National Water-Quality Assessment (NAWQA) Program
Seasonal patterns of pesticide occurrence in surface waters vary in different areas of the U.S. In Figure 4, examples of several relatively well-documented seasonal patterns are shown. Herbicides and insecticides used on rice in California have a distinct seasonal concentration pattern in streams because of the release of irrigation water at specific times. This is shown for the herbicide molinate in Figure 4A. In streams draining the Central Valley of California, there is a seasonal appearance of diazinon and other organophosphorus insecticides used on orchards in January and February (Figure 4B). Rainfall following an application results in movement of the insecticides to surface waters, and pulses of elevated concentrations continue downstream to San Francisco Bay.
The seasonal cycle of herbicide concentrations in midwestern reservoirs is somewhat different than in rivers. This is shown for atrazine in Figure 4D. Reservoirs in the Midwest receive much of their water during the spring runoff period, when concentrations of herbicides in tributary streams are relatively high. Concentrations may remain elevated in reservoirs much longer than in streams, because the herbicides are not flushed from the system as quickly. Relatively high levels of atrazine, cyanazine, metolachlor, and several transformation products of atrazine and alachlor have been observed in reservoirs long after inputs of these compounds from agricultural fields have declined (Goolsby and others, 1993).
The seasonal pattern of diazinon concentrations in the Illinois River (Figure 4E) is different from the pattern for the agricultural herbicides. Concentrations in 1991 were elevated throughout the summer and fall, with peak concentrations occurring in the fall. Diazinon is an organophosphorus insecticide used primarily in non-agricultural applications in the Midwest, including lawn care. The Illinois River drains a major urban area (Chicago), and the seasonal pattern is probably the result of applications to lawns by home-owners and commercial applicators.