Active capping demonstration in the Anacostia river,Washington, D.C.

An active capping demonstration project in Washington, D.C., is testing the ability to place sequestering agents on contaminated sediments using conventional equipment and evaluating their subsequent effectiveness relative to conventional passive sand sediment caps. Selected active capping materials include: (1) AquaBlokTM, a clay material for permeability control; (2) apatite, a phosphate mineral for metals control; (3) coke, an organic sequestration agent; and (4) sand material for a control cap. All of the materials, except coke, were placed in 8,000‐ft test plots by a conventional clamshell method during March and April 2004. Coke was placed as a 1.25‐cm layer in a laminated mat due to concerns related to settling of the material. Postcap sampling and analysis were conducted during the first, sixth, and eighteenth months after placement. Although postcap sampling is expected to continue for at least an additional 24 months, this article summarizes the results of the demonstration project and postcap sampling efforts up to 18 months. Conventional clamshell placement was found to be effective for placing relatively thin (six‐inch) layers of active material. The viability of placing high‐value or difficult‐to‐place material in a controlled manner was successfully demonstrated with the laminated mat. Postcap monitoring indicates that all cap materials effectively isolated contaminants, but it is not yet possible to differentiate between conventional sand and active cap layer performance. Monitoring of the permeability control layer indicated effective reductions in groundwater seepage rates through the cap, but also showed the potential for gas accumulation and irregular release. All of the cap materials show deposition of new contaminated sediment onto the surface of the caps, illustrating the importance of source control in maintaining sediment quality. © 2006 Wiley Periodicals, Inc.     

Active capping technology—New approaches for in situ remediation of contaminated sediments

This study evaluated pilot‐scale active caps composed of apatite, organoclay, biopolymers, and sand for the remediation of metal‐contaminated sediments. The active caps were constructed in Steel Creek, at the Savannah River Site near Aiken, South Carolina. Monitoring was conducted for 12 months. Effectiveness of the caps was based on an evaluation of contaminant bioavailability, resistance to erosion, and impacts on benthic organisms. Active caps lowered metal bioavailability in the sediment during the one‐year test period. Biopolymers reduced sediment suspension during cap construction, increased the pool of carbon, and lowered the release of metals. This field validation showed that active caps can effectively treat contaminants by changing their speciation, and that caps can be constructed to include more than one type of amendment to achieve multiple goals. © 2012 Wiley Periodicals, Inc.     

Investigation of NAPL transport through a model sand cap during ebullition

Nonaqueous‐phase liquid (NAPL) migration from sediments to the surface of water bodies has been reported frequently at sites with sediments contaminated with NAPLs, such as coal tar and creosote. Commonly, transport of NAPL from sediment is facilitated by gas ebullition caused by anaerobic biodegradation of organic matter in the sediment. A remedy often specified for these sites is a sand cap, and sand caps amended with sorbent materials (such as organoclays) are being pilot‐tested. This article discusses a laboratory study to assess the effectiveness of a sand layer for controlling NAPL migration. The study used a test column composed of a Plexiglas tube containing a tar source that was buried beneath a 30‐cm‐thick layer of fine sand. Water was added to the column until 5 cm of standing water covered the sand layer. To simulate ebullition, air was injected into the base of the sand column at approximately 200 mL/min. It was observed that the gas and NAPL migrated primarily through channels and fractures in the sand, and was not filtered through a network of stable pores. Tar migrated through the sand layer in 12 hours and accumulated on the water surface for several hours before losing its buoyancy and settling back down to the sand surface. After ending the tar migration experiment, the test column was frozen to preserve structures in the sand. The study showed that the tar migrated through the simulated sand cap in small (2‐mm) channels only a few sand grains thick. The results of this laboratory work call into question the effectiveness of sand caps for controlling NAPL migration from sediment in the presence of ebullition. © 2009 Wiley Periodicals, Inc.     

Evaluation of Active Cap Materials for Metal Retention in Sediments

This study evaluated chemically active amendments used to construct active caps for remediating contaminated sediments. Three experiments assessed the effects of apatite, organoclay, zeolite, and biopolymers (chitosan and xanthan) on metal mobility, retention, and speciation. The first showed that the amendments individually and in mixtures (2 percent dry weight) reduced the concentrations of Cr, Co, Ni, and Pb in water extracts from reduced sediment. The second experiment, which used sequential extraction procedures to evaluate the effects of the amendments on metal speciation, showed that the amendments reduced the potentially mobile fractions of Pb, Zn, Ni, Cr, and Cd that are likely to be bioavailable. Last, column studies showed that active caps composed of the amendments prevented the diffusive transport of metals from contaminated sediment over six months. In addition, there was a “zone of influence” beneath the caps in which water extractable concentrations of metals declined substantially compared with untreated sediment. © 2014 Wiley Periodicals, Inc.     

Sand Cap Placement and Cap Thickness Monitoring: A Case Study at a Confined Disposal Facility

Thin sediment capping is a commonly used technique to prevent mobilization of contaminants from sediments into the environment. A 70‐m‐deep subaqueous confined disposal facility (CDF, 350,000 m²) at Malmøykalven, Oslofjord, which received dredged contaminated sediments from Oslo Harbor, was capped with 148,900 m³ of sand in 2009. This research serves as a case study regarding some of the key considerations involved with the cap placement and monitoring of the cap layer. Uncertainty is included in all the cap thickness monitoring methods and a combined use of them provided a better understanding of the cap coverage and structure at the site. An open water disposal model (STFATE) was used to simulate the behavior of the barge‐released cap material. The modeling results were consistent with field observations regarding the material spread, and the results provided insight into the relatively high material losses calculated. Better knowledge obtained of material settling resulted in cap properties and cap monitoring methods that are useful when planning similar operations. ©2015 Wiley Periodicals, Inc.     

Experimental landfill caps for semi-arid and arid climates.

The United States EPA Subtitle D municipal solid waste landfill requirements specify that the permeability of a cap to a landfill be no greater than the permeability of the underliner. In recent years the concept of the evapotranspirative (ET) cap has been developed in which the cap is designed to store all rain infiltration and re-evapotranspire it during dry weather. Concern at the long period required for landfilled municipal solid waste to decompose and stabilize in arid and semi-arid climates has led to an extension of the concept of the ET cap. With the infiltrate-stabilize-evapotranspire (ISE) cap, rain infiltration during wet weather is permitted to enter the underlying waste, thus accelerating the decomposition and stabilization process. Excess infiltration is then removed from both waste and cap by evaporation during dry weather. The paper describes the construction and operation of two sets of experimental ISE caps, one in a winter rainfall semi-arid climate, and the other in a summer rainfall semi-arid climate. Observation of the rainfall, soil evaporation and amount of water stored in the caps has allowed water balances to be constructed for caps of various thicknesses. These observations show that the ISE concept is viable. In the limit, when there is insufficient rainfall to infiltrate the waste, an ISE cap operates as an ET cap.     

Pilot‐scale demonstration of in situ capping of PCB‐containing sediments in the lower Grasse River

A fish‐consumption advisory is currently in effect in a seven‐mile stretch of the Grasse River in Massena, New York, due to elevated levels of PCBs in fish tissue. One remedial approach that is being evaluated to reduce the PCB levels in fish from the river is in situ capping. An in‐river pilot study was conducted in the summer of 2001 to assess the feasibility of capping PCB‐containing sediments of the river. The study consisted of the construction of a subaqueous cap in a seven‐acre portion of the river using various combinations of capping materials and placement techniques. Optimal results were achieved with a 1:1 sand/topsoil mix released from a clamshell bucket either just above or several feet below the water surface. A longer‐term monitoring program of the capped area commenced in 2002. Results of this monitoring indicated: 1) the in‐place cap has remained intact since installation; 2) no evidence of PCB migration into and through the cap; 3) groundwater advection through the cap is not an important PCB transport mechanism; and 4) macroinvertebrate colonization of the in‐place cap is continuing. Additional follow‐up monitoring in the spring of 2003 indicated that a significant portion of the cap and, in some cases, the underlying sediments had been disturbed in the period following the conclusion of the 2002 monitoring work. An analysis of river conditions in the spring of 2003 indicated that a significant ice jam had formed in the river directly over the capping pilot study area, and that the resulting increase in river velocities and turbulence in the area resulted in the movement of both cap materials and the underlying sediments. The pilot cap was not designed to address ice jam–related forces on the cap, as the occurrence of ice jams in this section of the river had not been known prior to the observations conducted in the spring of 2003. These findings will preclude implementation of the longer‐term monitoring program that had been envisioned for the pilot study. The data collected immediately after cap construction in 2001 and through the first year of monitoring in 2002 serve as the basis for the conclusions presented in this article. It should be recognized that, based on the observation made in the spring of 2003, some of these conclusions are no longer valid for the pilot study area.The occurrence of ice jams in the lower Grasse River and their importance on sediments and PCBs within the system are currently under investigation. © 2003 Wiley Periodicals, Inc.     

Groundwater Impacts on Surface Water and Sediment—Gowanus Canal,Brooklyn, New York

The Gowanus Canal Superfund Site in Brooklyn, New York, is an approximately 1.5‐mile (1.61‐km) long estuary that was historically converted into a canal for industrial and commercial purposes. Three manufactured gas plants (MGPs) were formerly located on the Gowanus Canal and discharged waste into it. Surface sediments remain highly contaminated with polycyclic aromatic hydrocarbons (PAHs) long after the MGPs were razed. A hydrogeologic assessment indicates that groundwater passes through the deeper coal tar–contaminated sediment prior to discharging to the canal. This study was undertaken to investigate if groundwater passing through coal tar–contaminated sediment could be responsible for the ongoing contamination of both surface sediments and surface water in the canal. PAH compound distributions in surface water samples collected from the tidal canal at low tide were compared with PAH compounds found in adjacent groundwater‐monitoring wells, point sources (combined sewer overflows [CSOs]), and surface sediments. The results indicate a strong correlation between PAH contaminant distributions in groundwater, sediment, and surface water, indicating that contaminated groundwater passing through the deeper coal tar–contaminated sediments is the primary mechanism contributing to the contamination of both surface sediment and surface water in the canal. Therefore, any sediment remediation efforts in the Gowanus Canal that fail to evaluate and control the upward transport processes have a high chance of failure due to recontamination from below.  ©2016 Wiley Periodicals, Inc.     

Novel shoreline cap for controlling sheen and dissolved‐phase constituent discharge

A novel, multilayered shoreline cap was designed and installed to mitigate the release of petroleum light nonaqueous phase liquid (LNAPL) and dissolved‐phase groundwater constituents to the Willamette River in Portland, OR. Releases of LNAPL related to upland impacts caused occasional sheens on a portion of the river within the Portland Harbor Superfund Site. The frequency and volume of sheens decreased following the installation of an upland sheet pile barrier wall, but occasional sheens related to LNAPL impacts stranded downgradient of the wall continued–prompting the design of a shoreline remedy. Because the site is located within the Portland Harbor Superfund Site, the cap was designed to mitigate sheen and to meet the objectives specified in the Portland Harbor Record of Decision including limiting the discharge of certain dissolved‐phase constituents of interest. The cap design was the first instance of combining an oleophilic bio‐barrier to mitigate sheen and an activated carbon layer to capture dissolved‐phase constituents. No sheens have been visually observed since cap installation.     

Sequestration of metals in active cap materials: A laboratory and numerical evaluation

Active capping involves the use of capping materials that react with sediment contaminants to reduce their toxicity or bioavailability. Although several amendments have been proposed for use in active capping systems, little is known about their long‐term ability to sequester metals. Recent research has shown that the active amendment apatite has potential application for metals‐contaminated sediments. The focus of this study was to evaluate the effectiveness of apatite in the sequestration of metal contaminants through the use of short‐term laboratory column studies in conjunction with predictive, numerical modeling. A breakthrough column study was conducted using North Carolina apatite as the active amendment. Under saturated conditions, a spike solution containing elemental As, Cd, Co, Se, Pb, Zn, and a nonreactive tracer was injected into the column. A sand column was tested under similar conditions as a control. Effluent water samples were periodically collected from each column for chemical analysis. Relative to the nonreactive tracer, the breakthrough of each metal was substantially delayed by the apatite. Furthermore, breakthrough of each metal was substantially delayed by the apatite compared to the sand column. Finally, a simple 1‐D, numerical model was created to qualitatively predict the long‐term performance of apatite based on the findings from the column study. The results of the modeling showed that apatite could delay the breakthrough of some metals for hundreds of years under typical groundwater flow velocities. © 2012 Wiley Periodicals, Inc.     

Simulation of flow and mercury transport in Upper East Fork Poplar Creek,Oak Ridge,Tennessee

As a result of nuclear processing activities started back in the 1950s, the environment in the vicinity of the Y‐12 National Security Complex (Y‐12 NSC) in Oak Ridge, Tennessee, and surrounding watersheds has been contaminated by nearly 1,000 tons of elementary mercury. To comply with the state and federal surface water quality standards, a significant reduction in mercury concentration to parts‐per‐trillion levels has been proposed. In order to analyze the mercury cycle in the environment and provide forecasting capabilities for the flow and transport of mercury within the Upper East Fork Poplar Creek (UEFPC) watershed, an integrated surface and subsurface flow and transport model has been developed using the hydrodynamic and transport numerical package, MIKE, developed by the Danish Hydraulic Institute. The model has been constructed and calibrated using an extensive collection of historical records (i.e., hydrological data, and mercury concentration measurements in groundwater, soil, and sediment) obtained from the Oak Ridge Environmental Information System database. Daily fluctuations in stream flow, as a result of scattered rainfall, flooding, and flow augmentation, resuspend the contaminated streambed sediments and/or erode the polluted streambank soil and provide a secondary source of mercury to the creek. In order to investigate the significance of sediment‐mercury interactions on the fate and transport of mercury within the UEFPC study domain, simulations were performed for two different cases (i.e., with and without consideration of sediment‐mercury interactions). Computed total suspended solids and mercury concentrations at the integration point of the creek are compared with the corresponding historical records in both cases. As confirmed by the numerical simulations, a substantial portion of the mercury detected in the river is likely in the form of sediment particle–bound mercury (i.e., mercury particulates). © 2012 Wiley Periodicals, Inc.     

Hazardous chemical site remediation through capping: Problems with long-term protection

The capping of waste management units and contaminated soils is receiving increasing attention as a low-cost method for hazardous chemical site remediation. Capping is used to prevent further groundwater pollution by existing waste management units and contaminated soils through limiting the moisture that enters the wastes. In principle, for wastes located above the water table, the construction of an impermeable cap can prevent leaching of the wastes (leachate generation) and groundwater pollution. In practice, appropriately designed and constructed RCRA caps can provide for only short-term prevention of groundwater pollution. Alternative approaches are available for capping of wastes that can be effective in preventing moisture from entering the wastes and concomitant groundwater pollution. These approaches recognize the inability of the typical RCRA cap to keep wastes dry for as long as waste constituents will be a threat and, most importantly, provide the necessary funds to effectively address all plausible worst-case scenario failures that could occur at a capped waste management unit or contaminated soil area.     

In‐well sediment incubators to evaluate microbial community stability and dynamics following bioimmobilization of uranium

An in‐well sediment incubator (ISI) was developed to investigate the stability and dynamics of sediment‐associated microbial communities to prevailing subsurface oxidizing or reducing conditions. Herein we describe the use of these devices at the Old Rifle Uranium Mill Tailings Remedial Action (UMTRA) site. During a seven‐month period in which oxidized Rifle Aquifer background sediment (RABS) were deployed in previously biostimulated wells under iron‐reducing conditions, cell densities of known iron‐reducing bacteria, including Geobacteraceae, increased significantly, showing the microbial community response to local subsurface conditions. Phospholipid fatty acid (PLFA) profiles of RABS following in situ deployment were strikingly similar to those of adjacent sediment cores, suggesting ISI results could be extrapolated to the native material of the test plots. Results for ISI deployment with laboratory‐reduced sediments showed only slight changes in community composition and pointed toward the ability of the ISI to monitor microbial community stability and response to subsurface conditions. © 2009 Wiley Periodicals, Inc.     

Phytocapping: Importance of Tree Selection and Soil Thickness

An alternative landfill capping technique known as ‘Phytocapping’ (establishment of perennial plants on a layer of soil placed over the waste) was trailed in Rockhampton, Australia. In this technique, trees were used as ‘bio-pumps’ and ‘rainfall interceptors’ and soil cover as ‘storage’ of water. The environmental performance of the phytocapping system was measured based on its ability to minimise water percolation into the waste. The percolation rate was modelled using HYDRUS 1D for two different scenarios (with and without vegetation) for the thick and thin caps, respectively. Results from the modelling showed percolation rates of 16.7 mm year^?1 in thick cover and 23.8 mm year^?1 in thin cover, both of which are markedly lower than those expected from a clay cap. Results from monitoring and observations showed that 19 trees out of 21 tree species grew well in the harsh landfill environment. Top ten performing species have been identified and are recommended to be grown on phytocaps in the Central Queensland region.     

Assessment of factors affecting household solid waste generation and its composition in Gulberg Town,Lahore, Pakistan

This paper analyzes and compares the findings of the characterization study of collected solid waste from households of three different socioeconomic groups in Lahore, Pakistan, over the four seasons, i.e. Spring (March–April, 2008), Summer (May–June, 2008), Monsoon (August–September, 2008) and Winter (December 2008 and January 2009). The generation rate of waste was 0.96 kg/cap/day for high-income, 0.73 kg/cap/day for middle and 0.67 kg/cap/day for low-income group. The average of total household solid waste (HSW) generation is 0.79 kg/cap/day (including 0.75 kg/cap/day for spring, 0.77 kg/cap/day for summer, 0.86 kg/cap/day for monsoon and 0.76 kg/cap/day. The breakdown for the major physical components of the waste shows that organic waste accounts for the largest proportion (67.46 %). The relations between waste generation rates by physical category and subcategory, in addition to factors such as socioeconomic groups (population density levels, household income and household size), seasonal variation, and daily variation (difference of HSW generation among days of a week) were also analyzed. Statistical analysis shows that there was no significant difference in overall waste generation among days of a week. A significant difference between the seasons for food waste, cardboard, PET, HDPE, other hazardous waste, battery cells, and dust and stone (p < 0.001) was found. The generation rates were found to be higher when compared to other developing countries.     

Sustained treatment: Implications for treatment timescales associated with source‐depletion technologies

Sustained treatment is an emerging concept used to describe enhancements in attenuation capacity after the conclusion of the active treatment period for a given source‐depletion technology. The term includes mechanisms that lead to contaminant transformation or destruction over extended periods of time, such as endogenous biomass decay, slow diffusion of remedial amendments from low‐permeability zones, and the formation of reactive mineral species. This “value‐added” treatment continues after the end of capital expenditures at a site, and it provides additional insight in determining if monitored natural attenuation is a viable long‐term option for a site. This article identifies several sustained treatment mechanisms, examines technology‐specific factors that contribute to sustained treatment, and explores the potential timescales of sustained treatment relative to active treatment. As demonstrated in post‐treatment site data obtained during a comprehensive source‐depletion technology performance survey, enhanced bioremediation is the most promising in promoting sustained treatment, and this beneficial effect can extend for several years due to factors such as slow biomass decay. There is little evidence that other commonly used technologies (thermal treatment, in situ chemical oxidation, surfactant‐enhanced remediation, or cosolvent flushing) result in any significant sustained treatment. An exception would be a cosolvent flushing project where large quantities of biodegradable cosolvent are left in the subsurface at the end of the project, which could result in sustained long‐term dechlorination activity. In the case of in situ chemical oxidation, factors that contribute to a higher incidence of concentration rebound mask any potential sustained treatment effects. © 2011 Wiley Periodicals, Inc.     

Spatial Distribution of Geobacteraceae and Sulfate‐Reducing Bacteria During In Situ Bioremediation of Uranium‐Contaminated Groundwater

Analysis of the physiological status of subsurface microbial communities generally relies on the study of unattached microorganisms in the groundwater. These approaches have been employed in studies on bioremediation of uranium‐contaminated groundwater at a study site in Rifle, Colorado, in which Geobacter species typically account for over 90 percent of the microbial community in the groundwater during active uranium reduction. However, to develop efficient in situ bioremediation strategies it is necessary to know the status of sediment‐associated microorganisms as well. In order to evaluate the distribution of the natural community of Geobacter during bioremediation of uranium, subsurface sediments were packed into either passive flux meters (PFMs) or sediment columns deployed in groundwater monitoring wells prior to acetate injection during in situ biostimulation field trials. The trials were performed at the Department of Energy's (DOE's) Rifle Integrated Field Research Challenge site. Sediment samples were removed either during the peak of Fe(III) reduction or the peak of sulfate reduction over the course of two separate field experiments and preserved for microscopy. Direct cell counts using fluorescence in situ hybridization (FISH) probes targeting Geobacter species indicated that the majority of Geobacter cells were unattached during Fe(III) reduction, which typically tracks with elevated rates of uranium reduction. Similar measurements conducted during the sulfate‐reducing phase revealed the majority of Geobacter to be attached following exhaustion of more readily bioavailable forms of iron minerals. Laboratory sediment column studies confirmed observations made with sediment samples collected during field trials and indicated that during Fe(III) reduction, Geobacter species are primarily unattached (90 percent), whereas the majority of sulfate‐reducing bacteria and Geobacter species are attached to sediment surfaces when sulfate reduction is the predominant form of metabolism (75 percent and 77 percent, respectively). In addition, artificial sediment experiments showed that pure cultures of Geobacter uraniireducens, isolated from the Rifle site, were primarily unattached once Fe(III) became scarce. These results demonstrate that, although Geobacter species must directly contact Fe(III) oxides in order to reduce them, cells do not firmly attach to the sediments, which is likely an adaptive response to sparsely and heterogeneously dispersed Fe(III) minerals in the subsurface. © 2013 Wiley Periodicals, Inc.     

Environmental bioavailability of hydrophobic organochlorines in sediments—A review

A number of hydrophobic organochlorines, such as hexachlorobenzene and polychlorinated dibenzo‐p‐dioxins/dibenzofurans (PCDD/Fs), have been reported to be persistent and bioaccumulative; however, their availability to biota appear to be limited due to strong sorption to soil/sediment and sequestration with age. Studies to date have shown that the bioavailability of hydrophobic organic chemicals (HOCs) in sediments is highly variable, depending not only on a chemical's lipophicity (K_ow), but also molecular steric conformation and sediment characteristics. A subdomain of sediment organic carbon, so‐called black carbon (BC), which has much higher affinity to planar HOCs than amorphous organic carbon, has been found to be the predominant repository of many HOCs. The sediment/soil‐bound HOCs are composed of a rapid and reversible desorbing labile fraction and a slow‐desorbing, or resistant‐to‐desorbing, nonlabile fraction. The latter can account for up to 98 percent of the total. A number of chemical extraction methods have been under development to measure the actual bioavailable concentrations in soil/sediment and have shown some correlation to the results of bioaccumulation and/or biodegradation tests. To date, most of the published studies on this subject have focused on polynuclear aromatic hydrocarbons (PAHs). This review summarizes the governing processes and the testing methodologies relevant to the environmental bioavailability of hydrophobic organochlorines in soils and sediments. © 2004 Wiley Periodicals, Inc.