Hydrogeochemical changes during artificial groundwater well recharge.

Artificial groundwater recharge is a relatively economic and efficient method for solving shortages and uneven spatial-temporal distribution of water resources. Changes in groundwater quality during the recharge process are a key issue that must be addressed. Identifying the hydrogeochemical reactions that occur during recharge can be vital in predicting trends in groundwater quality. However, there are few studies on the evolution of groundwater quality during artificial recharge that comprehensively consider environmental, chemical, organic matter, and microbiological indicators. Based on an artificial groundwater recharge experiment in Xiong'an New Area, this study investigated the hydrogeochemical changes during groundwater recharge through a well. The results indicate that (1) as large amounts of recharge water (RW) were injected, the groundwater level initially rose rapidly, then fluctuated slowly, and finally rose again. (2) Water quality indicators, dissolved organic matter (DOM), and microbial communities were influenced by the mixture of RW and the background groundwater before recharge (BGBR), as well as by water-rock interactions, such as mineral dissolution-precipitation and redox reactions. (3) During well recharge, aerobic respiration, nitrification, denitrification, high-valence manganese (Mn) and iron (Fe) minerals reduction dissolution, and Mn2+ and Fe2+ oxidation-precipitation occurred sequentially. (4) DOM analysis showed that protein-like substances in the BGBR were the main carbon sources for aerobic respiration and denitrification, while humic-like substances carried by the RW significantly enhanced Mn and Fe minerals reduction dissolution. Therefore, RW quality significantly affects groundwater quality after artificial groundwater well recharge. Controlling indicators, such as dissolved oxygen (DO) and DOM, in the RW can effectively reduce harm to groundwater quality after recharge. This study is of theoretical and practical significance for in-depth analysis of the evolution of groundwater quality during artificial well recharge, prediction of trends in groundwater quality during and after recharge and ensuring groundwater quality safety.

Prediction and evaluation of groundwater level changes in an over-exploited area of the Baiyangdian Lake Basin, China under the combined influence of climate change and ecological water recharge.

Groundwater (GW) and surface water (SW) are important components of water resources and play key roles in social and economic development and regional ecological security. There are currently several stresses placing immense pressure on the GW resources of the Baiyangdian Lake Basin (BLB) in China, including climate change. A series of ecological and environmental challenges have manifested in the plain area of the BLB due to long-term over-exploitation of GW, including regional declines in GW level, aquifer drainage, land subsidence, and soil secondary salinization. Climate change may aggravate environmental challenges by altering GW recharge rates and availability of GW. This study applied the fully processed and physically-based numerical models, MODFLOW and the Soil & Water Assessment Tool (SWAT) in a semi-coupled modeling framework. The aim of the study was to quantitatively analyze changes to shallow GW levels and reserves in the plain area of BLB over the next 15 years (2021-2035) under climate change and different artificial recharge schemes. The results indicated that GW storage and levels are rising under the different GW recharge schemes. The maximum variation in the GW level was 20-30 m under a rainfall assurance rate of 50% and water level in the depression cone increased 14.20-14.98 m. This study can act as a theoretical basis for the development of a more sustainable GW management scheme in the plain area of the BLB and for the management and protection of aquifers in other areas with serious GW overdraft.

The Blocking Effect of Clay in Groundwater Systems: A Case Study in an Inland Plain Area.

In order to increase understanding of the hydrogeochemical effects that influence changes in the quality of salt water, we investigated the distribution of saline and fresh water in an inland plain area and, in particular, the scarcity of fresh water resources. Taking the inland plain in Jiyang County as a specific case study, samples of undisturbed clay and underground saline water from different depths were collected to examine hydrogeological changes. A wide variety of methods was used to analyze the blocking effect of clay on the chemical characteristics of the groundwater. These include real-time monitoring for field water quality, tests for isothermal adsorption, a factor analysis model, physiochemical analysis, and correlation analysis. Our results show that the optimal adsorption isotherm of clay for Na⁺, Ca2+ and Mg2+ in groundwater conform to the established Henry and Langmuir equations for adsorption isotherms. The influence of clay mineral types and content on the blockage of Na⁺, Ca2+ and Mg2+ in groundwater samples were evident at different depths, with the clay adsorption capacity increasing in line with increases in the clay mineral content. Clay at different depths was found to have the strongest blocking effect on Na⁺ in groundwater, being systematically greater than its effect on Ca2+ and Mg2+. It is believed that the blocking effect of clay has an important influence on the hydrochemical zoning of groundwater in inland plains and the formation of saline water in groundwater systems. This study therefore provides concrete evidence in support of this supposed effect.