## 4 Discussion
### The Analysis Based on Mass Flux
Comprehensive results could reflect the intensity and trend of natural attenuation. The distribution and concentrations were able to verify the trend analysis results compared to the monitoring results. Natural attenuation occurred in the groundwater of the petroleum-contaminated site, and the TPH concentrations decreased continuously. Attenuation strength in the plume was higher than the edge, and attenuation patterns were different. 740, it is an anaerobic condition, and there is no oxygen available; thus, denitrification can happen in the area [54]. However, biodegradation in the site is a complex process. pH and temperature are changeable, so [PERSON] [55] studied the relationship between Eh and various ions under different pH values, as shown in Figure 11. Thus, microbial functional areas of the site can be speculated. Furthermore, electron acceptor concentration is also an effective way to determine the microbial functional areas. Electron acceptors of the site have background values, and anomalies are usually caused by microorganisms. Therefore, concentrations of electron acceptors can reflect the microbial functional areas, at least those existing or taking place. Based on [PERSON] and [PERSON]'s research, we can determine that the site was utterly anaerobic. Denitrification and manganese reduction occurred in all monitoring wells of the site. Until September 2009, iron reduction occurred in all of the wells except Z19 and S1, and sulfate reduction and methanogenesis did not occur during the monitored period. However, there was a zone whose sulfate concentration was lower than the background value; ORP was unable to prove it, so we considered that sulfate reduction had occurred before the monitored period. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Time (d)** & **0** & **32** & **69** & **105** & **115** & **194** \\ \hline \multirow{10}{*}{CP1-CP2} & Mass flux (mg) & 243,868 & 168,081 & 108,778 & 95,533 & 80,997 & 82,453 \\  & Adsorbed mass (mg) & 81,289 & 56,027 & 36,259 & 31,844 & 26,999 & 27,484 \\  & Total mass (mg) & 325,157 & 224,108 & 145,037 & 127,377 & 107,996 & 109,937 \\  & Mass flux decreased (mg) & 0 & 75,787 & 135,090 & 148,335 & 162,871 & 161,415 \\  & Adsorbed mass decreased (mg) & 0 & 25,262 & 45,030 & 49,445 & 54,290 & 53,805 \\  & Biodegradation (mg) & 0 & 101,049 & 180,120 & 197,780 & 217,161 & 215,220 \\  & Total attenuation (mg) & 81,289 & 157,076 & 216,379 & 229,624 & 244,160 & 242,704 \\  & Biodegradation ratio (\%) & 0 & 64 & 83 & 86 & 89 & 89 \\  & Attenuation rate (\%) & 25 & 48 & 67 & 71 & 75 & 75 \\ \hline \multirow{10}{*}{CP2-CP3} & Mass flux (mg) & 139,415 & 87,636 & 50,803 & 45,171 & 39,551 & 34,223 \\  & Adsorbed mass (mg) & 46,472 & 29,212 & 16,934 & 15,057 & 13,184 & 11,408 \\  & Total mass (mg) & 185,887 & 116,848 & 67,737 & 60,228 & 52,734 & 45,631 \\  & Mass flux decreased (mg) & 0 & 51,780 & 88,613 & 94,244 & 99,865 & 105,192 \\  & Adsorbed mass decreased (mg) & 0 & 17,260 & 29,538 & 31,415 & 33,288 & 35,064 \\  & Biodegradation (mg) & 0 & 69,040 & 118,150 & 125,659 & 133,153 & 140,257 \\  & Total attenuation (mg) & 46,472 & 98,251 & 135,084 & 140,716 & 146,336 & 151,664 \\  & Biodegradation ratio (\%) & 0 & 70 & 87 & 89 & 91 & 92 \\  & Attenuation rate (\%) & 25 & 53 & 73 & 76 & 79 & 82 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The result of mass flux and the contribution of each attenuation. Figure 10: Redox potentials for various electron acceptors (based on Norris and Drapela) [32; 53]. Suppose all electron acceptors in each monitoring well had an unchanged background value. Within the monitoring period, all the monitoring wells' concentrations were at a lower level, except S1. Manganese had a similar trend with iron, so the main boundary was unable to be distinguished, and manganese was assigned to the same boundary. An apparent low value area was able to be obtained through sulfate concentrations change analysis. This area is downstream of WPP and could maintain stability during the monitoring period.