Pesticide Contamination in the Potomac Watershed

Samuel A. Miller, Kaycee E. Faunce, Larry B. Barber, Jacob A. Fleck, Daniel W. Burns, Jeramy R. Jasmann, Michelle L. Hladik, Factors contributing to pesticide contamination in riverine systems: The role of wastewater and landscape sources, Science of The Total Environment, Volume 954,2024, 174939, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2024.174939.

The article below is excerpted from the research article cited above.

The Potomac River Watershed is the second largest watershed (36,750 km²) within the Chesapeake Bay watershed and contributes about 15 % of the total streamflow to the Bay. The mainstem of the Potomac River has minimal flow regulation and is freshwater and the major source of drinking water for the  Washington, D.C. metropolitan area- Loudoun Water, Fairfax Water, Leesburg Water, WSSC and the Washington Aqueduct.

 Population in the Potomac River Watershed more than doubled from 1970 (3.2 million) to 2020 (6.9 million). The majority of development during this period occurred in the Washington, D.C. metropolitan area, where about 84 % of the watershed’s population resided as of 2020. To accommodate population growth, forest and agricultural lands have been reduced and public water supply and wastewater treatment systems have been expanded drawing on the Potomac River itself. A majority of public-supply withdrawals (80 %) in the Potomac River Watershed are from surface water.

The Potomac River Watershed contains a varied landscape consisting of steep mountains, rolling hills, broad valleys, and plains. The majority (53 %) of the basin is forested, 23 % is agricultural, and 13 % is developed whereas smaller remaining portions contain water, wetlands, and barren land cover. 

According to Miller et al wastewater discharges, although treated and in compliance with existing regulations, can be a continuous source of organic contaminants, including pesticides, to the Potomac River and  rivers worldwide. Pesticides can be introduced to wastewater treatment plants by what goes down-the-drain or flushed down the toilet at homes and businesses. Urban and suburban stormwater enter the watershed and may include sources of pesticides from residential lawns and gardens, leaky septic systems, or outdoor pest control, including applications to impervious surfaces. Separate sanitary sewer systems can also contain water sourced from groundwater via infiltration through cracked or broken sewerage pipes.

Frequently, pesticides are transformed or not completely removed during conventional wastewater treatment which are not designed to address these contaminants and are discharged in the wastewater treatment plant effluent to receiving waterbodies. In addition, some wastewater treatment plants receive industrial discharges. Although pesticides are frequently introduced to aquatic environments from agricultural and developed land-use nonpoint sources, wastewater treatment plant-derived pesticides are of particular concern because they are continuously discharged to receiving waters that often serve as source water for drinking water plants. Discharge from the largest wastewater treatment plant in the study area, UOSA,  is one of the largest indirect potable water reuse sources in the world (Jeffrey et al., 2022).

An integrated model was developed for the Potomac River watershed to determine the amount of accumulated wastewater percentage of streamflow and calculate predicted environmental concentrations (PECs) for 14 pesticides in non-tidal National Hydrography Dataset Plus Version 2.1 stream segments. Model predicted environmental concentrations were compared to measured environmental concentrations from 32 stream sites that represented a range of land use to evaluate model performance and to assess possible non-WWTP loading sources. Statistical agreement between PECs and MECs was moderate- it was strongest for insecticides, followed by fungicides and herbicides.

Among the target pesticides, PECs generally had the best agreement with MECs (Fig. 6) for insecticides (ρ = 0.52, p < 0.01), followed by fungicides (ρ = 0.36, p < 0.01) and herbicides (ρ = 0.20, p = 0.14). Individual pesticides with significant correlations between PECs and MECs included dinotefuran (ρ = 0.87, p < 0.01), thiabendazole (ρ = 0.82, p < 0.01), fipronil (ρ = 0.61, p < 0.01), and tebuconazole (ρ = 0.53, p = 0.02). Significant correlations were found between these individual pesticide concentrations and water quality indicators of wastewater presence including the optical brightener-to-fluorescein ratio, and boron concentrations, confirming that wastewater treatment plants effluent likely is a source for these pesticides.

Target pesticide corrected-MECs that had the most significant correlation with estimated agricultural use included atrazine (ρ = 0.54, p < 0.01), clothianidin (ρ = 0.35, p = 0.05), metolachlor (ρ = 0.62, p < 0.01), and simazine (ρ = 0.70, p < 0.01). The lack of agreement between corrected-MECs and estimated pesticide use on agricultural land for the remaining target pesticides indicates there may be sources beyond wastewater treatment plant effluent and agriculture during stream base flow conditions.

Pesticides are commonly used within the Potomac River Watershed for non-agricultural purposes, such as: (1) maintenance of lawns, gardens, and golf courses, (2) defoliation of rights-of-way, and (3) structural pest control (Ator et al., 1998). Pesticide usage rates are not well documented for these categories, which makes source identification difficult.

The U.S. Environmental Protection Agency estimated in 2012 that more money was spent on insecticides used in home and gardens ($2,650,000,000) than in agriculture ($1,499,000,000) and industry ($700,000,000) combined, and overall, 27 million kilograms of pesticides were applied to home and garden areas (Atwood and Paisley-Jones, 2017). Homeowners frequently apply high rates of pesticides beyond the recommended doses that unintentionally leave residues that pose a variety of human and ecological health threats (Md Meftaul et al., 2020).

Water-quality samples were collected from 32 streams within the Potomac River Watershed during low-flow conditions to represent periods with the greatest effects from ACCWW and legacy pesticide sources. In the study watersheds,  turfgrass percentage of drainage area was found to be significantly correlated with all target fungicides, insecticides, and diuron. This study did not assess variability in results over time, which may be important at some sites based on local conditions. At best this model could be used as a screening tool and not much more.