Hydrogeochemical changes during artificial groundwater well recharge

Molecular characterization and environmental response of dissolved organic matter in reserve quiescent groundwater wells of the North China plain: Insights from spectroscopy and mass spectrometry

Dissolved organic matter (DOM) plays a critical role in aquatic ecosystems. However, the characteristics of DOM in groundwater source wells and interactions with environmental factors remain poorly understood. This study investigated the spectral properties, molecular composition, and environmental drivers across vertical groundwater gradients in Shijiazhuang using spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS), multivariate statistics and molecular network analysis. Three components were identified: two humic-like substances (C1, C3) and one protein-like component (C2) Humic-like substances exhibited significant vertical stratification, with bottom groundwater DOM showing higher humification and autochthonous characteristics. Multivariate statistical analysis indicated that NO3--N and dissolved oxygen (DO) were keystone factors influencing the vertical differences of DOM. Surface-layer DOM was driven by dissolved total phosphorus (DTP), pH, DO and NO3--N, while the bottom layer was jointly regulated by pH, total phosphorus (TP), total nitrogen (TN) and NO3--N. DOM components correlated significantly with fluorescence index (FI), humification index (HIX), chemical oxygen demand (CODMn) and dissolved total nitrogen (DTN). FT-ICRMS analysis revealed that DOM molecular composition was dominated by CHO (38.71 %-52.07 %) and CHON (22.30 %-34.44 %) compounds, with lignin-like (LIG) (60.91 %-80.56 %) serving as the core molecular formulae. Redundancy analysis (RDA) identified that TN, DO, and NH4+-N were key drivers regulating the DOM molecules distribution. Furthermore, molecular network analysis demonstrated that LIG molecular formulae played a crucial role in the network, significantly enhancing the chemical stability of the DOM molecular network. These findings elucidate DOM dynamics in groundwater systems at a molecular scale, providing critical insights for resource protection and risk management.

Effects of ecological water replenishment on microbial denitrification in aquatic environment of infiltration area

Ecological water replenishment (EWR) is a crucial strategy for solving regional water shortages, yet its ecological impacts warrant further exploration. This study investigated the microbial community succession mechanism in surface water (SW) and groundwater (GW), the denitrification potential of functional bacteria, and their responses to water replenishment events of the South-to-North Water Diversion (SNWD), the largest water transfer project in China. Nitrogen and antibiotic contamination, resulting from prolonged infiltration of reclaimed water, decreased following water replenishment event. During water replenishment, both SW and GW showed an increase of microbial denitrification capacities. Post-replenishment, SW microbial denitrification capacity continued to rise, while GW returned to the baseline levels. Total organic carbon (TOC) and antibiotic resistance genes (ARGs) were the primary factors influenced denitrification before and after water replenishment. Among the denitrification steps, NO2--N reduction was most affected, linked to microbial community reassembly and resource utilization alteration after water replenishment. Furthermore, random forest analysis identified potential bacterial indicators and combinations sensitive to water replenishments highlighting key denitrification functional bacteria. These findings offer critical insights for optimizing water resource management and improving EWR effectiveness.

Cd migration in water-level fluctuation zones of Three Gorges reservoir: Interactions of periphytic biofilms and tryptophan

Periphytic biofilms (PBs) and dissolved organic matter (DOM) are key factors affecting the migration of Cd at the "water-sediment" interface. However, the specific effects of PBs and protein-like components of DOM on Cd migration within the "water-biofilm-sediment" system remain poorly understood. This study simulates the dissolution and re-immobilization of Cd at the "water-biofilm-sediment" interface in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR), aiming to clarify the role of PBs in Cd stability. The results indicate that the concentration of dissolved Cd increased by 0.96-fold (R2 = 0.992) with each doubling of L-tryptophan (L-Trp) concentration, as the functional groups of L-Trp (COOH and NH) formed complexes with Fe-Mn oxide-bound Cd in the sediment. In the presence of PBs and at an L-Trp concentration of 150 mg·L-1, PBs utilized L-Trp, resulting in a 16.7 % reduction in dissolved organic carbon (DOC) (p < 0.05) and an increase in protein content. Additionally, PBs contributed to a 35.12 % reduction in the peak concentration of dissolved Cd, thereby stabilizing the final Cd levels. Cd enrichment within PBs, facilitated by functional groups such as >CC<, OH, >CO, and CO, led to a significant increase in Cd content (0.43 mg·kg-1, p < 0.05). These findings suggest that L-Trp degradation and Cd enrichment by PBs act synergistically to promote the re-immobilization of dissolved Cd. This study offers a novel perspective on Cd migration in WLFZs and provides insights that may be applicable to other aquatic environments where PBs are present.

Carbon Isotopic Signatures of Aquifer Organic Molecules along Anthropogenic Recharge Gradients

The property of groundwater dissolved organic matter (DOM) subjected to anthropogenic groundwater recharge (AGR) might be affected by the water quality disparity between surface water and natural groundwater. However, the diverse molecular scenarios of groundwater DOM under uneven recharging levels remain largely unexplored. We combined molecular characteristics, carbon isotopic signatures of organic molecules, and end-member mixing analysis to explore the sensitivity and potential tracking capabilities of DOM to AGR along with recharging gradients. Our findings suggested that AGR enriched groundwater with diverse, saturated, labile, and sulfur-rich molecules, amplifying DOM abundance and intensity, which intensified with recharge gradients. Additionally, S-containing molecules and their indicators like CHOS% (with threshold values of 7.82%) exhibited high sensitivity and predictive power for AGR recognition. The major signatures (diversity, saturated degree, and stability) indicated by 13C-containing molecules were similar to the whole molecular pool. Notably, specific molecules (C12H10O5S and C15H16O12), although not detected in all groundwater samples, exhibit robust stability or favorable solubility, rendering them potential candidates as AGR-sensitive molecules. The R13C/12C ratio of 13C-containing C19H24O5 emerged as the most robust tracer, exhibiting a strong correlation with the recharge ratio and the smallest deviation from the theoretical mixing line, signifying its optimal suitability for precise groundwater DOM source apportionment. This study offers novel insights into AGR impacts and contributes to fostering a harmonious balance between human activities and water resource sustainability.

Chemical evolution characteristics and influencing factors of groundwater in the saline and fresh water funnel area in Hengshui City, North China

Both over-exploitation and exploitation reduction of groundwater can alter the conditions of groundwater recharge and discharge, thereby impacting the overall quality of groundwater. This study utilizes hydrogeochemical methods and statistical analysis to explore the spatial and temporal evolution characteristics and influencing factors of groundwater chemistry in the saline-freshwater funnel area of Hengshui City under exploitation reduction. The results showed that: With the exception of the deep freshwater funnel area in the western region, which exhibits a trend of water quality deterioration (Cl- accounted for more than 25%), groundwater quality in the other funnel areas demonstrates an improving trend (HCO[Formula: see text] accounted for more than 25%). The origins of hydrochemical components in the groundwater of the two types of funnel areas are influenced by rock weathering. Only the shallow saline water funnel area in the eastern region is impacted by evaporation. And groundwater hydraulic conditions play a distinct role in controlling the evolution of water chemistry in different types of funnel areas. It is mainly manifested in changing the water-rock reaction speed in the two types of funnel areas and diluting the groundwater ion concentration in the funnel area.

Key point of desert riparian forest development in the arid area: The response of phreatic water table depth to ecological water supply

Desert riparian forest vegetation maintains the fragile balance of ecosystems in extreme arid areas. Raising the phreatic water table through efficient ecological water supply is the key to the desert riparian forests in extreme arid areas. The main objective of this study is to explore an innovative framework in which the response of phreatic water table depth (PWTD) to ecological water supply flow (EWSF) can be effectively reflected. The framework is based on the computationally efficient integrated surface water - groundwater model (ISGWM), through which surface water processes, groundwater recharge and discharge processes, and PWTD changes under different EWSF can be accurately simulated. A large number of simulations were conducted to reflect the response of PWTD to EWSF, and to explore the suitable EWSF and its intra-annual process considering the efficiency of PWTD reduction. The applicability and advantages of ISGWM were evaluated for the Tarim River Basin (TRB), a typical inland river basin, northwest China. The response of PWTD to EWSF was studied in 55 gate control areas in the mainstream of TRB. The formulas of suitable EWSF of 55 gates with different initial PWTDs were fitted. It is more beneficial to concentrate the ecological water supply in flood season (Jun.-Oct.). Overall, the ISGWM can accurately describe the multi-process of surface water and groundwater interactions. This study can provide scientific support for water resources management and allocation in the TRB and other similar inland river basins.

Multi-objective double layer water optimal allocation and scheduling framework combing the integrated surface water - groundwater model

Balancing the water consumption of agricultural and ecological is the key point of sustainable social and economic development in an inland river basin. The growth of desert riparian forests in inland river basins mainly depends on a certain phreatic water table depth (PWTD). The main object of this study was to allocate and schedule water resources to regulate the PWTD and satisfy agricultural water demand. Therefore, a multi-objective double layer optimal allocation and scheduling framework based on the computationally efficient integrated surface water-groundwater model (ISGWM), which can simulate the surface water processes, groundwater recharge and discharge processes, and PWTD changes, was constructed and applied to the mainstream of Tarim River Basin (TRB). The top layer model of the framework is an optimal ecological water allocation model, and its optimal allocation results are used as the initial solution of the bottom layer model. The results show that under 5 different inflow frequencies, the agricultural water shortage rate is 0, 17.38 %, 17.41 %, 14.06 %, and 19.94 %, respectively. The PWTD regulation has a great performance. After the optimal scheduling, the proportions of good growth of the control area behind the gate under different inflow frequencies were 98.18 %, 98.18 %, 98.18 %, 90.91 %, and 94.55 %. Agricultural water shortage is mainly due to the non-uniformity distribution of intra-annual inflow and the lack of controlling hydraulic engineering. The regulation of PWTD can guarantee the growth of desert riparian forests on both sides of the mainstream of TRB. Besides, we explored the feasibility of exploiting groundwater to supplement agricultural water consumption. The groundwater exploitation should be controlled within the scope of not causing excessive increase of PWTD (difference between PWTD and target depth <1 m), due to the groundwater exploitation to supplement agricultural water will lead to the increase of PWTD. Overall, this framework, which regulates the PWTD with the change of ecological water supply based on the ISGWM, provides a new idea for the allocation and scheduling of agricultural and ecological water resources in arid inland river basins. It also provides a new method for the coupled cooperative operation of surface water and groundwater.

The impacts of climate change on groundwater quality: A review

Groundwater has been known as the second largest freshwater storage in the world, following surface water. Over the years, groundwater has already been under overwhelming pressure to satisfy human needs for artificial activities around the world. Meanwhile, the most noticeable footprint of human activities is the impact of climate change. Climate change has the potential to change the physical and chemical properties of groundwater, thereby affecting its ecological functions. This study summarizes existing research affiliated with the possible effects of a changing climate on the quality of groundwater, including changes in water availability, increased salinity and pollution from extreme weather events, and the potentiality of seawater intrusion into coastal aquifers. Previous works dealing with groundwater-induced responses to the climate system and climate impacts on groundwater quality through natural and anthropogenic processes have been reviewed. The climate-induced changes in groundwater quality including pH, dissolved oxygen level, salinity, and concentrations of organic and inorganic compounds were assessed. Some future research directions are proposed, including exploring the potential changes in the occurrences and fate of micropollutants in groundwater, examining the relationship between the increase of microcystin in groundwater and climate change, studying the changes in the stability of metals and metal complexation, and completing studies across different regional climate regions.