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J Field Studies of Coastal Terrain Along the Atlantic, Gulf of Mexico, and Arctic Coasts of the United States, broad coastal plains are transected by streams, scarps, and terraces. In some parts of these regions, local ground-water flow systems are associated with scarps and terraces, and freshwater wetlands commonly are present. Other parts of coastal regions are affected by tides, resulting in very complex flow and biogeochemical processes. Underlying the broad coastal plain of the mid-Atlantic United States are sediments 600 or more feet thick. The sands and clays were deposited in stratigraphic layers that slope gently from west to east. Ground water moves regionally toward the east in the more permeable sand layers. These aquifers are separated by discontinuous layers of clay that restrict vertical ground-water movement. Near land surface, local ground-water flow systems are associated with changes in land slope, such as at major scarps and at streams. Studies of the Dismal Swamp in Virginia and North Carolina provide examples of the interaction of ground water and wetlands near a coastal scarp. The Suffolk Scarp borders the west side of Great Dismal Swamp. Water-table wells and deeper piezometers placed across the scarp indicated a downward component of ground-water flow in the upland and an upward component of ground-water flow in the lowland at the edge of the swamp (Figure J-1A). However, at the edge of the swamp the direction of flow changed several times between May and October in 1982 because transpiration of ground water lowered the water table below the water level of the deep piezometer (Figure J-1 8). Rhode River, Maryland Great Dismal Swamp, Virginia A FEET 70 FEET 70 ~ l l 42 Suffolk Scarp ~ o / Great Dismal Swamp ObservatiOn Well 14 42 2B Direction of ground-water flow \ 2B 14 --- -- o DATUM IS SEA lEVel o 0.5 1 MilE B ~ 32 w ~ zul -,> zw 29 o~ ~;:5 <V> >w ~> 26 wo <<<Xl w< ~ :;: 23 __~~?:!.~?_!'_l!.~<!:~_n______n___n \ Potentiometric suriace of deeper ground water A M J J A SON D J F M AcM J 1982 1983 Figure J-1. Ground-water discharge at the edge of the Great Dismal Swamp in Virginia provides an example of local ground-water ffow systems associated with coastal scarps (A). The vertical components of flow can change direction seasonally, partly because evapotranspiration discharges shallower ground water during part of the year (B). (Modified from Carter, Virginia, 1990. The Great Dismal Swamp-An illustrated case study, chapter 8, in Lugo, A.E., Brinson, Mark, and Brown, Sandra, eds., Ecosystems 01 the world, 15: Forested wetlands, Elsevier, Amsterdam, p. 201-211.) (Reprinted with permission from Elsevier Sclence-NL, Amsterdam, The Netherlands.) ,-.,- Great Dismal Swamp, Virginia. (Photograph by Virginia Carter,) 44