Local geology, not disruptive human activities, may be to blame for elevated nitrate levels in some streams and lakes, report researchers at the University of California, Davis.
Nitrate contamination in streams and lakes is a serious environmental and human-health issue worldwide. Excessive nitrate levels can cause massive algae blooms that rob these 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."
"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," said 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."
Holloway and her colleagues describe their findings in the Oct. 22 issue of the journal Nature. The researchers studied a watershed in California's Sierra Nevada mountain range, southeast of Sacramento. This particular watershed stretches for about 40 miles along the Mokelumne River from the high-elevation resort of Kirkwood down to the foothill town of Jackson. The researchers hoped to pinpoint the primary source of elevated nitrate levels in downstream reservoirs.
Holloway collected water samples from 35 streams throughout the watershed and compared the varying nitrate levels with a geologic map of the area. There appeared to be a direct correlation between high levels of stream-water nitrate and the presence of nitrogen-rich bedrock.
She found that the highest concentrations of stream-water nitrate occurred in the lower portion of the watershed, an area dominated by metavolcanic and metasedimentary rocks such as phyllite, slate, biotite schist, breccia and greenstone. When the researchers tested these rocks, they found that they contained a significant amount of nitrogen that could be released as the rocks weathered.
These are rocks that millions of years ago began as volcanic material and ocean sediment at the bottom of a deep-sea trench. Over the ages, nitrogen-rich organic materials from the ocean drifted down to become part of the sediment, which in time was transformed by heat and pressure into the rocks that now compose much of the western slope of the Sierra Nevada mountain range.
However, rocks found in the upper Mokelumne River watershed -- where stream-water nitrate levels were not unusually high -- are igneous rocks such as granite and diorite. These rocks, formed long ago by the crystallization of the earth's molten rock or magma, turned out to be relatively low in nitrogen.
The researchers also found that levels of both stream-water nitrate and soil nitrogen peaked during early fall and winter. "It appears that the early rains flush out the nitrogen that weathers from the rock during the summer," said Holloway.
The data further indicated that more than 90 percent of the Mokelumne River watershed's nitrates originate in the lower part of the watershed, suggesting that a small amount of nitrogen in rock can have a disproportionately large effect on nitrate in water.
The discovery that the highest concentrations of nitrates and nitrogen were found in the lower streams and bedrock answered one important question for Holloway and colleagues. 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.
"The data clearly suggest that forestry activities are not the source of elevated nutrients, including nitrogen, in this particular watershed," said Dahlgren, a UC Davis soil science professor, who specializes in the biogeochemistry of forest soils.
If the elevated levels of stream-water nitrates are coming >from the naturally occurring bedrock, steps may be taken to mitigate the problem, Dahlgren says.
"At one time, native oaks grew throughout the lower Mokelumne River watershed," he explained. "Oak trees have deep root systems that can draw up the nitrogen released into the soil by weathering of the rocks."
That nitrogen would be stored in the oaks' leaves, which eventually fall to the ground. Periodic wildfires would burn the leaf litter, releasing the nitrogen into the atmosphere, he explained.
"Perhaps more oak trees need to be planted to replace the oaks that were removed to make way for livestock grazing in this watershed," suggested Dahlgren. "And we may want to rethink the benefits of allowing some naturally occurring fires to burn through this area."
Holloway and Dahlgren plan to continue their studies on stream-water nitrates generated by high-nitrogen bedrock, hoping to better understand the individual mechanisms and process-rates involved. Collaborating with them on the study were William Casey, a UC Davis professor in the Department of Land, Air and Water Resources, and Birgitte Hansen at the University of Aarhus in Denmark.
Materials provided by University Of California, Davis. Note: Content may be edited for style and length.
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