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==Downscaled High Resolution Datasets for Climate Change Projections==   
+
==Sediment Capping==   
Global climate models (GCMs) have generated projections of temperature, precipitation and other important climate change parameters with spatial resolutions of 100 to 300 km.  However, higher spatial resolution information is required to assess threats to individual installations or regionsA variety of “downscaling” approaches have been used to produce high spatial resolution output (datasets) from the global climate models at scales that are useful for evaluating potential threats to critical infrastructure at regional and local scales.  These datasets enable development of information about projections produced from various climate models, about downscaling to achieve desired locational specificity, and about selecting the appropriate dataset(s) to use for performing specific assessmentsThis article describes how these datasets can be accessed and used to evaluate potential climate change impacts.
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Capping is an ''in situ'' remedial technology that involves placement of a clean substrate on the surface of [[Contaminated Sediments - Introduction | contaminated sediments]] to reduce contaminant uptake by benthic organisms and contaminant flux to surface waterSimple sand caps can be effective in reducing exposure of benthic organisms and in limiting oxygen transport into the contaminated sediments, resulting in precipitation of metal sulfides. Amendments are sometimes included in caps to reduce cap permeability and groundwater upwelling, to increase contaminant sorption or biodegradation, or to improve habitat.   
 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
  
 
'''Related Article(s):'''
 
'''Related Article(s):'''
* [[Climate Change Primer]]
+
*[[Contaminated Sediments - Introduction]]
 +
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
 +
*Sediment Risk Assessment
 +
*[[Passive Sampling of Sediments]]
  
'''Contributor(s):''' [[Dr. Rao Kotamarthi]]
+
'''Contributor(s):'''  
 +
*Danny Reible
  
 
'''Key Resource(s):'''
 
'''Key Resource(s):'''
* Use of Climate Information for Decision-Making and Impacts Research: State of our Understanding<ref name="Kotamarthi2016">Kotamarthi, R., Mearns, L., Hayhoe, K., Castro, C.L., and Wuebble, D., 2016. Use of Climate Information for Decision-Making and Impacts Research: State of Our Understanding. Department of Defense, Strategic Environmental Research and Development Program (SERDP), 55pp. Free download from: [https://www.serdp-estcp.org/content/download/38568/364489/file/Use_of_Climate_Information_for_Decision-Making_Technical_Report.pdf SERDP-ESTCP]</ref>
+
*Processes, Assessment and Remediation of Contaminated Sediments<ref name="Reible2014">Reible, D. D., Editor, 2014. Processes, Assessment and Remediation of Contaminated Sediments. Springer, New York, NY. 462 pp. ISBN: 978-1-4614-6725-0</ref>
  
* Applying Climate Change Information to Hydrologic and Coastal Design of Transportation Infrastructure, Design Practices<ref name="Kilgore2019">Kilgore, R., Thomas, W.O. Jr., Douglass, S., Webb, B., Hayhoe, K., Stoner, A., Jacobs, J.M., Thompson, D.B., Herrmann, G.R., Douglas, E., and Anderson, C., 2019. Applying Climate Change Information to Hydrologic and Coastal Design of Transportation Infrastructure, Design Practices. The National Cooperative Highway Research Program, Transportation Research Board, Project 15-61, 154 pages. Free download from: [http://onlinepubs.trb.org/Onlinepubs/nchrp/docs/NCHRP1561_DesignProcedures.pdf The Transportation Research Board]</ref>
+
* Guidance for In-Situ Subaqueous Capping of Contaminated Sediments<ref name="Palermo1998">Palermo, M., Maynord, S., Miller, J. and Reible, D., 1998. Guidance for In-Situ Subaqueous Capping of Contaminated Sediments. Assessment and Remediation of Contaminated Sediments (ARCS) Program, Great Lakes National Program Office, US EPA 905-B96-004. 147 pp. [[Media: USEPA_905-B96-004.pdf | Report.pdf]]</ref>
  
* Statistical Downscaling and Bias Correction for Climate Research<ref name="Maraun2018">Maraun, D., and Wildmann, M., 2018. Statistical Downscaling and Bias Correction for Climate Research. Cambridge University Press, Cambridge, UK. 347 pages[https://doi.org/10.1017/9781107588783 DOI: 10.1017/9781107588783]&nbsp;&nbsp; ISBN: 978-1-107-06605-2</ref>
+
==Introduction==
 +
[[File:SedCapFig1.png|thumb|left|470px|Figure 1. Conceptual sketch of a cap configuration]]
 +
Capping is an ''in situ'' remedial technology for contaminated sediments that involves placement of a clean substrate on the sediment surface.  Capping contaminated sediments following [[Wikipedia: Dredging | dredging operations]] and capping of dredged material to stabilize contaminants has been a common practice by the United States Army Corps of Engineers since the 1970s.  Beginning in the 1980s, in Japan and subsequently elsewhere, capping has been used more widely as a remedial approach to improve the quality of the bottom substrate and reduce contaminant exposures to benthic organisms and fish.  The USEPA published a capping guidance document in 1998 that summarizes past uses of sediment capping and outlines its basic design<ref name="Palermo1998"/>.   Although capping technology has developed substantially in the past 20 years, this early reference still provides useful information on the approach and its applicationsA more recent summary of capping is described in Reible 2014<ref name="Reible2014"/>.
  
* Downscaling Techniques for High-Resolution Climate Projections: From Global Change to Local Impacts<ref name="Kotamarthi2021">Kotamarthi, R., Hayhoe, K., Wuebbles, D., Mearns, L.O., Jacobs, J. and Jurado, J., 2021. Downscaling Techniques for High-Resolution Climate Projections: From Global Change to Local Impacts. Cambridge University Press, Cambridge, UK. 202 pages.  [https://doi.org/10.1017/9781108601269 DOI: 10.1017/9781108601269]&nbsp;&nbsp; ISBN: 978-1-108-47375-0</ref>
+
Capping serves to contain contaminated sediment solids, isolate contaminants from benthic organisms and reduce contaminant transport to the sediment surface and overlying water. The clean substrate may be an inert material such as sand, a natural sorbing material such as other sediments or clays, or be amended with an active/reactive material to enhance the isolation of the contaminants. Amendments to enhance contaminant isolation include permeability reduction agents to divert groundwater flow, sorbents to retard contaminant migration through the capping layer or provide greater accumulation capacity, or reagents to encourage degradation or transformation of the contaminants.  
  
==Downscaling of Global Climate Models==
+
The basic concept of a cap is illustrated in Figure 1.  The Figure also illustrates that a cap is often a thin layer or layers relative to water depth and generally causes little disturbance to the underlying sediments or body of water in which it is placed.   Depending upon the erosive forces to which the cap may be subjected, the surface layer may be composed of relatively coarse material to withstand those erosive forces.  
Some communities and businesses have begun to improve their resilience to climate change by building adaptation plans based on national scale climate datatsets ([https://unfccc.int/topics/adaptation-and-resilience/workstreams/national-adaptation-plans National Adaptation Plans]), regional datasets ([https://www.dec.ny.gov/docs/administration_pdf/crrafloodriskmgmtgdnc.pdf New York State Flood Risk Management Guidance]<ref name="NYDEC2020">New York State Department of Environmental Conservation, 2020. New York State Flood Risk Management Guidance for Implementation of the Community Risk and Resiliency Act. Free download from: [https://www.dec.ny.gov/docs/administration_pdf/crrafloodriskmgmtgdnc.pdf New York State]&nbsp;&nbsp; [[Media: NewYorkState2020.pdf | Report.pdf]]</ref>), and datasets generated at local spatial resolutions.  Resilience to the changing climate has also been identified by the US Department of Defense (DoD) as a necessary part of the installation planning and basing process ([https://media.defense.gov/2019/Jan/29/2002084200/-1/-1/1/CLIMATE-CHANGE-REPORT-2019.PDF DoD Report on Effects of a Changing Climate]<ref name="DoD2019">US Department of Defense, 2019. Report on Effects of a Changing Climate to the Department of Defense. Free download from: [https://media.defense.gov/2019/Jan/29/2002084200/-1/-1/1/CLIMATE-CHANGE-REPORT-2019.PDF DoD]&nbsp;&nbsp; [[Media: DoD2019.PDF | Report.pdf]]</ref>). More than 79 installations were identified as facing potential threats from climate change. The threats faced due to changing climate include recurrent flooding, droughts, desertification, wildfires and thawing permafrost.  
 
  
Assessing the threats climate change poses at regional and local scales requires data with higher spatial resolution than is currently available from global climate models. Global-scale climate models typically have spatial resolutions of 100 to 300 km, and output from these models needs to be spatially and/or temporally disaggregated in order to be useful in performing assessments at smaller scales. The process of producing higher spatial-temporal resolution climate model output from coarser global climate model outputs is referred to as “downscaling” and results in climate change projections (datasets) at scales that are useful for evaluating potential threats to regional and local communities and businesses.  These datasets provide information on temperature, precipitation and a variety of other climate variables for current and future climate conditions under various greenhouse gas (GHG) emission scenarios. There are a variety of web-based tools available for accessing these datasets to evaluate potential climate change impacts at regional and local scales.
+
Although a cap is typically thin compared to the water depth, it generally must be thicker than the biologically active zone (BAZ) of the sediments. The biologically active zone is that zone in which benthic organisms live and interact with the sediment.  Their activities tend to mix the BAZ (known as [[Wikipedia: Bioturbation | bioturbation]]) over the course of a few years and thus a cap that is thinner than the BAZ will tend to become intermixed with the underlying contaminated sediments.  Processes other than bioturbation including diffusion, advection or groundwater upwelling, hyporheic exchange near the interface, biogenic gas production and migration and underlying sediment consolidation can all lead to contaminant migration into and through a cap.  These occur at different rates and intensities and their assessment and evaluation ultimately governs the effectiveness of a cap and the feasibility of its use as a sediment remediation technology for a particular site.
  
[[File: Kotamarthi2w2Fig1.jpg | thumb |left| 450px | Figure 1Typical processes and spatial scales of Regional scale Climate Models. The models may calculate circulation in the atmosphere, cloud processes, precipitation, and land-atmospheric and ocean-atmospheric processes on a limited portion of the Earth, with boundary conditions provided by a Global Climate Model.<ref name="Kotamarthi2016"/>]]
+
In general, capping is an effective remedial technology for contaminants that are strongly associated with the sediment solids including hydrophobic organic compounds such as high molecular weight [[Polycyclic Aromatic Hydrocarbons (PAHs) | polycyclic aromatic hydrocarbons (PAHs)]], [[Wikipedia: Polychlorinated biphenyl | polychlorinated biphenyls (PCBs)]], [[Wikipedia: Dioxins and dioxin-like compounds | dioxins]] and [[Wikipedia: DDT | DDTx]], but also [[Metal and Metalloid Contaminants | heavy metals]]Hydrophobic organic compounds tend to strongly associate with the organic fraction of sediments so organic rich sediments or the addition of organic phases to the capping material can be very effective at containing these contaminants. Many of the common heavy metals of concern, including cadmium, copper, nickel, zinc, lead and mercury, tend to be associated with insoluble sulfides under strongly reducing conditions.  Since oxygen penetration into a capping layer is typically limited to a few cm or less at the surface, a cap serves to drive the underlying contaminated sediment toward strongly reducing conditions and, particularly in marine and estuarine sediments, encourage sulfate reduction leading to the formation of these insoluble sulfides.  The low solubility of these sulfides encourages retention by a capping layer and makes the cap extremely effective as a remedial approach for sediments with elevated concentrations of heavy metals.
==Methods for Downscaling==
 
  
{| class="wikitable" style="float:right; margin-left:10px;text-align:center;"
+
A variety of tools have been developed to evaluate the processes leading to sorption and retardation of contaminants as well as processes leading to contaminant migration and release. The original references quantifying contaminant behavior in a sediment cap were explored in a series of papers in the early 1990s<ref name="Wang1991">Wang, X.Q., Thibodeaux, L.J., Valsaraj, K.T. and Reible, D.D., 1991. Efficiency of Capping Contaminated Bed Sediments in Situ. 1. Laboratory-Scale Experiments on Diffusion-Adsorption in the Capping Layer. Environmental Science and Technology, 25(9), pp.1578-1584.  [https://doi.org/10.1021/es00021a008 DOI: 10.1021/es00021a008]</ref><ref name="Thoma1993">Thoma, G.J., Reible, D.D., Valsaraj, K.T. and Thibodeaux, L.J., 1993. Efficiency of Capping Contaminated Bed Sediments in Situ 2. Mathematics of Diffusion-Adsorption in the Capping Layer. Environmental Science and Technology, 27(12), pp.2412-2419. [https://doi.org/10.1021/es00048a015 DOI: 10.1021/es00048a015]</ref>. Since that time, design tools have been continuously improved. [https://www.depts.ttu.edu/ceweb/research/reiblesgroup/downloads.php CapSim] is a commonly used and current tool developed by Dr. Reible and collaborators. This tool can evaluate contaminant release from uncapped, capped, and treated sediments for purposes of design and evaluation. The model formulation and structure is described in Shen et al. 2018<ref name="Shin2018">Shen, X., Lampert, D., Ogle, S. and Reible, D., 2018. A software tool for simulating contaminant transport and remedial effectiveness in sediment environments. Environmental Modelling and Software, 109, pp. 104-113.  [https://doi.org/10.1016/j.envsoft.2018.08.014 DOI: 10.1016/j.envsoft.2018.08.014]</ref>. One common use of such a tool is to evaluate the effect of various cap materials and thicknesses on the performance of a cap.
|+Table 1. Physical and chemical properties of TCP
 
|-
 
!Property
 
!Value
 
|-
 
| Chemical Abstracts Service (CAS) Number || 96-18-4
 
|-
 
| Physical Description</br>(at room temperature) || Colorless to straw-colored liquid
 
|-
 
| Molecular weight</br>(g/mol) || 147.43
 
|-
 
| Water solubility at 25°C</br>(mg/L)|| 1,750 (slightly soluble)
 
|-
 
| Melting point</br>(°C)|| -14.7
 
|-
 
| Boiling point</br>(°C) || 156.8
 
|-
 
| Vapor pressure at 25°C</br>(mm Hg) || 3.10 to 3.69
 
|-
 
| Density at 20°C (g/cm<sup>3</sup>) || 1.3889
 
|-
 
| Octanol-water partition coefficient</br>(log''K<sub>ow</sub>'') || 1.98 to 2.27</br>(temperature dependent)
 
|-
 
| Organic carbon-water partition coefficient</br>(log''K<sub>oc</sub>'') || 1.70 to 1.99</br>(temperature dependent)
 
|-
 
| Henry’s Law constant at 25°C</br>(atm-m<sup>3</sup>/mol) || 3.17x10<sup>-4</sup> to 3.43x10<sup>-4</sup>
 
|}
 
  
{| class="wikitable" style="float:right; margin-left:10px;text-align:center;"
+
==Cap Design and Materials for Chemical Containment==
|+Table 2. Advantages and limitations of TCP treatment technologies
+
An inert material such as sand can be effective as a capping material where contaminants are strongly associated with solids and where the operative site specific transport mechanisms do not lead to rapid contaminant migration through such a material. Additional contaminant containment can often be achieved through the placement of clean sediment, e.g. dredged material from a nearby location.  Other materials as cap layers or amendments may be useful to address particularly mobile contaminants or when particular degradative mechanisms can be exploited. The Anacostia River was the site of a demonstration that first tested “active” or “amended” capping in the field<ref name="Reible2003">Reible, D., Constant, D.W., Roberts, K. and Zhu, Y., 2003. Active capping demonstration project in anacostia DC. In Second International Conference on the Remediation of Contaminated Sediments: October.  Free download available from: [https://www.researchgate.net/profile/Danny-Reible/publication/237747790_ACTIVE_CAPPING_DEMONSTRATION_PROJECT_IN_ANACOSTIA_DC/links/0c96053861030b7699000000/ACTIVE-CAPPING-DEMONSTRATION-PROJECT-IN-ANACOSTIA-DC.pdf ResearchGate]</ref><ref name="Reible2006">Reible, D., Lampert, D., Constant, D., Mutch Jr, R.D. and Zhu, Y., 2006. Active Capping Demonstration in the Anacostia River, Washington, DC. Remediation Journal: The Journal of Environmental Cleanup Costs, Technologies and Techniques, 17(1), pp. 39-53.  [https://doi.org/10.1002/rem.20111 DOI: 10.1002/rem.20111]  Free download available from: [https://www.academia.edu/download/44146457/Remediation_Journal_Paper_2006.pdf Academia.edu]</ref>. Amended caps are often the best option when groundwater upwelling or other advective processes promote significant mobility of contaminants and the addition of sorbents can slow that contaminant migration<ref name="Ghosh2011">Ghosh, U., Luthy, R.G., Cornelissen, G., Werner, D. and Menzie, C.A., 2011. In-situ Sorbent Amendments: A New Direction in Contaminated Sediment Management. Environmental Science and Technology, 45(4), pp. 1163-1168. [https://doi.org/10.1021/es102694h DOI: 10.1021/es102694h]  Open access article from: [https://pubs.acs.org/doi/pdf/10.1021/es102694h American Chemical Society]&nbsp;&nbsp; [[Media: Ghosh2011.pdf | Report.pdf]]</ref>.  Although a variety of materials have been proposed for sediment caps, a far smaller number of options have been successfully employed in the field.
|-
+
! Technology
+
Metals migration is very site dependent due to the potential for many metals to complex with other species in the interstitial water and the specific metal speciation present at a site.  Often, the strongly reducing environment beneath a cap renders many common metals unavailable through the formation of metal sulfides.  In such cases, a simple sand cap can be very effective.  Amended caps to manage metal contaminated sediments may be advantageous when site specific conditions lead to elevated metals mobility, but should be supported with site specific testing<ref name="Viana2008">Viana, P.Z., Yin, K. and Rockne, K.J., 2008. Modeling Active Capping Efficacy. 1. Metal and Organometal Contaminated Sediment Remediation. Environmental Science and Technology, 42(23), pp. 8922-8929. [https://doi.org/10.1021/es800942t DOI: 10.1021/es800942t]</ref>.
! Advantages
 
! Limitations
 
|-
 
| ZVZ
 
| style="text-align:left;" |
 
* Can degrade TCP at relatively high and low concentrations
 
* Faster reaction rates than ZVI
 
* Material is commercially available
 
| style="text-align:left;" |
 
* Higher cost than ZVI
 
* Difficult to distribute in subsurface ''in situ'' applications
 
|-
 
| Groundwater</br>Extraction and</br>Treatment
 
| style="text-align:left;" |
 
* Can cost-effectively capture and treat larger, more dilute</br>groundwater plumes than ''in situ'' technologies
 
* Well understood and widely applied technology
 
| style="text-align:left;" |
 
* Requires construction, operation and maintenance of</br>aboveground treatment infrastructure
 
* Typical technologies (e.g. GAC) may be expensive due</br>to treatment inefficiencies
 
|-
 
| ZVI
 
| style="text-align:left;" |
 
* Can degrade TCP at relatively high and low concentrations
 
* Lower cost than ZVZ
 
* Material is commercially available
 
| style="text-align:left;" |
 
* Lower reactivity than ZVZ, therefore may require higher</br>ZVI volumes or thicker PRBs
 
* Difficult to distribute in subsurface ''in situ'' applications
 
|-
 
| ISCO
 
| style="text-align:left;" |
 
* Can degrade TCP at relatively high and low concentrations
 
* Strategies to distribute amendments ''in situ'' are well established
 
* Material is commercially available
 
| style="text-align:left;" |
 
* Most effective oxidants (e.g., base-activated or heat-activated</br>persulfate) are complex to implement
 
* Secondary water quality impacts (e.g., high pH, sulfate, </br>hexavalent chromium) may limit ability to implement
 
|-
 
| ''In Situ''</br>Bioremediation
 
| style="text-align:left;" |
 
* Can degrade TCP at moderate to high concentrations
 
* Strategies to distribute amendments ''in situ'' are well established
 
* Materials are commercially available and inexpensive
 
| style="text-align:left;" |
 
* Slower reaction rates than ZVZ or ISCO
 
|}
 
  
 +
For hydrophobic organic contaminants, cap amendments that directly control groundwater upwelling and also sorbents that can remove migrating contaminants from that groundwater have been successfully employed.  Examples include clay materials such as AquaBlok<sup>&reg;</sup> for permeability control, sorbents such as [[Wikipedia: Activated carbon | activated carbon]] for truly dissolved contaminants, and [[Wikipedia: Organoclay | organophilic clays]] for separate phase contaminants. 
  
 +
The placement of clean sediment as an ''in situ'' cap can be difficult when the material is fine grained or has a low density.  Capping with a layer of coarse grained material such as clean sand mitigates this issue although clean sands have minimal sorption capacity.  Because of this limitation, sand caps may not be sufficient for achieving remedial goals in sites where contamination levels are high or transport rates are fast due to pore water upwelling or tidal pumping effects. Conditions such as these may require the use of “active” amendments to reduce transport rates.
 +
   
 +
Capping with clean sand provides a physical barrier between the underlying contaminated material and the overlying water, stabilizes the underlying sediment to prevent re-suspension of contaminated particles, and can reduce chemical exposure under certain conditions.  Sand primarily provides a passive barrier to the downward penetration of bioturbating organisms and the upward movement of sediment or contaminants.  Although conventional sandy caps can often be an effective means of managing contaminated sediments, there are conditions when sand caps may not be capable of achieving design objectives.  Some factors that reduce the effectiveness of sand caps include:
  
There&nbsp;are&nbsp;two&nbsp;main&nbsp;approaches to downscaling. One method, commonly referred to as “statistical downscaling”, uses the empirical-statistical relationships between large-scale weather phenomena and historical local weather data. In this method, these statistical relationships are applied to output generated by global climate models. A second method uses physics-based numerical models (regional-scale climate models or RCMs) of weather and climate that operate over a limited region of the earth (e.g., North America) and at spatial resolutions that are typically 3 to 10 times finer than the global-scale climate models. This method is known as “dynamical downscaling”.  These regional-scale climate models are similar to the global models with respect to their reliance on the principles of physics, but because they operate over only part of the earth, they require information about what is coming in from the rest of the earth as well as what is going out of the limited region of the model. This is generally obtained from a global model.  The primary differences between statistical and dynamical downscaling methods are summarized in Table 1.
+
*erosion and loss of cap integrity
 +
*high groundwater upwelling rates
 +
*mobile (low sorption) contaminants of concern (COCs)
 +
*high COC concentrations
 +
*unusually toxic COCs
 +
*the presence of tidal influences
 +
*the presence of non-aqueous phase liquids (NAPLs)
 +
*high rates of gas ebullition
  
It&nbsp;is&nbsp;important&nbsp;to&nbsp;realize that there is no “best” downscaling method or dataset, and that the best method/dataset for a given problem depends on that problem’s specific needs. Several data products based on downscaling higher level spatial data are available ([https://cida.usgs.gov/gdp/ USGS], [http://maca.northwestknowledge.net/ MACA], [https://www.narccap.ucar.edu/ NARCCAP], [https://na-cordex.org/ CORDEX-NA]). The appropriate method and dataset to use depends on the intended application. The method selected should be able to credibly resolve spatial and temporal scales relevant for the application. For example, to develop a risk analysis of frequent flooding, the data product chosen should include precipitation at greater than a diurnal frequency and over multi-decadal timescales. This kind of product is most likely to be available using the dynamical downscaling method.  SERDP reviewed the various advantages and disadvantages of using each type of downscaling method and downscaling dataset, and developed a recommended process that is publicly available<ref name="Kotamarthi2016"/>. In general, the following recommendations should be considered in order to pick the right downscaled dataset for a given analysis:
+
Of these, the first three are common limitations to capping and often control the ability to effectively design and implement a cap as a sediment remedial strategy. In these cases, it may be possible to offset these issues by increasing the thickness of the cap. However, the required thickness can reach infeasible levels in shallow streams or navigable water bodies. In addition, increased construction costs associated with thick caps may become prohibitive.  As a result of these issues, caps that use alternative materials (also known as active caps) to reduce the thickness or increase the protectiveness of a cap may be necessary. The materials in active caps are designed to interact with the COCs to enhance the containment properties of the cap. 
  
* When a problem depends on using a large number of climate models and emission scenarios to perform preliminary assessments and to understand the uncertainty range of projections, then using a statistical downscaled dataset is recommended.   
+
[[Wikipedia: Apatite | Apatites]] are a class of naturally occurring minerals that have been investigated as a sorbent for metals in soils and sediments<ref name="Melton2003">Melton, J.S., Crannell, B.S., Eighmy, T.T., Wilson, C. and Reible, D.D., 2003. Field Trial of the UNH Phosphate-Based Reactive Barrier Capping System for the Anacostia River. EPA Grant R819165-01-0</ref><ref name="Reible2003"/><ref name="Knox2012">Knox, A.S., Paller, M.H. and Roberts, J., 2012. Active Capping Technology—New Approaches for In Situ Remediation of Contaminated Sediments. Remediation Journal, 22(2), pp.93-117.  [https://doi.org/10.1002/rem.21313 DOI: 10.1002/rem.21313]  Free download available from: [https://www.researchgate.net/profile/Anna-Knox-2/publication/233374607_Active_Capping_Technology-New_Approaches_for_In_Situ_Remediation_of_Contaminated_Sediments/links/5a7de4c5aca272a73765c344/Active-Capping-Technology-New-Approaches-for-In-Situ-Remediation-of-Contaminated-Sediments.pdf ResearchGate]</ref>.  Apatites consist of a matrix of calcium phosphate and various other common anions, including fluoride, chloride, hydroxide, and occasionally carbonate. Metals are sequestered either through direct ion exchange with the calcium atom or dissolution of hydroxyapatite followed by precipitation of lead apatite.  [[Wikipedia: Zeolite | Zeolites]], which are microporous aluminosilicate minerals with a high cationic exchange capacity (CEC), have also been proposed to manage metal species<ref name="Zhan2019">Zhan, Y., Yu, Y., Lin, J., Wu, X., Wang, Y. and Zhao, Y., 2019. Simultaneous control of nitrogen and phosphorus release from sediments using iron-modified zeolite as capping and amendment materials. Journal of Environmental Management, 249, p.109369[https://doi.org/10.1016/j.jenvman.2019.109369 DOI: 10.1016/j.jenvman.2019.109369]</ref>.
* When the assessment needs a more extensive parameter list or is analyzing a region with few long-term observational data, dynamically downscaled climate change projections are recommended.
+
 
 +
It is possible to create a hydrophobic, sorbing layer for non-polar organics by exchanging a cationic surfactant onto the surface of clays such as zeolites and bentonites,. Organoclay is a modified bentonite containing such substitutions that has been evaluated for control of non-aqueous phase NAPLs and other organic contaminants<ref name="Reible2007">Reible, D.D., Lu, X., Moretti, L., Galjour, J. and Ma, X., 2007. Organoclays for the capping of contaminated sediments. AIChE Annual Meeting.  ISBN: 978-081691022-9</ref>.  An organoclay cap has been implemented for sediment remediation at the McCormick and Baxter site in Portland, OR<ref name="Parrett2005">Parrett, K. and Blishke, H., 2005. 23-Acre Multilayer Sediment Cap in Dynamic Riverine Environment Using Organoclay an Adsorptive Capping Material. Presentation to Society of Environmental Toxicology and Chemistry (SETAC), 26th Annual Meeting.</ref>.  A similar organic sorbing phase can be formed by treating zeolites with surfactants but this approach has not been reported for contaminated sediments.  
  
==Uncertainty in Projections==
+
Activated carbon is a strong sorbent of hydrophobic organic compounds and has been used as a [[In Situ Treatment of Contaminated Sediments with Activated Carbon | treatment for sediments]] or as an active sorbent within a capping layer<ref name="Zimmerman2004">Zimmerman, J.R., Ghosh, U., Millward, R.N., Bridges, T.S. and Luthy, R.G., 2004. Addition of Carbon Sorbents to Reduce PCB and PAH Bioavailability in Marine Sediments: Physicochemical Tests. Environmental Science and Technology, 38(20), pp. 5458-5464.  [https://doi.org/10.1021/es034992v DOI: 10.1021/es034992v]</ref><ref name="Werner2005">Werner, D., Higgins, C.P. and Luthy, R.G., 2005. The sequestration of PCBs in Lake Hartwell sediment with activated carbon. Water Research, 39(10), pp. 2105-2113. [https://doi.org/10.1016/j.watres.2005.03.019 DOI: 10.1016/j.watres.2005.03.019]</ref><ref name="Abel2018">Abel, S. and Akkanen, J., 2018. A Combined Field and Laboratory Study on Activated Carbon-Based Thin Layer Capping in a PCB-Contaminated Boreal Lake. Environmental Science and Technology, 52(8), pp. 4702-4710. [https://doi.org/10.1021/acs.est.7b05114 DOI: 10.1021/acs.est.7b05114] Open access article available from: [https://pubs.acs.org/doi/pdf/10.1021/acs.est.7b05114 American Chemical Society]&nbsp;&nbsp; [[Media: Abel2018.pdf | Report.pdf]]</ref><ref name="Payne 2018">Payne, R.B., Ghosh, U., May, H.D., Marshall, C.W. and Sowers, K.R., 2019. A Pilot-Scale Field Study: In Situ Treatment of PCB-Impacted Sediments with Bioamended Activated Carbon. Environmental Science and Technology, 53(5), pp. 2626-2634. [https://doi.org/10.1021/acs.est.8b05019 DOI: 10.1021/acs.est.8b05019]</ref><ref name="Yan2020">Yan, S., Rakowska, M., Shen, X., Himmer, T., Irvine, C., Zajac-Fay, R., Eby, J., Janda, D., Ohannessian, S. and Reible, D.D., 2020. Bioavailability Assessment in Activated Carbon Treated Coastal Sediment with In situ and Ex situ Porewater Measurements. Water Research, 185, p. 116259.  [https://doi.org/10.1016/j.watres.2020.116259 DOI: 10.1016/j.watres.2020.116259]</ref>. Placement of activated carbon for sediment capping is difficult due to the near neutral buoyancy of the material but it has been applied in this manner in relatively low energy environments such as Onondaga Lake, Syracuse, NY<ref name="Vlassopoulos2017">Vlassopoulos, D., Russell, K., Larosa, P., Brown, R., Mohan, R., Glaza, E., Drachenberg, T., Reible, D., Hague, W., McAuliffe, J. and Miller, S., 2017. Evaluation, Design, and Construction of Amended Reactive Caps to Restore Onondaga Lake, Syracuse, New York, USA. Journal of Marine Environmental Engineering, 10(1), pp. 13-27.  Free download available from: [https://www.researchgate.net/publication/317762995_Evaluation_design_and_construction_of_amended_reactive_caps_to_restore_Onondaga_lake_Syracuse_New_York_USA ResearchGate]</ref>. Alternatives in higher energy environments include placement of activated carbon in a mat such as the CETCO Reactive Core Mat (RCM)<sup>&reg;</sup> or Huesker Tektoseal<sup>&reg;</sup>, or as a composite material such as SediMite<sup>&reg;</sup> or AquaGate<sup>&reg;</sup>.  In the case of the mats, powdered or granular activated carbon can be placed in a controlled layer while the density of the composite materials is such that they can be broadcast from the surface and allowed to settle to the bottom.  In a sediment treatment application, the composite material would either be worked into the surface or allowed to intermix gradually by bioturbation and other processes.  In a capping application, the mat or composite material would typically be combined or overlain with a sand or other capping layer to keep it in place and to provide a chemical isolation layer away from the sediment surface.  
{| class="wikitable" style="float:right; margin-left:10px;text-align:center;"
 
|+Table 2.  Downscaling model characteristics and output<ref name="Kotamarthi2016"/>
 
|-
 
!Model or</br>Dataset Name
 
!Model<br />Method
 
!Output<br />Variables
 
!Output<br />Format
 
!Spatial</br>Resolution
 
!Time</br>Resolution
 
|-
 
| colspan="6" style="text-align: left; background-color:white;" |'''Statistical Downscaled Datasets'''
 
|-
 
| [https://worldclim.org/data/index.html WorldClim]<ref name="Hijmans2005">Hijmans, R.J., Cameron, S.E., Parra, J.L., Jones, P.G. and Jarvis, A., 2005. Very High Resolution Interpolated Climate Surfaces for Global Land Areas. International Journal of Climatology: A Journal of the Royal Meteorological Society, 25(15), pp 1965-1978.  [https://doi.org/10.1002/joc.1276 DOI: 10.1002/joc.1276]</ref>
 
|Delta||T(min, max,</br>avg), Pr||NetCDF||grid: 30 arc sec to</br>10 arc min||month
 
|-
 
| Bias Corrected / Spatial</br>Disaggregation (BCSD)<ref name="Wood2002">Wood, A.W., Maurer, E.P., Kumar, A. and Lettenmaier, D.P., 2002. Long‐range experimental hydrologic forecasting for the eastern United States. Journal of Geophysical Research: Atmospheres, 107(D20), 4429, pp. ACL6 1-15. [https://doi.org/10.1029/2001JD000659 DOI:10.1029/2001JD000659]&nbsp;&nbsp; Free access article available from: [https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2001JD000659 American Geophysical Union]&nbsp;&nbsp; [[Media: Wood2002.pdf | Report.pdf ]]</ref>
 
|Empirical Quantile</br>Mapping||Runoff,</br>Streamflow||NetCDF||grid: 7.5 arc min||day
 
|-
 
| [https://cida.usgs.gov/thredds/catalog.html?dataset=dcp Asynchronous Regional Regression</br>Model (ARRM v.1)]<ref name="Stoner2013">Stoner, A.M., Hayhoe, K., Yang, X., and Wuebbles, D.J., 2013. An Asynchronous Regional Regression Model for Statistical Downscaling of Daily Climate Variables. International Journal of Climatology, 33(11), pp. 2473-2494. [https://doi.org/10.1002/joc.3603 DOI:10.1002/joc.3603]</ref>
 
|Parameterized</br>Quantile Mapping||T(min, max), Pr||NetCDF||stations plus</br>grid: 7.5 arc min||day
 
|-
 
| [https://sdsm.org.uk/ Statistical Downscaling Model (SDSM)]<ref name="Wilby2013">Wilby, R.L., and Dawson, C.W., 2013. The Statistical DownScaling Model: insights from one decade of application. International Journal of Climatology, 33(7), pp. 1707-1719. [https://doi.org/10.1002/joc.3544 DOI: 10.1002/joc.3544]</ref>
 
|Weather Generator||T(min, max), Pr||PC Code||stations||day
 
|-
 
| [https://climate.northwestknowledge.net/MACA/ Multivariate Adaptive</br>Constructed Analogs (MACA)]<ref name="Hidalgo2008">Hidalgo, H.G., Dettinger, M.D. and Cayan, D.R., 2008. Downscaling with Constructed Analogues: Daily Precipitation and Temperature Fields Over the United States. California Energy Commission PIER Final Project, Report CEC-500-2007-123. [[Media: Hidalgo2008.PDF | Report.pdf]]</ref>
 
|Constructed Analogues||10 Variables||NetCDF||grid: 2.5 arc min||day
 
|-
 
| [http://loca.ucsd.edu/ Localized Constructed Analogs (LOCA)]<ref name="Pierce2013">Pierce, D.W., Cayan, D.R. and Thrasher, B.L., 2014. Statistical Downscaling Using Localized Constructed Analogs (LOCA). Journal of Hydrometeorology, 15(6), pp. 2558-2585. [https://doi.org/10.1175/JHM-D-14-0082.1 DOI: 10.1175/JHM-D-14-0082.1]&nbsp;&nbsp; Free access article available from: [https://journals.ametsoc.org/view/journals/hydr/15/6/jhm-d-14-0082_1.xml American Meteorological Society].&nbsp;&nbsp; [[Media: Pierce2014.pdf | Report.pdf]]</ref>
 
|Constructed Analogues||T(min, max), Pr||NetCDF||grid: 3.75 arc min||day
 
|-
 
| [https://www.nccs.nasa.gov/services/data-collections/land-based-products/nex-dcp30 NASA Earth Exchange Downscaled</br>Climate Projections (NEX-DCP30)]<ref name="Wood2002"/>
 
|Bias Correction /</br>Spatial Disaggregation||T(min, max), Pr||NetCDF||grid: 30 arc sec||month
 
|-
 
| colspan="6" style="text-align: left; background-color:white;" |'''Dynamical Downscaled Datasets'''
 
|-
 
| [http://www.narccap.ucar.edu/index.html North American Regional Climate</br>Change Assessment Program (NARCCAP)]<ref name="Mearns2009">Mearns, L.O., Gutowski, W., Jones, R., Leung, R., McGinnis, S., Nunes, A. and Qian, Y., 2009. A Regional Climate Change Assessment Program for North America. Eos, Transactions, American Geophysical Union, 90(36), p.311.  [https://doi.org/10.1029/2009EO360002 DOI: 10.1029/2009EO360002]&nbsp;&nbsp; Free access article from: [https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2009EO360002 American Geophysical Union]&nbsp;&nbsp; [[Media: Mearns2009.pdf  | Report.pdf]]</ref>
 
|Multiple Models||49 Variables||NetCDF||grid: 30 arc min||3 hours
 
|-
 
| [https://cordex.org/about/ Coordinated Regional Climate</br>Downscaling Experiment (CORDEX)]<ref name="Giorgi2009">Giorgi, F., Jones, C., and Asrar, G.R., 2009. Addressing climate information needs at the regional level: the CORDEX framework. World Meteorological Organization (WMO) Bulletin, 58(3), pp. 175-183. Free access article from: [https://public.wmo.int/en/bulletin/addressing-climate-information-needs-regional-level-cordex-framework World Meteorological Organization]&nbsp;&nbsp; [[Media: Giorgi2009.pdf | Report.pdf]]</ref>
 
|Multiple Models||66 Variables||NetCDF||grid: 30 arc min||3 hours
 
|-
 
| [https://esrl.noaa.gov/gsd/wrfportal/ Strategic Environmental Research and</br>Development Program (SERDP)]<ref name="Wang2015">Wang, J., and Kotamarthi, V.R., 2015. High‐resolution dynamically downscaled projections of precipitation in the mid and late 21st century over North America. Earth's Future, 3(7), pp. 268-288[https://doi.org/10.1002/2015EF000304 DOI: 10.1002/2015EF000304]&nbsp;&nbsp; Free access article from: [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015EF000304 American Geophysical Union]&nbsp;&nbsp; [[Media: Wang2015.pdf | Report.pdf]]</ref>
 
|Weather Research and</br>Forecasting (WRF v3.3)||80+ Variables||NetCDF||grid: 6.5 arc min||3 hours
 
|}
 
A&nbsp;primary&nbsp;cause&nbsp;of&nbsp;uncertainty in climate change projections, especially beyond 30 years into the future, is the uncertainty in the greenhouse gas (GHG) emission scenarios used to make climate model projections. The best method of accounting for this type of uncertainty is to apply a climate change model to multiple GHG emission scenarios (see also: [[Wikipedia: Representative Concentration Pathway]]).  
 
  
The&nbsp;uncertainties&nbsp;in&nbsp;climate&nbsp;projections over shorter timescales, less than 30 years out, are dominated by something known as “internal variability” in the models. Different approaches are used to address the uncertainty from internal variability<ref name="Kotamarthi2021"/>. A third type of uncertainty in climate modeling, known as scientific uncertainty, comes from our inability to numerically solve every aspect of the complex earth system. We expect this scientific uncertainty to decrease as we understand more of the earth system and improve its representation in our numerical modelsAs discussed in [[Climate Change Primer]], numerical experiments based on global climate models are designed to address these uncertainties in various ways. Downscaling methods evaluate this uncertainty by using several independent regional climate models to generate future projections, with the expectation that each of these models will capture some aspects of the physics better than the others, and that by using several different models, we can estimate the range of this uncertainty. Thus, the commonly accepted methods for accounting for uncertainty in climate model projections are either using projections from one model for several emission scenarios, or applying multiple models to project a single scenario.
+
As an alternative to a sorptive capping amendment, low-permeability cap amendments have been proposed to enhance cap design life by decreasing pore water advection.  Low permeability clays are an effective means to divert upwelling groundwater away from a contaminated sediment area but are difficult to place in the aqueous environment. Bentonite clays can be placed in mats similar to what is done to provide a low permeability liner in landfills. There are also commercial products that can place clays directly such as the composite material AquaBlok<sup>&reg;</sup>, a bentonite clay and polymer based mineral around an aggregate core<ref name="Barth2008">Barth, E.F., Reible, D. and Bullard, A., 2008. Evaluation of the physical stability, groundwater seepage control, and faunal changes associated with an AquaBlok<sup>&reg;</sup> sediment cap. Remediation: The Journal of Environmental Cleanup Costs, Technologies and Techniques, 18(4), pp.63-70.  [https://doi.org/10.1002/rem.20183 DOI: 10.1002/rem.20183]</ref>.
 +
 
 +
Sediment caps become colonized by microorganisms from the sediments and surface water and potentially become a zone of pollutant biotransformation over time. Aerobic degradation occurs only near the solids-water interface in which benthic organisms are active and thus there might still be significant benthic organism exposure to contaminants. Biotransformation in the anaerobic zone of a cap, which typically extends well beyond the zone of benthic activity, could significantly reduce the risk of pollutant exposure but successful caps encouraging deep degradation processes have not been demonstrated beyond the laboratory.  The addition of materials such as nutrients and oxygen releasing compounds for enhancing the attenuation of contaminants through biodegradation has also been assessed but not applied in the field.  Short term improvements in biodegradation rates can be achieved through tailoring of conditions or addition of nutrients but long term efficacy has not been demonstrated<ref name="Pagnozzi2020">Pagnozzi, G., Carroll, S., Reible, D.D. and Millerick, K., 2020. Biological Natural Attenuation and Contaminant Oxidation in Sediment Caps: Recent Advances and Future Opportunities. Current Pollution Reports, pp.1-14.  [https://doi.org/10.1007/s40726-020-00153-5 DOI: 10.1007/s40726-020-00153-5]</ref>. 
 +
[[File: SedCapFig2.png | thumb |600px|Figure 2. A conceptualization of a cap with accompanying habitat layer]]
  
A comparison of the currently available methods and their characteristics is provided in Table 2 (adapted from Kotamarthi et al., 2016<ref name="Kotamarthi2016"/>)The table lists the various methodologies and models used for producing downscaled data, and the climate variables that these methods produceThese datasets are mostly available for download from the data servers and websites listed in the table and in a few cases by contacting the respective source organizations.
+
==Cap Design and Materials for Habitat Restoration==
 +
In addition to providing chemical isolation and containment, a cap can also be used to provide improvements for organisms by enhancing the habitat characteristics of the bottom substrate<ref name="Yozzo2004">Yozzo, D.J., Wilber, P. and Will, R.J., 2004. Beneficial use of dredged material for habitat creation, enhancement, and restoration in New York–New Jersey Harbor. Journal of Environmental Management, 73(1), pp. 39-52.  [https://doi.org/10.1016/j.jenvman.2004.05.008 DOI: 10.1016/j.jenvman.2004.05.008]</ref><ref name="Zhang2016">Zhang, C., Zhu, M.Y., Zeng, G.M., Yu, Z.G., Cui, F., Yang, Z.Z. and Shen, L.Q., 2016. Active capping technology: a new environmental remediation of contaminated sediment. Environmental Science and Pollution Research, 23(5), pp.4370-4386.  [https://doi.org/10.1007/s11356-016-6076-8 DOI: 10.1007/s11356-016-6076-8]</ref><ref name="Vlassopoulos2017"/>.  Often, contaminated sediment environments are degraded for a variety of reasons in addition to the toxic constituents.  One way to overcome this is to provide both a habitat layer and chemical isolation or contaminant capping layer. Figure 2 illustrates just such a design providing a more appropriate habitat enhancing substrate, in this case by incorporation additional organic material, vegetation and debris, which is often used by fish species for protection, into the surface layer. In a high energy environment, it should be recognized that it may not be possible to keep a suitable habitat layer in place during high flow eventsThis would be true of suitable habitat that had developed naturally as well as a constructed habitat layer and it is presumed that if such a habitat is the normal condition of the waterbody that it will recover over time between such high flow events.
  
The most popular and widely used format for atmospheric and climate science is known as [[Wikipedia:NetCDF | NetCDF]], which stands for Network Common Data Form. NetCDF is a self-describing data format that saves data in a binary format. The format is self-describing in that a metadata listing is part of every file that describes all the data attributes, such as dimensions, units and data size and in principal should not need additional information to extract the required data for analysis with the right softwareHowever, specially built software for reading and extracting data from these binary files is necessary for making visualizations and further analysis. Software packages for reading and writing NetCDF datasets and for generating visualizations from these datasets are widely available and obtained free of cost ([https://www.unidata.ucar.edu/software/netcdf/docs/ NetCDF-tools]). Popular geospatial analysis tools such as ARC-GIS, statistical packages such as ‘R’ and programming languages such as Fortran, C++, and Python have built in libraries that can be used to directly read NetCDF files for visualization and analysis.  
+
==Summary==
<br clear="left" />
+
Clean substrate can be placed at the sediment-water interface for the purposes of reducing exposure to and risk from contaminants in the sediments. The cap can consist of simple materials such as sand designed to physically stabilize contaminated sediments and separate the benthic community from those contaminants or may include other materials designed to sequester contaminants even under adverse conditions including strong groundwater upwelling or highly mobile contaminantsThe surface of a cap may be designed of coarse material such as gravel or cobble to be stable under high flow events or designed to be more appropriate habitat for benthic and aquatic organisms. As a result of its flexibility, simplicity and low cost relative to its effectiveness, capping is one of the most prevalent remedial technologies for sediments.  
  
 
==References==
 
==References==
 
<references />
 
<references />
 +
 
==See Also==
 
==See Also==
 
[https://serdp-estcp.org/Program-Areas/Resource-Conservation-and-Resiliency/Infrastructure-Resiliency/Vulnerability-and-Impact-Assessment/RC-2242/(language)/eng-US Climate Change Impacts to Department of Defense Installations, SERDP Project RC-2242]
 

Latest revision as of 18:43, 8 November 2021

Sediment Capping

Capping is an in situ remedial technology that involves placement of a clean substrate on the surface of contaminated sediments to reduce contaminant uptake by benthic organisms and contaminant flux to surface water. Simple sand caps can be effective in reducing exposure of benthic organisms and in limiting oxygen transport into the contaminated sediments, resulting in precipitation of metal sulfides. Amendments are sometimes included in caps to reduce cap permeability and groundwater upwelling, to increase contaminant sorption or biodegradation, or to improve habitat.

Related Article(s):

Contributor(s):

  • Danny Reible

Key Resource(s):

  • Processes, Assessment and Remediation of Contaminated Sediments[1]
  • Guidance for In-Situ Subaqueous Capping of Contaminated Sediments[2]

Introduction

Figure 1. Conceptual sketch of a cap configuration

Capping is an in situ remedial technology for contaminated sediments that involves placement of a clean substrate on the sediment surface. Capping contaminated sediments following dredging operations and capping of dredged material to stabilize contaminants has been a common practice by the United States Army Corps of Engineers since the 1970s. Beginning in the 1980s, in Japan and subsequently elsewhere, capping has been used more widely as a remedial approach to improve the quality of the bottom substrate and reduce contaminant exposures to benthic organisms and fish. The USEPA published a capping guidance document in 1998 that summarizes past uses of sediment capping and outlines its basic design[2]. Although capping technology has developed substantially in the past 20 years, this early reference still provides useful information on the approach and its applications. A more recent summary of capping is described in Reible 2014[1].

Capping serves to contain contaminated sediment solids, isolate contaminants from benthic organisms and reduce contaminant transport to the sediment surface and overlying water. The clean substrate may be an inert material such as sand, a natural sorbing material such as other sediments or clays, or be amended with an active/reactive material to enhance the isolation of the contaminants. Amendments to enhance contaminant isolation include permeability reduction agents to divert groundwater flow, sorbents to retard contaminant migration through the capping layer or provide greater accumulation capacity, or reagents to encourage degradation or transformation of the contaminants.

The basic concept of a cap is illustrated in Figure 1. The Figure also illustrates that a cap is often a thin layer or layers relative to water depth and generally causes little disturbance to the underlying sediments or body of water in which it is placed. Depending upon the erosive forces to which the cap may be subjected, the surface layer may be composed of relatively coarse material to withstand those erosive forces.

Although a cap is typically thin compared to the water depth, it generally must be thicker than the biologically active zone (BAZ) of the sediments. The biologically active zone is that zone in which benthic organisms live and interact with the sediment. Their activities tend to mix the BAZ (known as bioturbation) over the course of a few years and thus a cap that is thinner than the BAZ will tend to become intermixed with the underlying contaminated sediments. Processes other than bioturbation including diffusion, advection or groundwater upwelling, hyporheic exchange near the interface, biogenic gas production and migration and underlying sediment consolidation can all lead to contaminant migration into and through a cap. These occur at different rates and intensities and their assessment and evaluation ultimately governs the effectiveness of a cap and the feasibility of its use as a sediment remediation technology for a particular site.

In general, capping is an effective remedial technology for contaminants that are strongly associated with the sediment solids including hydrophobic organic compounds such as high molecular weight polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dioxins and DDTx, but also heavy metals. Hydrophobic organic compounds tend to strongly associate with the organic fraction of sediments so organic rich sediments or the addition of organic phases to the capping material can be very effective at containing these contaminants. Many of the common heavy metals of concern, including cadmium, copper, nickel, zinc, lead and mercury, tend to be associated with insoluble sulfides under strongly reducing conditions. Since oxygen penetration into a capping layer is typically limited to a few cm or less at the surface, a cap serves to drive the underlying contaminated sediment toward strongly reducing conditions and, particularly in marine and estuarine sediments, encourage sulfate reduction leading to the formation of these insoluble sulfides. The low solubility of these sulfides encourages retention by a capping layer and makes the cap extremely effective as a remedial approach for sediments with elevated concentrations of heavy metals.

A variety of tools have been developed to evaluate the processes leading to sorption and retardation of contaminants as well as processes leading to contaminant migration and release. The original references quantifying contaminant behavior in a sediment cap were explored in a series of papers in the early 1990s[3][4]. Since that time, design tools have been continuously improved. CapSim is a commonly used and current tool developed by Dr. Reible and collaborators. This tool can evaluate contaminant release from uncapped, capped, and treated sediments for purposes of design and evaluation. The model formulation and structure is described in Shen et al. 2018[5]. One common use of such a tool is to evaluate the effect of various cap materials and thicknesses on the performance of a cap.

Cap Design and Materials for Chemical Containment

An inert material such as sand can be effective as a capping material where contaminants are strongly associated with solids and where the operative site specific transport mechanisms do not lead to rapid contaminant migration through such a material. Additional contaminant containment can often be achieved through the placement of clean sediment, e.g. dredged material from a nearby location. Other materials as cap layers or amendments may be useful to address particularly mobile contaminants or when particular degradative mechanisms can be exploited. The Anacostia River was the site of a demonstration that first tested “active” or “amended” capping in the field[6][7]. Amended caps are often the best option when groundwater upwelling or other advective processes promote significant mobility of contaminants and the addition of sorbents can slow that contaminant migration[8]. Although a variety of materials have been proposed for sediment caps, a far smaller number of options have been successfully employed in the field.

Metals migration is very site dependent due to the potential for many metals to complex with other species in the interstitial water and the specific metal speciation present at a site. Often, the strongly reducing environment beneath a cap renders many common metals unavailable through the formation of metal sulfides. In such cases, a simple sand cap can be very effective. Amended caps to manage metal contaminated sediments may be advantageous when site specific conditions lead to elevated metals mobility, but should be supported with site specific testing[9].

For hydrophobic organic contaminants, cap amendments that directly control groundwater upwelling and also sorbents that can remove migrating contaminants from that groundwater have been successfully employed. Examples include clay materials such as AquaBlok® for permeability control, sorbents such as activated carbon for truly dissolved contaminants, and organophilic clays for separate phase contaminants.

The placement of clean sediment as an in situ cap can be difficult when the material is fine grained or has a low density. Capping with a layer of coarse grained material such as clean sand mitigates this issue although clean sands have minimal sorption capacity. Because of this limitation, sand caps may not be sufficient for achieving remedial goals in sites where contamination levels are high or transport rates are fast due to pore water upwelling or tidal pumping effects. Conditions such as these may require the use of “active” amendments to reduce transport rates.

Capping with clean sand provides a physical barrier between the underlying contaminated material and the overlying water, stabilizes the underlying sediment to prevent re-suspension of contaminated particles, and can reduce chemical exposure under certain conditions. Sand primarily provides a passive barrier to the downward penetration of bioturbating organisms and the upward movement of sediment or contaminants. Although conventional sandy caps can often be an effective means of managing contaminated sediments, there are conditions when sand caps may not be capable of achieving design objectives. Some factors that reduce the effectiveness of sand caps include:

  • erosion and loss of cap integrity
  • high groundwater upwelling rates
  • mobile (low sorption) contaminants of concern (COCs)
  • high COC concentrations
  • unusually toxic COCs
  • the presence of tidal influences
  • the presence of non-aqueous phase liquids (NAPLs)
  • high rates of gas ebullition

Of these, the first three are common limitations to capping and often control the ability to effectively design and implement a cap as a sediment remedial strategy. In these cases, it may be possible to offset these issues by increasing the thickness of the cap. However, the required thickness can reach infeasible levels in shallow streams or navigable water bodies. In addition, increased construction costs associated with thick caps may become prohibitive. As a result of these issues, caps that use alternative materials (also known as active caps) to reduce the thickness or increase the protectiveness of a cap may be necessary. The materials in active caps are designed to interact with the COCs to enhance the containment properties of the cap.

Apatites are a class of naturally occurring minerals that have been investigated as a sorbent for metals in soils and sediments[10][6][11]. Apatites consist of a matrix of calcium phosphate and various other common anions, including fluoride, chloride, hydroxide, and occasionally carbonate. Metals are sequestered either through direct ion exchange with the calcium atom or dissolution of hydroxyapatite followed by precipitation of lead apatite. Zeolites, which are microporous aluminosilicate minerals with a high cationic exchange capacity (CEC), have also been proposed to manage metal species[12].

It is possible to create a hydrophobic, sorbing layer for non-polar organics by exchanging a cationic surfactant onto the surface of clays such as zeolites and bentonites,. Organoclay is a modified bentonite containing such substitutions that has been evaluated for control of non-aqueous phase NAPLs and other organic contaminants[13]. An organoclay cap has been implemented for sediment remediation at the McCormick and Baxter site in Portland, OR[14]. A similar organic sorbing phase can be formed by treating zeolites with surfactants but this approach has not been reported for contaminated sediments.

Activated carbon is a strong sorbent of hydrophobic organic compounds and has been used as a treatment for sediments or as an active sorbent within a capping layer[15][16][17][18][19]. Placement of activated carbon for sediment capping is difficult due to the near neutral buoyancy of the material but it has been applied in this manner in relatively low energy environments such as Onondaga Lake, Syracuse, NY[20]. Alternatives in higher energy environments include placement of activated carbon in a mat such as the CETCO Reactive Core Mat (RCM)® or Huesker Tektoseal®, or as a composite material such as SediMite® or AquaGate®. In the case of the mats, powdered or granular activated carbon can be placed in a controlled layer while the density of the composite materials is such that they can be broadcast from the surface and allowed to settle to the bottom. In a sediment treatment application, the composite material would either be worked into the surface or allowed to intermix gradually by bioturbation and other processes. In a capping application, the mat or composite material would typically be combined or overlain with a sand or other capping layer to keep it in place and to provide a chemical isolation layer away from the sediment surface.

As an alternative to a sorptive capping amendment, low-permeability cap amendments have been proposed to enhance cap design life by decreasing pore water advection. Low permeability clays are an effective means to divert upwelling groundwater away from a contaminated sediment area but are difficult to place in the aqueous environment. Bentonite clays can be placed in mats similar to what is done to provide a low permeability liner in landfills. There are also commercial products that can place clays directly such as the composite material AquaBlok®, a bentonite clay and polymer based mineral around an aggregate core[21].

Sediment caps become colonized by microorganisms from the sediments and surface water and potentially become a zone of pollutant biotransformation over time. Aerobic degradation occurs only near the solids-water interface in which benthic organisms are active and thus there might still be significant benthic organism exposure to contaminants. Biotransformation in the anaerobic zone of a cap, which typically extends well beyond the zone of benthic activity, could significantly reduce the risk of pollutant exposure but successful caps encouraging deep degradation processes have not been demonstrated beyond the laboratory. The addition of materials such as nutrients and oxygen releasing compounds for enhancing the attenuation of contaminants through biodegradation has also been assessed but not applied in the field. Short term improvements in biodegradation rates can be achieved through tailoring of conditions or addition of nutrients but long term efficacy has not been demonstrated[22].

Figure 2. A conceptualization of a cap with accompanying habitat layer

Cap Design and Materials for Habitat Restoration

In addition to providing chemical isolation and containment, a cap can also be used to provide improvements for organisms by enhancing the habitat characteristics of the bottom substrate[23][24][20]. Often, contaminated sediment environments are degraded for a variety of reasons in addition to the toxic constituents. One way to overcome this is to provide both a habitat layer and chemical isolation or contaminant capping layer. Figure 2 illustrates just such a design providing a more appropriate habitat enhancing substrate, in this case by incorporation additional organic material, vegetation and debris, which is often used by fish species for protection, into the surface layer. In a high energy environment, it should be recognized that it may not be possible to keep a suitable habitat layer in place during high flow events. This would be true of suitable habitat that had developed naturally as well as a constructed habitat layer and it is presumed that if such a habitat is the normal condition of the waterbody that it will recover over time between such high flow events.

Summary

Clean substrate can be placed at the sediment-water interface for the purposes of reducing exposure to and risk from contaminants in the sediments. The cap can consist of simple materials such as sand designed to physically stabilize contaminated sediments and separate the benthic community from those contaminants or may include other materials designed to sequester contaminants even under adverse conditions including strong groundwater upwelling or highly mobile contaminants. The surface of a cap may be designed of coarse material such as gravel or cobble to be stable under high flow events or designed to be more appropriate habitat for benthic and aquatic organisms. As a result of its flexibility, simplicity and low cost relative to its effectiveness, capping is one of the most prevalent remedial technologies for sediments.

References

  1. ^ 1.0 1.1 Reible, D. D., Editor, 2014. Processes, Assessment and Remediation of Contaminated Sediments. Springer, New York, NY. 462 pp. ISBN: 978-1-4614-6725-0
  2. ^ 2.0 2.1 Palermo, M., Maynord, S., Miller, J. and Reible, D., 1998. Guidance for In-Situ Subaqueous Capping of Contaminated Sediments. Assessment and Remediation of Contaminated Sediments (ARCS) Program, Great Lakes National Program Office, US EPA 905-B96-004. 147 pp. Report.pdf
  3. ^ Wang, X.Q., Thibodeaux, L.J., Valsaraj, K.T. and Reible, D.D., 1991. Efficiency of Capping Contaminated Bed Sediments in Situ. 1. Laboratory-Scale Experiments on Diffusion-Adsorption in the Capping Layer. Environmental Science and Technology, 25(9), pp.1578-1584. DOI: 10.1021/es00021a008
  4. ^ Thoma, G.J., Reible, D.D., Valsaraj, K.T. and Thibodeaux, L.J., 1993. Efficiency of Capping Contaminated Bed Sediments in Situ 2. Mathematics of Diffusion-Adsorption in the Capping Layer. Environmental Science and Technology, 27(12), pp.2412-2419. DOI: 10.1021/es00048a015
  5. ^ Shen, X., Lampert, D., Ogle, S. and Reible, D., 2018. A software tool for simulating contaminant transport and remedial effectiveness in sediment environments. Environmental Modelling and Software, 109, pp. 104-113. DOI: 10.1016/j.envsoft.2018.08.014
  6. ^ 6.0 6.1 Reible, D., Constant, D.W., Roberts, K. and Zhu, Y., 2003. Active capping demonstration project in anacostia DC. In Second International Conference on the Remediation of Contaminated Sediments: October. Free download available from: ResearchGate
  7. ^ Reible, D., Lampert, D., Constant, D., Mutch Jr, R.D. and Zhu, Y., 2006. Active Capping Demonstration in the Anacostia River, Washington, DC. Remediation Journal: The Journal of Environmental Cleanup Costs, Technologies and Techniques, 17(1), pp. 39-53. DOI: 10.1002/rem.20111 Free download available from: Academia.edu
  8. ^ Ghosh, U., Luthy, R.G., Cornelissen, G., Werner, D. and Menzie, C.A., 2011. In-situ Sorbent Amendments: A New Direction in Contaminated Sediment Management. Environmental Science and Technology, 45(4), pp. 1163-1168. DOI: 10.1021/es102694h Open access article from: American Chemical Society   Report.pdf
  9. ^ Viana, P.Z., Yin, K. and Rockne, K.J., 2008. Modeling Active Capping Efficacy. 1. Metal and Organometal Contaminated Sediment Remediation. Environmental Science and Technology, 42(23), pp. 8922-8929. DOI: 10.1021/es800942t
  10. ^ Melton, J.S., Crannell, B.S., Eighmy, T.T., Wilson, C. and Reible, D.D., 2003. Field Trial of the UNH Phosphate-Based Reactive Barrier Capping System for the Anacostia River. EPA Grant R819165-01-0
  11. ^ Knox, A.S., Paller, M.H. and Roberts, J., 2012. Active Capping Technology—New Approaches for In Situ Remediation of Contaminated Sediments. Remediation Journal, 22(2), pp.93-117. DOI: 10.1002/rem.21313 Free download available from: ResearchGate
  12. ^ Zhan, Y., Yu, Y., Lin, J., Wu, X., Wang, Y. and Zhao, Y., 2019. Simultaneous control of nitrogen and phosphorus release from sediments using iron-modified zeolite as capping and amendment materials. Journal of Environmental Management, 249, p.109369. DOI: 10.1016/j.jenvman.2019.109369
  13. ^ Reible, D.D., Lu, X., Moretti, L., Galjour, J. and Ma, X., 2007. Organoclays for the capping of contaminated sediments. AIChE Annual Meeting. ISBN: 978-081691022-9
  14. ^ Parrett, K. and Blishke, H., 2005. 23-Acre Multilayer Sediment Cap in Dynamic Riverine Environment Using Organoclay an Adsorptive Capping Material. Presentation to Society of Environmental Toxicology and Chemistry (SETAC), 26th Annual Meeting.
  15. ^ Zimmerman, J.R., Ghosh, U., Millward, R.N., Bridges, T.S. and Luthy, R.G., 2004. Addition of Carbon Sorbents to Reduce PCB and PAH Bioavailability in Marine Sediments: Physicochemical Tests. Environmental Science and Technology, 38(20), pp. 5458-5464. DOI: 10.1021/es034992v
  16. ^ Werner, D., Higgins, C.P. and Luthy, R.G., 2005. The sequestration of PCBs in Lake Hartwell sediment with activated carbon. Water Research, 39(10), pp. 2105-2113. DOI: 10.1016/j.watres.2005.03.019
  17. ^ Abel, S. and Akkanen, J., 2018. A Combined Field and Laboratory Study on Activated Carbon-Based Thin Layer Capping in a PCB-Contaminated Boreal Lake. Environmental Science and Technology, 52(8), pp. 4702-4710. DOI: 10.1021/acs.est.7b05114 Open access article available from: American Chemical Society   Report.pdf
  18. ^ Payne, R.B., Ghosh, U., May, H.D., Marshall, C.W. and Sowers, K.R., 2019. A Pilot-Scale Field Study: In Situ Treatment of PCB-Impacted Sediments with Bioamended Activated Carbon. Environmental Science and Technology, 53(5), pp. 2626-2634. DOI: 10.1021/acs.est.8b05019
  19. ^ Yan, S., Rakowska, M., Shen, X., Himmer, T., Irvine, C., Zajac-Fay, R., Eby, J., Janda, D., Ohannessian, S. and Reible, D.D., 2020. Bioavailability Assessment in Activated Carbon Treated Coastal Sediment with In situ and Ex situ Porewater Measurements. Water Research, 185, p. 116259. DOI: 10.1016/j.watres.2020.116259
  20. ^ 20.0 20.1 Vlassopoulos, D., Russell, K., Larosa, P., Brown, R., Mohan, R., Glaza, E., Drachenberg, T., Reible, D., Hague, W., McAuliffe, J. and Miller, S., 2017. Evaluation, Design, and Construction of Amended Reactive Caps to Restore Onondaga Lake, Syracuse, New York, USA. Journal of Marine Environmental Engineering, 10(1), pp. 13-27. Free download available from: ResearchGate
  21. ^ Barth, E.F., Reible, D. and Bullard, A., 2008. Evaluation of the physical stability, groundwater seepage control, and faunal changes associated with an AquaBlok® sediment cap. Remediation: The Journal of Environmental Cleanup Costs, Technologies and Techniques, 18(4), pp.63-70. DOI: 10.1002/rem.20183
  22. ^ Pagnozzi, G., Carroll, S., Reible, D.D. and Millerick, K., 2020. Biological Natural Attenuation and Contaminant Oxidation in Sediment Caps: Recent Advances and Future Opportunities. Current Pollution Reports, pp.1-14. DOI: 10.1007/s40726-020-00153-5
  23. ^ Yozzo, D.J., Wilber, P. and Will, R.J., 2004. Beneficial use of dredged material for habitat creation, enhancement, and restoration in New York–New Jersey Harbor. Journal of Environmental Management, 73(1), pp. 39-52. DOI: 10.1016/j.jenvman.2004.05.008
  24. ^ Zhang, C., Zhu, M.Y., Zeng, G.M., Yu, Z.G., Cui, F., Yang, Z.Z. and Shen, L.Q., 2016. Active capping technology: a new environmental remediation of contaminated sediment. Environmental Science and Pollution Research, 23(5), pp.4370-4386. DOI: 10.1007/s11356-016-6076-8

See Also