February 11, 2019
Fiber Optic Temperature Sensor System to Evaluate EMNR/MNR
Philip Spadaro shared a presentation on the "Use of a Fiber Optic Temperature Sensor System to Evaluate EMNR/MNR at a Sediment Remediation Site" at the tenth International Conference on Remediation and Management of Contaminated Sediments.
Background/Objectives. The site is a shallow tidal inlet channel located in San Francisco, CA, that has been filled with construction debris and other materials leaving a narrow channel. Investigations identified polychlorinated biphenyls (PCBs) and lead as contaminants of concern (COCs) in sediments at the site. The Environmental Protection Agency-approved remedy allows enhanced monitored natural recovery (EMNR) or monitored natural recovery (MNR) to be employed, in lieu of dredging and capping, in areas where COC concentrations in surface sediments are marginally above remedial goals and technical studies can demonstrate short- and long-term effectiveness. A reliable assessment of the stability of the surface sediment layer and current site-specific sedimentation rate was needed to evaluate the potential for EMNR/MNR to be successfully employed at the site. Field data collection required measuring sedimentation rates of approximately 1 cm/yr. Traditional methods (such as time-series bathymetric surveys or sedimentation pins) do not meet the accuracy required for this project within the time frame, and the results of radioisotope methods are not deemed reliable because historical fill placement has disturbed the stratigraphy. TIG Environmental employed a novel fiber optic temperature sensor system at the site to evaluate sedimentation and erosion in combination with sedimentation pins and a marker horizon layer.
Approach/Activities. The fiber optic temperature sensor system consisted of a fiber optic cable laid in the sediment that measured temperature changes to determine whether the cable was surrounded by sediment or water. The cable was wrapped around a pole to enable measurement at different elevations. The vertical accuracy of the measurements was 4 mm. This approach allowed detection of changes in sediment elevation over time at multiple points along the length of cable. This method has been employed at several sites in Europe to evaluate sedimentation associated with maintenance of rivers and offshore wind farms, and at a site in the United States to evaluate groundwater leakage through capping layers. Informal research shows this method has not been utilized as part of a sediment remediation project. As part of this study, more than 2,000 m of cable were installed in a loop from the upland power source through five poles at various in-water locations. Measurements were taken twice daily (at high and low tide) for a six-month period. Sedimentation pins and the marker horizon layer method were also used to collect information on sedimentation rates at co-located points.
Results/Lessons Learned. Measurements showed a slow increase in sediment elevation at three of the five locations, and a slow decrease in sediment elevation at two of the locations. Overall, changes in sediment elevation were only a few centimeters. Sediments at the site were shown to be relatively stable over time, with little net sedimentation or erosion, although there is likely periodic small-scale sediment movement and elevation changes. An unexpected result was that the system was able to detect a density horizon below the top sediment elevation: the surface sediment layer was less dense and more permeable, while below this density horizon, permeability was reduced. These results were compared to the information gathered from the more traditional methods.