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Potentially toxic elements (PTEs), particularly metallic elements, are some of the most dangerous pollutants found in surface sediments due to their toxicity and persistence. Given the contamination-related issues for sediments in Central Asian countries (CACs), a combination of risk assessment and source apportionment was used in characterizing the pollution sources and source-specific risks of metallic elements in the sediments of CACs. Surface sediment samples collected from 254 sites across the CACs were analysed for PTEs. The calculation of enrichment factor (EF) and the assessments of health risks were devoted to identifying contamination and risk characteristics, including geomorphic- and watershed-driven variability. Multivariate statistical analyses were applied to quantitatively identify contamination sources. The results suggested that Cr, Cu and Ni were minimally enriched, Pb was moderately enriched, and Zn was significantly enriched. Interestingly, the EF of Pb was considerably higher in the northwest plain of the CAC region, but that of Zn was distributed across the study area. Source apportionment indicated that natural and anthropogenic sources accounted for 40.42% and 59.58% of the total contamination, respectively. Overall, the non-carcinogenic risks caused by PTEs were acceptable, but Cr and Pb exhibited higher non-carcinogenic effects in the Caspian Sea-Ural River and Ishim River regions. Moreover, fuel combustion, atmospheric deposition and industrial activities made higher contributions than other sources to the ecological risk of PTEs in the CACs. This study will advance our knowledge of spatial differences, contamination risks and source identification for PTEs in sediments and provide a scientific basis for policy makers to design environmental strategies in the CACs.

Thorough knowledge of hydrochemical characteristics and trace element concentrations in surface waters is crucial for protection of this resource, especially in arid regions. Uzbekistan lies in central, arid Central Asia, and is experiencing severe water scarcity as a consequence of warming climate and accelerated human impacts. Fifty-five surface water samples were collected from the Amu Darya Basin of Uzbekistan (ADBU) and measured for relevant variables, to determine hydrochemical characteristics and evaluate water quality. River water samples from the middle reach and Amu Darya Delta (ADD) were dominantly HCO3-Ca and SO4-Ca·Mg types, respectively, whereas water samples collected near the former shoreline and sewage outlets in the ADD were predominantly Cl-Ca·Mg and Cl-Na types, consistent with the distribution of sites that display high concentrations of trace elements and are seriously affected by human activities. Principal component analysis indicated that Pb and Cd in surface waters of the ADBU had industrial origins. Local agricultural activities are thought to have contributed to the concentrations of NO3, Zn, Ni, Hg, and Mn, through pesticide and fertilizer applications, and concentrations of Cu, Cr, As, and Co are controlled by mixed natural and anthropogenic sources. Water quality and health risk assessments indicated that unsuitable drinking waters are distributed mainly near the former shoreline and sewage outlets in the ADD, making human health risks in these areas high.

Central Asia (CA) has attracted global attention because of either water scarcity or ecosystem degradation. The Amu Darya Delta (ADD), one of the most important oases in CA, is endowed with valuable wetlands and biological resources that provide good ecosystem services to inhabitants. However, the region has experienced climate warming and large-scale anthropogenic changes since the last century. To assess the influences of anthropogenic interventions on the soil environment in this area, surface soil samples collected from the ADD were analysed for aliphatic hydrocarbon fractions and five heavy metals (HMs; including Cd, Zn, Cu, Ni and V). The results indicated that the n-alkanes extracted from surface soils were composed of homologous series from C14 to C35. Relatively high abundances of short-chain n-alkanes (n-C21) were observed in cluster 1 samples (mainly from the lakeshore of the Aral Sea), while significantly high abundances of mid-chain (n-C21 to n-C25) and long-chain (n-C26 to n-C32) n-alkanes were found in cluster 3 samples, which were distributed in urban and agricultural drainage areas. In addition, very-long-chain n-alkanes (n-C33) occurred in most surface soils, which might be a sign of a hot and arid climatic environment. Notably, almost all samples presented a clear even carbon dominance of short-chain n-alkanes, especially for cluster 1, which possibly represented the influence of hydrocarbon contamination and highly saline carbonate environments in addition to bacterial degradation. The biomarker indices and HM enrichment index indicated greater effects of crude oil pollution on cluster 1 (specifically samples 2, 4, 5, 6, 13, 16 and 34) and anthropogenic activities such as traffic emissions and agricultural drainage on cluster 3 samples. The results of this study provide evidence that the n-alkane composition and abundance in surface soils respond sensitively to anthropogenic interventions, arid climate and petroleum hydrocarbon pollution.

Potentially toxic elements (PTEs) in lake sediments are concerning because of their toxic effects on lacustrine ecosystems and human health. Baiyangdian Lake (BYDL), the "pearl of North China", plays a vital role in maintaining the ecological health of North China. Here, risk assessment and source identification of nutrients and PTEs in sediments were performed. The results showed that the sediments were highly contaminated with total organic carbon (TOC) and total nitrogen (TN), but contamination by total phosphorus (TP) was minor, and the ecological risk associated with Cd was considerable, especially in the northern region. The average noncarcinogenic hazard quotient of PTEs increased in the order of Zn Hg Cd Cu Ni Pb Cr As. Statistical analyses indicated that Al, Ti, Fe, Co, Cr, Hg, Ni, and rare earth elements were primarily from natural origins; Pb was derived from local tourism development and pollution by fishermen; and TOC, TN, TP, As, Cd, Cu, Zn, and Mn were mainly derived from industrial and agricultural activities. Additionally, the mean contribution rates of industrial wastewater and domestic sewage sources, agricultural sources, and natural sources to BYDL sediment pollution were 59.9%, 17.9%, and 22.2%, respectively. Spatially, industrial and domestic sewage sources contributed more in the northwestern region (average 79.8%) than in the southern region (average 35.5%), while agricultural sources contributed the most in the southwestern region (average 41.3%). These findings will advance our knowledge of the spatial differences, contamination risks and sources of nutrients and PTEs in BYDL and provide a scientific basis to help policy-makers establish a healthy ecological community in BYDL of the Xiong'an New Area.

As the core of arid Central Asia, Uzbekistan is experiencing prominent water scarcity with increasingly warming climate and accelerated human impact. To determine the hydrochemical characteristics and sources of toxic elements, as well as to assess water quality and health risks in Uzbekistan, 55 surface water samples were collected from the Amu Darya Basin of Uzbekistan (ADBU) and monitored for 20 parameters. A hierarchical cluster analysis showed that river water samples from the middle reach and Amu Darya Delta (ADD) were dominantly HCO3-Ca and SO4-Ca·Mg type, respectively. While the water samples collected near the former shoreline of the ADD and sewage outlets were dominantly Cl-Ca·Mg and Cl-Na types, which were consistent with the distribution of sites with high concentrations of toxic elements, seriously affected by human activities. Furthermore, principal component analysis indicated that the toxic elements of Pb and Cd in surface waters of the ADBU had industrial origins; local agricultural activities were considered to have contributed much of the NO3, Zn, Ni, Hg and Mn through pesticides and fertilizers; and Cu, Cr, As, and Co were controlled by mixed anthropogenic and natural sources. The results of water quality and health risk assessment also suggested that unsuitable drinking waters were displayed near the former shoreline of the ADD region and sewage outlets, and human health risks also occurred these areas.

Release and transport of contaminants in watersheds can have adverse effects on aquatic organisms and human health. Little attention, however, has been paid to chemical contamination of aquatic environments in arid regions by persistent organic pollutants. We analyzed the concentrations and distributions of organochlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs) in waters of the Ili-Balkhash Basin, in arid central Asia. ΣOCP concentrations ranged from 4.02 to 122.80 ng L−1 and ΣPAH concentrations were between 7.58 and 70.98 ng L−1. On a global scale, OCP and PAH concentrations in waters of the Ili-Balkhash system were relatively low, with only a few exceptions, i.e., highest concentrations near cities and relatively high values in some headwater areas. Source identification revealed that the dichlorodiphenyltrichloroethanes (DDTs) may come from recent use, whereas endosulfans stem from historic applications. Lindane, a common insecticide, may be responsible for hexachlorocyclohexanes (HCHs). Low-molecular-weight PAHs, primarily originating from wood and coal combustion and petroleum-derived sources, were the primary components of PAHs in waters. Furthermore, the primary sources of PAHs at different sites were identified using a Positive Matrix Factorization model: 1) oil leakage (33.9%), 2) biomass burning (29.5%), 3) coal combustion (22.6%), and 4) petroleum-powered vehicles (14.1%). Agricultural, industrial and domestic activities are all potential pollution sources. Besides, contaminated headwater areas indicate that long-range transport has probably become a non-negligible mechanism for pollutant distribution. Risk assessment showed low to moderate toxicity for aquatic organisms, but no marked carcinogenic or non-carcinogenic risks for human health.

Phytoremediation is an economic and promising technique for removing toxic pollutants from the environment. Freshwater sediments are regarded as the ultimate sink of the widely used PBDE congener decabromodiphenyl ether (BDE-209) in the environment. In the study, the aquatic macrophyte Scirpus validus was selected to remove BDE-209 from three types of sediments (silt, clay, and sand) at an environmentally relevant concentration. After 18 months of phytoremediation experiment, S. validus significantly enhanced the dissipation rates of BDE-209 in all the sediments compared to the controls. Average removal rates of BDE-209 in the three treatments of silt, clay, and sandy sediments with S. validus were respectively 92.84, 84.04, and 72.22%, which were 148, 197, and 233% higher than that in the control sediments without S. validus. In the phytoremediation process, the macrophyte-rhizosphere microbe combined degradation was the main pathway of BDE-209 removal. Sixteen lower brominated PBDE congeners (di- to nona-) were detected in the sediments and plant tissues, confirming metabolic debromination of BDE-209 in S. validus. A relatively higher proportion of penta- and di-BDE congeners among the metabolites in plant tissues than that in the sediments indicated further debromination of PBDEs within plants. The populations and activities of microorganisms in the sediments were greatly promoted by S. validus. Bacterial community structure in BDE-209-contaminated rhizosphere sediments was different from that in the control rhizosphere sediment, as indicated by the dominant proportions of β-proteobacteria, δ-proteobacteria, α-proteobacteria, Acidobacteria, and Chloroflexi in the microbial flora. All these results suggested that S. validus was effective in phytoremediation of BDE-209 by the macrophyte-rhizosphere microbe combined degradation in aquatic sediments.

Central Asia is the global hotspot resulting from either a scarcity of natural resources or environmental degradation. Tajikistan, however, is called the "central Asian water tower", is rich in water and minerals and plays a very important role in the ecology of Central Asia. Given the soil contamination issues in Tajikistan, the spatial distributions and sources of n-alkanes and heavy metals in surface sediments of Tajikistan were investigated. The evaluation of n-alkane and elemental indices helped to elucidate the origins of complex pollutants. The n-alkane indices were allowed to identify biogenic and petrogenic sources, and statistical methods were used to identify natural and anthropogenic sources of heavy metals. Enrichment factors were used to assess the contamination statuses of heavy metals. The results indicated that n-alkanes in the cluster I (sample 23) and II (samples 1, 5, 12, 14, 18, 22, 29, and 30) samples were affected by crude oil or incomplete fossil fuel combustion. Biomarker indices indicated strong contributions of petroleum sources to the n-alkanes in samples 14, 18, 23, and 29, but that n-alkanes in the other samples were mainly derived from higher plant waxes. Statistical analyses showed that cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn) were originated from industrial activities. Phosphorus (P) was closely related to local agricultural activities. Manganese (Mn) was derived from mining activities and industrial wastewater. The enrichment factors indicated that sediment was zero to minor contamination with Cr, Cu, Ni, and Pb, but moderate contamination with Cd and significant contamination with Zn. Interestingly, the samples contaminated with Cd included the cluster I and II samples, meaning the source of Cd contamination had agreed with the results of n-alkanes. The combined evaluation of n-alkanes and heavy metals suggested that their pollutant sources were crude oil contamination.

Climate variability influences both water quantity and quality in lakes and rivers, e.g., shifts in salinity caused by changes in the evaporation/precipitation ratio. However, knowledge about climate and human influences on water quality in some regions is scarce, which causes uncertainty when evaluating water resources and developing water management plans. We measured the chemical composition and stable isotope values (δD and δ18O) in water samples from the Ili-Balkhash Basin, arid Central Asia, to investigate factors that control hydrologic processes in the area, and explore how the arid environment influences local hydrochemistry and water resources. Isotope values in lake waters (−24.1‰ [δD] and −1.5‰ [δ18O]) are enriched relative to river waters (−84.6‰ [δD] and −12.4‰ [δ18O]), indicating that lake waters experience intense evaporation. In the upstream Ili River, waters with lowest δD and δ18O values are supplied mainly by glacial ice and snow melt, whereas lower reaches of the river are affected increasingly by evaporation and human water extraction. Waters of the Ili River and other small rivers that drain to Lake Balkhash are chemically similar, and of the Ca-HCO3 type. Gibbs diagrams indicate that mineral weathering is the key process that controls river water chemistry. On average, carbonate and silicate weathering contribute 55.1% and 26.7%, respectively, to the total dissolved ion load in the river waters, illustrating the strong control that regional geology exerts on streamwater chemistry. In contrast, lake waters are characterized as Na-Cl type, which results mainly from evaporation and some evaporite dissolution. Weathering in the area is controlled by climate, i.e. precipitation and temperature. Moreover, a negative correlation between lake water total dissolved solids and d-excess, and a systematic increase in chemical concentrations and stable isotope values downstream in the Ili River, reflect the fact that arid climate conditions promote evaporation and affect water quality. PCA analysis and mass balance calculation of water chemistry suggests that humans contribute only 6.6% of the major ion load to waters, which display increasing concentrations in the middle and downstream regions. There, effects of human-mediated weathering and transport of salts from agricultural irrigation are reinforced by higher temperatures and evaporation.