Envkon. SCI. T M . 1991. 25. 1794-1796
COMMUNICATIONS Herbicides in Surface Waters of the Midwestern United States: The Effect of Spring Flush E.
m. Thurman;
D. A. Goolsby,+ m. T. Meyer. and D. W. ~olpln'
U.S. Geological Survey, 4821 Quail Crest Place, Lawrence, Kansas 66049
Introduction Approximately three-fourths of all preemergent herbicides used in the United States are applied to row crops over a 12-state area, called the "corn belt" (I). The application of these compounds may cause widespread degradation of water quality (2). Because herbicides are water soluble, there is the potential for leaching into groundwater and surface water (3, 4), as well as aerial transport and Occurrence in precipitation (5). Monitoring studies in the Midwest have shown widespread detection of herbicides in groundwater and in surface water ( 3 , 4 ) ;however, little is known about the regional impact of herbicide application (6). The objective of our research was to assess the mag. nitude and persistence of herbicide runoff in the spring flush at the regional scale. Experimental Procedures Water samples were collected from 149 sites in 122 river basins during March and April (preplanting, 1989-1990), May and June (ptplanting, 198%1990), and October and November (harvest, 1989) a t U S . Geological Survey streamflow-gaging stations. We designed a stratified random-sampling procedure to ensure geographic distribution and broad-scale interpretation of the data. Drainage areas of the selected basins range from 260 kmz to more than 16OooO km2,and collectively the basins drain more than 500000 km2 of the Midwest (Figure 1). We screened the samples for triazine herbicides with a new, low-cost enyzme immunoassay (7) and then analyzed the samples for 11herbicides and 2 atrazine metabolites with a robotic procedure using solid-phase extraction and gas chromatography/mass spectrometry (7). Results and Discussion The most striking feature of the reconnaissance data (Table I; Figure 1) was that large concentrations of herbicides were flushed from cropland and were transported through the surface-water system as pulses in response to late spring and early summer rainfall. Median concentrations of the four major herbicides (atrazine, alachlor, cyanazine, and metolachlor) increased by 1 order of magnitude and then decreased to near preplanting levels by harvest sampling (Figure 2A,B). Second, several herbicides exceeded EPA-promulgated maximum contaminant levels (PMCL) for drinking water. For example, 52% of the sites exceeded the PMCL for atrazine (3 pg/L), 32% for alachlor (2 @g/L),and 7% for simazine (1pg/L). The PMCLs do not consider the cu-
'US.Geological Survey, Denver Federal Center, Bldg. 25, Lakewood. CO 80225. lU.S. Geological Survey, 400 South Clinton St., Room 269, Iowa City, IA 52244. 1794 Environ. Sci. Technol.. Vol. 25, NO. IO. 1991
EXPLAINhTION MEDIAN BASIN CONCENTRAilON IN MICROGRAMS PER LITER
0 0
-50 5010200
,200
-DRAINAGE BASIN *
BOUNDARY
SAMPLING SITES
Flgure 1. W m p M disbibutkm of total herbicides by GClMS for ttm
postplantlng sampling period. displayed by hydrologic cataloging unn. which is based on samples collected at gaging stations located near the terminus of the basin. Table I. Summary of Percent DetectionP of Herbicides' herbicide
preplanting
postplanting
harvest
alachlor atrazine desethylatrazine desisopropylatrazine cyanazine metolachlor metribuzin propazine prometon simazine
18 91 54 9
86 98 86 54
12 76 47
5
63
0
34 2 0
44
0
83 53 40 23
7
55
0
0
