Occurrence of toxic contaminant mixtures in surface water and groundwater in agricultural watersheds of the Chesapeake Bay

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The widespread use of pesticides and fertilizers, application of biosolids and manure, and large-scale animal feeding operations result in contaminant mixtures occurring in streams and rivers (figure 1). These nonpoint sources are affected by multiple processes (such as stream discharge, seasonality and management practices) that influence contaminant occurrence in surface and groundwater water. Exposure to these contaminant mixtures can negatively affect fish and wildlife and their habitats. The Chesapeake Bay Program (CBP) has a goal to reduce the effects of toxic contaminants on living resources within the Bay and its watershed. The USGS leads the CBP toxic contaminant outcome designed to increase our understanding of the effects and mitigation options for toxic contaminants.

Conceptual diagram describing contaminant sources

Figure 1. Conceptual diagram describing how contaminant sources and processes affect the occurrence of contaminant mixtures in surface and groundwater.

USGS Studies

The USGS undertook several coordinated studies to improve the understanding of contaminant mixtures in surface and groundwater in agricultural watersheds in the Susquehanna and Potomac Rivers (figure 2). Collectively the types of compounds sampled and analyzed in the studies ranged from pesticides and their degradates, phytoestrogens (natural estrogens found in legumes and other plants), steroid hormones (e.g. cholesterol) and bisphenol A (used in plastics). The objectives of respective studies were to:

Map of sampling locations

Figure 2. Map of sampling locations.
Image from McClure and others, 2020.

  1. Document the effects of stream flow and landscape variables on contaminant concentrations and determine the potential benefits of best management practices (BMPs) designed to reduce nutrients and sediment to streams in also reducing contaminants in surface waters. In this study scientists collected surface water at least monthly (during most of 2013-17) and analyzed for 301 organic contaminants (Smalling and others).
  2. Determine the spatial patterns of 28 organic contaminants and estrogenicity (indicator of estrogenic response) in surface water collected from 6 sites over 3 years. Scientists used a modeling approach to evaluate if contaminant co-occurrence varied seasonally and with changes in streamflow dynamics and agricultural land-use (McClure and others, 2020).
  3. Characterize the contribution of groundwater as a source of contaminant mixtures to surface waters. USGS scientists used thermal imaging to locate groundwater discharge zones at three locations (figure 3) (Thompson and others, 2021).
Thermal signature images from two groundwater upwelling zones

Figure 3. Example of the thermal signature images from two groundwater upwelling zones at one of our Susquehanna River sampling locations. Images show cooler groundwater (blue) entering warmer surface waters (orange and red) in July 2017. Image modified from Thompson and others, 2020

Major Findings

The findings from these coordinated studies are summarized for the occurrence of toxic contaminants, the factors affecting their occurrence, and the potential co-benefits of best management practices for nutrient and sediment reduction to reduce contaminants.

Occurrence of toxic contaminants

  • Of the 301 compounds analyzed in 370 surface-water samples, 109 (36%) were observed at least once. The compounds detected included 8 hormones, 27 pesticides, 25 pharmaceuticals, 38 organic waste indicator compounds, and 11 phytoestrogens and mycotoxins (Smalling and others, 2021.
  • Four pesticides (atrazine, metolachlor, fipronil and simazine) co-occurred frequently in surface water across sites. These findings provide baseline information on patterns of contaminant occurrence within agricultural watersheds of Chesapeake Bay. The highest probability of contaminant occurrence occurred in the spring and summer (McClure and others, 2020).
  • The most frequently detected chemicals in both ground and surface water were the phytoestrogens genistein (79%) and formononetin (55%), the herbicides atrazine (74%) and metolachlor (50%) and, the sterol, cholesterol (88%) (Thompson and others, 2021).
USGS scientists collecting surface water samples from Pine Creek

USGS scientists collecting surface water samples from Pine Creek. (Credit: Patrick Phillips, USGS, New York Water Science Center.)

Factors affecting occurrence of toxic compounds and estrogenicity

Of the contaminants measured, atrazine, metolachlor, formononetin, genistein, equol, cholesterol, and total estrogenicity were observed frequently enough to statistically compare to seasonal flow effects, landscape variables, and BMP intensity (figure 4). Some findings for these selected contaminants include:

  • Sites with greater amounts of agricultural lands and other landscape variables including pesticide use and cover crops were associated with greater average concentrations of herbicides and phytoestrogens.
  • Contaminant concentrations were often greater with higher seasonal stream flow, although the magnitude of this effect varied by contaminant across seasons and sites.
  • Co-occurrence patterns of contaminants, on the other hand, were not affected by seasonal flow or agricultural land-use near the sampling locations.
  • Groundwater discharge zones could be a unique exposure pathway of chemicals to surface- water systems and is a potential pathway for phytoestrogens entering streams. There was also a large amount of variability of chemical concentrations in groundwater discharge zones within a stream.

Potential co-benefits of best management practices (from Smalling and others, 2021)

The Smalling and others study incorporated the intensity of all available agricultural BMPs designed to reduce run-off, nutrients, and sediments to surface water to understand if BMPs in the upstream drainage at each site were related to frequently observed contaminants. The findings included:

  • Increased BMP intensity led to reductions in herbicide concentrations in streams suggesting nutrient and sediment BMPs potentially provide contaminant reduction.
  • Continued research in more watersheds could enhance the ability to make inferences about the potential co-benefit of BMPs in reducing contaminant loads to streams and rivers.
Relationship between contaminant concentrations and flow

Figure 4. Graphic showing the relationship between contaminant concentrations and flow during high flows in the spring and fall baseflow conditions. Image from Smalling and others, 2021

Management Applications

To reduce nonpoint source pollution and improve water quality, BMPs are voluntarily implemented by farmers throughout the Chesapeake Bay Watershed. The management applications from these USGS studies:

  • Show that nutrient and sediment BMPs in agricultural areas can also reduce co-occurring contaminants such as pesticides, which also may improve water quality and conditions for fisheries.
  • Document the contaminants that occur most frequently, providing information for prioritizing ecologically relevant contaminants that could be considered for future monitoring.
  • Document the occurrence of phytoestrogens in shallow groundwater discharge zones.
  • Highlight the importance of groundwater discharge zones as both habitat for spawning and as potential sources of contaminants to streams.
  • Can help inform the integrated management approaches need for surface and ground water in the Chesapeake Bay Watershed.

For more information

  • McClure, CM, Smalling, KL, Blazer, VS, Sperry, AJ., Schall, MK, Kolpin, DW, Phillips, PJ, Hladik, ML, Wagner, T. 2020. Spatiotemporal variation in occurrence and co-occurrence of pesticides, hormones, and other organic contaminants in rivers in the Chesapeake Bay Watershed, United States. Science of the Total Environment, 728, e138765. https://doi.org/10.1016/j.scitotenv.2020.138765.
  • Smalling, KL, Devereux, OH, Gordon, SE, Phillips, PJ, Blazer, VS, Hladik, ML, Kolpin, DW, Meyer, MT, Sperry, AJ, Wagner, T. 2021. Environmental and anthropogenic drivers of contaminants in agricultural watersheds with implications for land management. Science of the Total Environment. 774, e145687 https://doi.org/10.1016/j.scitotenv.2021.145687.
  • Thompson, T.J., Briggs, M.A., Phillips, P.J., Blazer, V.S., Smalling, K.L., Kolpin, D.W., Wager, T. 2021. Groundwater discharges as a source of phytoestrogens and other agriculturally derived contaminants to streams. Science of the Total Environment, 755, e142873. https://doi.org/10.1016/j.scitotenv.2020.142873.
  • Accompanying data release: Organic contaminants, including hormones, pesticides, pharmaceuticals and other endocrine disrupting compounds, and estrogenicity data in select surface water samples as well as organic contaminants in bed sediment samples collected in Chesapeake Bay watershed in parts of Maryland, Pennsylvania, and West Virginia, 2012-2018. https://doi.org/10.5066/P96L2GB0.

Read more


Contact Kelly Smalling (ksmall@usgs.gov)

Posted on April 20, 2021


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