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USGS - science for a changing world

USGS Circular 1316

Synthesis of U.S. Geological Survey Science for the Chesapeake Bay Ecosystem and Implications for Environmental Management

Chapter 8: The Occurrence of Pesticides in the Bay Watershed
By Judith M. Denver and Scott W. Ator

USGS Chesapeake

One of the CBP’s restoration goals is “to achieve and maintain the water quality necessary to support the aquatic living resources of the Bay and its tributaries and to protect human health.” The CBP developed a toxics reduction strategy to address contaminants as part of this goal. Some of the information needs of the toxics reduction strategy include (1) documenting the sources and occurrence of contaminants, and (2) understanding the potential for contaminants to adversely impact aquatic-dependent wildlife. The USGS had a science goal in 2001–06 to address the occurrence of selected contaminants to provide information to the CBP and also to support DOI needs about the impact of contaminants on wildlife. The USGS science goal mainly addressed pesticide occurrence in surface and ground water in the watershed by utilizing results of the USGS National Water-Quality Assessment (NAWQA) Program studies that were conducted during 1992–2004. Additionally, more recent results from USGS studies of emerging contaminants are presented. The USGS goal also addressed the impacts of selected contaminants on waterbirds and wildlife species. This chapter summarizes findings about pesticides and some selected emerging contaminants in the watershed. It provides an overview of the occurrence of pesticides in ground water and surface water, their relation to land use and other factors, and changes over time, followed by a summary of emerging contaminants. The next chapter focuses on the impact of contaminants on waterbirds and wildlife.

Results from NAWQA studies in the Susquehanna River Basin (Lindsey and others, 1998), Potomac River Basin (Ator and others, 1998), and the Delmarva Peninsula (Denver and others, 2004) revealed that synthetic organic pesticides, along with certain degradation products, have been widely detected at low levels (typically less than 1 microgram per liter) in ground water and streams in the Chesapeake Bay watershed. Pesticides and their degradates are generally detected more frequently in streams than in ground water; an example from the Delmarva Peninsula is shown in figure 8.1. The most commonly detected pesticides are herbicides used on corn, soybean, and small grain crops. Atrazine, metolachlor, and simazine are the most commonly detected pesticides in surface water, whereas atrazine is the most commonly detected pesticide in ground water (Hainly and Kahn, 1996; Ator and Ferrari, 1997; Ferrari and others, 1997; Denver and others, 2004). Pesticides also are detected in urban areas, where the use and detection of insecticides—such as diazinon, carbaryl, and chlorpyrifos—and the herbicide prometon are more common. Herbicides common to agricultural areas have also been widely detected in urban areas, though typically at lower concentrations. Pesticides are less commonly detected in forested areas; infrequent, low-level detections in such areas may be attributable to local use or to atmospheric transport from agricultural or urban areas (Majewski and others, 1998). Degradation products of pesticides also are found in ground water and streams, often at concentrations higher than those of their corresponding parent compounds (Ator and others, 2005; Denver and others, 2004). The occurrence and distribution of pesticides in the Bay watershed reflect usage patterns, environmental conditions, and the chemical and physical properties of the pesticides. Given that pesticide occurrence is closely tied to nutrient practices on agricultural and urban lands, these results could be used by resource managers to better integrate actions to reduce nutrients and pesticides to improve water quality in the Bay and its watershed.

The occurrence and distribution of pesticides in ground water are related to natural geologic and soil conditions, as well as usage patterns. Where applied, pesticides usually occur at higher concentrations in ground water in areas underlain by permeable soils and aquifer material than in areas underlain by less permeable materials (Ator and Ferrari, 1997; Lindsey and others, 1998; Debrewer and others, 2007). Results from these studies showed that concentrations were generally higher in agricultural areas overlying limestone or fractured crystalline bedrock (such as in the Great Valley or parts of the Piedmont Physiographic Provinces) or sandy sediments in the Coastal Plain. Lower concentrations were found in agricultural areas overlying unfractured sandstone and shale of the Piedmont and Appalachian Mountains or in fine-grained sediments underlying fine-grained, organicrich soil in the Coastal Plain. Once pesticide compounds enter ground water, they often take years to decades to be carried through the flow system and discharge to local streams and rivers.

Pesticides are present year round in streams of the Bay watershed, but the changes in pesticide concentrations over time generally reflect changes in application rates, as well as physical and chemical properties that control the movement of these compounds in the environment (Gilliom and others, 2006). Increasing or decreasing use of pesticides may cause relatively rapid corresponding changes in concentrations in overland runoff and streams during runoff periods. Changes in pesticide use will be more slowly reflected in ground water and, therefore, in streams during base-flow periods, however. Diazinon concentrations decreased 39 percent between 1998 and 2004 in Accotink Creek, in an urban area near Washington, D.C., coincident with reductions in diazinon use (fig. 8.2) (Phillips and others, 2007). No trends were apparent, however, between 1993 and 2002 in concentrations of several commonly used herbicides (atrazine, metolachlor, prometon, and simazine) or desethylatrazine in ground water in agricultural areas of the Great Valley underlain by carbonate bedrock (Debrewer and others, 2007), which indicates that usage of these compounds did not change significantly during the corresponding ground-water recharge period. The implication of these findings is that there will be varying lag times between management practices to reduce pesticides and improvements in water quality. For pesticides in the dissolved phase that are transported in runoff directly from a field to a stream, a very short response time between management actions and water-quality improvements may be expected. There will be a longer response time if the compound has been transported through ground water. Pesticides associated with sediment will have the longest lag time between management actions and improvements in water quality.

Figure 8.1 chart shows

Figure 8.1. Pesticides detected in surface water and ground water in the Delmarva Peninsula, 1999–2001 (modified from Denver and others, 2004). Synthetic organic pesticides, along with certain degradation products, have been widely detected in ground water and streams in the Bay watershed. Pesticide occurrence is closely tied with nutrient land practices on agricultural and urban lands, so there is potential to better integrate management actions to reduce both nutrients and contaminants to the Bay.


Over-application of herbicides on farm fields can result in excess toxins and nutrients reaching the waterways. Photograph by Jane Hawkey, IAN Image Library (

Figure 8.2 chart shows changes in diazinon
concentrations in Accotink
Creek, a small urban stream near
Washington, D.C.

Figure 8.2. Changes in diazinon concentrations in Accotink Creek, a small urban stream near Washington, D.C., 1997–2004 (modified from Phillips and others,2007). Pesticides are present year round, but changes in concentrations reflect application rates and properties affecting their movement.

In addition to pesticides, pharmaceuticals, hormones, and other organic wastewater compounds, are also of concern in the Bay watershed and the Nation. The USGS conducted a national study of emerging contaminants that included sites in the Bay watershed (Kolpin and others, 2002). During the study, samples were analyzed for 95 different emerging contaminants, including human and veterinary drugs, hormones, detergents, disinfectants, insecticides, and fire retardants. At least one of these contaminants was found in 80 percent of the Nation’s streams, with mixtures of the chemicals occurring at 75 percent of the sites. The most common groups detected were steroids, nonprescription drugs, and insect repellent. Only 14 compounds have human or ecological health criteria, and measured levels rarely exceeded any of the standards or criteria. However, little is known about the majority of the compounds or their mixtures.

The USGS also published results of a study on pharmaceutical compounds having antibiotic resistance to bacteria and their relation to nutrient cycling in sediments (Simon, 2005). The antibiotic oxytetracycline (OTC) was found in bottom sediments in two streams that were studied on the Eastern Shore of the Chesapeake Bay. OTC can produce changes in antibiotic resistance of indigenous bacteria and change the reaction rates of nitrate oxidation by soil and sediment bacteria. These results indicate that OTC in sediments decreases the ability of bacteria to alter nitrogen and phosphorous, which could result in increased loads of nutrients being delivered to the estuary.

Studies have recently begun to document the potential relation between emerging contaminants and the disruption of the endocrine system of fish in parts of the Bay watershed. Reconnaissance sampling for emerging contaminants at several sites in the West Virginia part of the Potomac River Basin detected antibiotics in municipal wastewater, aquaculture, and poultry-processing effluent (Chambers and Leiker, 2006). The highest number and the greatest concentrations were found in municipal effluent. Previous results from USGS sampling of the Potomac Basin by the NAWQA Program detected chlordane, DDT, and PCBs in streambed sediment and aquatic tissues (Ator and others, 1998). Sediment from over one-half of the sites contained concentrations that may pose adverse effects on aquatic life. There is a limited amount of information on these contaminants in the Bay watershed and their impact on the stream ecosystems and fish populations, however. Therefore, the USGS is beginning a more extensive study of the issue in the Bay watershed. More information can be found in the chapter on fish health.


Ator, S.W., Blomquist, J.D., Brakebill, J.W., Denis, J.M., Ferrari, M.J., Miller, C.V., and Zappia, H., 1998, Water quality in the Potomac River Basin—Maryland, Pennsylvania, Virginia, and West Virginia and the District of Columbia, 1992–96: U.S. Geological Survey Circular 1166, 38 p.

Ator, S.W., Denver, J.M., and Brayton, M.J., 2005, Hydrologic and geochemical controls on pesticide and nutrient transport to two streams on the Delmarva Peninsula: U.S. Geological Survey Scientific Investigations Report 2004–5051, 34 p.

Ator, S.W., and Ferrari, M.J., 1997, Nitrate and selected pesticides in ground water of the Mid-Atlantic Region: U.S. Geological Survey Water-Resources Investigations Report 97–4139, 8 p.

Chambers, D.B., and Leiker, T.J., 2006, A reconnaissance for emerging contaminants in the South Branch Potomac River, Cacapon River, and Williams River Basins, West Virginia, April–October 2004: U.S. GeologicalSurvey Open-File Report 2006–1393, 28 p.

Debrewer, L.M., Ator, S.W., and Denver, J.M., 2007, Factors affecting spatial and temporal variability in nutrient and pesticide concentrations in the surficial aquifer on the Delmarva Peninsula: U.S. Geological Survey Scientific Investigations Report 2005–5257, 44 p.

Denver, J.M., Ator, S.W., Debrewer, L.M., Ferrari, M.J., Barbaro, J.R., Hancock, T.C., Brayton, M.J., and Nardi, M.R., 2004, Water quality in the Delmarva Peninsula—Delaware, Maryland, and Virginia, 1999–2001: U.S. Geological Survey Circular 1228, 30 p.

Ferrari, M.J., Ator, S.W., Blomquist, J.D., and Dysart, J.E., 1997, Pesticides in surface water of the Mid-Atlantic Region: U.S. Geological Survey Water-Resources Investigations Report 97–4280, 8 p.

Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton, P.A., Martin, J.D., Nakagaki, Naomi, Nowell, L.H., Scott, J.C., Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 2006, Pesticides in the Nation’s streams and ground water, 1992–2001: U.S. Geological Survey Circular 1291, 172 p.

Hainly, R.A., and Kahn, J.M., 1996, Factors affecting herbicide yields in the Chesapeake Bay watershed, June 1994: Water Resources Bulletin, v. 32, no. 5, p. 965–984.

Kolpin, D.W., Furlong, E.T., Meyer, M.T., Thurman, E.M., Zaugg, S.D., Barber, L.B., and Buxton, H.T, 2002, Pharmaceuticals, hormones, and other organic wastewater contaminants in U.S. streams, 1999–2000: A national reconnaissance: Environmental Science and Technology, v. 36, no. 6, p. 1,202–1,211.

Lindsey, B.D., Breen, K.J., Bilger, M.D., and Brightbill, R.A., 1998, Water quality in the Lower Susquehanna River Basin, Pennsylvania and Maryland, 1992–95: U.S. Geological Survey Circular 1168, 38 p.

Majewski, M.S., Foreman, W.T., Goolsby, D.A., and Nakagaki, N., 1998, Airborne pesticide residues along the Mississippi River: Environmental Science and Technology, v. 32, no. 23, p. 3,689–3,698.

Phillips, P.J., Ator, S.W., and Nystrom, E.A., 2007, Temporal changes in surface-water insecticide after the phaseout of diazinon and chlorpyrifos: Environmental Science and Technology, v. 41, no. 12, p. 4,246–4,251.

Simon, N.S., 2005, Loosely bound Oxytetracycline in riverine sediments from two tributaries of the Chesapeake Bay, 2005: Environmental Science and Technology, v. 39, no. 10, p. 3,480–3,487.


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