124 * Wetlands: Their Use and Regulation 



wetlands around Hilton Head Island in South Car- 

 olina (43). 



A long-term effect of the disposal of contaminated 

 dredge spoil in or near wedands is the potential bio- 

 availability of toxic chemicals such as oil and grease, 

 pesticides, arsenic, and heavy metals, when the sed- 

 iments are resuspended periodically (1). Although 

 the bioavailability of these contaminants general- 

 ly is quite low, under certain conditions there may 

 be some long-term potential for bioaccumulation 



of these harmful substances within the food chain, 

 especially when contaminated dredged materials are 

 exposed to the air (27). 



For example, filling of wetlands by eroded soil 

 is also a factor associated with wetland conversions 

 from forestry, agricultural, and development prac- 

 tices in watersheds of the California coast; from 

 agricultural and development practices around the 

 Chesapeake Bay in Maryland; and from agricul- 

 tural activities in the prairie potholes and Nebraska. 



VARIABLES OF WETLAND-IMPACT MAGNITUDE 



The actual impacts of a specified construction or 

 development activity will vary geographically and 

 by season of the year according to regionally or 

 locally distinct characteristics of the physical- 

 chemical environment. The characteristics of bio- 

 logical populations and habitats and of the whole 

 wetland ecosystem also will modify the impacts. A 

 discussion of these variables has been included here 

 to illustrate both the site-specificity of wetland- 

 project impacts and the range of factors that must 

 be understood to make realistic impact assessments, 

 and to suggest how these variables may be manip- 

 ulated to mitigate project impacts. 



Physical and Chemical Variables 



Composition of Wetland Soils 



The physical characteristics of wetland soils will 

 have considerable influence on the severity of im- 

 pacts produced by different activities in wetlands. 

 Wetland bottom type is an important factor in spe- 

 cies diversity and productivity. For example, a proj- 

 ect that introduces large quantities of silt and clay 

 would have a significant impact by smothering pro- 

 ductive substrates. A wedand's chemistry also may 

 influence the magnitude of a project's impact. The 

 effects of dredging in marine or brackish waters are 

 likely to be less severe than in freshwater because 

 of the buffering capacity of these waters. Also, since 

 cold water generally has higher levels of dissolved 

 oxygen, the effects of activities that tend to deplete 

 the dissolved oxygen will be greater if water tem- 

 peratures are higher. 



Hydrologic Regime and Water Dynamics 



The hydrology of a wetland will affect substan- 

 tially the magnitude of impacts from activities in 

 wetlands. For example, wetlands that are hydro- 

 logicaUy isolated from ground water supplies, such 

 as perched bogs or playa lakes, will be more ad- 

 versely affected by excavation or dredging than wet- 

 lands that have sources of water besides precipita- 

 tion. Excavation in these isolated wedands may 

 damage the compact peat layer and/or clay layers 

 that seal the bottom of the wetland and hold water 

 within it (32). 



The construction of highways on wedand fill has 

 different impacts, depending on the particular wet- 

 land hydrology. Culverts placed through a highway 

 fill may cause flooding of the upslope side and 

 dewatering of the downslope side (44). In the Flor- 

 ida Everglades, however, the same type of highway 

 fill with drainage culverts may be able to accom- 

 modate the water that flows over the surface of the 

 wetland. 



Composition of Fill Material 



The disposal of solid wastes, however, carries the 

 risk of detrimental chemical and biological effects 

 due to leaching of the fill material. The magnitude 

 of adverse impacts depends on the actual waste 

 composition, which can vary physically and chem- 

 ically according to geographic region, community 

 standards, and seasonal variations. In general, 

 municipal solid wastes have a high proportion of 

 biodegradable animal and vegetable waste, rags. 



