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. Author manuscript; available in PMC: 2024 Oct 15.
Published in final edited form as: Environ Toxicol Chem. 2022 Mar 21;41(5):1311–1318. doi: 10.1002/etc.5310

Comparative Toxicity of Oil Spill Herding Agents to Aquatic Species

Matthew M Alloy a,e,*, Devi Sundaravadivelu b, Elizabeth Moso c, Peter Meyer d, Mace G Barron c
PMCID: PMC11474249  NIHMSID: NIHMS2021168  PMID: 35156233

Abstract

Chemical herding agents are surfactant mixtures used to coalesce spilled oil and increase slick thickness to facilitate mechanical recovery or in situ burning. Only two herders are currently listed on the United States’ National Oil and Hazardous Substances Pollution Contingency Plan or National Contingency Plan product schedule for potential use in spill response: the surface collecting agents Siltech OP-40 and ThickSlick 6535. Toxicity data for spill response agents are frequently available only for two estuarine species, mysid shrimp (Americamysis bahia) and inland silversides (Menidia beryllina), and are particularly limited for herding agents. Toxicity can vary over several orders of magnitude across product type and species, even within specific categories of spill response agents. Seven aquatic species were tested with both Siltech OP-40 and ThickSlick 6535 to evaluate acute herder toxicity and relative species sensitivity. The toxicity assessment included: acute tests with A. bahia and M. beryllina, the freshwater crustacean Ceriodaphina dubia, and the freshwater fish Pimephales promelas; development of the echinoderm Arbacia unctulate; and growth of a freshwater alga Raphidocelis subcapitata and marine alga Dunaliella tertiolecta. Siltech acute toxicity values ranged from 1.1 to 32.8 ppm. ThickSlick acute toxicity values ranged from 2.2 to 126.4 ppm. The results of present study show greater toxicity of Siltech compared to ThickSlick with estimated acute hazard concentrations intended to provide 95% species protection of 1.1 and 3.6 ppm, respectively, on empirical data and 0.64 and 3.3 ppm, respectively, with the addition of interspecies correlation data. The present study provides a greater understanding of species sensitivity of these two oil spill response agents.

Keywords: Oil spill, Herder, Toxicity, Species sensitivity distribution, Surface collecting agent

INTRODUCTION

Modern oil spill response includes the use of various chemical agents to assist response measures and mitigate the impacts of oil spills. Surface collecting agents, commonly referred to as herders, were developed to confine and condense surface slicks (Garrett, 1969). The United States’ National Oil and Hazardous Substances Pollution Contingency Plan or National Contingency Plan (United States, 2015) product schedule lists only two surface collecting agents, Siltech OP-40 and ThickSlick 6535 (hereafter referred to as Siltech and ThickSlick, respectively). These agents are intended to be sprayed around the boundary edge of a slick to prevent oil from spreading and to coalesce spilled oil and increase slick thickness to facilitate mechanical recovery or in situ burning. Siltech is a silicon-based surfactant composed primarily of 3-(Polyoxylethylene)propylheptamethy ltrisiloxane (CAS No. 67674-67-3). ThickSlick is a mixture of predominantly sorbitan monolaurate surfactant (CAS No. 1338-39-2) and 2-ethyl-1-butanol solvent (CAS No. 97-95-0).

Although herders have been tested for surface collection effectiveness for more than 50 years (Garrett, 1969), toxicity values have been reported for few aquatic species. Buist et al. (2018) reported 24 h median lethal concentrations (LC50) in a marine copepod, Calanus hyperboreus, exposed to the herders Siltech and ThickSlick, of less than 12.5 ppm and less than 600 ppm, respectively. The US National Contingency Plan product schedule lists toxicity values for the two herders to only two species: an estuarine invertebrate (mysid shrimp, Americamysis bahia) and fish (inland silverside, Menidia beryllina; US Environmental Protection Agency [USEPA], 2021). Consistent with the general trend in Buist et al. (2018), Siltech was more toxic to both mysids (Siltech 96 h LC50 = 6.4 ppm and ThickSlick 96 h LC50 286 ppm) and silversides (Siltech 96 h LC50 = 3.3 ppm and ThickSlick 96 h LC50 = 138 ppm). These limited toxicity data for oil spill herders make hazard and risk assessment challenging. Species vary in sensitivity to compounds (Newman et al., 2000; Wheeler et al., 2002), and differences in sensitivity between species to a specific toxicant can span several orders of magnitude for the same toxicant class or even the same compound (Barron et al., 2013; Wogram & Liess, 2001). Hazard and risk assessments based on extrapolation from one taxon to another without data from related species generally increase uncertainty, and toxicity extrapolation is generally more robust when data are available for multiple species.

The species sensitivity distribution pproachh was first developed to limit professional judgement in formulating water quality criteria (Stephen et al., 1985). The SSD approach models variation in sensitivity of species to a chemical without needing to know the origins of differing sensitivities. The fitting of individual species data to a cumulative probability distribution then facilitates the estimation of aquatic community sensitivity to a contaminant. The USEPA published guidelines for ecological risk assessment (USEPA, 1998), including guidance on the use of SSDs for risk characterization. Since then, the SSD approach has been adopted by other environmental protection agencies including the European Chemicals Agency (ECHA, 2016), Canadian Counsel of Ministries of the Environment (2007), as well as the Australian and New Zealand Environment and Conservation Council (2000). Fox et al. (2021) published a review detailing the limitations of SSD modeling, the advantages of the approach, the state of SSD modeling, and the recent developments.

The aim of the present study was to expand the knowledge base of aquatic species’ sensitivity to two globally available herders, Siltech and ThickSlick. Seven species encompassing four taxa including teleosts, crustaceans, an echinoderm, and algae were investigated. The toxicity assessment included: acute survival tests with A. bahia and M. beryllina, with the freshwater crustacean Ceriodaphnia dubia, and with the freshwater fish Pimephales promelas; developmental abnormality tests with the echinoderm Arbacia punctulata; and growth inhibition tests with the marine algae Dunaliella tertiolecta and the freshwater algae Raphidocelis subcapitata. Every tested species is an established laboratory organism with standard USEPA methods for culture and toxicity testing. The selection of organisms in the present study represents data from two Kingdoms and seven Orders of life.

MATERIALS AND METHODS

Test solution preparation

Primary stock solutions were prepared in glass containers homogenized by magnetic stirring with teflon stir bars to approximately 20% vortex. Herders were added to the appropriate diluent water (fresh or saltwater) by syringe to achieve the desired stock concentration. Test dilutions were prepared from stocks just prior to use in toxicity tests. Toxicity tests were performed using glass vessels with only nominal herder concentrations because of a lack of analytical methods compounded by unknown priority composition of each herder.

Test organisms

Algae.

Raphidocelis subcapitata (formerly known as Pseudokirchneriella subcapitata and Selenastrum capricornutum) was obtained from in-house cultures. Raphidocelis subcapitata was cultured and tested as per USEPA method 1003.0 (USEPA 2002a). In brief, synthetic fresh water and nutrient solutions were prepared and sterilized. Tests were initiated with a starting inoculum providing a cell density of 10 000 cells/ml in each replicate. Tests were conducted under continuous illumination by white fluorescents at 86 μE/m2/s and held at 25 °C for the duration. Test controls and treatments were replicated quadruplicate. Test chambers were 250 ml glass Erlenmeyer flasks with 50 ml solution. All test flasks were shaken twice daily by hand. Growth, as measured by increase in cell abundance, was assessed after 96 h by microscope using a hemocytometer.

Dunaliella tertiolecta were from in-house cultures; they were cultured and tested by methods adapted from USEPA method 1003.0 (2002a) with alterations. Culture water and testing water salinities were 20 ppt. Starting test inocula were 5000 cells/ml.

Invertebrates.

Americamysis bahia were obtained from inhouse laboratory cultures and tested by USEPA method 2007.0 (1994). In brief, tests were conducted at 20 ppt salinity and at 26 °C. Test organisms were 1–5-day old at test initiation. Test chambers were 500 ml glass beakers with 200 ml test solution and 10 A. bahia. Test controls and treatments were replicated in triplicate.

Arbacia punctulata were obtained from Aquatic Research Organisms and were cultured by methods described in the USEPA method 1008.0 (2002b). In brief, culture and test waters were made to 30 ppt at 20 °C. Gametes were combined at test initiation. Test chambers were 30 ml glass scintillation vials with 10 ml of test solution with approximately 1000 embryos. Test controls and treatments were replicated in quadruplicate.

Ceriodaphnia dubia were from in-house laboratory cultures and tested by USEPA method 2002.0 (USEPA 2002c). In brief, tests were conducted using reconstituted moderately hard water at 25 °C. Ceriodaphnia dubia were less than 24 h old at test initiation. Test chambers were 30 ml glass beakers with 20 ml test solution and five C. dubia neonates. Test controls and treatments were replicated in quadruplicate.

Fish.

Menidia beryllina were obtained from Aquatic Indicators and were cultured and tested according to USEPA method 2006.0 (2002c). In brief, tests were conducted at 20 ppt salinity at 25 °C. Menidia beryllina were between 9–14-day old at test initiation. Test chambers were 1 L glass beakers with 200 ml test solution and 10 M. beryllina fry. Test controls and treatments were replicated in triplicate.

Pimephales promelas were from in-house cultures and tested by USEPA method 2000.0 (2002c). In brief, tests were conducted in reconstituted moderately hard water at 25 °C. Pimephales promelas were between 9- and 14-day old at test initiation. Test chambers were 1 L glass beakers with 200 test solution and 10 P. promelas fry. Test controls and treatments were replicated in triplicate.

Quality assurance and quality control

All tests were conducted by the relevant methods without significant deviations. Data were internally reviewed for accuracy prior to data analysis. Water quality parameters were maintained within ranges as directed in the guidance documents (Supplemental Information, Table S1). All organisms were subjected to reference toxicant bioassays and showed responses within the ranges suggested in the guidance documents (Supplemental Information, Table S2). Data were cross-checked independently as part of USEPA’s quality assurance and quality control procedures and toxicity response data are included in the Supplemental Information.

Statistical approach and endpoint estimation

All statistical analyses were performed using the R statistical program (v3.5; R Core Team, 2021) and associated packages drc (Ritz et al., 2015) and tidyverse (Wickham et al., 2019). Effect concentrations derived were the no-observed-effect concentration (NOEC), lowest-observed-effect concentration (LOEC), and median lethal concentrations (LC50) with respective 95% confidence intervals (95% confidence interval [CI]) when relevant.

Species sensitivity distributions

The ssdtools R package was used for curve fitting, the estimation of hazard concentration at 5% (HC5)s, and CIs by parametric bootstrapping using 10 000 bootstrap samples (Thorley & Schwartz, 2018). Goodness of fit tests were performed to determine the most appropriate distribution to be used in generating each species sensitivity distribution (SSD). Datasets were fitted against a Gamma distribution, log-normal distribution, log-logistic distribution, log-Gumbel distribution, and Weibull distribution.

All SSDs are based on toxicity values from the present study, published literature (Buist et al., 2018), and the National Contingency Plan. The geometric mean was used in cases of multiple toxicity values from the same species. A second set of SSDs were calculated by adding values derived by interspecies correlation estimations (ICEs). Web ICE is a USEPA web-based ICE program (Raimondo et al., 2010). Only six of the seven species were surrogate species options (A. punctulata was outside the domain of Web ICE) for the program to extrapolate to close taxonomic neighbors and predict LC/median effect concentrations (EC50s) for species that lack hazard data. The data output of both the Siltech and ThickSlick ICEs are given in Supplemental Information, Tables S3 and S4. When Web ICE produced several predicted values for a single species from multiple surrogate species, the geometric mean of those values was used. Extrapolated species values with low coefficient of determination (<0.6) were included when there were other values for the same species with high (>0.6) coefficent of determination, otherwise they were omitted.

RESULTS

Toxicity of Siltech

All bioassays met the species-specific method quality control criteria of control survival, growth, or development. The echinoderm A. punctulata was the most sensitive to Siltech exposure with a development EC50 of 1.1 ppm. Raphidocelis subcapitata was the species least sensitive to Siltech with a growth EC50 of 32.8 ppm. Detailed descriptions of all species toxicity endpoints are given in Table 1.

TABLE 1:

Toxicity endpoint values (ppm) for seven species exposed to Siltech OP-40

Species Endpoint NOEC LOEC LC/EC50 (95% confidence interval)
P. promelas Survival 3 6 4.4 (1.3–7.6)
M. beryllina Survival 3 6 3.7 (2–5.5)
A. punctulata Development 0.5 1 1.1 (0.5–1.7)
C. dubia Survival 3 6 6.4 (4.5–8.2)
A. bahia Survival 7.5 15 11.5 (9.5–13.4)
R. subcapitata Growth 1.25 32.8a (21.3–44.4)
D. tertiolecta Growth 8 16 11.5 (10.7–12.2)
Species Sensitivity Distribution 1.4b (0.6–4.4)
a

Extrapolated value from data that did not achieve median effect at the highest concentration tested.

b

Fifth percentile (HC5) concentration.

NOEC = no-observed-effect concentration; LOEC = lowest-observed-effect concentration; LC50 = median lethal concentration; EC50 = median effect concentration; HC5 = hazardous concentration to 5% of the species.

Toxicity of ThickSlick

The freshwater crustacean C. dubia was the species most sensitive to ThickSlick exposure with an LC50 of 2.2 ppm. Menidia beryllina was the species least sensitive to ThickSlick with an LC50 of 126.4 ppm. Detailed descriptions of all species toxicity endpoints are given in Table 2.

TABLE 2:

Toxicity endpoint values (ppm) for seven species exposed to ThickSlick 6535

Species Endpoint NOEC LOEC LC/EC50 (95% confidence interval)
P. promelas Survival 30 60 53.7 (30.3–77.1)
M. beryllina Survival 75 150 126.4 (35.6–217.2)
A. punctulata Development 5 10 24.6 (6–43.3)
C. dubia Survival 1 2 2.2 (2–2.3)
A. bahia Survival 20 40 74.9 (68–81.7)
R. subcapitata Growth 6.25 12.5 14.3 (13.8–14.8)
D. tertiolecta Growth 0.25 0.5 12.7 (−11.7–37)
Species Sensitivity Distribution 2.6a (0.6–17.7)
a

Fifth percentile (HC5) concentration.

NOEC = no-observed-effect concentration; LOEC = lowest-observed-effect concentration; LC50 = median lethal concentration; EC50 = median effect concentration; HC5 = hazardous concentration to 5% of the species.

Species sensitivity distributions

The HC5s of two SSDs from the median effect concentrations using empirical toxicity data, the National Contingency Plan, and literature values for Siltech are presented in Figure 1. ThickSlick SSD is presented in Figure 2. Combining empirical data and Web ICE generated values produced a 54 species SSD for Siltech (Figure 3) and a 51 species SSD for ThickSlick (Figure 4). The HC5s are detailed in Table 3.

FIGURE 1:

FIGURE 1:

Siltech OP-40 species sensitivity distribution generated with only measured acute toxicity data. Dashed line indicates the fifth percentile hazard concentration. Shaded area is the 95% confidence region. NCP = National Contingency Plan.

FIGURE 2:

FIGURE 2:

Siltech OP-40 species sensitivity distribution generated with measured acute toxicity data and interspecies correlation estimation model predictions. Dashed line indicates the fifth percentile hazard concentration, and the shaded area is the 95% confidence region. NCP = National Contingency Plan; ICE = interspecies correlation estimation.

FIGURE 3:

FIGURE 3:

ThickSlick 6535 species sensitivity distribution generated with only measured acute toxicity data. Dashed line indicates the fifth percentile hazard concentration, and the shaded area is the 95% confidence region. NCP = National Contingency Plan.

FIGURE 4:

FIGURE 4:

ThickSlick 6535 species sensitivity distribution generated with measured acute toxicity data and interspecies correlation estimation model predictions. Dashed line indicates the fifth percentile hazard concentration, and the shaded area is the 95% confidence region. NCP = National Contingency Plan; ICE = interspecies correlation estimation.

TABLE 3:

Species sensitivity distribution hazardous concentration to 5% of the species values with 95% confidence intervals (ppm by volume) and their data sources

Herder Data sources Distribution Species HC5 (95% CI)
Siltech Empirical toxicity values Log normal 8 1.4 (0.6–4.4)
ThickSlick Empirical toxicity values Log normal 8 2.6 (0.6–17.7)
Siltech Empirical with Web ICE values Log-Gumbel 54 0.97 (0.7–1.4)
ThickSlick Empirical with Web ICE values Log-logistic 51 3.7 (1.9–7.2)

HC5 = hazardous concentration to 5% of the species; CI = Confidence intervals.

DISCUSSION

Although herders were first developed and evaluated in the 1970s, early compounds were unable to meet containment needs under wind and wave actions and did not see deployment in any spill (Aggarwal et al., 2017; Garrett, 1969). Because of the hiatus in development and testing and the lack of actual spill deployment, there is little known about herder toxicity beyond a few laboratory tests and small-scale field testing. Since 2004, there has been renewed interest in herder development and possible use via aerial application followed by insitu burning (Aggarwal et al., 2017). Recent studies have focused on the use of Siltech and ThickSlick for facilitating in situ burning of spilled oil because these are the only two herders listed on the US National Contingency Plan product schedule (Buist et al., 2018; Bullock, 2021; Rojas-Alva et al., 2020a, 2020b; van Gelderen et al., 2017). Herders have also received interest by oil spill responders and managers because of the potential applications in ice-affected waters and wetland areas (Aggarwal et al., 2017; Bullock et al., 2019).

Based on the review of available literature, toxicity data for Siltech and ThickSlick have only been reported for three aquatic species: the copepod Calanus hyperboreaus (Buist et al., 2018), the mysid shrimp A. bahia, and the inland silverside M. beryllina (USEPA, 2021). The present study provides same laboratory herder toxicity testing for the two species included on the US National Contingency Plan product schedule and expands the global knowledge base to include five additional species. The geometric mean of repeated species toxicity values was used in the generation of the first ever reported SSDs for each herder. In the case of M. beryllina, our toxicity results were similar to the values reported in the US National Contingency Plan product schedule (USEPA, 2021). However, this was not the case for A. bahia, where 95% confidence intervals for toxicity values from both herders did not overlap with those from the present study. This replication effort not only reduced uncertainty for M. beryllina but also indicates that additional testing of aquatic invertebrates may be prudent. Toxicity values were computed using nominal concentrations and did not account for the possible loss of toxicant over time during the 48–96 h static tests. Thus, our results for the two herders may be conservative, and the interpretation for hazard and risk assessments should consider the potential for our reported values to underestimate toxicity based on measured concentrations.

As with the hazard assessment of chemicals in general, limited diversity in the species of organisms with toxicity data can result in large uncertainties in understanding the potential hazards and risks to aquatic communities and habitats. Species sensitivity distributions are frequently used to develop community level estimates of chemical hazards when species-specific or habitat-specific toxicity data are not available. The minimum diversity of phyla and Kingdoms (or Domains) considered acceptable in an SSD approach varies with the taxa composition and number of data points and often relies on professional judgement. For example, US National Water Quality requires eight species with specified taxa diversity requirements (Belanger et al., 2017; Stephen et al., 1985). The ECHA recommends 10 species as the minimum with eight being acceptable for plant-oriented products (ECHA, 2016). Using only empirical hazard data from the present study and the limited available other information, SSDs for Siltech and ThickSlick were generated with eight aquatic species. The values of HC5 based on SSDs for only measured toxicity values for Siltech (1.4 ppm) and for ThickSlick (2.6 ppm). Within the Siltech SSD, M. beryllina was the second most sensitive species and would be protective of four-fifths of species based on the SSD. In comparison, A. bahia ranked fifth and would be protective of approximately 40% of species based on the SSD. Similarly, empirical toxicity data were used to construct a ThickSlick SSD with eight species. Within the ThickSlick SSD, M. beryllina ranked sixth out of eight in sensitivity and was estimated to be protective for a little more than a quarter of species, similar to what was found with respect to Siltech. Americamysis bahia ranked seventh and would be protective of less than a quarter of species based on the SSD. Thus, as previously reported, the standard test species (M. beryllina and A. bahia) may not be the most sensitive or reflective of an HC5 (Barron et al., 2013).

In general, a greater diversity of species and taxa included in an SSD is more likely to represent the true range of sensitivities to a particular chemical or agent. Because of the limited toxicity data available for the two herders, SSDs were augmented with estimates from the interspecies sensitivity modeling tool Web ICE. Augmented Web-ICE values resulted in a Siltech SSD with 54 species (HC5 = 0.97 ppm) and a ThickSlick SSD with 51 species (HC5 = 3.7 ppm). Augmented SSDs have been previously been reported to provide accurate HC5 estimates with similar uncertainty as SSDs generated with only measured values (Awkerman et al., 2014; Smetanová et al., 2014). Siltech’s HC5 value decreased by approximately 40% with the inclusion of extrapolated species and ThickSlick’s HC5 value increased by approximately 40%. Based on the Web-ICE augmented SSDs, Siltech appeared to be approximately fourfold more toxic than ThickSlick. The empirically based and augmented HC5 values for Siltech and ThickSlick were within the range of previously reported values for a variety of crude oils, oil products, and chemical dispersants (Barron et al., 2013; Bejarano, 2018; Hansen et al., 2014). For example, Barron et al. (2013) reported the HC5 of a 39 species SSD using data from five different crude and fuel oil SSDs to be 1 ppm and the HC5s of three different dispersants to be 5.2, 6.6, and 4.4 ppm. Testing with additional aquatic species and taxa are needed to better understand the toxicity and hazards of herders both within this class of spill response agents and in comparison to other categories of response agents, such as dispersants and surface-washing agents.

An aspect previously noted in the literature (Buist et al., 2008, 2018; Buist & Ross, 2010; Bullock et al., 2019; Fritt-Rasmussen et al., 2017; Lane et al., 2012) that herders have very low application rates compared with dispersants and other agent classes; thus, herders are expected to have correspondingly low concentration relative to crude oil in the water column. For example, the manufacture’s application recommendations for both herders are 15 L/km of area treated. The same report by Buist et al. (2018) gives the highest observed aqueous concentrations in their use experiments as being 0.02 ppm for Siltech and 0.01 ppm for ThickSlick. Although any one study cannot be completely authoritative, these limited monitoring data suggest that during intended use, it may be unlikely for herder concentrations in the water column to exceed the toxicity values determined in the present study. Herders have also been considered for use in spills in ice-affected waters and in wetland areas where use of alternative oil spill response measures may be less practical. Thus, assessing the hazards and risks to polar and wetland species are areas of potential future research.

Supplementary Material

Supplementary Material

Acknowledgment—

We thank R. Conmy and T. Parkerton for their review of a draft of this article. The present study was supported in part by an appointment to the Research Participation Program at the Office of Research and Development, US Environmental Protection Agency, administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the US Department of Energy and USEPA.

Footnotes

Supporting Information—The Supporting Information is availableon the Wiley Online Library at https://doi.org/10.1002/etc.5310.

DisclaimerThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in the present study. This manuscript has been subjected to US Environmental Protection Agency review and approved for publication. Approval does not signify that the contents reflect the views of the agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.

Data Availability Statement—

Data were cross-checked independently as part of the USEPA’s quality assurance and quality control procedures, and toxicity response data are publicly available at the USEPA’s Science Hub, as well as being included in the Supplemental Information associated with this publication. Data, associated metadata, and calculation tools are available from the corresponding author (alloy.matthew@epa.gov).

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary Material

Data Availability Statement

Data were cross-checked independently as part of the USEPA’s quality assurance and quality control procedures, and toxicity response data are publicly available at the USEPA’s Science Hub, as well as being included in the Supplemental Information associated with this publication. Data, associated metadata, and calculation tools are available from the corresponding author (alloy.matthew@epa.gov).

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