CHANGES IN THE HYDRAULIC CONDUCTIVITY OF A PASSIVE VERTICAL FLOW TREATMENT SYSTEM FOR ACID MINE DRAINAGE

Extensive mining in Western Pennsylvania has resulted in the contamination of many streams with acid mine drainage (AMD).  Oxidation of pyrite associated with coal deposits results in AMD characterized by low pH, high acidity, and elevated metals (Fe, Al) and sulfate concentrations.  Impacted streams are characterized by poor water quality and a limited biologic component.  Remediation has been most successful when the source is treated prior to entering the stream.  AMD is commonly treated by neutralizing the acidity and increasing the pH in a limestone matrix followed by precipitation of metals in a wetland.

A passive vertical flow system installed in 1996 at the former Brydon Mine in Butler Country, Pennsylvania consists of a 3ft bed of limestone and compost matrix overlain by1-3ft of water.   AMD is added to the system, allowed to filter through the matrix, collected by an underdrain, and discharged to a series of wetlands before it flows into a stream.  Influent flow rates vary between 10-30 gallons/minute. The outflow rate of the system is controlled by changes in the hydraulic head.  When constructed the hydraulic conductivity of the system was on the order of 10 ^-2 cm/sec but steadily decreased to 10 ^-4 cm/sec.  This resulted in a maximum outflow rate of 10 gallons/minute.  When the inflow rate exceeds this value the untreated excess is discharged directly into the wetlands.

The system was shut down and drained in the spring of 2002 to determine the source of flow reduction.  A trench was dug in the matrix and twenty-six undisturbed core samples were obtained.   When the matrix was placed in the system it was assumed to be a homogeneous mixture of limestone and compost.  However, the cores and trench revealed layers of limestone/compost mixture, limestone, organic material, cemented matrix, and iron oxides.  A constant head permeameter was used to determine the hydraulic conductivity of each core and isolate the restrictive layer(s).  Hydraulic conductivities ranged from 10^-1 to 10^-5 cm/sec.   Higher hydraulic conductivities  (10^-1 cm/sec) were measured from cores with limestone only, while lower hydraulic conductivities (10^-5 cm/sec) were from cores that contain either a layer of compressed organic material or cemented matrix.  Initial results show that the reduced flow through the system is due to less permeable layers within the matrix.