Anthropogenic Pb contribution in soils of Southeast China estimated by Pb isotopic ratios

Isotopic ratios were used to identify the source of Lead (Pb) contamination in rural soils from Southeast China. Enrichment of Pb in surface soils was detected from three sampling locations, with the 206Pb/207Pb ratio indicating recent anthropogenic input. The 206Pb/207Pb ratio from deeper soil profiles reflected the ratio from parent basalt. Mass fractions of anthropogenic-derived Pb for soil samples in the upper profiles was as high as 50%, implying that surface soils in the current study were impacted by anthropogenic activity. The 206Pb/207Pb and 208Pb/206Pb ratios were similar to anthropogenic sources including the combustion of coal, which has been common practice in the region for 2500 years. Considering the relatively short history of petroleum use in this area and the rural location of soils, anthropogenic Pb source from coal burning was considered to be the main cause of lead pollution.


Lead elemental and isotopic geochemistry. Lead concentration of soils and basalt is shown in
.
Pb concentrations of soil samples were higher than the parent bedrock (2.2 mg kg −1 ). Pb concentrations were up to 17.3 mg kg −1 , 15.6 mg kg −1 and 15.5 mg kg −1 of the A horizons for ZSJ, ZCR and ZAJ, respectively. Pb concentrations decreased with increasing soil depth. The results clearly demonstrate an enrichment of surface soil Pb concentrations.
For the deep soils, the 206 Pb/ 207 Pb ratios (Table 2) of the ZSJ, ZCR and ZAJ profiles (> 60 cm) are closer to basalt, implying an influence from the parent material with little anthropogenic Pb at depth. However, for the top soils, the Pb isotopic compositions were distinct from the parent material. The 208 Pb /206 Pb ratios of surface soil samples were higher than the parent material (2.079; Table 2). But the 206 Pb/ 207 Pb ratios of surface soil samples were much lower than the basalt (1.196) and increase with depth. The significantly low radiogenic 206 Pb/ 207 Pb ratio (1.175; n = 12) of the soils in the top 0-10 cm is close to anthropogenic Pb from fly ash in China 42 . Therefore evidence is provided here that the surface soils have been substantially influenced by anthropogenic Pb inputs.

Discussion
Characterizing anthropogenic Pb in soils. The ratio of 206 Pb/ 207 Pb was plotted against depth in comparison with Pb content (Fig. 1) illustrating that where higher Pb concentrations were detected (i.e. surface soils), there was a correspondingly lower 206 Pb/ 207 Pb ratio. The 206 Pb/ 207 Pb ratios decreased approximately with the increase of Pb concentration in soils (Fig. 1), suggesting an anthropogenic contribution to soil Pb concentrations. In order to help locate the source of Pb (i.e. naturally occurring from parent material, or anthropogenic), 206 Pb/ 207 Pb versus 208 Pb /206 Pb of soils, basalt and anthropogenic Pb sources were plotted (Fig. 2). The influence factors of human activities on Pb pollution mainly included smelting, automobile exhaust, coal combustion and www.nature.com/scientificreports/ so on. Firstly, the early Pb pollution was caused by emissions from the crude smelting technologies in copper production in Europe and China 43 . With the improvement of smelting technology and strict control of industrial pollution discharge, the contribution of smelting Pb to it is relatively small. Meanwhile, our research areas were remote from industrial areas, so smelting is not the main anthropogenic source of lead. Secondly, the anthropogenic Pb derived from the combustion of leaded petrol, often occurred in urban environments 44 , rather than in the rural areas. Our sample sites were far away from urban areas, so the effect of gasoline lead on it is relatively small. In addition, considering the shorter time usage of petroleum in China and the lower 206 Pb/ 207 Pb ratios for petroleum combustion (~ 1.11), its contribution to the change in soil Pb isotope ratios from ZSJ, ZCR and ZAJ could be considered as negligible 20  Because the relatively short history of petroleum use in this area and the rural location of ZSJ, ZCR and ZAJ, with little vehicular access, local anthropogenic Pb source from gasoline are likely to have only a very minor influence on soil contamination. However, coal usage had long history in China. Ancient mining and utilization of coal were begun at Spring and Autumn and Warring States (470 B.C.), especially in the Sui and Tang Dynasties, the scale of coal mining and utilization was further expanded 52 . Large coal mines distribution include Hancheng (Shaanxi Province), Taiyuan and Changzhi (Shanxi Province), Yangzhou (Jiangsu Province) and Huainan and Huaibei (Anhui Province). In addition, as the largest coal mine in Zhejiang Province, Changxing coal mine is the nearest to the research area. In the northern winter season, cold air from high latitudes is controlled by the continental high-pressure system, and propagates southward to form the strongest northerly dry and cold winter monsoon in the world. The northern winter monsoon can controls the atmospheric circulation 50 and carry the Pb pollutants from above coal mines to the study area during the dry season from November to April 53 . Meanwhile, Pb isotope ratios of the soils in this area were similar to that of anthropogenic Pb from coal combustion www.nature.com/scientificreports/ in China, particularly that of Jiangsu-Zhejiang region 41 , which is neighboring with Xinchang-Shengzhou Basin. Thus, we conclude that coal combustion is the main factor for the enhanced Pb contamination in surface soils.
In conclusion, three soil profiles from rural Southeast China been shown to have elevated surface Pb contamination. Using isotopic methodologies, this elevated Pb was shown to result mainly from anthropogenic activity. The 206 Pb/ 207 Pb values of deep horizons were close to the parent material suggesting contamination was restricted to the surface soil and did not leach through the profile. Our study suggested that the combustion of coal was the main source of soil contamination, and to avoid future contamination, lower particulate emissions will be required to avoid continued accumulation of Pb in surface soils in the region.

Study region and soil sampling. The study area is located in Xinchang-Shengzhou Basin, Southeast
China, between 120° 2′ E-121° 0′ E and 29° 1′ N-29° 5′ N (Fig. 4). It belongs to the southern fringe of the northern subtropics 54 and has a mean annual air temperature of 16.6 °C, with yearly extremes ranging from − 5.3 to 40.3 °C. The region has a mean annual precipitation of 1500 mm with nearly 70% falling during the wet season (April-September). Basalt is the dominant bedrock in the region 55 with the resulting soil most commonly derived from in situ weathering of basalt. The soil is classified as either Udic Ferrosols 56 , or Ultisol according to USDA Soil Taxonomy 57 . The soils support plants that are dominated by Machilus thunbergii and Camellia sp. Three basaltic weathering profiles i.e. native forest soils (ZCR and ZAJ) and farmland soil (ZSJ), were selected in a rural area of Chongren, Anjishan and Sanjie respectively, in Zhejiang province (Fig. 4, Table 1), with locations being relatively remote from cities and obvious influences of human activity. The typical basalt platforms in the study area are distributed in triangles. We chose the north, southeast and southwest of the triangle platform as the sampling sites, in order to make the sampling points have better typical representative. The parent rock from all profiles was fresh tholeiitic basalt, which was collected beneath the sampling profiles. Soils were excavated to bedrock and sampled from small concavities in an otherwise convex portion of the landscape by genetic horizon.
Laboratory analytical methods. Collected soil samples were air-dried, ground and passed through a 2 mm sieve. The soil pH was determined in a suspension of 1:2.5 soil:water solution (w/v). Soil bulk density was measured from the 100 cm −3 undisturbed soil cores by drying the cores for 24 h at 105 °C. A homogenized subsample of soil was digested with an acid solution (5 ml concentrated HNO 3 (65%, v/v), 5 ml concentrated HCl (30%, v/v) and 5 ml concentrated HF (40%, v/v)). Diluted and filtered samples were assayed using an inductively coupled plasma mass spectrometer (ICP-MS) at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry in the Chinese Academy of Science 58 . The standard reference materials were GSR-3, BCR-1, GXR-5 and GXR-6. Analytical uncertainties were less than ± 5%.
For the determination of Pb isotopes, soil samples (0.05 g) were digested in a mixture of 4 ml concentrated HNO 3 (65%, v/v) and 1 ml concentrated HF (40%, v/v) in Teflon vessels on a hotplate at 200 °C for 8 h. The vessel was then uncovered to allow evaporation to almost dryness. This procedure was repeated until the samples were completely dissolved 59 . Pb isotopes were measured on a GV Isoprobe-T thermal ionization mass spectrometer (TIMS) at the University of Science and Technology of China. The reagent blank was also measured and blank subtraction was done for the final intensity of each isotope of Pb in the sample. The relative standard deviations (RSD) of 10 replicate readings of samples were better than 1% for 206 Pb/ 207 Pb and 0.6% for 208 Pb/ 206 Pb.  www.nature.com/scientificreports/