###### Abstract  
We perceived a trend in the study and practice of petroleum-contaminate sites. Monitored natural attenuation (MNA) can reduce the contaminant concentrations in the soil and groundwater, and it is a method that can remediate the petroleum-contaminated site effectively. MNA is becoming a research focus. This study evaluated MNA using a series of lab-based bench-scale experiments and a large amount of monitoring data from field samplings. Based on the in-site total petroleum hydrocarbon (TPH) results, we used statistical methods, the Mann-Kendall test, and mass fluxes in order to evaluate the MNA of petroleum-contaminated sites in groundwater. The results showed that the TPH concentrations were decreasing, and the plume became smaller. The attenuation rate was from 0.00876 mg/d to 0.10095 mg/d; remediating the petroleum contamination site would cost 1.3 years to 10.6 years. The plume reached a quasi-steady state, and mass flux declined. The most essential process of MNA was biodegradation, and the second was sorption. During the monitoring period, 393 g of TPH was attenuated, including 355 g of TPH gradated by microbes. Biodegradation upstream of the plume was more serious. Iron(III) and manganese were the main electron acceptors utilized by microbes during the monitored period. MNA was in progress, and it can be an effective method to remediate the petroleum-contaminated site. Lab-based bench-scale experiments were performed with much monitoring data from the field samplings in order to understand the fate and transport mechanism of the petroleum contamination from the land surface to shallow groundwater according to site conditions. years can be costly but may cost less than other methods [9]. In China, the extensive study of MNA at the laboratory scale is moving into the study of petroleum-contaminated sites due to the great real-world demand [10; 11; 12]. MNA can be used as the sole remedial solution [13], or when other methods will not work or are expected to take almost as long. Sometimes, MNA is used as a final cleanup step after other methods for polishing purposes [14; 15]. Some of these naturally occurring physical, chemical, and biological processes can transform contaminants to less harmful forms or immobilize them in the subsurface, therefore reducing contaminant concentrations in groundwater [16; 17; 18]. Attenuation mechanisms encompass physical dilution, physicochemical sorption, ion exchange, chemical dissolution/precipitation or complexation, and microbial metabolic processes [19; 20]. Many studies found that aerobic and anaerobic biodegradation are the major processes for the reduction of contaminant mass in the subsurface [21; 22]. Biodegradation is generally considered the most important process for contaminated site remediation because it is destructive, unlike sorption, dilution, and volatilization [23]. Biodegradation would reduce the mass and toxicity of contaminants in the groundwater and soils. The biodegradation of aerobic microbes is more efficient than that of anaerobic microbes [24; 25]. Once oxygen is completely used or becomes limiting, microbes utilize nitrate, followed by manganese, iron, and sulfate [26]. However, if the rate of biodegradation is not significant enough to contain the plume, MNA will not be sufficient to protect aquifers and downgradient receptors [27]. So, ensuring the attenuation effect is the first and foremost task by estimating the trend of a plume, monitoring the concentration, and calculating the remediation period [28; 29; 30; 31]. Theories, such as the Mann-Kendall test, are often used to define the stability of a contaminant plume based on concentration trends at individual wells [32; 33; 34]. It is widely used in hydrology. There are multiple methods for estimating biodegradation. First-order kinetics may be able to approximately reproduce the mass and dimensions of contaminant plumes that follow from a far more complex degradation model but are often regarded as imprecise or even conceptually incorrect [16; 30; 35].