PRB Dimensions: Two trenches 0.6 m wide, 11 m deep and 76 m long, each. One trench was backfilled with a mixture of 50% sand with 50% sawdust. The other trench was backfilled with two mixtures; one 28% ZVI with 72% sand and the other 49% ZVI with 51% sand.
Description of Work:
Historical practices from a chemical and waste oil recycling business resulted in dangerous contamination of an area along the edge of the Helena River in Perth, Australia. Chemical contamination included chlorinated solvents (TCE) and petroleum hydrocarbons (BETX). Investigations discovered two groundwater plumes of contamination that merged at the site. Landcorp (an authority of the government of Western Australia), designed two sequential PRBs to treat the contaminated groundwater. The first PRB was about 0.6 m wide, about 11 m deep, 80 m long and filled with sawdust. The second PRB had similar dimensions but was 4 m down gradient and filled with ZVI (zero valent iron). As groundwater flows through the PRBs, nitrogen is removed in the first barrier and contaminates are removed in the second barrier. On some sites, nitrogen binds the ZVI reducing ZVI efficiency, but on this site removing the nitrogen with sequential barriers results in a more efficient and less costly system.
Landcorp issued a tender for a PRB with very few restrictions on the method of installation, so Bio-Polymer (BP) trenches competed directly with secant walls and other installation methods. Geo-Solutions teamed with Menard-Bachy using the bio-polymer trench method to win the project with a bid that was 40% less expensive than the second bid. The advantages of the BP methods are controlled dimensions of the trench, speed of installation and minimal cost.
Geo-Solutions shipped a small BP slurry mixer and BP ingredients along with a small team of BP slurry experts to Australia and began working with Menard-Bachy on the project in March of 2010. After creating a work pad and mixing facilities, the ZVI trench was started and completed in only 4 days. Excellent production was achieved despite encountering numerous large boulders. The sawdust/sand trench required 5 days of trenching due to encountering even more large boulders and unusually stable ground.
