Three types of columns were used in two different experiments to determine the influence of macropores and preferred channels on biodegradation of p-nitrophenol (PNP). The first set of experiments used one column that contained a single vertical artificial macropore with a diameter of 0.7 mm. The column was 5 cm in diameter and length. The macropore was an open cylindrical channel embedded in a homogeneous silt-loam soil with a < 0.1 mm pore fraction. Separate fibreglass wicks (0.6 mm diameter, 50 cm length) where used to sample the effluent near the macropore and in the surrounding soil matrix. The wicks were checked and found not to sorb organic chemicals. The biodegradation of PNP was tested using two different influent rates; a high application rate of 10 mg/L at 36 cm/day for the entire column resulting in apparent Darcy velocities of 2.5 cm/h for the macropore region and 1.3 cm/h for the matrix region. After 10 days the same column was subjected to a lower application rate of 10 mg/L at 16 cm/day, resulting in apparent Darcy velocities of 0.5 cm/h for the macropore region and 0.7 cm/h for the matrix region.

The lower macropore velocity was attributed to the lack of preferential flow near the macropore due to the lower application rate. Also, microbial acclimation for PNP biodegradation was tested at two temperatures, 25oC and 21oC.

The break-through curves for the matrix and macropore region is shown in Fig. 1 for high and low application rates of PNP. For the high application rate, the breakthrough curve for the macropore region occurred slightly before the matrix region and the peak concentration was obtained several hours earlier. For the low application rate, preferential flow did not occur. However, in the vicinity of the macropore, effluent was collected. Again the peak concentration of PNP was slightly higher than the matrix region, even though preferential flow was not generated.

Figure 1. PNP breakthrough curves in macropore and matrix regions.

Biodegradation of PNP in the matrix and macropore region is illustrated in Fig. 2. For the high application rate, the biodegradation rate increased faster in the macropore region compared with the matrix region. However, the macropore region required greater time until PNP was no longer present due to the higher flux. The importance of temperature is suggested by the higher peak PNP concentration for the low temperature (21oC). Presumably, the initial growth rate of microbes was slower at low temperature, allowing an initially higher concentration of PNP to pass through. At the high flow rate and high temperature, the biodegradation rates are greater and increase more rapidly than at low flow rates and temperatures.

A second set of experiments were conducted on paired sets of homogeneous and heterogeneous soil columns (4.1 cm diameter, 15 cm length). The homogeneous columns consisted of < 1.0 mm pore-fraction silt-loam. The heterogeneous columns contained a vertical channel of higher conductivity soil (1.7 - 2.0 mm fraction) surrounded by a soil matrix of the lower conductivity soil (0.6 to 1.0 mm fraction). Several different water velocities were used. Once again, the biodegradation rate was more rapid in columns with the channels than in the homogeneous soil columns. As the flow rate increased, the difference in biodegradation rates between the heterogeneous and homogeneous columns was more pronounced.

Figure 2. Biodegradation rates of PNP in macropore and matrix regions.