HYDROGEOLOGIC SETTING OF A FLORISTICALLY DISTINCTIVE GROUND-WATER SLOPE WETLAND ALONG THE FALL ZONE

Acidic ground-water slope wetlands are a distinctive feature of the Fall Zone landscape around Washington, D.C. Although these wetlands were never common, landscape disturbance and perturbations of the hydrologic system following urbanization have left even fewer ecologically functioning examples. Historically called ‘Fall Line magnolia bogs’, they host several regionally uncommon plants, such as sweetbay magnolia, poison sumac, swamp azalea, false hellebore, and sphagnum moss. The occurrence and distribution of these regionally rare wetlands are restricted to a specific hydrogeologic setting associated with dissected inner edge of the Coastal Plain along the Fall Zone. Such conditions are exemplified at Barcroft Park along Four Mile Run in Arlington, Virginia, one of the largest and best preserved remnants of this community. There, strongly acidic ground water discharges from pebbly sand at the base of the Potomac Formation, forming a line of seeps and springs over 1,000 feet wide and encompassing more than 25 feet of vertical and stratigraphic relief. Upward hydraulic gradients as great as 0.1 have been measured at the site, driving a total discharge of hundreds of gallons per minute. The seepage zone lies near the base of a 150-200-foot high, concave erosional scarp underlain at progressively higher elevations by Potomac Formation clays and Tertiary-Pleistocene river gravel. Tilting, faulting, and facies changes of the Potomac Formation produce a complex flow system, in which the ground water that supports this wetland does not recharge in woodlands immediately above the site, but up to 2 miles away in heavily urbanized Fairfax County, creating interesting management challenges. Despite lengthy residence times, the ground water is severely depleted in bases, and enriched in iron, manganese, and silica, reflecting the impact of deep Tertiary through Pleistocene weathering profiles. The distribution of moisture across this landscape creates a distinct, ground-water-driven ecotone manifested by systematic changes in community structure and composition. The hydrogeologic setting leads to processes within the wetland more commonly associated with northern bogs, such as the growth of sphagnum hummocks and their colonization by acid-tolerant wetland herbs and shrubs.