Sources and migration path of chemical compositions in a karst groundwater system during rainfall events

YANG PingHeng 0 1 YUAN DaoXian 0 1 YE XuChun 1 XIE ShiYou 1 CHEN XueBin 1 LIU ZiQi 1 0 The Karst Dynamics Laboratory , Ministry of Land and Resources, Institute of Karst Geology, Chinese Academy of Geological Sciences , Guilin 541004, China 1 School of Geographical Sciences, Key Laboratory of Eco-environments in Three Gorges Reservoir , Ministry of Education, Southwest University , Chongqing 400715, China Physical and chemical dynamics at Jiangjia Spring (JJS), the outlet of the Qingmuguan karst groundwater system in Chongqing, were monitored in situ during rainfall events to acquire a series of high-resolution data. Principal component analysis (PCA) was employed to identify the sources of chemical compositions in the karst groundwater. The coefficients of variations (CVs) of the physical and chemical data of JJS were utilized to interpret the migration path of the chemical compositions. The results showed that water-rock interactions, agricultural activities, and soil erosion were the main sources of the groundwater chemical compositions. Ions of potassium, sodium, nitrate, chloride and phosphate from agricultural activities together with ions of calcium, magnesium, strontium and bicarbonate derived from carbonate dissolution appear to be stored and regulated by the karst unsaturated zone in features such as fissures, pores and solution cracks. The concentrations of the ions remained relatively stable and they showed low CVs owing to their migration by diffuse flow to recharge the underground river. In contrast, concentrations of ions such as total iron, total manganese and aluminum from soil erosion were unstable and showed high CVs owing to their migration by overland flow to recharge the underground river directly via sinkholes. During heavy rainfall events, the nutrients from agricultural activities and sediment from soil erosion could quickly impair the aquatic ecosystem and pose serious threats to water quality. Therefore, it is necessary to reinforce management of the ecological system for better control of the influx of mass nutrients into the karst aquifer system. - The karst areas of SW China are typically vulnerable ecological regions that represent the largest consecutive distribution and most intense karstification in the world. Despite the areas being favored with good water and heat conditions in the subtropical zone, they have extremely heterogeneous water distributions due to long-term karstification. This has led to the formation of a karst soil and water resource environment with several issues including structural water shortage, infertile soil, and ecological vulnerability [1]. In recent years, the karst water resources in the area have begun to dry up and the pollution sources have become diversified under the obvious and increasing influences of anthropogenic activities and extreme climates [2]. This has become a hindrance to the sustainable economic and social development of the area. Hydrochemical compositions are the basis for investigations of groundwater and surface runoff sources and are crucial to groundwater resource evaluation. Accordingly, they play a significant role in the utilization and management of groundwater resources in a watershed as well as the protection and construction within the related ecological environments. Geochemical analysis is a commonly used The Author(s) 2013. This article is published with open access at Springerlink.com method for investigations of groundwater origin, recharge source, hydraulic connections, and water-rock interactions [312]. Karst geochemistry is a branch of geochemistry that studies the principles and application of chemical action in karst dynamic systems [10]. Previous studies using conceptual models of karst dynamics [13] and the historic perspective of research on karst groundwater geochemistry [1417] have primarily focused on the physical and chemical compositions (e.g. Ca2+, conductivity, hardness, etc.) associated with the water-rock interactions of karst groundwater. Because of limited research methods, the research parameters are less diversified and cannot comprehensively reflect the influences from substances of the surface environment on karst groundwater. Most studies to investigate the influences of anthropogenic activities on karst groundwater geochemistry have compared different chemical parameters of the karst water in different spaces within the same period, or at the same research site in different periods (e.g. a year, a season, etc.). However, such data in the studies are usually contingent, the prerequisite of comparisons are insufficient, and the descriptions are superficial and probably cannot comprehensively reflect the sensitivity of the karst system. In addition, comparison of hourly data (high-resolution monitoring) and monthly data results in a significant discrepancy invalidating the latter as an indicator of rapid changes in karst groundwater systems [18,19]. Therefore, investigations of karst groundwater systems with high sensitivity and vulnerability to influencing factors from the surface environment must be based on not only the chemical compositions in the karst groundwater system, but also the collection of high-resolution data describing those hydrochemical parameters. However, high-resolution monitoring could generate a tremendous volume of data and, in most cases, redundant information; therefore, a proper method for analysis is necessary. Principal component analysis (PCA) can combine a large number of correlated variables into a few uncorrelated parameters, transfer the former numerous variables into useful information and facilitate further analysis. So PCA is widely applied in the research of geosciences [2025]. The coefficient of variation (CV) describes the variation amplitude of a variable, with a larger variation in amplitude being associated with a larger CV, and vice versa. CVs of the chemical parameters of a karst spring reveal important information about groundwater recharge sources and migration paths [14,17,26]. During rainfall events, the geochemical behavior of karst groundwater responds quickly to the rainfall, and the groundwater system sufficiently exchanges substances and energy with the external environment [27]. In this study, a process for high-resolution data collected at Jiangjia Spring (JJS), the outlet of the Qingmuguan groundwater system in Chongqing, during rainfall events in late May 2009 is introduced. High-resolution hydrochemical data are presented and PCA is employed to identify the sources of chemical compositions. Based on the results of PCA and the characteristics of the CVs of physical and chemical parameters, the migration path of chemical compositions in the karst aquifer is proposed. These results of this investigation also help explain the interaction between karst groundwater and the surface environment, effectively forecast karst groundwater pollutants, and provide a scientific basis for rational e (...truncated)