Assessment of the retardation of selected herbicides onto Danube sediment based on small column tests

Purpose This study utilizes column tests to investigate the retardation of certain herbicides with different hydrophobicities (atrazine, alachlor and trifluralin) during transport through surface Danube sediment. The influence of water matrix on the retardation factor R d and Freundlich constant K f is investigated. The results are compared with batch tests to establish whether different methodologies result in similar or different conclusions. Materials and methods A stainless steel column was filled with natural Danube sediment. Three water matrices were investigated: synthetic, Danube surface, and deep groundwater rich in natural organic matter (NOM). The goal was to examine whether different water matrices would result in changes in the R d and corresponding K f values. After a tracer experiment, single herbicide solutions were tested in the three water matrices. Herbicides were analyzed by gas chromatography coupled with electron capture detector (GC/ECD). Retardation factors obtained in the column experiments were calculated using Transmod software (version 2.2). The K f values calculated were compared with the values obtained in previous batch experiments. Results and discussion A breakthrough curve (BTC) for trifluralin could not be obtained during the experiment. Atrazine R d values were almost the same in the natural matrices (54 and 55 for the ground and surface waters, respectively), and lower in the synthetic water (40). Alachlor R d values in the three water matrices were very similar (30–35). The corresponding K f values for alachlor (8.47–17.4) were lower than those of atrazine (13.5–27.9). These results differ from those obtained by earlier batch tests, which showed similar K f values for both atrazine (4.4–9.2) and alachlor (4.43–10.35) in all three matrices. In contrast to the results observed during the batch tests, the column tests exhibited higher K f values in the natural water matrices than the synthetic water, possibly due to the influence of dissolved organic carbon on herbicide sorption. Conclusions Of the three herbicides investigated, the smallest retardation was observed for alachlor. This was unexpected given the relative hydrophobicities of alachlor and atrazine. The potential risk of transport through the sediment may therefore be greater for alachlor than the other two herbicides. This was indicated neither by the batch tests nor from the K oc – K ow estimations. Both herbicides exhibited similar K d and K f values in the