Ground and surface waters in urban areas are susceptible to nitrate contamination from septic systems, leaking sewer lines, and fertilizer applications. Source identi®cation is a primary step toward a successful remediation plan in aected areas. In this respect, nitrogen and oxygen isotope ratios of nitrate, in conjunction with hydrologic data and water chemistry, have proven valuable in urban studies from Austin, Texas, and Tacoma, Washington.In Austin, stream water was sampled during strem¯ow and base¯ow conditions to assess surface and subsurface sources of nitrate, respectively. In Tacoma, well waters were sampled in adjacent sewered and un-sewered areas to determine if locally high nitrate concentrations were caused by septic systems in the un-sewered areas. In both studies, sewage was identi®ed as a nitrate source and mixing between sewage and other sources of nitrate was apparent. In addition to source identi®cation, combined nitrogen and oxygen isotopes were important in determining the signi®cance of denitri®cation, which can complicate source assessment by reducing nitrate concentrations and increasing d15 N values. The two studies illustrate the value of nitrogen and oxygen isotopes of nitrate for forensic applications in urban areas.# Published by Elsevier Science Ltd. on behalf of AEHS.

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Twenty-three pesticides were detected in water from urban streams during rainstorms, and the concentrations of five of these pesticides exceeded limits set to protect aquatic life. During rainstorms, 23 of 98 pesticides sampled for were detected in water samples from 12 study sites in 10 urban watersheds. Concentrations of five insecticides exceeded recommended maximum concentrations set by the National Academy of Sciences and National Academy of Engineering (NAS/NAE) (1973). In a few samples, concentrations of Diazinon, carbaryl, and Lindane exceeded U.S. Environmental Protection Agency (USEPA) and other chronic aquatic-life criteria. Pesticides used on lawns and gardens contribute to the occurrence of several pesticides in urban streams. According to 1997 sales data from home and garden stores, of the pesticides sampled for, Diazinon, 2,4-D, and MCPP are the most frequently purchased pesticides by residents of King County. MCPP and 2,4-D are also among those pesticides used by professional applicators for pest control in residential, recreational, and industrial areas. The presence of these pesticides in water samples from all of the 12 study sites shows that their widespread application impacts water quality in urban streams. Also, residents purchased and applied four of the five pesticides that exceeded recommended maximum concentrations set by the NAS/NAE (Diazinon, carbaryl, Malathion, and chlorpyrifos). Many pesticides found in urban streams might be the result of nonresidential applications. Almost half of the 23 pesticides detected in stream water had no retail sales according to a 1997 survey of pesticides sales from home and garden stores in King County. Two of these pesticides (atrazine and simazine) were found at more than 60 percent of the study sites. This indicates that these pesticides are being applied to nonresidential areas in urban watersheds such as rights-of-way, parks, and recreational areas.

Yields of suspended sediment from watersheds in the Quincy and Pasco Basins of Washington State have been reduced by the use of sprinkler irrigation on cropland previously in furrow irrigation. Mean daily yields of suspended sediment from nine watersheds sampled during April and May 1994 ranged from 0.4 kg/ha of irrigated cropland in a watershed with no furrow irrigation to 19 kg/ha in a watershed where 58% of irrigated cropland was in furrow irrigation. About 67% of the variation in the yields can be attributed to irrigation method. Temporal trends also indicated that use of sprinkler irrigation reduced sediment yields. Mean daily yields of suspended solids from one of the watersheds decreased from 0.3 kg/ha in 1975 to <0.2 kg/ha in 1988, corresponding with a decrease from about 65% to <50% in the use of furrow irrigation. Sampling in two watersheds suggests that the use of sprinkler irrigation reduces runoff losses of pesticides and N. For 10 of 13 pesticides and N, runoff losses from a watershed with mostly furrow irrigation exceeded runoff losses from a watershed with mostly sprinkler irrigation.