In situ-metoder

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As generally described above and in detail below (avsnitt genomförda in situ projekt), international knowledge of and experience with using in-situ remedies for remediating (managing) fiber sediments is extremely limited so far, both at laboratory and field scales.

Several different remedies are indeed internationally established and accepted for managing contaminated sediments in-situ, as summarized in Section 7.1.1. However, as further discussed in Section 7.1.2, these in-situ remedies have been applied more-or-less only to contaminated mineral-based (d.v.s. minerogenic) sediments, not to relatively organic-rich sediments, like fiber sediments.

Thus, in the absence of proven track records for remedy use, a fundamental question remains: How applicable and effective are already available in-situ remedies for also adequately managing fiberbank and fiber-rich sediments?

In an attempt to answer this question, the following information is presented below:

  • Summaries and descriptions of the available in-situ remedies.
  • A discussion of remedy use for managing fiber sediments in general.
  • A discussion of remedy use for managing fiber-rich sediments specifically.
  • A discussion of remedy use for managing fiberbank sediments specifically.

 

Överblick över tillgängliga åtgärdsmetoder

 

Det finns flera olika metoder för att hantera och behandla förorenade sediment in situ. Metoderna listas i Tabell 1 nedan tillsammans med en kort sammanfattning av viktiga aspekter för respektive metod. För mer ingående beskrivningar av åtgärdsmetoderna, se fördjupade metodbeskrivningar för in situ-åtgärder av förorenade sediment. 

Tabell 1. Tillgängliga åtgärdsmetoder för in situ-åtgärder av förorenade sediment.

Metod

Vad innebär metoden?

Hur reducerar metoden riskerna och bidrar till att uppnå långsiktiga åtgärdsmål?  

Övervakad naturlig självrening (ÖNS)

Förorenade sediment lämnas ostörda på platsen medan kontinuerlig övervakning av föroreningssituationen och nedbrytningsprocesserna görs enligt kontrollprogram.

Låter naturligt förekommande processer, i huvudsak översedimentering med nya rena sediment, reducera föroreningskoncentrationerna i sedimentytan till acceptabla nivåer.

Förstärkt ÖNS (FÖNS)

Utläggning av ett relativt tunt lager av kemiskt inert finkornigt material, t ex natursand, ovanpå förorenade sediment. Tjockleken på täckningen motsvarar vanligen bioturbationsdjupet, upp till ca 10-15 cm. Inte designad för att vara erosionsmotståndig.

Accelererar den naturliga återhämtningsprocessen, huvudsakligen genom att ”snabbspola” översedimenteringsprocessen.

Metoden är även känd som konventionell tunnskiktsövertäckning.

Isolationsövertäckning (konventionell och aktiv typer)

Utläggning av ett relativt tjockt lager av inerta (eller i kombination med aktiva material) ovanpå förorenade sediment. Ofta består täckningen av finare och grövre kornstorlekar, och erosionsmotståndigt skikt. Total tjocklek på täckningen varierar vanligen mellan 20–100 cm.

Reducerar vanligen risken genom fysisk isolering av bottenlevande fauna från sedimentbundna föroreningar, kemisk isolering av fauna från lösta föroreningar och skydd mot erosion, minskad transport och spridning av förorenade sediment.

AC-baserad tunnskiktsövertäckning

Utläggning av ett relativt tunt lager högadsorberande aktivt kol på ytan av förorenade sediment. Tjockleken på täckningen är i storleken cm, och mindre än bioturbationsdjupet.

Bygger på naturlig bioturbering för at att blanda ned det aktiva kolet med ytsediment och därigenom signifikant reducera biotillgängligheten och upptaget av föroreningar hos bottenlevande organismer i sediment. Metoden är även känd som ”in situ treatment”

A wide variety of equipment and methods can be used to place an equally wide variety of capping materials for the purpose of constructing different types of conventional or active caps (t.ex. Figures A and B).

cap placement

Figur A. Examples of equipment and methods used to construct various types of caps (sources: BioBlok Solutions AS, Norway [left], Parsons, USA [right]).

placing caps insitu USEPA

Figur B. Examples of equipment and methods used to construct active (t.ex. AC-amended) caps, specifically (source: US EPA).

Notera: “Klassisk in situ-behandling”, är ytterligare en tillgänglig in situ-metod. Metoden bygger på att med mekaniska metoder snabbt blanda in behandlingsmedlet ned i sedimentytan med syfte att accelerera behandlingsprocessen och minska biotillgängligheten. Den har dock aldrig blivit etablerad eller fullt accepterad på grund av varierande resultat (ofta ej uppnådda åtgärdsmål, och ibland förvärrande av föroreningssituationen). Metoden presenteras mer i detalj i metodbeskrivningen på Åtgärdsportalen men beskrivs därför inte vidare när det gäller möjligheterna för att åtgärda fibersediment.

For further reading, abundant references for each of the in-situ remedies can be found elsewhere on Åtgärdsportalen (here). Additional selected references are also provided below, where considered appropriate.

 Användning av tillgängliga in situ-metoder för att hantera fibersediment generellt

De senaste decennierna har internationellt sett ett stort antal in situ-åtgärder av förorenade sediment genomförts, både på pilot- och fullskala. De flesta har genomförts i USA, men även flera i Kanada och Norge. Antalet identifierade in situ-projekt som har genomförts för att åtgärda sediment till dagens datum (november 2019) är minst: Övervakad naturlig självrening (ÖNS), 28 projekt. Förstärkt naturlig självrening (FÖNS), 10 projekt. Isolationsövertäckning, 120 projekt. Aktiv isolationsövertäckning (beskrivs under isolationsövertäckning på Åtgärdsportalen), 40 projekt. AC-baserad tunnskiktöverstäckning, 15 projekt.  

Majoriteten av ÖNS and in-situ capping projekt (of different types) conducted to-date, worldwide, oavsett behandlingsmetod, har genomförts på minerogena sediment, där halterna TOC (total organic carbon) per kg/TS ofta var kring 5 % eller lägre (Reible, 2014; Russell, 2019). Ett litet antal av dessa projekt har planerats eller genomförts på förorenade sediment med TOC-halter betydligt högre än 5% (e.g. BBL, 2004; Rockne et al., 2010; Meric et al., 2014; NCSCAG, 2016; RERC, 2005). Ännu färre projekt har genomförts på förorenade sediment där den högre TOC-halten härrörde från fibermaterial (se kapitel [In situ-åtgärdsprojekt]).

Som en naturlig konsekvens av att majoriteten av projekt genomförts i platser med minerogena sediment har också alla aspekter av att designa och genomföra in situ-åtgärder i sediment utvecklats och anpassats för minerogena sediment, och inte sediment med så högt organiskt innehåll som fibersediment har. 

Den kunskap och de erfarenheter som har byggts upp internationellt genom åtgärder in situ av förorenade minerogena sediment kan ändå till viss del användas för att bedöma metodernas användbarhet på fibersediment på grund av att:

  • Många av de metalliska och organiska föroreningar som åtgärdats i minerogena sediment förekommer även på platser med fibersediment.
  • I likhet med många fibersediment, särskilt fiberbankssediment, så är många minerogena sediment mjuka med låg hållfasthet (e.g. Jersak et al., 2016 a) och det finns projekterfarenhet som visar på att åtgärder genom täckning ändå kan lyckas, när konstruktionsprocessen genomförts korrekt (e.g. Jersak et al., 2016 a, b).

Faktum är dock att det finns väldigt lite internationell erfarenhet av in situ-åtgärder av fibersediment specifikt, som? gör att stora osäkerheter kvarstår.

Most of the uncertainties relate to certain characteristics inherent to many fiberbank sediments, namely, their: very low bearing capacity, very high gas production, and low stability on slopes.

Use of available in-situ remedies for managing fiber-rich sediments

Fiber-rich sediments can display a broad range of characteristics in terms of composition, stratigraphic occurrence, and other attributes. Regardless, mineral (not organic) material comprises the great majority of sediment mass, and TOC content - although highly variable - tends to be lower than in fiberbank sediment because of the relatively lower quantities of fiber material present. Thus, it is reasonable to generally consider many fiber-rich sediments as more-or-less minerogenic in overall character.

Furthermore, if many fiber-rich sediments can be considered as minerogenic, it is also reasonable to then assume, at least initially, that many sites can be successfully managed using all of the established and accepted in-situ remedies (Table 1). Even if TOC content in fiber-rich sediment is relatively high compared to typical (non fiber-bearing) minerogenic sediments, there is still at least some project precedent for using the available in-situ remedies for managing more organic-rich sediments (Section 6.1.2).

Regarding ÖNS specifically: The natural process of over-sedimentation – a key requirement for this in-situ remedy – tends to occur atop fiber-rich sediments more often than atop fiberbank sediments; this is probably because fiber-rich sediments tend to be located in deeper, calmer waters (Norrlin and Josefsson, 2017). So, this provides at least some circumstantial evidence that ÖNS may be successfully used to manage at least some fiber-rich sediments.

Use of available in-situ remedies for managing fiberbank sediments

In contast to fiber-rich sediments, it is not assumed that all in-situ remedies have equal potential for successful use on fiberbank sediments, given their characteristics of very high gas production, very low bearing capacity, and inherently low stability on slopes. Consequently, each in-situ remedy needs to be considered separately, as described below.

Övervakad naturlig självrening, ÖNS (Figur C)

skiss MNR1

Figur C. ÖNS, in concept (source: J. Jersak).

 

Potential for successful use

Probably not an appropriate remedy for most fiberbank sediments.

Reasoning and available evidence

Some key sediment and site conditions favorable to, and indeed required for, successful use of ÖNS include:

  1. The site is low-energy in terms of aquatic erosion potential.
  2. New, clean minerogenic sediments naturally accumulate overtop the contaminated sediment bed over time (over-sedimentation).
  3. The sediment bed is physically stable over time.
  4. Contaminant concentrations in surface sediment are relatively low.
  5. Concentrations in sediment and resident benthic macroinvertebrates are already trending towards risk-based remediation goals.

Conditions a (low aquatic energy) and b (clean sediments accumulateon top):

Many fiberbank deposits (d.v.s. fiberbank sediments) occur in shallower, higher-energy waters. Because of this, over-sedimentation usually does not occur overtop the fiberbank sediments (e.g. Norrlin and Josefsson, 2017).

Condition c (sediment bed remains physically stable over time):

Work by SGU indicates that, due to their unique fibrous character, fiberbank sediments may be more erosion-resistant than typical minerogenic sediments. Regardless, this is no guarantee fiberbank sediment will remain stable over time in such higher-energy environments; in this regard, it is reasonable to assume cellulose material occurring in fiber-rich sediments originates, at least in part, from erosion and spreading of nearby fiberbank sediment. Furthermore, as land up-lift continues to occur over the coming decades ([Erosionsförhållanden]), surface waters will become even shallower and more erosive. As a result, the sediment bed will also become even more unstable over time.

Field investigations by SGU and others further indicates that fiberbank sediment deposits can be inherently geotechnically unstable, as indicated by past occurrence of submarine landslides in sloped areas ([Erosionsförhållanden]), So, the issue of sediment-bed instability relates not only to erodibility of surficial fiberbank sediment (as discussed above), but also to the overall stability of the deposited masses themselves.

Condition d (Contaminant concentrations in surface sediment are relatively low):

Contaminant concentrations in fiberbank surface sediments can be highly elevated (see section [Föroreningar och processer]). Även om koncentrationerna av föroreningar i vissa fiberbankar är låga tyder genomförda undersökningar på att fibrerna och de höga halterna TOC i fiberbankar skapar anaeroba och sura miljöer som i sig själva kan ha en negativ påverkan på bottenlevande organismer, och att nedbrytning av fiberbankarna inte kommer att ske på naturlig väg inom en överskådlig framtid.

Condition e (already trending towards of reaching risk-based remediation goals):

 Since over-sedimentation is unlikely and many of the contaminants are resistant to natural degradation combined with also being bioac cumulative (e.g. PCB, DDT, dioxine and mercury), it is not likely that contamination concentrations in sediments and resident benthic organismsare trending towards risk-based remediation goals. To this point, field observations made to-date indicate very limited to no benthic activity in fiberbank sediments, such that these anoxic and often highly-contaminated surface substrates can effectively be considered as “dead zones” in terms of benthic habitat (Section [Biologisk status]).

 

The formation, buildup, and release of large quantities of biogenic gas from fiberbank sediments (Section gasförekomst) may also be a major contributing factor to not only long-term instability of the sediments, but also greater contaminant release and contaminant spreading, se kapitel [föroreningsspridning] (e.g. Norrlin and Josefsson, 2017). Gas-facilitated contaminant transport processes include the following:

  • Re-suspension and migration of particle-bound contaminants.
  • Migration of contaminant-bearing porewaters.
  • Transport of hydrophobic Persistent Organic Pollutants (POPs) from the pore water to the gas bubbles (see GASFIB project).

One or more of these processes could potentially transfer contaminants from the sediment bed up into surface waters not only much more rapidly than by diffusion, but also even more rapidly than advective migration due to groundwater upwelling.

 

Förstärkt ÖNS, FÖNS (Figur D)

 fons EMNR

Figur D. FÖNS, SPI photograph. Sand layer thickness ~ 5 cm (source as indicated).

 Potential for successful use

Probably not an appropriate remedy for most fiberbank sediments.

Reasoning and available evidence

Conceptually, FÖNS is a close remedial relative to ÖNS in that it accelerates natural recovery processes by ”fast-forwarding” the over-sedimentation process.  Given the close connection between these two in-situ remedies, it is reasonable to assume that the same sediment and site conditions favorable to and required for using ÖNS (including those listed above) also apply to using FÖNS. Therefore, if ÖNS is probably not appropriate for use at most fiberbank sediments, neither is FÖNS.

Field observations by SGU indicate gas can release from fiberbank  surfaces even when over-sedimentation consisting of 1-20 cm-thick layers of mainly minerogenic material are in-place (Norrlin and Josefsson, 2017: more SGU refs?); that is, the gas can successfully releases through the over-sedimentation layer. If gas-facilitated contaminant transport could be occurring through un-capped fiberbank sediment (see ÖNS), it is reasonable to assume such transport could also be occurring through naturally capped sediment.

Furthermore, results of ongoing laboratory column studies at UU for the FIBREM project, Figur E (presented at conferences, but not yet published in scientific journals) show that gas release can occur and pockmarks can form on cap surfaces within months after placing relatively thin (FÖNS-scale) sand caps overtop two distinctly different types of fiberbank sediment.  

cap labtest

Figur E. Laboratory column capping studies. Cap thickness ~ 15 cm (source: UU).

Isolationsövertäckning, konventionell och aktiv typer (Figur F)

 

skiss caps isol AC

Figur F. Concept-level isolation caps, conventional, left, and active, right (source: J. Jersak).

Potential for successful use

It is uncertain if either or both types of these remedies could be appropriate for fiberbank sediments.

Reasoning and available evidence

Unpublished results of ongoing FIBREM project research at UU (see FÖNS) also indicate pockmarks can form on and gas release can occur from substantially thicker (isolation-scale) sand caps within months of placement overtop different fiberbank sediments. These results strongly imply gas-facilitated contaminant transport can also be occurring through thicker - not just thinner - placed caps.

 Given the inherent geotechnical instability of at least some fiberbank sediment deposits on slopes even when un-capped (see ÖNS), such instabilities - plus the potential for sliding - could besubstantially increased even further, when loading an unstable deposit with a relatively thick and heavy cap.

Furthermore, it is currently unknown if fiberbank sediments may have even lower bearing capacities than the softest-and-weakest minerogenic sediments; this is mainly because of measurement limitations at the low end of the value scale. But if so, techniques developed and refined for constructing isolation caps overtop very soft minerogenic sediments (Jersak et al., 2016 b) may need to be further modified when capping fiberbank sediment deposits, even if a deposit occurs on a relatively low - or even no - slope.

Regarding active isolation capping (t.ex. see Figur F): Incorporating proven-effective sorbents like activated carbon or organoclay into an isolation cap can improve a cap’s ability to chemically isolate contaminants over a longer period of time – at least when capping typical minerogenic sediments. However, when capping fiberbank sediments, it is possible that incorporating such active materials may not provide enough additional protection to counteract and overcome the effects of significant and sustained gas-facilitiated contaminant transport. Research is currently being conducted at UU to explore this issue, both as part of the FIBREM project and as part of the GASFIB project

Furthermore, regardless of what type of reactive material or amendment is added in, the isolation cap will still be relatively heavy, and thus could still pose significant risks in terms of geotechnical stability of the capped sediment, sloped or not.

To summarize, an isolation cap, regardless of its type or design, cannot be expected to meet its long-term remediation goals (Table 1) unless:

  • The cap can be constructed in a geotechnically stable manner and remain stable and physically in-tact over time, including on slopes, and
  • The rate and extent of un-controlled, gas-facilitated contaminant transport through the cap can somehow be significantly reduced and maintained as such over the long-term.

Practically speaking, it is not possible to restrict biogenic gas from forming, building up, and trying to release from capped fiberbank sediment. Therefore, the most logical way of reducing gas-facilitated contaminant transport through the cap is by somehow actively controlling the manner in which gas releases from the sediment. Research is currently being conducted by UU and others to explore these issues.

AC-baserad tunnskiktsövertäckning (Figur G)

ac cap

Figur G. AC-baserad tunnskiktsövertäckning, SPI photograph. Clay + AC layer thickness ~ 5 cm (source as indicated).

Potential for successful use

It is uncertain if this remedy could be appropriate for fiberbank sediments.

 Reasoning and available evidence

 AC-based thin-layer capping is essentially an active form of conventional thin-layer capping (FÖNS) which, in turn, is an accelerated form of ÖNS. Given the close connections between these in-situ remedies, it is reasonable to assume that the same sediment and site conditions favorable to and required for using ÖNS or FÖNS also apply to using their conceptually close remedial relative, AC-based thin-layer capping. Thus, at least in general principle, if neither ÖNS nor FÖNS are appropriate for managing most fiberbank sediments, neither is AC-based thin-layer capping.

However, iIn contrast to ÖNS and FÖNS, the potential for successful use of this particular in-situ remedy is not so logical and straight-forward.

Activated carbon (either in powdered or granular form) is internationally recognized as the most effective sorbent available for a wide variety of dissolved-phase, hydrophobic organic contaminants relevant to fiberbank sediments, namely, PCBs, PAHs, and dioxins/furans (e.g. Jersak et al., 2016 b). Furthermore, AC is an effective sorbent for methyl mercury (e.g. Gilmour et al., 2013; Bussan et al., 2016), which is also relevant to at least some fiberbank sediments impacted by mercury.

Additionally, since AC-bearing thin-layer caps are so thin and light-weight, the geotechnical stability concerns associated with AC’s use within the context of a much thicker, heavier isolation cap would not be an issue here.

Despite its potential promise as another remedial “tool in the toolbox”, significant key unknowns remain related to use of AC-based thin-layer capping for managing fiberbank sediments, including:

  • Could the amount of gas generated by fiberbank sediment simply be too high, and the extent of pockmark formation and gas-facilitated contaminant transport too extensive and widespread, such that even a marginally adequate level of reduction in contaminant exposure and bioavailability cannot be achieved?
  • Could the degree of mercury methylation occurring in the capped sediment be so extensive and sustained that the addition of even large amounts of AC cannot provide adequate long-term reductions in contaminant exposure and bioaccumulation?
  • Could high concentrations of naturally occurring dissolved organic carbon quickly saturate AC particle surfaces, thus significantly reducing their ability to effectively sorb dissolved-phase organic contaminants?

In an effort to address the above and other questions research is currently being conducted at UU, as part of the GASFIB project, to explore how AC-based thin-layer capping could be used for managing fiberbank sediments, including how its use could be optimized.

References

Blasland, Bouck, and Lee, Inc. (BBL). 2004. Pre-design investigation report for Silver Lake sediments. Volume 1. February, 2004.

Bussan, D., R. Sessums, and J. Cizdziel. 2016. Activated carbon and biochar reduce mercury methylation potentials in aquatic sediments. Bull. Environ. Contam. Toxicol. Vol. 4, pp. 536-539.

Gilmour, C., G.S. Riedel, G. Riedel, S. Kwon, and U.Ghosh. 2013. Activated carbon mitigates mercury and methylmercury bioavailability in contaminated sediments. Environ. Sci. Technology. Vol. 47, pp. 13,001-13,010.

Jersak, J., G. Göransson, Y. Ohlsson, L. Larson, P. Flyhammar, och P. Lindh (Jersak et al.). 2016 a. In-situ capping of contaminated sediments. Capping Sweden’s contaminated fiberbank sediments: A unique challenge. SGI Publication 30-5E (in English). www.swedgeo.se.

Jersak et al. 2016 b. Huvuddokument. In-situ övertäckning av förorenade sediment. Metodöversikt. SGI Publikation 30-1 (på Svenska). www.swedgeo.se.

Meric, D., S. Barbuto, T. Sheahan, J. Shine, and A. Alshawabkeh. 2014. Benchscale assessment of the efficiency of a reactive core mat to isolate PAH-spiked aquatic sediments. Soil Sediment Contam. Vol. 23, No. 1, pp. 1-23.

Newtown Creek Superfund Community Advisory Group (NCSCAG). 2016. Meeting summary, October 20, 2016.

Norrlin, J. and S. Josefsson. 2017. Förorenade fibersediment i svenksa hav och sjöar. SGU-rapport 2017:07. Juni 2017.

Reible, D. 2014. Chapter 2. Sediment and contaminant processes. In: Processes, Assessment and Remediation of Contaminated Sediments. D. Reible (Editor). SERDP and ESTCP Remediation Technology Monograph Series, C. Ward (Series Editor). Published by Springer.

Rockne, K., P. Viana, and K. Yin. 2010. Sediment gas ebullition and flux studies, Bubbly Creek, South Fork Branch, Chicago River. Volume 1 and 2: Report with Appendices A-D. July 2010.

Russell, K. 2019. Consultant, Anchor QEA LLC., personal communication. January 2019.

Rutgers Environmental Research Clinic (RERC). 2005. Kearny Marsh restoration project preliminary report. November 23, 2005.’’