The Delaware Department of Natural Resources & Environmental Control plans to begin a $750,000 restoration of Mirror Lake in autumn 2013 to clean up pollution that consists of sedimentation, stormwater runoff, chemical contaminants, excess nutrients, invasive plant species and bacteria.
Mirror Lake had sustained mercury and PCB contamination over the years that led to a prohibition against eating fish from Mirror Lake, Silver Lake Commission Chairman Dean Holden had told the city's Parks, Recreation and Community Enhancement Committee in mid September.
DNREC and the Silver Lake Commission had considered doing nothing and allowing time to clean up the mess, DNREC engineer Rick Greene said. But they decided that was not an option because it would take 74 years for PCBs in Mirror Lake to dissipate to the point where fish were edible.
The second and third options, respectively, were to dredge the lake at a high cost or cap contaminants, Greene said.
The preferred option was to sequester contaminants at half the cost of dredging using a layer of carbon sediment laid overtop the contaminated sediment, Greene said. That carbon treatment would reduce contamination by 70 percent to 90 percent within five years through a complex, biological process, he said.
The end result would be a clean, carbon sediment deposit that would serve as a new barrier to the contaminated sediment at the bottom of the lake, he said.
Greene and Holden presented the plan to Dover City Council back at its Sept. 24 meeting. They asked for the city's support of the plan and for some labor needed to see the project through to fruition. Council voted 8-0 in favor of the restoration.
The restoration plan also includes creating a channel that would accelerate the flow of water in the middle of the lake in order to move sediment along in a more natural manner, Greene said. Mirror Lake, similar to Silver Lake, was manmade by widening the St. Jones River, thereby taking away the usual, more rapid flow of water, he said.
In addition, DNREC would have to empty Mirror Lake and draw down Silver Lake as a temporary buffer, Greene said. This would involve installing a temporary dam and using high capacity pumps to "dewater" Mirror Lake.
The state would also improve aesthetics and the habitat at Mirror Lake by expanding and enhancing wetlands on either side of the lake, Greene said. The new wetlands would serve as a natural cap for the contamination, improve the lake's ecology and enhance the beauty of Mirror Lake with little maintenance.
The existing sand bar would also be incorporated into the design as opposed to any attempt to remove that sand bar, which DNREC acknowledged as a permanent fixture, he said.
The $0.75 million project would be funded for the most part with $680,000 secured from the state Hazardous Substance Cleanup Act and $63,000 from DNREC to develop plans and specs, Greene said. Labor for much of the construction would come from DNREC and the Kent Conservation District.
Page 2 of 2 - Federal and state permit applications would be in done in kind by DNREC, he added.
In addition to "enthusiasm," DNREC asked the city to draw down Silver Lake prior to dewatering, albeit carefully; to provide a lift truck to help load and unload Portadam frames, as well as two people to help set up and tear down the frames and plants.
Construction is scheduled to begin in November 2013 and is expected to end in December 2013,Greene said.
Greene thanked the Silver Lake Commission, City Manager Scott Koenig and the Dover Department of Parks & Recreation for their help and input on this restoration project.