Determination of Lateral Effects of Borrow Pits on Hydrology of Adjacent Wetlands

Abstract

Borrow pits in eastern North Carolina are constructed to serve as a source of fill for highway construction. The borrow pits are often located near or adjacent to wetlands. There is concern the borrow pits will act as long term drainage sinks by creating hydraulic gradients such that seepage is predominantly from the wetland to the pit. An approximate method to calculate the lateral effect of a drainage ditch on adjacent wetland hydrology was previously developed. The goal of this study was to develop the necessary inputs to the approximate method for borrow pit application and to compare results of the method to field data. Field testing of the method was conducted at eight borrow pits located adjacent to wetlands in eastern North Carolina. Four pits were closed for more than four years. The remaining pits were closed recently. Continuous water level recorders were installed in the pits and along a transect leading from the pit to adjacent wetlands. Data were collected from late 2004/2005 to present. Water level in the pits reached an equilibrium level in less than two years. Preliminary analysis of the data indicated that seepage is from the wetland to the pit at half the study sites. Observations at the Prime Ridge site show water levels in the eastern wetland greater than and in the western wetland less than water levels in the pit. This indicated no lateral effect due to the pit on the western wetland. Data also indicated that fluctuations of the water level on the eastern side may be more a function of precipitation and evapotranspiration than seepage to the pit.