Neonicotinoid Contaminants in Wisconsin Groundwater: Relationships to Landscape Cropping Systems

Background: Neonicotinoids are a popular and widely-used class of insecticides whose water-soluble nature and 20-year usage history has led to questions about their potential to accumulate in the environment and harm local ecosystems. Over 6.7 million pounds of neonicotinoid insecticides are now applied annually on 140 different crops in the United States, with the three most popular compounds, imidacloprid (IMD), clothianidin (CLO), and thiamethoxam (TMX), making up over 90% of agricultural usage nationally and generating over $4.6 billion in market activity in 2013. Neonicotinoid usage in the United States remained below 500,000 pounds per year until 2003, when the expansion of crop registrations and the introduction of additional active ingredients led to a rapid increase in total usage. Virtually all corn and soybean seeds planted in the United States are now treated with either IMD or TMX seed treatments intended to protect the developing seedlings from early-season pests. This heavy usage, combined with the water-soluble nature of neonicotinoids and their potential to harm beneficial wildlife, has brought their environmental fate (e.g., the life cycle of a chemical) into sharp focus. Contamination of surface and groundwater specifically, may occur from major agricultural sources such as spray drift during application, deposition of contaminated dusts released during drilling of treated seeds, surface runoff and leaching, greenhouse runoff, as well as human error or irresponsibility, sewer and storm water drainage, and residential usage. High water solubility and long environmental persistence times contribute to the potential for these compounds to migrate through the soil column and contaminate groundwater-fed streams, the consequences of which for aquatic invertebrates remain unknown.

Objectives: Objective 1: Utilize a calibrated groundwater flow model developed for the Central Sands Region to delineate groundwater-contributing areas to streams and relate local landscape compositions and associated detections of neonicotinoid insecticides. Objective 2: Refine our understanding of the spatial and temporal variations in groundwater and base flow along discrete sections of Fourteen Mile Creek relating groundwater flow to surface water insecticide detection frequency and concentrations.

Conclusions/Implications/Recommendations: This study demonstrated how groundwater models can aid in the delineation of groundwater contributing areas to surface water features and provide an approach for evaluating relationships between land-use activities and water quality. In this study, evaluations were performed for a series of repeat stream-monitoring locations, with corresponding delineated groundwater contributing areas of anywhere from 1 to over 50 square miles across the Central Sands region. Similarly, this same approach could be applied by researchers and regional stakeholders as a means of interrogating groundwater and surface water dynamics, water quality conditions, and land-use activity at a variety of scales. Model output provides a means to 1) field validate the presence of gaining and losing stream sections with a watershed, 2) improve our conceptual models of how groundwater and surface water interact, and 3) help practitioners design better monitoring of streamflow, stream water quality, groundwater quality, and overall watershed health.

Observations made along Leola Ditch were collected during dry, wet, and snowmelt conditions and suggest that nitrate and neonicotinoid concentrations in surface water may represent time-weighted, spatially averaged concentrations of groundwater from across the entire contributing area, which gradually discharge to surface water over time. The stream, in this case, integrates groundwater quality across the contributing area and may provide a convenient way to track groundwater quality over time. If true, long-term synoptic surveys of stream water quality (e.g., annual surveys over decades) could provide an approach for tracking changes in the overall quality of groundwater within the corresponding contributing areas. Taken together these techniques could help watershed managers looking to monitor long-term stream and groundwater quality and water resource technicians looking to optimize the design of watershed-scale sampling and monitoring programs.