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Title: Analytical Advection-Dispersion Model for Transport and Plant Uptake of Solutes in the Root Zone

item Skaggs, Todd
item Van Genuchten, Martinus

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2007
Publication Date: 11/4/2007
Citation: Skaggs, T.H., Jarvis, N.J., Pontedeiro, E.M., Van Genuchten, M.T., Cotta, R.M. 2007. Analytical Advection-Dispersion Model for Transport and Plant Uptake of Solutes in the Root Zone. Vadose Zone Journal. Vol 6:890-898

Interpretive Summary: Safe use of biosolids in agriculture requires accurate assessments of their impacts on human health and environmental quality. Comprehensive risk analyses utilize conceptual and mathematical models that predict the environmental fate and transport of biosolids. These comprehensive models are comprised of submodels that describe fate and transport in individual environmental subsystems, such as the atmosphere, root-zone, groundwater, etc. To maintain the complexity of the larger model at a manageable level, the submodels are often highly idealized or simplified; a challenge for researchers is to develop submodels that are simple enough to be used in risk assessments yet maintain the degree of realism needed for accurate risk assessment. In this work, we developed a new root-zone model that predicts the accumulation, leaching, and plant uptake of trace metals that are often a concern in the land application of biosolids. The model has relatively modest data requirements, but is considerably more realistic than root-zone models currently used in risk assessments. As an example, we demonstrated that the new model makes significantly different predictions for the uptake of cadmium by wheat. The new model should be beneficial to scientists and regulators assessing the impacts of land application of biosolids.

Technical Abstract: We develop an advective-dispersive solute transport equation that includes plant uptake of water and solute, and present an analytical solution. Assumptions underlying the transport model include linear solute sorption, first-order plant uptake, and a uniform soil water content. We examine the latter assumption in detail and demonstrate the effects of rooting depth, soil texture, and leaching fraction on the uniformity of the root-zone water content. The new analytical advective-dispersive model should be particularly useful for assessing the transport and uptake of strongly sorbing and persistent solutes, where the time-scale relevant for assessing environmental impacts is long (decades) and short-term fluctuations caused, e.g., by precipitation can be averaged. As an example, model predictions are made for the uptake of cadmium (Cd) by wheat grown in sludge-amended soil. The predictions of the advective-dispersive model are compared with those of a one-compartment model similar to that used in many risk and regulatory studies. The comparison shows that the one-compartment model significantly overestimates the long-term, steady-state Cd concentration in harvested wheat grain, and thus the analytical advective-dispersive model is recommended as the preferred tool for environmental risk assessment of strongly sorbing, persistent solutes.