- California ordered a major rewrite of Central Valley dairy regulations.
- The new rules will focus on manure spread across cropland.
- Dairies linked to unsafe wells may have to provide replacement water.
- Final regulations must be adopted within five years.
- Existing dairies may receive phased compliance schedules.
Friday, September 18, 2026 — California water regulators have ordered the Central Valley Regional Water Quality Control Board to rewrite its rules for controlling nitrate pollution from dairy operations.
The State Water Resources Control Board adopted the 131-page order
on September 15, 2026. The decision sends the Central Valley’s 2013 dairy regulations back to the regional board for revisions and establishes a new framework for regulating nitrogen discharges.
The order applies to all dairies under the Central Valley board’s authority, not just those formally enrolled under the 2013 regulations. It also covers dairies that use anaerobic digesters or co-digesters.
The state board said the new framework is needed because regulators now better understand how dairy manure affects groundwater.
Earlier regulations focused heavily on manure storage ponds. Newer research and monitoring indicate that the much larger source of nitrogen pollution is manure spread on cropland.
“The extensive over-application of dairy manure to dairy cropland by many dairies” has exposed weaknesses in the existing regulatory approach, the board stated.
Most Nitrogen Comes From Cropland.
A dairy cow produces about 120 pounds of solid manure and 36 pounds of liquid manure each day, according to the order. A 1,000-cow dairy produces an estimated 365,000 pounds of nitrogen in its manure each year.
Manure can be a valuable fertilizer when applied at rates crops can use. Problems develop when more nitrogen is placed on the land than crops can absorb.
The excess can move below plant roots and eventually reach groundwater as nitrate.
The Central Valley Dairy Representative Monitoring Program estimated that about 94 percent of the nitrogen loading from dairies comes from spreading manure on cropland. Manure ponds account for about 4 percent, while dairy production areas account for approximately 2 percent.
Total nitrogen loading from Central Valley dairies was estimated at roughly 79,000 tons per year.
A University of California, Davis study cited in the order estimated that Central Valley dairy cropland received an annual average of 890 pounds of manure nitrogen per acre. Cropland may also receive commercial fertilizer and nitrogen carried in irrigation water.
The state board said recent monitoring found an average nitrate concentration of 48 milligrams per liter in shallow groundwater beneath dairy operations. The median concentration was 35 milligrams per liter.
California’s drinking water standard is 10 milligrams per liter. Natural background nitrate levels are generally between 1 and 2 milligrams per liter, according to the order.
New Limit Tied to Drinking Water Safety.
The new regulatory framework establishes what the board calls a Nitrogen Discharge Limit.
Under that standard, nitrogen from any part of a dairy operation must eventually stop causing or contributing to groundwater nitrate concentrations above applicable water quality standards.
The requirement applies to manure spread on cropland, manure retention ponds, production areas, and other nitrogen sources associated with a dairy, including commercial fertilizer.
State water board staff must develop a formula for determining how much nitrogen may be applied to dairy cropland without causing groundwater to exceed the nitrate standard.
The formula will consider crop uptake, soil conditions, water moving below the root zone, natural nitrogen removal, and other available information. It also must include a safety margin and protections against creating local nitrate hot spots.
That technical work must be completed and independently reviewed within four years.
Existing Dairies Will Face Interim Limits.
Existing dairies will not be required to meet the final limits immediately. Instead, the Central Valley board must establish enforceable interim limits that become more protective over time.
The state board said the first interim requirement should be at least as protective as the current rule, which generally limits applied nitrogen to 1.4 times the amount crops remove.
The order includes examples of how the limit could gradually decrease, but those figures are illustrations rather than final requirements. The Central Valley board will decide the actual interim limits and deadlines during the rulemaking process.
Final compliance schedules must be as short as practical. In cases approved under the Central Valley’s existing nitrate-management program, a schedule may extend for up to 35 years from the date an exception and schedule are approved. Those schedules must be reviewed at least once every five years.
The board acknowledged that dairies cannot simply raise milk prices to cover new manure-management expenses. It also recognized that markets and treatment systems that can handle the region’s enormous volume of manure remain limited.
Exporting raw manure can be expensive because of its weight and volume. Composting, manure treatment, and other processes may make it easier to transport or reuse, but those methods are not yet operating at the scale needed across the industry.
The board said regulators, agricultural agencies, researchers and the dairy industry will need to work together to develop practical uses and markets for excess manure.
New and Expanded Dairies Face Stricter Treatment.
The order distinguishes between existing operations and new or expanded dairies.
Existing dairies may qualify for phased compliance schedules. New dairies and certain expansions will generally be expected to meet final land-application requirements without receiving the same extended schedule.
The Central Valley board must reopen the 2013 dairy regulations within 150 days so currently operating dairies that are not properly enrolled can report their present operations and herd sizes. Those dairies will have 60 days after the amendment to enroll.
A dairy that misses that opportunity may be treated as a new dairy for purposes of future compliance schedules.
The order also directs regulators to identify all operating dairies and their herd sizes within 180 days.
Drinking Water Wells Must Be Tested.
The new framework also addresses families, workers and other residents who rely on groundwater near dairy operations.
Within two years, the Central Valley board must issue orders requiring monitoring of certain on-farm and off-farm drinking water wells.
Dairies outside established nitrate-management zones generally will be required to offer testing for private domestic wells located within one mile of dairy cropland or manure ponds in areas considered at high risk for nitrate contamination. The state board estimated that approximately 97 wells initially meet those conditions.
If testing shows that nitrate pollution extends beyond one mile, regulators must expand the sampling area.
When a dairy is found to have caused or contributed to nitrate above the drinking water standard, it may be required to provide short-term and long-term replacement water. That could include bottled water, access to filling stations, treatment or other safe supplies.
Dairies participating in established management zones may satisfy these requirements through programs already testing wells and providing replacement water.
Special Rules for Wells Located on Dairies.
On-farm drinking water wells will receive additional attention because they may serve dairy workers, tenants and families living on the property.
Those wells initially must be tested for nitrate every year. If results remain below 8 milligrams per liter for three consecutive annual tests, testing may be reduced to once every five years.
If nitrate exceeds 10 milligrams per liter, water users must be formally notified. A dairy owner generally must provide that notice within 10 days.
When the dairy operator does not own the property, the operator may provide the result to the property owner within 24 hours. The property owner must then notify water users within the remaining nine days.
The Central Valley board is expected to act promptly when replacement water is needed.
Five-Year Deadline for Revised Dairy Rules.
The order establishes several deadlines for regulators:
- Orders examining possible connections between manure ponds and groundwater must begin within one year.
- Orders requiring drinking water well testing must be issued within two years.
- Draft dairy regulations and a draft environmental review must be released within three years.
- The nitrogen-loading formula and whole-farm accounting system must be developed within four years.
- Final revised dairy regulations must be adopted within five years.
The Central Valley and state water boards also must issue progress reports every six months. The reports must identify upcoming deadlines, document work completed and disclose any significant risk that a milestone will be missed.
The state board described California’s dairy industry as an important part of the Central Valley economy. It also said the industry must move toward manure-management practices that protect groundwater and the communities that depend on it.
The revised rules will be developed through a public process that includes environmental review and outreach to communities near dairy operations.
