U.S.Geological Survey Water-Resources Investigations Report 98-4017
Abstract
The occurrence, concentrations, and loads of dissolved pesticides in
storm runoff were compared for two contrasting land uses in the Tuolumne
River Basin, California, during two different winter storms: agricultural
areas (February 1994) and the Modesto urban area (February 1995). Both
storms followed the main application period of pesticides on dormant almond
orchards. Eight samples of runoff from agricultural areas were collected
from a Tuolumne River site, and 10 samples of runoff from uran areas were
collected from five storm drains. All samples were analyzed for 46 pesticides.
Six pesticides were detected in runoff from agricultural areas, and 15
pesticides were detected in runoff from urban areas. Chlorpyrifos, diazinon,
dacthal (DCPA), metolachlor, and simazine were detected in almost every
sample. Median concentrations were higher in the runoff from urban areas
for all pesticides except napropamide and simazine. The greater occurrence
and concentrations in storm drains is partly attributed to dilution of
agricultural runoff by nonstorm baseflow in the Tuolumne River and by storm
runoff from nonagricultural and nonurban land. In most cases, the occurrence
and relative concentrations of pesticides found in storm runoff from agricultural
and urban areas were related to reported pesticide application.
Pesticide concentrations in runoff from agricultural areas were more variable during the storm hydrograph than were concentrations in runoff from urban areas. All peak pesticide concentrations in runoff from agricultural areas occurred during the rising limb of the storm hydrograph, whereas peak concentrations in the storm drains occurred at varying times during the storm hydrograph. Transport of pesticides from agricultural areas during the February 1994 storm exceeded transport from urban areas during the February 1995 strom for chlorpyrifos, diazinon, metolachlor, napropamide, and simazine. Transport of DCPA was about the same from agricultural and urban sources, and the main source of transport for the other pesticides could not be determined because of concentrations less than the method detection limit.
Contents
Abstract
Introduction
Methods
Sample-Collection Strategy
Sample Collection, Processing, and Laboratory Methods
Pesticides in Storm Runoff
Occurrence
Variation in Concentrations
Loads
Occurrence of Pesticides in Relation to Application
Summary of Conclusions
References Cited