Most pesticides are man-made chemicals. They are used to control pests such as weeds (herbicides), insects (insecticides), fungi (fungicides), and so forth. Pesticides present in streams, even at very low concentrations, can make the water unsuitable for human consumption or render it toxic to aquatic organisms.
The USGS National Water Quality Assessment Program is evaluating the relation of land use to surface- and ground-water quality. The NAWQA study of the Apalachicola-Chattahoochee-Flint (ACF) River basin has conducted an investigation to determine the presence, persistence, and water-quality effects of pesticides in streams draining small watersheds representative of urban and agricultural land-use areas in the basin. This pamphlet summarizes preliminary information from that investigation.
Water samples were collected weekly from March 1993 through April 1994 at an urban watershed in Metropolitan Atlanta and an agricultural watershed in southwest Georgia. The USGS National Water Quality Laboratory analyzed the samples for 84 pesticides commonly used throughout the United States. Many of these pesticides are used for weed and insect control in the ACF River basin. However, analyses do not include all pesticides in current use, having excluded a number of commonly used pesticides such as paraquat, methanearsonate, glyphosate, DSMA, MSMA, and several chlorophenoxy herbicides.
Urban areas are a small part of the overall land area, yet they may have a greater effect on water quality in the Apalachicola-Chattahoochee-Flint River basin than agricultural areas. This study evaluated pesticide contamination in streams draining an urban watershed in Metropolitan Atlanta and an agricultural watershed in southwest Georgia.
18 herbicides and 7 insecticides were detected in water samples from
the urban watershed.
Median concentrations for each detected pesticide
were well below EPA standards and guidelines for drinking water (see
table). In one sample, the maximum concentration for the herbicide
simazine exceeded the maximum contaminant level (MCL) for drinking
water allowed by EPA drinking-water standards, however, the median
concentration for all samples was only 3 percent of the MCL. Atrazine
and diazinon had median concentrations that were only 1 percent of the
MCL and 3 percent of the lifetime health advisory, respectively.
However, maximum concentrations of five detected insecticides and
median concentrations of chlorpyrifos and diazinon exceeded guidelines
for protection of aquatic life.
Many compounds were present throughout much of the year, but with highest concentrations at, or just after their principal application periods. Atrazine and simazine, commonly used to control winter annual weeds on turf, were detected at highest concentrations in December and February. Pendimethalin and 2,4-D, commonly used for postemergent control of weeds on turf, were detected at highest concentrations during March and April. Prometon and tebuthiuron, commonly used to control vegetation on noncropland, were detected at highest concentrations in March and May. The highest concentration of each insecticide detected was in May.
16 herbicides and 4 insecticides were detected in water samples from
the agricultural watershed.
No pesticide concentrations exceeded EPA
standards or guidelines for drinking water. The insecticide carbaryl
was detected in one sample at a concentration that exceeded guidelines
for protection of aquatic life. However, median concentrations for all
detected pesticides were below the method detection limits and well
below standards and guidelines for drinking water and protection of
aquatic life.
Most herbicides detected were primarily used for control of weeds on cropland, although additional herbicides used for vegetation control in noncrop areas also were detected. The insecticides detected are used on a number of crops and orchards, as well as for use at homes.
Pesticides were detected with greatest frequency from March through July in the agricultural watershed. This corresponds with the principal application periods for preemergent and postemergent herbicides and foliar applications of insecticides.The herbicides atrazine and metolachlor were the most persistent, with detectable concentrations throughout most of the year.
The maximum contaminant level (MCL) is the maximum concentration allowed in drinking water according to EPA drinking-water standards. The lifetime health-advisory (HA) level is the maximum concentration in drinking water that would not cause adverse human-health effects, excluding cancer (based on a 150-pound adult consuming about 2 quarts of water per day for a period of 70 years). The standards for protection of aquatic life are recommended by the National Academy of Sciences and the National Academy of Engineering, and are based on the best available data regarding maximum concentrations in water that are not expected to adversely affect aquatic life.
To date, 2,4-D is the only pesticide detected in waters within the ACF River basin that has an estimate of cancer risk from drinking water. The estimated risk is based on the assumption that if a 150-pound adult drank about 2 quarts of water per day containing 100 micrograms per liter (µg/L) 2,4-D during an average 70-year life span, that person would increase his/her chances of contracting cancer by 1 in a million. The highest measured 2,4-D concentration in samples collected to date (July 1994) was 0.63 µg/L. The assessment of risk associated with human activities is very complex, however, the adjoining table helps to put into perspective the risk associated with 2,4-D in water as related to other selected human activities. This perspective is not intended to minimize the importance of detecting harmful chemicals in the water resources of the ACF River basin, but rather to indicate that even though there should be concern that pesticides are present, they are present at very low concentrations and generally do not pose serious health concerns.
Potential sources for pesticides in urban streams include: