Phenoxy herbicides’ interactions with river bottom sediments

Abstract

MCPA (2-methyl-4-chlorophenoxy acetic acid) and 2,4-D (2,4-dichlorophenoxy acetic acid) have a relatively high water solubility (> 20,000 mg L−1 at 20 °C) and a few studies have examined the affinity of these herbicides for river sediments. The aim of this study was to evaluate whether the concentrations of MCPA and 2,4-D quantified in bottom sediments were associated with the characteristics and herbicide sorption-desorption parameters determined for these sediments. Sixty surface bottom sediments samples (15 cm3) and water column samples (1 L) were collected from 12 sampling sites distributed across selected rivers in a prairie province of Canada, with each site being sampled four to seven times during the summer 2016. The concentrations of MCPA and 2,4-D in the sediment and water column samples were quantified by a gas chromatography coupled to triple quadrupole mass spectrometry. Sediment characteristics included determinations of the organic carbon content (OC), the percentages of sand, silt, and clay by the pipette method, as well as detailed particle size distributions (PSD) as determined using a laser diffraction particle size analyzer. Sorption of 2,4-D and MCPA by sediments was determined using standard batch equilibrium method. Sediments with larger OC contents had relatively larger sorption and smaller desorption when their unimodal PSD had a narrow range (1 to 60 μm), but relatively smaller sorption and larger desorption when their unimodal PSD had a broader range (1 to 1000 μm) thus coarser particle sizes. Sediments with smaller OC contents always had relatively smaller sorption and larger desorption. The detection frequencies and concentrations in sediments were substantially greater for MCPA than 2,4-D even though batch-equilibrium experiments showed that sediments sorbed significantly less MCPA than 2,4-D. Neither MCPA nor 2,4-D concentrations detected in sediments were significantly correlated with sediment properties or their sorption-desorption characteristics. However, the detection frequencies and concentrations in water column samples were also substantially greater for MCPA than 2,4-D. Relatively to 2,4-D, MCPA was more frequently detected in the sediments and in greater concentrations because the more frequent presence of MCPA in the water column allowed for greater opportunities for MCPA to partition to sediments. Thus, the water column loadings of MCPA and 2,4-D, and not sediment characteristics, are the driving force for determining their frequency and concentrations in sediments.