FOR PEAT’S SAKE, ENGINEERING GEOLOGY AND ENVIRONMENTAL CHALLENGES IN A PEAT BOG PRESERVATION PROJECT, CEDAR RIVER WETLAND 14, KING COUNTY WASHINGTON

A habitat protection project constructed in August 2001 was designed to improve the hydrology of a remnant peat bog. Because of the unique scope and setting of the project, the work posed design and construction challenges. Unusual aspects of the work included assessment of bog health, interpretation of groundwater data in peat, construction with heavy equipment in a wetland over blue clay substrate, structural capacity of the peat, isolating the living peat bog chemically from alkaline earth and rock, and archaeological impacts.

The original wetland, located southeast of Seattle, Washington was described in 1958 by Rigg in Peat Resources of Washington. About 80% of the 33-acre raised sphagnum peat bog was removed during mining operations which extended over a 25-year period ending in 1987. The site is archaeologically significant because of a 10,000 year old fluted projectile point that was found in 1983. The uniqueness of the sphagnum-dominated peat lands include the rare plant and invertebrate communities they support, and their paleorecord of pollen and seeds.

Measurement of groundwater levels revealed that the exposed vertical peat face was dehydrating the 6-acre remaining bog. These changes in the hydrology were revealed by accelerated growth of hemlock near the bog face, which was verified with a series of tree cores.

A 5-foot high berm 300 feet long was constructed along the mined face of the peat bog to protect and support an inert geomembrane that would seal the edge of the bog. This design was chosen from several alternatives because of the relatively simple use of material, effectiveness, and ease of construction. We did not try to manipulate the laggs that drain the perimeter of the bog because the hydrology of the bog system was not well understood. An unseasonal 2-inch rainfall event on the second day of construction created equipment access challenges; the ground in the work area became a supersaturated clay. A geotextile reinforced rock access pad was constructed to allow work to continue. Wetland hydrology was restored to the work area when construction was complete.