An Advection-Diffusion Model for Observed Solid Phase Transport of Roxarsone by Earthworms

The use of organoarsenic acids, such as roxarsone (ROX), as feed supplements in the poultry industry has increased the potential for arsenic to contaminate agriculture fields due to the application of poultry litter as fertilizer. The objective of this research is to quantify solid phase transport of ROX via earthworm bioturbation by calculating the biodiffusivity of arsenic in response to the burrowing activity of Lumbricus terrestris (common earthworm/nightcrawler).

The initial experiment consisted of two columns, 5cm (ID) x 30cm, packed with artificial soil and contaminated with 85 ppm arsenic as ROX at a depth of 10-20cm. One earthworm was introduced to each column. After 30 days, columns were cut into 2-3 cm intervals and sequential extractions on soil sections were completed to determine arsenic distribution. Concentration versus depth profiles show an overall advective movement of maximum concentration below the initial contaminated horizon. Arsenic concentration spread was asymmetrical across the mean concentration. Overall skewness towards the surface is interpreted to be caused by variation in bioturbation activity with depth.

A higher spatial resolution experiment was needed to quantify the solid phase diffusive transport by earthworms. An additional experiment was conducted with four groups consisting of six columns, each were constructed similar to the above; however, the contaminated layer was only 1cm thick at a depth of 5-6cm. The four groups consisted of both watered and non-watered columns, each having a group with and without earthworms to compare the effects of burrowing and leaching. After 45 days, columns were cut into 1cm intervals from 0-16cm and 2cm from 16-30cm depth. Sequential extractions on soil sections were completed to determine arsenic distribution. Normalized concentration versus depth profiles were modeled using a finite-difference solution of the 1-D diffusion equation with advection in MatLab. Advective and diffusive parameters were varied to fit observed profiles.