Local geology, not disruptive human activities, may be to blame for elevated nitrate levels in some streams and lakes, report UC Davis researchers. "Conventional wisdom has held that high nitrate concentrations in stream water are caused by atmospheric emissions, livestock feeding, agricultural runoff, timber harvest or industrial discharges," says JoAnn Holloway, a hydrology doctoral candidate in the laboratory of UC Davis biogeochemist Randy Dahlgren. "But the data from this study clearly point to the naturally occurring bedrock as a source of nitrate in our watershed." Nitrate contamination is a serious environmental and human-health issue worldwide. Excessive nitrate levels can cause massive algae blooms that rob surface waters of oxygen and lead to large fish kills. Furthermore, elevated nitrate levels in drinking water have been implicated in some human cancers as well as an infant blood disorder commonly known as "blue baby syndrome." The UC Davis researchers studied the Mokelumne River watershed in California's Sierra Nevada mountain range, southeast of Sacramento. Their study, funded by the California Department of Forestry and Fire Protection, aimed at determining if clear-cut timber harvests in the high elevations of the watershed might be contributing to elevated levels of nitrogen in downstream reservoirs. Holloway and her colleagues describe their findings in the Oct. 22 issue of the journal Nature.