The report says the goal of universal access to clean water has "yet to be attained" in parts of the San Joaquin Valley.
The study says groundwater is being contaminated by elevated nitrate levels from crop fertilizers and animal manure.
The U.N. study was coordinated by a human-rights group called the Unitarian Universalist Service Committee. Patricia Jones is with the committee:
"We have information about schools that have had to close their drinking water fountains in the San Joaquin Valley where the school districts themselves must buy bottled water placing a financial burden on the educational institution."
California state lawmakers are considering a bill that would tighten up water quality standards called the "Human Right to Water Measure".
Opponents say it would hurt California's agriculture industry.