Using hydraulic conductivity and micropetrography to assess water flow through peat-containing wetlands

Abstract

This study focuses on research designed to assess water flow through peat-containing wetlands using hydraulic conductivity (HC) and micropetrography. These goals were accomplished by examining a series of cores taken in a transect through a peat-containing wetland located at the Savannah River Site, South Carolina, and comparing these results with similar measurements from a selected suite of samples from a sample bank of highly characterized peats from other localities. The hydraulic conductivity of all samples was measured using ASTM method D4511-00. In addition, all samples were processed into oriented thin sections to determine porosity and other micropetrographic parameters. The results of this study demonstrate that differences in composition of peat layers found with depth in a peatland can affect water flow through the peatland. In general, the highest hydraulic conductivities tended to be found where the peat layers were high in fiber (fibric to hemic) and light in color (e.g., Nymphaea-dominated aquatic peats). For all core samples, a strong positive correlation was seen between hydraulic conductivity and percentages of macropores (pores >50 μm). The equation found in this study to produce the best prediction of hydraulic conductivity from macropore percentage was y=0.0002 x−0.0027, where y=hydraulic conductivity (cm/s) and x=percentage of macropores. This study further suggests that the percentages of macropores in peats could be a very useful tool in developing models to predict both direction and speed of water transport through in situ, multi-layered peatlands; whereas either total porosity or macroporosity could be equally useful as predictors of hydraulic conductivity in: (a) the upper 25 cm of peatlands, (b) in shallow constructed wetlands where peats are not greatly compacted by burial, or (c) in peat-based artificial filtering systems. Some applications of these relationships might include predicting flow of ground water in engineered wetlands used for acid mine drainage, predicting past or present flow through more deeply buried peat deposits, or predicting flow of water or gases through lignites or other low rank coal beds.