The USEPA adapted a risk assessment approach in biosolids regulations that includes the use of an uptake coefficient (UC) (i.e., the ratio of plant concentration to quantity of element added) to determine limitations on selected elemental additions. The nature of the risk assessment requires UCs to be constants. Our hypothesis was that the UC for Cu, Fe, Mo, Ni, P, and Zn for biosolids-amended dryland winter wheat ( Triticum aestivum L.) decreases with multiple biosolids applications at the same location. We applied up to 10 applications to two sites (designated North Bennett A and B) in eastern Colorado at rates from 2.24 to 11.2 Mg ha -1 per application from 1993 to 2013. Results indicated that grain concentrations for all six elements followed no discernible trend as the number of biosolids applications increased. The UC values for these elements compared with the number of biosolids applications followed exponential decay models ( R2 ranged from 0.329 to 0.879). Consequently, UC values will likely not provide constants for risk assessment where multiple biosolids applications are made on the same site. We found that the slope between cumulative elemental removal by grain (kg ha -1) to the cumulative amount of element added with biosolids (kg ha -1) provides a constant over the number of biosolids additions ( R2 ranged from 0.471 to 0.990). As compared with the USEPA approach, our strategy of looking at cumulative changes may provide better estimations of wheat-grain concentrations for risk assessment of biosolids-borne elements.
