Sediment

 

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Sedimenten (lera, silt, grus, organiskt material, etc) som ackumuleras på botten av hav, sjöar, hamnar och andra vattendrag är en kritisk komponent för alla akvatiska ekosystem. Sedimenten kan utgöra både föda och habitat för bottenlevande organismer, vilka i sig utgör föda för andra organismer i det akvatiska ekosystemet. Ett hälsosamt akvatiskt ekosystem bygger därför på att det finns en hälsosam och ren miljö i sedimenten. Sedimenten är dock även slutstationen för många av de miljögifter som släpps ut. Förorenade sediment kan ha signifikant och långvarande påverkan på hela det akvatiska ekosystemet och kan spridas till nya områden genom erosion orsakad av både naturliga och mänskliga aktiviteter.

De senaste decennierna har utsläppen vissa ämnen till akvatiska miljöer minskat kraftigt i flera länder, framförallt tack vare strängare miljölagstiftning. Trots det finns föroreningarna i sediment kvar långt efter att utsläppskällorna (som oftast är landbaserade) har är borta. Vilket framförallt beror på att många av de föroreningar som finns i sediment antingen icke nedbrytbara (så kallade persistenta föroreningar), eller bryts ned väldigt långsamt. Föroreningarna kan då finnas kvar i årtionden, eller längre. Förorenade sediment finns i hela landet, och kan delas in i två huvudsakliga typer: Minerogena (mineralbaserade) och fibersediment (cellulosabaserade som ofta delas in i fiberrika sediment eller fiberbankar).

I det här dokumentet beskrivs rekommendationer för systematiska förberedelser och genomförande av undersökningar av förorenade sediment i Sverige. Processer för riskbedömning av förorenade sediment beskrivs inte men återfinns bland annat här (Naturvårdsverket 2006) och på Åtgärdsportalen för fibersediment samt här (Lst Västernorrland 2017).

För en större förståelse hur resultaten från undersökningar kan användas i arbetet att genomföra en åtgärd av ett förorenade område, vänligen se metodbeskrivningarna av förorenade sedimentÅtgärdsportalen.

STEPS IN SEDIMENT INVESTIGATIONS

An accepted, step-by-step process should be followed when preparing for and conducting any sediment investigation – regardless of its objectives, scale, or budget.

Following an established procedure will insure appropriate sediment and other site data are obtained to meet objectives in the most efficient and cost-effective manner possible and practical. Not following an established procedure typically results in a time-consuming and costly investigation that, in the end, often does not produce enough appropriate data to adequately meet project objectives.

Steps in the process of preparing for and conducting sediment investigations – as presented in multiple, international sediment guidance documents (US EPA, 2001; US EPA, 2002; US EPA, 1991; Ohio EPA, 2012; NAVFAC, 2005; Chapman, 2011; MDEQ, 2002; DEFRA, 2017) - are summarized in Figure 1.

planering

Figure 1. Steps to follow in preparing for and conducting sediment investigations.

In practice, investigations of contaminated media, including sediment, rarely follow a linear path, as depicted in Figure 1. As an investigation evolves and new information becomes available, the project team typically needs to ”return” to and re-assess one or more previous steps before moving forward again. This iterative nature of the investigative process is discussed in more detail in Section 11.

STEP A: ESTABLISH PROJECT OBJECTIVES

A general (non media-specific) discussion of the critical importance of establishing project objectives first thing is found here.

For contaminated sediment sites, investigations are conducted for many reasons to meet a wide variety of objectives. Some of the more common types and examples of objectives for conducting sediment investigations are presented in Table 1.

Table 1. Common types and examples of objectives for conducting sediment investigations.

Type of objective

To

Examples

Searching

Identify

·        Occurrence and types of sediment contamination present

·        Occurrence, types, and locations of past and/or ongoing contaminant sources, land and/or in-water

Delineation

Map

·        Occurrence and spatial extent of erosional versus depositional areas, including deposited sediment thicknesses

·        Lateral and vertical extent of sediment contamination, including types and concentrations

Characterization or Assessment

Describe and quantify

·        Properties of the in-place sediment bed (t.ex. slope, total thickness, layering, water content, bulk density, compressibility, strength)

·        Key sediment properties (physical, geotechnical, chemical, biological) to assist in remedy selection and design

·        Occurrence, types, and spatial distribution of benthic organisms and communities (benthic populations)

·        Bioavailability of sediment contaminants to benthic populations

·        Risks that sediment contaminants pose to benthic populations

Monitoring    over time

Identify and quantify

·        Changes or trends in sediment contaminant concentrations, including new inputs

·        Changes or trends in occurrence, types, and spatial distribution of benthic populations

·        Changes or trends in risks posed to benthic populations

·        Rates of sediment deposition or erosion, short- or long-term

 

STEP B: BUILD PROJECT TEAM

A general (non media-specific) discussion of building a project team and determining the level-of-effort needed for an investigation is found here.

An initial investigation for a relatively small sediment project should at a minimum include the following team members: a project manager, en miljövetare/miljöingenjör, and two field personnel experienced with sampling equipment and methods. I mindre projekt kan samma person vara projekledare, miljövetare och göra fältarbete. Det är ofta värdefullt om den som planerat provtagningen även är med i fält.

Larger-scale/bigger-budget sediment investigations may – depending on project objectives – additionally require one or more of the following team members: an aquatic biologist, ecologist, or toxicologist; a chemist or geochemist; a geologist; a statistician; a sedimentologist; a hydrologist or hydrogeologist; an oceanographer; someone skilled in GIS; someone familiar with laboratory protocols; and/or someone with working knowledge of potentially relevant and applicable regulations from the authorities (local, regional, national).

STEP C: IDENTIFY DATA NEEDED

Obtaining one or more types of sediment data is obviously central to any sediment investigation. But for most investigations, at least some additional non-sediment site data are also needed to meet investigation objectives.

  • SEDIMENT DATA: WHAT TO OBTAIN AND HOW

When identifying what sediment data are needed to meet investigation objectives, there are several issues to consider and decisions to be made by the project team, including:

  • Categories and specific types of sediment parameters to be measured.
  • Measuring sediment parameters using whole sediment or using porewaters.
  • Zones of the sediment profile to focus on: Surface, sub-surface, or whole-profile.
  • How to obtain sediment data: grab sampling, core sampling, or in situ
  • Equipment for collecting sediment samples: Grab or core.

5.1.1              Categories and types of sediment parameters

Categories and types of sediment parameters for which data are commonly obtained to meet investigation objectives are summarized in Table 2.

Table 2. Categories and types of commonly evaluated sediment parameters.

Sediment parameter category

Sediment parameter

Contaminant

·        Total concentration

·        Simultaneously extracted metals (SEM)

·        Non-aqueous phase liquids (NAPLs)

Physical

·        Total deposited thickness, including stratigraphy

·        Particle size distribution

·        Particle density (specific gravity)

·        Total water/solids content

Geotechnical

·        Bulk density (unit weight)

·        Shear strength (bearing capacity)

·        Cohesiveness

·        Atterberg limits

·        Compressibility (consolidation)

Chemical

(non- contaminant)

·        Total organic carbon (TOC)

·        pH

·        Oxidation-reduction potential

·        Oxygen content

·        Acid volatile sulfide (AVS)

·        Total sulfides

·        Nutrients (nitrogen, phosphorus)

·        Ammonia

·        Salinity, conductivity

·        Alkalinity, hardness

Biological

·        Benthic macroinvertebrate assessment (species, abundance, community structure)

·        Toxicity testing (bioassays)

·        Bioaccumulation testing

The sediment contaminants involved will vary greatly from project to project. Nevertheless, they often comprise one or more metallic and/or organic contaminants. Organic contaminants are usually hydrophobic, with relatively low water solubility, and will therefore usually strongly bind to organic material

NOTE: Throughout the remainder of Section 5.1, emphasis is placed on obtaining data for sediment contamination. In-depth discussions of obtaining other types of sediment data, e.g. physical, geotechnical, biological (Table 2), are beyond the scope of Undersökningsportalen. Nevertheless, the subject of obtaining non-contaminant sediment data is addressed to a limited degree herein, when and where appropriate.

5.1.2  Measurements using whole sediment or porewaters

Sediment contaminants are typically measured using whole (bulk) sediment, where all phases - solid, liquid, and gas - occur together. This is because such an approach is appropriate for meeting most investigation objectives. 

To meet some objectives, however, it is necessary to also or instead measure contaminants (or sometimes non-contaminant chemical parameters) occurring in sediment porewaters. One common example of this is quantifying contaminant concentrations that are freely dissolved in porewaters to assess contaminant bioavailiability to macroinvertebrate organisms living in surface sediments. See section 5.1.4 for methods to measure contaminants in porewater.

Unless explicitly stated otherwise, use of bulk sediment samples for measuring sediment contaminants is assumed throughout in the following sections.

5.1.3.             Focus on surface, sub-surface, or whole-profile sediment

Contaminant (and other) data can be obtained for surface sediment, sub-surface sediment, or the entire sediment profile.

”Surface sediment” is defined as the profile’s well-mixed (bioturbated) zone and is often called the biologically active zone (BAZ). Thickness of surface (BAZ) sediment varies between sites, but typically ranges between about 5 and 15 cm.

”Sub-surface sediment” is defined as the profile zone extending from the base of the BAZ down to the interface between water-deposited sediment and original, in-place geologic strata.

Project objectives determine which zone(s) of the sediment profile an investigation should focus on. For example (and referencing Table 1):

A focus on surface sediment is appropriate for many assessment, monitoring, or searching investigations because:

  • Assessments often address biological status and responses to contamination in the BAZ.
  • Monitoring surface sediment detects changes in depositional or erosional conditions.
  • Monitoring sediment deposition (using sedimentation pans or traps) identifies and quantifies new inputs of particle-bound contamination, including sediment input rates.
  • Testing surface sediment is a cost-effective way to search or “screen for” contamination presence.

A focus on sub-surface (or whole-profile) sediment is appropriate for many delineation and characterization investigations because:

  • Determining how deeply a profile is impacted is integral to fully delineating the lateral and vertical extent of sediment contamination.
  • Studera tidstrender, och därigenom datera sediment eller få en uppfattning om sedimentationshastighet, samt om belastningen minskar eller ökar.
  • Characterizing sediment conditions throughout all or most of the profile is required for remedy selection and design. In fact, contaminant, geotechnical, and other sediment characteristics relatively deeper in the profile often have a greater influence than surface-sediment conditions when deciding: (i) whether or not to dredge and, if so, how deep, (ii) how dredged material should best be managed, usually based on its contamination status, or (iii) when determining how thick/heavy of a cap the sediment bed can physically support.

Zone(s) of the sediment profile on which a sediment investigation focuses has a controlling influence on the approaches that can and should be used to obtain sediment data.

5.1.4              Obtaining sediment data

Contaminant and non-contaminant sediment data (Table 2) can be obtained using different sampling methods in combination with different approaches for measurement.

Use of coring or grab methods first involves collecting either type of sediment sample from its original deposited location then measuring parameters remotely - that is, measuring parameters “ex situ”. The collected samples can either be relatively mixed (disturbed) or relatively in-tact (undisturbed). The degree of sample disturbance depends on the equipment used for its collection. In-depth decriptions of equipment for grab and core sampling are found here. Except grab or core samplers there are also other type of samplers or sampling methods, which are described here.

In contrast to the above, “in situ” sediment data are obtained in the field by directly measuring parameters within in-place, undisturbed sediment occurring in its original deposited location.

 Grab sampling

Most grab samplers provide relatively physically disturbed samples av ytsediment, compared to core samplers.

Eftersom proverna är relativt störda kan gripprovtagare användas för att ta upp prov för t.ex. undersökning av medelhalten föroreningar i de ytliga sedimenten. I vissa fall kan gripprovtagare behöva användas då sedimenten är för hårda för att provta med en rörprovtagare, men ger även då ett stört prov.

For sub-surface sediment, use of grab samples for measuring contaminants is usually not possible. This is because hand-operated grab samplers are typically only able to collect samples down to a maximum depth of about 10 to 15 cm below the sediment surface (plus, the depth interval of collection is often unclear). If contaminant data are needed for sub-surface sediment, core sampling is typically required. Much larger, winch-operated grab samplers are available for collecting larger bulk sediment samples from deeper in the profile, as described here. However, as with smaller, hand-operated grab samplers, such larger-scale grab samplers are collecting from an relatively unknown depth interval.

Core sampling

In contrast to grab samplers, box-core and especially cylindrical (e.g. acrylic tube) samplers are commonly used to provide relatively physically undisturbed sediment samples. Examples of core samplers are described [here].

Relatively undisturbed core samples are needed when maintenance of three-dimensional (3D) – and especially vertical - sediment integrity is critical for investigating contamination levels in separate layers, as when determining certain geotechnical parameters – namely bulk density, shear strength and compressibility (Table 2). Such samples are also often needed to quantify long-term sediment deposition rates by radioisotope profiling, e.g. 210Pb and 137Cs isotopes. Further discussion of sampling for these and other geotechnical and physical sediment parameters is beyond Undersökningsportalen’s scope. Nevertheless, discussions on the use of cores and coring techniques to obtain such non-contaminant sediment data can be found elsewhere (e.g. Fuchsman et al., 2014; Magar et al., 2009; Erickson et al., 2007).

If only needing sediment mass for measuring contaminants, a sediment core can give a very good picture how the contamination levels are vertically distributed with sediment depth. Such an approach gives information on both the newly added contaminants to the sediment surface and on the historically added contaminants which are buried deeper in the sediment. Depending on sample mass needed for contaminant analysis, and on core diameter, multiple cores may need to be collected and depth-discrete sections composited to obtain a large enough sample size for analysis.  

In some cases, relatively undisturbed sediment samples may be needed for making some contaminant measurements. A good example of this is when installing a vertical series of passive samplers down into a sediment core to provide “pseudo” in situ data on dissolved contaminant concentrations in porewaters at discrete depths within surface and sub-surface sediment (e.g. US EPA, 2017).

Ex situ measurements

Ex situ measurements can be made on whole (bulk) grab samples or portions of core samples using a couple different approaches.

By-far the most common approach for measuring sediment contaminants ex situ involves submitting samples to a qualified commercial laboratory for analysis. Detailed procedures for contaminant-specific laboratory analysis, including the pre-analytical steps often required (sample mixing, compositing, sub-sampling, etc.), are described elsewhere (e.g. US EPA, 2001, 2002; Ohio EPA, 2012; CSIRO, 2016).

Kontakta labbet innan provtagning för info om hur prov ska levereras och vilka kärl som behövs för de planerade analyserna samt hur mycket material som behövs. Anlitat labb ska vara ackrediterad enligt de standarder som är godkända i Sverige. Om det är viktigt hur labbet förbereder provet så måste man kontakta labbet och förhöra sig om det är möjligt. För info om metodik och provberedning kontakta det aktuellt labb.Another approach for obtaining ex-situ data involves field personnel making measurements while still on-site. Portable meters and field test kits are available for measuring a wide variety of sediment contaminants (and other, non-contaminant chemical parameters). In-depth discussions of field-based analytical methods for measuring selected chemical parameters are found here. On-site contaminant measurements may need to be combined with measurements made by a qualified laboratory. Nevertheless, using such field-based methods during preliminary site investigations (see Step F, Section 8) to obtain more-or-less real-time data can be a very useful and cost-effective tool for, t.ex., quickly screening sediment contamination status at a site.

In addition to analyzing bulk sediment samples, ex situ measurements can also or instead be made of contaminant concentrations dissolved in sediment porewaters. This can be accomplished using a couple different techniques.

technique involves using passive samplers, which also can be used in situ. Passive samplers are plastic materials inserted directly into a saturated bulk sediment sample and left in-place, during which dissolved contaminants slowly (passively) sorb into the sampler. After some time period for equilibration, the reacted sampler is then removed from the sediment, the contaminants chemically extracted from the sampler and concentrated, and the concentrate analysed. Further discussions of passive samplers and their use in sediment investigations can be found elsewhere (e.g. US EPA, 2012, 2017).

One technique involves using passive samplers, which also can be used in situ. Passive samplers are plastic materials inserted directly into a saturated bulk sediment sample and left in-place, during which dissolved contaminants slowly (passively) sorb into the sampler. After some time period for equilibration, the reacted sampler is then removed from the sediment, the contaminants chemically extracted from the sampler and concentrated, and the concentrate analysed. To-date, passive-sampler technologies have been developed and used mainly for hydrophobic organic contaminants (e.g. PCBs, PAHs, TBT, etc.), although progress is being made on samplers for metallic contaminants as well. Further discussions of passive samplers and their use in sediment investigations can be found elsewhere (e.g. US EPA, 2012, 2017).

Another technique involves first physically separating porewater from sediment solids (e.g. by centrifugation) then analysing contaminant concentrations in the isolated porewater; this approach assumes dissolved concentrations are high enough (relative to the analytical detection limit) to measure directly. But in cases where contaminant concentrations are relatively low, passive samplers can instead be placed into the isolated porewaters, as a means of concentrating the contaminants before analysis.

In situ measurements

In contrast to using collected grab or core samples to obtain ex situ measurements on bulk sediment or isolated porewater, in situ measurements made of sediment parameters in the field offer the lowest degree of physical disturbance possible.

Further discussions on equipment and techniques for in situ measurement of physical and geotechnical sediment parameters – e.g. using cone penetrometers or similar devices to measure deposit thickness, bulk density, shear strength, and/or compressibility - can be found elsewhere (Curtis et al., 2018; Dorvinen, 2016; Anchor QEA, 2011; CH2MHill, 2005; Palermo et al., 2004).

In situ techniques for use in investigations of contaminant and other chemical or biological conditions within in-place sediment deposits are described here.

Passive samplers (discussed above) can also be installed and used in the field for directly measuring contaminant concentrations in porewaters of in-tact surface and sub-surface sediments. There are multiple project examples where this approach has been used (Oen et al., 2011; US EPA, 2012, 2017).

Summary

Deciding on the most appropriate combination of approach(es) for obtaining data on sediment contamination – grab sampling, core sampling, ex situ measurement, and/or in situ measurement – should be based mainly on investigation objectives.

However, other factors also need to be considered when making such decisions, including: the specific contaminants-in-question, project size and scale, available resources, certain site conditions (mainly water depth and flow velocity), and what is (and is not) technically possible in terms of sampling equipment and methods.

     

5.1.5.             Equipment for collecting sediment samples: Grab or core

In-depth discussions on a variety of commonly used grab and core samplers, including the relative advantages and limitations associated with each sampler, are found here.

  • NON-SEDIMENT SITE DATA

Many sediment investigations require certain types of non-sediment site data to facilitate meeting objectives. Categories and types of such non-sediment site data are presented in Table 3.

Table 3. Categories and types of non-sediment site data.

Site parameter category

Parameter

Contaminant sources

·        Occurrence of land-based (primary) and/or in-water (secondary) sources

·        Source locations

·        Past versus ongoing sources

Surface-water characteristics

·        Water body configuration

·        Water depths, bathymetry

·        Flow dynamics (directions, patterns, normal and peak flow velocities)

·        Natural and human erosional forces (waves, ice scour, ship traffic)

Sediment-bed characteristics

·        Channel occurrence, location

·        Occurrence, lateral extent of erosional (hard-bottom) versus depositional areas

·        Types, locations of depositional areas (coarser- versus finer-grained)

·        Occurrence, lateral extent, and thickness of finer-grained sediment deposits

Biological conditions

·        Occurrence, types, spatial distribution of benthic populations

Some site data may already be available at the time of investigation planning. Some data may instead need to be obtained as part of the investigation scope. In the later case, data should be obtained as soon as possible, so the information can be used to help direct investigation planning.

Not all categories and types of non-sediment site data will be needed for all sediment investigations. As described below, requirements for one or more types of site data will depend on investigation objectives, project scale, and available resources. 

5.2.1              Contaminant sources

For all investigations, any existing information related to confirmed or suspected contaminant sources should always be identified and obtained when possible, and as soon as possible.

Having existing knowledge in-hand on contaminant source types and locations early on during investigation planning:

  • Helps identify sediment contaminants likely to occur at a site, if not known already.
  • Helps guide construction of a useful, sediment-focused conceptual site model.
  • Facilitates developing an appropriate overall strategy for obtaining need-to-know sediment contaminant data.

Sometimes very little to no information exists on contaminant sources at a site. In these cases, gathering sediment data and other information for the purpose of identifying types and locations of contaminant sources may actually be a project’s main objective, such as during some searching investigations.

Surface-water characteristics

For all investigations, having data in-hand on water depths and flow velocities expected at the site during field campaigns is essential. This is because maximum water depth has a significant influence on selecting appropriate sampling equipment. For example, use of some smaller, hand-operated grab- or core samplers is not practical or efficient in relatively deeper, and especially flowing, waters (this is discussed in more detail here). Also, difficulties in collecting hand-push core samples of surface sediment by divers increase with greater water depth.


For some investigations, additional detailed data may be needed on site bathymetry, flow dynamics, and other erosional forces. This is often the case for larger-scale/bigger-budget projects, or during later stages of even smaller-scale/lower-budget projects, when data are needed to support decisions related to remedy selection and design.

For many investigations, detailed data on certain surface-water characteristics – especially flow dynamics and other erosional forces – is usually not necessary. Instead, it may be adequate to simply have a general understanding of such surface-water characteristics and instead focus time and resources on investigating characteristics of the sediment bed, since the sediment bed integrates, and more-or-less “sums up”, flow dynamics and net-erosional forces collectively acting at the site over time.

Sediment-bed characteristics based on surface-water characteristics

Sediment beds beneath constantly high-flowing waters, e.g. in river channels, tend to be erosional and hard-bottom (free from deposited sediments) whereas beds beneath relatively lower-flow, lower-energy waters tend to be depositional. Coarser-grained sediments, like sands and gravels, often deposit in shallower waters, where flows are relatively higher. In contrast, finer-grained sediments, including silts, clays, and organic matter, deposit in svackor eller deeper waters, where flows are usually lowest.

The range in surface-water characteristics and thus sediment-bed characteristics occurring at a site depend greatly on where the site is set. Rivers and streams tend to be much more spatially heterogenous in both respects than lakes and ponds. Additionally, the size of the site, including where investigation boundaries are defined, will also have a strong influence on spatial uniformity of the bed. For example, even a relatively small site set in a river may display significant bed heterogeneity, whereas a much larger site set in a lake may not.

Knowing if and where depositional areas occur at a site, and also grain-size characteristics of the depositional areas, is critical to developing an accurate conceptual site model for use in guiding sediment investigations.

Sediment-bed characteristics can be investigated or inferred using various methods which range broadly, both in terms of level-of-sophistication and cost. Such methods may include:

  • Inferring likely occurrence and locations of depositional-versus-erosional areas based simply on above-water visual inspection of water-body size, lateral dimensions, and general characteristics of the adjacent upland landscape. Experienced field personnel are often highly skilled in this art, much in the same way an experienced hunter is highly skilled in ”reading” animal tracks in the snow.
  • Desk-top review of available information, e.g. reports or maps previously developed and published by consultants, authorities, or institutes.
  • Systematically probing the sediment bed from a boat using a long metal or pvc pole, or a grab sampler.
  • Conducting transect-based geophysical surveys using acoustic sub-bottom profiling and also perhaps side-scan sonar techniques, see Section 5.1.4.3.

Biological status

Having information already in-hand on a site’s biological (benthic) conditions is not necessary for meeting objectives for many sediment investigations. For example, meeting an objective of delineating and mapping the vertical/lateral extent of sediment contamination present does not require knowledge on benthic conditions.

However, as in the case of identifying and locating contaminant sources, collecting information on the occurrence, types, and spatial distribution of benthic populations present may in fact be the main objective for some sediment investigations.

 

STEP D: COMPILE AND REVIEW EXISTING DATA

A general (non media-specific) discussion of compiling and reviewing existing site data is found here.

The project team can potentially save considerable time and resources by investing a little up-front time trying to track down and obtain any site-specific information that may already be available and useful to the current investigation. Information can be provided by multiple authorities and agencies (including kommun, lst, ebh-stödet, smhi, sgu, sgi, and hav). Furthermore, the types of information provided can vary greatly, and can include consulting reports, site inspection summaries, water-quality surveys, etc. Informal conversations with past site owners or facility employees can also often provide very useful information.

Once the team has obtained and reviewed available data, they will need to answer the following types of questions:

  • Have sediment conditions changed significantly over time? For example, contaminant concentrations in surface sediment measured in the 1980s in an area of a lake known or suspected to be highly depositional may not be so useful today.
  • Are laboratory analytical detection limits from earlier investigations still useful today?
  • Har strömningsförhållandena förändrats över tid och i så fall hur?
  • STEP E: SET INVESTIGATION BOUNDARIES

A general (non media-specific) discussion of setting investigation boundaries is found here.

For sediment investigations in particular, setting what are in essence legal lateral boundaries of a particular site can be especially complicated, since it is sometimes unclear who technically owns such ”water-side” property, much less where in-water property lines are specifically drawn (assuming they exist). This is why everyone directly or indirectly involved – including the project team, the property owner, adjacent property owners, and all relevant authorities – must have a clear understanding of, and be in agreement on, where lateral boundaries for a sediment investigation at a site are to be set and why. Note, setting legal lateral boundaries of a site is completely separate from the issue of establishing the lateral extent of sediment contamination across an area (which obviously will not follow any such human-made construct).

Vertical boundaries for a sediment investigation must also be set and agreed to by all parties. The total sediment depth to be evaluated will determine what types of sampling equipment are appropriate for use in the investigation. In some cases, and if the project budget allows, it may be wise to collect “contingency” core samples beyond (deeper than) the currently needed total sediment depth. Doing so may, in the end, be more cost-effective if/when such deeper samples are eventually required for the project.

Lateral and vertical boundaries will determine the shape, dimensions, area, and volume of sediment to be investigated. These aspects, in combination with locations of known or suspected contaminant sources, will influence design of the strategy used for obtaining data on sediment contamination. In practice, lateral and vertical boundaries often need to be adjusted somewhat, based on field observations and/or results from follow-up investigations.

Temporal boundaries should also be considered for sediment monitoring investigations. For example, there needs to be clear agreement on how many years contaminant concentrations in surface sediment will be monitored at a site, as well as how frequently monitoring will occur during any given year and who is going to pay for such monitoring over time. 

Step F: CONDUCT A PRELIMINARY SITE INVESTIGATION

A general (non media-specific) discussion of conducting preliminary site investigations is found here.

A preliminary sediment investigation need not require extensive time or resources. On the contrary. Using a small boat and some inexpensive field equipment and techniques, two experienced field personnel can – during a one-day site visit – collect a wide variety of very useful sediment and other site data. The types of information that can be collected during a preliminary site investigation is summarized in Table 4.

Table 4. Sediment and other site data that can be collected during a preliminary site investigation.

Equipment or technique

Information collected

Measuring tape

·        Water depths

Sonar (the type fishermen use)

·        Water depths, bathymetry, presence/absence of channels

·        Some models also provide information on general sediment-bed conditions (hard bottom, relative grain size of surface sediment)

Bottom probing with long metal or pvc poles

·        Presence/absence of hard-bottom versus water-deposited sediment

·        General grain-size characteristics of sediment deposit (sand versus silts and clays). Inferred by sound and ”feel”

·        Thickness of water-deposited sediment

Hand-operated sediment grab sampler

·        Presence/absence of hard-bottom versus water-deposited sediment

·        Grain-size characteristics of water-deposited surface sediment (by visual and manual inspection)

·        Provides samples for contaminant screening

Field test kits

·        General data on contaminants in surface sediment, d.v.s. the type(s) present, concentration ranges, and locations at the site

Camera

·        Overall visual documentation of sediment and site conditions

Footnotes:

  1. Measuring tape such as those typically used to measure water-surface elevations in groundwater monitoring wells.
  2. Multiple types, brands, and models of fish-finder sonars, grab samplers, and field test kits are commercially available.

Visual inspections during a preliminary site investigation will also provide valuable information on water-body characteristics, including flow conditions and where depositional areas may most likely be located. Presence of depositional areas can then be confirmed using techniques or equipment summarized above.

Making a preliminary on-site visit also answers some even more basic questions related to practical aspects of site access, for example: Are there locked gates for which we need keys? Does the field crew need permission to cross property not owned by the client? Is there an adequate and nearby shoreline location for boat launch and take-out? Where is the nearest hospital?   

Obviously, conditions at some sites may limit use of one or more types of equipment or techniques. For example, water depths greater than approx. 4 to 5 m may preclude use of physical bottom probing with metal or pvc poles (although deeper probing may be possible with a grab sampler). Furthermore, faster-flowing waters may make grab sampling of surface sediments a challenge.

STEP G: DEVELOP A SEDIMENT-FOCUSED CONCEPTUAL SITE MODEL

A general (non media-specific) discussion of conceptual site models (CSMs) is found here.

At contaminated sediment sites, adequately understanding and accurately describing the complicated connections or links between contaminant sources → spreading processes → receptors is usually anything but a simple and straight-forward exercise. Reasons for this include:

  • Multiple contaminant sources often occur, both land (point and non-point) and in-water (sediment) sources.
  • Contamination sources are often geographically separated from one another.
  • Contaminant sources are often geographically separated from where contaminated sediment deposits occur.
  • Over time, contaminated sediments can erode, migrate, and re-deposit in other areas.
  • The aquatic forces controlling dynamics of sediment spreading (erosion, transport, re-deposition) at a site can change over time.

Regardless of the complexities, several assumptions can be made to first simplify construction of a sediment-focused CSM then facilitate its practical use in designing an appropriate strategy for obtaining sediment data. Such assumptions include: 

  • Sediment contaminants tend to concentrate in finer-sized sediment fractions, along with reactive materials like organic carbon, clays, oxides, and sulphides.
  • Focus on spreading dynamics of finer-grained, not coarser-grained, sediment.
  • Focus on depositional areas, specifically where finer-grained sediment deposits dominate.
  • The final, ”→ receptors” link is obviously a critical aspect of a CSM. Nevertheless, for current purposes, adequately understanding and describing the spatial link between contaminant sources and deposited contaminated sediment is most relevant. Thus, we focus herein on the first link – namely the ”contaminant sources → spreading” part – of a sediment-focused CSM.

Figure 2, referred to as the graphical portion of a “site-wide” CSM (Magar et al., 2009), illustrates the types of spatial relationships that can occur between a contaminant source (e.g. a factory), finer-grained deposits of contaminated sediment occurring in lower-energy ”backwater” areas (left), and the adjacent higher-flow, higher-energy river channel (center).

CSM Magar2009

Figure 2. Example of the graphical portion of a “site-wide” CSM (from Magar et al., 2009).

Step H: DESIGN AN OVERALL SAMPLING STRATEGY

Many different factors need to be considered, weighed, and balanced in designing an appropriate site-specific strategy for collecting data on sediment contamination. These factors include:

  • Overall project scale and available resources
  • Total size and configuration of site
  • General site characteristics
    • Aquatic setting (river, lake, bay), water depths, flow velocities
  • Available information on
    • Contaminant sources, d.v.s. locations, primary, secondary, past, ongoing
    • Finer-grained (possibly contaminated) sediment deposits, d.v.s. locations, sizes, thicknesses
    • Spatial positions of source locations relative to deposit locations.
  • Type(s) of contamination data needed (bulk sediment? porewater? both?)
  • Zones of the sediment profile to be investigated
    • Surface, sub-surface, and/or whole-profile
  • Approaches, including sampling equipment, to be used to obtain sediment data
    • Grab sampling, core sampling, and/or in situ measurements

General (non media-specific) discussions of designing an overall sampling strategy are found here.

Table 6 presents a few example scenarios for selecting different probability-based or targeted sampling designs specifically for contaminated sediment sites. Terminology clarifications: Probability-based in general = ”random” designs, probability-based systematic = ”grid” designs, and biased = ”targeted” or ”judgmental” designs.  

Table 6. Example scenarios for selecting sediment sampling designs (adapted from US EPA, 2001).

If you are.....

and you have.....

consider using.....

in order to.....

conducting an initial screening investigation and you know that the spatial scale of contamination is relatively small

a limited budget and/or a limited time schedule

targeted sampling

assess whether further investigation is needed using a probability-based sampling design.

wanting to determine where sediment contamination occurs, and not

adequate budget for the number of samples needed

grid sampling

have coverage of the area of concern, and have a certain level-of-confidence you would have detected a ”hot spot” of a certain size.

wanting to estimate the average concentration of contamination X in surface sediments

adequate budget

grid sampling

also produce information on spatial patterns.

knowledge of spatial contaminant patterns

stratified sampling

increase precision of the estimate with the same number of samples, or achieve the same precision with fewer samples and lower cost.

delineating lateral boundaries of a contaminated area

a field screening method

stratified sampling

simultaneously uses all observations in estimating the average contaminant concentration.

In general, and as discussed elsewhere (e.g. US EPA, 2001, 2002):

  • When locations of both contaminant sources and finer-grained sediment deposits are known, biased (targeted or judgmental) sampling designs may be more appropriate.
  • When locations of contaminant sources are uncertain or unknown, but locations of finer-grained sediment deposits are known, stratified (probability-based or random) sampling designs may be more appropriate. The ”strata” concept in this context applies laterally (sub-areas of finer-grained deposits, coarser-grained deposits, or hard-bottoms) and/or vertically (different zones of the sediment profile).
  1. When a sediment site consists entirely of finer-grained sediment deposits - all of which could be contaminated, to some degree – a simple (probability-based) sampling design may be most appropriate.

STEP I: EXECUTE SAMPLING STRATEGY

General (non media-specific) discussions of executing the sampling strategy are found here.

As for investigations of contaminated soil or groundwater, the process of preparing for and conducting sediment investigations is, in practice, almost never linear, as conceptually shown in Figure 1. Instead, the process is iterative in some way. For any given sediment investigation, there are countless reasons that could motivate the project team to return to earlier steps in the investigative process.

A few examples of some of the more common motivations for altering course when conducting sediment investigations include the follows:

  • When identifying the contamination and other site data needed (Step C), it may become obvious that the original investigation objectives (Step A) are too broad and ambitious for the time and resources available. This motivates a return to Step A and re-consideration of objectives.
  • It may have originally been assumed that the site sediment bed comprised a patchwork array of conditions, d.v.s. finer-grained sediment deposits, sandy deposits, and hard-bottom areas adjacent to one another. However, results of the preliminary field investigation (Step F) reveal the entire site consists of fine-grained sediment, all of which could potentially be contaminated to some degree. This motivates a return to Step B and re-consideration of the level-of-effort and resources required.
  • Results of the preliminary field investigation (Step F) may reveal the un-expected presence of extensive aquatic vegetation, sunken timber, or other debris across much of the sediment bed, which may preclude use of selected equipment for collecting grab and/or core samples. This motivates a return to Step C and re-consideration of how best to obtain sediment contamination data.
  • The search for existing site information (Step D) may disclose past occurrence of a previously unknown contamination source very close to the site. Thus, “new” contaminants now need to be added to the list of contaminants of concern requiring investigation. This motivates a revision of the preliminary field investigations (Step F) and, if contaminants are present and at significant concentrations, a re-evaluation of investigation objectives (Step A).

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