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

Abstract

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 batch tests, and lower values in the natural water matrices. In comparison, the column tests showed higher K_f values, with higher values in the natural matrices than in the synthetic water matrix.

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