Non-reactive hydraulic assessments during a freeze-thaw laboratory based simulation of a permeable reactive barrier

All solutions were pumped into and out of the Darcy boxes using peristaltic pumps and acid washed Masterflex FDA vitron tubing. Dry media was weighed in 1 kg batches and homogenised by shaking and turning end over end in a ziplock bag for 1 minute. To ensure that the media was always saturated, known amounts of Milli-Q water followed by the homogenised media were added to each box in approximately 1 cm layers. 20 mm of space was left at the top of the boxes to allow for frost heave and other particle rearrangement processes.

The process of each solution flow assessment took approximately 2.5 days. For the entire duration the flow rate of the upstream pump was set at 18.1 mL min-1. The height in the feed weir was maintained as closely as possible to 30 mm below the top of the box by fine adjustment of the downstream pump.

During this time the electrical conductivity (EC) of the effluent was logged at 5 second intervals. Initially, Milli-Q water was passed through the box until the EC reduced to a constant value. After approximately 10 hours of water flow a conductivity-based pulse tracer test was conducted on the box.

This was performed by changing the feed solution to 0.05 M sodium bromine for 20 minutes. Between 95% and 103% of the tracer was recovered in all tests as measured by an e curve method described by Levenspiel (1999). Residence times were determined using the exit age distribution method.

The remaining assessment consisted of increasing step concentrations of copper and zinc solutions. This reactive tracer data is presented in Statham et al. (unpublished manuscript). After the sampling, metal clamps were tightened along the length at the base and top of the boxes to increase structural integrity when exposed to freeze-thaw cycling.

The Perspex sides and bases of both Darcy boxes were covered with insulated panels of 25 mm of extruded polystyrene and the boxes were placed in a Sanyo MIR-153 laboratory incubator. The incubator was programmed to cycle through 4 days at -12 degrees C followed by 3 days at 10 degrees C. These temperatures were based on the lower limit of operation of the machine and a realistic field condition. Levenspiel, O. (1999) Chemical Reaction Engineering. 3rd Edition.

John Wiley and Sons, New York.