Soil contamination around porphyry copper mines; an example from a semi-arid climate

Extraction and processing of metalliferous ores can cause severe disturbances and contamination in natural ecosystems but little known about such effects for copper mines in semi-arid areas of southwest Asia. This study was conducted on seven sites around Sungun Copper Mine area, northwest Iran. Electrical Conductivity (EC), pH, nitrogen, phosphorous, potassium, organic matter, soil texture along with 16 metal and metalloids concentrations were measured in 94 soil samples. Results showed that sites were distributed along a gradient of contamination from low contamination in natural hillsides to high contamination in Waste Rock Dump, Oxide Dump, alongside Pakhir and Sungun Rivers. Nutrient (N, K, Ca, Mg) de�ciency occurred in disturbed sites. The main contaminant sources were Waste Rock Dump, mine pit drainage, and Oxide Dump. The results of Non-metric multidimensional scaling ordination showed elevated Cd, Zn, Fe, Cu, Pb, As, Mo, Mn, Co, S concentrations, high EC, and higher sand percentage in the sites affected by mine waste and acid mine drainage. Geo-Accumulation and Potential Ecological Risk Indices indicate that Pakhir riverside, Sungun riverside and Oxide Dump have severe to moderate levels of environmental risks. Positive correlations between some metal elements imply their common sources and similar reaction pathways could also be a reason for their similar geochemical behaviour in transport and deposition and their interdependence. Overall, the de�ciency of organic matter nutrients along with the soil sandy texture in contaminated sites of Sungun Copper Mine are the main limiting factors in managing metal mobility and soil remediation.


Introduction
As an important portion of ecosystems, soil is the source of water and essential nutrients for soil organisms and plants, and the geochemical sink or lter for contaminants (Terrones-Saeta et al., 2021; Bermudez et al., 2010).Toxic metals that cause soil contamination turns into environmental problems worldwide due to the increasing rates of industrial and particularly mining activities (J.Adewumi et al., 2020;Sun et al., 2020;Khalid et al., 2017).A potential source of metal and metalloid contaminants is mining operations and cause primary concern for environmental sustainability (Jung et al., 2001).Unlike many organic contaminants, heavy metal that is de ned as metals and metalloids accumulate in soil because they are environmentally persistent by association with organic matter and sorption to mineral surfaces (Ahdy and Khaled, 2009;Grafe and Naidu, 2008).Soils in mined lands have typical characteristics such as de ciencies in carbon compounds and nutrients and elevated contents of toxic mineral elements (Rodríguez et al., 2009;Huang et al., 2012).Metalliferous mines are the source of signi cant amounts of metalliferous wastes, such as rock dumps, wastes of mineral processing and smelters, lead to contamination and degradation in vicinity of mine area (Dinelli et al., 2001;Randelovic et al., 2014).Spatial heterogeneity of mine wastes, soil physicochemical properties and differences with natural geochemical background values are main aspects of soil contamination investigation process in mined lands (El Azhari et al., 2017).Most mine wastes are not chemically or physically stable, and cause problems in surrounding environments as dust, erosion into surface waters, and through leaching and acid mine drainage (Otte and Jacob, 2008).Soil pH ≤ 7 and elevated metals and metalloid elements concentration due to acid mine drainage (AMD) exacerbate environmental problems by increasing the toxic elements bioavailability (Rezaie and Anderson, 2020).Determination soil mineral elements content and their mobility in contaminated soils is a practical approach in eco-toxicological evaluation of polluted areas (Clemente et al., 2008).For better management, including improved soil quality, of mined lands, an essential step is to determine the metal concentrations in soils in order to monitor their distributions and manage their effects and possible toxicities (Radi et al., 2023;Yun et al., 2016).But soils' metal(loides) elements mobility and toxicity along with the elements total concentrations depend on their speci c chemical forms, and edaphic and environmental characteristics such as soil acidity or alkalinity, organic matter, salt concentrations and climatic condition (Gabarron et al., 2017;Mehrabi et al., 2015;Zu et al., 2014;Chrastny et al., 2012).
Open pit mines cause serious environmental problems through vegetation cover and top soil removal which leads to exposure of rocks and minerals to weathering which were buried in geological eras, rocks and minerals grinding, and nal extraction of the metal (Karaca et al., 2018).Soil contamination and acid mine drainage cause considerable environmental degradation with severe impacts on communities and biodiversity (Kefeni et al., 2017).There are many reports of porphyry copper deposits for which mining activities have caused soil contamination through elevated levels of some metal and metalloid elements but little known about such effects in semi-arid ecosystems in southwest of Asia (Bech et al., 1997;Oyarzún et al., 2001;Maturana et al., 2004;Kelm et al., 2009 (Aghili et al., 2018).As there is no study about soil contamination around the Sungun copper mine.This study aimed to (i) evaluate metal and metalloid contamination and other physicochemical properties of soils around Sungun copper mine, Iran, (ii) determine the contamination indices and gradient, (iii) to discuss the relations between metal and metalloid elements and their association with soil physical and chemical characteristics.

Study area
Sungun Cu-Mo deposit in north-west of Iran, 120 km Northwest of Tabriz (46 ο 30′ E to 46 ο 52′ E, 38 ο 34′ N to 38 ο 45′) hosts Iran's second important and large copper mine (Fig. 1).The mineralization at Sungun Copper Mine is largely hosted by diorite/granodiorite and monzonite/quartz-monzonite porphyry (Hezarkhani, 2011).Original porphyritic texture through silici cation process altered into heavily dense rock that is cut by quartz veins, which is together with pyrite, molybdenite and, to some extent chalcopyrite (Nabi Bidhendi et al., 2007).The approximate amount of mine ore is about 500 million tons (Hezarkhani et al., 1999), which has been operated since 2006.The elevation of the study area varies from 1645 m to 2700 m (ASL).Study area has a semi-arid climate, and annual mean precipitation is about 400 mm, also average monthly temperatures in summer and winter varies between 33°C and − 20°C (Moore et al., 2011).Mining area is located near Arasbaran protected natural heritage and its vegetation mostly consists of Asteraceae, Poaceae, Fabaceae, Apiaceae, Brassicaceae, and Lamiaceae families (Ghorbani et al., 2018).Based on eld observations and according to the potential of contamination soil sampling performed in different areas surrounding Sungun copper mine (Figs. 2 and 3).These were: (i) alongside Pakhir River which is in downstream of mine waste rocks and mine pit drainage; ii) alongside Sungun River that is not affected by mine drainages; (iii) Oxide Dump; (iv) Waste Rock Dump, (v) undisturbed hillside next to Waste Dump; (vi) natural hillside near mine pit; and (vii) natural hillside far away from the mine pit.

Soil sampling and laboratory analysis
Sample collection was conducted by 94 samples from 0-15 cm, and 15-30 cm depths.Topsoil samples were collected from 6 point in Waste Rock Dump, 5 point from Oxide Dump, 5 point alongside Sungun River, 7 point alongside Pakhir River, 9 point in hillside next to Waste Rock Dump, 5 point from hillside near mine pit and 10 point in the natural hillsides far from mine pit.A mixture of four soil samples in each point were used to form a single composite sample that were kept in Clear Polyethylene bags until chemical analysis.The samples dried in room temperature then grinded and sieved through < 0.2mm (80mesh).The fraction of soil samples < 2mm were used for physicochemical characterization.Topsoil samples pH and electrical conductivity evaluated in a 1:2.5 (w/v) soil/ water suspension by Adwa (AD 1000) pH meter and ELMETRON (CC 501) conductivity meter, respectively.Organic matter (%) and total nitrogen (%) were determined using titration and colorimetric method (Walkley and Black, 1934) and Kjeldahl method (Bremner and Mulvaney, 1982).The calcium carbonate (CaCO 3 ) contents were determined by titration with hydrochloric acid (HCl).Potassium (K) concentrations were measured by ame photometry (Colwell, 1963).Topsoil available phosphorus concentration evaluated by Olsen method (Olsen et al., 1954).Soil particle size analysis conducted by Hydrometer (Bouyoucos, 1951).Total concentrations of Al, As, Ca, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, P, Pb, S, and Zn in soil were measured by ICP-OES (Varian 735, Margesin and Schinner, 2005).

Contamination indices
Evaluation of soil indices of Geo-Accumulation (I geo ) (Muller, 1969) and Potential Ecological Risk (RI).These indices are e cient to determine soil contamination and quality degradation because of anthropogenic causes (Sur et al., 2022).
Cn is the measured and Bn is the background value (Kabata-Pendias, 2011) of element n concentration in soil, because of lithological variations and variations in background data the factor 1.5 is added to the equation.The Geo-Accumulation index have seven classes (Table 1).Response index of heavy metals' potential toxicity in soil is RI= .The ecological risk factor ( ) has ve classes (Table 2).

Statistical Analysis
The R ® statistical computing language used for statistical analysis (Oksanen et al., 2011).Shapiro-Wilk W test, dedicated to checking normality and then log-transformation performed on non-normal distributed data before analyses.Two-factor ANOVA performed to determine the depth and the sites effects on physical, chemical and mineralogical characteristics of soil samples.Interaction of sites and soil depths was not signi cant.Duncan multiple range test was applied to compare means of soil data in different sites.Relationships between pair-wise distances of sites and soil properties and dissimilarities (ordination space) analysed by NMDS method.Then, sites were shown with ellipses on the gradient of soil contamination.To evaluate the association between soil characteristics and levels of metals and metalloids Spearman's correlation coe cient was used.

Soil properties and contaminant concentrations
The majority of the soil properties and metal and metalloid elements concentrations showed signi cant differences among studied sites (Tables 3 and 4).The soil pH varied from 6.40 in alongside Pakhir River sites to 7.01 in the Waste Rock Dump (Table 3).Alongside Pakhir River and Oxide Dump had signi cantly highest values of electrical conductivities (EC).In contrast, the Waste Dump and natural hillsides had low EC values (Table 3).Soil texture analysis showed that the samples related to Pakhir River and Sungun River contained greater sand (91.446% and 86.968%) and the lowest silt (7.707% and 9.724%) and clay (0.265% and 1.807%) contents, respectively.The greatest amount of clay was found in Oxide Dump and natural hillside far from the mine pit (Table 3).The Oxide Dump and three natural hillsides were found to have high silt content (24.520%, 19.753%, 27.729%, and 18.526%, respectively).Natural hillside located a few meters from mine pit showed high levels of calcium carbonate, with no signi cant differences between other sites (Table 3).The N-P-K contents indicated that natural hillsides had greater levels of N (0.537%, 0.344%, and 0.359%) and K (1.875, 1.746 and 1.620 µg g -1 ) compared with those of other sites in the mining area (Table 3).In contrast, the two riversides and Oxide Dump showed to have greater P concentrations than other sites.The soil organic matter contents ranged from 0.083% in waste dump to 5.84% in natural hillside located in the vicinity of Waste Dump.In mining affected sites (alongside Pakhir and Sungun River, Oxide Dump, and Waste Rocks), the organic matter concentrations were signi cantly lower (0.470%, 0.506%, 0.553%, and 0.083%, respectively) than that in the natural hillsides (5.840%, 2.270% and 1.780%; Table 3).(Range)  Mean trace elements in the soils of different sites showed that Pakhir riverside and the natural hillside far away from the mine pit had the highest and lowest Co concentrations respectively (18.83 µg g -1 and 11.6 µg g -1 ,; Table 4).
There was no signi cant different between all other studied sites.The three natural hillsides had identically elevated concentrations of Cr in the soil (46.31, 43.98, and 39.58 µg g -1 ).The values related to Cu, Fe, Zn, Cd, and Pb concentration were the greatest in Pakhir riverside (Table 4).Soil samples from two natural hillsides near the Waste Rock Dump and mine pits had higher Mg content (13138.26µg g -1 ).The highest concentrations of Mo (69.07 µg g - 1 ) and Ce (70.26 µg g -1 ) were found in Sungun riverside and Waste Dump, respectively (Table 4).
The maximum S concentrations were found in the two riversides and Oxide Dump, with signi cant differences from other sites.The average As concentrations in the soil of the two riversides were signi cantly greater than that in all other sites.Elevated concentrations of Mn, Ni, Al, and Ca were recorded in natural hillsides, but they were not signi cantly different from some of the mining affected areas (Table 4).

Contamination gradient
The NMDS ordination of sampled plots according to their soil properties revealed that the arrangement of sites in the ordination space is strongly in uenced by soil properties (Fig. 4).The ellipses of two riversides (S1 and S2) particularly Pakhir riverside are located in the upper right corner showing an association with the positive ends of two axes.These axes are mainly de ned by Cd, Zn, Fe, Cu, Pb, As, Mo, Mn, Co, S concentrations, electrical conductivity, and sand content.In contrast, other sites (S4: Waste Rock Dump, S5: natural hillside close to Waste Rock Dump, S6: natural hillsides near the mine pit, S7: natural hillside far away from mine pit) are assembled together in the opposite direction associated with higher silt and clay contents, and higher K, Mg, N and organic matter concentrations, and higher pH.The Oxide Dump had a greater association with the positive end of axis 1 (As, Cu, Mo, P and Pb concentrations and electrical conductivity) and also the negative end of two axes (higher silt content, pH, and Mg, CaCO 3 , K, and N concentrations).These result shows a spatial gradient of soil contamination around Sungun Copper Mine area.

Contamination indices
The Geo-Accumulation Index values for Cu and Pb indicated that the soil alongside Pakhir River was extremely contaminated (Table 5).Also the I geo values for As shows strongly contamination of soil alongside Pakhir riverside and Sungun riverside and also the average of the Geo-Accumulation Index of Mo suggested that soil alongside Sungun River was extremely strongly contaminated by this trace element (Table 5).According to I geo index values, elements of As, Mo, Pb and Ce are sources of moderately to strongly contamination of soil in Waste Rock Dump and Oxide Dump.Regard to all analyzed metal and metalloids elements Natural hillsides showed no contamination to moderate contamination rates (Table 5).6).Alongside Sungun River there was a high ecological risk factor due to As (Table 6).
Despite Cd showed moderate ecological risk other metals had low rates of this factor in all other sites.(RI) showed that soil alongside Pakhir River and Sungun River had serious and sever potential ecological risk level, respectively (Table 7).A moderate potential ecological risk level was found for Oxide Dump and low-grade for other sites (Table 7).

Correlation matrix
The relationship between soil parameters showed some signi cant positive and negative correlations (Fig. 5), which is consistent with the results of NMDS ordination (Fig. 4).Soil pH had positive association with Ca and a negative relation with Cd, Fe, and Pb (Fig. 5).As, Cd, Ce, Co, Cu, Fe, Mn, Mo, Ni, Pb, S, and Zn concentrations were signi cantly associated with soil electrical conductivity (Fig. 5).Soil organic matter was had positive correlation with Al, Cr, and Mg concentrations but negative relation with Ce, Cu, Mo, Pb, S, and Zn (Fig. 5).Except for Ca, other metal elements in soils were signi cantly correlated with each other (Fig. 5).

Discussion
Soil physicochemical characteristics are often used for assessing the soil quality.According to the results soil pH mean values were signi cantly different among the studied sites.The lowest pH was found alongside Pakhir River (6.40) and Oxide Dump (6.55) and the highest in Waste Rock Dump (7.01).This could have been caused by sul de oxidation that leads to hydrogen ion release or dissolution of minerals that promote acidic pH (Kavehei et  According to the present ndings, the main concern about waste rocks in the long term is acid mine drainage that leads to mobility of metals/metalloids and release of sulfate. Our results showed that mine disturbed sites had lower ranges of N, K, Ca, and Mg concentrations.This is an indication of soil fertility degradation because of litter layers and organic material loss (Narhi et al., 2012, Wang et al., 2021).We found that concentrations of these nutrients were greater in the soil of natural hillsides and there was a considerable correlation among N, K content with soil OM content, demonstrate their organic origin.Phosphorous concentrations were greater in alongside Sungun River and Oxide Dump, as copper phosphates can be considered a possible reason (Abreu et al., 2008).Phosphate compounds are known to be effective in decreasing mobility of potential toxic minerals like as Pb, Cd, and Zn (Andrunik et al., 2020).Positive correlations were observed between these elements and soil phosphorus concentration in both correlation analysis and NMDS.
The highest tolerable levels of soil contamination for plants are Co (10 µg g -1 ), Cr (100 µg g -1 ), Cu (100 µg g -1 ), Fe (35000µg g -1 ), Mn (550 µg g -1 ), Ni (100 µg g -1 ), Zn (300 µg g -1 ), As (7 µg g -1 ), Cd (5µg g -1 ), Mo (10 µg g -1 ), Pb (100 µg g -1 ), Ce (10 µg g -1 ) ( The three natural hillsides had lower ranges of chalcophile element concentrations than those in mine-affected sites.The data for three natural hillsides showed that the one close to Waste Dump had greater concentrations of N and OM.This might be associated with the presence of nitrogen-xing plant species from the Fabaceae family and higher vegetation cover that add litter material to surface soil (Ghorbani et al., 2018;Buta et al., 2019;Chen et al., 2020).Also, lower metal element concentrations were found in a natural hillside far from mine, probably due to higher clay contents, as clay fraction and organic matter in soil is a major factor for metal sorption (Rieuwerts, 2007; of heavy metals in soil and reference values to evaluate pollution suggested by Kabata-Pendias (2011) .Also ( ) is the index of assessing a given contaminant potential ecological risk that is measured by Eq. response and contamination factors respectively.Hakanson (1980) reported the The values of heavy metals as: Cd = 30, As = 10, Cu = Pb = Ni = 5, Cr = 2, Zn = 1.

Table 1
2Where is pollution coe cient for a certain heavy metal that is determinant of contamination in study area but is not e cient to show the hazards and ecological effects of metal elements.and are measured

Table 3
Means of soil physico-chemical characteristics in Sungun Copper Mine, Iran.Range of data are shown in parentheses.Means followed by different letter in each row are signi cantly different.Sites are shown in Fig. 2.

Table 4
Means of soil metal and metalloids elements in Sungun Copper Mine, Iran.Range of data are shown in parentheses.Means followed by different letter in each row are signi cantly different.Sites are shown in Fig. 2.

Table 5 I
geo values for soil metal and metalloids elements in Sungun Copper Mine, Iran.Sites are shown in Fig.2.
for Cu, As, Cd, Pb (Table

Table 6
values for soil metal and metalloids elements in Sungun Copper Mine, Iran.Sites are shown in Fig.2.

Table 7
General level of Potential Ecological Risk Level in different sites of Sungun Copper Mine, Iran.Sites are shown in Fig. 2.
NMDS ordination, the arrows of pH and CaCO 3 indicate that they are the main factors contributing to the separation of Waste Rock Dump from other sites, while Pakhir riverside, Sungun riverside, and Oxide Dump are mainly affected by S concentration.Lower content of calcium carbonate (CaCO 3 ), which can neutralise the acidity produced by the dissolution of sulphides (Likus-Cieslik et al., 2017; Candeias et al., 2017), was measured in the sites affected by mine drainage, namely Pakhir riverside, Sungun riverside, Oxide Dump and Waste Rock Dump.Reduction in soil acidity in result of higher CaCO 3 contents decrease toxic mineral elements solubility and stimulate metal immobilization in soil (Khan and Jones, 2008).RI calculated value for heavy metal toxicity is higher in sites with higher values of metal(loides) like alongside Pakhir River and Sungun River indicating main sources of environmental problems around Sungun Copper Mine (Weeks and Comber, 2005).The high values of RI and I geo indicated that this mining area have great potential to affect the ecological function of surrounding area such as Arasbaran protected area by mobility and transportation of metal(loides) through Pakhir and Sungun Rivers which can also lead changes in hydrogeochemical processes and their contribution on degradation of water quality (Ogunkunle and Fatoba, 2013; Aghili et al., 2018; Punia and Singh, 2021).Mining drainage is main cause of metal(liodes) elements increase in surface soil and surrounding water sources that leads to ecological risks (Lu et al., 2021).Moderate levels of ecological risks, mainly related to Cd and Cu is also reported from Cyprus' old Agrokipia copper mine (Hadjipanagiotou et al., 2020).CaCO3 content was negatively correlated with trace metals like Pb, Cd, As, Zn, and Cu content that shows carbonates in uence on decomposition and stabilization of toxic minerals in soils with elevated content of CaCO 3 (Gasparatos et al., 2015; Bashir et al., 2019; Talaab et al., 2019).
al., 2021; Kulikova et al., 2019; Aghili et al., 2018; Sanderson et al., 2012).In the Waste Rock Dump, the neutral range of pH indicates abundance of carbonates over sul des along with the absence of AMD (Perlatti et al., 2015).As shown in (Kabala et al., 2020)2)a-Pendias, 2011).Previously we found the native vegetation cover potential for phytostabilization in these sites(Alizadeh et al., 2022).According to these values our results prove that soil of contamination with highest degrees has occurred in mine drainage affected sites, including two riversides and the Oxide Dump which is different from results reported from Europe's largest copper mine in Poland(Kabala et al., 2020)but in accordance with data reported from Sarcheshmeh and Miduk copper mines from arid areas of Iran (Khorasanipour and Aftabi, 2011; Moore et al., 2014), and copper mine in Malaysia (Ali et al., 2004).It is more by soils, due to competition for negatively charged surfaces of clay minerals between H + and dissolved metals (De Matos et al., 2001; Pandey et al., 2007; Gonzalez-Fernandez et al., 2011; Sungur et al., 2014).Most metals and metalloids (Co, Cd, Zn, Fe, Cu, Pb, As, and Mo) in the NMDS plot are associated with higher sand content, and are correlated with soil sand content, indicating that metals do not tend to retained by large soil particles (Romzaykina et al., 2021; Cevik et al., 2009).Fe and Cu contents of soil have positive association because Fe-oxides enhance xation of metals, especially Cu (Arenas-Lago et al., 2014).The considerable positive correlations of metals including Cd, Cu, Fe, Mo, Pb, and Zn with S concentration, and also along with each other in the NMDS plot shows that these elements are mainly sul de-associated (Kim et al., 2003; Alakangas et al., 2010) Metal element correlation with each other also implies they had the same sources and similar weathering pathways, also suggesting similar origin, geochemical behaviours for the transport, deposition, and interdependence of these elements (Lienard et al., 2014; Kalil et al., 2013; Demovka et al., 2019; Antunovic et al., 2023).Also, mineral elements like Cu, Pb, and Zn regularly co-exist because of same process of geochemistry digenesis (Han et al., 2015; Pu et al., 2010).
(Edraki et al., 2014))drainage occurrence in conditions that sul dic, especially Fe-sul des (FeS2) or pyritic substances such as oxidised wastes and waste rocks are accumulated(Kavehei et al., 2021).Mine wastes because of physicochemical characteristics and geochemical transformations lead to release of H + and metal(loides) with enormous environmental problems on mine water and vegetation covers that should be in priority of stabilization(Edraki et al., 2014).The negative correlation of soil clay aggregates with Co, Fe, Mn, and Zn total amounts, and also the lower ranges of pH in contaminated sites, shows that the decrease in soil pH and lower Ca concentrations can reduce metal adsorption