U.S. Department of the Interior USGS Fact Sheet FS-011-03

U.S. Geological Survey March 2003

Issues

In 1996, atrazine (6-chloro-N-ethyl-N’-

(1-methylethyl)-1,3,5-triazine-2,4-diamine) 

was the most heavily applied herbicide in 

the United States, with a total use of about 

33 million kilograms (Capel and Larson, 

2001). Atrazine is a highly soluble herbicide. The greatest area of use is in the Corn 

Belt of the upper Midwest, including the 

upper Mississippi and Illinois River Basins 

(Meade, 1995). The waters in these basins 

are sources of drinking water downstream 

either directly from the Mississippi River 

mainstem at New Orleans or by way of distributaries draining the lower Mississippi 

River such as Bayou Lafourche and the Gulf 

Intracoastal Waterway (GIWW) (fig. 1). 

The U.S. Environmental Protection Agency 

(USEPA) has established a maximum conNATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Atrazine in Southern Louisiana Streams, 1998-2000

INTRODUCTION

The landscape of southern Louisiana 

has been sculpted by the Mississippi 

River. The historical and ongoing riverine processes of delta lobe formation and 

abandonment have produced the swamps, 

marshes, and alluvial plains that are typically associated with southern Louisiana. The Mississippi River drains the 

midcontinental United States, which covers 41 percent of the continental United 

States, and the riverine deposits transported by the Mississippi River to southern Louisiana over the last 7,500 years 

have resulted in highly fertile soils for 

agriculture. Important crops in Louisiana 

are sugarcane in the southeastern area, 

corn in the south-central area, and rice 

and sugarcane in the southwestern area. 

These crops, especially corn and sugarcane, are routinely treated with the herbicide atrazine for broadleaf weed control. 

Atrazine can be applied at different times 

of the year, early in the growing season as 

a preplanting or pre-emergent herbicide 

for control of most germinating broadleaf 

weeds, or later in the growing season for 

post-emergent weed control. Atrazine 

acts as a selective herbicide by inhibiting 

photosynthesis (U.S. Environmental Protection Agency, 2002, 1986). 

taminant level (MCL) of 3 μg/L 

(micrograms per liter) for atrazine 

in drinking water. However, 

because MCL’s are based on 

annual average concentrations of 

quarterly samples, one or more 

exceedances don’t necessarily 

indicate noncompliance. Currently (2003) atrazine is classified 

as “not likely to be carcinogenic to 

humans” (U.S. Environmental 

Protection Agency, 2002). There 

is some evidence that atrazine 

may have endocrine disruption 

(feminization) effects on amphibians at concentrations less than 

Sugarcane in the upper Terrebonne Basin.

Slow-flowing bayou in the Barataria Basin.

Herbicide

Solubility, in 

milligrams per 

liter

Soil half-life, 

in days

Maximum 

contaminant level, 

in micrograms per 

liter

Health advisory 

level, in 

micrograms per 

liter

Atrazine 33 60 3 3

Meade, 1995.

1 μg/L (Hayes and others, 2002). Additional information on atrazine can be 

found at URL http://www.epa.gov/pesticides/reregistration/atrazine

The U.S. Geological Survey (USGS), 

as part of the National Water Quality 

Assessment Program (NAWQA), is investigating the occurrence, distribution, and 

transport of pesticides, including atrazine, 

in southern Louisiana. The Louisiana 

Department of Agriculture and Forestry 

has an atrazine monitoring program in the 

upper Terrebonne Basin. The Louisiana 

Department of Environmental Quality 

(DEQ) has identified Bayou Grosse Tete, a 

tributary in the basin, as a stream of concern due to elevated atrazine concentrations. Because of this stream’s location in 

the upper part of the Terrebonne Basin, the 

waterway is hydrologically isolated from 
U.S. Department of the Interior USGS Fact Sheet FS-011-03

U.S. Geological Survey March 2003

the Mississippi River, and concentrations 

of atrazine in the waterway almost certainly originate from local inputs. 

This report presents atrazine data collected at 20 sites in 1998-2000 from 

selected streams in the NAWQA Acadian-Pontchartrain Drainages study area 

(fig. 1). These data are compared to atrazine data from the Mississippi River at 

Baton Rouge to evaluate seasonal fluctuations in the concentrations and to determine the sources of atrazine in streams in 

the study area. 

Study Area

The Acadian-Pontchartrain Drainages study area comprises most of southern Louisiana, excluding the Pearl and 

Sabine River Basins (fig. 1). Basins in the 

study area include the Barataria, Terrebonne, and Mermentau River.

The Barataria and Terrebonne Basins 

consist of a series of interdistributary 

Figure 1. Location of study area in southern Louisiana.

basins separated by natural and constructed levees. Jean Lafitte National 

Historical Park, located in the Barataria 

Basin, has been isolated from direct Mississippi River inputs due to natural processes of levee formation and a complex 

series of artificial levees constructed primarily for flood control. However, this 

hydrologic isolation has changed with 

the completion in July 2002 of the Davis 

Pond Diversion. As much as 10,000 ft3

/s 

(cubic feet per second) will be diverted 

from the Mississippi River through the 

diversion into the upper Barataria Basin 

during the spring and early summer. 

Bayou Lafourche, which as recently as 

300 years ago was a major distributary of 

the Mississippi River, was hydrologically 

isolated from the river in 1904. During 

1998-2000, about 200 to 250 ft3

/s were 

either pumped or siphoned from the river 

into Bayou Lafourche for drinking-water 

supply and industrial uses.

 In addition to the Mississippi River, the 

hydrology of southern Louisiana also is influenced by the Atchafalaya River, a distributary 

of the Mississippi River. The Atchafalaya 

River flows through south-central Louisiana 

to the Gulf of Mexico (fig. 1). Some of the 

Atchafalaya River water is distributed east 

and west through the GIWW. The amount of 

water moving through the GIWW and into 

canals and bayous of the Terrebonne Basin 

can reach 12,000 ft3

/s (C.M. Swarzenski, 

USGS, written commun., 2002). The sources 

of this water can be from the upper Mississippi, the Missouri, and the Ohio Rivers or 

other tributaries to the Mississippi River in 

Mississippi, Alabama, and Louisiana. 

 The Penchant marshes, located in the 

Terrebonne Basin, receive freshwater from 

the Mississippi River by way of the Atchafalaya River into the GIWW. The GIWW distributes a west-to-east flow of Atchafalaya 

River water from the Terrebonne Basin across 

Bayou Lafourche and into the Barataria 

Basin. 
U.S. Department of the Interior USGS Fact Sheet FS-011-03

U.S. Geological Survey March 2003

The Mermentau River Basin is located 

in southwestern Louisiana. Locally, the area 

is called Coastal Prairie or Wet Prairie. The 

Mermentau River Basin is characterized by a 

heavy clay soil and low gradient streams. 

There were originally 2.5 million acres of 

wet prairie in Louisiana, and less than 

100 acres of undisturbed wet prairie remain 

in the Mermentau River Basin (Allain and 

others, 2000). Most of the basin now is primarily used for growing rice, sugarcane, and 

soybeans.

SOURCES AND INTERACTIONS

Southern Louisiana is unique in that 

there is a combination of atrazine inputs 

from local sources and from sources in the 

midcontinental United States (midcontinental sources). The amounts and timing of 

these local inputs in Louisiana streams differ 

from the other regions of the Nation, where 

atrazine concentrations increase in early 

spring and then gradually decline to concentrations near minimum detection levels. 

Therefore, an assessment of the occurrence 

and distribution of atrazine concentrations in 

southern Louisiana streams must take into 

account the timing, probable sources, and 

routes of atrazine introduction into the environment.

The Mississippi River

Figure 2 shows the pattern of atrazine 

concentrations in the Mississippi River at 

Baton Rouge (site 19), 1999-2000. The 

graph includes data from February to May 

2000 from the Mississippi River near St. 

Francisville, located about 30 river miles 

upstream from Baton Rouge. The graph 

indicates a relatively consistent seasonal pattern of peak concentrations arriving in Louisiana in May and June. This period of 

increased atrazine concentrations in streams 

and rivers has been referred to as the spring 

flush. The increased concentrations are the 

result of usage, rainfall, and subsequent runoff in agricultural areas of the midcontinental United States. The amplitude and 

duration of the concentration peaks are the 

result of the timing of pesticide applications 

and rainfall patterns for that particular year. 

For example, reduced-rainfall years in the 

midcontinental United States such as 

occurred in 2000, will produce lower peaks 

as compared to wet years as less pesticide 

will runoff into receiving waterbodies.

Bayou Grosse Tete

Bayou Grosse Tete at Rosedale (site 4) 

is located in an agricultural area in the 

northern part of the Terrebonne Basin. 

Corn, sugarcane, and soybeans are the predominant crops. The bayou was sampled 

monthly or twice monthly for atrazine during 1999-2000. Figure 3 shows the pronounced peaks in April after atrazine had 

been applied to corn and sugarcane. The 

vary with the type of crop, acreage planted, 

and the timing and amount of rainfall. The 

primary use of atrazine on both corn and 

sugarcane is as a pre-emergent herbicide 

(Louisiana State University Agricultural 

Center, 1998), but there may be several 

additional applications in the northern Terrebonne Basin during the growing season 

to control broadleaf weeds. The acreage 

planted in either sugarcane or corn can vary 

greatly in response to prices and crop disease (Larry LeJeune, Louisiana Department of Agriculture and Forestry, oral 

commun., 1999). 

Figure 2. Atrazine concentrations in the lower Mississippi 

River near St. Francisville and at Baton Rouge.

Figure 3. Atrazine concentrations representing local sources at Bayou Grosse Tete at 

Rosedale in the northern Terrebonne Basin.

peak concentration determined in 1999 at site 4 was 

41.3 μg/L on April 9. In 

comparison, the peak concentration determined in the 

Mississippi River at Baton 

Rouge (site 19) was 1.41 μg/

L on May 25, over a month 

later. Similarly in 2000, the 

peak concentration determined at site 4 was 37.7 μg/

L on April 10, and the peak 

concentration determined at 

site 19 was 1.17 μg/L on 

June 20. The annual pattern 

in Louisiana, especially the 

amplitude of the peak, will 
U.S. Department of the Interior USGS Fact Sheet FS-___-2003

U.S. Geological Survey

Site 

numberSite name

Atrazine, in micrograms per liter

March May June August

Date Concentration Date Concentration Date Concentration Date Concentration

1 Mississippi River near St. Francisville 3-30 0.11 5-6 1.31 6-24 0.41 8-5 0.21

2 Atchafalaya River at Melville 3-16 .13 5-7 1.04 6-28 .19 8-5 .22

3 Bayou Grosse Tete at Frisco 3-21 .81 5-10 .37 6-18 .35 8-30 .20

4 Bayou Grosse Tete at Rosedale 3-21 .88 5-10 1.36 6-18 2.80 8-30 .21

5 Choctaw Bayou near Brusly 3-21 7.20 5-10 1.73 6-21 2.06 8-30 .19

6 Gulf Intracoastal Waterway at Jack Miller Landing 3-21 1.84 5-11 1.44 6-21 1.06 8-30 .43

7 Bayou Pierre Part at Pierre Part 3-22 3.44 5-11 1.20 6-22 .71 8-31 .36

8 Belle River at Belle River 3-22 3.50 5-11 .80 6-22 .65 8-31 .42

9 Cancienne Canal at Bayou Crab 3-22 6.82 5-12 1.02 6-22 1.02 8-31 .20

10 Bayou Lafourche at Thibodaux 3-21 .18 5-11 15.60 6-22 .69 8-31 .31

11 Grassy Lake at Simon Pass 3-23 1.84 5-12 1.07 6-22 .91 8-31 .39

12 Bayou Boeuf at Amelia 3-22 .68 5-12 1.18 6-22 .79 8-31 .50

13 Bayou Chevreuil near Chackbay 3-21 .38 5-11 .17 6-22 .84 8-31 .14

14 Turtle Bayou near Bayou Penchant 3-22 .01 5-10 .05 6-9 .41 8-25 .18

15 Bluebird Canal near Bayou Copesaw 3-22 .13 5-10 .12 6-9 .63 8-25 .10

16 Interior Canal at Salvador Wildlife Management Area 3-22 .01 5-10 .10 6-9 .02 8-25 .07

17 Keyhole Six near Westwego 3-22 .01 5-10 .09 6-9 .01 8-25 .09

Figure 4. Distribution of atrazine concentration at selected suface-water sites in the Barataria and Terrebonne Basins, March through

August 1999.
U.S. Department of the Interior USGS Fact Sheet FS-011-03

U.S. Geological Survey March 2003

Barataria and Terrebonne Basins

Four atrazine surveys were conducted in 

the upper to mid reaches of the Barataria and 

Terrebonne Basins at approximately 6-week 

intervals from March to August 1999. Atrazine concentrations at 11 sites (sites 3-13, 

fig. 4) were compared to atrazine concentrations at the Mississippi River near St. Francisville (site 1) and the Atchafalaya River at 

Melville (site 2).

The results for March show elevated 

atrazine concentrations at sites 3, 4, 5, and 6, 

ranging from 0.81 to 7.20 μg/L. These sites 

are located in a mixed corn- and sugarcaneproducing area. Similarly, sites 7, 8, 9, and 

11, with concentrations ranging from 3.44 to 

6.82 μg/L, are located in predominantly sugarcane-producing areas. When compared to 

atrazine concentrations of 0.11 and 0.13 μg/L 

from the Mississippi and Atchafalaya Rivers, 

respectively, the concentrations and timing 

indicate local sources of atrazine.

The results for May indicate mixed 

sources of atrazine in southern Louisiana 

streams. Atrazine concentrations in the Mississippi and Atchafalaya Rivers are higher 

than in March (1.31 and 1.04 μg/L), and concentrations at sites 3, 5, 6, 7, 8, 9, and 11 are 

all lower than in March. However, the conIn June, atrazine concentrations at the 11 

sites are generally less than the earlier peaks, 

but the concentrations continue to remain 

well above detection limits, from 0.35 to 

2.80 μg/L. This indicates that by June the 

interaction of local sources, midcontinental 

sources, hydrologic modifications, and low 

gradients in the coastal zone have produced a 

mixture of prolonged low-level and elevated 

atrazine concentrations. This scenario is 

more complex than the traditional concept of 

a spring flush, illustrated most clearly by the 

Mississippi River mainstem.

The concentrations in late August, 

although at a lower level than June, show that 

atrazine is still present in the hydrologic system. Atrazine concentrations continue to be 

greater than 0.3 μg/L at sites 6, 7, 8,10, 11, 

and 12.

Barataria-Terrebonne Interior Marsh

Atrazine from the flush of pesticides 

from midcontinental sources enters coastal 

parts of the Barataria and Terrebonne Basins 

through the Lower Atchafalaya River. 

Locally-applied atrazine also enters the same 

coastal wetlands primarily through Bayou 

Boeuf and Bayou Lafourche. Atrazine from 

both sources flows into the GIWW and is carried east and south into the interior marshes 

by the natural hydraulic gradient. LocallyFigure 5. Atrazine concentrations, Barataria and Terrebonne Basins.

applied atrazine enters the GIWW (site 

6) in late March (figs. 4, 5), and the flush 

from midcontinental sources arrives 

more than a month later as evidenced by 

concentrations in the Atchafalaya River. 

Freshwater wetlands in the western 

part of the Terrebonne Basin, therefore, 

receive water from two sources, both 

containing substantial quantities of atrazine at different times. In contrast, the 

hydrologically-isolated freshwater wetlands in the eastern part of the Barataria 

Basin do not receive water containing 

elevated levels of atrazine. For example, 

atrazine concentrations measured in June 

at two locations in the Terrebonne Basin, 

sites 14 and 15, were elevated, whereas 

atrazine concentrations measured at sites 

16 and 17 located in the eastern part of 

the Barataria Basin were not (figs. 4, 5). 

The marsh type that dominates in both 

basins is peat marsh, locally referred to 

as flotant for its buoyant properties. Currently it is unknown if the herbicide atrazine has any adverse effects on flotant. 

Peat accumulation and substrate integrity 

are largely the result of root productivity, 

and atrazine is known to inhibit root productivity.

centration in Bayou Lafourche 

(site 10) increased from 0.18 to 

15.6 μg/L. As this bayou receives 

water directly from the Mississippi 

River, this increase is caused by a 

combination of local and midcontinent sources, with local sources 

apparently predominating. The 

March and May surveys missed 

the April peak concentrations 

measured in Bayou Grosse Tete 

(fig. 3), but atrazine concentrations 

derived from local sources probably peaked between late March 

and mid-May. Atrazine concentrations peaked in the Mississippi 

River during late May.
U.S. Department of the Interior USGS Fact Sheet FS-011-03

U.S. Geological Survey March 2003

Tropical Storm Francis

Atrazine concentrations were measured at five sites immediately after Tropical Storm Francis passed through southern 

Louisiana in September 1998 (fig. 6). 

Although there is no widespread application of atrazine in southern Louisiana in 

September and the streams sampled were 

bank-full or flooding, atrazine concentrations were still elevated, ranging from 0.62 

to 0.95 μg/L at the three sites (Bayous 

Lafourche, Boeuf, and Grosse Tete) in the 

Terrebonne Basin. Dawson Creek (site 

20), located in the City of Baton Rouge, 

and Bayou Lacassine (site 18), located in 

the Mermentau River Basin, had concentrations of 0.06 and 0.08 μg/L, respectively. 

Atrazine is applied in Baton Rouge as a 

landscaping herbicide and in the Mermentau River Basin for general broadleaf weed 

control. The results indicate that these 

streams might be affected by low levels of 

atrazine. Apparently atrazine is retained 

within these basins for months and is 

flushed out during storms. 

CONCLUSIONS

The occurrence and distribution of 

atrazine in streams in southern Louisiana is 

the result of mixing of local and midcontinental sources. The combination of lowgradient coastal plains and interbasin 

hydrologic connections, in conjunction 

with spring flush inputs from the Mississippi and Atchafalaya Rivers, result in a 

prolonged period of low-level and elevated 

atrazine concentrations in local streams. 

This may have disturbing implications for 

cumulative environmental effects from 

prolonged exposure to low-level concentrations of atrazine. Amphibian populations may be particularly at risk from 

atrazine effects as an endocrine disrupter 

through the prolonged impact of low-level 

atrazine concentrations below 1 μg/L .

REFERENCES

Allain, Larry, Vidrine, Malcolm, Grafe,

Vicki, Allen, Charles, and Johnson,

Steve, 2000, Paradise lost? The

coastal prairie of Louisiana and Texas:

U.S. Fish and Wildlife Service, 39 p.

Capel, P.D., and Larson, S.J., 2001, Effect

of scale on the behavior of atrazine in

surface waters: Environmental Science

and Technology, v. 35, no. 4, 

p. 648-657.

Hayes, T.B., Collins, Atif, Lee, Melissa, 

Mendoza, Magdelena, Noriega, 

Nigel, Stuart, A. A., and Vonk, Aaron, 

2002, Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically 

relevant doses: Proceedings of the 

National Academy of Sciences, v. 99, 

no. 8, p. 5476-5480.

Louisiana State University Agricultural 

Center, 1998, Suggested weed control 

guide for 1998: Baton Rouge, La., 

Louisiana Cooperative Extension 

Service, 183 p.

Meade, R.H., ed., 1995, Contaminants in 

the Mississippi River, 1987-92: U.S. 

Geological Survey Circular 1133, 

140 p.

U.S. Environmental Protection Agency, 

1986, Water quality advisory, Atrazine: Washington, D.C., Criteria and 

Standards Division, Office of Water 

Regulations and Standards, 31 p.

---2002, Revised human health assessment, Atrazine: Reregistration Branch 

3, Health Effects Division, Office of 

Pesticide Programs, accessed September 16, 2002, at URL http://

www.epa.gov/pesticides/reregistration/atrazine/

By Dennis K. Demcheck and 

 Christopher M. Swarzenski

Figure 6. Atrazine concentrations after Tropical Storm Francis, Sept. 12-13, 1998.

For additional information, contact:

District Chief

U.S. Geological Survey

3535 S. Sherwood Forest Blvd., Suite 120

Baton Rouge, LA 70816

Telephone: (225) 298-5481

Fax: (225) 298-5490

Home pages: http://water.usgs.gov/nawqa

http://la.water.usgs.gov