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act the pOllutants confained in agricultural runoff. Billions more will be spent to meet tougher drinking water standards expected in the future. Of particular concern is the organic material carried otf farm fields by agricultural runotf, a problem exaggerated when the Della's peat soils are irrigated. The organic material reacts with chlorine and other chemicals traditionally used to purify water to torm trihalomethanes, or THMs, which are suspecfed human carcinogens. Among the options to reduce THMs are more sophisticated treatment facilifies or constructing diversion points higher in the watershed, or at the top of the Delta, where the water is cleaner - both expensive propositions. The Della is not the only place where pollution is threatening water supplies. Some communities that are dependent on groundwater polluted with pesticides and solvents - in the agricultural Central Valley and the industrial South Coast plain - have had to drill expensive new wells, or look for surtace EmID Once a vast swamp crisscrossed by sloughs and streams, the Della is now a controlled jigsaw puzzle of leveed islands and channels. Upstream dams moderate flows in all but the wettest of winters and reservoir releases freshen the waterways through the long summers. The Delta - a natural migratory path for fish and water fowl - also is the major crossroads for G..lifornia's engineered water system. Cargo ships and recreational boats ply the waters. Grow- ers divert water for island crops, the North Bay Aqueduct draws water for growing cities to the west and the federal Central Valley Project and State Water Project pump sloughs uphill to fill southbound aqueducts leading to southern cities and Central Valley farms. The transactions make for complex water quality problems. More than 44,000 square miles of California drain into the Delfa, bringing with it the pollutants from above, many of fhem from nonpoint sources - sediment, pesticides, metals from mines and oil from city streets. Delfa farmers add fheir own pollutants, notably organic matter flushed out of the peat soils. The organics react with chlorine when the water is In California and the West, where satisfying societal water needs has always been a controversial chal- lenge, the need to replace polluted water sources is expected to further increase the pressure on existing water supplies. purified for drinking. A byproducf of this reaction is trihalomethanes, or THMs, a suspected human carcinogen. water supplies to meet their needs. Disease-causing bacteria, viruses and protozoa - such as Giardia and Cryptospori- dium - can make their way into drinking water supplies from areas grazed by domes- tic animals. Feedlots and sewage plants have caused infectious outbreaks among residents in the Midwest and other parts of the U.S. While fish kills are a \'isible effecI of pol/uriol/. rest'archers are .'ifil/ examining lu)\\' much POl/lllioll is 10 blame for diminishing nllmbers and di\'crsity (~rspecies. The water quality problems are compounded by the Della's natural role in the ecosystem. As tides come and go, the fresh water mixes with the salt water. But as runoff is sfored or diverted up- stream, and as large diversion facilities in the soufhern Della pull water into the aqueducts, the natural rhythms are altered. All of these changes have had major effects on the Delta's ability to pertorm its natural functions and meef human needs. While biologists and engineers struggle to restore the conditions that nurture fish and wildlife, water providers are struggling to ensure that water diverted from the Delta is safe to drink. Among the allernatives that have been consid- ered are sophisticated new treatment processes and moving the water project intakes farther upsfream. Refeasing more water for fish and wildlife has the added benefit of increasing drinking water quality. But fhat option trades water quality problems for additional water supply problems. 13