## 3 Results and Discussion
### Relationship between Sludge-Borne Hydrocarbon and Soil-Borne Hydrocarbon  
It is interesting to note that there was only a loose relationship in TPH between the oily sludge and the underlying surface soil layer for the 16 sampling locations (Figure 6). The differential spatial distribution pattern in TPH between the surface soil layer and the overlying oily sludge may be explained by two reasons. Firstly, the TPH concentration in the sludge was determined by the number of impurities present in the original crude oil and the incorporation of sediments from atmospheric deposition driven by the wind into the sludge. Moreover, the sludge pit received petroleum with different chemical compositions from different spill events and sediments at different amounts, and sludge derived from drilling mud tended to contain fewer petroleum hydrocarbons than that  
Figure 4: Distribution of various (**a**) aliphatic hydrocarbon fractions and (**b**) aromatic hydrocarbon fractions in the surface layer (0–5 cm) of the contaminated soil.  
derived from spilled oil. Secondly, the porosity of the surface soil might vary horizontally, causing horizontal variation in soil-borne TPH.  
Figure 5: Changes in various (**a**) aliphatic hydrocarbon fractions and (**b**) aromatic hydrocarbon fractions along soil profile 16.  
Figure 6: TPH relationship between the sludge and the underlying surface soil layer.  
Further analysis shows that the petroleum hydrocarbon composition differed between the sludge and the underlying soil. On average, the soil had a greater proportion of shorter-chain hydrocarbon fractions (either aliphatics or aromatics) compared to the sludge (Figure 7). This inconsistency reflects differential dissipation rates of the petroleum hydrocarbon fractions between the sludge and the soil. The volatility of petroleum hydrocarbons increased with increasing temperature [24; 25]. The sludge was directly exposed to the atmosphere and received solar radiation. This favored the evaporation of semi-volatile hydrocarbon fractions, as compared to the soil-borne counterparts, which were trapped in soil pores via adsorption by soil particles [26].  
There was a poor relation of the aliphatic fraction of C16-C21 between the sludge and the surface soil layer (R\({}^{2}\) = 0.16) (Figure 8a). Although the relationship improved for the longer-chain hydrocarbon fractions (Figure 8b,c), the R\({}^{2}\) values were all less than 0.6. In comparison with aliphatic hydrocarbons, closer relationships between the sludge and the surface soil layer were observed for the aromatic hydrocarbon fractions. There was also a trend whereby the R\({}^{2}\) increased with an increasing carbon number (Figure 9). Meanwhile, the poor relationship between the sludge and the soil for the semi-volatile shorter-chain hydrocarbon fractions was partially attributed to the differential dissipation rates via volatilization. The same phenomena could not be explained by this factor because the longer-chain hydrocarbon fractions were non-volatile. Therefore, it was likely that various hydrocarbon fractions did not migrate downwards proportionally due to differences in soil conditions between different locations.