The Navy, Department of Defense (DoD), and other government and private entities are in the process of identifying, assessing, and remediating a large number of hazardous waste sites that are the result of decades of waste management practices resulting in the release of contaminants to soil, sediment, and groundwater in coastal environments. At contaminated sediment sites, it is generally accepted that the affinities of contaminants for fine-grained sediment result in high contaminant concentrations in areas that are characterized by fine sediments. In contrast, at groundwater-surface water interaction (GSI) sites, groundwater discharge of more mobile, dissolved-phase contaminants is often associated with course grained, permeable sediment units. Knowledge of grain size at sediment study sites can provide lines of evidence that can be applied to identify potential areas of contaminated sediment and contaminant discharge zones.
Field surveys for grain size can require a full sampling regime with substantial analytical costs. The sediment friction-sound probe (SED-FSP) technology can be used to quickly and cost-effectively acquire grain size information. The objective of this project was to field demonstrate the effectiveness of the SED-FSP for direct, in-situ measurement of grain size at contaminated sediment and GSI sites. This was accomplished through development of a commercial prototype friction-sound probe, verification of sensor performance in the laboratory, and field demonstration and validation.
The friction-sound intensity at a particle/sensor interface has been shown to be a linear function of the radius of particles in contact with the sensor surface and the velocity of the probe. The SED-FSP technology employs this relationship to infer grain size by measuring the acoustic response as a probe with an imbedded microphone penetrates a sediment matrix. The microphone signal is processed through an on-board electronics interface package and transmitted to recording software. A pneumatic drive unit mounted on an aluminum frame assembly is used to drive the probe into the sediment bed at a controlled speed. Grain size is determined by comparing the acoustic response to responses of prepared sediments of known grain sizes, the calibrations are performed prior to the field deployment.
Three types of sites were selected to field demonstrate the technology: (1) a GSI site, (2) a contaminated sediment site, and (3) a contaminated sediment thin-layer containment cap where the vertical profiling capabilities of the technology were demonstrated. Site surveys of the areas were conducted with the SED-FSP system and responses were used to generate grain size maps. For two of the areas, the system was used to generate grain size depth profiles. Validation of the technology was accomplished by comparing SED-FSP response to laboratory validated measurements of site sediments and through comparison to previously conducted site surveys.
The SED-FSP technology was demonstrated at three locations: Naval Base San Diego (NBSD) at the mouth of Chollas Creek in San Diego Bay, North Island Naval Air Station (NASNI) Installation Restoration (IR) Site 9, and the Active Capping Pilot Study Site on the Anacostia River in Washington, D.C. At Chollas Creek, 20 stations were acquired including collection of validation samples at all stations. The resulting survey showed that the largest grain sizes measuring in the medium sand range were acquired at the mouth of the creek trending to finer sediments into San Diego Bay and upstream into Chollas Creek. These results were supported by an earlier site assessment performed in 2004 by Space and Naval Warfare Systems Command (SPAWAR) Systems Center – Pacific (SSC-PAC) investigators, which found the same trends.
Two surveys were performed at the NASNI IR Site 9 location. During the first field effort, the SED-FSP was deployed at 27 locations during which validation samples were collected. The SED-FSP succeeded in determining size classifications for the validation sediments to greater than 85% accuracy; in all instances where the response was not validated, the SED-FSP under predicted grain size. During the second deployment at NASNI, the SED-FSP was used to survey the entire study area, which included twelve transects, nine to twelve stations per transect. The results were used to generate grain size maps of four depth layers, which were used as evidence supporting previous assessments of contaminant transport at the site. The results were also used to support the sampling plan for a comprehensive assessment of IR Site 9 that is anticipated in the near future.
At the Active Capping Pilot Study Site on the Anacostia River, a sand cap that had been installed in March 2004 was investigated. The purpose of the deployment was to demonstrate the capability of the SED-FSP to acquire grain size measurements in subsurface sediments, to delineate the capping material/native sediment interface, and to provide information on the capping thickness. Of the 44 core sections submitted for validation, the SED-FSP correctly predicted 42 size classification results. The SED-FSP identified the subsurface capping material/sediment interface and confirmed that its thickness and boundaries have remained intact. This was confirmed with the sediment cores, which showed that the capping material remained intact with little dispersion beyond the cap boundaries or into the underlying native sediment.
The SED-FSP implementation costs are similar to costs associated with sediment sampling deployments. The key cost drivers are labor, field deployment costs, transportation/shipping, and capital equipment costs. The capital costs would be expected to be recovered quickly as they are low. The demonstrations were performed at full-scale; therefore, scale-up is a non-issue. Costs related to sample analysis relate to data reduction by the user by spreadsheet or other processing software, costs are not incurred for sample analysis as the SED-FSP performs this function in real-time.
Prior to each of the field demonstrations, the SED-FSP was calibrated using prepared sediments of known grain sizes. When employed in the field, it was found that the system tended to under predict grain size based on analysis of validation samples. Recalibration of the system using a limited number of site sediments as calibration samples resulted in the unit performing within the performance metrics. Therefore, site-specific calibrations are required using site sediments.
Field testing of the unit confirmed applicability of the technology where fine sediments were differentiated from sandy sediment and between sub-classifications of sands; sediments in the clay range (< 3.9 micromoles [µm]) were not acquired either as a SED-FSP response or as results of laboratory analysis of site samples. Laboratory testing also showed that the SED-FSP did not resolve or accurately predict sizes of this range and smaller. The unit should therefore be considered for use where differentiation of sands and fines are required. Differentiation of silt (3.9-63 µm) and clay sizes was not validated.