Resident atrazine mass

A complete description of the processes that govern the fate of agrochemicals in the Bay is still beyond our current scientific knowledge. Simplified analyses are thus often used to gain some insight into the trends of chemical concentrations in the Bay. The next section presents a descriptive siunmaiy of the relevant physical and chemical processes occurring in the Bay. A compilation of data on atrazine inputs to Ae Bay is summarized in the following section. These data are used to estimate a resident atrazine mass, which is compared to estimations made fi om measured field values. We then... 

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. 

The lack of a standard protocol for sediment sampling introduces additional variability in reported concentrations to the natural variability encountered in the Bay. Kroll and Murphy (34) detected atrazine in 93% of their sediment samples taken in 1978 at a Midland site, with a maximum measured concentration of 2.15 ppb. Boynton et ed. (36) measured atrazine concentrations below 1 ppb in the sediment layers in the eastern and western tributaries of the Bay in 1980. More recently, Eskin et al. (37) reported no detectable atrazine concentrations in 40 stations covering the main stem and tributaries of the Bay. The difference between this work and earlier studies rtuQr reflect a removal of atrazine from the sediment layer, or, sittqrly, better accuracy and reliability in the analytical methods. It is reasonable to adopt the most recent results and to consider the atrazine mass resident in the sediment layer insignificant. 

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