You are here: Home › Stressors › Pollution › COAST › Assessment Studies
Current Use Pesticide Study
While synthetic organic pesticides are often implicated for their potential to adversely affect the health of coastal ecosystems, evidence to support this is primarily limited to the organochlorine insecticides that were banned in the 1970s and 1980s. In contrast, evidence of aquatic ecosystem impact from contemporary pesticides is scarce and limited to site specific contamination during isolated storm and runoff events (Pait et al., 1992; Johnson et al., 1994).
The National Oceanic and Atmospheric Administration (NOAA) through its National Status and Trends (NS&T) Program, Mussel Watch Project, monitors chemical contamination in coastal waters of the United States, and the Great Lakes. The goal of the project is to assess the status and trends of chemical contamination through the collection and analysis of bivalve tissues at over 280 sites biennially and sediments on a less frequent basis. Among the more than 80 chemicals monitored are many of the banned organochlorine insecticides. Except for gamma hexachlorocyclohexane (lindane), no contemporary pesticides were monitored in the Mussel Watch Project prior to 1993 at which time two contemporary pesticides, chlorpyrifos and endosulfan, were added to the list of compounds.
We report in this study the results four years of chlorpyrifos monitoring (1994 to 1997) in bivalve tissue and sediment, and one year (1997) of endosulfan monitoring in bivalve tissue. The spatial distribution of these two compounds and the mechanisms by which they are transported to coastal environments are evaluated. Concentrations of these compounds are compared with respect to their proximity to human population, freshwater discharges from fluvial and estuarine drainage areas, and pesticide and land use characteristics. Analysis of the temporal tends in concentrations will be determined after at least six years of monitoring has been completed.
Background
Pesticides, unlike most compounds monitored in the NS&T Program, are intentionally released into the environment to control plant and animal pests. Once released they may transform into other compounds of equal, greater or lesser toxicity and they may leave the site of application by different pathways including surface runoff to streams and rivers, leaching to groundwater and subsequent discharge to surface waters, and volatilization followed by short or long-range transport in the atmosphere and deposition. The chemical and physical properties of chlorpyrifos and endosulfan, and estimates of their usage (the amount of pesticide applied to control plant or animal pests) and loadings (the amount of pesticide delivered down stream or down wind, through fluvial or atmospheric transport, respectively) are discussed below.
Chlorpyrifos. The broad-spectrum organophosphate insecticide, chlorpyrifos is moderately persistent in soils with a half-life usually between 60 and 120 days, but that can range from 2 weeks to over 1 year, depending on the soil type, climate, and other conditions (Howard, 1991; Wauchope et al., 1992). When applied to moist soils, the volatility half-life of chlorpyrifos is short with 62 to 89 percent of the applied chlorpyrifos remaining on the soil after 36 hours (Racke, 1992). The principal transformation product, 3,5,6-trichloro-2-pyridinol (TCP), absorbs weakly to soil particles and appears to be moderately mobile and persistent in soils (Racke and Robbins, 1991). TCP is also a transformation product of the herbicide triclopyr (Racke and Robbins, 1991). The organophosphate insecticides including chlorpyrifos are transformed to their respective oxons by photolytic processes. The oxons have been measured in fog (Glotfelty et al., 1987; Schomburg et al., 1991) at concentrations 20 times the parent compound. Volatilization is also the primary route of loss of chlorpyrifos from water. Volatility half-lives of 3.5 and 20 days have been estimated for pond water (Racke, 1992). In general, pesticides with organic carbon partitioning coefficients (Koc) less than 500 tend to remain dissolved while those with Koc values of more than 1,000 are primarily on suspended-sediment particles. The Koc of chlorpyrifos (9,930) is not remarkable relative to endosulfan, nor is its water solubility (1.18 mg/l) suggestive of mobility via surface runoff.
Endosulfan. Endosulfan is a chlorinated hydrocarbon insecticide and acaricide of the cyclodiene subgroup. It is a contact poison to a wide variety of insects and mites. Although it may also be used as a wood preservative, it is used primarily on a wide variety of food crops including tea, coffee, fruits, and vegetables, as well as on rice, cereals, maize, sorghum, and other grains. Endosulfan has a high Koc of 12,400 (USDA ARS Pesticide Properties Database) and like chlorpyrifos, adsorbs to particles in surface runoff. Technical endosulfan is made up of a 70/30 isomeric mixture of alpha and beta isomers (endosulfan I and endosulfan II, respectively). Alpha endosulfan is the more volatile and predominate in air, whereas both alpha- and beta-endosulfan are found in water. Endosulfan isomers have different degradation times in soil. The alpha and beta isomers have 35 and 150 day half-lives, respectively, under neutral conditions and persist longer under more acidic conditions (Kidd and James, 1991). In addition alpha endosulfan is actually two enanti-omers (Schmidt, et al., 1997). Transformation products of endosulfan include endosulfan sulfate, endosulfan diol, endosulfan ether, endosulfan alpha-hydroxy ether, and endosulfan lactone. The primary transformation products found in surface waters are endosulfan sulfate and endosulfan diol (NRCC, 1975). Endosulfan and its sulfate are highly toxic to aquatic organisms (Day, 1991). In two aquatic invertebrates, the reported 96-hour LC50 values were, respectively, 5.8 mg/l and 3.3 mg/l (Johnson and Finley, 1980). Bioaccumulation of endosulfan by the mussel (Mytilus edulis) is reported to be 600 times the ambient water concentration (USNLM, 1995).