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\begin{document}

\title{Reemission of inorganic pollution from permafrost? -- a freshwater
hydrochemistry study in the lower Kolyma basin (North-East Siberia)}



\author[1]{Danuta Szumińska}%
\author[1]{Krystyna Koziol}%
\author[1]{Sergiej R. Chalov}%
\author[2]{Vasilii A. Efimov}%
\author[3]{Marcin Frankowski}%
\author[4]{Sara Lehmann-Konera}%
\author[5]{Żaneta Polkowska}%
\affil[1]{Uniwersytet Kazimierza Wielkiego w Bydgoszczy Instytut Geografii}%
\affil[2]{Moskovskij gosudarstvennyj universitet imeni M V Lomonosova}%
\affil[3]{Uniwersytet im Adama Mickiewicza w Poznaniu Wydzial Chemii}%
\affil[4]{Uniwersytet Marii Curie-Sklodowskiej w Lublinie Wydzial Nauk o Ziemi i Gospodarki Przestrzennej}%
\affil[5]{Politechnika Gdanska Wydzial Chemiczny}%


\vspace{-1em}



  
  \date{May 24, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Permafrost regions are under particular pressure from climate change
resulting in widespread landscape changes, which impact also freshwater
chemistry. We investigated a snapshot of hydrochemistry in various
freshwater environments in the lower Kolyma river basin (North-East
Siberia, continuous permafrost zone) to explore the mobility of metals,
metalloids and non-metals resulting from permafrost thaw. Particular
attention was focused on heavy metals as contaminants potentially
released from the secondary source in the permafrozen Yedoma complex.
Permafrost creeks represented the Mg-Ca-Na-HCO \textsubscript{3}-Cl-SO
\textsubscript{4} ionic water type (with mineralisation in the range
600-800 mg/L), while permafrost ice and thermokarst lake waters were the
HCO \textsubscript{3}-Ca-Mg type. Multiple heavy metals (As, Cu, Co, Mn
and Ni) showed much higher dissolved phase concentrations in permafrost
creeks and ice than in Kolyma and its tributaries, and only in the
permafrost samples and one Kolyma tributary have we detected dissolved
Ti or Hg. In thermokarst lakes, several metal and metalloid dissolved
concentrations increased with water depth (Fe, Mn, Ni and Zn - in both
lakes; Al, Cu, K, Sb, Sr and Pb in either lake), reaching 1370 \selectlanguage{greek}µ\selectlanguage{english}g/L Cu,
4610 \selectlanguage{greek}µ\selectlanguage{english}g/L Mn, and 687 \selectlanguage{greek}µ\selectlanguage{english}g/L Zn in the bottom water layers.
Permafrost-related waters were also enriched in dissolved phosphorus (up
to 512 \selectlanguage{greek}µ\selectlanguage{english}g/L in Yedoma-fed creeks). The impact of permafrost thaw on
river and lake water chemistry is a complex problem which needs to be
considered both in the context of legacy permafrost shrinkage and the
interference of the deepening active layer with newly deposited
anthropogenic contaminants.%
\end{abstract}%



\sloppy


\textbf{1. INTRODUCTION}

Permafrost loss has been observed widely in the Northern Hemisphere
since the second half of 20\textsuperscript{th} century
(Streletskiy\emph{et al.} , 2015) and it has impacted markedly both
terrestrial and aquatic environments (Mann \emph{et al.} , 2022; Vonk
\emph{et al.} , 2015). Further significant changes are predicted in
permafrost environments during the 21\textsuperscript{st} century (Smith
\emph{et al.} , 2022; Streletskiy \emph{et al.} , 2021; Teufel \&
Sushama, 2019) as the climate warms (Overland J., 2017; Overland
\emph{et al.} , 2014). Such changes will be both vertical and
horizontal, resulting in deeper active layers (Abramov \emph{et al.} ,
2021), shrinking permafrost extent, and the retreat of its respective
zones (Lim \emph{et al.} , 2019; Streletskiy, 2021). The progressing
permafrost degradation leads to important and lasting changes in
geomorphological processes (Rudy\emph{et al.} , 2017; Tananaev \&
Lotsari, 2022), hydrological phenomena (Rudy \emph{et al.} , 2017;
Suzuki \emph{et al.} , 2021) and biogeochemical cycles (Grosse \emph{et
al.} , 2016; Mann \emph{et al.} , 2012; Vonk \emph{et al.} , 2015).

Hydrochemical changes in permafrost regions are increasingly probable,
leading to both temporary and permanent hazards to the surface water
quality in the Arctic (Brubaker \emph{et al.} , 2012;
Gunnarsd\selectlanguage{ngerman}óttir\emph{et al.} , 2019). Permafrost active layer deepening
and the formation of new thermokarst lakes, taliks and drainage pathways
(Dzhamalov \& Safronova, 2018; in't Zandt \emph{et al.} , 2020; Tananaev
\& Lotsari, 2022) all lead to changes in the migration of chemical
compounds (Frey \& Mcclelland, 2009; Monhonval \emph{et al.} , 2021)
both horizontally and vertically (Ji \emph{et al.} , 2021; Tananaev
\emph{et al.} , 2021). The newly formed drainage pathways may leach
chemical compounds from layers previously disconnected from groundwater
flow (Ji\emph{et al.} , 2021; Lim \emph{et al.} , 2019). Thermoerosion
phenomena lead to intense erosion and thaw slump formation. Such
phenomena impact the fluvial transport of suspended sediment and
chemical compounds along the main hydrological pathways, to the
estuaries and the ocean (Chalov\emph{et al.} , 2018; Frey \& Mcclelland,
2009; Kokelj \emph{et al.} , 2013; Toohey \emph{et al.} , 2016). Both
natural and antropogenic chemical compounds may become mobilised from
permafrost into surface waters (Kosek \emph{et al.} , 2019;
Lehmann-Konera \emph{et al.} , 2018), and the anthropogenic compounds
may be of both local and long-range-transport origin (Potapowicz
\emph{et al.} , 2019; Szopi\selectlanguage{polish}ń\selectlanguage{english}ska\emph{et al.} , 2016).

The studies on the hydrochemical impacts of climate change in Siberian
Arctic rivers have focused mainly on the organic carbon (OC) transport,
including the dissolved OC (DOC), suspended particulate OC (POC) and
riverbed sediment OC (SOC) (e.g., Holmes \emph{et al.} , 2012;
Jong\emph{et al.} , 2022; Mann \emph{et al.} , 2012; Wild \emph{et al.}
, 2019). Compared to the research on OC, the recognition of inorganic
elements transport, including heavy metals, is relatively poor and
limited to selected elements (Ji \emph{et al.} , 2021; Kuchmenko
\emph{et al.} , 2002; Lim \emph{et al.} , 2019). Systematic research on
the potential accumulation of heavy metals in permafrost has only been
conducted on mercury (Wang \emph{et al.} , 2022). Therefore, we present
a study on the possible release of contaminants from permafrost into
freshwater. Our objective was to perform a detailed hydrochemical
assessment of permafrost thaw impact on the inorganic chemicals
occurrence in the lower Kolyma River basin. We investigate this question
in various freshwater environments: thermokarst lakes, permafrost ice,
permafrost thaw creeks, and in watercourses representing various
proportions of permafrost thaw supply.

\textbf{2. METHODS}

\textbf{2.1. Study area and sampling}

The study region is located in the lower Kolyma watershed (Figure 1) in
a transitional zone between boreal forest (northern taiga) in the south
and tundra ecosystems in the northern Yakutia. Kolyma is one of the
largest pan-Arctic watersheds with a total area of 653,000
km\textsuperscript{2}, a total river length of 2,129 km, and annual
water discharge of 109 \selectlanguage{ngerman}± 7 km\textsuperscript{3} (Holmes \emph{et al.} ,
2012), fully underlined by continuous permafrost (Mann \emph{et al.} ,
2012). The Arctic coastal lowlands of northeastern Yakutia constitute
vast accumulative plains formed mostly by the Late Pleistocene
loess-like Yedoma Ice Complex, characterized by a high ice and organic
matter content (Abramov \emph{et al.} , 2021). Permafrost thickness in
the area reached 500-600 m, and mean annual ground temperatures (MAGT)
before the 1990s were of \selectlanguage{english}-8 to \selectlanguage{english}-11degC within the tundra and \selectlanguage{english}-7degC to \selectlanguage{english}-8degC
in the taiga (Yershov \emph{et al.} , 1991). The very high ice content
(up to 95\% by volume) of Yedoma deposits in the Kolyma lowland
(Strauss\emph{et al.} , 2021) makes them extremely vulnerable to climate
warming. In 2000-2016 (Ran \emph{et al.} , 2022), MAGT in the studied
region increased significantly against the former century due to climate
warming, and now ranged from \selectlanguage{english}-8degC to \selectlanguage{english}-4degC. Mean annual air temperature
(MAAT) in Cherskii (68.75degN, 161.28degE, 28 m asl) increased from \selectlanguage{english}-12 degC
in the 1960s to \selectlanguage{english}-10 degC in the 2000s, with a positive trend of 0.0472 degC
per year (Sakai \emph{et al.} , 2016).

MAAT in Cherskii in the year of sampling (2021) amounted to -9.8degC and
was lower than the MAAT in the years 2006-2021 (\selectlanguage{english}-8.9degC) (\emph{Suppl.
Mat. 1, Table S2} ). However, the maximum air temperature in 2021 was
the highest in the years 2006-2021 (t\textsubscript{max} = 33.4degC) and
it occurred directly before our sampling (\emph{Suppl. Mat. 1, Table S2}
,\emph{Figure S1} ). In the study area, the minimum air temperature
ranged from \selectlanguage{english}-40.2degC to \selectlanguage{english}-49.7degC and in 2006-2021 was observed in January,
February or December. The maximum air temperature was observed in
June-August, ranging 25.9degC -- 33.4degC (\emph{Suppl. Mat. 1, Table S2} ).
The mean monthly air temperature was higher than 0degC from May to
September (in 2006-2021) (\emph{Suppl. Mat. 1, Figure S2} ).

Samples have been collected between 19th and 22nd July 2001 (28 in
total). River waters were collected in the four cross sections of Kolyma
(Cherskii, samples K11-K13; Yermolovo, K21-K23; Duvanny Yar, K31-K33,
K31b-K33b; Kolymskoye, K4), and in the mouth sections of the Anyui (A1,
A2) and Omolon (O1) tributaries. In addition, a meltwater creek and ice
were sampled from a permafrost cliff located at Duvanny Yar (P1-P3), and
lake waters were collected from two thermokarst lakes (L11-L15, L21-L24)
(Figure 1, a detailed description in \emph{Suppl. Mat. 1, Table S1} ).
The Kolyma river samples were taken at the middle depth of three
vertical profiles in the channel cross sections, except of Kolyma
upstream of the permafrost cliff, where only one sample (from the middle
of the channel) was collected. Water samples were pumped out with a
filterless submersible 12V pump (details in Chalov \emph{et al.} ,
2022). In both lakes, samples were taken along a vertical profile
located above the deepest point of the lake. Among the sample sites,
direct permafrost influence was expected at the permafrost cliff, in
Lakes 1 and 2 and in Kolyma cross section K3 (directly below permafrost
cliff).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} Location of the study area and sampling sites. Please
note that all K-symbol samples come from Kolyma river cross sections.

\textbf{2.2. Analytical methods}

All water samples, due to their turbidity, were filtered through
0.7-\selectlanguage{greek}µ\selectlanguage{english}m-pore-size glass fiber filters (Whatman GFF, pre-combusted in
400\selectlanguage{ngerman}°C, 24 h).. Samples were kept in cool storage (+4°C) and analysed
approximately two months since collection time. Specific electric
conductivity (SpC), pH and dissolved oxygen were measured in the field
with a multiparameter YSI Pro Plus probe with a 4-sport Quattro Cable
(11m).

Dissolved organic carbon (DOC) was determined through a total organic
carbon (TOC) measurement in filtrate with a TOC-VCSH/CSN analyser
(Shimadzu; the method of catalytic oxidation with oxygen at 680°C and
nondispersive infrared spectroscopy detection). Calibration was
performed with a potassium phthalate standard (Shimadzu), with an
R\textsuperscript{2} = 0.998. CVs of five sample replicates were below
2\%. Samples with DOC concentrations above the analytical range were
diluted with MilliQ water (Merck Life Sciences). Only a few samples with
a high suspended sediment content were analysed for POC (details
in\emph{Suppl. Mat. 1} ).

Major ions (F\textsuperscript{-}, Cl\textsuperscript{-},
NO\textsubscript{2}\textsuperscript{-}, Br\textsuperscript{-},
NO\textsubscript{3}\textsuperscript{-},
PO\textsubscript{4}\textsuperscript{3-},
SO\textsubscript{4}\textsuperscript{2-}, Na\textsuperscript{+},
NH\textsubscript{4}\textsuperscript{+}, K\textsuperscript{+},
Ca\textsuperscript{2+} and Mg\textsuperscript{2+}) were determined using
Dionex ICS 3000 (Dionex, USA), calibrated with dilutions of a 1000 mg
L\textsuperscript{-1} single-ion standard solutions. Analytical accuracy
was checked with the TraceCERT® Multi Anion Standard 1 for IC
(69734-100ML) and Multi Cation Standard 1 for IC (91286-100ML) (all
solutions by Merck Life Science).

Elemental analysis was performed in acidified (with high-purity nitric
acid, Suprapur, Merck Life Science) filtrate using inductively coupled
plasma optical emission spectrometry (ICP-OES 9820, Shimadzu, Japan) for
Be, P, S and Ti. Inductively coupled plasma mass spectrometry (ICP-MS
2030, Shimadzu, Japan) was applied for Ag, Al, As, Ba, Bi, Cd, Co, Cr,
Cu, Fe, Hg, Li, Mn, Mo, Ni, Pb, Sb, Se, Si, Sr, V and Zn. For
calibration of the ICP-MS, the ICP IV multielement standard (Merck Life
Sciences) and single-element standards of As, Be, Sb, Se, Mo, and V
(Sigma-Aldrich, USA) were used. Single-element 1000 mg
L\selectlanguage{english}\textsuperscript{-1} standards (Sigma-Aldrich, USA)were applied for the
ICP-OES. Internal standards controlling for the MS signal were solutions
of Sc, Rh, Tb, and Ge in suprapure 1\% HNO\textsubscript{3} (Merck).
Accuracy of the analysis was checked with Sigma-Aldrich CRMs (certified
reference materials) Trace Metals ICP-Sample 1 and Trace Metals
ICP-Sample 2, with 98-104\% recoveries. High-concentration samples were
diluted with Milli-Q water (Direct 8 Purification System Water, Merck
Life Sciences, France). All metals, non-metals and metalloids
concentrations measured have been corrected for a procedural blank value
if it exceeded the method limit of quantification (LOQ); in all other
determinations, the blank correction was not required. The details of
all measurement conditions and QA/QC parameters, including method LOQ,
are given in the Supplementary material (\emph{Suppl. Mat. 1, Table S3}
).

\textbf{2.3. Statistical methods}

Cluster analysis was performed using Statistica version 13 (TIBCO
Software Inc.) on a selection of variables listing elemental, ionic and
DOC concentrations , with the selection criterion that only variables
with 33\% or fewer values \textless{} LOQ (or below blank) were taken
into account to avoid artifacts in grouping. Prior to cluster analysis,
values \textless{}LOQ (or below blank) were replaced by half LOQ (or
blank, whichever was higher). All data was standardised (z-transformed)
within variables. Clustering was performed using Ward's method and
squared Euclidean distance. This analysis may be cross-compared with the
principal component analysis presented in \emph{Suppl. Mat. 1} .

Enrichment factors (EFs) relative to average concentration of elements
in the suspended sediment of World rivers (after Viers \emph{et al.} ,
2009 and Taylor, 1964) have also been calculated and are shown in Table
S4 (\emph{Suppl. Mat. 1} ).

\textbf{3. RESULTS}

\textbf{3.1. Electric conductivity, pH and dissolved organic carbon
(DOC)}

SpC in the collected samples exhibited a wide range of values, from
\textless{}50 \selectlanguage{greek}µ\selectlanguage{english}S cm\textsuperscript{-1} in Lake 2 to 1405 \selectlanguage{greek}µ\selectlanguage{english}S
cm\textsuperscript{-1} in a sample from permafrost-fed creek P2
(\emph{Suppl. Mat. 2} ). Conductivities in the Kolyma water ranged
125-165 \selectlanguage{greek}µ\selectlanguage{english}S cm\textsuperscript{-1}, and 65-69 \selectlanguage{greek}µ\selectlanguage{english}S cm\textsuperscript{-1}in
its tributaries Anyui and Omolon. Finally, both Lakes 1 and 2 showed
SpCs \textless{}100 \selectlanguage{greek}µ\selectlanguage{english}S cm\textsuperscript{-1}, except the bottom layers
of Lake 1 with a steep incline in SpC values, reaching 318 \selectlanguage{greek}µ\selectlanguage{english}S
cm\textsuperscript{-1} near the bottom. Both lakes were thermically
stratified, with much cooler water near the bottom (especially Lake 1,
changing from 15-16\selectlanguage{ngerman}°C at the surface to \textasciitilde{}5°C near the
bottom). The bottom layers of Lake 1 were also strongly depleted in
oxygen, which was confirmed by the precipitation of iron hydroxide in
the collected samples, and infused with methane (Stepanenko \emph{et
al.} , 2011; Walter \emph{et al.} , 2006).

Dissolved organic carbon (DOC) concentration showed consistent values
within each one sample type, while it differed widely between the
various water bodies. Particularly high DOC concentrations (max = 127
mg/L) were found in the permafrost-fed creeks at Duvanny Yar (Figure 2).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2.} A DOC concentration boxplot grouped by the water
bodies sampled: Anyui, Kolyma and Omolon are river names; DY =
permafrost-fed creeks draining a Yedoma cliff at Duvanny Yar; Dy\_I --
ice collected from Yedoma at the same site; Lake\_1 = the thermokarst
lake close to Cherskii,; Lake\_2 = a thermokarst lake at Duvanny Yar.
Please note the logarithmic scale of DOC concentration .

\textbf{3.2. Major ions}

Chloride and sulphate were the predominant anions in sampled waters (and
presumably also bicarbonate, which was not measured, yet its
concentrations estimated from the ionic balance approximated the
concentrations of calcium ions, and if recalculated to mass, they were
the main ions in the collected samples, reaching 311 mg
L\textsuperscript{-1} in P2). Phosphate, nitrate, nitrite and bromide
have only been detected in a few samples each, therefore they are not
presented in graphs (Figure 3). The highest dissolved solid
concentrations have been found in Yedoma-fed creeks (\textgreater{}100
mg/L of chloride and sulphate both), and sulphate was relatively high in
most Kolyma river samples. Among cations, calcium was predominant, at
the average level of 12.4 mg L\textsuperscript{-1} in the Kolyma river,
while magnesium was usually second, exceeding Ca\textsuperscript{2+}only
in P2 sample at 74.2 mg L\textsuperscript{-1} in the permafrost cliff
creeks (DY; Figure 4). Permafrost-fed creeks exhibited relatively high
concentrations (approx. 50 mg L\textsuperscript{-1} or more) of all
three cations: Ca\textsuperscript{2+}, Mg\textsuperscript{2+} and
Na\textsuperscript{+} (Figure 4). If the ionic water type is defined,
using the {[}mval{]} concentrations of major ions amounting to at least
20\% of the anion or cation total (\emph{Suppl. Mat. 2} ), three groups
of objects emerge: (1) thermokarst lakes and permafrost ice represented
the HCO\textsubscript{3}-Ca-Mg water type; (2) in the rivers, water
types varied between SO\textsubscript{4}-HCO\textsubscript{3}-Ca-Mg,
Ca-Mg-HCO\textsubscript{3}-SO\textsubscript{4} and
Ca-Mg-Na-HCO\textsubscript{3}-SO\textsubscript{4}, except sample A2
(SO\textsubscript{4}-Ca-Mg); (3) permafrost creeks were of
Mg-Ca-Na-HCO\textsubscript{3}-Cl-SO\textsubscript{4} type.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3.} Major anions in the Kolyma region freshwaters.
Nitrite, nitrate, bromide and phosphate, detected only in a few samples,
are not shown. See \emph{Suppl.mat.} \emph{2} for a full data table and
Figure 2 caption for site symbols.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4.} Major cations concentrations in freshwater samples
from the lower Kolyma basin. Sampling sites named as in Figure 2.

\textbf{3.3. Metals, metalloids and non-metals}

Elemental concentrations exhibited a large variability (Figure 5), with
maxima typically found in P1-P2 (Duvanny Yar) samples (Be, Co, Sb, Se,
Sr, V), in the lower layers of lake waters (As, Cd, Cu, Fe, Mn, Ni, Si,
Zn) or in single samples from Kolyma river (Ba, Li, Mo). Only Al showed
the highest concentration in permafrost ice (sample P3). Multiple heavy
metals showed much higher concentrations in permafrost creeks and ice
(As 1.24-4.23 \selectlanguage{greek}µ\selectlanguage{english}g L\textsuperscript{-1}, Cu 1.82-11.4 \selectlanguage{greek}µ\selectlanguage{english}g
L\textsuperscript{-1}, Co 0.57-3.99 \selectlanguage{greek}µ\selectlanguage{english}g L\textsuperscript{-1}, Mn
122-1300 \selectlanguage{greek}µ\selectlanguage{english}g/L, \selectlanguage{greek}µ\selectlanguage{english}g L\textsuperscript{-1}Ni 3.36-16.5 \selectlanguage{greek}µ\selectlanguage{english}g
L\textsuperscript{-1}) than in Kolyma and its tributaries (As 0.22-0.81
\selectlanguage{greek}µ\selectlanguage{english}g L\textsuperscript{-1}, Cu 0.70-5.72 \selectlanguage{greek}µ\selectlanguage{english}g L\textsuperscript{-1}, Co
0.01-0.05 \selectlanguage{greek}µ\selectlanguage{english}g L\textsuperscript{-1}, Mn 0.61-14.1 \selectlanguage{greek}µ\selectlanguage{english}g
L\textsuperscript{-1}, Ni 0.2-1.87 \selectlanguage{greek}µ\selectlanguage{english}g L\textsuperscript{-1}(\emph{Suppl.
Mat. 2} ). Permafrost samples also contained metals that were not
typically found in other samples: Ti (15.7-30.3 \selectlanguage{greek}µ\selectlanguage{english}g
L\textsuperscript{-1}) and Hg (0.128-0.131 \selectlanguage{greek}µ\selectlanguage{english}g L\textsuperscript{-1}). In
the lakes, several metals concentrations increased with water depth: Fe,
Mn, Ni and Zn concentrations did so in both lakes; Al, K, Sr in Lake 1,
and Cu, Pb, Sb in Lake 2. The only instances of an opposite direction of
change were Mo, Na and Sb in Lake 1. The highest concentrations of
selected elements in this study were observed in lake bottom waters,
e.g. {[}Cu{]} = 1370 \selectlanguage{greek}µ\selectlanguage{english}g/L, {[}Mn{]} = 4610 \selectlanguage{greek}µ\selectlanguage{english}g/L, and {[}Zn{]} = 687
\selectlanguage{greek}µ\selectlanguage{english}g/L. Bottom lake waters, permafrost creeks and ice were also enriched
with phosphorus (up to 34.7 \selectlanguage{greek}µ\selectlanguage{english}g/L in Lake 2, up to 512 \selectlanguage{greek}µ\selectlanguage{english}g/L in creeks and
43.3 \selectlanguage{greek}µ\selectlanguage{english}g/L in ice) (\emph{Suppl. Mat. 2} ).

Enrichment factors (EF) of all the collected samples indicate
significant or higher enrichment in heavy metals (As, Cd, Co, Cu, Mn,
Ni, Pb and Zn). The highest EFs in the dataset were found in lake bottom
waters for As, Co, Cu, Mn, Ni, Pb and Zn (\emph{Suppl. Mat. 1, Table S4}
). Noteworthy is also the extremely high enrichment of the permafrost
ice and creeks, as well as Maly Anyui (A2) sample in Hg. Furthermore,
the concentration level of Cd was higher in the collected samples
connected to permafrost than elsewhere: in permafrost creeks, permafrost
ice, lower layers of lake water and in a few samples from the Kolyma
river in the K3 cross section. In that particular cross section, the
enrichment in Cd was higher in the mid-stream (samples K31 and K32) than
in the river bottom waters (K31b, K32b), testifying to the limited
mixing of dissolved elements in the vertical profile of the river,
suggesting their local supply.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5.} Elemental concentrations in the collected samples.
Note the logarithmic scale of concentrations. Boxplots show quartiles
and full range without outliers, points show all data. Sampling sites
marked as in Figure 2.

\par\null

\textbf{3.4. Cluster analysis}

To explore the patterns in elemental and ionic constituent
concentrations, together with DOC, we performed cluster analysis.
Grouping variables (Figure 6a), at the 33\% maximum distance level,
yielded 5 clusters: C1 (Zn \& Cu), C2 (Mo, Li, Cd \& Ba), C3 (Si, Ni, Mn
\& Fe), C4 (Be, Co, As, Al and K\textsuperscript{+}), and C5 (V, DOC,
Se, Sb, Sr and all other ions). At the 67\% level, C1 and C2 form one
cluster together, and another is formed by C3 and C4, joined. Grouping
samples (Figure 6b), on the other hand, resulted in three distinct
clusters at the 33\% maximum distance level: P1 \& P2; L14 \& L15; and
all other samples. Notably, permafrost ice P3 was classified as
relatively similar to the samples from the bottom of Lake 2 and one
Kolyma river sample (K31). The variables distinguishing Lake 2 and the
P3 ice from other samples were the concentrations of DOC, Al, Co, As,
Mn, Ni, Fe, Zn, while the variables setting the permafrost creeks P1 and
P2 apart were the concentrations of Si, Sr, Sb, V and the ions
Na\textsuperscript{+}, Mg\textsuperscript{2+}, F\textsuperscript{-}, and
SO\textsubscript{4}\textsuperscript{2-} (cf. \emph{Figures S3 \& S4,
Suppl. Mat. 1} ).

An additional cluster analysis (Figure 7), performed for river waters
alone (Kolyma, Omolon and Anyui), yielded three clusters at 67\% maximum
distance level. Cluster c1 contained rock-derived elements, organics and
some more typically anthropogenic contaminants (Si, Mn, Fe, Zn, Sb, Cu,
V, Co, As, Ni and DOC); c2 grouped Li, Mo, Cd, Ba, Be, Al and Sr; and c3
joined Sr, Se, and all inorganic ions. Around 33\% maximum distance, the
c1 cluster divided into c1a (Si, Mn \& Fe) and c1b (Zn, Sb, Cu, V, Co,
As, Ni and DOC); cluster c2 consisted of c2a (Li, Mo \& Cd) and c2b (Ba,
Be \& Al); and cluster c3 was composed also of two sub-clusters: c3a
(Sr, Se, K\textsuperscript{+}, Mg\textsuperscript{2+},
Ca\textsuperscript{2+} \& SO\textsubscript{4}\textsuperscript{2-}) and
the saline c3b (Cl\textsuperscript{-}, Na\textsuperscript{+} and
F\textsuperscript{-}).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6.} Cluster analysis results for all collected samples:
a.) clustering variables; b.) grouping samples (objects).

\textbf{Figure 7.} Results of the cluster analysis performed on river
samples data alone (Kolyma, Anyui and Omolon).

\textbf{4. DISCUSSION}

Permafrost regions have experienced dynamic environmental changes
induced by climate change, especially in the north-eastern Siberia
(Nitzbon \emph{et al.} , 2020). Recent climate-induced increases in thaw
propagation have triggered changes in local relief in the Yedoma
uplands, including soil subsidence (G\selectlanguage{ngerman}ünther \emph{et al.} , 2015),
activation of thermokarst and thermoerosion processes (Grigoriev\emph{et
al.} , 2009; Morgenstern \emph{et al.} , 2021), and the expansion of
pond and thermokarst lake areas (Nitze \emph{et al.} , 2017;
Veremeeva\emph{et al.} , 2021). In the large rivers, an increase in
runoff is both expected and already observed: in the Kolyma, mean annual
discharge has increased over the 2010--2020 by 27.7\% (94.6 to 120.7
km\textsuperscript{3} year\textsuperscript{-1}) compared to a baseline
period of 1971--2000 (Mann \emph{et al.} , 2022). The same authors
estimated an increase of 50\% (±25\%) in Kolyma's mean annual runoff by
2100, in line with climate projections. Climate warming resulted also in
an increase in Kolyma's sediment transport rates from 20 to 100 t
km\textsuperscript{-2} year\textsuperscript{-1} in 1941-2000
(Chalov\emph{et al.} , 2018), which can be also attributed to mining
impact (Walling \& Fang, 2003) or changes due to permafrost thaw
(Chalov\emph{et al.} , 2021).

The data presented here shows the hydrochemical patterns of different
waters in an environment shaped by permafrost seasonal and long-term
changes. The ionic composition of all sampled waters is likely impacted
by both suprapermafrost waters and an inflow of deeper, intra- and
subpermafrost waters; especially the permafrost creeks P1 and P2 are
likely connected to unfrozen groundwater (Cochand \emph{et al.} , 2019).

Among the studied samples, the permafrost creeks differ from river
waters by the high content of DOC, major cations and anions, and
selected metals and metalloids, including heavy metals. The pathways of
OC transport and transformation in Siberian freshwater have been
thoroughly analysed recently, e.g. by Wild \emph{et al.} (2019),
Mann\emph{et al.} (2022), or Jong \emph{et al.} , (2022). Mann \emph{et
al.}(2022) stated that permafrost-derived OC additions will
significantly enhance inland OC turnover over the upcoming decades,
however the fate of OC is uncertain due to additional transformation
factors (Keskitalo\emph{et al.} , 2022). In the large rivers, OC occurs
mainly as DOC: for Kolyma, the dissolved share has been calculated by
Mann \emph{et al.}(2022) as 80\%. Similar values are reported by others
(Jong \emph{et al.} , 2022; Keskitalo \emph{et al.} , 2022). Keskitalo
\emph{et al.}(2022) have concluded, based on isotopic evidence, that the
smaller fraction by mass, the POC, is more affected by the legacy
Yedoma-derived OC, than is DOC. Similarly, Wild \emph{et al.} (2019)
claim that POC in the large Siberian rivers may originate from the
direct erosion of the old Yedoma layers, although they also estimate the
legacy OC share in the DOC and POC fractions in the Kolyma to be
approximately equal. Bröder \emph{et al.} (2020) remarks that Kolyma
transports more suspended POC at higher discharges, and both the total
POC and legacy POC concentrations are higher there than in a small
stream (the average POC ages in Kolyma and a small stream were estimated
at \textasciitilde{}2,840 and \textasciitilde{}590 years, respectively).
These claims are supported by Jong \emph{et al.} (2022), who observed an
increased contribution of POC in the TOC transport in large rivers and
connected it to the increased direct erosion of the river banks, while a
smaller stream exhibited a higher share of DOC. Therefore, the POC in
Kolyma likely originates from recent vegetation and old permafrost
erosion combined. Mann \emph{et al.} (2022) suggest also that the OC
flux and total export will rapidly increase due to climate change and
permafrost degradation.

In our research, the DOC concentration in a permafrost ice sample
amounted to 9.13 mg L\textsuperscript{-1} and it was approximately 11
times less than the concentrations in the creeks draining the permafrost
cliff. Similar DOC values of 154-336 mg L\textsuperscript{-1} and 103.4
mg L\textsuperscript{-1}, for creeks draining the permafrost cliff at
Duvanny Yar, were obtained by Vonk \emph{et al.} (2013) and Jong\emph{et
al.} (2022), respectively, and the concentrations of POC in the same
samples amounted to 6700-9240 mg L\textsuperscript{-1}, eclipsing the
DOC contribution. (This is also consistent with the additionally sampled
POC in P1 and P2 samples, amounting to 8500 and 12000 mg
L\textsuperscript{-1}, respectively -- methods are reported in
the\emph{Supp. Mat. 1.} ) Conversely, Jong \emph{et al.} (2022) obtained
DOC concentrations (2.76 to 4.97 mg L\textsuperscript{-1}) at
approximately double the POC concentrations (1.49 to 2.73 mg
L\textsuperscript{-1}) for Kolyma below Duvanny Yar, and similar results
for Maly and Bolshoy Anyui (3.16-4.43 mg L\textsuperscript{-1} and
1.29-1.7 mg L\textsuperscript{-1}, respectively). In our research,
thermokarst lakes were studied also, with DOC concentrations similar to
a small stream studied by Jong \emph{et al.} (2022), representing soil
leaching and active layer drainage ({[}DOC{]} =
21.5~mg~L\selectlanguage{english}\textsuperscript{-1}). The legacy DOC from permafrost can be
approximated by the DOC concentration in permafrost ice (9.13 mg
L\textsuperscript{-1}), yet the extra 90\% of DOC in permafrost creeks
may originate both from permafrost active layer (i.e. relatively recent
atmospheric deposition and microbial processes) and the exclusion of
various organic compounds during groundwater freeze. POC from the ice
may also be a source of DOC in thaw waters. The cluster analysis of our
data has shown similarities in the water chemistry between the
permafrost ice sample (P3), the bottom layer of water in Lake 2 and a
sample from Kolyma below Duvanny Yar (K31, sampled mid-depth away from
the cliff side, where the main river current flows), which suggests the
influence of legacy permafrost thaw upon the Kolyma river in this cross
section. At the same time, the connection between the other lake samples
and the samples from the permafrost creeks suggests a general permafrost
waters influence over the lake waters, although it is probably a
combined influence of the active layer drainage and deeper talik-type
waters.

In the collected permafrost ice, a contrastingly high level of aluminium
concentration has been noted as compared to the other samples; a
relatively high concentration of Al was also found in the permafrost
creeks and the sampled lakes. Aluminium co-occurs with
K\textsuperscript{+}, Be, but also with Co and As, frequently classified
as heavy metals. Arsenic occurred at elevated (against other samples)
concentrations in the lake bottom waters of this study. In the lake
waters, permafrost creeks and permafrost ice, we found also elevated
concentrations of other heavy metals (Co, Mn, Ni, Zn, Cu and Sb). Zinc,
copper and cadmium showed elevated values also in the Kolyma, while the
bottom layer waters of the thermokarst Lake 2 exhibited also a very high
concentration of Pb. Monhonval \emph{et al.} (2021) characterised the
chemical composition of Yedoma waters, by exploring the mineral element
stock in sediments of the Yedoma. They found a relatively high
concentration for Si, followed by Al, Fe, K, Ca, Ti, Mn, Zr, Sr, and Zn.
Moreover, the stock of Al and Fe (598 +- 213 and 288 +-104 Gt) was at the
same order of magnitude as the OC stock (327--466 Gt). Our surface water
and ice samples from the Kolyma watershed exhibited similarly high
concentrations of Si and Fe, as well as Al. The cluster analysis of all
the collected samples shows that in cluster C3, the elements Si and Fe
were grouped together with Ni and Mn (Figure 6a). However, in river
waters they correlated with Zn, Sb, Cu, V, Co, As, and DOC (cluster c1,
Figure 7). Both clusters may be connected to the suprapermafrost waters
of the active layer, which are flowing intensely into the thermokarst
lakes and rivers. Ji \emph{et al.} (2021) studied elements in permafrost
soils and concluded that Mn, Ca, Mg, Al, and Ti showed the highest
mobility from soil to suprapermafrost water and further into the ponds
and flowing waters. Furthermore, they considered Mn a proxy for
predicting the processes occurring in the active layer during
summer-autumn thaw. Conversely, the long-term research in the Yukon
River basin (1982-2014), underlain by discontinuous permafrost, shows a
relationship between the deepening of permafrost active layer and
significant increases in the Ca, Mg, and Na annual flux in the Yukon
River and its tributary Tanana, and in the SO\textsubscript{4} and P
annual flux in the Yukon (Toohey \emph{et al.} , 2016). The authors
described increased active layer expansion, weathering, and sulfide
oxidation due to permafrost degradation throughout the studied basin.
Frey \& Mcclelland (2009) forecast also a significant increase in the
concentrations of major ions due to permafrost degradation and lowering
water tables (except for nitrate in Siberia). In the studied samples
from the Kolyma watershed, clusters C5 and c3 (in the all sample and
river sample analysis, respectively; Figure 6a, Figure 7) indicate the
likely influence of rock weathering on water composition. The visible
division within the c3 cluster into c3a (Sr, Se, K\textsuperscript{+},
Mg\textsuperscript{2+}, Ca\textsuperscript{2+} and
SO\textsubscript{4}\textsuperscript{2-}) and c3b (Cl\textsuperscript{-},
Na\textsuperscript{+} and F\textsuperscript{-}) may represent two
separate water supply factors, namely the shallow and deep groundwater
flow (Douglas \emph{et al.} , 2013). An extra contribution to the total
sulphate load in the rivers may also result from the wet deposition
connected to wildfires, which have influenced considerably the
deposition of sulphur (and nitrogen) compounds in the regions of Siberia
and the Russian Far East (Berezin \emph{et al.} , 2013). On the other
hand, the lower contribution of Ca\textsuperscript{2+} in thermokarst
lake waters may result, according to Monhonval \emph{et al.} (2021),
from the leaching of soluble elements such as Ca during former thaw
periods.

In this study, mercury was found only in permafrost creeks, permafrost
ice and in the Maly Anyui. Recent research has shown that Hg may be
released from continental permafrost with climate-change-induced thaw,
and there are very different quantitative estimates of Hg fluxes
connected to various local sources (permafrost is considered one source
among the many; Campeau \emph{et al.} , 2022; Ci \emph{et al.} , 2020;
Mu\emph{et al.} , 2020; Schuster \emph{et al.} , 2018). Schuster
\emph{et al.} (2018) estimated that the entire Northern Hemisphere
permafrost area contains 1,656 +- 962 Gg Hg, of which 793 +- 461 Gg Hg
(\textasciitilde{}47\%) is frozen in permafrost. Except in permafrost
ice, our samples may contain mercury of various origin, both from
permafrost thaw and from atmospheric deposition (Schuster \emph{et al.}
, 2018), and in the case of Maly Anyui it may also be connected to
mining. (Gold mining occurs in parts of both the Maly and Bolshoy Anyui
catchments, yet higher production comes from around the Maly Anyui
tributary Karalveem River). However, the low Hg concentrations
(\textless{}LOD) in the other collected samples (from Kolyma, Omolon and
the lakes) indicate that any atmospheric sources of Hg would likely be
only local and periodic (e.g., forest fires) (Francisco Lopez \emph{et
al.} , 2022). Furthermore, it cannot be excluded that most Hg in the
analysed samples occurred in the particulate form, which has not been
investigated here. Lim \emph{et al.} (2019), who have analysed the POC
and Hg fluxes in the rivers of various permafrost zones, indicate
minimal particulate Hg export at the sporadic to discontinuous
permafrost zone. According to these authors, the current climate
warming, the northward shift of permafrost boundaries and the increasing
active layer depth in Western Siberia, may result in an enhanced
particulate Hg export by small rivers to the Arctic Ocean by a factor of
two over the next 10-50 years. The possible origin of the Hg may be also
connected to mining areas (both abandoned and active) distributed over
Maly Anyui river.

Besides mercury, also As and Cd are heavy metals sometimes mentioned as
posing a new hazard due to permafrost thaw (Zhang \emph{et al.} , 2021).
In this study, a potential source of those is the erosion of a
permafrost cliff, given the significant enrichment of As and Cd in
permafrost ice, the extremely high EF in permafrost creeks, and very
high to extremely high EF in the deep waters of thermokarst lakes (Table
S4, \emph{Suppl. Mat. 1} ). The supply of Cd may be connected to the
drainage of mixed suprapermafrost water, which may include atmospheric
deposition, and deeper water both. With the active layer deepening,
atmospheric pollutants deposited in the past, which have been excluded
from the freeze-thaw cycle for a time (Ji \emph{et al.} , 2021), may be
remobilised (Edwards \emph{et al.} , 2021; Li \emph{et al.} , 2020;
Lim\emph{et al.} , 2019; Rubino \emph{et al.} , 2016). The notable
increase in Cu, Ni, Pb and Zn concentrations in deep lake waters
indicate the likely source of these elements in permafrost thaw, talik
water, or enhanced mobility of suprapermafrost water eluting them from
soil. Importantly, it is not only in the Lake 1 near the Cherskii
research station and the settlement which experienced the elevated
concentrations of these metals in the bottom waters, but also in the
further from any human activity Lake 2, showing that such contamination
is probably irrelevant of local infrastructure.

In the studied freshwaters, it was evident that thermokarst lakes form
at least temporary traps for heavy metals connected to one or several
types of permafrost influence. Thermokarst lakes form one of the more
dynamic elements of the continental permafrost landscape, and their mass
emergence is among the more frequently observed impacts of climate
change in the Northern Hemisphere (in't Zandt \emph{et al.} , 2020;
Karlsson \emph{et al.} , 2012; Plug \emph{et al.} , 2008), including the
Kolyma region (Veremeeva \emph{et al.} , 2021). Due to the dynamically
increasing number of thermokarst lakes, and their predicted drainage
with the advancing climate change and deepening active layer, the
sedimentation in these lakes and the biogeochemical processes happening
in them (in't Zandt \emph{et al.} , 2020) will likely be of high
importance in studies of continental permafrost areas.

\textbf{5. CONCLUSIONS}

In our study of the dissolved inorganic chemistry in the lower Kolyma
basin, we found that the ionic HCO\textsubscript{3}-Ca-Mg water type was
prevalent (representing both lakes and permafrost ice), and the river
and permafrost creek waters had also a significant admixture of
sulphate. The highest concentrations of DOC (9.13-127 mg/L) were found
in permafrost ice, permafrost creeks and thermokarst lakes, i.e. the
water bodies connected to permafrost thaw. Among the collected water
samples, a few heavy metals were detected at higher concentrations in
permafrost-related samples than elsewhere, i.e. As, Cd, Co, Fe, Mn, Ni,
and V. The occasionally detected Cr, Hg, Pb and Ti were also typically
associated with the permafrost-related origin of a sample. Mercury
occurred also at a relatively high concentration in the Maly Anyui
tributary of Kolyma. The highest concentrations of most of the studied
elements were found in the lake bottom waters, suggesting that
thermokarst lakes act as local traps for elements, including heavy
metals. Further evidence for the origin of these metals could be
provided in the future through sampling lake bottom sediments as well.

The impact of changes in permafrost regions, occurring due to climate
change, need to be considered with respect to both the remobilisation
from older permafrost (of organic carbon and other chemical components)
and the interference with newly deposited anthropogenic contaminants.
Such impacts are likely to be complex and modified by the local
sedimentation processes in the local watercourses and lakes. The further
forecasted climate change may result in the formation of new flow
pathways for dissolved chemicals migrating between supra-, intra- and
subpermafrost waters. Further research into the concentration of
dissolved and particulate phases in permafrost thaw and active layer
waters is warranted to diminish the uncertainties related to this field
of knowledge or to estimate the total load of e.g. heavy metals in the
permafrost regions, available to remobilisation.

\textbf{Funding}

This research was funded by INTERACT, H2020-EU.1.4.1.2.
(\emph{PollAct,}grant no. 730938) and by the Project Supporting
Maintenance of Research Potential at Kazimierz Wielki University. K.K.'s
fieldwork was supported by similar Research Potential Maintenance funds
of the Gdansk University of Technology. S.Ch. and V.E. acknowledge the
support of RSF for fieldwork (project 21-17-00181).

\textbf{Acknowledgements}

We are grateful to F. Pawlak and B. Knoebelsdorf for their aid in sample
preparation, and to J. Potapowicz for EF calculations. Special thanks to
M. Szopi\selectlanguage{polish}ń\selectlanguage{english}ska and the North-East Science Station for help during
fieldwork.

\textbf{Conflict of Interest Statement}

The authors declare no conflict of interest.

\textbf{Data Availability Statement}

The data is currently under restricted access in the Zenodo Open Data
repository: https://zenodo.org/record/7452043, to be released upon
manuscript publication.

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