The appropriate disposal of material dredged from navigation projects is a nationwide issue but has important implications for the use, management and protection of waters in the Great Lakes Basin. Confinement of contaminated dredged material, determined to pose an unacceptable risk to the environment, is a federal policy and is recognized as necessary by state and local governments. Concern over environmental effects of dredging and disposal of dredged material, the increasing unavailability of suitable disposal sites and dredging=s role in supporting waterborne commerce have combined to elevate the issue in the region’s public policy agenda.
The Beginning of A Confined Disposal Program
Until the mid-1960’s, much of Great Lakes dredged material was redeposited offshore away from navigation channels. Some of it though was used for nearby land filling and to replenish beach sand lost to littoral drift and wave action. Rising concern about Great Lakes water quality and possible connection to polluted sediments resulted in a shift of policy on disposal of dredged material. During the latter half of the 1960’s the Corps of Engineers, in cooperation with the Federal Water Pollution Control Administration (predecessor of US EPA), began to study its Great Lakes dredging activities and related in-water disposal of dredged materials. This investigation effort also included a Buffalo District report, released in 1969, that concluded in-water disposal of polluted dredged material was A presumptively undesirable for the Great Lakes.
The Corps investigation coupled with growing public concern about water quality in the Great Lakes and elsewhere spurred Congress to take action. With the passage of the River and Harbor Act of 1970 (P.L.91-611), the current era in U.S. disposal of contaminated dredged material from the Great Lakes was launched. Section 123 of this legislation authorized the construction of diked disposal facilities for the Great Lakes. The Corps has constructed 43 confined disposal facilities around the Great Lakes. Of this total,16 were constructed on land and 27 were built as in-water facilities sometimes at shore-adjacent locations. Under current dredged material evaluation procedures, about half of the material removed each year is considered polluted or otherwise not suitable for open water disposal and placed in confined disposal facilities. This amount, averaging around 2.5 million cubic yards (1.92 million cubic meters) would fill 500,000 standard dump trucks and, if parked end-to-end, the line of trucks would stretch from Windsor, Ontario to Spokane, Washington.
CDFs, Their Design and Performance
The size and design of each CDF is site-specific, depending on the location, the nature and potential amount of sediments and how it will be used or function once it is full and/or no longer receiving dredged material. Dikes for in-water CDFs are usually constructed in layers with heavy, protective stone on the outside and progressively smaller stones to sand on the inside. Some CDFs incorporate liners or steel sheet pile in the dike walls. As dredged material is pumped or placed in a CDF, the sediments settle out and the accompanying water evaporates or percolates through the walls or into the ground. When permeability is reduced over time because of sediment sealing, a variety of water release mechanisms including overflow weirs and filter cells are used.
Great Lakes ecosystem health depends, among others things, on a high level of contaminant isolation. A major concern relates to how effective CDFs are in keeping the material from recontaminating the surrounding environment. Contaminants often bind with fine sediments such as silt and clay. To the extent that this form of pollution is confined to the CDF and bioaccumulation of pollutants by plants and animals in or near CDFs is not significant, then CDFs are presumed to be relatively efficient. There is no systemwide, continual monitoring program for Great Lakes CDFs. However, CDF water quality monitoring generally occurs during dredging and disposal operations and 12 CDFs do have monitoring wells in dike walls.
Many environmental studies have been conducted at selected CDFs around the Great Lakes with interesting results. A 1993 CDF report prepared by the Corps summarized the overall environmental status of CDFs as reported in studies to date by stating, The results of water quality monitoring has confirmed that CDFs are highly efficient at retaining the sediment solids and attached contaminants.” CDF effluents typically have suspended solids levels between 10 and 150 mg/1.@ With respect to contaminant losses from in-water CDFs, detailed studies at several facilities show a high level of efficiency at keeping pollution within the CDF itself.
Studies of contaminant transfer to plants and animals in and near CDFs is the focus of continuing research. For example, the Times Beach CDF at Buffalo has had more biological studies than any other Great Lakes CDF. This 46 acre facility was used for only 4 years before becoming a nature preserve in 1976. Dozens of studies since have shown some mobility of heavy metals through the plant community into soil invertebrates but organic contaminants appear to be decreasing in concentration in the upland area. Even though CDFs appear to be relatively efficient in retaining contaminants, more research is needed to describe the potential contaminant exposure pathways associated with CDFs and test mitigation strategies such as capping or otherwise eliminating or neutralizing sediment exposure.
A Continuing Need for CDFs
The use of confined disposal facilities in the Great Lakes continues to be necessary. Originally, the main CDF program authorized by P.L. 91-611 envisioned use of such facilities for a 10 year period. It was believed that progress in pollution control particularly from municipal and industrial point sources would sufficiently reduce the contamination of sediments and thereby eliminate (or reduce) the need to use CDFs. However, the extensive accumulation of contaminated bottom sediments particularly in industrial harbor areas and tributary river reaches and continuing land use practices that create erosion, sedimentation and related pollution require on-going confinement of large quantities of polluted dredged material.
Confinement is the principal means to address the overall problem of disposal of contaminated sediments derived from dredging for navigation purposes, but other strategies exist. Remediation or clean up of polluted material from a CDF or elsewhere has been demonstrated to be technologically feasible through various means, but is relatively expensive compared to placement/storage in a CDF. Sediment reduction at the source, particularly through soil conservation practices, offers much promise, both as a means to reduce polluted sediment transport and, where possible, lessen navigation-related dredging requirements.
Additional CDF capacity beyond that which exists now or alternatives will be needed. Some CDFs have been filled to capacity and are no longer being used and all but two of the 26 CDFs built and used under P.L. 91-611 will be full or at design capacity by the year 2006. Without more CDF capacity, much of the maintenance dredging for navigation purposes could come to an end with serious consequences for commercial navigation and the regional economy. The Lake Carriers’ Association indicates that even slight decreases in available depth reduces significantly a vessel’s carrying capacity. For example, with respect to the workhorse vessel of the Great Lakes fleet, the 1000-footer, it loses 270 tons of cargo for each inch reduction in draft. Passage of the Water Resources Development Act of 1996 provided for the continued use of CDF’s in the Great Lakes and elsewhere and established a uniform level of cost sharing for construction.
Several communities around the Great Lakes are taking a “conservation-of-space” approach to the problem of future CDF capacity as well as planning for new facilities. For example, in the shared harbor at Duluth, Minnesota and Superior, Wisconsin, the CDF known as the Erie Pier Dredged Materials Placement Facility, has been the site of a successful sediment cleaning and sorting activity. Coarser-grained material (clean) is separated from the finer particles and reused offsite for construction, and road maintenance. In Toledo, Ohio a Harbor Planning Group made up of local, state and federal officials has developed a long-term management strategy for its dredged material disposal problems. With an average of 850,000 cubic yards of sediment needed to be dredged each year, this large volume has spawned a three-pronged approach: 1) continued short-term use of the harbor CDF for nearly three-quarters of the sediment with the rest determined to be suitable for open lake disposal; 2) concerted effort to continue reuse of dredged material subject to cleaning and combined with sewage sludge to create top soil material along with efforts to better manage CDF sediments through dewatering to facilitate compaction (to create additional capacity); and 3) plan for a new CDF-type facility that would function as a shoreline protection structure and also create wildlife habitat and eliminate the need for future open lake disposal.