USGS

Water Quality in the Potomac River Basin, Maryland, Pennsylvania, Virginia, West Virginia, and the District of Columbia, 1992-96

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MAJOR ISSUES AND FINDINGS IN THE POTOMAC RIVER BASIN
Nutrients and pesticides in streams and ground water -- A regional perspective

In the Potomac River Basin, the quality of streams and ground water is affected by a variety of natural and human processes. Several major types of chemicals found in water in the basin include nutrients, trace elements, pesticides, chlorinated industrial compounds, and volatile organic compounds. Agricultural and urban land-use practices broadly characterize the distribution of these sources, because most contamination results from the disposal of human and animal wastes or from the use and disposal of other chemical compounds. Water, sediment, and tissue samples were collected throughout the basin and analyzed for nutrients, pesticides, metals, or other potential contaminants that are associated with urban and agricultural sources.

Much of the analysis done by the NAWQA Program within the Potomac River Basin focused on nutrients and pesticides because these compounds are of primary concern to environmental managers within the basin. Elevated concentrations of nutrients and pesticides can render water unsafe to drink and can have adverse environmental effects. In 1987, Maryland, Pennsylvania, Virginia, the District of Columbia, and the Federal government established a goal of reducing nutrient loads to Chesapeake Bay. The Potomac River is the second largest tributary to Chesapeake Bay, and information from the NAWQA Program can be used to monitor progress toward this goal. Only a small amount of data was available for pesticides in the Potomac River Basin prior to the NAWQA Program. Data collection was designed to fill this gap in information.

Nutrients (Nitrogen and Phosphorus)

Although nitrogen and phosphorus occur naturally, elevated concentrations of these nutrients in streams and ground water of the Potomac River Basin often result from human activities. Major sources of nitrogen and phosphorus to the basin in 1990 included commercial fertilizers and manure. Atmospheric deposition from the combustion of fossil fuels accounts for an additional 32 percent of nitrogen inputs. Municipal wastewater-treatment plants are locally important sources of nutrients to many streams, but they contributed about 12 percent of nitrogen and 4 percent of phosphorus inputs to the basin in 1990 (fig. 9).

Pie charts: sources of nitrogen and phosphorus (9,956 bytes)

Figure 9. Major inputs of (A) nitrogen and (B) phosphorus to the Potomac River Basin, 1990. Most nitrogen and phosphorus inputs are derived from commercial fertilizers and manure, although a large percentage of nitrogen is derived from atmospheric deposition as well (Blomquist and others, 1996).

Nutrient inputs to the Potomac River Basin are related to land use. The Great Valley (fig.3), which is 45 percent cropland, received the largest estimated inputs from non-point sources of both nitrogen and phosphorus per unit area in 1990, followed by the Piedmont (including the Triassic Lowlands), which is 43 percent agricultural and 25 percent urban (Vogelmann and others, 1997; Hitt, 1994). Inputs to these areas were mostly attributed to commercial fertilizers and manure. Dominantly forested areas of the basin received smaller inputs of nitrogen (mostly from atmospheric deposition) and phosphorus per unit area in 1990. (fig. 10).

Bar chart: Nitrogen (9,637 bytes) Bar chart: Phosphorus (8,692 bytes)

Figure 10. Inputs of (A) nitrogen and (B) phosphorus from non-point sources to areas of the Potomac River Basin, 1990 (modified from Blomquist and others, 1996).

In most waters of the Potomac River Basin, concentrations of nutrients do not pose a threat to human health or wildlife. A Maximum Contaminant LeveL (MCL) of 10 milligrams per liter (mg/L) as nitrogen has been established for nitrate (U.S. Environmental Protection Agency, 1994a). (MCLs are standards for levels of contaminants in finished public water supplies and are not directly applicable to untreated streams or ground water. MCLs are cited in this report for reference only.) Drinking water containing nitrate in excess of this concentration may cause health problems in infants and small children. Nitrate concentrations in sampled streams of the Potomac River Basin were generally well below the MCL, even during high flows. Ground water in agricultural areas of the basin underlain by carbonate rock, however, is particularly susceptible to nitrate contamination (Ferrari and Ator, 1995). Nearly 25 percent of ground-water samples from domestic wells in such areas contained nitrate in excess of 10 mg/L. Excessive nitrogen or phosphorus in streams can cause eutrophication, a condition whereby aquatic plants and algae are overproduced. This algae can smother larger plants, consume dissolved oxygen, block sunlight from the water, and produce toxins that are harmful to other aquatic life (Allaby, 1989). To control eutrophication, the U.S. Environmental Protection Agency (1986) recommends that total phosphorus concentrations in flowing waters not exceed 0.1 mg/L, a concentration exceeded in only 12 percent of samples from small streams in the Potomac River Basin at low flow but in a greater percentage of samples from larger streams at varying flow conditions.

Nutrients are present in waters of the Potomac River Basin in many forms, often at concentrations suggestive of human-derived sources (figs. 11, 12). Maximum natural or "background" concentrations of nutrients in ground water of the basin are estimated at 0.4 mg/L (as nitrogen) for nitrate, 0.1 mg/L (as nitrogen) for ammonia, and 0.07 mg/L (as phosphorus) for orthophosphate, the most common form of dissolved phosphorus in natural waters (Ator and Denis, 1997). On the basis of data from across the United States, maximum natural concentrations of nutrients in streams have been estimated at 0.6 mg/L for nitrate and 0.1 mg/L for ammonia (Mueller and others, 1995).

Box plot: Nitrate concentrations, by environmental setting (17,696 bytes)

Figure 11. Among samples from small streams at low flow and ground water in the Potomac River Basin, the highest nitrate concentrations were found in areas of agriculture and carbonate bedrock.

Box plot: Nitrate concentrations, by watershed (12,945 bytes)

Figure 12. Time-series data show a wide variation in nitrate concentrations in selected Potomac River tributaries from 1993 through 1995, but concentrations typically are highest in those tributaries draining agricultural land and carbonate bedrock.

Elevated nitrogen concentrations in streams and ground water are common in the Potomac River Basin in areas of intensive row cropping and carbonate bedrock. Nitrate is the most common form of nitrogen in waters of the basin. Nitrate concentrations may vary considerably during the year at any given location because of seasonal variations in precipitation and inputs, but they generally exceed natural concentrations more often in streams and ground water in agricultural areas of the basin than in areas with other land uses. This is particularly evident in areas underlain by carbonate bedrock, where natural waters are more susceptible to such contamination (figs. 11, 12). Elevated nitrate concentrations in streams and ground water in some urban areas indicate that urban sources also contribute nutrients to the basin, although relatively few samples were collected in these areas during this study (Ator and Denis, 1997; Miller and others, 1997). Because nitrogen sources are typically surficial, concentrations are often greater in shallower than in deeper ground water (Blomquist and others, 1996). This is not evident from the data, however, likely because mostly shallow wells were sampled.

Nitrate concentrations that exceed natural concentrations are more common in streams and ground water in agricultural areas in the northeastern part of the Potomac River Basin than in other agricultural areas that contain lesser percentages of row cropping (fig. 13) (Ator and Denis, 1997; Miller and others, 1997). In 1992, row crops accounted for less than 20 percent of agricultural land in most counties of the basin, but as much as 47 percent of such land in counties in the northeastern part (U.S. Department of Commerce, 1995). Nitrogen is typically applied in greater quantities to crops in the form of fertilizer and manure than to pastures in the form of manure.

Map: Nitrate concentrations, and agricultural land planted in corn, barley, oats, wheat, sorghum, or soybeans (23,091 bytes)

Figure 13. Within agricultural areas, most elevated nitrate concentrations in ground water and in small streams at low flow were found in counties with the highest percentages of agricultural land devoted to row crops.

Ammonia concentrations are typically lower than nitrate concentrations but occasionally exceed natural concentrations. Elevated ammonia concentrations in small streams during low flow in carbonate areas were likely related to agricultural and urban land uses (Miller and others, 1997). Ammonia in ground water in forested areas may form from nitrogen that is deposited from the atmosphere or from organic nitrogen in leaves and other natural debris (Ator and Denis, 1997).

Organic nitrogen and phosphorus concentrations are typically low in waters of the basin, except in streams during high flows. Organic nitrogen (fig. 14) and phosphorus concentrations can be elevated for short periods in streams during high flow, but they were detected at low concentrations or were undetectable in most ground-water samples.

Plot: Flow, nitrate, and organic nitrogen, June 1996 (6,861 bytes)

Figure 14. Organic nitrogen concentrations were elevated for a short time during high flow in Conococheague Creek in June 1996 but quickly decreased as the flow subsided. Concentrations of nitrate, a much more soluble form of nitrogen, were slightly depressed during the high flow.

Streams draining primarily agricultural or urban areas yield the greatest quantities of nutrients to the Potomac River. Although the Shenandoah River contributes the greatest loads of total nitrogen and phosphorus among streams at which data were collected over time, Conococheague Creek, Muddy Creek, and the Monocacy River, which drain primarily agricultural watersheds, yield the largest loads of total nitrogen per square mile. Accotink Creek, an urban stream, yields the largest loads of total phosphorus per square mile, followed by Muddy and Conococheague Creeks and the Monocacy River (fig. 15).

Bar chart: Total nitrogen yield, per year, per stream (17,158 bytes)

Figure 15. Estimated yields of nitrogen and phosphorus for selected streams in the Potomac River Basin, 1994-95 (Lizarraga, in press). The Monocacy River and Conococheague and Muddy Creeks, which drain primarily agricultural watersheds, and Accotink Creek, which drains a small urban watershed, yield the greatest amounts of nitrogen and phosphorus per square mile to the Potomac River.

Phosphorus is less soluble than most forms of nitrogen and is often bound to stream sediment. Accotink Creek yields the most sediment per square mile among Potomac tributaries at which data were collected over time (Lizarraga, in press).

Pesticides

Each year, 4.94 million pounds of synthetic organic pesticides are applied to agricultural lands within the Potomac River Basin, including 2.88 million pounds of herbicides, 1.09 million pounds of insecticides, and 0.97 million pounds of fungicides. Additionally, 1.24 million pounds of oil and 0.47 million pounds of sulfur are used primarily on apples and peaches to control insects and fungi, respectively. Pesticides are also used for non-agricultural purposes such as maintenance of golf courses, lawns, and gardens; defoliation of rights-of-way; and control of disease-carrying or defoliating insects. Application rates for these uses are not as well documented.

Table 1: Major agricultural pesticides used in the Potomac River Basin and results of pesticide analyses

[F, fungicide; H, herbicide, I, insecticide; A, alfalfa; Ap, apples; C, corn; Ch, cherries; O, other hay; P, pasture; Pe, peaches; S, soybeans; T, tobacco; W, wheat; D, detected; N, not detected, ---, not analyzed]

Pesticide (Trade names1)

Type

Estimated amount applied2 (lb/yr)

Estimated acres treated

Major target crops

Sample result

Ground water

Streams

Atrazine (AAtrex, Gesaprim)

H

697,000

492,000

C

D

D

Metolachlor (Dual, Pennant)

H

539,000

323,000

C, S

D

D

Captan (Clomitane, Captanex)

F

319,000

37,600

Ap, Pe

---

---

Alachlor (Lasso, Alanox)

H

295,000

165,000

C, S

N

D

Chlorpyrifos (Dursban, Lorsban)

I

281,000

253,000

C, A, Ap

D

D

2,4-D (Weed-B-Gon, Chloroxone)

H

205,000

376,000

P, O, C, W

N

D

Mancozeb (Dithane DF, Nemispor)

F

202,000

43,300

Ap

---

---

Simazine (Aquazine, Princep)

H

168,000

126,000

C, Ap, Pe

D

D

Cyanazine (Bladex, Fortrol)

H

167,000

113,000

C

N

D

Metiram (Carbatene, Polyram DF)

F

148,000

8,230

Ap, Pe

---

---

Paraquat (Cyclone, Total)

H

133,000

336,000

C, A, Ap, S

---

---

Carbofuran (Furadan, Curaterr)

I

132,000

143,000

C, A, S

N

D

Butylate (Genate Plus, Sutan)

H

121,000

28,000

C

N

D

Glyphosate (Roundup, Rattler)

H

120,000

108,000

C, P, S, Ap

---

---

Ziram (Corozate, Thionic)

F

109,000

16,000

Ap, Pe

---

---

Azinphos-methyl (Guthion, Carfene)

I

88,000

49,800

Ap, Pe

N

D

EPTC (Eptam, Alirox)

H

83,300

19,700

C, A

D

D

Dicamba (Banvel, Metambane)

H

77,100

253,000

P, C, O

N

N

Pendimethalin (Prowl, Stomp)

H

73,700

79,300

C, S, T

N

D

Methomyl (Lannate, Methomex)

I

67,800

75,000

Ap, C, W

N

N

1Use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
2Amount of active ingredient (Gianessi and Puffer, 1990, 1992a, 1992b).

 

Cornfields and apple orchards receive nearly 75 percent of agricultural pesticides applied in the Potomac River Basin (table 2). Apples receive the heaviest average rate of pesticide application, followed by potatoes and peaches. The agricultural areas with the lowest average rates of pesticide application are pastures and hayfields.

 

Table 2: Pesticide application to major crops in the Potomac River Basin

Crop

Estimated acres1

Estimated amounts of pesticides used (lb/yr)2

Most heavily applied pesticide

Fungicides

Herbicides

Insecticides

Total

Corn

416,000

2,500

2,090,000

354,000

2,450,000

Atrazine

Apples

50,000

812,000

110,000

372,000

1,300,000

Captan

Alfalfa

247,00

03

102,000

212,000

314,000

Chlorpyrifos

Soybeans

157,000

583

275,000

38,400

314,000

Metolachlor

Pasture

1,510,000

03

149,000

03

149,000

2,4-D

Peaches

8,790

79,600

11,300

31,900

123,000

Captan

Other Hay

545,000

03

73,800

1,260

75,000

2,4-D

Wheat

123,000

1,880

21,000

22,400

45,300

2,4-D

Potatoes

1,220

9,310

6,410

13,600

29,400

Mancozeb

Cherries

1,600

16,600

721

2,090

19,400

Captan

1Based on 1992 agricultural census data (U.S. Department of Commerce, 1995).
2Based on Gianessi and Puffer, 1990, 1992a, 1992b.
3No reported use.

 

Commonly used pesticides are present in ground water of the Potomac River Basin, typically at low concentrations. Pesticide compounds were detectable in ground-water samples from throughout the basin, although concentrations rarely exceeded 1 microgram per liter (µg/L). Atrazine was the most commonly detected pesticide in ground water, in 34 percent of samples (fig. 16). Simazine, metolachlor, and prometon were also detected in at least 15 percent of samples, and 10 other compounds (p,p'-DDE, EPTC, metribuzin, tebuthiuron, DCPA, pebulate, dichlorbenil, dicamba, terbacil, and propoxur) were detected in less than 10 percent of samples. Deethylatrazine (a byproduct of atrazine degradation) was detected in nearly all ground-water samples that contained detectable atrazine, often at higher concentrations.

Plot: Atrazine Concentration (6,889 bytes)

Figure 16. Concentrations of atrazine in ground water in sampled areas of the Potomac River Basin. Atrazine was most commonly detected in ground water in agricultural areas of the Great Valley.

A wider variety of pesticides was detected more frequently in small streams than in ground-water samples. Pesticides were commonly detected in samples from small streams of the basin that were collected during periods of low flow in late summer, although concentrations were usually less than 1 µg/L. Atrazine (fig. 17), metolachlor, simazine, and prometon were each detected in more than half of these samples. In addition, tebuthiuron, diazinon, and carbaryl were each detected in at least 10 percent of samples, and 18 other pesticide compounds were detected in less than 10 percent of the samples. Only atrazine and metolachlor were measured at concentrations greater than 1 µg/L. Deethylatrazine was detected in 67 percent of samples from small streams at low flow and in 86 percent of such samples that contained detectable atrazine.

Plot: Atrazine concentrations during low flow (9,641 bytes)

Figure 17. Concentrations of atrazine in small streams during low flow in sampled areas of the Potomac River Basin. The highest concentrations of atrazine were typically detected in streams of the Great Valley, although the highest concentration was detected in the Triassic Lowlands.

Concentrations of pesticides in streams and ground water of the Potomac River Basin are usually not threatening to human health or most ecosystems, based on current standards and understanding. Established MCLs (U.S. Environmental Protection Agency, 1994a) for atrazine (3 µg/L), simazine (4 µg/L), and alachlor (2 µg/L) were only occasionally exceeded in stream samples collected during this study of the Potomac River Basin, most commonly during storms. The MCL of 70 µg/L for 2,4-D was never exceeded. There are currently no established drinking-water criteria for the other pesticides in common use in the basin, although established lifetime health-advisory levels (U.S. Environmental Protection Agency, 1996) for cyanazine and diazinon were infrequently exceeded. Although criteria for the protection of aquatic life do not reflect all possible effects of pesticides in streams and have not been established for many compounds, degradation products, and mixtures, established criteria (International Joint Commission Canada and United States, 1977; Canadian Council of Resource and Environment Ministers, 1991; U.S. Environmental Protection Agency, 1991a) were exceeded for atrazine, chlorpyrifos, cyanazine, diazinon, malathion, methylazinphos, and metolachlor. Aquatic-life criteria were exceeded almo1st exclusively in samples collected during storms in May, June, or July from Accotink Creek, the Monocacy River, or Muddy Creek. These streams drain predominantly urban or agricultural areas and were among the most intensely sampled for pesticides within the Potomac River Basin by the NAWQA Program.

Pesticides were commonly detected in streams and ground water in agricultural areas of the Potomac River Basin; samples from forested areas rarely contained detectable pesticides. Much of the agriculture in the basin is in the Great Valley, Piedmont, and Triassic Lowlands (fig. 3). In carbonate areas of the Great Valley, atrazine (figs. 16, 17), deethylatrazine, simazine, prometon, and metolachlor were each detected in more than half of the samples from ground water and small streams. Pesticides were also commonly detected in ground water and streams of the Piedmont and Triassic Lowlands. In the mostly forested Valley and Ridge, however, detections of pesticide compounds in streams were much less frequent, and there were virtually no detections in ground water. Six compounds (atrazine, deethylatrazine, p,p'-DDE, metribuzin, prometon, and simazine) were each detected in a single ground-water sample from the Valley and Ridge, and dichlorbenil was detected in four samples. Only atrazine, deethylatrazine, metolachlor, prometon, simazine, and tebuthiuron were detected in small-stream samples from the Valley and Ridge.

Pesticide concentrations in streams and ground water of the Potomac River Basin are greatest in areas of intense crop production - particularly in the corn-producing northeastern counties and in the Great Valley. Pesticide concentrations in ground water and small streams were typically higher in samples from the more heavily cropped areas than other agricultural areas (fig. 18). Among larger streams, the highest incidence of detections and elevated concentrations of atrazine and metolachlor also occurred predominantly in these areas, particularly in the Monocacy River and Opequon, Conococheague, and Antietam Creeks.

Map: Atrazine concentrations, and agricultural land planted in corn, barley, oats, wheat, sorghum, or soybeans (23,714 bytes)

Figure 18. Within agricultural areas, most elevated atrazine concentrations in ground water and in small streams at low flow were found in counties with the highest percentages of agricultural land devoted to row crops.

Streams in the Potomac River Basin are affected by pesticide applications in urban as well as agricultural areas. Many pesticides have been detected in samples from Accotink Creek, which drains a small, urban watershed near Washington, D.C. Herbicides detected at this site include atrazine, metolachlor, MCPA, oryzalin, and prometon. Simazine was detected most often and at the highest concentrations, occasionally exceeding the MCL of 4 µg/L. Insecticides--including diazinon, carbaryl, and chlorpyrifos--were also detected, year-round. Samples from Accotink Creek contained the highest concentrations of the insecticides diazinon and malathion measured in the Potomac River Basin and the highest concentrations of oryzalin and MCPA measured by the NAWQA Program. Pesticides found in Accotink Creek are generally used on rights-of-way, turf, golf courses, and for landscaping, and as additives to asphalt and other building materials

Maximum concentrations of most pesticides occur in streams during the spring and early summer months, although atrazine and metolachlor are present year round in agricultural areas. The Monocacy River at Bridgeport, Md., for example, drains 173 square miles, almost 80 percent of which is agricultural. Pesticide concentrations in this stream are related to flow, but the major controlling factor is the seasonal application (fig. 19). During a storm in May 1994, concentrations of alachlor (3.1 µg/L), atrazine (25 µg/L), cyanazine (3.0 µg/L), and metolachlor (23 µg/L) in the Monocacy River were the highest measured in any water sample during this study. Other pesticides frequently detected in the Monocacy River included simazine, prometon, and linuron. Of the 45 pesticides for which samples from this site were analyzed, 19 were detected at least once during the period of most intensive sampling in 1994. Pesticide concentrations in the Potomac River also were highest in the spring, during the time of applications. Concentrations declined through the summer months and were nearly undetectable in late fall and winter.

Graph: Atrazine, metolachlor, and stream flow (14,174 bytes)

Figure 19. In the Monocacy River at Bridgeport, Md., atrazine and metolachlor were present at detectable concentrations during most of the year but concentrations were typically highest during the late spring following application.

 

Three major floods occurred in the Potomac River Basin in 1996 and had different effects on water quality because they occurred during different seasons (fig. 20).

Record high flow for a single day occurred in January in the Potomac River at Washington, D.C., due to intense rainfall and rapid snowmelt caused by unseasonably warm weather following a blizzard. In June, basins of the Monocacy River and Conococheague Creek, which are dominantly agricultural, received intense rainfall over a 5-day period. Flow from the June storm in the Monocacy River at Bridgeport, Md., reached a level expected to occur only once in 200 years. Intensive rainfall from a tropical storm caused flooding in September in the mostly agricultural Shenandoah River Basin and points west.

Nitrogen loads in the Potomac River at Washington, D.C., were greater during the January flood than during June or September. Record flow, including runoff from previously frozen land, carried an estimated 18 million pounds of nitrogen, including large amounts of ammonia and organic nitrogen which are typically associated with manure and wastewater discharges.

The Potomac River at Washington, D.C., carried 25 times as much atrazine during the June flood as in September, even though the flow was three times greater in September. Over a 5-day period, the river carried an estimated 3,300 pounds of atrazine and 3.3 million pounds of nitrogen. On two consecutive days following the June storm, atrazine was measured in the river at concentrations greater than the MCL of 3 µg/L. Pesticides and fertilizers are generally applied to the land in the spring, whereas nitrogen may be applied in the form of manure throughout the year.

Long-term impacts of the unprecedented flooding within the Potomac River Basin during 1996 have yet to be determined; however, the USGS is currently studying aquatic vegetation downstream in the Potomac estuary, and State and local governments are planning to sample drinking-water sources for pesticides more frequently during peak application periods.

Bar charts: Storm flow, total nitrogen load, and atrazine load during 3 major floods (17,754 bytes)

Figure 20. Three major floods in the Potomac River Basin during 1996 had different effects on water quality in the Potomac River at Washington, D.C., because they occurred at different times of the year.

Within the Potomac River Basin, streams and ground water in the Great Valley show the greatest water-quality effects from agricultural sources (fig. 21) because the valley is intensely used for crop and animal production and because carbonate bedrock permits rapid transportation of constituents to ground water and streams. Much of the Great Valley is underlain by carbonate rock which commonly contains open conduits (solution channels) created and enlarged by the action of acidic water moving through rock fractures. Rain is naturally acidified by the dissolution of carbon dioxide in the atmosphere and can be further acidified by the introduction of nitric and sulfuric acids into the atmosphere from the combustion of fossil fuels. Chemicals at the land surface in agricultural areas such as the Great Valley can be quickly and easily carried to ground water through conduits in bedrock. Once chemical constituents reach the ground water in carbonate rocks, they may move quickly with ground-water flow to streams, springs, or water-supply wells. Nutrient and pesticide concentrations in ground water of carbonate areas are often highly variable because the ground water responds so quickly to precipitation and the application of fertilizers and pesticides. In addition, soils derived from carbonate rocks tend to contain little organic carbon and therefore have a reduced ability to bind to organic chemicals, such as pesticides (Barbash and Resek, 1996).

Plot:Nitrate concentration vs. percentage of cropland within 0.25 miles upgradient of sampled well (8,418 bytes)

Figure 21. Nitrate concentrations in ground-water samples from the Potomac River Basin increase with increasing percentages of cropland, but are typically higher in samples from carbonate rocks like those of the Great Valley than in samples from rocks of other types.

Photo (57,281 bytes)


U.S. Geological Survey Circular 1166

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Suggested citation:
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, West Virginia, and the District of Columbia, 1992-96: U.S. Geological Survey Circular 1166, on line at <URL: http://water.usgs.gov/pubs/circ1166>, updated June 10, 1998 .

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