I think the nitrate limit may be different from what you stated. This source says Nitrate MCL is 45mg/L: https://www.waterboards.ca.gov/rwqcb3/water_issues/programs/ag_waivers/docs/resources4growers/nitrate_info%20guide_102913.pdf
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Re: “Nitrate levels in groundwater have SLO County water quality officials concerned about rural residential wells”
I think the nitrate limit may be different from what you stated. This source says Nitrate MCL is 45mg/L: https://www.waterboards.ca.gov/rwqcb3/water_issues/programs/ag_waivers/docs/resources4growers/nitrate_info%20guide_102913.pdf