For flows of 12,000 cfs and a deepening of 4 feet, the maximum increases above US 17 would be 0.03 ppt for each river system. At 12,000 cfs, maximum salinity increases below US 17A, where the majority of marshes are saline, would be 1.72 ppt in the Front River and 1.32 ppt in the Back River. Salt Water Intrusion Into Aquifers Studies were conducted to determine impacts of harbor deepening on the fresh water aquifer. Two borings were taken to define the soil stratum at critical locations. Information from these borings, and others previously taken, show that more than 50 feet separate the top of the stratum bearing the aquifer and the bottom of the deepest proposed channel. According to work performed by the U.S. Geological Survey, Paul Huddleston of the Georgia Geologic Survey, and Dr. Vernon J. Henry of Georgia State University, the surfaces of the early Miocene and late Oligocene Age aquifers appear to be sufficiently deep to prevent damage by even a project constructed to a depth of 50 feet mean low water (mlw). An offshore geological structure known as the Beaufort Arch created an uplift of the Tertiary sediments in the vicinity of the Savannah light tower. This uplift resulted in the uppermost confined aquifer in the area, Parachucla formation (of early Miocene Age), surfacing at about -90 feet mlw. A more valuable aquifer, the Late Eocene aged Ocala Limestone (Upper Floridan) Aquifer, would be expected to be at no higher elevation than 190 feet mlw in this area. These clays are The uppermost aquifer is confined by the highly impermeable middle Miocene clays of 40 to 70 feet in thickness. overlain by sandy clays and soft limestones. Miocene are soft granular Pliocene and Pleistocene age, along with current soft deposits of the Holocene age. This is the area that would be dredged. Introduction of water into the upper Floridan Aquifer would require contact with a fissure, or fault, or ancient stream channel which would lead to this strata, an extremely remote to never-likely-to-happen situation. The only other way for water to be introduced into the Upper Floridan Aquifer would be to remove the entire Miocene Age cap and expose the underlying limestone. This would require dredging the channel to -100 feet mlw. Based on this information, no impact to the upper confined aquifer, nor the Floridan Aquifer is expected to result from construction of the project. Wildlife Implementation of Plans 1 and 5 should not have any significant impacts on wildlife resources. Disposal of dredged material using Plan 1 would involve existing inner harbor disposal areas and an EPA-approved ocean disposal site, while Plan 5 would use the existing approved ocean disposal site. Plan 2 would have adverse impacts on wildlife since this plan involves the use of a new inner harbor upland disposal site. Proposed Disposal Area 9 consists of 1,020 acres and is located between the road to Old Fort Jackson and the road to Elba Island, north of the Islands Expressway. The area is mostly wetlands, consisting of 580 acres of giant cordgrass and 84 acres of mixed brackish marsh. The rest of the area consists of 255 acres of deciduous forest, 14 acres of spoil, and 87 acres developed. The wetland areas have been degraded due to industrial pollution and mosquito control ditching. However, these areas still support wildlife species characteristic of marshes, such as small mammals and birds. The area was evaluated with the U.S. Fish and Wildlife Service (FWS), using their Habitat Evaluation Procedures (HEP). Although Area 9 rated low to moderate for most of the evaluation species, no model was available for such marsh users as crab, shrimp, and fish, which would increase the number of species in the moderate to high range. Area 9 was also evaluated under FWS mitigation guidelines. Although the type of marsh in Area 9 is usually rated in Resource Category 2, it was placed in Resource Category 3 because it had been degraded. FWS guidelines recommend that habitat found in Resource Category 3 be replaced by habitat of sufficient value to eliminate the total loss of habitat value. Recommended compensation would be in kind and might involve the conversions of less valuable habitat to wetland areas similar to those found in Area 9. The remainder of Area 9 was placed in Resource Category 4. Mitigation would involve minimizing the loss of habitat value by confining the impacts to as small an area as possible. Wetlands The primary concerns for wetlands center around the potential for physical destruction of wetlands during project construction or dredged material disposal and possible changes in plant species composition due to the predicted salinity increases that would be caused by the plans. A discussion of each of these potential areas of impact is provided below: Impacts from Construction. The Savannah District has examined drawings indicating the location of the top of the Savannah Harbor channel after construction of the Widening Project. This review found that the top of the channel at all locations along the harbor is far enough from existing wetland areas to preclude any excavation of wetlands or any potential for sloughage of wetland areas into the channel during construction. Therefore, project construction is not expected to have any impact on wetlands. Impacts from Disposal. As discussed above, Plan 2 would eliminate 919 acres of upland and wetland vegetation consisting of 580 acres of giant cordgrass, 84 acres of mixed brackish marsh, and 255 acres of deciduous forest. Plan 1 would use existing high ground disposal areas and our approved ocean disposal site; Plan 5 would only use the approved ocean disposal site. Therefore, disposal of dredged material under plans 1 and 5 would have no impact on wetlands. These disposal areas provide sufficient disposal capacity for the life of this project (50 years). Impacts Due to Salinity Increases. The Savannah National Wildlife Refuge, which lies just above U.S. Highway 17, consists of approximately 26,500 acres, of which approximately 5,700 acres are impounded wetlands managed for waterfowl and 5,400 acres are unimpounded marshes. The major intake for the impounded marshes of the refuge is at Lucknow Canal, Back River mile 24.5. With the Section 1135 project modification in place, the maximum salinity levels at the intake would be approximately 0.11 ppt. The LAEMSED model indicates that deepening of the harbor by 4 feet would result in only a 0.04 ppt increase in salinity at this site. Therefore, freshwater would be available to the refuge at all times at this intake structure with any of the proposed construction plans. The unimpounded marshes of the refuge extend down the Back River to about mile 18.5. A model developed by the U.S. Fish and Wildlife Service (Pearlstine 1990) indicates that construction of the Section 1135 project will result in conversion of approximately 4,000 acres of these marshes to freshwater marshes. Based on the LAEMSED model, the maximum salinity level that would occur at Back River mile 18.5 after construction of the Section 1135 project modification would be approximately 2.64 ppt. The LAEMSED Model indicates that maximum salinity levels of 2.64 ppt or less would not occur until approximately Back River mile 19.0 for a two foot deepening and mile 19.5 for a four foot deepening. Based on this comparison alone, Plan 1-40 has the potential to reduce the extent of saline/brackish to fresh marsh conversion by about 0.5 mile, and Plan 1-42 could reduce the extent of conversion by about 1 mile. However, according to Pearlstine, other factors such as the topography of the marsh area and distance from the river channel will influence the impacts of this salinity increase. If the predicted impact does occur, there would be a 6 percent (250 acre) reduction in freshwater marsh conversion due to a 2 foot deepening and a 12 percent (560 acre) reduction due to a 4 foot deepening. This potential reduction in conversion of saline/brackish marsh to fresh marsh is not considered to be significant since the area in question is at the lower end of the predicted area of marsh conversion where model prediction are more suspect. Also, the 250-560 acres of marsh would convert from a saline/brackish area to an intermediate marsh, which is characterized by having both brackish and freshwater species with salinity levels of from 0.5-3.0 ppt. The conversion of saline marsh to intertidal marsh would also be reduced; however, the saline marsh would convert to brackish marsh from mile 14 to mile 18. Figure 23 shows the model predicted marsh class distributions to result from construction of the Section 1135 Project. Figure 24 shows the marsh class distribution predicted to result from the proposed deepening project with the Section 1135 project modification in place. Fisheries The proposed dredging and disposal operations have the potential to impact fisheries in the following ways: impacts due to dredging, disposal, and salinity increases in the system. Impacts due to the dredging proposed for Plans 1 and 2 would center around the potential for physical contact with the dredge or the plume created by its operation. Since adult fish are mobile and the dredging impacts are very localized, the potential for the adult fish being harmed due to these factors is very low. However, eggs and larval fish are not as mobile. Therefore, there is a potential for them being impacted either by being entrained by the dredge or being smothered/physically damaged by the materials in the dredge plume. Of particular concern are the anadromous species which migrate up the Savannah River to spawn. |