E2 was faced with a challenge: Perform long-term monitoring of an underwater cap placed atop contaminated sediments in conditions with near zero visibility for the divers. To ascertain the effectiveness of the pilot-scale test caps over time, samples must be regularly collected from beneath the cap, which is designed to protect the caps from damage.
E2 created a system that was installed with the cap material that allows:
- the divers to locate the sampling locations in near-zero visibility in the lake,
- verify the locations are correct,
- access the underlying contaminated sediments through the armor layer of gravel,
- provide consistent locations for all monitoring over the two-year program,
- and provide locations for measuring porewater and seepage.
The solution was to install Armor Basket Clusters (ABCs) which each contain six sampling ports for collecting sediment cores, and two sampling ports for collecting porewater samples. Performance monitoring requires that representative samples of native sediment, cap material, and newly deposited sediments atop the armor layer of the cap be obtained with minimal disturbance. E2 designed a system of double interlocking baskets (“basket within a basket”) that were installed concurrent with the cap. The double baskets permit divers to remove the inner basket containing the cap armor material, isolate it, and bring it to the surface for sampling. With the inner basket removed, the outer basket restrains the surrounding armored layer and permits access to the fine-grained layer of the cap and underlying native sediments via a piston vibracore sampler.
Pore water samples are collocated with each sample basket cluster, assuring that the data points are properly paired over the two years of monitoring. To allow the divers to find the ABC and collect the sample from the right port, a system of beads and bobbers was installed which had a unique combination, essentially a braille system for the divers. The beads and bobbers are on 2 ft high wire antennas so the divers can locate them within very low visibility conditions that result from algal blooms.
Three monitoring events have been completed and the systems have provided excellent quality samples while maintaining the integrity of the armor layers. The divers have been able to quickly locate and collect the samples despite the poor visibility while maintaining the integrity of the cap.
“Because the cap is armored with gravel, traditional sampling methods would be very challenging…E2’s …Armored Basket Clusters (ABCs)…enable ports for the sampling to be accessed by simply removing an armor-filled basket, inserting the appropriate sampling device, collecting the sample, and then replacing the basket. The system is highly efficient, significantly reduces the time spent on station collecting samples, and is very effective in obtaining high quality samples from the cap material and below. The ABC system also incorporates pre-positioned seepage funnels with flow tubes emerging from the armor layer. This makes it a simple matter to connect a seepage flow meter to the flow tube and monitor the seepage through the cap. The foresight and innovation employed by E2 in preinstalling the ABCs and the seepage systems has enabled the ongoing monitoring of the pilot cap over multiple sampling events, yielding high quality monitoring data in a cost-effective manner.
— Bart Chadwick, Coastal Monitoring Associates