legal document called a Prospective Purchaser Consent Decree (PPCD). It’s a cleanup agreement for someone who buys already-contaminated property.
Many commercial and industrial sites have such agreements, but this one has an unusual twist. Normally, a property owner – current or past – conducts the required cleanup work. Northlake Ship Yard opted instead to deposit funds into a bank trust account over several years. Ecology is using that account to conduct the cleanup. This is our first cleanup under such an arrangement.
What’s down there
A previous owner used ground-up slag from the Tacoma Smelter to sandblast ships. The slag was inexpensive and plentiful many years ago. It also was rich in arsenic, a toxic metal. Sandblasting also produces other residue from ships’ metal and paint. Today it is illegal to sandblast over the water.
We estimate that our contractor, Redside Construction of Port Gamble, will haul away about 8,000 cubic yards of this material all told. That may take about three months. Unless we find the contamination is much higher than we’ve seen in our sampling, we expect it all can go to a regular landfill.
A clean cleanup
Each scoop of mud is up to 60 percent water. We can’t haul that in our trucks. It’s too heavy. And, we don’t want water from this muck on our streets. So the mud lands first on a barge so that water can slowly drain back to the lake.
About every three days, we’ll have a truck-out day. Covered trucks will haul moist, but not soaking, mud. There might be 20 to 30 truckloads on each of these days.
Silt, and plenty of it
We’re going to raise clouds of silt in the water from digging and from the water draining off of the muck-drying barge. To keep this silt from getting into the rest of the lake, we will surround the project with a fine-mesh silt curtain. It hangs from floats and is weighted at the bottom.
The curtain will stay in place until the work is done and we have tested the water to make sure all silt has settled. Also, we monitor the water outside the curtain continuously, so we can stop work and fix any problems we find.
We expect our mucking with the lake bottom will release some oily substances that will float to the surface. The work area may contain sheen, a thin coating of oil on the surface of the water.
To keep that out of the rest of the lake we will add another barrier: a bright yellow or orange oil-spill containment boom.
We’ll have spill response equipment on hand. Often, though, sheen is so thin that cleanup materials cannot remove it from the water. We will clean the sheen whenever possible.
Many people keep an eye on the busy waters of Lake Union and the Ship Canal. Ecology follows up on reports of oil slicks and sheen in this area on a regular basis. We’re making every effort to keep sheen from this cleanup out of the rest of the lake.
Odds and ends
We expect now and then to pause our muck picking to remove the odd log, barrel or whatnot. We know of some such objects from surveys of the lake bottom. Still, the mud may hide any and every kind of object, so we’ll see what turns up.
That’s another good reason for the silt curtain and the spill gear. You can never know.
Before we leave
We expect to dig down about two feet to remove the contaminated mud and grit. We’ll close by covering the area with a half foot of clean sand. This will make the lake bottom more suitable for nature’s return.