Impact of river reconstruction on groundwater flow during bank filtration assessed by transient three-dimensional modelling of flow and heat transport

Abstract

Bank filtration (BF) is an established indirect water-treatment technology. The quality of water gained via BF depends on the subsurface capture zone, the mixing ratio (river water versus ambient groundwater), spatial and temporal distribution of subsurface travel times, and subsurface temperature patterns. Surface-water infiltration into the adjacent aquifer is determined by the local hydraulic gradient and riverbed permeability, which could be altered by natural clogging, scouring and artificial decolmation processes. The seasonal behaviour of a BF system in Germany, and its development during and about 6 months after decolmation (canal reconstruction), was observed with a long-term monitoring programme. To quantify the spatial and temporal variation in the BF system, a transient flow and heat transport model was implemented and two model scenarios, ‘with’ and ‘without’ canal reconstruction, were generated. Overall, the simulated water heads and temperatures matched those observed. Increased hydraulic connection between the canal and aquifer caused by the canal reconstruction led to an increase of ~23% in the already high share of BF water abstracted by the nearby waterworks. Subsurface travel-time distribution substantially shifted towards shorter travel times. Flow paths with travel times <200 days increased by ~10% and those with <300 days by 15%. Generally, the periodic temperature signal, and the summer and winter temperature extrema, increased and penetrated deeper into the aquifer. The joint hydrological and thermal effects caused by the canal reconstruction might increase the potential of biodegradable compounds to further penetrate into the aquifer, also by potentially affecting the redox zonation in the aquifer.

Seasonal dynamics modifies fate of oxygen, nitrate, and organic micropollutants during bank filtration — temperature-dependent reactive transport modeling of field data

“…Values for the calibrated hydraulic conductivities are presented in Table 2. In our study, only a slight adjustment of hydraulic conductivities was desired, as being at hand from a 3D model already, and with our calibrated hydraulic heads, we were able to get mostly better agreement than Wang et al (2020). The simulation of the hydraulic heads justifies our simplifying assumptions with respect to the model boundaries and the aquifer's hydrogeology (Fig.…”

“…The transient hydraulic heads for the outflow boundary condition at the pumping well (PW in Fig. 1 c) correspond to the simulated results from Wang et al (2020). For temperatures and concentrations, default no-dispersive flux boundaries of PHT3D were used for this outflow boundary.…”

“…1 c). A silt layer from the Weichsel cold period is fragmentary due to periglacial processes and erosion so that silts occur as lenses in the area and the local layer is implemented as a thin zone in the upper part of the first aquifer, according to Wang et al ( 2020 ).

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“…We applied a transient 2D water flow, heat, and reactive transport model to simulate the hydrochemical processes at the bank filtration site. The transect was extracted from a calibrated 3D flow and heat transport model of the study area (Wang et al 2020 ) following a flow line from the canal towards a pumping well (Fig. 1 a).…”

“…The spatial and temporal variations in groundwater flux, as well as temperature and partly redox conditions at this bank filtration site, were studied already by Wang et al (2020) and Munz et al (2019). According to Munz et al (2019), the hydrological system is seen to be fairly stable in time with a high share of bank filtrate ranging between 61% (W04 HIGH ) and 100% (W07 HIGH ) and average subsurface residence times ranging between 18 d (W07 HIGH ) and 438 d (W04 LOW ).…”