Materials and methods
Study area description
To test the integrated approach, a typical agricultural watershed in Eastern China was selected as the case study area.For a specific sampling site, the MLR-APCS model can be expressed as follows:
Ci  1/4  X0i th?pj 1/4 1Xji  APCSj	d3TH
where Ci is the excessive concentration of trace metal i in sediment; APCSj is the absolute principal component score of anthropogenic source j; Xji is the regression coefficient; X0i is the contribution of undefined anthropogenic sources by PCA; and the combined term Xji x APCSj represents the contribution of anthropogenic source j.
Lead isotopic mixing model
Lead isotopic composition was characterized to discriminate the different Pb sources, and their respective contributions were calculated by using mixing model.The calculation equations in this software are as follows:
f s  1/4  p1 f 1 th p2 f 2 th?th pn f n	d4TH
1  1/4  p1 th p  th	pn	d5TH
where fs is the Pb isotopic composition of sediment sample; f1, f2, and fn are the Pb isotopic compositions of different sources; p1, p2, and pn are the contributions of different sources (%).In this study, there are 2 Pb isotopic compositions (206Pb/207Pb and
208 207 Pb/ Pb) and 4 sources (background soil, chemical fertilizer, livestock manure, and atmospheric deposition).If there are n isotopic compositions and n + 1 sources, we can find certain contribution values, but if there aren + 1 sources, feasible solutions can be obtained according to isotopic mass balance (Phillips and Gregg 2003).The total concentrations of Cd, Cu, Ni, Pb, Cr, Zn, Fe, and Al in the digestion solutions were then analyzed by an inductively coupled plasma-atomic emission spectroscopy (ICPAES; IRIS Intrepid II XSP, Thermo Electron, USA).With the EF value for each trace metal, its excessive concentration in sediment can be determined as follows (Huang et al. 2018):
X excessive  1/4  X sediment-Ysediment dX=YTHbaseline  1/4  X sediment d1-1=EFTH d2TH
Multiple linear regression of absolute principal component scores
The MLR-APCS model was used to estimate respective contributions of major anthropogenic trace metal sources in the river sediments.Because the source discrimination power of Pb stable isotopes is mainly due to 206Pb, 207Pb, and 208Pb (Sangster et al. 2000), they were measured for sediment, soil, and anthropogenic source samples by an inductively coupled plasma-mass spectrometry (ICP-MS; Agilent 7500, Agilent Technologies, USA).1 Location of the study area showing land uses and sampling
sites
For sediment and soil samples, they were digested in a closed poly-tetrafluoroethylene system with an acid mixture of HNO3 (5 mL)-HF (1 mL)-HClO4 (1 mL) at 180 ?C for 10 h (Huang et al. 2015).The method detection limits, defined as 3 times the standard deviation of blank measurements, were 0.1, 0.4, 0.4, 1.4, 0.5, 1.2, 8.9, and 8.9 mg/L for Cd, Cu, Ni, Pb, Cr, Zn, Fe, and Al, respectively.Quantitative source identification methods
Enrichment factor (EF)
The EFs were calculated to determine the excessive concentrations of trace metals in the river sediments.In addition, 5 atmospheric deposition samples were collected from long-term monitoring sites, which evenly distributed in the watershed for collecting total suspended particulates (Fig.Currently, the dry farmland has occupied the largest watershed area, where wheat is the main crop being cultivated and rainfall, surface, and groundwater supply all irrigation water.APCS were obtained from the principal component analysis (PCA), which is a data dimensionality reduction analysis and can identify the major trace metal sources (Mico et al. 2006; Cai et al. 2019b).A multi-element standard solution (GSB04-1767-2004, Guobiao Testing & Certification Co., Ltd., China) was used to establish calibration curves.For quality control, each digestion batch included 2 reagent blanks, 1 standard reference material, and 1 duplicate sample to assess the accuracy and precision of the measurements.Replicate analysis of the standard reference material GBW-07304 (National Research Center for Geo analysis, China) showed good accuracy, with overall recovery rates ranging from 97 to 106%.Metal concentration and Pb isotope analyses
Fig.1). 1).1). 1).