The Navy has identified over 150 contaminated sediment sites in its Installation Restoration (IR) and Base Realignment and Closure (BRAC) programs. Of these sites, approximately 25 to 30 percent have known or suspected metals contamination. Two common remediation strategies for metals contaminated sediments include capping and monitored natural recovery (MNR), which leave the contamination in place and therefore require long-term monitoring to evaluate remedy effectiveness. Given the extent of sediments contaminated by metals across the United States, and the costs associated with the sampling during long-term monitoring, there is a need for improved methods for in situ metals detection in sediments.
Electrochemical sensors have been tested in a wide range of aquatic settings, including marine sedimentary porewater; however, their use has been on time scales of hours to days and long-term monitoring applications have not been evaluated. The objective of this study was to demonstrate and validate the Analytical Instrument Systems, Inc. (AIS) microelectrode geochemical observatory (MGO) for the long-term monitoring of metals in contaminated sediments.
The MGO was evaluated for performance criteria during a laboratory evaluation phase, and a pre-field deployment at Old Woman Creek (OWC) in Huron, Ohio. The AIS electrode was tested in the laboratory with metals spiked synthetic marine porewater, and with marine porewater and sediment that was collected by extruding core samples. Electrode testing was done in triplicate with standards solutions and porewater that were spiked with metals. The electrode was evaluated in solutions spiked with metals at a minimum of four concentrations per metal. The electrode was tested in sediment to measure background levels of metals and also with sediment spiked at 10-times background levels.
Attempts to ruggedize the gold electrode before field deployment proved futile as the gold wire was exceptionally fragile. The AIS system was deployed at OWC for 1 month. OWC is a relatively pristine wetland containing mainly manganese and iron in the sediment, as opposed to the target metals; therefore, the amalgam electrode was used at the site.
During the early stages of the laboratory testing, it became evident that the off-the-shelf SnapTrodes™ lacked both an accurate and precise response as well as the sensitivity to detect the selected target metals at their maximum contaminant level (MCL). After the SnapTrodes™ were found to be ineffective, goldmercury amalgam electrodes were built following a design from literature. The electrode was able to detect iron, manganese, and reduced sulfur compounds but not the target metals. After testing the amalgam electrode, 5 mm gold electrodes were constructed following procedures from literature. The gold electrode was able to produce calibration curves for copper, lead, and zinc in standard solutions. The 5 mm gold electrodes also were able to produce calibration curves with lead and zinc in Bremerton porewater.
The unit was able to detect manganese, iron, and reduced sulfur in the sediment for a few days before the system shut down. The system had to be restarted multiple times. In addition to hardware issues, the electrode became fouled over a relatively short period of time (weeks) and produced incoherent, noisy scans.
This work was predicated on the off-the-shelf SnapTrode™ components functioning properly. While the team proposed using mature, commercially available components, it became clear during the initial phase of this project that adapting open water technology for sediments application remains a challenge. Although the team expanded on resident expertise by fabricating two different electrodes, further work is needed to design an electrode with the required functionality across a broad range of metals. Ruggedization of the electrode will support the design of a field deployable probe. Refinement of the field design will yield cost savings measures via alternative power sources, such as solar and remote data collection capability. Each of these areas requires further, suitably-funded studies to advance this technology to a field-ready condition.