Atrazine in Chesapeake Bay

The three major pathways for atrazine to enter the Chesapeake Bay are surface runoff, groundwater inflow, and wet and dry atmospheric deposition. Processes such as hydraulic flushing, air-water transfer and chemiod reactions will then influence the final concentration and distribution of atrazine in the Bay. 

In particular, horizontal advection and horizontal diffusion in the Chesapeake Bay are comparable while vertical difiiision is a fast process that acts over short distances, and a model must account for all three. In this environment, atrazine that is discharged to the surface waters could be horizontally distributed over a distance of 1 km over a period of one week, since the time scale of horizontal advection-difiusion processes is 10 -10 s (approximately 3 hours). As atrazine is distributed horizontally, it also mixes vertically down the water coluitm. With the estimates of verticd diffiisivity for the Bay that are available in the literature, for a depth of 10-20 m the time scale for vertical diffusion processes is on the order of 15 minutes, and can be as short as 3 minutes. The sidfidic vraters are in the sediment porewaters and atrazine needs to be transported to the water-sediment inter ce in order to encounter and react with reduced sulfiir species. The characteristic horizontal and vertical scales that describe the flow in the Bay indicate that it is possible for atrazine to reach the depth of the water-sediment interface before it is horizontally transported out of the system. The subsequent exchange at the water-sediment interface depends on many factors, including half-life of atrazine, the hydraulic residence time of the bottom layer, turbulent processes, and other characteristics of the water column above the sediment layer. Simple box models cannot capture the dynamics necessary to describe these exchanges that ultimately govern the te of atrazine in the Bay. 

Hall Jr., L.W., R.D. Anderson, J. Kilian, and D.P. Tierney (1999). Concurrent exposure assessments of atrazine and metolachlor in the mainstem, major tributaries and small streams of the Chesapeake Bay watershed Indicators of ecological risk. Environ. Monit. Assess., 59 155-190. 

Sensitive species of aquatic flora experience temporary adverse effects at concentrations as low as 1.0-5.0 p,g/L however, most authorities agree that potentially harmful levels, i.e., >10.0p,g/L for long periods, have not been documented and are probably unrealistic under current application protocols and degradation rates. The observed declines in submerged aquatic vegetation in the Chesapeake Bay are not now directly attributable to atrazine use. Atrazine indirectly affects aquatic fauna at... 

Assessing Atrazine Input and Removal Processes in the Chesapeake Bay Environment An Overview... 

Systematic investigation of pesticides in the Chesapeake Bay watershed began in the 1970 s in response to the observed decline in SAV and fish populations during that period. Investigations typically spanned a few years and tested for specific families of herbicides (e.g., dUoro-s-triazines and chloroacetanilides) and insecticides (e.g., organophosphates and chlorinated hydrocarbons) in various media. Detection limits and consistency in the methods of data collection improved with time. The data that are sununarized below consist of measurements of concentrations of atrazine in the iiqruts to surface waters at several locations throughout the Bay. We should stress that data are sparse spatial and temporal distributions have to be estimated and/or extrapolated from measurements in the top 0.5-1.0 m of the surface layer. 

The total amount of atrazine entering the whole Bay via the various pathways is 4,700 kg/yr. The estimates are summarized in Table II. The Nation Oceanic and Atmospheric Administration (NOAA) reports that approximately 481,382 kg (1,061,707 lb) of atrazine is plied to the Chesapeake Bay drainage area annually (77). Using this value and assuming that the atrazine loading is 1% of that mass, aiq>roximately 4,800 kg/yr of atrazine are washed into the Bay. This value closely agrees with the one presented here. 

To date, only sporadic temporal and spatial measurements exist for atrazine concentration in the surface waters of the Chesapeake Bay, and the quality of the data only allows for an order-of-magnitude estimate of the resident atrazine mass. To obtain this estimate, we assume that the bulk of this herbicide resides in the water column and the sediment layer, with negligible amounts in biota and the surface microlayer, a layer of up to 1 mm thick at the air-water inter ce. 

An overview of the behavior of atrazine, an extensively used com herbicide, in the Chesapeake Bay environment has been presented. Atrazine has a differential effect on varying species of submerg grasses and could shift species composition in areas where it teaches the Bay or tributaries in large quantities. There is concern that it can become an added stressor to an enviromnent alrea heavily affected by excess nutrients, rapid changes in land use and population growth. There is also concern about the effects of various... 

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