A CLUSTER-BASED METHOD FOR UNDERSTANDING SPATIAL AND TEMPORAL PATTERNS AND CONTROLLING FACTORS OF GROUNDWATER GEOCHEMISTRY IN A REGIONAL AQUIFER

This study presents a new cluster-based method for investigating the spatial and temporal patterns and controlling factors of groundwater geochemistry in the confined aquifer of the Jianghan Plain, China. The cluster-based approach is conceptually straightforward and easy to implement. Different from other clustering approaches that either use temporal means or conduct clustering for data of different sampling times separately, a unique feature of our approach is that it identifies clusters for the entire dataset obtained in space and time. This is particularly useful for understanding groundwater geochemistry that evolves in space and time simultaneously. Based on 13,024 groundwater geochemistry data collected over 23 years from 29 monitoring wells distributed over the Jianghan Plain, seven statically distinguished clusters are delineated, and the delineation is confirmed to be reasonable by using principal component analysis and geochemical analysis. Based on the spatial distribution of the clusters, the Jianghan Plain is separated into four geochemical zones along the regional groundwater flow path, namely, the recharge zone, the transition zone, the flow-through zone, and discharge-mixing zone. This is a major contribution of this study. The groundwater geochemistry is controlled by water-rock interactions and anthropogenic activities. Temporal changes of groundwater geochemistry are significantly affected by the operation of the Three Gorges Dam, which increases river discharge to groundwater in the discharge-mixing zone after 2009. This impacts should be further investigated in future studies. Our study provides a systematic and easy-to-implement method for understanding groundwater geochemistry at a regional scale and over a long monitoring period.