Assessment of Pesticide Contamination in Suburban Drinking Water Wells in Southeastern Wisconsin

Background/Need

The wide use of pesticides for agricultural and residential activities has a significant impact on groundwater quality in the United States. Currently, the occurrence of pesticides in groundwater has been monitored in Wisconsin with an emphasis on agricultural land by multiple agencies. However, limited efforts have focused on monitoring residential (e.g., home and garden) pesticides in groundwater in suburban areas, where a substantial portion of the land is for non-agricultural use. Considering the commonly over use of home and garden pesticides, and the widespread presence of private drinking water wells in suburban areas, the impact of residential pesticide on groundwater quality in suburban areas remains insufficiently evaluated.

Objectives

The primary objective of this project was to evaluate the impact of home and garden pesticides on local groundwater quality in suburban areas in Southeastern Wisconsin. Specifically, we used Milwaukee metropolitan area as the model study area because of the substantial portion of non-agricultural land and the large population in the suburban area that use groundwater as the drinking water source.
Methods: Groundwater samples were collected from 16 active private wells in the Milwaukee metropolitan area: including two in Milwaukee County, four in Ozaukee County, six in Washington County, and four in Waukesha County. These wells were selected primarily based on their location within well-kept, more densely populated, suburb neighborhood away from agricultural fields to ensure that the groundwater collected was representative of residential pesticide application. Four sampling events were performed during June 2019 to February 2020 to capture seasonal dynamics. Groundwater samples were analyzed for seven representative home and garden pesticides, including four herbicides (2,4-dichlorophenoxyacetic acid (2,4-D), methylchlorophenoxypropionic acid (MCPP), dicamba, and 2-methyl-4-chlorophenoxyacetic acid (MCPA)), and three insecticides (carbaryl, malathion, and imidacloprid). These pesticides were selected based on their popular use, relatively high water solubility, low soil affinity and long half-life. The pesticide concentrations in groundwater were determined using liquid-liquid extraction, followed by liquid chromatography tandem mass spectrometry (LC-MS/MS) measurement.

Results and Discussion

One or more of the targeted pesticides have been detected in groundwater in seven of the sixteen wells during the four sampling events. Specifically, there were six and two wells that detected one or more of the targeted pesticides during the June/July 2019 and August 2019 sampling events, respectively. No pesticides were detected in any wells during the November 2019 and February 2020 sampling events. Results suggested a seasonal trend of residential pesticide occurrence in groundwater. The most frequently detected pesticide was 2,4-D, showing up in three separate wells, followed by malathion showing up in two wells; carbaryl, dicamba, imidacloprid, MCPA, and MCPP each making an appearance once. The concentrations of all detected pesticides were below any known groundwater standards, indicating a relatively low risk of residential pesticide contamination in private drinking water wells.

Conclusions/Implications/Recommendations

This study suggested that groundwater may be more susceptible to residential pesticide contamination during the late spring and early summer months. This time frame is when homeowners and professional lawn care companies apply the most pesticides to lawns. Recharge into the local groundwater also typically takes place during this time. However, this study did not observe any strong correlation or trend between the hydrology, chemical properties and/or chemical application to the wells. Although the severity and frequency of detection does not compare to those done in an agricultural setting, testing for residential pesticides should continue to be monitored for historical trends and potential health-based implications. Specifically, long-term monitoring activities are recommended to fully evaluate the seasonal variability of residential pesticide occurrence in groundwater.