THE GROUNDWATER GEOCHEMISTRY OF THE GREAT DISMAL SWAMP, A PEAT WETLAND DISCHARGING TO TWO MID-ATLANTIC ESTUARIES, USA

The Great Dismal Swamp, Virginia and North Carolina, USA, is a 45,325 hectare peatland characterized by several principal forested-wetland types underlain by groundwater that flows through the peat and discharges into ditches which then drain through adjacent streams to Chesapeake Bay and Albemarle Sound. Most groundwater is derived from precipitation that falls directly on the swamp. Thus, precipitation and reactions between the groundwater and peat have a dominant effect on the water geochemistry affecting pH, concentrations of nutrients, and concentrations of other constituents in water discharging from the swamp.

Studies of the swamp hydrology and water chemistry are revealing the likely effects of the swamp on the quality of receiving waters. Results of a completed, local study provide background water chemistry of the swamp. An ongoing study of the carbon budget as it relates to climate change is evaluating the transport of carbon dioxide, methane, dissolved organic carbon, nutrients, and major ions through groundwater to the ditches from the Atlantic white cedar, pocosin, and maple/gum forest communities. These results along with those of an ongoing flow-modeling study will help estimate export rates for nutrients, carbon, and other constituents from the swamp to nearby receiving waters.

The current soil and groundwater chemistry of the swamp, which are characterized by highly degraded peat, low dissolved oxygen concentrations, and acidic conditions, have been affected by historical drainage and other processes. Dissolved oxygen concentrations typically are less than 1.0 mg/L at the water table as a result of peat decomposition. Although the pH can be between 4.0 and 4.5, the pH of most water is between 3.1 and 3.8. The combination of low dissolved oxygen concentrations and acidic conditions probably slows bacterial decomposition of the peat, release of constituents from the peat, and oxidation of ammonia to nitrate. Typically, concentrations of phosphorus are less than 0.02 milligrams per liter (mg/L), nitrite plus nitrate concentrations are less than the 0.016 mg/L as nitrogen (N) reporting level, ammonia concentrations are 0.02 to 1.0 mg/L as N, and organic nitrogen concentrations are between 2.0 and 3.0 mg/L as N in groundwater.