Biogeochemical Mechanism Of Abnormally Low Dissolved Inorganic Carbon In Petroleum-contaminated Groundwater

Evaluating the potential ability of the contaminant degradation is one of element tasks for scientifically managing and controlling the petroleum-contaminated sites.Carbon balance method is one of the common methods to evaluate contaminants degradation abilities.The basic theory of the method is that petroleum hydrocarbon degradation will increase the dissolved inorganic carbon(DIC)in the groundwater,and therefore,the quantitative evaluation of petroleum contaminants degradation will be got by calculation the incremental concentration of DIC.However,there were lower DIC concentrations in a petroleum contaminated aquifer,namely,abnormally low dissolved inorganic carbon phenomenon,in previous study.In order to evaluate the contaminant-degrading capacity of abnormally low DIC site,it is necessary to study the mechanism of abnormally low DIC phenomenon formation.In order to study the formation mechanism,more than 20 monitoring wells were set up to in the study site and parameters about hydrochemistry,isotope,microorganisms,and so on were monitored at 3 time-points in 2 years period.The correlation analyses between hydrochemical data were used to find the hydrochemical parameters relevant to the DIC and to put forward the probable mechanism affecting the DIC concentrations.The proposed mechanism was further confirmed by isotope analyses,microbiological analyses and microcosm experiment.Based on theoretical aknowledges in the proposed mechanism,a contaminant degradation assessment model rooted in carbon balance was proposed,and degradation velocities at different period in the study site were evaluated.The results showed that the DIC is positively correlated to the hardness and negatively correlated to the p H.Combining to 13C-DIC analysis,a conjecture is proposed that autotrophic metabolism is the dominant factor for abnormally low DIC phenomenon in the groundwater.Autotrophic metabolism process including the assimilation DIC by autotrophic microorganisms and the precipitation induced by autotrophic microorganism producing alkaline microenvironment.Microbial genome analysis shows that there was a large quantity of autotrophic microorganisms in the aquifer,and the pathways of assimilating DIC mainly include reductive acetyl-Co A pathway,3-hydroxypropionate bicycle,and 3-Hydroxypropionate/4-Hydroxybutylate cycle.From microbial species analyses,the microorganisms involving in these pathways include Microbacterium,Novosphingobium,Reyranella,etc,and most of them are facultative autotrophic microorganisms,which can both absorb organic carbon,such as petroleum contaminants,and assimilate inorganic carbon.From microbial function analyses,the microorganisms can oxidize ammonium,hydrogen,ferrous and reduced sulfur.Among them,the sulfur oxidizing bacteria had the highest abundance and proved to have the ability to decrease the concentrations of DIC by microcosm experiment.Based on these mentioned above,an understanding that autotrophic metabolism lead to the phenomenon of abnormally low DIC was got and a carbon balance model considering the autotrophic metabolism was proposed.In the closed aquifer contaminated by petroleum,the model about DIC evolution can be divided into four processes: heterotrophy function,heterotrophy and precipitation function,autotrophy function,autotrophy and precipitation function.Based on the model,multi-periodical assessment results showed that the model is excellent anti-interference performance,simplicity and feasibility.DIC consuming and producing velocities was gradually increasing with time in the autotrophy dominated area,and DIC producing velocities was gradually decreasing with time in the heterotrophy dominated area.DIC producing velocities was gradually decreasing with time in the whole aquifer,and as a result,carbon sequestration effect was formed by the autotrophic metabolism in the study site.