## 3 Results and Discussion
### Three-Dimensional Diffusion Models  
On the sides of national highways or near the gas stations, they are the main sites of pure petroleum pollution, and they are usually quite spacious and environmentally similar within the same place, then assuming that the diesel oil can permeate freely. Then, in the next set of experiment, 5.3 g diesel oil was quickly dropped on three soil samples with gravimetric moisture of 0%, 5%, and 10% by a pipette, and the density of the soils is about 2 g per cubic centimeter. Then we kept the sealed beaker in the shade for 15 days, and observed its diffusion every day. After 7 days later, the variation of the diesel diffusion could not be detected by LIF. After 15 days of standing, the LIF spectra of three-dimensional distributions of diesel oil in three different soil samples were detected. The intervals of the longitudinal penetration depth detection were 1 cm, and the fluorescence peak intensities of diesel oil versus the longitudinal penetration depths down the center point are shown in Figure 5.  
Figure 4: The calibration curve between LIF intensity and diesel oil concentrations. Figure 3: The fluorescence spectra of diesel oil in different soil moisture. (**a**) The fluorescence spectra; (**b**) the change regularity of the fluorescence peak intensity. Figure 5 indicates that the diesel oil deposits in 0-6 cm below the surface of the soil. When the soil moisture is 0%, the fluorescence intensity of diesel oil increases with increasing soil depth, but after 2 cm depth later, the fluorescence intensity of diesel oil gradually decreases with depth. When the soil moisture is 5% or 10%, the fluorescence intensity of diesel oil gradually decreases with increasing soil depth. Then the fluorescence intensities were taken into the calibration curve, and the concentrations of diesel oil at different depths were calculated. As shown in Figure 6, when the soil moisture is 0%, the concentrations of diesel oil in soils versus the longitudinal penetration depths are conformed to quadratic law, but after 2 cm depth later, the concentration of diesel oil gradually decreases with depth. When the soil moisture is 5% or 10%, the concentration of diesel oil gradually decreases with the penetration depth increasing and a nearly linear relationship exists between the concentrations of diesel oil and the longitudinal penetration depths. The square correlation coefficients of the calibration curves are all above 0.95, which implies that the diesel oil accumulation in soils decreases linearly with the penetration depth increasing, and the longitudinal penetration depth decreases with the increasing of soil of humidity. The intervals of the lateral diffusion detection were 1 cm, and the fluorescence peak intensities of diesel oil versus the lateral diffusion breadths outward from the central point in different humidity soils are shown in Figure 7.  
As shown in Figure 7, we can detect the fluorescence of diesel oil 5 cm away from the central point of the pollution source. Then the fluorescence intensities were taken into the calibration curve, and the concentrations of diesel oil at different breadths were calculated. Figure 8 indicates that there are good linear relationships between the concentrations of diesel oil and the lateral diffusion breadths away from the central point of the pollution source in different humidity soils. The square correlation coefficients of the calibration curves are all above 0.98, which implies that the amounts of the lateral diffusion of diesel oil in soils decreases linearly with the transverse diffusion distance increasing. As shown in Figure 8, we can observe that the lateral diffusion breadth increases with the increasing of soil humidity. Figure 5: The fluorescence intensity of diesel oil in different longitudinal penetration depths. Figure 6: The concentrations of diesel oil in different longitudinal penetration depths.