## 4 Discussion
### The Analysis Based on Mass Flux  
The TPH concentrations were given by the data of six monitoring periods. Aquifer thickness was equal to the well depth minus the aeration zone depth. The hydraulic gradient was calculated by the water level. Mass flux results are shown in Figure 9. The mass flux of TPH generally reduced over time, only increasing slightly in January 2010. The fluctuation occurred mainly when the TPH concentration was relatively small. Test accuracy was impacted by the fluctuation. According to the changes of mass flux, it could be seen that CP1 \(>\) CP2 \(>\) CP3. The mass flux of CP1 was 1415 mg/d in September 2009, which reduced to 524 mg/d in January 2010, and then it rebounded slightly to 580 mg/d. CP2 and CP3 had the same trend (Figure 9). This indicated that the TPH concentrations through each section decreased. Natural attenuation was carried on in the contaminated plume, and TPH in the site reached the half-steady state from time to time. Figure 8: Distributions of the trend analysis results of TPH concentrations in the monitoring wells. \begin{table}
\begin{tabular}{c c c} \hline  & **Concentration Trend** & **Confidence Level (\%)** \\ \hline Z1-1 & Probably decreasing & 93 \\ Z6 & Probably decreasing & 97 \\ Z7 & No trend & 69 \\ Z8 & Stable & 83 \\ Z9 & Probably decreasing & 97 \\ Z10 & No trend & 79 \\ Z11 & No trend & 56 \\ Z16 & Probably decreasing & 97 \\ Z19 & Probably decreasing & 93 \\ Z20 & Probably decreasing & 96 \\ Z21 & Decreasing & 99 \\ Z22 & Probably decreasing & 97 \\ S1 & Probably decreasing & 93 \\ \hline \end{tabular}
\end{table} 
Table 3: Results of the Mann–Kendall trend analyses. The main attenuation processes were adsorption, volatilization, biodegradation, and dispersion. Based on the research of [PERSON] (2002) [39], the volatilization of TPH in groundwater is negligible. Once the medium and environment are fixed, adsorption and dispersion are stable, so the decrement of mass flux can be considered as the approximate contribution of biodegradation. We calculated the biodegradation using Formulas (9)-(11), and the results are listed in Table 4. Based on the mass flux data in time and space, biodegradation of TPH and natural attenuation were able to be analyzed. However, if the mass flux is reduced, the soil particle will resolve part of the petroleum hydrocarbons, and then the biodegradation of capacity will be the sum of reduction and desorption. We were able to calculate desorption by using Formula (12). From September 2009 to March 2010, biodegradation due to mass flux reduction between CP1 and CP2 was 215 g; total flux reduction was 242 g through calculus, and biodegradation occupied 89% of the total attenuation. This explained why microbial degradation was a major attenuation mechanism in the site. Similarly, from September 2009 to March 2010, biodegradation calculated by mass flux between CP2 and CP3 was 140 g; total mass reduction was 152 g, and biodegradation occupied 92% of the total attenuation. During this process, the adsorption capacity between CP1 to CP2 was from 81 g down to 27 g, and adsorption shrank. In the monitoring process, adsorption and biodegradation declined continuously, but the biodegradation ratio increased. The biodegradation ratio at the upstream was 64-89%, and the biodegradation rate downstream was 70-92%. The average biodegradation rate in the upstream was 0.4911 mg/d/m\({}^{3}\), and it was 0.338 mg/d/m\({}^{3}\) downstream. The attenuation rate upstream was higher than downstream. In short, the following were found: (1) Calculations showed that attenuation stably occurred in the plume, the mass flux of each CP was reducing, and there were more contaminants upstream than downstream. (2) Adsorption and biodegradation were the main attenuation processes; biodegradation was more efficient. (3) Biodegradation increased, and adsorption decreased with time. The biodegradation capacity was 383 g.  
The three findings above were able to be used to estimate the natural attenuation of petroleum-contaminated sites.