HEALTH CONSULTATION
Sediment Operable Unit
ST. LOUIS RIVER/INTERLAKE/DULUTH TAR
NATIONAL PRIORITY LIST (SUPERFUND SITE)
ST. LOUIS, ST. LOUIS COUNTY, MINNESOTA
The St. Louis River Interlake-Duluth Tar (SLRIDT) Site (the site) is a National Priorities List (Superfund) site on the St. Louis River in Duluth, Minnesota. Beginning in about 1904, pig iron, coke, and gas were produced at this site from iron ore and coal.
Over the years, different areas of the site have been open water. Currently the site contains three embayments (see Attachment #1): Stryker Bay (approximately 35 acres), Hallett Dock Company Slip #6 (Slip #6; approximately 23 acres), and Hallett Dock Company Slip #7 / Keene Creek (Slip #7; approximately 27 acres); as well as about 130 acres of land between the inlets. The site lies about four miles upstream from the St. Louis River's outlet into Lake Superior. It is bounded by residential and light industrial areas to the north; residential areas to the west; by the St. Louis River to the south; and by a shipping dock to the east.
Approximately 960 and 4200 people live within 0.5 mile and 1 mile of the site, respectively (2000 census).
Stryker Bay, the westernmost boundary of the site, extends about one-half mile north-south and about 1000 feet in the east-west direction. Typically, within 100 feet of the shoreline, the depth of the bay drops to approximately 4 feet, and the depth across most of the bay remains a fairly constant 4 - 6 feet. Stryker Bay is shallower at the northern end and forms a small wetland at the northern tip. Storm sewers and a small stream feed this wetland. Stryker Bay is not in a shipping area. Furthermore, there are no commercial docks for large lake vessels upstream of Stryker bay on the St. Louis River. There are a number of residences along the western shore of the bay. Some of these residences have private, recreational docks in the bay. The bay is intermittently used for recreation, including swimming, water-skiing, and fishing.
Slip #6 is located in the middle of the site and extends approximately one-half mile north from the St. Louis River , with a relatively uniform width of 250 feet. The Hallett Dock Company currently uses Slip #6 as a loading facility for ships traversing the Great Lakes, as well as ocean-going ships.
The western portion of the Slip #7 / Keene Creek (Slip #7) embayment is part of the site. Slip #7 has been used in the past as a loading facility. The eastern side of Slip #7 is currently used for docking barges. Keene Creek has been redirected so that it no longer flows into Slip #7. Area residents have been observed fishing in this bay.
Seiches (a sudden rise in water level), caused by wind forces shifting water in Lake Superior, can cause 7.9 hour periodic water level oscillations of 3 centimeters (cm) to 25 cm in the St. Louis Harbor area. This oscillation is considered the driving force for mass transport in the lower St. Louis Harbor (Stortz and Sydor, 1980). Near the site, seiches can cause water level fluctuations of up to 15 cm. As the water level changes, relatively strong currents can be expected in narrow areas like the mouth of Stryker Bay (MDNR, 2001). These currents will increase mixing and turbidity and affect the sedimentation process. Furthermore, large seiches can result in a reversing of St. Louis River flow. For example, upstream flow (26.5 cm/sec) at the site of the old Arrowhead Bridge (1/2 mile downstream from the site) is calculated to be similar to flow downstream (28.2 cm/sec) when a 15 cm seiche is modeled (Stortz and Sydor, 1980). Therefore, contamination from this site can be assumed to be dispersed upstream, as well as downstream from the site.
Concerns presently being addressed at this site are related to contaminated sediments associated with historic industrial activities. The forum for addressing these concerns is a Peer Review process that was created by the Minnesota Pollution Control (MPCA) Citizens' Board in November 1999. The Peer Review Team (PRT) includes experts in groundwater, dredging, capping and pricing of remedial alternatives. Data is presented by the MPCA and the potentially responsible parties (PRPs) to the PRT for review. At the request of the MPCA, MDH is also reviewing documents presented to the PRT and attending PRT meetings. An extensive amount of study has been conducted by the PRPs to provide information about the effectiveness and cost of proposed remedies. This health consultation reviews the product of those studies: the Data Gaps Report (Service, 2002).
MDH is specifically concerned about potential human exposure to contaminants at the present time, during any cleanup activities, and in the future. Therefore, our recent questions have been on data gaps that may significantly affect estimates of human exposure to contaminants or the longterm effectiveness of remedial actions. MDH has previously raised concerns about the cleanup of these sediments: in testimony to the Minnesota Pollution Control (MPCA) Citizens' Board (Attachment #2, (MDH, 1999)); and in a health consultation (MDH, 2001b), written in cooperation with the U.S. Agency for Toxic Substances and Disease Registry (ATSDR).
One of the focuses of the Data Gaps Report (Service, 2002) is the potential public exposure to contaminants that would be emitted during dredging of contaminated sediments. This health consultation is a review of experimental data, the proposed emissions model and conclusions of the Data Gaps Report. In addition, this Health Consultation contains a brief review of chemicals of concern; identifies routes of potential exposure; reviews concerns that are dependent on the choice of remedial option, and; provides MDH recommendations.
Human health may be adversely impacted when people are exposed to significant amounts of toxic chemicals. Because individual chemicals behave differently in the environment, potential routes of exposure are chemical dependent. Therefore, quantification of human health risks associated with contaminated sediments may require analysis of several routes of exposure. These include ingestion of chemicals partitioned into sediments or surface water; dermal exposure to chemicals in sediment or surface water; inhalation of volatile chemicals partitioned first into surface water and then into air, and/or; ingestion of fish or other foods contaminated by chemicals. If exposures are limited by removal of contaminants from areas where exposures may occur or by creating barriers to exposure, the probability of adverse health effects is also limited.
Chemicals of concern
Previous MDH documents have identified polycyclic aromatic hydrocarbons (PAHs) and mercury as the primary chemicals of concern for human health at the SLRIDT site. Polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), octachlorostyrene (OCS), and hexachlorobenzene (HCB) are also potential chemicals of concern at this site and most contaminated aquatic sites in the St. Louis River area. A survey for these 5 chemicals has not been conducted at SLRIDT. All 7 of the above listed chemicals and chemical groups are addressed in greater detail in the 2001 MDH Health Consultation (MDH, 2001b).
The chemical listed above are of concern not only because of potential risks associated
with exposure to them at this site, but also because they may be transported offsite into
the St. Louis River and then to Lake Superior. Canada and the United States have
developed a Binational Toxics Strategy (BNS) "...toward the goal of virtual elimination
of persistent toxic substances resulting from human activity, particularly those which
bioaccumulate, from the Great Lakes Basin, so as to protect and ensure the health and
integrity of the Great Lakes ecosystem" (http://www.epa.gov/glnpo/p2/bns.html 
). The
chemicals of concern are all Level I substances targeted by the BNS.
Naphthalene and similar PAHs
MDH has previously reviewed polycyclic aromatic hydrocarbon contamination data at this site and discussed toxicity issues (MDH, 2001b). PAHs are a class of generally hydrophobic, non-polar organic compounds that result from the burning of organic materials. They exist in mixtures in the environment. These mixtures have been shown to have both carcinogenic and non-carcinogenic effects on humans and animals. Historically, chemical analysis has been restricted to a handful of individual carcinogenic PAHs (cPAHs) and non-carcinogenic PAHs (nPAHs) out of the hundreds of PAHs found in the environment. However, in early 2000, MDH began recommending PAH analysis include an extended list of 25 cPAHs with available California Office of Environmental Health Hazard Assessment (OEHHA) potency slopes or potency equivalency factors (PEFs) (CA OEHHA, 2002). MDH finalized recommendations in a 2001 memo to MPCA (MDH, 2001a) and cited this memo in the 2001 Health Consultation on SLRIDT sediments (MDH, 2001b). Recent samples from SLRIDT have not been analyzed for these additional cPAHs (see below).
Naphthalene is the PAH of most concern during cleanup activities because it is very volatile and prevalent in sediments. Additional PAHs, primarily methylated naphthalenes, will have similar volatilities, but they do not appear to be as prevalent in sediments at this site.
Naphthalene is currently classified as an nPAH by EPA, but the International Agency for Research on Cancer (IARC) classifies naphthalene as possibly carcinogenic to humans (Group 2B) due to the evidence of carcinogenicity in laboratory animals exposed for long periods of time, at high concentrations (IARC, 2002). Current MDH criteria have been developed from data showing naphthalene to cause nasal and respiratory effects in rats exposed to low concentrations for 2 years, or exposed to much higher amounts for 4 hours. Using standard risk assessment methods, MDH has developed a shortterm (acute) health criterion of 200 micrograms per cubic meter (µg/m3), and a longterm (chronic) health criterion of 9 µg/m3 for naphthalene (Attachment #3; MDH, 2003). Exposures to air concentrations below these values are expected to be safe. A few people may be affected by naphthalene at concentrations around the odor threshold (between 200 and 440 µg/m3). It is likely that most people will not be affected by acute naphthalene exposures well above the acute criterion of 200 µg/m3.
Mercury
Mercury and methyl mercury are neurotoxins and exposures are of particular concern
during fetal and early postnatal development. The methylated form of mercury
accumulates in biota because it is readily absorbed and only slowly excreted. As a result,
concentrations of methyl mercury are the highest in animals at the top of the food chain.
Ingestion of fish tissue containing high levels of methyl mercury is the dominant
exposure to mercury for most people. The MDH Fish Consumption Advisory has more
restrictive advice for women of child-bearing age and children than for the general
population. In general, women of child-bearing age and young children are advised not
to eat walleyes greater than 25 inches in length, and to limit eating walleyes 15 to 20
inches from the lower St. Louis River to one meal a month or less. In addition, the
advice for the general population advises limiting consumption of walleye larger than 20
inches to once a month. This advice is somewhat less stringent than fish consumption
advice for the St. Louis River prior to 2001, and is based on changes in methods of
calculating fish consumption advice as well as data from a limited number of fish.
Limited sediment mercury data show that mercury in sediments in Stryker Bay is elevated above current background levels (e.g. REMAP, 1995). Furthermore, as discussed in the previous Health Consultation (MDH, 2001b), industrial activities at this site were, potentially, a large source of mercury to the local area and the region.
Mercury is of concern if it is converted to methyl mercury and enters the food chain. Methyl mercury accumulates in the food chain, with high trophic level fish having much higher concentrations of methyl mercury than sediments (USGS, 2001). Methyl mercury concentrations in sediments can change over the course of hours, days, months and years, because it is produced from inorganic mercury by bacteria (Gilmour et al., 1992) and it is reconverted to inorganic mercury biotically and abiotically (Matilainen and Verta, 1995; Sellers et al., 1996; Regnell et al., 1998). Therefore, changes in methylation or demethylation rates may have a greater impact on fish tissue concentrations than the amount of inorganic mercury in sediments.
The National Oceanic and Atmospheric Administration (NOAA, 1996) has stated:
In order to select an effective remedial alternative, it may be necessary to characterize the major pathways to receptors of concern and aspects of the aquatic system that enhance methylation and influence mercury availability.
The role of [mercury] speciation in determining concentrations in, and toxicity to, biota may need to be understood prior to attempts to control the geochemical cycling of mercury within a waterbody. Remediation attempts have been unsuccessful at sites where these factors have been ignored.
Additional chemicals of potential concern
MDH recommended in the 2001 Health Consultation (MDH, 2001b) that regular sediment analyses include an extended list of 25 carcinogenic PAHs (including 6 already analyzed and 19 additional cPAHs), and that sediments at the site be screened for a number of chlorinated organic chemicals (i.e. polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), octachlorostyrene (OCS), and hexachlorobenzene (HCB)). These additional chemicals of concern listed in Table #1, were not addressed in the Data Gaps Report.
Table 1. Recommended additions to chemical analysis list for sediments
|
Polycyclic Aromatic Hydrocarbons |
Chlorinated organics |
| Benzo[j]fluoranthene | Polychlorinated biphenyls |
| Dibenz[a,j]acridine | Polychlorinated dibenzodioxins |
| Dibenz[a,h]acridine | Polychlorinated dibenzofurans |
| Dibenz[a,h]anthracene | Octachlorostyrene |
| 7H-Dibenzo[c,g]carbazole | Hexachlorobenzene |
| Dibenzo[a,e]pyrene | |
| Dibenzo[a,h]pyrene | |
| Dibenzo[a,i]pyrene | |
| Dibenzo[a,l]pyrene | |
| 7,12-Dimethylbenz[a]anthracene | |
| 3-Methylcholanthrene | |
| 5-Methylchrysene | |
| 1-Nitropyrene | |
| 4-Nitropyrene | |
| 1,6-Dinitropyrene | |
| 1,8-Dinitropyrene | |
| 5-Nitroacenaphthene | |
| 6-Nitrochrysene | |
| 2-Nitrofluorene |
Potential routes of exposure
Six different potential routes of exposure to chemicals from contaminated sediments in the St. Louis River and at the SLRIDT site have been identified. Based on MDH sediment screening criteria, a limited number of important (driving) pathways were identified for each chemical of potential concern. Table #2 lists the potentially significant routes of concern for those chemicals.
Table 2. Significant Exposure Pathways of Concern (based on sediment screening evaluation)
| Chemical / Chemical Group |
Ingestion of chemicals in sediment | Dermal exposure to chemicals in sediment | Ingestion of chemicals in fish tissue | Ingestion of chemicals in water | Dermal exposure to chemicals in water | Inhalation of chemical vapors |
| Polycyclic aromatic hydrocarbons (PAHs) | * | * | * | * | ||
| Polychlorinated dibenzo-dioxins (PCDDs) | * | |||||
| Polychlorinated dibenzo-furans (PCDFs) | * | |||||
| Polychlorinated biphenyls (PCBs) | * | |||||
| Hexachlorobenzene (HCB) | * | * | * | |||
| Octachlorostyrene (OCS) | * | |||||
| Mercury / methyl mercury | * |
REMEDIAL ACTION-SPECIFIC ISSUES OF CONCERN
The Data Gaps Report (Service, 2002) contains reviews and reports of experimental data. Currently, two general remedial actions are being discussed for the contaminated SLRIDT sediments: dredging and removal of contaminated sediment to a contained disposal facility (CDF) - either onsite or offsite; or capping contaminated sediments in place. Dredging and capping have the potential to cause future exposures in very different ways.
Dredging
Dredging will move most of the contaminated materials to a structure (e.g., tomb) designed to isolate them from the environment. The Peer Review Team had lengthy discussion about the redeposition, or fallback, that may be expected during dredging (PRG, 2002). While there is evidence that dredging will leave contamination behind, MDH has been unable to locate any studies on the extent of "fallback" at dredging sites. The Data Gaps Report does not address this issue, and discussion of the magnitude of the problem and possible solutions is needed.
On the other hand, the movement of contaminants away from the dredging area can be somewhat controlled during dredging (e.g. by use of hydraulic dredging), and if necessary, controls can be added so that there is very little movement of these suspended materials offsite (National Research Council, 1997; pg 113-116). The biggest public health concern during dredging operations is that volatile contaminants (e.g. naphthalene) may escape from the water/sediment slurry and be emitted into the air (National Research Council, 1997; pg 112); and people can be exposed by breathing the contaminated air.
Capping
Capping of contaminated sediments will leave 100% of the contaminants on site, under a blanket of possibly 3-4 feet of clean materials. Long-term capping of contaminants in an aquatic environment, particularly a shallow environment, can be more difficult than entombment in an upland area. Water flow around or through contaminated sediments and a cap can move contaminants or erode a cap. In addition, application of a cap and compaction of underlying layers will displace porewater, and possibly non-aqueous phase liquid (NAPL). Such displacement could potentially cause a short-term release and introduce contaminants into the new overlying materials. If, as suggested in the Data Gaps Report, emissions outside of the dredged cell are greater than emissions in the active cell, then small disturbances of the sediment may have a disproportional effect on air emissions. In any event, it is expected that there will be a pulse of contaminants into the aquatic environment when a cap is applied.
The site is in an area with a vertical groundwater gradient toward the surface, and groundwater activity below, within and above contaminated sediments is of potential concern. Groundwater can potentially transport contaminants and it can also erode the cap. Of additional concern are frost or ice penetration into the cap and the effect of plant growth, including woody plant growth, in the cap. These activities can create channels through parts of a cap, and also break up the matrix of a cap. Such activities could increase the mixing and erosion potential. Proposed capping materials include silt, which would increase the ability of the cap to grow vegetation and the effects of ice intrusion, and sand, that would decrease plant growth and matrix-disrupting effects of frost on the cap. If the contaminants remain on site, monitoring and repair of the cap will be necessary for many generations, or essentially forever.
Discussion
If sediments are moved into a CDF, the footprint of contamination can be significantly reduced and future exposures can be limited or eliminated by controlling contaminant access and interactions with groundwater, surfacewater, frost, air, plants and animals with engineering controls. If contaminants are capped in place, the contamination footprint remains at its current size, and controls are generally limited to the top of the contaminated sediments.
If contaminants are left on site, monitoring releases will be necessary while the contaminants remain. Institutional controls have often proven to be ineffective (Environmental Law Institute, 1999). MDH is concerned that a cap or CDF may not be maintained, monitored and repaired, and contaminants may become a health hazard in the future. Therefore, MDH favors a remedial action that permanently restricts exposures to levels below health concern at this site.
Hybrid remedial designs have been discussed and MDH recognizes that combinations of dredging and capping may be considered. Unless the issues inherent to both remedies are addressed in a hybrid plan, hybrid remedial actions may fail to adequately limit risks.
Air emissions experiments and proposed model
Introduction
Concerns about emissions during dredging were discussed in a 2001 health consultation (MDH, 2001b). The health consultation recommended: "...monitoring of emissions should be conducted during the entire [dredging] operation and provisional plans should be prepared for further limiting emissions should emissions exceed levels of concern."
PAHs that volatilize from a slurry of sediment and water will be largely the light, sparingly soluble PAHs that have high Henry's Law Constants (Hk). These will primarily be naphthalene and methylated naphthalenes. Not only does the Henry's Law constant (an equilibrium constant) decrease for larger PAHs, but molecular diffusion of these larger molecules is slower, thereby decreasing the speed at which they traverse the boundary layers.
The Data Gaps Report Air Analysis reports on emissions experiments conducted in the laboratory and applies the resulting data to two models. The Data Gaps Report presents a model for air emissions during dredging, then uses emissions estimates from that model as inputs for the EPA ISCST3 air dispersion model. MDH has not reviewed the application of the air dispersion model to emissions data from this site. MPCA technical staff are better qualified to review air dispersion models. This review focuses on experiments and theory used in the Data Gaps Report to develop emissions estimates for dredging.
The greatest errors in modeling ambient air concentrations are usually the inputs into the dispersion model. When the source is contaminated sediments, errors can be in the estimates of sediment concentrations from which emission estimates are extrapolated, or errors in the model used to estimate emissions during dredging. Available data on PAH concentrations as model inputs for Data Gaps Report are very limited. For the purpose of modeling air emissions, Stryker Bay was divided into 15 dredging cells, each approximately 3 acres. There are no sediment data for 2 of the 15 proposed dredging cells in Stryker Bay. In addition, analytical data from a total of 38 point locations was used to characterize some 35 acres of the bay. Furthermore, data are not available on whether and where the contamination is in a non-aqueous phase liquid (NAPL). Therefore confidence in the emission estimates, even from a proven emissions model, would be limited.
Experimental data review
The Data Gaps Report contains experimental data for PAH and volatile organic compound (VOC) emissions from sediment slurries. Emission data are reported for four different slurries with targeted concentrations of 45% (0.45 kgsed/kgslurry), 8%, 8% and 1% solids by weight (i.e. different volumes). One of the 8% slurries was stirred throughout testing, while the remainder were stirred immediately prior to testing and allowed to settle out during the experiments. Slurries were made from a single homogenized sediment sample and diluted with St. Louis River water. The sediment used as the PAH and naphthalene source was taken from one of the most contaminated areas in Stryker Bay. The Data Gaps Report does not indicate whether the sediment used in the experiment contained NAPL.
The sediment slurry concentrations used to estimate emissions at different locations during dredging seem reasonable representations of sediment solids concentrations in dredged materials. However, the accuracy of the experimental slurry concentrations and factors which result in slurry concentration variation during dredging were not discussed in the Data Gaps Report, and MDH does not have experience with dredging operations. The 1% slurry, in particular, may be a very high estimate for suspended solid concentration in the vicinity of a dredging operation, and emissions from that slurry likely overestimate emissions in areas not being dredged. The National Research Council Report on contaminated sediments states that in most studies, suspended solids are less than 100 milligrams per liter (mg/L) except in immediate proximity to the dredging head, and, in most field studies, less than 10 mg/L has been measured 100 meters from the dredging head (National Research Council, 1997; pg 110). Therefore, while the model indicates that contribution of emissions distal to dredging is small relative to total emissions in the model, it actually exceeds emissions in the active cell (see Table #4). Basing estimates on emission rates from a 10 grams per liter (g/L) (1%) slurry in the inactive cells may overestimate suspended solids contributions from this part of the model by 2 to 3 orders of magnitude.
In addition because a very small amount of particulates are transported outside of the dredging area, naphthalene in water outside the active cell will be dissolved. Dissolved naphthalene volatilizes rapidly, so it is unlikely that significant amounts of dissolved naphthalene will remain in water far from the active cell. Therefore, the use of experimental data from a 1% slurry to describe volatilization over about 42 acres outside of a 3 acre dredge area, appears to be a large overestimation. Use of 1% slurry emissions data for cells bordering the active cell should provide a very conservative estimate of non-active emissions from those cells. Current emissions from the rest of Stryker would probably provide a reasonable emission rate estimate for the non-adjacent cells in Stryker Bay.
Experiments described in the Data Gaps Report Air Model appendix were intended to help quantify naphthalene emissions from dredging operations and a CDF. Emission rates from sediments were estimated using an experimental apparatus that allowed a steady air stream to flow over sediment / water slurries and subsequent air sample collection for analysis. In this manner it is possible to measure potential air emissions under controlled conditions.
It is critical to an experiment that the initial quantities of the chemicals of interest are known. In addition in complicated experiments where errors can occur, it is useful to measure the amount of chemical in different compartments (e.g. sorbed and dissolved) at the beginning and end of the experiment. Minimally, the amount of chemical in the sediment slurries at the beginning of each experiment and the amount remaining at after each experiment should have been measured. In this way the mass balance of an experiment could have been checked. Monitoring mass balance is extremely important in experiments with multiple physical compartments and a potential different preparation of samples or volatile losses.
In the Data Gaps Report air emissions experiments, not only was no mass balance performed, the actual quantity of chemicals available in the slurries prepared for the experiments is uncertain. While Data Gaps Report Table (DGR Table) A1-1 shows a reasonable match between the targeted and measured solids composition of the slurries, DGR Table A2-3 shows a very poor agreement between their targeted chemical composition of the slurries and the measured chemical composition of the slurries. Because water used to make up the slurries contained very little PAHs, the sediment fraction should initially contain all the PAHs in the slurries. Furthermore, the PAH concentration in the sediment (solid) fraction should be the same for all slurries. A total of 2.1 kg of material, sediment, and water, was used in each slurry mixture tested (DGR Section A1-1.3). Dried sediment has about 1.5 times the density of water; therefore, the total amount of sediment in each experiment was approximately 980, 172, 189 and 32 grams (g) for the 45%, 8% mixed, 8% quiescent, and 1% slurries (calculated from measured solids composition (Table A1-1) and total mass of 2.1 kg). The volume of each experimental slurry was not reported.
The targeted amount of each chemical (including naphthalene) in each experiment can be calculated from the assumed concentration in source sediments and the amount of source sediments in each slurry. DGR Table A2-3 shows the results of chemistry analyses of source sediments. This table shows analytical data for two 'solids' samples (bulk and 45% sediment; reported in milligram chemical / kilogram sediment (mg/kg)) and three 'liquid' samples (micrograms per liter (µg/L)), for which no solids or moisture data are available. The Data Gaps Report implies that DGR Table A2-3 includes all of the chemical data for each experimental slurry. If this is the case, there is a large discrepancy between the measured amount of naphthalene and the targeted amount of naphthalene in the experimental slurries. Table #3 below compares the total amount of naphthalene in each slurry (calculated from DGR Table A2-3 and total experimental mass of 2.1 kg for slurries) with its targeted amount.
Table 3. Total naphthalene in slurries: Measured and Target
| Solids in Slurry | Bulk sediment | 46.80% (of 2.1 kg) | 8.20% (of 2.1 kg) | 9.00% (of 2.1 kg) | 1.50% (of 2.1 kg) |
| Measured naphthalene | (1300 mg/kg) | 10811 mg/test | 32 mg/test | 173 mg/test | 15 mg/test |
| Target naphthalene | (11000 mg/kg) | 10811 mg/test | 1894 mg/test | 2079 mg/test | 347 mg/test |
| Ratio- measured:target | 12% | 100% | 1.7% | 8.3% | 4.3% |
If there are no more analytical data on the chemical mixtures in the slurries than the data described in DGR Table A2-3, comparison of emission data between experiments is problematic. In addition, the use of data from these experiments to describe emissions during dredging becomes problematic as well. Note in Table #3 (and DGR Table A2-3 and DGR Section A2-3.2) that bulk sediment was analyzed. However, chemical analysis of the bulk sample showed about 9 times less naphthalene than the 45% sediment samples. While the Data Gaps Report explained that the bulk sample was not mixed properly, there is no way to tell if the sediment added to the other slurries was mixed. The Data Gaps Report says "for purposes of air modeling, it is assumed the source sediment is as characterized by sample '45-SED'." Without offering data or corroboration, the Data Gaps Report suggests that the chemical concentrations measured in the 45% sediment sample are "more consistent with the other analytical results." Mass balance is very important in experiments that characterize contaminant movement, and failure to accurately measure chemicals in source media makes the usefulness of these experiments to describe air emissions during dredging questionable.
There is another possible explanation for the confusing 8% and 1% slurry data in DGR Table A2-3. The laboratory report on these slurry samples says that the media were liquid, and suggests by the absence of solid and moisture data that the analysis may have characterized dissolved chemicals including naphthalene. This would suggest that the solubility of naphthalene in the slurries was up to 86 mg/L (from DGR Table A2-3). Clearly this is not possible, as the solubility of naphthalene in water is 31 mg/L. Any co-solvent effect that could elevate the amount of naphthalene in solution significantly would require considerable (maybe 1% or more) organic solvent dissolved in the aqueous mixture. The data discrepancies outlined in Table #3 suggest: non-homogeneous sediment; different pretest preparation of samples; different methods for sampling slurries for analysis, or; analytical problems. Therefore, it is difficult to draw any conclusions from the chemical data.
Emissions model description
The Data Gaps Report proposes a model for use in quantifying naphthalene emissions from sediments during dredging. This model is based on a proposed linear relationship between sediment concentration of naphthalene and the amount of emissions up to a maximum emission rate, above which any increase in naphthalene concentration in sediment will not increase emissions. The Data Gaps Report describes the sediment naphthalene concentration at which the maximum emission rate is first achieved as the "breakpoint." Further, the Data Gaps Report says that the emissions experiments described above were conducted at naphthalene sediment concentrations above the "breakpoint." The Data Gaps Report asserts that as the concentration of naphthalene in sediment increases, the movement of naphthalene from sediment to water becomes restricted as the naphthalene concentration in water approaches the solubility of naphthalene in water. Therefore, the "breakpoint" where maximum emissions occur can be calculated from the maximum solubility of naphthalene in water.
The following rearranged equilibrium partitioning equation was the basic equation used in the Data Gaps Report to calculate the naphthalene sediment concentration "breakpoint."
Solubility x Koc x foc = [naphthalenesed] (@ "breakpoint")(1) -equation #1
The Data Gaps Report modified the above equilibrium partitioning equation to include an additional factor (a solubility correction factor) that represents solubility interactions between different PAHs (% naphthalene in the non-aqueous phase liquids (NAPL)). Justification for this additional factor of 0.052 is not clear. Furthermore, if a "breakpoint" is used in this model without sound justification, it should have been supported experimentally. The resulting equation used to calculate "breakpoints" for the Data Gaps Report (Section A2-3.3.3 Source Strength Adjustment) is:
Solubility x % naphthalene in NAPL x Koc x foc = [naphthalenesed] (@ "breakpoint") -equation #2
The 'solubility' proposed in the Data Gaps Report (7 mg/L) is a naphthalene concentration measured in a sample from the site. It is stated that this measured concentration is "the highest measured naphthalene solubility (7 mg/L) in any of the solubility related tests (DRET, SBLT)" (DGR Section A2-3.3.3). Actually, 22 mg/L naphthalene was recorded in a 0.7 m filtered sample from the SBLT (DGR SBLT Table 4-4). See Appendix 2 of this health consultation for further discussion.
The dissolved naphthalene concentration in the air emission experimental slurries was not measured, and the assumption of a dissolved concentration less than the solubility of naphthalene is problematic. Measuring the solubility of naphthalene is difficult since the chemical is volatile. The solubility for naphthalene in water is about 31 mg/L (31.7 mg/L; ATSDR, 1995). While this solubility cannot be decreased, it can be increased above 31 mg/L if there is substantial co-solvent present. However, when a NAPL is present, the equilibrium concentration in water may be considerably less than the solubility (i.e. 5-11 mg/L: more discussion in a later section of this document).
In addition, the Data Gaps Report proposes using a site-specific Koc. Determination of Koc from laboratory experiments is not a simple task. MDH reviewed the Koc experiments conducted for the Data Gaps Report, and concluded that they were calculated using incorrect data as well as data from experiments that contained errors in methods. Appendix 2 contains a discussion of the derivation of the Data Gaps Report "site-specific Koc," including discussion about why Koc cannot be determined from samples containing NAPL.
The Data Gaps Report emissions model assumes that the naphthalene concentration in sediment in the experimental slurries was the target or "nominal" concentration, and calculated the hypothetical emissions total during dredging for 2 different "breakpoints" (238 mg/kgsed and 1000 mg/kgsed naphthalene). Figure #1 shows the proposed relationship between naphthalene sediment concentrations and total naphthalene air emissions (first 2 hours) proposed by the Data Gaps Report for the 238 mg/kgsed "breakpoint." Note that the actual emissions may be different under different experimental conditions, but that the "breakpoints" in the Data Gaps Report model remain the same regardless of experimental conditions (more discussion of this below).
Also, note that for a single experimental data point (triangles in Figure #1), the model proposes 2 lines: the maximum emission rate between 238 and 11,000 mg/kg naphthalene in sediment and; a line from the maximum emission rate to zero for sediments with 0 - 238 mg/kg naphthalene. While there are no experimental data that support this model, an analysis conducted in Appendix 1 of this Health Consultation demonstrates that experimental data demonstrates that the model is incorrect, especially in predicting emissions from slurries with low suspended solids concentrations.
For comparison, Figure #2 graphically compares 8% mixed slurry emissions modeled with "breakpoints" at 238 mg/kgsed, 1000 mg/kgsed, 5,000 mg/kgsed and 15,000 mg/kgsed naphthalene. Note that use of the Data Gaps Report model with a 15,000 mg/kgsed "breakpoint" results in the same predicted emission rates as a model without a "breakpoint" for naphthalene concentrations at the site (< 15,000 mg/kg naphthalene). Note in Figure #2 that there is one experimental datapoint (triangle) from which the Data Gaps Report proposed an emissions model.
Table #4 (below) shows the effect of using different breakpoints to calculate air emissions during dredging.
Table 4. Total modeled naphthalene emissions during 1 year of Stryker Bay dredging
|
DGR model "breakpoint" |
15,000 mg/kg naphthalene (same as no "breakpoint") | 5000 mg/kg naphthalene | 1000 mg/kg naphthalene | 238 mg/kg naphthalene | ||||
| kg/yr | %total | kg/yr | %total | kg/yr | %total | kg/yr | %total | |
| Active emissions during dredging | 947 | 12% | 1,403 | 7% | 1,582 | 5% | 2,302 | 7% |
| Emissions from inactive cells | 1,195 | 15% | 1,769 | 9% | 1,996 | 6% | 2,904 | 9% |
| CDF active emissions | 5,161 | 66% | 15,482 | 77% | 25,888 | 81% | 25,888 | 77% |
| Quiescent CDF pool emissions | 469 | 6% | 1408 | 7% | 2,354 | 7% | 2,354 | 7% |
| Total naphthalene air emissions | 7,752 | 100% | 20,062 | 100% | 31,821 | 100% | 33,448 | 100% |
| Total naphthalene dredged (calc from cell avgs) |
223,152 | 223,152 | 223,152 | 223,152 | ||||
| Percent total naphthalene dredged that is emitted | 3.5% | 9.0% | 14.3% | 15.0% | ||||
It is clear that the Data Gaps Report model is very sensitive to the choice of a "breakpoint". In other words, a low "breakpoint" assumes a very high rate of increase in air emissions as sediment concentrations of naphthalene increase. Appendix 1 of this health consultation demonstrates that the proposed model breakpoint of 238 mgnaphthalene/kgsediment predicts values that do not agree with the experimental data from the Data Gaps Report. If the Data Gaps Report model is correct, better experimental confirmation than presently exists is needed. In order to accept this model right now, one has to accept very implausible (and likely incorrect) assumptions that are not supported by the poor quality experimental data.
Partitioning: equilibrium versus dynamic systems
In a closed system at equilibrium, concentration ratios between all compartments are constant and all net fluxes between compartments are zero. When a closed system is disturbed and no longer exists at equilibrium, there is movement of chemicals between compartments, and ratios of chemical concentrations between compartments will change. Concentrations under non-equilibrium conditions are not determined by equilibrium ratios, but are determined largely by the chemical flux between compartments. At steady-state, as at equilibrium, concentration ratios between compartments are also constant; however there is a constant rate of movement of contaminants from one phase to another. Chemical concentration ratios between compartments at steady-state may be very different than ratios at equilibrium. And certainly, chemical ratios between compartments in a dynamic open system are not at equilibrium.
Movement of naphthalene from sediment to air requires the chemical to move between different compartments: sediment, water, and air. The speed at which a chemical can move from one compartment to the next is determined by the amount of mixing of the bulk medium (e.g. sediment, water or air), the molecular diffusivity of a chemical through the mediums, and the distance that the chemical must diffuse. Any movement through a compartment that is not aided by mixing must be accomplished by diffusion. Therefore, diffusion is usually confined to boundary layers that border the bulk media. Diffusion is also important in stagnant sediments.
Models of mass transfer across these boundary layers and into the different media describe slow diffusion across the boundary layers and rapid transfer from one layer to another to maintain equilibrium conditions across extremely small areas of adjacent boundary layer surfaces. Therefore, as boundary layers become thinner (a result of mixing) or molecular diffusivity increases (a function of temperature, typically a minor factor), flux from one compartment to another increases. This was demonstrated experimentally by the data from the 8% mixed slurry (assuming targeted naphthalene concentrations): mixing increased the emission of chemicals from 8% sediment slurries. These experimental emission rates are marked with triangles in Figure #1. (However, note in Figure #1 that the measured emissions from the 45% and 1% slurries are similar, and the emissions from the 8% quiescent slurry is about 40% less. This suggests either inconsistent preparation of samples, non-homogenous source sediments, problems with sample collection, analytical errors, or the presence of a NAPL.)
Experimental data from the Data Gaps Report suggest that emissions were not restricted by a solubility limit. Solubility remains constant over time. If, as suggested in the Data Gaps Report, solubility restricts movement of contaminants from sediment to air, experimental data should show steady emissions of naphthalene. As shown in Figures #3-#6 (chemical emission rates corrected for chemical loss and normalized to emission rates over the first 2 hours), all experiments had decreasing volatilization of naphthalene over time. Changes in emission rates must be a result of changes in the system. There are two apparent potential causes for this rate decrease: 1) Mixing during the test was insufficient to maintain the initial rate of transport of naphthalene through the water column, and 2) there are two (or more) compartments of sediment-adsorbed naphthalene that release naphthalene at different rates.
If the lack of mixing decreases the naphthalene renewal at the water-air surface or solubility limits desorption from sediments, the emission rate reduction over time should be less for the 8% mixed slurry than from the 8% quiescent slurry. As shown in Figures #4 and #5, the reduction in emission rate for the 8% quiescent slurry may be less than the reduction for the 8% mixed slurry; however the difference is small and likely insignificant (56% and 91% for hours 2-6 and 6-24, respectively, versus 47% and 85%). Therefore, it is unlikely that the lack of mixing or solubility limitations are responsible for most of the decrease in emission rates through time.
If naphthalene is sorbed to sediment particles differently, with different affinity for different types of particles (e.g. different types of organic carbon) or with longer diffusion paths as more desorbs, then experimental data may show less of a decrease in emission rate in experiments with the 45% slurry than in other experiments. However, if the rate of transfer through the water column is sufficiently fast, and there is no solubility limitation to the desorption from sediment, the difference may be insignificant. While decreases in the naphthalene emission rate may have been the least in the 45% slurry experiment, variation between all experiments was not large (see Figures #3-#6). Because water mixing does not appear to limit emission rate decreases, by default, slowed diffusion of naphthalene from sediments over time is the suggested explanation for the emission rate decreases.
There is considerable information in the environmental chemistry literature that shows PAHs, in general, have different affinities for organic carbon of different origins. PAH partitioning into water has been shown to be decreased in areas with significant tar or pyrogenic materials (Maruya et al., 1996). In addition, research by Gustafsson et al. (1997) and others has shown that soot or black carbon in sediments does not allow PAHs to desorb as readily as natural or other organic carbon.
In summary, the Data Gaps Report experimental data suggest that there is probably not a solubility limit to naphthalene emissions under conditions similar to those encountered in these experiments. Further, if there is a solubility limit under some conditions, the solubility limit will change as experimental conditions change. The known mechanisms whereby chemicals partition between different phases or media, as well as very limited experimental data, suggest that even if there is a sediment concentration above which there will be no increase in emissions (i.e. a "breakpoint"), this sediment concentration could be very different under different experimental conditions. Therefore, the single "breakpoint" concept is of little utility when attempting to model emissions during dredging.
While experiments in the Data Gaps Report were not designed to find a possible cause of decreasing naphthalene emission rates over time, the data do suggest that emission rates may be limited by diffusion and desorption of naphthalene from the sediments.
Naphthalene partitioning model
Naphthalene partitioning from sediments can be very different if contamination exists as a NAPL, or if it is sorbed to organic particulates in the sediment. Equilibrium between naphthalene in a NAPL and water, and equilibrium between sediment organic carbon and water, are two distinctly different conditions. If there is a NAPL in the sediments, then partitioning should be calculated between the NAPL and water, as well as directly between the NAPL and air. If there is no NAPL, then the important partitioning is between the organic carbon in the sediment and water. These are important distinctions, because the presence of a NAPL can lead to very different emission rates than will be encountered when dredging sediment-sorbed naphthalene. The concentration of naphthalene in water at equilibrium (i.e. in a non-dynamic system) with naphthalene sorbed to sediment is:
dissolved naphthalene (mg/L) = [naphthalene] (mg/kg) / Koc (L/kg) / foc
for dissolved < 31 mg/L -equation #3
The concentration of naphthalene in water at equilibrium with naphthalene in a NAPL can be approximated by (Schwarzenbach et al., 1993):
dissolved naphthalene (mg/L) » Cwsat * Exp((+)DSm/R * ((Tm / T) - 1) * fNAPL * gorg mix
for dissolved < 31 mg/L -equation #4
Where:
| Cwsat (mg/L) | = water solubility (31 mg/L) |
| DSm (J/mol/K) | = entropy of melting (~48 - 59 J/mol/K) |
| R (J/mol/K) | = gas constant (8.3145 J/mol/K) |
| Tm (K) | = melting temperature (353.75 K) |
| T (K) | = temperature (293.15 K) |
| fNAPL | = fraction of naphthalene in NAPL |
| gorg mix | = fugacity of naphthalene in the organic mixture (~1 - 1.8) |
Note that the naphthalene concentration in water (e.g. 5 - 11 mg/L @ fNAPL = 5.2 % naphthalene) is almost totally dependent on the fraction of naphthalene in the NAPL and therefore, the dissolved concentration (at equilibrium) will remain constant as long as a similar composition NAPL is present. Experimental data from the Data Gaps Report Sequential Batch Leaching Report shows up to 22 mg/L naphthalene in water (DGR SBLT Table 4-4; see Appendix 2 for discussion). Dissolved naphthalene was not measured in the air emissions experiments, and it is possible that samples contained NAPL. As concentrations of naphthalene decrease in sediments, there is probably a point where there is no longer a NAPL. At those locations, the concentration in water is determined (again, at equilibrium) by the equilibrium partitioning equation (equation #3) above. Figure #7 shows potential dissolved naphthalene concentrations at equilibrium with sediment, with and without NAPL.
NAPL in a slurry can also emit volatile chemicals directly to the air. The potential for this type of air release was not studied in the Data Gaps Report, or by MDH during our review.
The Data Gaps Report emissions model is structured to model emissions from contaminants that are sorbed to sediments, or solid phase contaminants. The presence of a NAPL in the sediments or slurries is not discussed in the air emissions sections of the Data Gaps Report, but NAPL could have large effect on emissions calculations. When attempting to apply the Data Gaps Report experimental data to the emissions model, estimates of dissolved naphthalene could be off by a factor of 6 depending on the presence or absence of a NAPL. If there is a NAPL, there is likely a maximum naphthalene concentration in water that is below the solubility of naphthalene. This maximum is not related to the naphthalene sediment concentration, but is reached when there is a NAPL present, and is absent when there is no NAPL. Sediment-sorbed naphthalene will be associated with generally higher dissolved concentrations of naphthalene than will naphthalene from a NAPL, because naphthalene in a PAH NAPL has less fugacity than naphthalene sorbed to natural organic carbon. If there is no NAPL, the dissolved naphthalene concentration adjacent to sediment containing 11,000 mg/kg naphthalene may be about 31 mg/L.(2) The presence or absence of a NAPL is critical in evaluating experimental data, and it may also be needed to calculate conservative and realistic emission estimates from available physical and chemical data on naphthalene.
Additional air emission issues
Increasing windspeed will increase emissions from dredging areas and containment facilities. But increased windspeed also increases the dilution of contaminants, thereby decreasing potential exposures. Therefore, inputs to the dispersion model are potentially complex, and the results are not easily predictable. The highest acute exposures typically occur when there is an atmospheric inversion and the windspeed is low. Because emissions from water are somewhat dependent on windspeed (wind causes a thinning of the water and air-side boundary layers), emissions at low windspeed may be below those predicted by a simple emission model. Conversely, emissions at high windspeed may exceed those in the emissions model.
Analysis of the relative relationship between increasing dilution and increasing emissions as functions of windspeed should be conducted.
Henry's Law constants at different temperatures as well as ratios of Henry's Law and ratios of molecular diffusivities at other temperatures with their values at 20º are listed in the Table #5 below (EPA, 2001).
| Temperature | Hk - naphthalene | Hk Ratio (to 20º C) | Molecular diffusivity ratio (to 20º C) |
| 22.4º C | 40.48 pa/m3/mol | 120% | 101% |
| 20º C | 33.85 pa/m3/mol | - - | - - |
| 17.3º C | 27.56 pa/m3/mol | 81% | 98% |
| 8.5º C | 13.63 pa/m3/mol | 40% | 93% |
These data suggest that temperature-dependant changes in the rate of volatilization will be related to changes in Henry's Law, and will not be appreciably affected by changes in molecular diffusivity. However, given that Henry's Law is an equilibrium constant, the temperature dependence of the rate of volatilization is very dependent on naphthalene concentrations in the water / air boundary layers. These data suggest that emissions may be significantly decreased if the cells containing the highest quantities of naphthalene are dredged in the early spring or the late fall.
Review of the Data Gaps Report suggests that numerous steps can be taken to decrease the potential exposure of the public to emissions during dredging. These include:
Summary of additional MDH reviews of the Data Gaps Report
Dredging Elutriate Test Report
The purpose of the Dredging Elutriate Testing (DRET) was to "measure the effects of St. Louis River/Interlake/Duluth Tar Site (SLRIDT) sediments on St. Louis River water quality when mixed together in the immediate area of the disturbance"(Service, 2002; Appendix DRET, Executive Summary). MDH did not review the DRET Report entirely, but confined review to sections and tables that characterized sediment samples. MDH has the following comments:
Minimally, the Data Gaps Report should have identified and addressed the uses and limitations of these data.
Ice Report
MDH reviewed the original Ice Report in December 2001. Other than the addition of observations during the winter of 2001-2002, there are no notable changes in the new report. The two winters of observation now include an "atypical winter," 2000-2001, and "an uncharacteristically warm (winter) with minimal snow cover," 2001-2002.
The Report includes mention that "where the ice thickness was equal to the water depth the ice was typically frozen to the bottom, eliminating the potential for ice movement." However, there is no discussion of frozen sediments. In addition, while the Ice Report states that thermal expansion under minimal snow cover "can create significant lateral forces that produce ice movement, bottom gouge, ridge formation, and ride-up," the report also states that "Duluth always has a snow cover." This is not entirely true, as evidenced by conditions this winter (Attachment #4; MDH and MPCA, 2003).
The Ice Report does not address concerns raised by MDH with regard to ice intrusion into sediments or a cap, the potential formation of ice feet, or lenses, and the effect of ice on the sediment matrix. These issues were discussed in the 2001 MDH Health Consultation (MDH, 2001b).
Additional Data Gaps Report Review
MDH has reviewed most sections of the Data Gaps Report that directly apply to human exposure to contaminants during dredging and the longterm protectiveness of a remedy at the SLRIDT site. Currently we are in the process of reviewing some related sections of the Data Gaps Report, and if we have additional contributions to make to the ongoing discussions we will write an additional review. In lieu of a full review, MDH offers the following comments.
Groundwater model
MDH expects to complete a review of the groundwater sections of the Data Gaps Report soon. Initial areas of concern are:
Extent of sediment contamination
The Data Gaps Report (GH 7-3.1, GH 7-3.2) states that there is a lack of association between metals data in layers and total PAH (tPAH) data in the same layers and concludes that there is no association between releases of tPAH and metals onsite. The lack of co-location of contaminants does not exclude the probability that both metals and organics were discharged by facilities that operated at this site. Co-location of contaminants assumes that transport of these contaminants to sediments is the same, and that movement in sediments is the same. While movement of non-polar organics can be easily modeled, the partitioning and movement of metals and metallic compounds/species can be more complicated. Some metals may remain relatively insoluble as long as conditions remain reduced and there is sufficient sulfide. Groundwater and seiche flow through contaminated sediments can make the prediction of metallic species and movement in sediment even more difficult.
Information not contained in the Data Gaps Report
Mercury methylation
There are limited data on the mercury contamination in sediments at the SLRIDT site. Any sediment sampling and analysis performed at this site should include analysis of mercury.
In the MDH 2001 Health Consultation (MDH, 2001b) on sediments at the SLRIDT site, MDH discussed concerns about building a wetland over sediments contaminated with mercury. The most serious concern related to mercury at the SLRIDT site is that creating a wetland cap may increase the methylation rates of mercury from the site. Wetlands have been shown to be the major source of methyl mercury in watersheds (Rudd, 1995; Saint Louis et al., 1996). In addition, field studies have suggested that groundwater flow may increase the flow of mercury into a system (Krabbenhoft et al., 1998). Mercury in sediments can form many different sulfide complexes, including some that are soluble (Paquette and Helz, 1997; Benoit et al., 1999). While MDH does expect that mercury concentrations in surficial sediments will be lowered with remedial action, we are concerned that methylation rates may be increased. If Stryker Bay is converted into a wetland because of the construction of a cap, methylation rates are expected to rise. If methylation rates increase, it will be important to limit the amount of mercury available for methylation by maintaining low concentrations of total mercury in surficial sediments. The Data Gaps Report only addresses mercury by suggesting, with the one-dimensional groundwater model, that the movement of mercury through a cap will be minimal.
Dredging also offers some challenges for containment of mercury. Some mercury will be resuspended during dredging. However, using a conservative estimate, the total amount of mercury discharged in decanted dredged water will be at most about 35 grams (MDH, 2001b). A CDF should be designed to isolated contaminants, including mercury, from groundwater and surfacewater. Following dredging, maintenance of current water depths should not result in a long-term increase in methylation.
MDH General Comment on the Data Gaps Report
Review of the experimental studies in the Data Gaps Report generally shows inadequate laboratory procedures and incomplete explanation of methods and results in sections reviewed by MDH. A significant amount of information in the reviewed studies is imported from other sections or reports that have not been reviewed by MDH. MDH is concerned that there is a cascading of effects that is not clearly presented in the Data Gaps Report - where errors in methods, analysis, or interpretation are carried over and impact other calculations and interpretations. This segregation of errors, and lack of full disclosure of the sources and limitations of data, can lead to improper and unjustified conclusions.
For instance, computation of the naphthalene site-specific Koc used in the Data Gaps Report Air Analysis Appendix contains significant errors. Appendix 2 of this health consultation contains a review of the site-specific Koc derivation. This review concludes that the data presented are not sufficiently reliable to replace a more general, published, carefully derived Koc of 933 L/kg (ATSDR, 1995) with the Koc (3200 L/kg) used in the Data Gaps Report.
As mentioned elsewhere in this health consultation, any partitioning study should include a mass balance component or study. Not only did the air emissions experiments not include a mass balance study, only the emissions appear to have been appropriately characterized. Furthermore, the presence or absence of a NAPL will significantly affect experimental results and air emission model predictions. The Data Gaps Report contained no discussion of the presence or absence of NAPL in sediments that may be dredged. Chemical composition data for the slurries used in the experimental studies were not reliable. This makes drawing quantitative conclusions impossible. Some inferences can be made by comparing emission rates within individual studies, as done in Figures #3-#6, and by then comparing normalized results, but any conclusions can not be stated with confidence.
As part of an MDH Health Consultation, issues related to children's health are addressed explicitly. Children are exposed to chemicals differently than adults. Children will splash and play in water, sometimes drinking it and any suspended particles. They also eat, drink and breathe more than adults on a body weight basis. Therefore, they may have significantly greater exposures than adults. In addition children are not fully developed, and developing organs may be more sensitive to chemical hazards.
Scenarios for exposure to chemicals in sediments or other media are evaluated by MDH using a child receptor. Activities of a child and child intake rates are used in determining potential exposures by all routes of exposure. In addition, criteria used to evaluate the toxicity of chemicals are calculated to be protective of children and other sensitive sub-populations.
The Data Gaps Report contains a large amount of information characterizing a number of issues related to cleanup of the SLRIDT site. MDH reviewed sections of the Data Gaps Report that discussed topics previously raised by MDH. These included sections on potential air emissions during dredging; ice impacts on a cap over contaminated sediments and; the effect of groundwater on remedial options.
MDH review of the air emissions experiments identified several instances for which sampling methods were poorly described or not available, targeted naphthalene concentrations in experimental samples were not achieved, and there was incomplete or missing data. Problems encountered in the experiments or in data analysis were not identified or discussed in the text of the report. In addition, the proposed Data Gaps Report air emission model contains significant errors and is inconsistent with the experimental data in the Report.
No chemical mass balance was performed in the experiments conducted for the Data Gaps Report. This is a significant weakness: there is no way to verify that the design and methods allowed analyses of intended compartments. It is not possible to resolve apparent errors in chemical analyses of the 1% slurry and both 8% slurries without additional data. The Data Gaps Report suggests that target concentrations better represent the actual concentrations of contaminants in sediment than do the analytical measurements, so it is assumed that there was an error in methods in sampling the slurries. These errors do not affect the conceptual or the comparative analysis of normalized data within samples, but they may affect between sample comparisons and the quantification of emissions by any model that uses the experimental data. If analyses were bad, the experimental data are useless for describing a functional relationship between: naphthalene in sediments, naphthalene in water, and naphthalene in air.
The Data Gaps Report proposes that there is a sediment concentration of naphthalene above which no additional naphthalene will be emitted from a slurry. A consequence of this proposal is that the modeled relationship of air emissions to sediment naphthalene has a very rapid rise at very low naphthalene concentrations. In addition, Data Gaps Report proposes that this "breakpoint" is the same under different experimental conditions. This analysis is not substantiated by experimental data, and is likely incorrect. Further, the model uses one experimental point to determine two lines (see Figure #1); therefore the experimental data neither support or refute the model, as one point is "consistent" with any model (see Figure #2). While solubility may have an effect on naphthalene emissions, this effect is very difficult to approximate in a dynamic system. Changing experimental conditions will change any solubility limit. Therefore, while the output of the model is very sensitive to a single parameter (bulk chemical solubility), the model contains no adjustment to account for: dispersion of naphthalene in the water due to mixing of the slurry, or; additional dilution by added water (i.e. the model uses a single sediment concentration, solubility limit, regardless of the amount of suspended solids in the slurry).
Furthermore, the Data Gaps Report errs by including a correction factor of 0.052 when calculating a theoretical, solubility-limited "breakpoint." In addition, it is likely that there was NAPL present in some experiments. This is suggested by data that shows high concentrations of naphthalene in sediments, yet water concentration of naphthalene in those experiments well below the solubility of naphthalene.
The general conclusion of MDH analysis of the air emissions data and model is that the public can be protected from exposure to chemicals at levels of concern during dredging. However, the effort necessary to achieve protection is not well understood.
Review of the air emissions model in the Data Gaps Report suggests that:
The Ice Report in the Data Gaps Report was essentially unchanged from the Ice Report previously submitted by the PRPs. It did contain new field observations from last winter. Analyses of ice intrusion into sediment, the possible formation of ice feet or lenses, and the disruption of the sediment matrix, which have been identified by MDH as processes that may impact the longterm effectiveness of a cap, are not included in the Data Gaps Report.
MDH has also begun reviewing selected areas of the Groundwater Report and expects to have full comments shortly. Concerns are generally related to the groundwater-aided movement of contaminants through a cap or from a CDF, and the potential for groundwater to contribute to the erosion and ultimate failure of a cap or a CDF.
Discharges of contaminants from the sediments during placement of a cap have not been addressed. Furthermore, a review of fallback or the redeposition of residual contamination on sediments after dredging needs to be conducted. This would help us develop a common understanding of these problems.
Mercury issues related to this site are not discussed in the Data Gaps Report. MDH is mainly concerned about the increase in methylation that can occur in wetland areas that would be created by capping. This may be offset because mercury movement from areas under a cap will be below those currently experienced, but the mobility of mercury is not well understood and therefore cannot be accurately assessed.
Sediment samples taken for the Data Gaps Report were not analyzed for all chemicals that have been identified as potential chemicals of concern by MDH. These include an extended list of carcinogenic PAHs and chlorinated organics.
Carl Herbrandson, Ph. D.
Toxicologist
Site Assessment and Consultation Unit
Environmental Surveillance and Consultation Section
Minnesota Department of Health
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This St. Louis/Interlake/Duluth Tar Site Health Consultation was prepared by the Minnesota Department of Health under a cooperative agreement with the Agency for Toxic Substances and Disease Registry (ATSDR). It is in accordance with approved methodology and procedures existing at the time the health consultation was begun.
Alan W. Yarbrough
Technical Project Officer, SPS, SSAB, DHAC
ATSDR
The Division of Health Assessment and Consultation, ATSDR, has reviewed this public health consultation and concurs with the findings.
Roberta Erlwein
Chief, State Program Section, SSAB, DHAC, ATSDR
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