Abstract
Abstract Bank filtration induced by groundwater pumping results in redox zonation along the groundwater flow path. Besides the river water, recharge from other sources can change local redox conditions; therefore, redox zonation is likely to be complex within the riverbank filtration (RBF) system. In this study, hydrodynamics, hydrogeochemistry, and environmental stable isotopes were combined together to identify the redox conditions at an RBF site. The recharge characteristics and redox processes were revealed by monitoring the variations of water level, δ2H and δ18O, and redox indexes along shallow and deep flow paths. The results show that local groundwater is recharged from river, regional groundwater, and precipitation. The responses of redox zonation are sensitive to different sources. In the river water recharge zone near shore, O2, , Mn(IV), Fe(III), and are reduced in sequence, the ranges of each reaction are wider in deep groundwater because of the high-velocity deep flow. In the precipitation vertical recharge zone, precipitation intermittently drives O2, , and organic carbon to migrate through vadose zone, thereby decreasing the groundwater reducibility. In the regional groundwater lateral recharge zone in the depression cone, the reductive regional groundwater is continuously recharging local groundwater, leading to the cyclic reduction of Mn(IV) and Fe(III).
Highlights
Riverbank filtration (RBF) involves water extraction by pumping wells at riverside fields to enhance water supply by stimulating river recharge to groundwater
Many factors affecting the formation of redox zonation, such as fluctuations in river stage and groundwater level, intensity of pumping, lithology and structure of aquifer medium, and water temperature, have all been considered in previous studies, yet the influence of recharge conditions of local groundwater, such as water from precipitation and regional groundwater conditions have been neglected (Burt et al ; Massmann et al ; Kohfahl et al ; Wang et al )
This study aims to: (1) carry out survey and analysis of the hydrodynamics of the river and groundwater around the Kaladian well field by combining δ2H and δ18O analyses to identify the spatial distribution characteristics of recharge; (2) survey the spatial distribution of the redox environment, sensitive components in river water and groundwater, and the soluble N and C components in the aquifer medium, as well as analyze their relationships with hydrodynamic conditions; and (3) partition the redox zonation along the groundwater flow path based on the spatial distribution of recharge and redox conditions, as well as clarify the hydrogeochemical processes within each zone
Summary
Riverbank filtration (RBF) involves water extraction by pumping wells at riverside fields to enhance water supply by stimulating river recharge to groundwater This can attenuate or degrade pollutants, such as suspended solids, inorganic or organic substances, poisonous heavy metals, pathogenic viruses, and bacteria; it is considered an efficient and natural treatment technology for water quality improvement (Hiscock & Grischek ; Tufenkji et al ; Trauth et al ; Muz et al ). Rivers are rich in O2 and organic matter; driven by Gibbs free energy, the organic matter supplies electrons to the lowest unoccupied molecular orbital locations during river filtration; O2, NOÀ3 , Mn(IV), Fe(III), and SO24À are reduced in sequence (Kedziorek & Bourg ; Farnsworth & Hering ) This process can cause incomplete removal of organic carbon, suspended solids, and inorganic pollutants, and release of heavy metals, such as Fe, Mn, and As, from sediment or the aquifer medium to groundwater (Gandy et al ; Su et al ). In addition to river water, groundwater recharge from other sources can change the local redox conditions, and redox zonation is likely to be more complex within the RBF system (Kedziorek & Bourg ; Buzek et al )
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