Submitted to: Society of Environmental Toxicology and Chemistry Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: May 25, 2000
Publication Date: N/A
Predicting pesticide contamination of groundwater requires knowledge of pesticide sorption and degradation dynamics and hydrological processes within the soil, the subsoil, the lower vadose zone, and groundwater. Although substantially less information exists concerning pesticide degradation in the subsoil and lower vadose zone, it is apparent that the degradation rates of many pesticides decrease substantially in the subsurface compared to rates measured in surface soils. The underlying factors controlling biodegradation in these environments are foremost the microbial communities, and secondly the environmental factors governing the activity of those communities. Subsurface microbial communities are dominated by bacteria, with fungi either absent, or present at very low levels. Bacterial biomass often decreases with depth near the surface, only to increase in sediments near the water table. FAME analysis reveals transitions in the bacterial communities as depth below the surface increases. Compounds that support microbial growth, such as 2,4-D, are often mineralized quickly after lag periods of variable duration. During these lag periods, small populations of degrader microorganisms increase above threshold levels, which produces measurable biodegradation. Other compounds, such as atrazine, are degraded very slowly. Degradation may be accomplished by individual microorganisms or by consortia, resulting in accumulation of metabolites or in mineralization. Rates of chloroacetanilide degradation can be stimulated by additions of secondary carbon sources that support cometabolism.