Determining Hydraulic Conductivities in a Vertical Flow System

Vertical flow systems (VFS) are a type of passive treatment unit process in which acid mine drainage (AMD) flows vertically through layers of organic matter and limestone. Located in the coal belt of eastern Oklahoma, the Red Oak VFS was constructed to address AMD exiting an abandoned underground coal mine. The passive treatment system consists of five cells, alternating between surface flow (cells 1, 3, and 5) and vertical flow (cells 2 and 4) designs. The VFS included an approximately 0.6-m limestone drainage layer containing a network of pipe overlain by approximately 1-m of composted horse manure mixed with limestone. Design surface water elevations were approximately 1-m above the organic matter surface. In spring 2004, cell 4 appeared to be experiencing hydraulic conductivity problems as outflow discharge rates decreased and water levels increased, based on measured flows and staff-gauge readings, respectively. It was hypothesized that accumulated solids (either sulfides or degraded organic material) were causing clogging in the substrate. This investigation was conducted to evaluate these possible hydraulic conductivity problems. A well point screened throughout the substrate layer and water column and containing a pressure transducer was installed. A series of field dropping head permeameter tests was conducted to determine hydraulic conductivity. Although vertical flow cells have been shown to result in substantial water quality improvement, concerns about the long-term physical viability of these systems warrants further investigation. Additional