Grain size and heavy metal assessment in barchan dunes surrounding the Talatan PV power generation area, Qinghai Province

Heavy metal contamination in sediments near photovoltaic (PV) power generation areas poses potential environmental risks, requiring detailed characterization and source apportionment to facilitate sustainable management. This research explores the characteristics of sediment grains and the concentrations of heavy metals present in surface sediments from barchan dunes proximal to the Talatan Photovoltaic (PV) power generation area, located within Gonghe County, Qinghai Province. The sediments displayed an average grain size within the medium sand range, with marginal differences discernible between the windward and leeward slopes. With the exception of Mn, Cu, Zn, V, Pb, and Ba, concentrations of the remaining metals were found to surpass the baseline levels established by Chinese aeolian soil data, alluding to potential anthropogenic influences. Through employing coefficients of variation in multivariate statistical analysis, it was identified that the concentrations of Cr and Co were significantly elevated, suggesting potential anthropogenic contamination, which may be associated with photovoltaic industrial activities. Specifically, the elevated concentrations of Cr and Co suggested anthropogenic contamination, potentially associated with photovoltaic industrial activities. Utilizing a combination of Correlation Coefficient Analysis, Principal Component Analysis, and Cluster Analysis, three potential sources of heavy metals were identified: (1) industrial origin for elements such as Cu, Cr, Ni, Zn, and As; (2) elements with limited direct application in the photovoltaic industry but associated with materials and energy storage, namely Ba, V, and Mn; (3) anthropogenic inputs related to construction materials and battery storage systems in the photovoltaic park, specifically Co and Pb. The findings offer a significant understanding of the heavy metal characteristics and sources in proximity to the Talatan PV power generation area, emphasizing the impact of human activities on environmental quality. These insights underscore the necessity for enhanced monitoring and management of industrial activities to mitigate potential environmental impacts. Further research is recommended on a broader spatial scale to yield a more comprehensive understanding of this subject.


Introduction
Solar photovoltaic (PV) technology, in its quest for sustainable energy solutions, has rapidly gained momentum as an effective renewable energy source.China has positioned itself as the global frontrunner in PV applications, driven by its geographic advantage and robust policies [1][2][3].Particularly, the arid to semi-arid regions of northwestern China, with abundant solar radiation and vast land resources, have transformed into pivotal hubs for solar power generation [4][5][6][7].
The Gonghe Basin, located in Qinghai Province on the Qinghai-Tibet Plateau, epitomizes this transformation [8].With an expansive PV Industrial Park covering 609.6 km 2 and a staggering power generation capacity of 31.14 billion kW•h, it stands as a significant locale for solar photovoltaic energy [9].Complementing the Longyangxia Hydropower Station, this park significantly advances the sustainable socio-economic development of western China [10].However, these advancements also bring forth ecological challenges.The Gonghe Basin grapples with desertification, with 62.84% of its area comprising mobile and semi-fixed sand dunes [11].The development of the PV industry, particularly in the Talatan Desert region of the basin, inevitably impacts the local wind and sand environment [12].As a case in point, by August 2015, the Talatan PV station had become the largest solar installation in China, emphasizing the scale and implications of such developments [13].
However, alongside the advantages of PV technology, there are mounting concerns about its environmental effects, especially concerning the heavy metals used in PV panels.Metals like cadmium and lead in solar cells raise questions about potential leaching and environmental repercussions [14][15][16][17].The lifecycle of PV panels, from production to disposal, introduces pollution emissions, calling for comprehensive scrutiny [18][19][20].
With this backdrop, our study delves into the Talatan PV area.We collected surface sediment samples from barchan dunes to discern the impact of the PV park infrastructure on granulometric characteristics and heavy metals including chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), lead (Pb), vanadium (V), and barium (Ba) concentrations within the surface sediments of barchan dunes.The research also attempts to uncover the potential origins of these metals, linking them to PV applications and other human activities.

Study area
The study area is located in the Talatan area of Gonghe Basin, Gonghe County, Hainan Prefecture, Qinghai Province, approximately 10 km east of the Longyangxia Reservoir and 18 km north of Gonghe County (figure 1).As a meeting point of the Asian winter and summer monsoons and the westerly circulation, the region lies on the northwestern edge of the Asian Monsoon area and exhibits a cold, dry, and semi-arid continental climate.Characterized by scarce rainfall, cool temperatures, abundant sunlight, and significant diurnal temperature differences, the region has an average annual temperature of 4.1 °C, with a highest recorded temperature of 33.7 °C and the lowest at −33.5 °C.Annual precipitation amounts to 246.3 mm, concentrated between July and September, while the annual evaporation rate is 1,716.7 mm.Boasting an average solar radiation of 6,564.26MJ•m −2 annually and a mean annual sunshine duration of 2,907.8h, the region predominantly experiences winds from the west and northwest.The average yearly wind speed measures at 2.7 m•s −1 , with maximum speeds surpassing 40 m•s −1 .Windy conditions prevail for approximately 50.6 days per annum, with some years recording as many as 97 days of strong winds.Moreover, sandstorms occur on an average of 20.7 days each year [21][22][23].

Sample collection
Three barchan dunes of distinct developmental scales were selected to the east of the Talatan PV power generation area (labeled as Barchan I, II, and III) (figure 2).The morphological parameters of each barchan are presented in table 1. Sediments were collected from 3-4 profiles on the surface of each dune (figure 2).The sediment sampling locations for each profile included: the base of the windward slope, the lower part of the windward slope, the upper part of the windward slope, the dune top/ridge line (the highest position on the central axis is referred to as the dune top, while the highest position on other profiles is termed the ridge line), the middle of the leeward slope, and the base of the slope.A total of 142 samples were collected.

Analytical methods
Sample particle-size composition was established utilizing a Mastersizer 2000 laser size analyzer.The initial process involved treatment with 30% H 2 O 2 to eliminate organic constituents.Thereafter, carbonate components were dissolved using 10% HCl.Prior to conducting grain-size measurements, sample solutions underwent dispersion via 0.5 N (NaPO 3 ) 6 , aided by an ultrasonic vibrator [24].
The process of sample preparation necessitated the thorough air-drying of the samples, followed by grinding and sieving them to a fineness of 75 μm.A sample of 4 grams was measured and introduced into the core of the column apparatus in conjunction with boric acid, then compressed at a pressure of 30 t/m 2 for a duration of 20 s.This resulted in a final processed sample of 4 cm in diameter and 8 mm in thickness, which was subsequently subjected to analysis by Wavelength Dispersive x-ray Fluorescence Spectrometry (XRF, PANalytical PW2403 apparatus) [25].Analytic outcomes are expressed in terms of oxide compounds, with the exception of trace elements, which are reported in elemental form.To maintain the integrity of quality assurance and control (QA/QC), the established procedure incorporated the analysis of duplicate samples and standard reference materials (GSS1 and GSD12).These standards were procured from the Center of National Standard Reference Material of China and subjected to identical processing as the field samples.The precision of the analyses, verified through the relative standard deviation of duplicate samples, consistently fell within the 3%-5% range.Furthermore, the accuracy, gauged by the relative error of the standard reference materials, was consistently maintained below 5%.

Statistical analysis
To scrutinize the interrelationships among heavy metals in the barchan dune surface sediments of Gonghe, Qinghai Province, and to deduce their potential sources, a suite of multivariate statistical analyses-Pearson's correlation coefficient analysis, Principal Component Analysis (PCA), and Cluster Analysis (CA)-were undertaken using SPSS version 13.0.Initially, PCA simplified the multidimensional data by reducing correlated variables into principal components (with eigenvalues>1), revealing underpinning correlations among original variables through eigenvalue and eigenvector extraction.Subsequent to standardizing heavy metal concentration data via z-score transformation and computing Euclidean distances for similarity assessment among variables, CA, employing Ward's method, clustered observations into exclusive groups, effectively categorizing heavy metals in the studied sediments based on their source similarities.

Grain size characteristics
The windward slope surface sediments display an average grain size between 1.28 and 2.35 Φ, with a mean of 1.75 Φ, falling within the medium sand range.The standard deviation ranges from 0.02 to 0.27, averaging at 0.12, indicating slight fluctuations.Sorting coefficients lie between 0.39 and 1.35, with an average of 0.54, pointing to a fairly sorted condition [26].Skewness ranges from −0.06 to 0.33, with a mean of 0.06, primarily positively skewed.Kurtosis spans from 0.93 to 1.45, with an average of 0.97, suggesting moderate peakedness.From the foot of the windward slope to the ridge line, the grain size changes insignificantly in the left wing profile, while the remaining profiles exhibit certain fluctuations, with inconsistent trends.The windward slope sediments are mainly composed of medium sand (55.87%), followed by fine sand (37.28%), coarse sand (4.87%), very fine sand (1.16%), silt clay (0.79%), and very coarse sand (0.02%), with no gravel present [27,28].
The leeward slope surface sediments exhibit an average grain size from 1.39 to 2.67 Φ, with a mean of 1.94 Φ, also falling within the medium sand range.Sorting coefficients range from 0.44 to 1.40, with an average of 1.64, indicating fairly sorted condition [26].Skewness spans from 0.03 to 0.38, with a mean of 0.09, showing a positive skew.Kurtosis ranges from 0.92 to 1.13, with a mean of 0.97, implying moderate peakedness.In terms of mechanical composition, the proportion of medium and fine sand is the highest, accounting for 45.25% and 43.78% respectively, while the content of coarse sand, very fine sand, and silt clay decreases in sequence, accounting for 4.52%, 3.31%, and 3.13% respectively, with no very coarse sand or gravel present [27,28].
The grain size of the dune surface sediments ranges from 1.28 to 2.67 Φ, with the leeward slope sediments being slightly finer than those of the windward slope.The dominant grain sizes are medium and fine sand, consistent with the Gonghe Formation dune sand samples in the Gonghe Basin and Talatan, but the content of fine sand is far less than the two [10,29].Compared to previous studies (2.54 to 3.34 Φ), the average grain size is slightly coarser [30][31][32][33].Grain size frequency curves are predominantly unimodal, and grain size parameters show little variation across different locations.Compared to the background values of Chinese aeolian soil elements, all measurements exceed their counterparts except for Mn, Cu, Zn, V, Pb, and Ba [34].Notably, when benchmarked against international standards like those of Northern Europe, Australia, and Finland, the concentrations of some of these metals, particularly Cr, Ni, and As, are observed to be higher than the referenced background values for these regions [35,36].Furthermore, when contrasted with surface deposits from the Tengger Desert's interior, our study's average concentrations of Cr, Mn, Co, Ni, Cu, Zn, and Pb are higher [37].

Element concentrations
In the barchan dune surface sediments of Gonghe, the average metal concentrations, normalized to the respective background values of Chinese aeolian soil, follow the sequence: The study observes that the arithmetic means of Cr, Co, Ni, and As in the sediment samples exceed their corresponding background values by factors of 2.2, 2.7, 1.1, and 1.1 respectively, when compared to Chinese aeolian soil elements.Distinct coefficients of variation (CVs) emerge for Cr and Co, pointing to a significant spatial variation in their distribution across the study area.This may indicate a potential influence of human activities on the concentrations of Cr and Co.

Correlation analysis of heavy metals
Table 3 presents the interrelations between various metals in the surface sediments of Gonghe's barchan dune, as indicated by Pearson's correlation coefficients.Cu, Cr, Ni, Zn, and As notably exhibit a statistically significant positive correlation at the P < 0.01 significance level.Mn also demonstrates a robust positive correlation with Ba and V at the P < 0.01 significance level.In contrast, Co inversely correlates with Cr, Ni, Cu, Zn, As, and V at the same significance level.Furthermore, Pb is significantly positively correlated with As at the P < 0.05 level.
The data reveals that these inter-elemental associations shed light on the intricate dynamics and interactions within the dune ecosystem, offering vital insights into the origins and transportation pathways of these metals [38].The pronounced positive correlations among Cu, Cr, Ni, Zn, and As indicate potential shared or similar pollution sources for these elements.In parallel, the strong interrelation among Mn, Ba, and V may also hint towards common pollution sources among them.On the other hand, the inverse correlation of Co with Cr, Ni, Cu, Zn, As, and V signifies a possibility of differing pollution sources for Co, separating it from the aforementioned metals.
The observed elemental correlations and disparities within the sediments point towards complex underlying processes, entwining both natural and anthropogenic activities.Specifically, a simultaneous rise in heavy metal elements with approximate pollution sources might suggest that certain heavy metal pollutants related to photovoltaic production processes are affecting the nearby sedimentary environment.From an environmental and ecological perspective, the accumulation of these metals urgently warrants attention, considering the substantial risks they pose.These metals can be ingested by organisms, permeating and potentially amplifying through the food chain, thereby affecting local biota and potentially introducing risks to human health through mechanisms of bioaccumulation and biomagnification.

Principal component analysis
Principal Component Analysis (PCA), facilitated via varimax rotation with Kaiser Normalization, was harnessed to decipher both anthropogenic and geogenic origins of heavy metals, employing a statistical methodology that discerns predominant factors accounting for maximal variance in multidimensional datasets.Prior to PCA implementation, the Kaiser-Meyer-Olkin (KMO) and Bartlett's test of sphericity (KMO = 0.628, P = 0.000) were employed, ensuring data suitability for PCA.Eigenvalues and eigenvectors were extracted from the correlation matrix to ascertain significant factors and the percentage of variance explicated by each, utilizing SPSS v19.0 software; findings are articulated in table 4. Three eigenvalues >1 elucidated approximately 71% of the total variance, while the fourth eigenvalue (0.76) explicates about 8% thereof-a noteworthy contribution to cumulative variance elucidation.Consequently, the initial four eigenvalues, elucidating approximately 79% of the total variance, were selected for ensuing analysis, and eigenvalues <0.87 were discarded to establish a probable number of contributing source factors.
Table 4 delineates the three factor loadings with varimax rotation and communalities.The inaugural factor, accounting for 32.0% of the total variance, is predominantly loaded with Zn, Ni, Cu, As, and Cr.The secondary factor, predominantly influenced by Ba, V, and Mn, represents 23.8% of the total variance.The tertiary factor, primarily characterized by Co, constitutes 15.1% of the total variance, while the quaternary factor, primarily loaded by Pb, explicates 7.6% thereof.Interrelations among the heavy metals, grounded on the initial two principal components, are depicted in figure 3.

Cluster analysis
Following this, Ward's method was implemented for hierarchical clustering on the standardized dataset.The CA outcomes for the investigated heavy metals are depicted in figure 4, three clusters emerge: (1) Mn-V-Ba, (2) Ni-Zn-Cu-Cr-As, and (3) Co-Pb, which align well with the outcomes from the PCA.

Heavy metal source identification
Relative to the standard background values of aeolian soil elements in China, the observed increases in concentrations of Cr and Co within the surface sediments of the barchan dune suggests a possible anthropogenic  influence as a contributing source of these elements [38,39].Meanwhile, the concentrations of other elements including Mn, Ni, Cu, As, Pb, V, and Zn in majority of samples are either slightly elevated or roughly equivalent to their respective background values.This indicates that their primary origin can be traced to natural sources, predominantly local soil.Outcomes from correlation coefficient analysis indicate that elements such as Cu, Cr, Ni, Zn, and As share a common source, while Ba, V, and Mn can be traced back to another mutual origin.
Interpretations from PCA and CA support these inferences.Through the triangulation of correlation coefficient analysis, PCA, and CA, we can discern three distinct origins, i.e. (1) Cu, Cr, Ni, Zn, and As exhibit significant correlation and extensive utilization in photovoltaic manufacturing, suggesting an industrial source.The conspicuous presence of Cr intimates its use in thin-film solar cells.(2) Ba, V, and Mn, found in lower concentrations, have limited direct applications in the photovoltaic industry but contribute to related materials and energy storage.
(3) The distinctly elevated levels of Co and Pb suggest anthropogenic sources, possibly associated with construction materials and battery storage systems within photovoltaic parks, and potential pollution stemming from equipment disposal.One cluster comprises elements such as Cu, Cr, Ni, Zn and As, demonstrating potent positive correlations in both PCA and correlation coefficient analysis, with further grouping evident in CA.The mean concentrations of these elements in sediment samples, namely Cu, Ni, Zn, and As, exhibit values that slightly exceed or are nearly akin to their corresponding background levels.Notably, Ni maximum concentration approaches twice its background value.Cu is mainly used for wires and connectors in the photovoltaic industry.Due to its high electrical conductivity and good corrosion resistance, it is an ideal material for cables and connectors in photovoltaic systems.Additionally, Cu can be used to manufacture conductive strips for solar cell backplates, improving cell conversion efficiency [40].Ni and Zn application in the photovoltaic industry is primarily focused on coatings and protective films.They can be used to create a corrosion-resistant layer for solar cells, protecting them from external environmental damage and extending their service life [41,42].As's application in the photovoltaic industry is limited, mainly used for manufacturing multi-junction solar cells.Gallium arsenide (GaAs) is a commonly used semiconductor material with high conversion efficiency and radiation resistance, often used in solar cells [43].Cr of the first group in all sediment samples is obviously higher than the background value of aeolian soil elements in China.Cr is often used as a buffer layer or interface layer material in thin-film solar cells, such as CIGS (copper indium gallium selenide) cells.These layers help improve cell performance, such as enhancing carrier transport, reducing defect density, and increasing cell stability [44].
Ba, V, and Mn compose a second cluster of elements.Their mean concentrations in the samples each present as relatively low when juxtaposed with their respective background values from Chinese aeolian soil elements.These elements do not have many direct common applications in the photovoltaic industry, but in some respects, they can play a role in photovoltaic-related materials or devices: The application of Ba in the photovoltaic industry is not widespread.However, certain compounds containing Ba (such as barium titanate) have application value in the fields of piezoelectric and ferroelectric materials [45].The application of V in the photovoltaic industry is primarily focused on energy storage.Vanadium redox flow batteries are reversible charge-discharge energy storage devices with high energy density, long cycle life, and scalability [46].The application of Mn in the photovoltaic industry is mainly reflected in lithium-ion batteries [47].Although the applications of Ba, V, and Mn in the photovoltaic industry differ, they all relate, in varying degrees, to energy storage and management within photovoltaic systems.
A third cluster of elements, Co and Pb, delineate themselves from the other heavy metals, thereby alluding to a primary anthropogenic origination.Elevated concentrations of Co, discovered in the majority of sediment samples, signal that the primary source of Co in barchan dunes' surface sediments are rooted in humanassociated activities.These activities span from the use of construction materials to battery storage systems in photovoltaic industrial parks, with an additional consideration of the potential environmental pollution triggered by the disposal of photovoltaic modules and other related equipment.To be precise, in the context of constructing and operating photovoltaic industrial parks, Co and Pb exhibit similar provenances.Building materials rich in Co and Pb can be deployed in the construction of these parks, while certain paints, pigments, and corrosion-resistant materials may also introduce these heavy metal elements [48,49].Energy storage systems, such as lithium-ion and lead-acid batteries, present within the photovoltaic industrial parks, serve as notable reservoirs of Co and Pb [50,51].Lastly, operational activities within these parks may generate waste imbued with Co and Pb.The disposal of photovoltaic modules, batteries, and other related equipment can thereby contribute to environmental pollution [52].

Implications on the environment
This study explores heavy metal pollution in surface sediments of barchan dunes near the Talatan PV power area in Qinghai, highlighting potential environmental and ecosystem impacts.
Firstly, the super-background concentrations of heavy metals in the intake system imply that these elements might have undergone long-term accumulation, or these areas are under the influence of continuous pollution sources.Particularly, the higher concentrations of Co and Cr indicate that human activities (especially the construction and operation of photovoltaic industrial parks) have to some extent altered the distribution and migration of these elements in the study area.Continuous invasion of heavy metals may adversely affect the soil microbes and vegetation in this area, hinder their normal functions, and possibly further affect a broader ecosystem by amplifying impacts through the food chain to higher organisms.
Secondly, considering the uses of different heavy metals in the photovoltaic industry, as well as their emissions in various processes, attention must be paid to the potential long-term impacts of these elements on soil, groundwater, and air.For instance, the potential permeation and migration of Cr, Ni, and Zn might cause pollution in groundwater, while wind-blown sand and dust may spread these heavy metals to wider areas [53,54].
In terms of management strategies, it is recommended to adopt a series of preventive and control measures to stop or at least slow down the further accumulation of heavy metals.Including but not limited to: 1. Implementing strict emission standards and regulation in photovoltaic industrial parks, reducing the emission of metals such as Co, Cr, and Ni; 2. Enhancing the management and recycling of waste, such as old photovoltaic panels and energy storage batteries, to prevent them from becoming potential sources of environmental pollution; 3. Implementing vegetation cover and soil restoration strategies to reduce wind erosion and migration of heavy metals; 4. Formulating more refined spatial planning for areas where heavy metals exceed standard levels to prevent further spread of pollution.
Lastly, future research can further explore the migration and transformation mechanisms of these heavy metals in different environmental media (such as air, soil, and groundwater), as well as their specific impacts on local and regional ecosystems, to provide support for implementing more targeted environmental management strategies.

Conclusion
This study provided a comprehensive analysis of grain characteristics and heavy metal concentrations in surface sediments from barchan dunes proximal to the Talatan PV power generation area in Gonghe County, Qinghai Province, with sediments chiefly comprising medium to fine sand and exhibiting minor grain size variations between windward and leeward slopes.
It was identified that all metals, with the exception of Mn, Cu, Zn, V, Pb, and Ba, surpassed the background values for Chinese aeolian soil.The amplified concentrations of metals such as Cr and Co hint at anthropogenic inputs, likely tethered to industrial activities.Notably, the variability in heavy metal concentrations within the study area underscores a discernible impact of human activities on their distribution.
Through the application of correlation coefficient analysis, PCA, and CA, three principal sources of these metals were discerned: an industrial origin is suspected for elements like Cu, Cr, Ni, Zn, and As; a confined direct application in the photovoltaic industry yet related to materials and energy storage for Ba, V, and Mn; and anthropogenic input possibly correlated to the construction materials and battery storage systems of the photovoltaic park for Co and Pb.
The findings unravel crucial insights into the heavy metal characteristics and sources in surface sediment from barchan dunes near the Talatan PV power generation area, underscoring the tangible impact of human activities, particularly those related to the photovoltaic industry, on environmental quality.The highlighted potential adverse impacts on the environment not only necessitate meticulous monitoring and management of these activities but also demand the consideration and incorporation of the management strategies proposed in section 3.6, emphasizing emission regulation, waste management, and environmental restoration.
However, while this study sheds light on localized impacts and potential contaminant pathways, it operates within a particular geographical scope.Thus, future research should broaden its spatial scale for a more encompassing understanding of this topic and delve deeper into the migration and transformation mechanisms of heavy metals across different environmental media, such as air, soil, and groundwater, as well as their particular impacts on local and regional ecosystems, thereby scaffolding more targeted and efficacious environmental management strategies.

Figure 1 .
Figure 1.Location of study area and sampling sites in Gonghe, Qinghai Province.

Figure 2 .
Figure 2. The positions of barchan dunes and sampling profiles.
Table 2 presents a comparison of heavy metal concentrations in surface sediments from the barchan dunes in Gonghe, Qinghai Province, with reference values for aeolian soil elements from various regions.The table shows that, it is evident that the mean values for Cr, Mn, Co, Ni, Cu, Zn, As, Pb, V, and Ba in the surveyed samples are 53.3,257.3, 22.8, 12.8, 6.1, 27.3, 4.7, 10.8, 35.2, and 276.6 mg kg −1 , respectively.

Table 2 .
Metals concentrations in surface sediments from barchan dune in Gonghe, Qinghai Province.

Table 3 .
Pearson's correlation matrix for metal concentrations.The lower-left figures are correlation coefficients, and the upper-right figures are P values.

Table 4 .
Rotated component matrix for the data from the surface sediments of the barchan dune (PCA loadings > 0.4 are shown in bold).