Coupled controls of climate, lithology and land use on dissolved trace elements in a karst river system

Highlights

• Al and Mn are the dominant elements causing water quality deterioration.

• As, Sr, U and V are the largest contributors to non-carcinogenic risk.

• Carbonate and urban land distributions control on contents of some trace elements.

• Hydrological seasonality is an important regulator for trace elements.

Abstract

As an important factor affecting water quality, dissolved trace element (DTE) is not well studied in the karst region. Here, eighteen DTEs were analyzed to investigate their contents, seasonal-spatial variations, sources and controlling factors, and to evaluate water quality and health risks in a karst river system (Chishui River, southwestern China). The results showed that dissolved trace element concentrations (DTECs) in this river were at a relatively low level compared with other Chinese rivers. Except for Cr, Ni, Sr and U, other DTEs had a higher median concentration in the wet period than in the dry period. Water quality and health risk assessments indicated that the water quality in this river had no extensive heavy metal pollution, and the DTECs were below the hazard level. Correlation analysis and principal component analysis identified that Al, Co, Cu, Li, Mn, Ni, U, Mo, Sb and Sr were mainly derived from natural sources, while As, Ba, Cr, Ga, Pb, Rb, V and Zn were dominated by anthropogenic sources. Carbonate and urban land distributions control the DTECs of river waters, indicated by correlations of DTECs with coverage of carbonate and urban land within the draining area. Further analyses found that for mono-lithological sub-catchments with low urban land coverage, the water samples from carbonate catchments had a higher concentration of Co, Cr, Ni, U, Mo, Li and Sr and a lower concentration of Ga, Pb, Sb, Cu, As, Rb, V, Mn, Zn, Al and Ba than the water samples from silicate catchments. These differences would be changed with the increasing of urban land coverage in mono-lithological sub-catchments, suggesting that the impacts of urban land on DTEs are not limited to As and Rb indicated by correlation analyses. Seasonal changes in contents of DTEs and in their correlations with analyzed factors reflect the regulations of seasonal hydrologic conditions. This study highlights that the DTECs of river waters in the Chishui River basin, a region with low level of urbanization, are significantly related to urban land distribution. Future urban planning and watershed management need consider the impacts of urbanization on the DTEs and water quality in rivers in karst areas.

Introduction

The rapid process of urbanization and the intensification of agricultural activities worldwide have led to water pollution, notably heavy metal pollution in the aquatic environment (Krishna et al., 2009, Lemly, 1996, Liu et al., 2013, Pekey et al., 2004). Dissolved trace elements (DTEs) are essential for natural ecosystems and human health. For example, Al concentrations are related to the abundance of fish in rivers (Gaillardet et al., 2003). Se from industrial and agricultural sources have been found to poison fish and wildlife (Lemly, 1996). As in groundwater put at least 100 million people at risk for cancer and other As-related diseases (Bhattacharya et al., 2007). Natural sources of DTEs in river waters include bedrock weathering, volcanism and atmospheric precipitation etc., most of which are controlled by geology (Gaillardet et al., 2003, Gonneea et al., 2014, Krishna et al., 2009). Anthropogenic sources include mining, mineral processing, metal smelting and waste incineration etc (Huang et al., 2007, Li and Zhang, 2010b, Liu et al., 2013, Meng et al., 2016). Trace metals (TEs) tend to be trapped in the aquatic environment and accumulated in sediments, thus threatening the aquatic ecosystems (Pekey et al., 2004, Zeng and Han, 2020, Zeng et al., 2020). Some TEs, e.g., Cu and Zn, are considered to be biological micronutrients necessary for normal growth and function of organisms; in contrast, some TEs, e.g., As, Cd, Hg, Pb, Se and Sn, are considered harmful to human health and aquatic life (Bhattacharya et al., 2007, Lemly, 1996, Pekey et al., 2004). Humans have exploited many TEs from natural accumulation sites and used them, and they are thus an indicator that is highly sensitive to human impacts (Gaillardet et al., 2003).

Globally, karst landscapes cover roughly 20% of ice-free land (Hartmann and Moosdorf, 2012), and supply potable water for 25% of the world's population (Ford and Williams, 2013). Due to the rapid chemical weathering rate, carbonate rocks have been affecting the hydrochemical characteristics of many rivers in the world (Gaillardet et al., 1999). Additionally, for this reason, heavy metal elements are rapidly released as the carbonate weathering, and then migrate and enrich into the environment through precipitation, adsorption and complexation (Wu et al., 2020). Carbonate diagenesis can lead to a higher iron oxide content, making Cr, Ni, Cu and Zn more easily enriched in sediments and soils in carbonate watersheds by stronger metal adsorption (Qu et al., 2020). Therefore, although the contents of most of the heavy metals in carbonate bedrock are relatively low, carbonate weathering is one of the major causes of metal pollution in karst areas in southwestern China (Qu et al., 2020, Wu et al., 2020). Previous studies have found that some DTEs are mainly derived from silicates and evaporites (Gaillardet et al., 2003, Ma et al., 2020, Wang et al., 2015), and showed that the contents of some DTEs are closely related to carbonate minerals, e.g., Sr and U (Chen et al., 2020, Gonneea et al., 2014, Han and Liu, 2004, Neal et al., 2006, Palmer and Edmond, 1993), but there are still few systematic studies of DTEs for karst-dominated watersheds.

Karst terrains are highly susceptible to anthropogenic and climatic changes (Sullivan et al., 2019). Substantial research documented that, in addition to lithology, land use also significantly affects the hydrochemistry of karst rivers (Jiang et al., 2008, Xu et al., 2020, Yue et al., 2019). In addition, the major ion geochemistry of karst river waters is closely related to the hydrologic condition, which is regulated by climate change (Zhong et al., 2017). Research on the Xijiang River found that the concentrations of some DTEs (e.g., Mn, Zn and Pb) showed significant seasonal variations, which are significantly affected by the monsoon climate (Liu et al., 2017). More than half of the flux of DTEs occurs in the high flow season in the Pearl River, which is mainly controlled by the high discharge (Zeng et al., 2019). Guizhou province, the central karst region of southwestern China, was one of the provinces with the fastest Gross Domestic Product (GDP) growth rate in China in 2019 and is in the process of rapid urbanization. The impacts of urbanization on dissolved trace element concentrations (DTECs) in this karst region are not yet well known. For better water quality, it is necessary to explore the responses of DTEs in karst river waters to the impacts of both natural and anthropogenic factors (Gonneea et al., 2014, Liu et al., 2017, Park et al., 2005, Sullivan et al., 2019, Zhu et al., 2018).

So far, geochemical characteristics of DTEs and their contamination to the aquatic environment have been studied in many watersheds in China (Meng et al., 2016, Xiao et al., 2014, Xiao et al., 2019, Wang et al., 2017, Zeng et al., 2015). As the third-largest river (in terms of length (6300 km)) in the world and the largest river in China, many studies have been conducted to investigate TEs of river waters and sediments in the Yangtze River basin (Li and Zhang, 2010b, Qu et al., 2019, Wang et al., 2011, Wu et al., 2009, Zeng et al., 2015). Chishui River is the only undammed first level tributary in the Yangtze River basin and an important rare fish reserve, so it has unique ecological status in China (Chi et al., 2017, Jiang et al., 2011). Chishui River basin has abundant natural resources, especially the reserves of coal and pyrite (Ren et al., 2009). With the development of economy, the environmental protection in this basin is facing unprecedented pressure. Moreover, the Chishui River flows through the karst terrains, so that severe soil erosion has resulted in a fragile ecological environment. Chishui River basin is famous for its developed liquor industry and high aquatic biodiversity, and has nearly 10 million people living in there (Chi et al., 2017, Ren et al., 2009). For these reasons, the importance of water quality for regional drinking water safety and the economic development in this basin is self-evident.

Based on the above considerations, our study is based on the quantitative analyses of lithology and land use, to investigate the eighteen selected DTEs (Al, As, Ba, Co, Cr, Cu, Ga, Li, Mn, Mo, Ni, Pb, Rb, Sb, Sr, U, V and Zn) of river waters in the Chishui River basin, southwestern China. In this investigation, the aims are to: (1) investigate the contents and seasonal-spatial variations of DTEs and to evaluate water quality and health risks through the Water Quality Index (WQI) and Hazard Quotient/Index (HQ/HI), respectively; (2) explore the possible sources of eighteen selected DTEs using multivariate statistical analyses; (3) identify the main controlling factors of DTECs. The results can provide important information for local water quality protection, and can help to better understand the geochemical behaviour of DTEs and their response to the impacts of natural and anthropogenic factors in the context of global change.