Formation And Evolution Of Microbial Induced Iron Clogging In Wells Of Groundwater Heat Pump Systems

Groundwater heat pump（GWHP）heating and cooling technologies utilize shallow groundwater for energy exchange.GWHP technologies play a vital role in reducing fossil fuel consumption and curbing CO₂ emissions,thus making substantial contributions to carbon neutrality goals.However,the sustainability of GWHP systems has been restricted by technical issues,most notably well clogging,leading to a sharp decrease in recharge rate to aquifer and poor water circulation.Iron clogging is a widespread issue in the GWHP systems,especially in areas with the high concentration of iron in groundwater.Iron clogging typically manifests at the interfaces of porous media,filters,and well water,often occurring swiftly.Microorganisms are common in iron clogging and are known be involved with the iron clogging.In order to ensure a sustainable development of the GWHP,it is crucial to understand formation and mechanisms of iron clogging in recharge wells of GWHP systems through microbial regulation.Therefore,this study,based on a heavily clogging GWHP system in Shenyang,undertook comprehensive research.First,field surveys of operational GWHP wells analyzed the characteristics and the formation mechanism of iron clogging.Subsequently,laboratory sand tank experiments were conducted to explore the spatiotemporal evolution of iron clogging.Through the batch experiment,the primary controlling clogging factors influenced by iron oxidizing bacteria were elucidated.Furthermore,the cumulative hydrogeochemical model for iron clogging formation was constructed.The main findings and conclusions are summarized as follows:1.The field investigation of the GWHP system site revealed significant disparities in well clogging across different operational modes.Notably,recharge wells demonstrated the most severe clogging,followed by pumping wells,while observation wells exhibited minimal clogging.Moreover,with increasing depth,clogging intensified in recharge and pumping wells,with the deepest aquifer areas experiencing the most substantial clogging.The main components of the iron logging were identified as iron oxides,specifically goethite,ferrihydrite,hematite,and calcite.2.The spatial variation in the degree of clogging originates from different formation mechanisms,（i）where shallow iron clogging in pumped and recharge wells may originate from Fe（0）in the well equipment.（ii）Iron clogging in the deeper layers of recharge wells is formed at the groundwater-filter interface,which is affected by the oxidizing effect of microorganisms and the interaction of iron-containing minerals in the formation,and the iron clogging originates from divalent iron minerals in the aquifer.（iii）Deep iron clogging of pumping wells occurs at the mixing of well water,and the redox reaction triggered by hydrodynamic mixing prompts the rapid oxidation of Fe²⁺in anaerobic groundwater to form iron clogging.3.The evolution of clogging materials during groundwater recharge was revealed through sand tank simulation experiments and can be summarized in three stages:（1）In the initial stage,the permeability of the medium decreases slowly.Low-crystallinity calcium carbonate forms on the filter surface.Abundant denitrifying bacteria are present,primarily driven by oxygen and nitrate.（2）In the adaptation and rapid proliferation stage,iron-oxidizing bacteria adapt to the environment,leading to a rapid drop in permeability.Fe²⁺decreases in the recharge system,resulting in a significant increase in oxidation-reduction potential（ORP）and pH.（3）During the stable development stage of iron clogging,permeability becomes relatively stable.Both iron-oxidizing and iron-reducing bacteria play important roles in the stable development of clogging.Overall,in the early stages of clogging,upper layer clogging materials consist of ferrihydrite,while lower layers are primarily composed of calcium carbonate and low-crystallinity iron oxides.In the later stages of recharge,upper layer clogging materials mainly consist of SiO₂ and higher-crystallinity goethite and hematite,with lower layers having lower crystallinity iron oxides.4.Considering the significant role of iron-oxidizing bacteria in mediating iron clogging,we conducted experiments to study their growth and oxidation kinetics under varying conditions.Our findings revealed that temperature,pH,and cell density were the primary factors affecting iron-oxidizing bacteria-mediated clogging.Under low-temperature conditions,the polysaccharide content of extracellular eolymeric substances（EPS）secreted by iron-oxidizing bacteria increased,leading to a slow decrease in the permeability coefficient.Under weakly alkaline conditions,iron oxidizing bacterial biofilm and EPS increased,promoting iron clogging formation.Equations were developed to describe the effects of pH and dissolved oxygen（DO）on the rate of Fe²⁺oxidation,providing a theoretical explanation for the mechanism of iron clogging mediated by iron-oxidizing bacteria.5.The process of clogging formation and accumulation in pumping and recharge wells was simulated using the software PHREEQC.The model results were consistent with both field and laboratory experiments.In the pumping well,the interplay of mixing effects led to the rapid short-term deposition of most clogging materials within the wellbore.Conversely,in the recharge well,iron minerals gradually released ferrous ions,resulting in the gradual yet sustained buildup of iron oxides.Consequently,clogging within the pumping well was comparatively less pronounced than in the recharge well.This phenomenon was further facilitated by the catalytic activity of iron-oxidizing bacteria,which promoted the accumulation of iron oxides.