Integrative Monitoring of Neonicotinoid Insecticides in Baseflow-Dominated Streams on the Wisconsin Central Sand Plain

Background/need: Neonicotinoid insecticides are used widely across the Wisconsin Central Sand Plain (WCSP) to control various pests that inflict damage to crops such as potatoes, corn and soybeans. They are often used as seed coatings, but also sprayed upon foliage, or applied as granules in furrows. Neonicotinoids are systemic insecticides that are considered very water soluble and mobile in groundwater. Coarse sandy soils on the WCSP, a shallow water table, and irrigation increase the propensity for leaching of water-soluble pesticides and nutrients from soil to groundwater. The WCSP has many natural streams that are often connected by drainage ditches to lower the water table for agricultural purposes. These streams and ditches are primarily fed by groundwater discharge and supply a cold-water habitat for aquatic life. The US Environmental Protection Agency (EPA) has adopted acute and chronic aquatic life benchmarks (ALB) for neonicotinoids to aquatic invertebrates, plants, and fish. Of the neonicotinoids targeted, imidacloprid has the most stringent EPA ALB for invertebrates. Acute exposure criteria for aquatic invertebrates often relate to a 48-hour exposure period while chronic assessment criteria are often set at 7-40 days, depending upon species. As a result, traditional grab sampling may not provide an accurate representation of water quality over an extended period which is necessary to address chronic criteria. An evaluation of a passive sampling device to assess chronic toxicity criteria is included in this study.

Objectives: Phase One of this study was conducted in 2019 – to assess the presence and concentrations of five neonicotinoid pesticides in twenty streams and ditches in the WCSP using both grab sampling and polar organic compound integrative samplers (POCIS). The results were compared with land use in the watersheds.

Phase Two of this study began in late spring of 2020 in one watershed to evaluate seasonal changes in surface water quality for a period of 12 months. POCIS were deployed at ten sites in the Tenmile Creek – South Branch watershed for 30-day periods. Hyporheic water was collected four times, at three locations using mini-piezometers at depths ranging from 30-60 cm across the width of the stream.

Methods: Study sites were selected to survey a wide range of land use activities, position in the watershed, and both on the east and west side of the surface water divide on the WCSP. Surface water grab samples were collected in one-liter amber glass and stored in a refrigerator at 4^oC until the time of analysis. Samples were extracted using solid phase extraction techniques and analyzed by liquid chromatography/mass spectrometry. Similarly, POCIS were disassembled, and the sorption media removed, extracted, and analyzed by LC/MS.

Conclusions from Phase One: Imidacloprid and clothianidin were commonly detected and concentrations often exceeded the US EPA chronic aquatic life benchmarks for invertebrates. Thiamethoxam was frequently detected but usually at concentrations below the chronic aquatic life benchmark. Dinotefuran was infrequently detected and at concentrations near detection limits. Acetamiprid was not detected. Neonicotinoid concentrations in surface water were generally higher in streams and ditches west of the surface/groundwater divide. These areas west of the divide are dominated by irrigated agriculture with ditches connecting natural surface water drainages. Samples collected west of the divide were from both natural streams and ditches. Samples collected east of the divide were all from natural streams.

Surface water grab samples were collected at the time of deployment of POCIS and again upon retrieval approximately 30 days later. Comparisons were made between POCIS-derived TWA concentrations of the three commonly detected neonicotinoid insecticides and surface water grab samples at 20 locations. The average grab sample concentrations (n=2) varied linearly with the POCIS-derived concentration (n=1). Linear regressions between POCIS and grab samples had an r2 of: imidacloprid = 0.93, clothianidin = 0.96, and thiamethoxam = 0.95.

Neonicotinoid concentrations increased concomitant with agricultural land cover in the groundwater contributing areas for the stream locations. Because neonicotinoids have only been used for the last 25 years, a linear regression model for neonicotinoid concentrations was developed using only the agricultural land cover in the groundwater contributing areas within 10- and 20-year travel time of the stream. The regression model was able to explain about 60% of the variation in neonicotinoid concentrations. That model was used to project future neonicotinoid concentrations by accounting for the percentage of agricultural land in the groundwater contributing area that would be delivering neonicotinoids to the stream. That model suggests that concentrations may double or triple at some of the stream locations over the next 40 years.

Conclusions from Phase Two: During the 12 months of continual monitoring, TWA neonicotinoid insecticide concentrations were relatively stable for clothianidin, imidacloprid and thiamethoxam in surface water of the South Branch of Tenmile Creek. The standard deviation of 12 monthly measurements at 10 sites ranged from less than 1.0 to 21 ng/L. Hyporheic water collected from the streambed had neonicotinoid insecticide concentrations that varied widely consistent with neonicotinoids being present in only a fraction of the total groundwater flowpaths because of travel time and land cover variations.

The agreement between POCIS time-weighted concentrations and grab samples indicate that both provide reliable ways to measure neonicotinoid concentrations in these baseflow-dominated streams. The sensitivity of analytical methods is sufficient to evaluate concentrations of the most abundant neonicotinoids with grab sampling. While POCIS provide a temporally weighted concentration that can incorporate variations during the deployment period, for baseflow dominated streams such as those in the WCSP, those variations are likely small and conventional grab sampling at low flow conditions provides an assessment of the typical concentrations that are likely to be encountered by aquatic biota.