The construction of highways requires soil to fill low areas, to build overpasses and ramps, and for a variety of other purposes. When the required fill is unavailable from cuts made during the construction process, it is commonly obtained from ""borrow pits"", usually located close to the highway. In some cases the borrow pits are located in the vicinity of wetlands. This is often the case in eastern North Carolina, where surface elevations are low and much of the landscape is poorly drained under natural conditions. In such locations it may be nearly impossible to find a site for a borrow pit that is not adjacent to, or in the vicinity of, wetlands. While the North Carolina Department of Transportation's specifications for highway construction clearly state that wetlands in the vicinity of borrow areas are to be delineated, and that a 25 foot buffer between the borrow and adjacent wetlands is to be maintained, there is concern that the borrow pit may serve as a long-term drainage ""sink"" for the wetland. The concern is that drainage to the pit, after closure, will affect wetland hydrology in a strip of land adjacent to the pit. The purpose of this proposed research project is to develop methods for determining the lateral effects of borrow pits on the hydrology of adjacent wetlands.
Methods were developed in a previous NC DOT sponsored research project to estimate the lateral effect of highway drainage ditches on wetland hydrology. The methods are based on solutions to the Boussinesq equation and are described by Skaggs and Chescheir (2002). It seems reasonable to apply these same methods to determine the lateral effects of borrow pits. However, there are differences and unknowns that must be resolved before the methods developed for highway drainage ditches can be used for determining the lateral impacts of borrow pits. One of the main differences concerns the water level in the borrow pit relative to the water table elevation in the wetland. After the borrow material has been removed and the pit is ""closed"", the elevation of the water level in the pit will control the hydraulic gradient and determine whether the pit serves as a sink or a source for seepage to or from adjacent wetlands. One of the objectives of this research is to determine the ""equilibrium"" water level elevation in the pit, and how it varies with weather conditions and season. Once this elevation is known, the methods developed in previous research can be applied to estimate the lateral effect.
The proposed three-year research project has both short-term and long-term objectives. In the short term, methods are needed immediately to estimate lateral effects for current and pending operations.
Methods will be developed to predict the water level in the pit and how it varies with time. This is considered a long-term objective and will involve both field surveys of pits that have been closed for several years and monitoring of recently closed pits. We expect to develop methods to determine both the ""equilibrium"" water level and the time required after closure to attain that level. Water balance models will be developed to predict fluctuations of the water level in the pit on a daily and seasonal basis.
