Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/1997
Publication Date: N/A
Interpretive Summary: Herbicides are widely used in Midwestern agriculture and are potential contaminants of groundwater. The soil and subsoil act as a natural barrier to prevent herbicide movement from the soil surface into groundwater. Herbicides can bind to the soil particles in a process called sorption and are degraded into non-toxic forms by soil microorganisms. This research was conducted to determine the ability of microorganisms living in the subsoil and aquifer to degrade the herbicide alachlor. We removed samples of subsoil and aquifer material from a field site near Aurora, SD, which overlays the Big Sioux Aquifer, added alachlor and measured the time required for the degradation under different conditions. Microorganisms were able to slowly degrade alachlor in the unsaturated and saturated aquifer sediments. Additions of soluble organic carbon stimulated the microbial community and increased alachlor degradation substantially. Analysis of the metabolites in the samples showed that an energy-requiring microbial enzyme system is responsible for some of the degradative activity. Alachlor was less strongly sorbed in the aquifer sediments than surface soils. This knowledge will be used in the design of well protection strategies which will allow efficient weed control and agricultural production while protecting the Nation's groundwater.
Technical Abstract: Degradation and sorption of alachlor, two processes that control its fate in soil, were determined in a Brandt silty clay loam (fine-silty over sandy or sandy skeletal, mixed Pachic Udic Haploborall) soil and subsurface sediments (2C horizon materials from the capillary fringe and saturated zones of the aquifer) collected from a shallow Big Sioux aquifer site near Aurora, SD. Alachlor sorption to surface soil was about 13 times greater than sorption to sediments <4-mm in size. About 22% of the sorbed alachlor desorbed from the surface soil, whereas none desorbed from the sediments. About 2.5% of the phenyl ring of alachlor mineralized in surface soil after 112 days at 10 C. Ring mineralization was about 0.7% and 2.0% in the capillary fringe and saturated zone sediments, respectively, when carbon (added at 1.3 ug-C g**-1 soil as lyophilized algae biomass) was added. Polar metabolites of alachlor were detected in methanol/water extracts from soil and aquifer sediments with the greatest amounts in C-amended treatments. These data indicate that bacterial populations in the Big Sioux aquifer are sufficiently diverse to degrade complex organic molecules. Carbon was the factor most limiting alachlor metabolism in subsurface sediments.