Trends and Transformation of Nutrients and Pesticides In a Coastal Plain Aquifer System, United States

The distribution of redox conditions as well as usage of agricultural chemicals affect the transport of nutrients and pesticides in ground water to different degrees at four local-scale sites with similar corn and soybean agriculture but different hydrogeologic setting in the Northern Atlantic Coastal Plain. Settings ranged from predominantly well-drained soils overlying thick, sandy surficial aquifers to predominantly poorly drained soils with complex aquifer stratigraphy and high organic-matter content. Agricultural chemicals were the predominant dissolved ions in ground water at all sites. Specific conductance was strongly correlated with reconstructed nitrate (the sum of nitrogen in nitrate and nitrogen gas) (R-squared = 0.81, p <.0001) and is indicative of the relative degree of agricultural effects on ground water. Trends in nitrate are primarily related to changes in manure and fertilizer use over time at the well-drained sites where aerobic conditions are present throughout the unconfined surficial aquifer system. Reducing conditions in ground water affect nitrate concentrations at the poorly drained sites, where nitrate is present in young ground water but completely removed over time through denitrification. The presence and concentrations of atrazine, metolachlor and some of their common degradates were related to soil and aquifer characteristics. Median concentrations were higher at well-drained sites than at the poorly drained sites with concentrations of degradates generally higher than those of the parent compounds at all sites. The percentage of deethylatrazine to total atrazine increased over time at one well-drained site, indicating a greater degree of atrazine degradation in older water. This trend was not evident at the other three sites. These results show the importance of understanding the potential effects of hydrogeologic setting as well as chemical usage in studies of the transport and transformation of agricultural chemicals in a Coastal Plain setting.