Current methods for assessing stormwater exposure, fate, and transport are limited. Compliance programs generally focus on end-of-pipe monitoring for first-flush conditions. These measurements are often highly variable and provide little insight into actual environmental exposure, impact, fate, and transport. Methods for assessing particle transport and fate at Department of Defense (DoD) coastal sites also is limited. Sediment transport models have been applied on larger scales but are highly complex and usually of inadequate resolution to resolve small stormwater discharges. Field based methods that have been used include sediment sampling near outfalls, sediment traps placed near outfalls, and geochronology analysis of sediment cores collected in suspected depositions zones. Sediment sampling near the outfalls provides indirect evidence of potential fate of particles, but lacks any direct linkage, and is often confounded by high spatial heterogeneity of contaminant levels in sediments. Sediment traps in active harbor areas provide a measure of sediment deposition that incorporates all forms of transport and resuspension, but are unable to distinguish source deposits from these other depositional sources. Geochronology in active harbor areas is often confounded by historical disturbance of the sediments by dredging and resuspension events, and thus cannot be reliably applied in many of the areas of interest. Because of the limitations of current methods, there is a high degree of uncertainty in the exposure, transport, and fate associated with these sources, and new tools are needed that can help the DoD better manage these challenges.
The objective of this project was to demonstrate a family of technologies adapted from the oceanographic and environmental arenas that could significantly improve the ability to address contaminant source exposure, transport, and fate challenges at DoD coastal sites in a relatively simple and cost-effective way. Through the integration of these technologies, the goal was to develop and demonstrate a spectrum of new capabilities.
The demonstration focused on the following three key technologies:
- Drifting Exposure System (DrEx): Surface global positioning system (GPS) drifter with position data telemetry, composite sample collection, and passive sampler capabilities for exposure characterization.
- Drifting Particle Simulator (DPS): GPS drifter with position data telemetry, buoyancy control, bottom detection, and passive sampling capability for measurement of depositional footprints and sampling of source related particles.
- Sediment Deposition Detector (SeDep): Sediment bed scour sensor with high resolution differential pressure sensor, shore cable or in situ data logging, and coupled sediment trap capabilities for simultaneous quantitative measurement of deposition rates and sampling of depositing particles.
Performance analysis focused on the ability of the systems to provide improved exposure, transport, and fate assessment for potential sources. A key aspect of this performance was the ability to quantify the linkage between ongoing sources and potential recontamination of sediment.
DrEx: Performance for the DrEx system was evaluated during two demonstration events at the mouth of Paleta Creek at Naval Base San Diego (NBSD). The first was conducted during a relatively large storm event while the second was conducted during a smaller event. In both cases, ten drifters were released into the first flush of the storm event to track the stormwater plume and collect samples during exposure periods that ranged from about 12 hours for the large event to about six hours for the small event. Overall, performance results from the DrEx demonstrations indicated: the GPS tracking data from the DrEx systems provided a clear visualization of the area of the bay with connectivity to the stormwater plume; the onboard sensor data from the DrEx units were very useful in evaluating the dynamics of the stormwater plume, the dilution of the plume over time, and the influence of other stormwater sources in the general vicinity; the composite samples from the events provided an effective means for characterizing exposure conditions within the first-flush portion of the discharge plume from both a chemical and toxicological perspective; and the units stayed within the plume and recorded conditions that were consistent with the exposure that would be expected for the first-flush portion of the plume as it disperses into the receiving water.
DPS: Performance of the DPS system was evaluated during two demonstration events in Pearl Harbor. The first event was conducted at the Oscar Pier site in Pearl Harbor and the second event was conducted at the Waiau Generating Station site. During each event, DPS units were deployed at approximately hourly intervals throughout a full 24-hour tidal cycle. The DPS units were tracked until they made bottom contact, and the resulting bottom contact map was used to construct deposition footprints for each of the outfall sites where the demonstrations were performed. Overall, performance results from the DPS demonstrations indicated: the GPS tracking and bottom contact data collected using the DPS systems provided a clear visualization of the area of the harbor with connectivity to the outfall, the depth, spatial and time scales of the transport area, and the spatial location and size characteristics of the deposition footprint; the DPS system can act as a platform for sensors to track background or storm event conditions during the trajectories of the drifters; and the results compare favorably to model simulations for particle transport and settling performed for the same sites.
SeDep: Performance of the SeDep system was evaluated during a single demonstration event at the Oscar Pier site in Pearl Harbor. Based on the DPS deposition footprint results, ten SeDep units were deployed in the nearfield area of the footprint with an additional unit deployed further into the harbor and further out toward the entrance to characterize the far field areas of the footprint. Deposition data were collected continuously over the 42-day period of the deployment. Overall, performance results from the SeDep demonstrations indicated: the SeDep sensor systems provided a unique temporal quantification of cumulative sediment deposition under conditions that are representative of DoD harbors subject to stormwater and other sediment transport processes; the sediment traps that were collocated with the SeDep sensors provided an effective means of collecting deposited sediments, and the deposition rates were consistent with expectations regarding the typical rates in Pearl Harbor; and in conjunction with the deposition data, the sediment trap and surface sediment chemistry were useful in evaluating the potential for recontamination at the deployment site.
These technologies will provide a broad new set of capabilities that are highly applicable to characterizing the exposure, transport, and fate of stormwater contaminant sources. They are targeted to reduce DoD total ownership costs by (1) avoiding costly best management practices (BMPs) and containment systems targeted toward stormwater discharges based solely on end-of-pipe discharge violations, and (2) avoiding major costs associated with recontamination of remediated sediment sites. These potential cost impacts are significant considering that installations such as Naval Base San Diego are faced with first-flush capture requirements for stormwater that could cost in excess of $300 million and Pearl Harbor is preparing to invest millions in sediment remediation while potential source control uncertainties persist.
The over-arching strategy for implementation of the technologies was based on several key components including: the technology is well demonstrated and documented, standard operating procedures are developed and available, equipment is available on the open market, technology service providers are available to DoD users, and regulators have visibility of the technology. The equipment, described in this document, is currently available from vendors with the exception of the SeDep system. For this system, a relationship with a commercial vendor was not finalized, although the system is based directly on the commercially available system. Service providers collaborated in the project and have experience with the equipment.
The technologies, particularly the SeDep system, would still benefit from further demonstration by early adopters under a broader range of applications, and would also benefit from further exposure to regulatory agencies that have oversight of stormwater and/or sediment cleanups.