This is not unexpected since humic acid components of organic carbon matter are known to undergo ionic complexation with clay mineral ion-exchange sites. This leads to a diminution in the number of mineral surface reactive sites and to more restricted access to the sites still available for complexation. Nevertheless, it is clear that naturally occurring clay mineral surfaces still retain the capacity for significant sorption of moderately hydrophilic herbicides such as atrazine and alachlor. Therefore, when modeling movement of these herbicides in groundwater systems, it is important to incorporate sorption data for both organic carbon and clay mineral surface content. For example, retardation of atrazine and alachlor in the present subsoil calculated by ignoring sorption to the mineral portion are low by factors of 2.6 and 2.8.
Actual retardation is about three times slower because of mineral sorption.
This is important when trying to determine the extent and time course of groundwater contamination resulting from herbicide usage. The project was supported in part by General Purpose Revenue Funds of the State of Wisconsin through the University of Wisconsin System.