Heavy metals in the Arctic: Distribution and enrichment of five metals in Alaskan soils

Clarice R Perryman; Jochen Wirsing; Kathryn A Bennett; Owen Brennick; Apryl L Perry; Nicole Williamson; Jessica G Ernakovich

doi:10.1371/journal.pone.0233297

. 2020 Jun 3;15(6):e0233297. doi: 10.1371/journal.pone.0233297

Heavy metals in the Arctic: Distribution and enrichment of five metals in Alaskan soils

Clarice R Perryman ^1,^2,^*,^#, Jochen Wirsing ^3,^#, Kathryn A Bennett ^1,^2,^‡, Owen Brennick ^4,^‡, Apryl L Perry ^1,^2,^‡, Nicole Williamson ^5,^6,^‡, Jessica G Ernakovich ⁴

Editor: Yi Hu⁷

PMCID: PMC7269202 PMID: 32492035

Abstract

Metal contamination of food and water resources is a known public health issue in Arctic and sub-Arctic communities due to the proximity of many communities to mining and drilling sites. In addition, permafrost thaw may release heavy metals sequestered in previously frozen soils, potentially contaminating food and water resources by increasing the concentration of metals in freshwater, plants, and wildlife. Here we assess the enrichment of selected heavy metals in Alaskan soils by synthesizing publicly available data of soil metal concentrations. We analyzed data of soil concentrations of arsenic, chromium, mercury, nickel, and lead from over 1,000 samples available through the USGS Alaskan Geochemical Database to evaluate 1) the spatial distribution of sampling locations for soil metal analysis, 2) metal concentrations in soils from different land cover types and depths, and 3) the occurrence of soils in Alaska with elevated metal concentrations relative to other soils. We found substantial clustering of sample sites in the southwestern portion of Alaska in discontinuous and sporadic permafrost, while the continuous permafrost zone in Northern Alaska and the more populous Interior are severely understudied. Metal concentration varied by land cover type but lacked consistent patterns. Concentrations of chromium, mercury, and lead were higher in soils below 10 cm depth, however these deeper soils are under-sampled. Arsenic, chromium, mercury, nickel and lead concentrations exceeded average values for US soils by one standard deviation or more in 3.7% to 18.7% of the samples in this dataset. Our analysis highlights critical gaps that impede understanding of how heavy metals in thawing permafrost soils may become mobilized and increase exposure risk for Arctic communities.

Introduction

The Arctic is warming at double the rate of other areas of the globe [1]. Increasing temperatures are driving numerous ecological, cryospheric, and hydrological changes in the northern latitudes. One notable consequence of rising temperatures in the Arctic and sub-Arctic is widespread thaw of permafrost [2,3]. Permafrost thaw not only exacerbates the decomposition and liberation of carbon (C) to the atmosphere [4] and adjacent water bodies [5], but heavy metals stored in permafrost can also be released by thaw and transported into surface waters [6,7]. Heavy metals are present in Arctic soils due to weathering [8], atmospheric deposition [9], and anthropogenic activities including mining and/or smelting [10,11]. In permafrost-affected landscapes, metals may accumulate in frozen soils. As warming and permafrost thaw continue, the potential risk to human health posed by the liberation of heavy metals into surface waters and ecosystems may intensify.

Previous work demonstrates the potential of permafrost thaw as a driver of heavy metal liberation, which depends on each element’s solubility and redox chemistry as well as how thaw alters the hydrology of an affected area. Schuster et al. [12] showed that northern permafrost soils store more mercury (Hg) than all other soils, the atmosphere, and the oceans combined. Elevated Hg concentrations have been found downstream of thaw slumps [7], illustrating the potential for permafrost thaw to pose added danger to human health as this Hg may infiltrate water and food resources. Other metals may be similarly released by permafrost thaw. High concentrations of iron, manganese, nickel, and zinc have been found in permafrost-affected soils in northern Siberia [13], and elevated concentrations of iron, aluminum, chromium, and lead have been observed in areas that experienced rapid thaw [6]. Additionally, increases in soil organic matter (SOM) degradation during permafrost thaw can impact metal mobility as metals bound to SOM are released [14]. Plants also take up metals from soil, and do so more rapidly under warming [15] and elevated CO₂ conditions [16,17]. While the ability of permafrost thaw to mobilize heavy metals is increasingly recognized, further work is needed to assess the presence of heavy metals in Arctic soils due to their potential to impact human health.

Subsistence hunting, fishing, and foraging are practices that are both nutritionally and culturally important to Arctic peoples; increasing levels of heavy metals in Arctic landscapes, due to liberation from thawing permafrost, would threaten the safety of this practice. Heavy metals and other toxins have been identified in Arctic freshwater ecosystems [18–20], wildlife [21], indigenous foods [22], and indigenous peoples [23,24]. Kenny et al. [25] report that country foods—especially caribou, beluga, ringed seal, and fish—are an important source of protein and vitamins for communities. Due to bioaccumulation, these species contain increased levels of arsenic, mercury, cadmium, lead, and selenium [22] and persistent organic pollutants [26] which have been found in blood and breast milk in communities that consume country foods. Drinking water resources may also be threatened by metal liberation through permafrost thaw, given previous observations of thaw-driven increases in freshwater metal concentration [7,27] and widespread reliance on surface water resources throughout the Arctic [28].

We focus on five heavy metals that have both documented occurrences in Arctic food and water resources and have substantial human health effects: arsenic (As), chromium (Cr), mercury (Hg), nickel (Ni), and lead (Pb). Arsenic is a carcinogen that can accumulate within the body [29] and has been found in concentrations 14 times higher than the safe threshold levels in some commonly consumed Alaskan fish [22]. Lead and mercury cause neurological damage that is most significant in young children and passes easily from nursing mothers to infants [30,31]. Nickel and chromium are both carcinogens and allergens, and chromium can have negative impacts on kidney health [32,33]. The presence of these metals in the Arctic food chain and their subsequent human health effects are well documented, but the amount of heavy metals stored in Arctic soils and the risk posed by potential metal liberation through permafrost thaw remains largely unknown. While previous work has examined the concentration of heavy metals in soils in sites within the permafrost zone, to date there has been a lack of data synthesizing the prevalence of heavy metals in permafrost-affected soils, and therefore the potential pool of heavy metals that could be released as permafrost thaws on a global scale. Furthermore, studies that quantify the concentration and/or thaw-driven liberation of heavy metals in Arctic soils are generally conducted in remote areas [7,13], so it is unclear if available data on soil metals includes areas in proximity to communities that may be affected by the release of these metals through permafrost thaw.

Our original aim was to conduct a meta-analysis bringing together publicly available, machine readable data to assess the occurrence of heavy metals in soils across the pan-Arctic, but a lack of such datasets in public repositories precluded this analysis. As such, in this study we synthesize available data on the concentration of As, Cr, Hg, Ni, and Pb in soils in Alaska between 60–70°N, where Arctic tundra is prevalent, to assess the occurrence of elevated metal concentrations and the risk of heavy metal exposure in permafrost-affected soils in this region. The objectives of this study were to (a) to identify the spatial distribution of sites across the Alaskan Arctic where soil concentrations of As, Cr, Hg, Ni, and Pb have been quantified; (b) determine the heterogeneity of soil heavy metal concentrations across land cover types, depth in the soil profile, and in different permafrost zones; and (c) compare soil metal concentrations from Alaska to average concentrations found in other soils in the United States.

Materials and methods

Database search

We searched the Arctic Data Center, National Ice and Snow Data Center, and the United States Geological Survey (USGS) databases for datasets containing data for soil concentrations of As, Cr, Hg, Ni, and Pb. The Arctic Data Center and the NSIDC repositories did not contain any databases that met our search criteria (e.g., soil metal concentrations). The USGS had three datasets matching our search criteria: Alaskan Geochemical Database Version 2.0, the PLUTO Soil Database, and the National Geochemical Survey Database. The Alaskan Geochemical Database Version 2.0 (AGDB2), superseded the PLUTO Soil Database and the National Geochemical Survey Database, so only the AGDB2 database was relevant to our query. Our subsequent analysis was performed using the AGDB2 “Best Value” soils data https://pubs.usgs.gov/ds/759/[34].

Data preparation and analysis

All data cleaning, analysis, and figure creation were performed using R (v. 3.5.2) [35]. Maps were created using ArcGIS (v. 10.7.1). Data cleaning was performed using the tidyverse R package [36] unless otherwise specified. The data were filtered to retain information on: concentrations of As, Cr, Hg, Ni, and Pb, sample depth, location (latitude, longitude), source (land cover type), collection year, and analytical method. The AGDB2 “Best Value Soil” dataset we used in our analysis was pre-screened by USGS chemists for quality control. The USGS Best Value ranking system selected the method providing the most accurate value for each sample that had been analyzed using more than one analytical method. Samples missing latitude, longitude, depth, and land cover type information were removed. Rows containing negative and/or absent values for As, Cr, Hg, Ni, and Pb were also excluded from the analysis. In the instances where groups of samples had identical concentration values, these data were removed as they were assumed to be erroneous. The resulting data table contained 1,151 rows representing individual samples; however, not every sample had a value for the concentration of all 5 metals. Sampling depths were converted into centimeters (cm) and manually binned into three depth groupings (0–10 cm, 10–30 cm, and > 30 cm) in Excel. Land cover type had been coded by individual research projects over the course of data collection (c. 1995–2010) at USGS. As such, classifications were not consistent across data sources or between samples in close proximity, varying from information on land cover type to drill sample type. To simplify these labels, the classifications describing similar land cover types were grouped together to match broader classifications from recent land-cover maps of Alaska [37,38] in accordance with their location within the state. For example, all previous mine categories were condensed into a single “mine-impacted” group. The resulting land cover type codes are: forested, glacial deposit, tundra, wetland, and mine-impacted. The post-processing data included in our analysis can be found in the S1 File.

Metal concentrations were checked for normality and homogeneity of variance within land cover types and depths using the Shapiro-Wilks Test and Bartlett Test. Both raw and log-transformed data failed to meet the assumptions for ANOVA. The Kruskal-Wallis Test from the agricolae R package [39], which uses a Bonferroni correction and the Fisher's Least Significant Difference post-hoc test was used to assess differences in metal concentrations between land cover types and/or depths [40]. Significance was determined at the α < 0.05 level. While analyses were performed on untransformed data, data presented in boxplots are natural log transformed due to the very large range of metal concentrations in the data set. All figures were created using the ggplot2 R package [41].

We also assessed the proportion of samples in the dataset with elevated metal concentrations using reported average concentrations in the A horizon of soils from across the United States from the USGS [42] (Table 1). We compared soil metal concentrations in the USGS AGBD2 to values from uncontaminated soils across the United States to assess how Alaskan soil metal concentrations compared relative to other biomes and climatic regions. Maps of sample locations showing the distribution of sampling sites, the land cover types from which the samples were collected, and the location of samples with elevated metal concentrations were created in ArcGIS (v. 10.7.1). We also mapped the locations of the sampling sites in the AGBD2 alongside locations of sites with known contamination from the Alaska Department of Environmental Conservation [43] to see if co-location with contaminated sites affected the concentrations we report.

Table 1. Summary of metals concentrations.

Metal	Number of observations	Range (mg kg^-1)	Mean ± SD (mg kg^-1)	Median (mg kg^-1)	IQR (mg kg^-1)	US average (mg kg^-1)^a
As	961	0.39–14900	188 ± 1120	11.0	7.0–20.0	6.6 ± 19.6
Cr	1072	1.0–350	59.2 ± 38	56.0	33.3–78.0	37 ± 89
Hg	800	0.01–6090	30.4 ± 0.06	0.06	0.04–0.09	0.04 ± 0.17
Ni	1073	2.0–702	44.6 ± 61	28.3	19.0–41.6	18.5 ± 54.4
Pb	1081	2.0–720	21.1 ± 41.1	13.0	9.0–19.0	22.2 ± 46.6

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IQR = interquartile range; first quartile–third quartile

^aValues from Smith et al., 2013, Geochem. and Mineralogical Data for Soils of the Conterminous US, USGS Data Series 801

Results

Sample distribution and metal concentrations across depth and land cover types

Sample locations in the dataset used for analysis were primarily in south and southwestern Alaska (Fig 1) and few sampling localities are in proximity to towns or settlements. The dataset also contained very few (< 10) samples located in the continuous permafrost zone; the bulk of sample locations were located in discontinuous and sporadic permafrost. Metal concentrations are summarized in Table 1.

Metal concentrations varied between sampling depth intervals of 0–10 cm, 10–30 cm, and below 30 cm for all metals except As (Fig 2). While Ni concentrations are highest above 10 cm (χ²_[2] = 63.38, p < 0.001; Kruskal-Wallis multiple comparisons of means and Fisher’s Least Significant Difference tests for all comparisons reported here), Cr, Hg, and Pb have higher concentrations at depth. Concentrations are highest in the 10–30 cm depth interval for Cr (χ²_[2] = 43.14, p < 0.001) and Pb (χ²_[2] = 11.39, p = 0.003) and below 30 cm for Hg (χ²_[2] = 6.68, p = 0.035). Metal concentrations also varied between land cover types (S1 Fig). Wetland sites had the lowest concentration of both As and Pb (χ²_[4] = 99.76, p < 0.001; χ²_[4] = 11.91, p < 0.001). Forested sites had higher concentrations of Cr and Ni and lower concentrations of Hg than other vegetated land cover types. Mine-impacted soils have higher concentrations of all metals except Ni, for which mine-impacted sites and forested sites had comparable concentrations (p < 0.001 for all).

Comparison to US soil averages

Concentrations of As, Cr, Hg, Pb, and Ni in soils across Alaska exceed US average soil concentrations [42] (Fig 3 and Table 1). We quantified the percentage of samples with metal concentrations greater than reported US averages, as well as those that exceeded the reported averages by a factor of 2, a factor of 10, and by 1 standard deviation of the national average.

Over 70% of soil samples had concentrations of As, Cr, Hg, and Ni greater than published average values of soil metals concentrations across the United States. Further, 18% of samples had Hg concentrations higher than 0.1 mg kg^-1, compared to 12% found in a synthesis of soils in the western USA by Obrist et al. [47]. Over 25% of samples had concentrations that exceeded average concentrations of As, Cr, Hg, and Ni by a factor of 2, and nearly 10% of soil samples in the AGDB2 had As concentrations that exceeded the US average by a factor of 10. Lead concentrations were generally lower than the US average, and only 16.3% of samples reported here were greater than the reported US average concentration. Across the metals, 3.7% to 18.7% of the samples in the database had elevated concentrations that exceeded the US average values by one standard deviation or more (Fig 3), with As having the largest and Cr having the smallest proportion of samples with such elevated concentrations. Sampling locations in which soils exceed reported averages by one standard deviation or more are distributed throughout the study region (Fig 3).

Discussion

An overarching challenge in synthesizing large datasets such as the AGDB2 are the large ranges and variances found in the data, as shown in Table 1. Despite the large variance in the data, the heavy metal concentrations in the soils in the AGDB2 dataset are within the range of those previously reported in other parts of the Arctic. For example, the median values we report for As, Ni, and Pb of 11.0, 28.3 and 13.0 mg kg^-1, respectively, fall within the range previously reported in permafrost affected soils from the Lena Delta in Siberia of As ranging from 1.29 to 11.3 mg kg^-1, Ni from 5.12 to 30.2 mg kg^-1, and Pb from 2.41 to 13.1 mg kg^-1 [13]. The interquartile range (IQR) we report for Ni and Pb is the same order of magnitude as the range of concentrations reported by Barker et al. [27] in soil from the Imnavait Creek watershed in Alaska of 13.9–23.7 mg kg^-1 and 4.8–13.1 mg kg^-1, respectively. Halbach et al. [48] measured a suite of metal concentrations in Svalbard, and they report median As, Cr, and Ni concentrations approximately 2 to 4 times lower than the median concentrations reported in this study. Mercury and Pb concentrations from Svalbard [48] of 0.023 to 0.107 mg kg^-1 and 10.6 to 10.7 mg kg^-1 are the same order of magnitude as our reported medians of 0.06 mg kg^-1 and 13.0 mg kg^-1. Similar levels of soil Hg have been reported from numerous sites across the Arctic, including the Canadian High Arctic [49], the Taimyr Peninsula [50], Lena Delta [13] in Siberia, Alaska [12], and Sweden [51]. The median and IQR of Hg concentrations reported here are in agreement with previous work which finds that Hg in Arctic soils tends to range from below detection (~0.01 mg kg ^-1) to below 0.1 mg kg^-1.

To contextualize soil metal concentrations and their relevance to human health, we defined “elevated concentrations” as those greater than the average value for US soils by one standard deviation or more. Soil metals with elevated concentrations are distributed across the study area in Alaska regardless of land cover type and depth, and for the metals examined in this study, between 3.7% and 18.7% of samples had elevated concentrations (Fig 3). While we originally intended to contextualize our findings against human health guidelines, we decided not to for two primary reasons. One, health guidelines are generally for oral and/or dermal exposure and may exceed concentrations at which metal liberation through hydrologic and biogeochemical cycles is a concern. Furthermore, the acceptable heavy metal concentrations in soils allowed by health guidelines vary widely across countries; for example, Kamunda et al. [52] report maximum allowable limits of the metals examined in this study in soils from different nations that vary over one to two orders of magnitude. Given the lack of international consensus amongst health professionals and the fact that these comparisons are highly sensitive to the standards selected, we chose to forgo a comparison to health standards. Instead, we chose to base our definition of “elevated concentrations” on observations of other soils.

Concentrations of As, Cr, Hg, Ni, and Pb in mine-impacted areas of Alaska were especially elevated (Fig 2), likely due to heavy metal air pollution from mines and subsequent wet/dry deposition onto soils [10,53]. However, soils with metals concentrations that exceed average values from the contiguous United States are distributed across the study region and are not isolated to mine-impacted sites or sites with close proximity to sources of anthropogenic contamination (S2 Fig). The proportion of samples with elevated concentrations were comparable when calculated with and without mine-impacted sites (S2 Fig), indicating that the small number (n < 20 for all metals, S1 Fig) of mine-impacted sites did not disproportionately impact the proportion of samples with elevated concentrations. This suggests that “pristine” soils in Alaska (i.e. those from outside areas with heavy anthropogenic disturbance) can also contain elevated levels of heavy metals which may be released through warming and thaw. Aside from anthropogenic contamination, heavy metals in soils can be derived from lithogenic pools that become distributed across surface environments through natural biogeochemical cycles and near-surface processes (i.e. erosion, weathering, pedogenesis) [54]. Furthermore, the bias towards surficial soils in the AGDB2 (Fig 2) impedes distinguishing if elevated concentrations are a result of background levels or from surface contamination.

Regardless of the source of heavy metals, their potential liberation as permafrost thaw and climatological drivers alter hydrological and biogeochemical cycles in Arctic and sub-Arctic regions is a pressing concern. Critical gaps in publicly available data for heavy metals in these soils preclude a thorough assessment of the pool of heavy metals that may be released from thawing permafrost. Over 99% of sample locations within AGDB2 are located in sporadic, isolated, and discontinuous permafrost, which combined make up only ~50% of Alaska [46]. Soil metal concentrations in the AGDB2 severely underrepresent soils in continuous permafrost. This limits our ability to infer whether soils in the continuous permafrost zone in Alaska sequester heavy metals. Likewise, 74% of the soil samples in AGDB2 were surface samples between 0–10 cm. Metal concentrations tended to be higher below 10 cm depth (Fig 2), suggesting that assessing metal concentrations in surface soil samples alone may not accurately reflect heavy metal concentrations through the soil profile. Most previously published work on heavy metals in permafrost-affected soil focuses on the seasonally thawed layer and/or surface soil, which further inhibits inference about the sequestration of heavy metals in permafrost. In two studies that did investigate deeper soil horizons, metals were found to be higher both just above the permafrost table [13] and within permafrost [27], a reflection that the prevalence of heavy metals in permafrost soil profiles may be complicated by site lithology and hydrology, mobility of the heavy metal in question, and previous thaw history. We suggest that future research on soil metal concentrations should include sampling of deeper soil, including permafrost.

Future work should attempt to constrain how heavy metals in soils move through terrestrial, aquatic, and human ecosystems in a warming Arctic, and the implications of widespread elevated soil metal concentrations to communities in the Alaskan Arctic. Heavy metals have long been found in human blood and tissues in Arctic communities [28], largely through exposure to heavy metals through food [55]. Arctic communities are particularly susceptible to threats to water security from climate change [28], and potential liberation of metals through permafrost thaw may additionally threaten water quality in the Arctic [7,27]. Our study shows that high concentrations of heavy metals are distributed in soils across Alaska, suggesting permafrost thaw and associated changes in the environment may present an unaccounted-for pathway of heavy metal exposure. This conclusion is supported by previous observations in Alaska of suspended river and coastal sediments enriched in heavy metals [56,57], increased elemental and sediment loading into freshwater ecosystems as permafrost thaws [58,59], and of a seasonal signature of stream water metal concentrations that coincides with the depth of seasonal thaw [27]. Given recent observations of widespread and rapid permafrost thaw due to ongoing warming [60] and increases in wildfires that accelerate thermokarst expansion [61], more attention to the threat of heavy metal liberation from thawing permafrost is needed. A critical first step is ameliorating gaps in the existing data, which underrepresent continuous permafrost, deeper soils horizons, and locations near human communities.

Conclusion

While the metal concentrations in both USGS datasets include large variances, our synthesis of publicly available data on soil concentrations of As, Cr, Hg, Ni, and Pb in Alaska indicates that metal concentrations in soils in southern and southwestern Alaska often exceed average US soil concentrations by a factor of 2 or more. The prevalence of As, Cr, Hg, Ni, and Pb in soils largely in the discontinuous and sporadic permafrost zone emphasizes the need for better understanding of how permafrost thaw alters the mobility and cycling of heavy metals. This synthesis highlights three gaps in publicly available data of heavy metal concentrations in permafrost-affected soils in Alaska: the lack of sampling near human populations, limited data on metal concentration in the continuous permafrost zone, and a bias in previous sampling towards surface (upper 10 cm) soils. Filling these data gaps will be necessary for understanding the potential for liberation of heavy metals from permafrost thaw into food and water resources that may present an unaccounted-for pathway of heavy metal exposure for communities in a warming Arctic.

Supporting information

S1 Fig. Metal concentrations by land cover type.

Concentrations of As (A), Cr (B), Hg (C), Ni (D), and Pb (E) across different land cover types. All concentrations are displayed on a natural log scale. Chi-squared test statistic and alpha value results of the Kruskal-Wallis tested used to determine differences by land cover type for each metal are reported above each plot, DF = 4 for all. Letters above boxes represent groups determined by the Fisher's Least Significant Difference test. Numbers below boxes represent the number of observations.

(EPS)

Click here for additional data file.^{(3.4MB, eps)}

S2 Fig. Location of known contaminated sites and impact of mine sites on the proportion of samples with elevated metal concentrations.

(Left) Overview map of study area including known contaminated sites from the Alaska Department of Environmental Conservation (https://dec.alaska.gov/Applications /SPAR/PublicMVC/CSP/Search/). (Right) Comparison of the percent of sampling sites in our cleaned dataset that had metal concentrations over US national averages by one standard deviation or more (upper right) and human health guidelines (upper left) with and without sites denoted in the dataset as mine-impacted included. Reprinted map layers from the US Census Bureau [44], Alaska Department of Natural Resources [45], and the North Slope Science Initiative for the US Fish and Wildlife Service [46].

(EPS)

Click here for additional data file.^{(26.3MB, eps)}

S1 File. Post-processing data from the USGS Alaskan Geochemical Database Version 2.0 used in our analysis.

(CSV)

Click here for additional data file.^{(168.8KB, csv)}

Acknowledgments

We would like to thank the members of the New England Arctic Network (NEAN) for their feedback throughout the development of this work, especially NEAN members Jack Dibb, Katherine Duderstadt, Robyn Barbato, Ruth Varner, Julie Bryce, Florencia Fahnestock, and Stacey Doherty. We would also like to thank the Arctic Science IntegrAtion Quest (ASIAQ) for their support and the academic editor and 5 reviewers of this manuscript for their time and thoughtful feedback. Database searches, data analysis, and preliminary manuscript preparation was conducted through a course offered by the UNH Department of Natural Resources and the Environment, and we thank the department for their support of the course.

Data Availability

The raw data files used in this analysis can be accessed through the United States Geological Survey (https://doi.org/10.3133/ds759). We used a subset of the Alaskan Geochemical Database Version 2.0 "Best Value Soil" dataset in our analysis. All data used in our analysis are within the paper and its Supporting Information files.

Funding Statement

The author(s) received no specific funding for this work.

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PLoS One. doi: 10.1371/journal.pone.0233297.r001

Decision Letter 0

Yi Hu

12 Dec 2019

PONE-D-19-30690

Heavy metal in the Arctic: Characterizing the distribution of five metals in soil in the Alaskan Arctic

PLOS ONE

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Reviewers' comments:

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Comments to the Author

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Reviewer #1: No

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: I Don't Know

Reviewer #5: Yes

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Reviewer #1: No

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

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Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: see attachement

The manuscript “Heavy metal in the Arctic” addresses important scientific problem and uses unique set of data which should be presented to the scientific community

However the treatment of the data, comparison with other available studies and discussion of mechanisms and consequences are not at the level suitable for an academic publication

Reviewer #2: The manuscript presents relevant new information and has high quality. However, the manuscript presents few points that need to be revised. The detailed comments are listed below:

Keywords

1. Delete all keywords already in title.

Introduction

1. Line 114 � Delete “to”.

Materials and Methods

1. Line 128: Delete “Alaskan Geochemical Database Version 2.0”.

Results

1. Table 1 presentation should be improved.

Discussion

1. Line 243 and all manuscript: mg/kg � mg kg-1.

2. Second paragraph: What implications can these results have on the environment?

Transfer to surface and subsurface waters?

Line 310: It does not reduce SOM's ability to retain metals, it only reduces its soil contents. Rewrite.

Reviewer #3: PLOS ONE

Manuscript Number: PONE-D-19-30690

Title: Heavy metal in the Arctic: Characterizing the distribution of five metals in soil in the Alaskan Arctic

The article is clearly presented and straightforward. While the authors did not synthesize any of their own data, they do present a large publically available dataset in a concise way that has not been done before for Alaska. Particularly, the key finding that there is a bias in sampling towards the upper 10 cm of soils and that may be biasing values to be preferentially lower for some metals versus others that tend to accumulate more in the deeper subsurface is an important finding.

Overall, comments are relatively minor. There are two major edits that need to be addressed: (1) the authors need to be more clear when comparing Alaska values to ‘US averages’ and ‘health standards’, it is confusing and somewhat misleading as written (explained further below in the line items) and (2) the authors need to clarify background values for the different classification ‘bin’ types – there should be published values on background concentrations for Alaska soils (also would be important to compare to crustal abundances for the metals studied).

Line items:

Title: Is there a reason the first ‘metal’ mentioned is not plural – i.e. ‘Heavy metals in the Arctic’ – sounds odd as it is since you are studying more than one metal.

Lines 62-64: I would clarify that there are also heavy metals present in the Arctic naturally in the soils at low concentrations in addition to anthropogenic sources.

Line 83: consider changing ‘peoples’ to ‘communities’

Lines 86 and 90: consider changing ‘country’ to subsistence-derived’

Line 95: is there a verb missing between ‘and’ and ‘substantial’?

Lines 150-153: Can you go more in depth on the classification systems that you re-coded? For the re-coded ‘tundra’ category does the included ‘shrub-covered’ and ‘grassland/grazing’ mean there were underlain with permafrost? Go more in-depth about the original USGS land-cover categories and what they meant and why you chose to include those groups together. I would assume the ‘tundra’ re-coding includes only places with permafrost? As it is written, it is unclear what those categories actually mean.

Line 165: What do you mean by ‘there are no conclusive U.S. standards’… for what? Drinking water? Soils? Fish? The US has drinking water standards, soil standards, etc. What do you mean by that statement?

Lines 171-172: Please discuss the criteria for how ATSDR determined the average concentrations of these metals in soils. It will help with your later comparisons for Alaska. Was Alaska included in the ATSDR calculations?

Figure 2: resolution looks blurry – it looks different than the other images in terms of resolution

Line 223: Which human health standard are you referring to? This whole paragraph refers to human health standards and US averages – list those standards and averages for referring to

Figure 3: Alaska is known to have high arsenic values naturally in their soils (in addition to mining activities) – I think it’s important to note the background values for all these metals. Arsenic in Alaska, for example, is always going to have higher than US average soils because there is an abundance of arsenic in soils up here – not necessarily due to ‘contamination’ – that is known. There needs to be a discussion of background values and crustal abundances for all the metals examined here. Also, please include what ‘health limit’ you are referring to on the figure somewhere – either in the caption or on the plot. The value and also what type of limit – drinking water, etc. It is very unclear, as is.

Methods section: you mentioned that you separated published values based on analytical method – any information on that in the document? Are some analyses different than others? There should be some mention of that in the results, etc.

Section ‘Comparison to health standards and US averages’: Does the US average include Alaska and Hawaii samples or not?

Discussion, 1st paragraph: Please note somewhere within this text that you are comparing average values for the entire state of Alaska (including legacy and active mining sites and likely some military sites) with concentrations from pristine sites (citations 12, 13, 45, 46) – the values should be different so that is not a new finding. The paragraph is a bit misleading as written – what would be more appropriate is to compare areas in Alaska with no known anthropogenic sources (Hg will be difficult to do this with) to the citations 12, 13, 45, and 46 and vice versa – compare legacy contamination with the ‘high’ values found in this report. Or only look at the ‘arctic’ values where there is little contamination.

Line 262: except Hg? You mentioned earlier that Hg was higher in the 0-10 cm bin.

Lines 268-271: Remove or reword the statement about permafrost acting as a barrier to metal migration – this statement as written assumes the metal concentrations found in the two studies mentioned (Antcibor and Barker) migrated down into permafrost and that is why the concentrations are higher. Some of these soils are very old and are a result of glacial deposits or wind-blown deposition processes over time and not as a result of metals leaching downward from the surface.

Line 291: Include citation for arsenic mobility, same for the next line on chromium

Reviewer #4: My review is submitted as an attachment. I suggest major changes in the manuscript and request that the authors consider revising some of their health standards and US soil average values. I do believe that the paper should be published after changes submitted.

Reviewer #5: Summary

Heavy metals in the environment can result in detrimental effects on the health of the public. Metals have been deposited across the Alaskan Arctic over time, becoming sequestered in permafrost. Melting permafrost due to climate change could result in the release of these metals to the environment and increase exposure to humans and wildlife. This study evaluated soils concentrations of heavy metals across the Alaskan Arctic using publicly available data. Soil heavy metal concentrations in the Alaskan Arctic were generally found to be elevated compared the average concentrations of the contiguous US. This study provides a novel synthesis of soil heavy metal concentrations across the Alaskan Arctic and identifies important gaps for future research efforts. Additionally, this study improves the understanding of health risks associated with possible heavy metal releases from permafrost thaw in the Alaskan Arctic.

Major comments

It’s difficult to determine from the methods, but were concentrations of all 5 heavy metal known for any/some/many of the included sampling locations? This could be made clearer in the method. While it may be outside the scope of this study, it would be interesting to look at the relationship among the metals sampled at the same location (if this occurs).

Minor comments

Line 47: Change “concentration” to “concentrations”

Line 76 (88, 113): Be consistent in including the element symbols in the first mention and using the symbol in subsequent mentions

Line 101: It would be good to focus on health effects that may result from exposure due to release from permafrost, it does not seem relevant to include “respiratory problems from inhalation”

Line 111: The term “permafrost-affected soils” seems strange, maybe use different terminology?

Line 164 -170 and 216: It would be useful to include how you determined the health standards based on types of exposure. For instance, you reference oral and dermal exposure in line 216 for As. This information should be included in the methods, did you use oral and dermal health standards are each metal used? Were other types for exposure standards used?

Line 170-175: Include that these averages are for uncontaminated soils (as explained in line 278)

Line 198 and 208: Change “sample sources” to “land-cover types”, be sure to keep the terminology consistent

Line 212: Change “source” to “type”

Line 298-299: Remove “have been found”

Lines 321-328: It would be good to include some discussion about the depth profile (> 10, 10-30, <30 cm) and how that might relate or be important to transport from melting permafrost

Line 334: change “concentration” to “concentrations”

Fig 1. The pink color of the mine-impacted sites is hard to see, particularly due to the similarity to the pink color of the town/settlements

Fig. 3. This is figure is blurry

Fig. 3. There are some orange dots in the Ni map, are those supposed to be there?

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Reviewer #2: No

Reviewer #3: No

Reviewer #4: No

Reviewer #5: No

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Attachment

Submitted filename: Soil Alaskan Arctic 2019.docx

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Attachment

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PLoS One. 2020 Jun 3;15(6):e0233297. doi: 10.1371/journal.pone.0233297.r002

Author response to Decision Letter 0

14 Mar 2020

Our responses to all Reviewer comments can be found in our Response to Reviewers document. A summary of the revisions and improvements is also provided in our Cover Letter for the revised manuscript.

Attachment

Submitted filename: AKSoilMetals_ReviewerResponses.docx

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PLoS One. doi: 10.1371/journal.pone.0233297.r003

Decision Letter 1

Yi Hu

3 Apr 2020

PONE-D-19-30690R1

Heavy metals in the Arctic: Distribution and enrichment of five metals in Alaskan soils

PLOS ONE

Dear Ms. Perryman,

We would appreciate receiving your revised manuscript by May 18 2020 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.
A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.
An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

We look forward to receiving your revised manuscript.

Kind regards,

Yi Hu

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

Reviewer #3: All comments have been addressed

Reviewer #4: (No Response)

Reviewer #5: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

**********

6. Review Comments to the Author

Reviewer #1: The comments of 1st reviewer were adequately adressed, and the arguments of authors (where they disagree) are well taken. The paper is in the shape which is suitable for publication; it represents clear added value.

Reviewer #2: (No Response)

Reviewer #3: (No Response)

Reviewer #4: The revised manuscript (PONE-D-19-30690R1) is improved due to efforts by the authors to address numerous suggestions from the reviewers. An overreaching difficulty in writing the paper was trying to work with very large ranges and standard deviations (SD) in both the Alaska and the US soil data. I encourage the authors to mention this challenge in the beginning of the Discussion and again in the Conclusions.

I have a few suggestions listed below in order of appearance in the text.

1. Line 40 and many others. I do not think that various forms of the word “liberate” (e.g., lines, 40, 64, 69, 72 and several more throughout the text) best explain the process of “release” or “mobilization” of metals from previously frozen soils. I prefer not to see the much overused word “liberate” in the manuscript.

2. Line 41. “exacerbate contamination” implies that the soils are already contaminated. Although this is certainly true in a few cases, it does not seem to be the norm nor is it adequately proven. Therefore, “… potentially contaminating food and water” … seems more defendable.

3. Lines 46-57. After listing Items 1), 2) and 3), I prefer to see responses to each to follow in the same order. Therefore, the first sentence after the list would be “We found substantial clustering …”.

4. Overall, the Abstract, Introduction and Discussion could use some careful editing with a few hours of help from each of the other authors.

5. Table 1. Better use of significant figures is essential. Use of more than 3 significant figures seems unacceptable (e.g., 14900.0 [6 sig figs] goes to 14900 [3 sig figs]; 112.73 [5 sig figs] goes to 113 [3 sig figs]).

Pb (mean ± SD) is much more realistic at 21 ± 41, and so on.

Table 1 really prompted my introductory comment about challenges of the large ranges and SDs. Care is certainly required when comparing two data sets with such large ranges and SDs.

To me, range seems better than maximum and minimum (e.g., 2‒720 for Pb).

As an aside, in rethinking the statistics, I probably would have used medians and median

absolute deviations (MAD, as in Smith et al., 2013). No need for the authors to make this change.

6. Line 257 and many more. I previously commented “I see too much use of the word ‘comparable’. All data are comparable. The point is whether means for one set of data are statistically different or not from another set of data.”

I still see "comparable" used too often (e.g., lines 257, 259, 264, 267)

7. I think the authors make their point and thus the Conclusion seems to work well.

Reviewer #5: (No Response)

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Oleg S. Pokrovsky

Reviewer #2: Yes: Tadeu Luis Tiecher

Reviewer #3: No

Reviewer #4: No

Reviewer #5: No

PLoS One. 2020 Jun 3;15(6):e0233297. doi: 10.1371/journal.pone.0233297.r004

Author response to Decision Letter 1

30 Apr 2020

Our response to all reviewer comments can be found in our uploaded Response to Reviewers document. A summary of changes in this revision is also included in our Cover Letter for the resubmission.

Attachment

Submitted filename: AKSoilMetals_ReviewerResponses2.docx

Click here for additional data file.^{(16KB, docx)}

PLoS One. doi: 10.1371/journal.pone.0233297.r005

Decision Letter 2

Yi Hu

4 May 2020

Heavy metals in the Arctic: Distribution and enrichment of five metals in Alaskan soils

PONE-D-19-30690R2

Dear Dr. Perryman,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

Within one week, you will receive an e-mail containing information on the amendments required prior to publication. When all required modifications have been addressed, you will receive a formal acceptance letter and your manuscript will proceed to our production department and be scheduled for publication.

Shortly after the formal acceptance letter is sent, an invoice for payment will follow. To ensure an efficient production and billing process, please log into Editorial Manager at https://www.editorialmanager.com/pone/, click the "Update My Information" link at the top of the page, and update your user information. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, you must inform our press team as soon as possible and no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

With kind regards,

Yi Hu

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Reviewer #4: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #4: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #4: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #4: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #4: Yes

**********

6. Review Comments to the Author

Reviewer #4: Great persistence on your part produced a much improved, interesting and useful manuscript.

Congratulations.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #4: No

PLoS One. doi: 10.1371/journal.pone.0233297.r006

Acceptance letter

Yi Hu

8 May 2020

PONE-D-19-30690R2

Heavy metals in the Arctic: Distribution and enrichment of five metals in Alaskan soils

Dear Dr. Perryman:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please notify them about your upcoming paper at this point, to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

For any other questions or concerns, please email plosone@plos.org.

Thank you for submitting your work to PLOS ONE.

With kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Prof. Yi Hu

Academic Editor

PLOS ONE

Associated Data

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

Supplementary Materials

S1 Fig. Metal concentrations by land cover type.

(EPS)

Click here for additional data file.^{(3.4MB, eps)}

S2 Fig. Location of known contaminated sites and impact of mine sites on the proportion of samples with elevated metal concentrations.

(EPS)

Click here for additional data file.^{(26.3MB, eps)}

S1 File. Post-processing data from the USGS Alaskan Geochemical Database Version 2.0 used in our analysis.

(CSV)

Click here for additional data file.^{(168.8KB, csv)}

Attachment

Submitted filename: Soil Alaskan Arctic 2019.docx

Click here for additional data file.^{(15.2KB, docx)}

Attachment

Submitted filename: Reviewer 1 for PONE-D-19-30690.docx

Click here for additional data file.^{(21.4KB, docx)}

Attachment

Submitted filename: AKSoilMetals_ReviewerResponses.docx

Click here for additional data file.^{(133.1KB, docx)}

Attachment

Submitted filename: AKSoilMetals_ReviewerResponses2.docx

Click here for additional data file.^{(16KB, docx)}

Data Availability Statement

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PERMALINK

Heavy metals in the Arctic: Distribution and enrichment of five metals in Alaskan soils

Clarice R Perryman

Jochen Wirsing

Kathryn A Bennett

Owen Brennick

Apryl L Perry

Nicole Williamson

Jessica G Ernakovich

Roles

Abstract

Introduction

Materials and methods

Database search

Data preparation and analysis

Table 1. Summary of metals concentrations.

Results

Sample distribution and metal concentrations across depth and land cover types

Fig 1. Overview map of study area.

Fig 2. Metal concentrations by sample depth.

Comparison to US soil averages

Fig 3. Location and proportion of samples over US averages for soil metal concentrations.

Discussion

Conclusion

Supporting information

Acknowledgments

Data Availability

Funding Statement

References

Decision Letter 0

Yi Hu

Roles

Author response to Decision Letter 0

Decision Letter 1

Yi Hu

Roles

Author response to Decision Letter 1

Decision Letter 2

Yi Hu

Roles

Acceptance letter

Yi Hu

Roles

Associated Data

Supplementary Materials

Data Availability Statement

ACTIONS

PERMALINK

RESOURCES

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