Hydrochemical Characteristics and Water Quality Assessment of a Zn-Pb Mine Waters in Yunnan Southwest China

Water samples (rivers, streams, mine adits, tailings ponds) from Jinding zinc-lead polymetallic mining area and Bi River Basin were analysed for 32 indicators including major hydrochemical components and heavy metals. The correlation analysis of water chemical parameters and Gibbs diagram show that the water composition of the Bi River is influenced by human activities in addition to the rock weathering. In the streams, the main cations and anions are derived from the rock weathering of formations rich in gypsum, calcite and dolomite. Nemerow comprehensive pollution index method was performed to evaluate water quality. The water of streams and one pond is clean. Mine adits water pollution level is medium-heavy pollution. The water samples flowing through the mining area and from tailings ponds are heavy polluted.


Introduction
Mining activities, such as mining, mineral processing and smelting, always have an impact on the environment [1] [2]. Contaminative mine drainage waters have become one of the major hydrogeological and geochemical problems in the world [3].
Jinding mine is a world-class zinc-lead polymetallic deposit in Lanping County, Yunnan Province, Southwest China. However, mining activities have a serious impact on the water environment, especially Bi River, which flows through the mining area [4][5] [6]. The objective of our study is to reveal the influence of the development of Jinding Zn-Pb mine on surface water and groundwater.

Brief introduction of study area
Jinding Zn-Pb mine is located in E99°25′02″, N26°24′50″, and concentrated in 6.8km 2 area. There are one open-pit, two underground stopes, six mineral processing plants and two smelters distributing in the valley and both sides of Bi River which flows through west part of Jinding mine. Bi River is the mother river of Lanping County and Yunlong County and the first level tributary of Mekong River. The overall length of Bi River is 169.5km, while mean annual runoff is 11.4×10 8 m 3 /s. Due to historical reasons, the mining activities have resulted in serious pollution of Bi River. In addition, there are many streams and creeks in the mining area, and all of them are branches of Bi River.

Sampling and chemical analyses
In this study, 33 water samples were collected from the mining area and Bi River Basin in January 2016, composed of 2 samples from adits, 3 samples from tailings ponds, 18 samples from Bi River, 9 samples from branches of Bi River and 1 sample from a pond (figure 1).
The sampling process followed Water quality-Guidance on sampling techniques (HJ 494-2009) strictly. All samples were analyzed at the Ministry of Land and Mineral Resources Supervision and Test Center Kunming, ISO 9001 and ISO 17025 Accredited Lab in China. 32 indexes including major hydrochemical components and heavy metals were tested, namely total dissolved solids (TDS), total hardness (TH), pH, appearance, smell and taste, turbidness, chroma, Linear Alkylbenzene Sulfonates (LAS), volatile phenol, CN -, HCO 3 -, CO 3

Data processing
The minimum, maximum and average values of all test indexes of surface water and groundwater in dry season were calculated. Piper diagram was drawn by AquaChem 3.70, and Gibbs diagram was drawn by Origin 8.5. Pearson correlation coefficient between main hydrochemical parameters was performed by SPSS 19 (IBM Corp., New York, USA).

Water quality assessment
Single factor index and Nemerow multi-factor index method was used to evaluate waters quality. The formula of Nemerow multi-factor index method was calculated as follows: where I i is single factor pollution index and NI is Nemerow comprehensive index; ρ i and S i represent measured mass concentration of water samples and reference standard values respectively. Reference standard values of surface water and underground water were ascertained due to limits of Ⅳ type in Environmental quality standards for surface water (GB3838-2002) and Quality standard for ground water (GB/T 14848-93) (  pH of water samples from branches of Bi River, the tailings pond and the pond ranges from 7.3 to 8.12, both of which are weakly alkaline. pH of Bi River ranges from 7.6 to 8.29 except that when it flows through the mining area (S26) with the value of 6.72. pH of water samples from adits ranges from 7.65 to 7.72. Mean concentration of total dissolved solids (TDS) in streams (branches of Bi River) and the pond is 274.7mg/L and 247.2mg/L, respectively, which can be considered as background value. TDS in waters from tailings pond and adits is on the high side obviously, and mean concentration is 3139mg/L and 2396.5mg/L, respectively. TDS ranges from 253 to 4268 mg/L in Bi River, and the average is 618.8mg/L. The value becomes very high rapidly when Bi River flows through Jinding mining area, showing that TDS was influenced by mining activities distinctly. The change of total hardness (TH) is similar as TDS (figure 2).
The correlation matrix of hydrochemical parameters in branches of Bi River shows that, SO 4 2has significant positive correlation with Ca 2+ and Mg 2+ , and correlation coefficients are 0.927 and 0.905, respectively (table 3). HCO 3has significant positive correlation with Ca 2+ , and correlation coefficient is 0.804. Ca 2+ has significant positive correlation with Mg 2+ , and correlation coefficient is 0.873. In combination with the geological background of Lanping, the main anion in streams maybe derived from the dissolution of gypsum, calcite and dolomite in the strata.

(4) Causes of formation of hydrochemical characteristics
Gibbs boomerang diagram, a semilog plot identified by Gibbs in 1970, has been one of the important tools to investigate geochemical processes [9]. Figure 4 shows that most of 33 water samples lie in the region of Na + /(Na + +Ca 2+ )or Cl -/(Cl -+ HCO 3 -) value less than 0.5, and TDS values are moderate. This indicates that the ionic composition is mainly derived from the rock weathering process. The value of Cl -/ (Cl -+ HCO 3 -)in tailings ponds water is inclined to be evaporation-crystallization type, which is consistent with evaporation more than precipitation in dry season in Lanping.

Concentrations of heavy metals and Single index assessment of water quality
Concentrations of Linear Alkylbenzene Sulfonates, volatile phenol, CN -, Se, Hg, Ag, Cr 6+ in any water samples are not detected or trace. Concentrations of Fe, Mn and some heavy metals are listed in table 4.
According to Function Division of Water Environment for Surface Water in Yunnan Province (2010-2020), the function of Bi River (headwater to entrance to Mekong River) is industrial and agricultural water, and Ⅳ type is operative standard, which can be used as drinking water by proper treatment besides industrial and agricultural purpose.   The statistical results show that any test index of streams and the pond is not out of limits. Concentrations of SO 4 2-, NO 3 -, Zn, Cd and Mn in tailings ponds water are out of limits with over-limit ratio of 100%, and maximum excess multiples are 8.5, 1.3, 4.8, 21 and 109, respectively. Concentrations of SO 4 2-, Zn, Cd and Mn in Bi River are out of limits with over-limit ratios of 11.1%, 11.1%, 44.4% and 61.1%, and maximum excess multiples are 10.5, 36.7, 35 and 416, respectively.
Compared with surface waters, concentrations of Fe, Mn and heavy metals are not out of limits. However, SO 4 2-, TH and TDS are out of limits with over-limit ratios of 100%, 100% and 50%, and maximum excess multiples are 4.5, 1.6 and 0.4, respectively.

Nemerow comprehensive index assessment
11 indexes were selected to perform comprehensive assessment, namely SO 4 2-, F -, NO 3 -, Cl -, Pb, Zn, As, Cd, Cu, Fe and Mn. The results of assessment were shown in table 5. It is shown that all samples from streams and the pond are clean. More than half of samples from Bi River are also clean, mainly distributing upstream and downstream far from Jinding mine (figure 5). 3 samples from Bi River are on the warning line and mild pollution, which distribute in the midstream. There are 2 samples from Bi River flowing through the mining area are polluted seriously, especially Nemerow pollution index (NI) of S26 reaches up to 30.24. Nemerow pollution index of tailings ponds water is 13.84～15.82, indicating heavy pollution. Nemerow pollution index of adits water is 2.64 and 3.92, and pollution degree is moderate pollution and heavy pollution, respectively.   All samples from streams and the pond Bi River：S18, S16, S17, S08, S14, S25, S06, S21, S20, S19 Ⅱ 0. 7＜NI≤1. 0 warning line still clean Bi River：S29, S15, S30 Ⅲ

Conclusions
 According to the Piper trilinear chart, the content of alkaline earth metals in all water bodies is more than that of the alkali metal, and the cation is the absolute advantage of Ca 2+ . The strong acid group anion is in accordance with weak acid ion, and the anion is dominated by SO 4 2and HCO 3 -. The change trend of the water chemical type in the Bi river basin reflects that the mining activity is the main reason for the large release of SO 4 2-. The correlation analysis of water chemical parameters and Gibbs diagram show that the water composition of the Bi River is influenced by human activities in addition to the rock weathering. In the stream, the main cation and anion are derived from the rock weathering of formations rich in gypsum, calcite and dolomite.
 Concentrations of Linear Alkylbenzene Sulfonates, volatile phenol, CN -, Se, Hg, Ag, Cr 6+ in any water samples are not detected or trace. Compared with water environment quality standard of the grade Ⅳ, none of test index of water samples from streams and one pond exceeded the standard. The content of SO 4 2-, NO 3 -, Zn, Cd and Mn in the water of the tailings pond exceeded the standard, while the content of SO 4 2-, Zn, Cd and Mn in the water of the Bi River exceeded the standard. The mining activities have a direct impact on the pollution of the Bi River, which has a sudden pollution from the mining area, and the downstream pollution is rapidly decreasing. None of the heavy metals and Fe, Mn in the mine water exceeded the standard, but the total hardness, the total dissolved solids and SO 4 2exceeded the standard.
 Nemerow comprehensive pollution index method was performed to evaluate water quality. The water of streams and one pond is clean. More than half of the water samples from the Bi River are also clean, which mainly distributed in the upper reaches of the mining area and downstream from the mine. The water with warning line and light pollution distributed in the middle reaches. The water samples flowing through the mining area are heavy polluted. All the tailings pond water samples pollution level is heavy polluted. Mine adits water pollution level is mediumheavy pollution.