Experiments using packed soil columns with an artificial macropore and undisturbed soil columns were conducted to measure the biodegradation of 2, 4-D. The packed columns were 7.0 cm diameter, 10 cm long, and packed with silt-loam (< 1.0 mm fraction) and had a 0.8 cm diameter cylindrical artificial macropore. The undisturbed columns were 10.0 cm diameter, about 8 cm long, and consisted of a silty clay with earthworm burrows. The undisturbed columns were collected from a farm near Cornell University. A 50 mg/L solution of 2, 4-D solution was continuously applied at 8.2 cm/d for packed columns and 5.5 cm/d for undisturbed soil columns to the surface of each column for 480 hours and breakthrough curves indicating the loss of the pesticide in the effluent was obtained from both the macropore and matrix regions of the soil columns. In the packed soil columns with the artificial macropore, the biodegradation rate of 2, 4-D in the matrix region was initially higher than that for the macropore region (see Fig. 1). However, with time, the biodegradation rate in the macropore region increased and surpassed that for the matrix region, presumably due to the increased microbial activity. The matrix and macropore regions had a larger difference in the rate of 2, 4-D mass biodegraded when flow through the macropore was increased (see Fig. 2). Complete loss of the 2, 4-D in both flow paths was observed after continuous application for 400 hours and biodegradation to non-detectable levels was also observed in both regions of the undisturbed soil columns.

Figure 1. Illustrates that 2, 4-D mass loss is greater as the solute flux increases. The 2, 4-D flux through Packed Column #2 was greater than Packed column #1 and the fluxes in the macropores was greater than that in the matrix.

Figure 2. 2,4-D biodegradation curves for packed soil columns with artificial macropores.

During the early phase of each experiment, the biodegradation rate in the macropore region was lower than the matrix region and is probably due to the low initial population of 2, 4-D degraders throughout the soil and the slow movement of solutes in the matrix allowed more biodegradation at low microbial populations, whereas the rapid movement of the solute was not conducive to immediate biodegradation. However, as the microbial populations increased with time in the macropores, presumably as a result of increased oxygen availability and greater mass of substrate passing through, the biodegradation rate of 2, 4-D in the macropore region exceeded the rates in the matrix regions.

Fig. 3 shows the observed break-through curves for a packed soil column for matrix flow and the predicted break-through curves that would be obtained using the CDE for a range of first-order decay coefficients obtained from the literature. Clearly, the predicted break-through curves do not appear similar to the observed break-through curve and neither predict the peak concentration nor eventual loss of all 2, 4-D. This indicates that in cases of continuous application of solutes, such as slow, steady leaks from chemical storage tanks, which allow microbial populations to increase significantly, the CDE cannot be used with first-order decay coefficients to accurately predict the peak concentration nor the total mass delivered to groundwater.

Figure 3. Comparison of packed columns #1 matrix BTC to BTC's predicted using the CDE with first-order decay coefficients, k (t-1). Theoretical curves for k = 0, 0.0021, 0.1386, and 0.1783 are shown.

These studies show that macropores and channels containing highly conductive soil may provide better conditions for biodegradation of pesticides. Microbial growth may occur along the entire length of the column and would result in microbial activity deeper in the soil profile. The courser material in the channels might allow for a thicker mass of micro-organisms to grow. However, the fast flow may also have adverse effects because cells might not be able to utilise all of the substrate that rapidly moves past and might also be detached with the flow. Also, the walls of the macropore may only allow the development of a thin film of micro-organisms. Another apparent beneficial effect of macropores is that they may permit a greater diffusion of oxygen in the surrounding soil matrix at greater depth and this increased aeration may contribute to higher biodegradation rates.