Kinetic Study on Clogging of a Geothermal Pumping Well Triggered by Mixing-Induced Biogeochemical Reactions

Iron clogging mechanism around injection wells during groundwater recharge in shallow iron-rich aquifer: insight from spatiotemporal evolution

Iron deposits in the filters and porous media around injection wells could induce severe clogging, thereby limiting the sustainability of groundwater heat pump (GWHP) systems. During the water injection in shallow iron-rich aquifers, iron clogging exhibits both spatial and temporal evolution in terms of its composition and formation processes. This study employed a two-dimensional sand tank experiment with continuous multicomponent groundwater to explore the spatiotemporal evolution of clogging around the injection well. The results suggested that upper-layer clogging was dominated by high-crystallinity iron oxides, goethite, and lepidocrocite, primarily caused by oxidation corrosion of filter. The evolution of lower-layer clogging occurred in stages with distinct material-mechanism-impact relationships. The initial phase (0-15 days): clogging comprised of calcium carbonate and low-crystallinity iron oxides induced by oxygen enrichment and CO₂ depletion during the early injection period, causing a 10% hydraulic head difference to increase. Mid-stage (15-35 days): microbial aggregates, SiO₂, and lower-crystallinity iron oxides dominated as hydrodynamic shifts elevated pH and redox potential, accelerating microbial growth and iron oxide deposition, amplifying hydraulic head difference to 60%. Stabilization phase (after 35 days): biofilms and extracellular polymeric substances (EPS) secretion by microbial communities consolidated pore occlusion, sustaining stable hydraulic resistance. Each stage transitioned from abiotic mineral precipitation to biologically reinforced clogging, progressively restricting permeability through synergistic physicochemical and microbial processes. This study highlighted a novel understanding of the evolution of iron clogging within an injection well located in a shallow iron-rich aquifer, and proposed region- and stage-specific strategies for clogging mitigation and control.

Mechanisms of well iron clogging in groundwater heat pump systems: Insights from video imaging, hydrogeochemical analysis, and geochemical modeling

Groundwater heat pump (GWHP) systems are increasingly popular as low-carbon and environmentally friendly technologies, but well clogging induced by iron remains a significant issue. This study investigated the clogging characteristics and biogeochemistry of three typical wells (pumping, injection, and observation wells) in an operating GWHP system using video imaging, sampling, and analysis of hydrogeochemical and microbial data. The results revealed that iron-induced well clogging is a complex process involving physical, chemical, and microbial factors. Pumping wells experience clogging due to water mixing with varying redox conditions, resulting in hematite-based iron oxide deposits. Injection wells exhibit higher clogging severity, with transformed oxidation and accumulation of reduced iron minerals at the solid-liquid interface, resulting in darker colored clogs with magnetite. Clogging in both extraction and injection wells is closely related to iron-rich aquifer sections, where severe clogging occurs. Shallow clogging due to iron oxide is limited and attributed to the oxidation of zero-valent iron in well casing material. Iron-oxidizing bacteria and iron-reducing bacteria were detected in the consolidated deposits of clogged wells, indicating their involvement in the clogging formation process. Moreover, a strong correlation was observed between the presence of nitrate-reducing bacteria in the water phase and the severity of clogging, suggesting a possible link between iron oxidation and nitrate reduction in the system. Geochemical modeling results further supported the observed clogging severity in GWHP systems and confirmed varying clogging mechanisms in different wells and depths. These findings contribute to the understanding of clogging in GWHP operations, aiding in robust water utilization and energy-saving efforts, and supporting global carbon reduction initiatives.

Microscale Chaotic Mixing as a Driver for Chemical Reactions in Porous Media

Mixing-induced reactions play a key role in a large range of biogeochemical and contaminant transport processes in the subsurface. Fluid flow through porous media was recently shown to exhibit chaotic mixing dynamics at the pore scale, enhancing microscale concentration gradients and controlling mixing rates. While this phenomenon is likely ubiquitous in environmental systems, it is not known how it affects chemical reactions. Here, we use refractive index matching and laser-induced fluorescence imaging of a bimolecular redox reaction to investigate the consequence of pore scale chaotic mixing on the reaction rates. The overestimation of measured reaction rates by the classical macrodispersion model highlights the persistence of incomplete mixing on the pore scale. We show that the reaction product formation is controlled by microscale chaotic mixing, which induces an exponential increase of the mixing interface and of the reaction rates. We derive a reactive transport model that captures experimental results and predicts that chaotic mixing has a first order control on reaction rates across a large range of time scales and Péclet and Damköhler numbers. These findings provide a new framework for understanding, assessing, and predicting mixing-induced reactions and their role on the fate and mobility of environmental compounds in natural porous media.

Groundwater Microbiology of an Urban Open-Loop Ground Source Heat Pump with High Methane

Ground source heat pumps (GSHPs) are low-carbon alternatives to gas boilers for decarbonizing heating. Open-loop GSHP systems abstract groundwater, pass it through a heat exchanger, and return it to ground or surface water. Groundwater samples from the top and base of an abstraction and a recharge borehole of an open-loop GSHP system in Cardiff, UK were assessed, and compared to two local boreholes in the same aquifer. Groundwater samples were taken when the GSHP system was active (once) and inactive (twice) and analyzed for changes in geochemistry, viable cell counts, and microbial community (16S rRNA gene sequencing). The GSHP had a distinct geochemistry and microbial community compared to the control boreholes, and the abstraction borehole showed greater variability than the recharge borehole. The microbial community of the GSHP system showed an increase in relative abundance of genera involved in oxidation of methane and methylated compounds, of which Methylotenera was the most abundant (up to 83.9% of 16S rRNA gene sequences). There were also changes in genera associated with nitrification (Nitrospira, Nitrosomonas) and those with potential for sulfur and iron cycling (Rhodoferax). Methane concentration was analyzed after identification of methylotrophs and found that methane concentrations were up to 2855 μg L^-1 , thus likely having had a significant impact on the bacterial communities present. Understanding the microbiology and biogeochemistry of GSHP systems provides insight into potential issues with local infrastructure and long-term system performance, and supports modeling to maximize efficient and sustainable use of the subsurface.

Water Quality Assessment Bias Associated with Long-Screened Wells Screened across Aquifers with High Nitrate and Arsenic Concentrations

Semi-arid regions with little surface water commonly experience rapid water table decline rates. To hedge against the falling water table, production wells in central Mexico are commonly installed to depths of several hundred meters below the present water table and constructed as open boreholes or perforated casings across their entire length. Such wells represent highly conductive pathways leading to non-negligible flow across chemically distinct layers of an aquifer-a phenomenon known as ambient flow. The objectives of this study were to estimate the rate of ambient flow in seven production wells utilizing an end-member mixing model that is constrained by the observed transient chemical composition of produced water. The end-member chemical composition of the upper and lower layers of an urban aquifer that overlies geothermal heat is estimated to anticipate the future quality of this sole source of water for a rapidly growing urban area. The comprehensive water chemistry produced by seven continuously perforated municipal production wells, spanning three geologically unique zones across the city of San Miguel de Allende in Guanajuato State, was monitored during one day of pumping. The concentration of conservative constituents gradually converged on steady-state values. The model indicates that, relative to the lower aquifer, the upper aquifer generally has higher specific conductance (SC), chloride (Cl), nitrate (NO₃), calcium (Ca), barium (Ba) and magnesium (Mg). The lower aquifer generally has a higher temperature, sodium (Na), boron (B), arsenic (As) and radon (Rn). Ambient flow ranged from 33.1 L/min to 225.7 L/min across the seven wells, but this rate for a given well varied depending on which tracer was used. This new 3D understanding of the chemical stratification of the aquifer suggests that as water tables continue to fall, concentrations of geothermally associated contaminants of concern will increase in the near future, potentially jeopardizing the safety of municipal drinking water.

Investigation into the Cause of Iron-Related Clogging of Groundwater Bores Used for Viticulture in the Limestone Coast, South Australia

Iron-related clogging of boreholes, pumps and dripper lines is a significant and costly problem for irrigators worldwide. The primary cause of iron-related clogging is still debated. Previous studies have described complex interactions between biological clogging and inorganic iron/manganese oxide precipitation. This case study examined groundwater bores used for viticulture irrigation in the Limestone Coast region, a highly productive wine growing area in the SE of South Australia. Iron clogging of bore screens, pumps and dripper systems has been a persistent problem in the region and the issue is perceived to be growing, with irrigators suggesting the widespread introduction of iron-related bacteria (IRB) through drilling equipment to be the root cause of the problem. Analysis of the groundwater microbiology and inorganic chemistry found no apparent correlation between the presence of IRB and the clogging status of wells. In fact, IRB proved to be widespread throughout the limestone aquifer. However, a clear correlation could be found between clogging affected bores and the redox potential of the groundwater with the most severely affected bores strongly oversaturated in respect to iron oxide minerals. Elevated dissolved concentrations of Fe(II) thereby tended to be found in deeper bores, which also were generally more recently drilled. Following decades of less than average rainfall, a tendency to deepen bores in response to widespread declines in water levels has been documented for the SE of South Australia. The gradually widening clogging problem in the region is postulated to be related to the changes in climate in the region, with irrigators increasingly driven to rely on deeper, anoxic iron-rich groundwater resources.

Joint response of chemistry and functional microbial community to oxygenation of the reductive confined aquifer

Oxygen can enter into reductive aquifer through natural and artificial processes. However, the joint response of groundwater chemistry and functional microbial communities to oxygenation is not well understood due to the gap between taxonomic and functional microbial composition. Here, two wells named CZK15 and CZK22 at the second confined aquifer in Central China were in situ aerated, and the chemical parameters of groundwater and microbial communities in bio-trapping sand sediment were analysed during aeration. The microbial metabolic functions related to C, N, S, Fe transformation were predicted by Functional Annotation of Prokaryotic Taxa (FAPROTAX) approach and some key functional genes, such as phe, nah, narG, and soxB were verified by the real-time quantitative Polymerase Chain Reaction (qPCR) method. The biomass was promoted, microbial diversity fluctuated, and microbial composition changed remarkably with aeration mainly constrained by reduction-oxidation (redox) variation and SO₄^2- concentration. Among functional microbes, aerobic chemoheterotrophs including aromatic compound degraders (also especially for relative abundance of phe and some nah gene) and methylotrophs are dramatically enriched interpreting dissolved oxygen (DO) consumption and total organic carbon (TOC) decomposing in sediment. Whilst fermenters and methanogen expectedly decreased during aeration. Denitrifying microbes and narG gene relative abundance increased corresponding to the NO₃^- increase after aeration, while microbes for N₂ fixation, ammonification, and nitrification decreased relating to the source of NH₄⁺. The sulfide oxidation causing increased SO₄^2- was reflected by the blooming of sulfur-oxidizing microbes and soxB gene. Some sulfate reducers persisted in sediment after aeration due to sufficient SO₄^2- as substrate. Fe(II) was mainly chemically oxidized as iron-oxidizing microbes were of low abundance and tended to decrease with aeration. The iron-reducing bacteria Geobacteraceae increased with aeration corresponding to the increased Fe(III) oxides formation. The findings of this study could have important implications in understanding the biogeochemical behaviours with cyclic redox conditions.

Effects of a Groundwater Heat Pump on Thermophilic Bacteria Activity

Groundwater samples were collected from the tubular wells of a groundwater heat pump (GWHP), and the psychrophilic, mesophilic, and thermophilic bacteria inhabiting the collected groundwater were cultured and isolated. Using the isolated bacteria, we analyzed temperature-dependent changes in autochthonous bacteria based on the operation of the GWHP. Microbial culture identified eight species of bacteria: five species of thermophilic bacteria (Anoxybacillus tepidamans, Bacillus oceanisediminis, Deinococcus geothermalis, Effusibacillus pohliae, and Vulcaniibacterium thermophilum), one species of mesophilic bacteria (Lysobacter mobilis), and two species of psychrophilic bacteria (Paenibacillus elgii and Paenibacillus lautus). The results indicated A. tepidamans as the most dominant thermophilic bacterium in the study area. Notably, the Anoxybacillus genus was previous reported as a microorganism capable of creating deposits that clog above-ground wells and filters at geothermal power plants. Additionally, we found that on-site operation of the GWHP had a greater influence on the activity of thermophilic bacteria than on psychrophilic bacteria among autochthonous bacteria. These findings suggested that study of cultures of thermophilic bacteria might contribute to understanding the bio-clogging phenomena mediated by A. tepidamans in regard to GWHP-related thermal efficiency.