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All urban creeks in the San Francisco Bay Area are on California's 303(d) list of impaired water bodies due to observations of aquatic toxicity, primarily due to runoff of the common insecticide diazinon. Although the main culprit is diazinon, there are other pesticides of concern that are used in urban watersheds, and the toxic effects of some of these pesticides tend to be additive. There is also concern that a TMDL that focuses on a single pesticide, such as diazinon, will create consequential incentives to use other equally or more toxic pesticides. As such, this TMDL is designed to remedy and prevent impairment of urban creeks caused by urban pesticides, not just diazinon, found in urban runoff.

This urban-pesticide TMDL comes with the “usual” challenges associated with runoff TMDLs; it also comes with additional challenges unique to pesticide TMDLs. Relative to the former, we contend that it does not make sense to establish “end-of-pipe” mass-load allocations per unit time for runoff discharges since runoff volume rates are climate dependent. The preferable approach is to establish load reduction via an allocation of responsibility that is demonstrated by a commitment to implement specific and/or general BMPs with recognized pollution prevention or control value. There are also challenges associated with regulatory responsibility and constraints on regulation of pesticides. Although municipalities are responsible for discharges of pollutants from their storm drain systems, there are prohibitions and restrictions on regulating many uses of pesticides at the municipal level. Another complication associated with use and regulation of pesticides is delineation of the true sources, and the relative responsibilities of users, retailers, distributors, formulators and manufacturers of pesticides.

Despite these challenges, the TMDL process has provided a workable framework to resolve water quality impairment associated with impairment of urban waterways by pesticides. We have demonstrated use of toxicity identification evaluations to determine cause of observed toxicity in urban creeks. We have demonstrated use of toxicity tests and enzyme assays designed for specific pesticides to track sources. We have not limited our consideration of sources to storm drains. Instead, we have investigated particular pesticide formulations and applications that have potential to be discharged with urban runoff. We have estimated that observed levels of pesticides in an urban creek can be attributed to just 0.3% of the pesticides applied in the watershed. Fortunately, 99.7% of the applied pesticides do not reach the creek; the bad news is that pesticides are designed to be toxic, and it takes very little to cause toxicity in creeks.

One of the more positive outcomes of this TMDL has been unprecedented collaboration among state pesticide regulators, state water quality regulators, the pesticide industry, professional pesticide applicators, and municipalities. The net result and benefit is a BMP-based TMDL and a TMDL allocation and implementation scheme based on shared responsibility, commitment, and accountability. Most importantly, the TMDL provides a mechanism to measure effectiveness of integrated pest management actions, the ultimate solution to urban creek pesticide toxicity.
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Document Type: Research Article

Publication date: 2005-01-01

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