Wild & Scenic Mokelumne Update

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Scientists test foothill wells
Many Sierra foothills wells are drilled in fractured rock aquifers. Often, little is known about the quality of the water in those wells, its age, or its extent.

To help provide more information about those wells, scientists from the U.S. Geological Survey, working for the California State Water Resources Control Board, recently published a study of groundwater quality in the Mokelumne, Cosumnes, and American River watersheds.

The study team sampled water from 67 domestic wells and one domestic spring between August 2016 and January 2017. The wells in the study unit typically were 30 to 150 meters deep, with water levels typically 2 to 42 meters below the ground’s surface. The study was made public in March.

The study found fecal contamination in 10 percent of the wells tested. However, the scientists said the levels found were below the benchmark for drinking water set by the state as unhealthy and that more testing needs to be done.

As might be expected in wells drilled mostly in fractured Sierra rock formations, minerals such as iron and manganese were found in high concentration.

“About 8 percent of the groundwater resources used for domestic drinking water had high concentrations of nitrate, and less than 2 percent had moderate concentrations,” the study said. Nitrates are generally associated with such products as fertilizer and can be unhealthy for some individuals.

Regarding the age of the water in the wells tested, the study noted that, “Tritium concentrations in groundwater can be used to estimate whether the groundwater was recharged primarily before 1950 (‘old’) or after 1950 (‘young’). About three-quarters of the study-unit well samples had tritium concentrations indicating ‘young’ groundwater.”

This is good news for owners of young wells who may be concerned about their wells going dry, but those wells are also more vulnerable to pollution from surface contaminants such as nitrates. All of the well owners who participate in the study volunteered to do so. Each received a full report on their well prior to publication of the report.

“We were happy to participate in the study,” said Foothill Conservancy President Katherine Evatt. “We learned a lot about our well water and were happy to help expand the level of knowledge about Sierra foothill groundwater.”

The entire report can be viewed online.

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