Predictive Modeling of Polyacrylamide (PAM) Transport and Fate in Irrigation Canals - L. Chen, J. Zhu and M.H. Young

As soon as PAM is released into the irrigation canal water, a series of complex physical, chemical and biological processes take place immediately. These inter-related processes significantly impact the partitioning of PAM, the fate of PAM in the environment, as well as the functioning of PAM as a seepage reduction agent. To monitor its fate and therefore safeguard especially the aquatic environment, and optimize its effect of reducing water seepage loss into the unlined canal bottom, it is critical to quantify PAM processes after its release into irrigation water. Continuous field monitoring is usually not cost-effective and practically not feasible. This study seeks to predict PAM behavior in the aquatic environment through numerically quantifying important processes and therefore provide an effective, efficient and economic alternative for this purpose.
There are a number of important processes that dictate PAM fate. We shall first briefly describe these processes in the following.
• Dissolution: PAM is usually sprinkled into the moving irrigation canal water in the form of powder. It gradually dissolves in the water. Only dissolved portion of PAM can settle to the canal bottom and be effective in reducing the seepage loss.
• Advection: The most significant process that controls PAM transport in water is advection, in which flowing water carries both solid and liquid forms of PAM along the irrigation canals.
• Dispersion: Dispersion is caused by longitudinal velocity shear. As a result of the non-uniform velocity distribution along the cross-section, the solute spread is faster at high velocity zone, leading to greater separation of the solute mass than by diffusion. Molecular and turbulence diffusion are usually merged into dispersion in modeling studies.
• Flocculation and Settling: If suspended sediment exists in the flowing water, liquid phase PAM will enhance the flocculation of very fine sediment and result in flocs of PAM and sediment mixture, which will settle in the canal bottom rather quickly. The implications of floc settling are two-fold. On one hand, it reduces PAM mass from being transported further in water phase. On the other hand, this helps seal the unlined canal bottom and therefore reduces water seepage loss.
• Other Mechanisms for PAM Mass Change: Other processes that control the total PAM mass in the canal water include infiltration of dissolved PAM into the bottom soil, resuspension of settled PAM back into the water, possible adhesion of liquid phase PAM onto the bounding solid interfaces, and degradation of PAM due to physical or biological impacts.