Different canal systems have different problems, says Rhonda Haag, who oversees Monroe County’s $5 million pilot program to improve water quality in Keys canals.
At Sexton Cove Canal 29, off mile marker 106 in Key Largo, a crew from Adventure Environmental Inc. continues a months-long effort to place dirt and rocks into an over-dredged waterway.
One crane lifts fill onto a special conveyor belt that carries the material to a barge, where a second crane moves the fill to the water.
The canal, about 225 yards long, has depths that range from 20 to 34 feet — far too deep to maintain water quality in a canal with little tidal flow.
“Below the surface, yellow-colored water maintains [low-oxygen] conditions and elevated levels of hydrogen sulfide” that limit the life that can flourish, says a project summary.
The $1.36 million demonstration project aims to backfill the canal to shallower depths “to promote flushing and reduce [or] eliminate stratification,” the unwanted layer of stagnant water.
Previous scientific studies “have indicated that approximately only the upper 6 feet of the canals will naturally flush in the shallow Keys nearshore environment,” the summary says.
The Sexton Cove project, between Bunting Drive and Pigeon Drive, is the most ambitious canal-restoration effort so far undertaken by Monroe County. It is expected to finish in June, Haag said.
The county’s restoration projects were launched in 2013 to find effective techniques for improving water quality in more than 100 Keys canal systems considered to have significant environmental problems.
The handful of demonstration projects may help secure millions of dollars from Deepwater Horizon fines dedicated to environmental improvements, county commissioners said.
A new project estimated to cost just over $2 million will use a vacuum dredge to remove decayed vegetation from the bottom of two Big Pine Key canals.
County commissioners reviewed plans for the project at their April 15 meeting in Key West.
“Due to persistent currents and wind direction, a thick weed wrack becomes entrapped in the canal and sinks to the bottom where it decays,” a county report says.
“As the seaweed decomposes, oxygen is consumed, leaving the canal in an uninhabitable condition” for sea grasses and fish. Nearby residents also may notice the pungent smell of hydrogen sulfide.
A barge-mounted vacuum will remove bottom sediment from a 400-yard canal between Witters Lane and Bailey Lane, and a 200-yard canal between Avenues I and J. The “wet slurry” from the canal bottom will be taken to a container for drying.
Some of the dehydrated material could be used for fill on the golf course at Marathon’s Florida Keys Country Club. Other material that slightly exceeds state standards for arsenic and copper will be trucked to a landfill upstate.
When dredging is complete, a 6-inch layer of sand will be placed on the bottom to provide a base for natural sea life.