Spatial variation in thiamethoxam detections in groundwater resources in Wisconsin's Central Sands vegetable production region

Neonicotinoids are a popular and widely-used class of insecticides whose water-soluble nature and 18-year usage history has led to questions about their accumulation in groundwater resources.  Wisconsin  vegetable  growers  continue  to  rely  heavily  on  neonicotinoid  insecticides  for  the control of damaging populations of key insect pests through a combination of in-ground and foliar applications.  Thiamethoxam, one of the three most widely-used neonicotinoids, has been detected in Wisconsin monitoring wells at concentrations nearing 9.0 µg/L and also in private potable wells where levels have not exceeded 1.61 µg/L.  The reported risks to human health at these levels is considered minor, however previous research has demonstrated quantifiable, sub-lethal effects on native pollinator species when exposed to similar levels in pollen and nectar.  To date, no analysis had been conducted to quantify the spatial distribution or granularity of these detects in central Wisconsin’s groundwater, or to correlate crop use and pesticide inputs with specific well-site detections.  In cooperation with six commercial crop producers, over 50 active irrigation wells in seven discrete clusters (3.5 km min. separation) in the Wisconsin River drainage basin were sampled, and thiamethoxam concentrations were determined using competitive ELISA kits.  Detections ranged from 0.012 µg/L to 1.766 µg/L, and significant between-cluster variation and well-to-well variation was observed, indicating that thiamethoxam is present in Wisconsin groundwater, but the level of contamination varies greatly among sites.  Currently, relationships between these discrete groundwater detections and the USDA NASS annual crop usage patterns are being explored.