Summary of Pesticide Data from Streams and Wells in the Potomac River Basin
by Colleen A. Donnelly and Matthew J.Ferrari
Eighty-five water-soluble pesticides and pesticide degradation products were analyzed in 384 surface-water and Groundwater samples collected from the Potomac River Basin during March 1993 through September 1996. Thirty-nine of these compounds were detected in surface-water samples and 16 were detected in Groundwater samples. At least one pesticide was detected in 86 percent of the streams sampled and 45 percent of the wells sampled. Pesticides were detected more frequently and at higher concentrations in surface water than in Groundwater. The following four herbicides and one degradation product were the most frequently detected pesticides in both surface water and Groundwater: atrazine and metolachlor, which are used primarily on corn and soybean crops; prometon, which is used primarily in nonagricultural (urban and suburban) areas; simazine, which is used in both agricultural and nonagricultural areas, and desethylatrazine, which is one of the degradation products of atrazine. Insecticides were detected more frequently in surface water than in Groundwater. Diazinon, chlorpyrifos, and gamma-HCH (Undone) were found in more than 10 percent of surface-water samples, but in none of the Groundwater samples.
Contamination of streams and Groundwater by pesticides is a major concern to human and aquatic health. Pesticides (for example, herbicides and insecticides) are chemicals used to control unwanted organisms such as weeds and insects. Pesticides in surface water and Groundwater, even at very low concentrations, can render the water unfit for human consumption and make it toxic to aquatic organisms. Pesticides in surface water and Groundwater in the Potomac River Basin were analyzed by the U.S. Geological Survey (USGS) as part of the National Water Quality Assessment (NAWQA) program. This report presents the results of pesticide sampling that was done for the Potomac NAWQA study.
An estimated 4.94 million pounds of pesticides are used annually for agricultural purposes in the Potomac River Basin (Gianessi and Puffer, 1990; 1992a-b). Nonagricultural applications of pesticides are difficult to quantify, but the U.S. Environmental Protection Agency estimated that nationwide, agricultural applications accounted for 75 percent of the total pesticide usage in 1993 (Asplin, 1994). Atrazine and metolachlor are the most widely applied agricultural pesticides in the Potomac River Basin, with estimated annual applications of 697,000 pounds and 539,000 pounds, respectively (Gianessi and Puffer, 1990; 1992a-b), and are the two pesticides most frequently detected in this study (fig. 1). Of the 20 most widely applied agricultural pesticides in the Potomac River Basin, 13 were detected in either Groundwater or surface water and 1 was not detected; the samples were not analyzed for the other 6 pesticides (fig. 1). A complete list of pesticides and degradation products for which the samples were analyzed is shown in table 1. Of the 85 compounds for which samples were analyzed, 43 were detected in at least one sample.
|Dacthal (mono acid)||alpha-HCH||cis-Permethrin|
The Potomac River Basin drains 14,670 square miles in parts of four states — Maryland, Pennsylvania, Virginia, and West Virginia — and the District of Columbia. Major land uses in the basin include forest (50 percent), agriculture (35 percent), and urban (10 percent) (Hitt, 1994; fig. 2). For the purposes of NAWQA water-quality investigations, the Potomac River Basin was subdivided into eight subunits based on physiographic and geologic characteristics (Blomquist and others, 1996). Seven physiographic provinces and subprovinces are included in the Potomac River Basin — the Appalachian Plateau, Valley and Ridge, Great Valley, Blue Ridge, Piedmont, Triassic Lowlands, and Coastal Plain. Differences in the geology of the Great Valley subprovince were considered important enough to further subdivide that subprovince into carbonate and noncarbonate subunits. Four of the subunits — Valley and Ridge, Great Valley Carbonate, Piedmont, and Triassic Lowlands — were selected for sampling emphasis. A more detailed discussion of the Potomac NAWQA sampling design may be found in Gerhart and Brakebill (1996). A detailed discussion of the national NAWQA sampling design guidelines may be found in Gilliom and others (1995).
Three sampling approaches — multiple-sample monitoring, subunit synoptic surveys, and a survey of major tributaries — were used by the USGS to assess the quality of surface water in the Potomac River Basin (Gerhart and Brakebill, 1996). For the multiple-sample monitoring, 11 sites were sampled repeatedly and were designated as either "fixed indicator" or "fixed integrator" sites. Indicator sites drain small to intermediate size watersheds (less than 400 square miles) having relatively homogeneous environmental settings. Integrator sites drain relatively large areas (greater than 400 square miles) and represent the combined effects of all natural and human water-quality factors in the watersheds they drain (Gerhart and Brakebill, 1996). The water at these fixed sites was monitored throughout the study for nutrients, major inorganic elements, and suspended sediment (Shelton, 1994). Four of these sites were intensively monitored for pesticide concentrations (table 2). The frequency of sample collection at the Muddy Creek monitoring site is shown in figure 3; samples were collected at similar frequencies at the Accotink Creek and the Monocacy River at Bridgeport, Md. fixed sites. The Shenandoah River at Millville, W. Va., was sampled less frequently than Muddy Creek, Accotink Creek, and the Monocacy River. Samples were collected even less frequently at the other fixed sites (table 2). In addition, four fixed sites were sampled in June 1996 during high-flow conditions resulting from extremely heavy, local rainfall in the Conococheague Creek and Monocacy River watersheds.
|Station Name||Drainage Area
|Number of Samples||Period of Sampling||Watershed Description|
|North Branch Potomac River at Cumberland, Md.||875||1||6/94||Integrator site for the Appalachian Plateau subunit|
|South Fork South Branch Potomac River near Moorefield, W. Va.||283||4||6/94-8/95||Indicator site for the forested areas of the Valley and Ridge subunit|
|South Branch Potomac River at Springfield, W. Va.||1,471||4||6/94-9/96||Integrator site for the Valley and Ridge subunit|
|Conococheague Creek at Fairview, Md.||494||6||6/94-6/96||Integrator site for the northern Great Valley|
|Muddy Creek at Mount Clinton, Va.||14.2||39||3/93-5/95||Indicator site for the agricultural areas of the Great Valley Carbonate|
|Shenandoah River at Millville, W. Va.||3,040||23||3/93-9/96||Integrator site for the southern Great Valley|
|Catoctin Creek at Taylorstown, Va.||89.6||2||6/94-7/94||Indicator site for the agricultural areas of the Piedmont subunit|
|Monocacy River at Bridgeport, Md.||173||42||6/92-6/96||Indicator site for the agricultural areas of the Triassic Lowlands subunit|
|Monocacy River near Frederick, Md.||817||5||6/94-6/96||Integrator site for the Piedmont and the Triassic Lowlands subunits|
|Potomac River at Washington, D.C.||11,560||10||6/94-9/96||Integrator site for the Potomac River|
|Accotink Creek near Annandale, Va.||23.5||42||6/94-8/95||Indicator site for the urbanized areas of the Piedmont subunit|
The four subunits (Valley and Ridge, Great Valley Carbonate, Piedmont, and Triassic Lowlands) selected for sampling emphasis encompass most of the Potomac River Basin. Synoptic sampling (single samples collected over a relatively short period of time) of small streams (those draining generally less than 10 square miles) during low-flow conditions in late August or September was conducted in each of these subunits. Synoptic samples were collected over a period of 3 years from the following subunits: Great Valley Carbonate subunit (27 samples), September 1993; Piedmont (25 samples) and Triassic Lowlands (12 samples) subunits, August 1994; Valley and Ridge subunit (25 samples), August 1995.
In addition to the subunit surveys, 23 major tributary sites were sampled synoptically during June 1994 under stable-flow conditions (Fisher, 1995). Sampling under stable-flow conditions made spatial comparisons possible.
Groundwater investigations in the Potomac NAWQA study focused on synoptic sampling in two of the subunits — the Piedmont and Triassic Lowlands — as well as synoptic sampling of agricultural land-use areas within the Great Valley Carbonate and Valley and Ridge subunits. Synoptic samples were collected over a period of 3 years (Great Valley Carbonate subunit, June through September 1993; Piedmont and Triassic Lowlands subunits, June through August 1994; Valley and Ridge subunit, June through July 1995). Wells were randomly selected for sampling within the target area of each subunit (either a specific land use within the subunit or the entire subunit) to create a spatially unbiased network within that area, and were sampled using trace-level protocols (Koterba and others, 1995). Three additional wells within the forested areas of the Valley and Ridge were sampled to provide information on the backGroundwater-quality conditions in that subunit.
The laboratory analyses for the pesticide data presented in this report were performed by the USGS National Water Quality Laboratory (NWQL), in Denver, Colo. All surface-water and Groundwater samples were analyzed by the NWQL for selected pesticides and their degradation products by gas chromatography with detection by mass spectrometry (Zaugg and others, 1995). Surface- water samples from 8 fixed sites and Groundwater samples from the Great Valley Carbonate, Triassic Lowlands, Piedmont, and five wells from the Valley and Ridge subunits also were analyzed for additional pesticides by high-performance liquid chromatography with detection by ultraviolet spectroscopy (Wemer and others, 1996). The different analytes detected by these methods and the results of those analyses are shown in table 3. Some detectable concentrations of pesticides are qualified as "estimated".
Estimated concentrations occur when the actual concentration is greater than or less than the calibrated range for the laboratory analysis method. All concentrations of carbaryl, carbofuran, desethylatrazine, dichlobenil, and methyl azinphos are reported as estimated due to comparably small or variable recovery in the analysis (Zaugg and others, 1995; Wemer and others, 1996).
Pesticides in Surface Water
Selected pesticides and degradation products (table 1) were analyzed in 279 water samples collected from 112 stream sites in the Potomac River Basin. Sampling locations and the number of compounds detected at each site are shown in figure 4. The number of pesticides detected in surface-water samples ranged from 0 to 27; sixteen sites had no pesticides detected. Pesticides were detected most frequently in the Great Valley Carbonate, Triassic Lowlands, and Piedmont subunits — areas with a high percentage of agricultural land use. Conversely, pesticides were detected less frequently in the Valley and Ridge subunit, an area that is heavily forested. Thirty-nine compounds were detected in surface-water samples, with detection frequencies ranging from 0.4 to 88.2 percent of all samples (table 3). The compounds detected most frequently (fig. 5) and at the greatest number of sites were atrazine (92 sites), metolachlor (83 sites), simazine (83 sites), desethylatrazine (80 sites), and prometon (68 sites) (table 3). These five pesticides were detected more. frequently and at higher concentrations in surface water than in Groundwater. All other pesticides were detected less frequently and at fewer than 25 percent of the surface-water sites.
More pesticides were detected at fixed monitoring sites, where samples were collected most frequently, than at sites sampled only once (synoptically). For example, 10 or more pesticides were detected at 7 of the 11 fixed sites (figure 4). The greatest number of pesticides (27) were detected at the Monocacy River at Bridgeport, Md., and Accotink Creek near Annandale, Va. More pesticides were detected and often at higher concentrations during periods of surface-water runoff after storms. The highest pesticide concentrations also occurred during the spring/summer application period. Typical results from one of the frequently sampled surface-water monitoring sites. Muddy Creek at Mount Clinton, Va., are shown in figure 3. The highest concentration of atrazine in Muddy Creek was 14.9 micrograms per liter (µg/L) in May 1993, during a storm that followed a seasonal application of pesticides. Median concentrations of atrazine, desethylatrazine, and metolachlor analyzed in all surface- water samples were 0.057 µg/L, 0.028 µg/L, and 0.024 µg/L, respectively. The median concentrations of all other pesticides were at the detection limit (table 3).
Federal drinking-water standards have been established for 11 of the 85 compounds analyzed in the NAWQA study (U.S. Environmental Protection Agency, 1994; table 4). Concentrations of three of those compounds — alachlor, atrazine, and simazine — were higher than the maximum contaminant levels (MCL's) in 17 samples collected. MCL's are strictly applicable only to treated drinking water and are used here only as a point of reference. These elevated levels were detected at six sites and persisted for short periods of time during periods of surface-water runoff conditions following spring storms.
|Pesticide||MCL, in Micrograms per Liter||Number of Detections in surface-Water Samples Above MCLa||Number of Surface-Water sites Where Detected Above MCL|
|a No Pesticides were detected above MCL's in any
bMonocacy River at Bridgeport, Md.
cConococheague Creek at Fairview, Md.; Muddy Creek at Mount Clinton, Va.; Monocacy River at Bridgeport, Md.; Monocacy River at Frederick, Md.; and Potomac River at Washington, D.C.
dMuddy Creek at Mount Clinton, Va. and Accotink Creek near Annandale, Va.
Pesticides in Groundwater
Water samples were collected from 105 wells in the Potomac River Basin and analyzed for selected pesticides and degradation products (table 1). Sampling locations and the number of compounds detected at each site are shown in figure 6. Sixty wells had no detectable concentrations of pesticides or degradation products. The greatest number of pesticides detected in any Groundwater samples was six. Figure 6 also shows that pesticides were detected most frequently in the Great Valley Carbonate subunit.
Sixteen different compounds were detected in the Groundwater samples, with the number of detections ranging from 1 to 38 (table 3). Desethylatrazine, atrazine, simazine, metolachlor, and prometon were the compounds detected most frequently, with 38,36,23,20, and 16 detections, respectively (fig. 7). Except for the compound p,p'-DDE, which had 10 detections, all other compounds detected were found in three or fewer samples. The USGS found that detections of these chemicals occurred at concentrations that are substantially below current drinking-water MCL's (U.S. Environmental Protection Agency, 1994). Again, only 11 of the compounds analyzed have established MCL values. However, multiple pesticides were frequently detected in the same well and the health effects of combinations of pesticides in drinking water are not well understood.
Asplin, A. L., 1994, Pesticide industry sales and usage - 1992 and 1993 market estimates: U.S. Environmental Protection Agency, Office of Pesticide Programs, Biological and Economic Analysis Division, Economic Analysis Branch Report, 733-K-94-001,33 p.
Blomquist, J. D., Fisher, G.T. , Denis, J.M., Brakebill, J.W., and Werkheiser,W.H., 1996, Water-quality assessment of the Potomac River Basin: Basin description and analysis of available nutrient data, 1970-90: U.S. Geological Survey Water-Resources Investigations Report 95-4221,88 p.
Fisher, G.T. , 1995, Selected herbicides in major streams in the Potomac River Basin upstream from Washington, D.C.: U.S. Geological Survey Fact Sheet 95-107,4 p.
Gerhart, J.M., and Brakebill, J.W., 1996, Design and implementation of a sampling strategy for a water-quality assessment of the Potomac River Basin: U.S. Geological Survey Water-Resources Investigations Report 96-4034,31 p.
Gianessi, L.P. and Puffer, C.A., 1990 (revised April 1991), Herbicide use in the United States: Washington, D. C., Resources for the Future, Quality of the Environment Division, 128 p.
_____ 1992a. Fungicide use in U.S. Crop Production: Washington, D. C., Resources for the Future, Quality of the Environment Division [variously paged].
_____ 1992b. Insecticide use in U.S. Crop Production: Washington, D.C., Resources for the Future, Quality of the Environment Division [variously paged].
Gilliom, R.J., Alley, W.M., and Gurtz,M.E., 1995, Design of the National Water-Quality Assessment program: Occurrence and distribution of water-quality conditions: U.S. Geological Survey Circular 1112,33 p.
Hitt, K.J., 1994, Refining 1970's land-use data with 1990 population data to indicate new residential development: U.S. Geological Survey Water-Resources Investigations Report 94-4250,15p.
Koterba, M.T., Wilde, F.D., and Lapham.W.W., 1995, Groundwater data-collection protocols and procedures for the National Water-Quality Assessment program: Collection and documentation of water-quality samples and related data: U.S. Geological Survey Open-File Report 95-399, 113 p.
Shelton,L., 1994, Field guide for collecting and processing stream- water samples for the National Water- Quality Assessment program: U.S. Geological Survey Open-File Report 94- 455,42 p.
U.S. Environmental Protection Agency, 1994, National Primary Drinking Water Standards: U.S. Environmental Protection Agency, Office of Water, EPA 810-F-94-001,2 p.
Wemer, S.L., Burkhardt, M.R., and DeRusseau,S.N., 1996, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory — Determination of pesticides in water by Carbopack-B solid-phase extraction and high-performance liquid chromatography: U.S. Geological Survey Open-File Report 96-216,91 p.
Zaugg, S.D., Sandstrom, M.W., Smith,S.G.,and Fehlberg,K.M., 1995, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory — Determination of pesticides in water by C-18 solid-phase extraction and capillary-column gas chromatography/mass spectroscopy with selected-ion monitoring: U.S. Geological Survey Open-File Report 95-181,49 p.
The authors wish to thank the landowners within the Potomac River Basin who participated in the study. Thanks also is given to the following USGS personnel for their contributions to this report: Members of the NAWQA study-unit team and volunteers who assisted in the collection of water-quality samples throughout the Potomac River Basin; Janet M. Denis, for the extensive help she gave with data management; and Lisa D. Olsen, Richard J. Wagner (Tacoma, Wash.), and Sheryl Protani, for their technical and editorial contributions. Layout and camera-ready illustrations were designed by Tim Auer.
For further information contact:
U.S. Geological Survey
5522 Research Park Drive
Baltimore, MD 21228