| Abstrakt: |
As a highly researched topic in hydrochemistry, rock weathering is a key factor in controlling the hydrochemical characteristics in a watershed. In karst watersheds, the weathering of carbonate rocks has a significant impact, leading to high levels of Ca2+ and Mg2+ cations, as well as HCO3- anions in water. In small non-karst watersheds, the dominant cations are Na+ or K+, with lower levels of Ca2+, and the main anions are Cl-, NO3-, and others. Calcium is a vital element that plays a crucial role in the structure and functioning of karst ecosystems. Therefore, understanding the differences in Ca2+ concentrations and the interactions between external water and karst water is essential for accurately assessing the hydrochemical impact of bedrock on karst watersheds. The Shuibian river is a typical karst river located in the northern part of Guangdong Province, China. It spans a distance of 78 kilometers with an average gradient of 1.8‰. Its geographical coordinates are E112° 42' 13.511"--E113° 15' 3.730", and N24° 21' 28.22"--N23° 55' 34.61". The total area of its basin is approximately 857 square kilometers. The region where the river is situated falls under a subtropical monsoon climate, characterized by hot and humid summers. The prevailing warm and moist airflow comes from the south in summer. The average annual temperature ranges from 17.5 ° to 20.7 °, and the average annual precipitation is between 1,830--1,879 mm. The climate exhibits distinct wet and dry seasons. The wet season lasts from March to September, accounting for over 70% of the annual rainfall, while the dry season occurs from October to February, with less precipitation. The research area is predominantly composed of four different types of bedrock: sandstone, carbonate rock, granite, and conglomerate. To explore the impact of weathering processes of these various bedrock types on the chemical composition of surface water in karst regions, this study focuses on a representative karst watershed called the Shuibian river in northern Guangdong Province. The main objective of this study is to investigate the changes in calcium concentration ([Ca2+]) in the water body and understand the mechanisms of calcium migration within the watershed. To achieve this, a total of 28 sampling points were strategically placed along the main stream of the Shuibian river, with an additional 25 sampling points along 9 significant tributaries. These sampling points were spaced at an interval of approximately 3 kilometers covering a distance of 78 kilometers downstream from the upper reaches. Sampling had been conducted over a period of five years, from January 2016 to July 2020, encompassing both the dry season (January) and the wet season (July) each year. A total of 683 water samples were collected during these time periods. In addition to measuring water width, depth, and flow velocity at each sampling point, on-site measurements of conductivity, salinity, temperature, pH, and dissolved oxygen were also performed. By analyzing the data collected through this comprehensive five-year study, the aim is to determine the contribution of weathering processes of different rock types to the transport of calcium in the main stream water, as well as to understand the underlying mechanisms of calcium migration in the karst watershed. The results indicate as follows: (1) There are significant differences in the [Ca2+] levels of the tributaries with different types of bedrock, especially during the dry season. The [Ca2+] levels in the tributaries with carbonate rock are 3.8 times higher than those in tributaries with granite, 4.7 times higher than those in tributaries with sandstone, and 14.9 times higher than those in tributaries with conglomerate rock. (2) After the convergence of tributaries with different types of bedrock into the main stream, there is a delayed hydrochemical change that exhibits seasonal variations. Specifically, when the tributaries with carbonate rock converge, the [Ca2+] levels in the main stream water increase with a delay, while other tributaries dilute the [Ca2+] levels, resulting in a more pronounced dilution effect during the wet season compared to that during the dry season. (3) Through statistical analysis, it is found that the tributaries with carbonate rock account for 28% of the watershed land area, but they contribute 83% to the [Ca2+] levels in the watershed water. This indicates that the weathering of carbonate rock in karst watersheds plays a significant role in constraining the migration of calcium elements. (4) From 2016 to 2020, the average calcium flux transported by the Shuibian river to the Lianjiang river was 7.2x104 t•a-1. The average transport during the wet season was 5.8x104 t•a-1, while during the dry season, it was 1.4x10x104 t•a-1. However, it is important to note that the increase in soluble Ca2+ concentration caused by the dissolution of deposited CaCO3 in the water is approximately 30% of the total output. Therefore, the calcium migration process in the watershed water, which is transported in the form of CaCO3, also requires special attention. [ABSTRACT FROM AUTHOR] |
