Environmental Monitoring and Pest Management Branch
Department of Pesticide Regulation
Simazine and diuron are herbicides
that have been found in surface and ground water in California.
Because they have traditionally been used on highway rights-of-way
during the rainy season, this is a potential source of surface
and ground water pollution. The Department of Pesticide Regulation
(DPR), which is responsible for preventing pesticide contamination
of surface and ground water, conducted a study in Glenn County
northern Central Valley, in cooperation with the California Department
of Transportation (Caltrans), the agency responsible for weed
control along all State and interstate highways in California.
Simazine and diuron were applied
together in a spray to a 2.4 meter wide strip next to the highway
pavement at three sites, and simulated rain was applied to the
treated areas. Investigators then measured concentrations of
simazine and diuron in water running off the experimental sites,
and in soil cores. Similar measurements were made at other treated
sites after natural rainfall events.
Where artificial rainfall ran off
sites, simazine and diuron were detected in the runoff waters.
Simazine concentrations in runoff from sites with artificial
rainfall ranged from 78 to 574 parts per billion (ppb). For reference,
the U.S. Environmental Protection Agency (US EPA) health advisory
for simazine is 4 ppb, which is the same as the Maximum Contaminant
Level (MCL) established by the California Department of Health
Services (DHS). Diuron concentrations ranged from 144 to 1770
ppb. The US EPA health advisory for diuron is 10 ppb. DHS has
not established an MCL for this herbicide. Where natural rainfall
ran off sites, simazine and diuron were also detected in the runoff
waters. Concentrations ranged from 29 to 337 ppb simazine, and
46 to 2849 ppb diuron.
Soil samples taken at sites receiving
artificial rainfall contained simazine and diuron residues both
before and after experimental application of these herbicides.
Before application, residues in soil ranged from none detected
to 694 micrograms simazine per kilogram of soil, and none detected
to 145 micrograms diuron per kilogram of soil. Immediately after
application, residues ranged from 6.7 to 104 micrograms simazine
per kilogram of soil, and 57 to 874 micrograms diuron per kilogram
of soil. The high degree of variability of soil residues is probably
attributable to the complex infiltration and transport processes
in soils. After approximately 321 mm of total seasonal precipitation,
residues per kilogram of soil were greatly reduced, to 57 micrograms
simazine and up to 94 micrograms diuron. The maximum depth at
which herbicide was found at any of the 3 sites was 0.3 m.
Natural rain runoff from one quadrant
of a freeway interchange was also sampled during several storms.
Only simazine was applied at this site. Samples were collected
from a flume that discharged runoff into a drainage canal, which
flows through the Sacramento National Wildlife Refuge. Simazine
concentrations averaged 105 ppb after 100 mm of rain had fallen,
and 83 ppb after the heaviest rainfall sampled.
During the time that they were cooperating
in this study, Caltrans was also in the process of adopting an
integrated vegetation management program designed to reduce its
use of chemical pesticides for vegetation management. As a result
of this program, adopted in 1992, Caltrans has changed the way
it manages weeds along rights-of-way. A vegetation management
strategy is identified for each treatment site that is as self-sustaining
as possible over the long term. Control actions are selected
for the near-term which reduce dependence on chemical pesticides.
Naturally occurring controls on pests are used where feasible.
Because the vegetation management strategy leads to reduced-risk
pest management practices, it is consistent with the Department
of Pesticide Regulation=s
pest management strategy, one element of which is to encourage
pesticide users to adopt reduced-risk pest management practices.
Where highway treatment sites do
receive applications of simazine and diuron, the results of this
study indicate that further research is desirable, especially
to assess the potential hazard to aquatic life in surface waters
receiving runoff from rights-of-way.
Simazine and diuron runoff from highway rights-of-way in California is a potential source of environmental contamination because these preemergence herbicides are widely used during the rainy season from November to March. A study to investigate this concern was conducted in Glenn County in California's northern Central Valley, in cooperation with the California Department of Transportation, which is responsible for weed control along all State and Interstate highways. Simazine and diuron were applied together in a spray to a 2.4-m-wide strip next to the highway pavement, at the rate of 2.02 kg simazine active ingredient ha-1 and 3.59 kg diuron ha-1. Concentrations of simazine and diuron in highway runoff were measured during both simulated and natural rainfall. Simulated rain (13 mm in 1 hr) was applied to plots on treated highway shoulders at three sites. At one site, none of the artificial rainfall ran off the plot. At the other two sites, 5-12% and 17-46% of the applied water ran off. Simazine concentrations in runoff at these two sites, respectively, ranged from 78-447 and from154-574 :g L-1; diuron concentrations ranged from 144-1175 and 348-1770 :g L-1. Total mass of herbicide leaving the plots in runoff accounted for 0.2-1.8% and 1.6-2.3% of total simazine applied at each of the two sites, respectively, and for 0.2-3.2% and 2.5-5.4% of the diuron. Soil was sampled to a depth of 3 m at the site where no runoff occurred, and to 1 m at the other sites. Soil was sampled to a depth of 3 m at the site where no runoff occurred, and to 1 m at the other sites. Herbicide was not found below 0.3 m depth at any of the 3 sites. Of the total 38 samples taken from the top 0.3 m of soil, 13 contained simazine (maximum concentration 694 ug kg-1, found prior to herbicide application) and 17 contained diuron (maximum concentration 874 ug kg-1, just after rainfall simulation). Natural rain runoff was sampled at a fourth site during several winter storms. Concentrations ranged from 29-337 :g L-1 simazine and 46-2849 :g L-1 diuron. The largest amounts removed in any sampled period were 5.3% of the applied simazine and 8.4% of the diuron in one 28-hr period. Natural runoff from one quadrant of a freeway interchange was also sampled during several storms. Only simazine was applied at this site. Samples were collected from a flume that discharged runoff into a drainage canal. The first runoff sample was taken after a total of 100 mm of rain had fallen, and simazine concentration averaged 105 :g L-1 in 52-66 m3 of runoff water collected. The greatest mass discharge in any sampled period was 155-200 m3 of runoff in 20 hr, with an average concentration of 83 :g L-1 simazine. Further research should assess the potential hazard to aquatic life in receiving waters.