Formatted Title

Evaluation of MFR and ZVI as Treatment Alternatives for the Organochlorine Herbicides Dicamba and Acetochlor

Background/Objectives

A grade crossing incident in South Georgia with a truck-flatbed trailer transporting bulk totes of the agricultural herbicides Engenia^® and Warrant^® resulted in a significant spill at the crossing. Engenia^® (by BASF) is a liquid suspension that contains 3,6-dichloro-2-methoxybenzoic acid (dicamba) at 48.4% by weight. Warrant^® is a liquid suspension that contains 2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide (acetochlor) at a concentration of 33% by weight. Additional substances in the Engenia^® includes N,N-bis-(3-aminopropyl)methylamine salt at 10-15% by weight to reduce volatility of the dicamba by binding with the acid form. Engenia^® is most often applied to crops that are genetically modified for resistance to the herbicidal effect of dicamba. Dicamba was commonly used before the introduction of crops resistant to Roundup’s^® active ingredient, glyphosate. A resurgence in the use of dicamba is underway as weeds have become resistant to Roundup’s^® glyphosate and dicamba resistant crops are becoming more commonly cultivated.

Following the incident, emergency response (ER) services performed a limited cleanup of visibly impacted ballast and soil. An initial assessment of soil and groundwater identified impacts to both media in the release area post-ER. A residential drinking water supply well was identified immediately downgradient to the release. The release area was subsequently listed in Georgia Environmental Protection Division’s (EPD) Hazardous Site Inventory. Georgia EPD has a Type 1 Risk Reduction Standard for dicamba in groundwater of 570 µg/L and in soil of 2.95 mg/kg. No state cleanup standard for acetochlor has been promulgated. EPA’s Regional Screening Level (THQ=0.1) for acetochlor in tapwater is 35 µg/L.

Approach/Activities

Dicamba is volatile, soluble in water, and tends not to sorb to soils. There are very few studies of methods for decomposition or remediation of dicamba. Organochlorine herbicides and pesticides have been successfully treated using zero valent iron (ZVI) and chemical oxidation. To evaluate, comparative bench tests were performed that examined treatment efficacy against dicamba and acetochlor in site matrix materials using modified Fenton’s reagent (MFR, stabilized hydrogen peroxide catalyzed by chelated iron) or ZVI. Efficacy testing of soil/groundwater slurry vessels was performed with aggressive doses of MFR (25 g/kg soil) and ZVI (30 g/kg soil). Groundwater and soil matrix starting concentrations for dicamba were 87.5 g/L and 467 g/kg, respectively. Acetochlor baseline concentrations were 25 g/L and 190 g/kg.

Results/Lessons Learned

Reductions of dicamba in groundwater and soil were 99.9% and 94.7%, respectively, by MFR and 46.3% and 98.8%, respectively, by ZVI. Acetochlor reductions were the inverse in groundwater and soil with ZVI apparently more effective. Reductions in acetochlor were observed to be 96.3% (groundwater) and 80% (soil) with ZVI, compared with 44% (groundwater) and 48.4% (soil) concentration reduction using MFR. These observations prompted subsequent phases of testing exploring combined MFR and ZVI treatments for the soil/groundwater matrix impacted with dicamba and acetochlor.  Phase 2 testing examined the effect of varying dose as ZVI alone and with sulfonated components as an in-situ chemical reduction (ISCR) process, and varying ZVI dose combined with Modified Fenton’s Reagent (MFR) as an ISCR+ISCO process. Based on these results, a final Phase 3 test sought to refine results and examined the use of MFR at 7.5 g/kg and ZVI at 15 or 30 g/kg doses, achieving 70 to nearly 80% reductions in dicamba mass and 60 to 75% reductions in acetochlor mass across soil and groundwater matrices.