## 3 Results
### Contamination and Attenuation Processes
#### 3.1.4 Hydrogeochemistry and Biogeochemistry  
Based on the investigation of groundwater level, the groundwater flow direction was from the southeast to the northwest (Figure 1). The main anion in the site was the bicarbonate ion, and the content was usually not less than 60%, except Z22. The distributing of bicarbonate ions associated with the TPH concentration, which was high at the downstream of WPP, reaching 2 mg/L, such as with Z6, Z7, Z8, and Z22. In peripheral monitoring wells (Z10, Z11, Z19, Z20, and S1), bicarbonate ion concentrations were lower than 1 mg/L (Figure 6). The evapotranspiration process reacted in the area where the bicarbonate ion concentration was high, and the cations were either sodium ions or sodium ions and calcium ions. Chloride ions were another major anion in the study area, and they were widely distributed downstream of the WPP. After the cleanup the source, the enrichment area of chloride ions became more significant. The main cations in the groundwater were sodium ions and calcium ions. The sodium ions were distributed in the west of the study area, and calcium ions were distributed in the east of the study area (Figure 6). The enrichment of calcium ions and bicarbonate was the result of the leaching process, and the enrichment of sodium ions and chloride ions was the result of the evapotranspiration process. The distribution of the hydrochemical type was formed by recharging not only from the upper gradient area but also from the second Songhua River [50]. During the monitoring period, the concentration of dissolved oxygen (DO) was very low. DO concentrations in the high-TPH wells were lower than in the low-TPH wells, and DO concentrations typically were less than 1 mg/L. In the heavily contaminated area, especially near the WPP, the DO concentrations were less than 0.1 mg/L (Z1-1, Z6, Z7, and Z8). These data fully prove that aerobic biodegradation was occurring at this site. The decreasing trends of nitrate concentrations also can be observed. Nitrate concentrations in the contaminated areas ranged from 1.2 to 14.0 mg/L (Table 1). Higher TPH and lower nitrate were detected near the source, and DO was depleted. The nitrate concentrations decreased from September 2009 to January 2010, and they were stable from January to March 2010 (1 mg/L). The ammonium concentrations increased from September  
Figure 6: Chemical types and distributions of chlorine. to November 2009. The average ammonium concentrations ranged from 0.142 mg/L to 1.03 mg/L. This showed that denitrification was occurring on the site. Parallel results were measurable for the distribution of manganese in groundwater in September 2009. Manganese is the production of manganese dioxide, which is a component of soil. Mn(II) occurs because of the metabolism by microbes [51]. Downstream of WPP, the average manganese concentrations increased (1.51 to 1.93 mg/L). The average iron(III) concentrations were as high as 0.337 to 7.79 mg/L in wells, while background levels were approximately 1.112 mg/L. Iron(III) concentrations can be used as an indicator of anaerobic degradation of the petroleum hydrocarbons at the site. Sulfate can be reduced to sulfide by microbes. The background level of sulfate was 143 mg/L. In wells downstream of WPP (Z1-1, Z6, Z7, Z8, Z11, and Z16), the sulfate concentrations were less than 50 mg/L, lower than the farther wells. Oxidation reduction potential (ORP) indicates that the groundwater environment was weak oxidation to weak restoration (36.4-163.0) in September 2009. ORP was lower downstream of WPP, and the lowest was Z1-1. This illuminates that petroleum contaminated Z1-1 first, and Z1-1 experienced natural attenuation for a long time. ORP decreased from September 2009 to January 2010 (36.81 dropped average) and increased from January to March 2010 (35.01 raised averagely) in some wells (Z1-1, Z6, Z7, Z8, etc.) downstream of WPP. This showed that electron acceptors were depleted, and biodegradation was unable to continue.