Dissipation of herbicides in surface soils and subsurface sediment slurries and development of treatment methods

The wide use of pesticides has led to the contamination of soils, subsoils, surface and ground waters. This motivated research on pesticide dissipation and remediation of pesticide contaminated soils and groundwater. The herbicide dissipation was studied in boreal surface soils, subsurface sediment slurries and pilot-scale sediment columns. The sediments were from a groundwater area that was contaminated with atrazine, desethylatrazine (DEA), simazine and 2,6-dichlorobenzamide (BAM). The aim was to enhance dissipation by microbes (indigenous / Pseudomonas ADP), sonication (0, 5, 10, 20 or 30 min, 43 kHz, 320 W) or the following additives: surfactant methyl-β-cyclodextrin, a mixture of zero-valent iron and organic matter (EHC®), peat or a compost-peat-sand (CPS) mixture. Microbes enhanced atrazine (half-lives 57–181 d) and terbutryn (70–291 d) degradation in surface soils. Atrazine was also degraded chemically in four soils (half-lives 120–183 d). Oxygen did not affect atrazine half-lives significantly, while terbutryn was only degraded in aerobic soils. Herbicide dissipation was poorest in the soil that had the lowest amounts of organic matter and nitrogen. Sonication did not decrease the atrazine concentrations, compared with the non-sonicated controls, despite of indications of microbial and chemical degradation. Surfactant did not stimulate atrazine degradation by indigenous sediment microbes, and Pseudomonas ADP degraded atrazine even without surfactant. EHC® enhanced chemical atrazine dissipation under aerobic conditions and, in columns, cleaned atrazine-, DEA- and BAM-contaminated groundwater for about a month. Peat and CPS enhanced the chemical dissipation of atrazine and simazine. Peat also enhanced hexazinone dissipation. Only trace amounts of atrazine, simazine and hexazinone could be extracted from peat and sediment, which indicates that the dissipation was caused by chemical degradation and/or unextractable bound residue formation. In conclusion, the addition of peat, CPS and Pseudomonas ADP were the best approaches for reducing pesticide concentrations. EHC® could also be useful in remediation of small quantities of pesticides, preferably in the presence of oxygen. However, the usability of peat, CPS and EHC® in remediation should be further studied to avoid the possible adverse effects of organic matter additions to drinking water quality.