Challenged by an environmental organization, the Napa Sanitation District has agreed to do additional studies of how its operations might contaminate local waterways.

The agreement between the district and Northern California River Watch will resolve such questions scientifically rather than through litigation, officials on both sides said.

Northern California River Watch is concerned that incidents involving sewage overflow, storage pond capacity and irrigation runoff have polluted the Napa River and feeder streams, said Jerry Bernhaut, the group’s attorney.

While Napa Sanitation is a well-run operation, he said, virtually all sewer districts play a role in the degradation in water quality throughout the Bay Area.

Michael Abramson, Napa Sanitation District general manage, said the agency is in compliance with the Clean Water Act, but River Watch raised “some good questions. They understand the issues,” he said.

The district will undertake $200,000 worth of additional studies over the next seven years to assess its impact on waterways, Abramson said.

The district, which incurred $45,000 in attorney and consultant expenses in negotiating the settlement, will reimburse River Watch $60,000 for its attorney’s fees.

“It was in the ratepayer’s best interest to continue to improve our operation” rather than fight a more expensive legal battle with River Watch, Abramson said.

One study will assess whether biosolids sprayed on land in Jamieson Canyon are making their way into the river. The district will redouble efforts to make sure that customers who irrigate with reclaimed water do not let overflows reach the river.

Because of concern that old pipes leak raw sewage, the district will look for “human markers” in streams near district lines.

Finally, the district will sponsor studies of excessive nutrients in Upvalley streams and ways to reduce them. This is outside the district’s service area, but could contribute to the overall health of the river, Abramson said.

“We were very positively impressed with the level of professionalism and competence displayed by district staff and with their genuine interest in operating their plant in a manner that protected public health and the environment,” Bernhaut said in a prepared statement.

Napa Sanitation District treats sewage from the greater Napa area, with a treatment plant on the east bank of the Napa River, south of the Butler Bridge.

Because the district does not discharge treated waste water into the river during the low-flow dry months, the district argued that it did not contribute to the river’s problem with excessive nutrients. The river is considered an “impaired” waterway due to high levels of nutrients that can cause algae blooms and deprive water life of oxygen.