Adsorption characteristics of seston in irrigation water: implications for the use of aquatic herbicides

Abstract

Adsorption of three important aquatic herbicides by seston (suspended particulate matter) in water from the Murrumbidgee Irrigation Areas of New South Wales was investigated. Seston was extremely reactive: the adsorption of terbutryn and glyphosate was generally at least an order of magnitude higher than reported in the literature for soils. although adsorption is expected to have little effect in reducing concentrations of terbutryn in practice. Adsorption of glyphosate was decreased by addition of phosphate, which may contribute to the difference in adsorbing power observed for seston from different sources. Since phosphate and glyphosate compete for adsorption sites, a high adsorption maximum for glyphosate may reflect the high capacity of seston to adsorb and transport phosphates. Although several other factors may also be involved, adsorption by seston alone was sufficient to completely inactivate diquat in irrigation drainage waters, at concentrations up to 0.5 mg 1-1. However, once the strong adsorption capacity of seston (about 8 mg g-1) was saturated. an increasing proportion of the initial concentration remained in solution, so this herbicide could be effective if sufficient were used. The general implications of adsorption on the transport, behaviour and persistence of organic compounds and pollutants in the aquatic system are also discussed.