Subaqueous Capping Research Articles

Effects of organic matter addition on methylmercury formation in capped and uncapped marine sediments

In situ subaqueous capping (ISC) of contaminated marine sediments is frequently proposed as a feasible and effective mitigation option. However, though effective in isolating mercury species migration into overlying water, capping can also alter the location and extent of biogeochemical zones and potentially enhance methylmercury (MeHg) formation in Hg-contaminated marine sediments. We carried out a boxcosm study to investigate whether the addition of organic carbon (OC) to Hg-contaminated marine sediments beneath an in situ cap would initiate and/or enhance MeHg formation of the inorganic Hg present. The study was motivated by ongoing efforts to remediate ca. 30,000 m2 of Hg-contaminated seabed sediments from a Hg spill from the U864 WWII submarine wreck. By the time of sinking, the submarine is assumed to have been holding a cargo of ca. 65 tons of liquid Hg. Natural organic matter and petroleum hydrocarbons from fuels and lubricants in the wreck are potential sources of organic carbon that could potentially fuel MeHg formation beneath a future cap. The results of our study clearly demonstrated that introduction of algae OC to Hg-contaminated sediments, triggered high rates of MeHg production as long a there was sufficient OC. Thus, MeHg production was limited by the amount of organic carbon available. The study results also confirmed that, within the six-month duration of the study and in the absence of bioturbating fauna, a 3-cm sediment clay cap could effectively reduce fluxes of Hg species to the overlying water and isolate the Hg-contaminated sediments from direct surficial deposition of organic matter that could potentially fuel methylation.

In-situ subaqueous capping of mercury-contaminated sediments in a fresh-water aquatic system, Part I—Bench-scale microcosm study to assess methylmercury production

Bench-scale microcosm experiments were designed to provide a better understanding of the potential for Hg methylation in sediments from an aquatic environment. Experiments were conducted to examine the function of sulfate concentration, lactate concentration, the presence/absence of an aqueous inorganic Hg spike, and the presence/absence of inoculums of Desulfovibrio desulfuricans, a strain of sulfate-reducing bacteria (SRB) commonly found in the natural sediments of aquatic environments. Incubations were analyzed for both the rate and extent of (methylmercury) MeHg production. Methylation rates were estimated by analyzing MeHg and Hg after 2, 7, 14, 28, and 42 days. The production of metabolic byproducts, including dissolved gases as a proxy for metabolic utilization of carbon substrate, was also monitored. In all treatments amended with lactate, sulfate, Hg, and SRB, MeHg was produced (37ng/g-sediment dry weight) after only 48h of incubation and reached a maximum sediment concentration of 127ng/g-sediment dry weight after the 42 day incubation period. Aqueous phase production of MeHg was observed to be 10ng/L after 2 day, reaching a maximum observed concentration of 32.8ng/L after 14 days, and declining to 10.8ng/L at the end of the incubation period (42 day). The results of this study further demonstrates that, in the presence of an organic carbon substrate, sulfate, and the appropriate consortia of microorganisms, sedimentary Hg will be transformed into MeHg through bacterial metabolism. Further, this study provided the basis for evaluation of an in-situ subaqueous capping strategy that may limit (or potentially enhance) MeHg production.

Sediment dynamics and pollutant mobility in rivers: An interdisciplinary approach

AbstractCharacteristic dynamic features of sediment‐related processes in rivers include dramatic effects of stormwater events on particle transport, rapid and far‐reaching effects of sulphide oxidation during resuspension, and biological accumulation and potential release of toxic chemicals. Pollutant mobility is the net result of the stabilizing and mobilizing effects in both hydraulic and chemical fields. In practice, emphasis has to be given to fine‐grained sediments and suspended matter as these materials exhibit large surface areas and high sorption capacities. Organic materials are highly reactive. Degradation of organic matter will induce oxygen depletion and might enhance formation of flocs and biofilms. Study of variations of sediment and water chemistry should predominantly include changes of pH and redox conditions, competition of dissolved ions and processes such as complexation by organic substances. Major questions relate to the potential reduction of sorption sites on minerals and degradation of organic carrier materials. All these processes will influence solution/solid equilibrium conditions and have to be studied prior to modelling the overall effects of pollutants on the water body and aquatic ecosystems. With respect to handling and remediation of contaminated river sediments, either in‐place or excavated, a chemical and biological characterization of the material, of the (disposal) site and of the long‐term processes is crucial. Passive techniques (e.g. in situ stabilization, subaqueous capping) provide economic advantages as there are no operation costs following their installation. However, the success of these ecological and geochemical engineering approaches is mainly based on an in‐depth knowledge of the underlying processes.

Simulation of the migration of dissolved contaminants through a subaqueous capping layer: model development and application for As migration

TRANSCAP-1D is a numerical model that simulates the vertical migration of dissolved contaminants through a subaqueous capping layer. The model was developed for the prediction of the long-term effectiveness of a sediment cap. It considers advection, diffusion, chemical reactions, and the effect of the burrowing activity of the benthic fauna. The sediment is represented as a dual porosity medium composed of sediment pores and biologically formed tubes. The numerical model was calibrated with the concentration profiles of dissolved arsenic measured in the sediments at two sampling stations in the Saguenay Fjord, in Québec, Canada, where a major flood event caused the natural capping of contaminated sediments. Thereafter several numerical simulations with variable bio-irrigation depth and variable thickness of the capping layer were performed to test the effect of the uncertainty and variability of the input values on the model response. These simulations indicate that the depth of bio-irrigation in the sediment and the release from mineral dissolution are major factors controlling the distribution of dissolved contaminants in the sediment column. Key words: Saguenay Fjord, migration, numerical model, capping layer, sediment, bio-irrigation, heavy metals, arsenic.

Use of active barriers to reduce eutrophication problems in urban lakes

Excessive concentration of phosphorus is one of the main causes of algal blooms and eutrophic conditions in lakes. In many urban lakes, it appears that a large proportion of the phosphorus in the water column comes from the sediments, particularly when these are anaerobic. Sub-aqueous capping is a relatively new method that has become an attractive option for isolating contaminated sediments from the environment, thus preventing or delaying the release of contaminants into surface waters. Active barrier materials (i.e. capping layers that consist of one or more reactive components) are gaining increasing attention for their greater efficiency in inactivating contaminants held in sediment layers. This paper reports laboratory bioreactor experiments to test the effectiveness of three forms of calcium carbonate (CaCO3) in reducing the release of phosphorus from anaerobic sediment from Lake Carramar, a small urban lake in Melbourne. Two of the CaCO3 active barrier materials tested proved to be quite effective, the most effective materials being the fine particle size, precipitated forms of CaCO3. Over the 20-day experimental period, a 2% layer of the German material SoCal reduced the amount of phosphorus released by almost 100 times over that occurring with no barrier. The Australian product ESCal, while not as effective as the SoCal, still reduced the phosphorus released by around 15 times that with no barrier. A finely ground Lilydale limestone was essentially ineffective in reducing phosphorus release from the sediments. A preliminary cost-benefit analysis suggests that SoCal is unlikely to be attractive for use in Australia, given the estimated application cost of around 3,800 dollars per tonne. However, although the ESCal is slightly less effective in retaining phosphorus, its potential application cost estimated at 2,000 dollars per tonne, makes it an attractive option. On the basis of these most promising preliminary results, we intend to further test the use of the ESCal. Further investigations will include: longer term laboratory studies using ESCal, optimisation of the barrier layer and methods for applying this material, mesocosms and full lake studies, and risk assessment studies to ensure there are no adverse ecological effects from its use.

Design criteria and theoretical basis for capping contaminated marine sediments

Contamination of rivers, waterways and harbors, directly affects their recreational and commercial uses. Contaminated sediments can exert a significant oxygen demand, support a poor diversity of benthic organisms and adversely affect local (overlying) and downstream water quality. In-situ, subaqueous capping is an attractive, non-intrusive and cost-effective method of remediating contaminated sediments. The successful design of an underwater cap requires the proper application of hydraulic (armor and filter equations), chemical (diffusive and advective/dispersive transport equations), and geotechnical (settlement and stability equations) engineering principles. Theoretical considerations for cap and armor design are presented in this paper and illustrated using an example application of a cap design for a confidential contaminated harbor site.

Concept of subaqueous capping of contaminated sediments with active barrier systems (ABS) using natural and modified zeolites

In this study the concept of subaqueous capping of contaminated sediments of lakes, rivers and coastal waters with active barrier systems (ABS) in order to minimise the contaminant release into the surface water is developed. This concept is supposed to provide a low-cost alternative to conventional methods in water protection. Active barrier systems, i.e. reactive geochemical barriers on the basis of low-cost materials, shall actively inhibit contaminant release from the sediment into the surface water, without the hydraulic contact between sediment and surface water being disturbed. Theoretical considerations and first experimental results regarding the retention of Pb 2+ by four different zeolitic rocks suggest that natural zeolite as a reactive material in sediment capping meets all the economical and technical requirements posed by the active barrier concept. Natural zeolites are capable of demobilising large amounts of cationic pollutants by sorption, as shown for Pb 2+, and, furthermore, they are capable of demobilising non-polar organics and anionic contaminants, when the zeolite surface is pre-treated with cationic surfactants. Active barrier systems on the basis of natural zeolites thus can be applied to nearly any type of contaminated sediment. Additionally, the physical characteristics of zeolitic rocks, as grain size and density, facilitate their use in subaqueous applications.

Assessment of sediment and porewater after one year of subaqueous capping of contaminated sediments in Hamilton Harbour, Canada

In this manuscript, we present data from a demonstration in situ capping site (100 m × 100 m) in Hamilton Harbour, Lake Ontario, Canada. A layer of clean medium to coarse sand with the average thickness of 35 cm was placed at the site in the summer of 1995. Concentration of Zn, Cr, and Cd in the original sediments reached values over 6000, 300 and 15 μg/g, respectively. The predicted consolidation of the uppermost one meter of sediment was about 14 cm, which was in good agreement with values obtained from comparisons of moisture content values of pre-capping and post-capping cores. A thin layer of fresh moderately contaminated sediments has started to develop on the top of the cap. In general, the concentrations of elements were greater in porewater than in the overlying water, e.g., the concentration of Fe and soluble reactive phosphorus were 1000 times, and those of Mn 100 times greater. There was a significant reduction in the vertical fluxes of all the trace elements after the capping of the contaminated sediments.

Assessment of sediment and porewater after one year of subaqueous capping of contaminated sediments in Hamilton Harbour, Canada

In this manuscript, we present data from a demonstration in situ capping site (100 m x 100 m) in Hamilton Harbour, Lake Ontario, Canada. A layer of clean medium to coarse sand with the average thickness of 35 cm was placed at the site in the summer of 1995. Concentration of Zn, Cr, and Cd in the original sediments reached values over 6000, 300 and 15 μg/g, respectively. The predicted consolidation of the uppermost one meter of sediment was about 14 cm, which was in good agreement with values obtained from comparisons of moisture content values of pre-capping and post-capping cores. A thin layer of fresh moderately contaminated sediments has started to develop on the top of the cap. In general, the concentrations of elements were greater in porewater than in the overlying water, e.g., the concentration of Fe and soluble reactive phosphorus were 1000 times, and those of Mn 100 times greater. There was a significant reduction in the vertical fluxes of all the trace elements after the capping of the contaminated sediments.

Subaqueous capping of very soft contaminated sediments

A proposed 100 m × 100 m site for a subaqueous capping pilot-scale project in Hamilton Harbour is located in a water depth of about 15 m where the capping material will be in a relatively low-energy environment with little potential for erosion of the cap. The cap will consist of a 0.5 mm diameter clean sand that will be placed on the bottom with a tremie pipe discharge system mounted on a spudded barge. The design thickness of 0.5 m will provide an effective isolation of contaminants from the overlying water and biota. Consolidation following the placement of the cap is estimated to occur within the period of about 10–50 days. The settlement of the cap surface will range from about 14 to 21 cm. The results of a laboratory capping test, as well as theoretical results, suggest that the selected-placement technique will not disturb very soft contaminated sediments. The effectiveness of the cap will be tested by physical, chemical, and biological monitoring to ensure that the cap is placed as intended and that it prevents contaminant migration to the aquatic environment. Key words : subaqueous capping, cohesive sediments, contaminant isolation, in situ, consolidation, placement technique.

Considerations for Capping Subaqueous Dredged Material Deposits

Capping is the controlled, accurate placement of contaminated dredged material at a disposal site which is covered by a cap of clean isolating material. Capping projects are typically categorized as either level-bottom placement or contained aquatic disposal. Field experience with subaqueous capping is limited, but 11 sites have been identified where the technique has been applied. Site selection considerations for capping projects are similar to those for any open-water disposal. The influences of several site characteristics on capping have been identified and discussed. Modeling methods are available to aid in evaluating sites and designs. Additional information on cap materials, placement, and monitoring techniques are provided.