Progress of Implementation of Nitrate TMDL for the San Lorenzo River, Santa Cruz County, California

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A watershed-related issue examined by the ENVS 560/L Watershed Systems class at CSUMB.


The San Lorenzo River TMDL for nitrate was the first TMDL written for a water body in the central coast region. The TMDL was first initiated in 2000, when the Central Coast Water Quality Control Board (CCWQCB) approved an amendment to the original Central Coast Basin Plan (Basin Plan). The amendment changed the Basin Plan's unrealistic nitrate concentration goal (which was so low that is was below background concentrations), to the target concentration required by a TMDL for the San Lorenzo. The CCWQCB approved the nitrate TMDL for the San Lorenzo River in September of 2000, and it was finally approved by the EPA in 2003.


...Name the smallest watershed (or other applicable region) within which the issue occurs...

Resource/s at stake

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Laws, policies, & regulations

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... What elements of the biophysical system are/were involved?...


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Action Taken Since TMDL Approval

Much of the implementation plan for the San Lorenzo nitrate TMDL was adopted from the existing Nitrate Management Plan (NMP), which was developed for the Basin Plan in 1995 [1]. It was required that Santa Cruz County submit a Report on Nitrate Management Plan Implementation to the CCRWQCB in 2005, 2010, and 2020 [2].

In 2008, the County of Santa Cruz submitted a program status report for the Wastewater Management Plan, which included a review of the Nitrate Management Plan's progress. Implementation of the NMP was expected to reduce nitrate levels by 15-20% over the next 10 years, followed by a reduction of 10% in the next 10 years. Observed nitrate trends show that reductions are occurring slower than desired, with an 11% decrease over the past 15 years. Cen Lomons and Boulder Creek sites has experienced significant reductions of up to 60%. The report suggested that "no significant adverse impacts resulting from nitrate loading at the current level have been identified".

The following nitrate reduction measures have been directly taken from the program status report:

Manage Wastewater Disposal for Nitrogen Reduction

  1. Maintain the existing requirement of a one acre minimum parcel size for new development served by septic systems in the San Lorenzo Watershed (Ongoing)
  2. Implement improved wastewater disposal management through the San Lorenzo Wastewater Management Plan (Ongoing).
  3. Complete ongoing efforts to improve treatment procedures at Boulder Creek Country Club Treatment Plant to reduce nitrate discharge by using wastewater reclamation on the golf course. (The treatment process was refined and fully operational by May 1998. The improvements provide for treatment for nitrogen removal, with the possibility of wastewater reclamation on the golf course much of the year. Effluent has generally not been used for reclamation on the golf course, due to strict regulations. However, the effluent that is delivered to leachfields for disposal has significantly lower nitrogen levels. Nitrogen levels in Boulder Creek are 60% less than the levels from the mid 1990’s. )
  4. Maintain the new requirement for shallow leachfields for new and repaired septic systems (less than 4 feet in sandy areas, and 4-6.5 feet in other areas). (Ongoing)
  5. Implement enhanced technology for at least 50% nitrogen removal for septic system in sandy soils:
      • Require septic systems serving new or expanded uses in sandy soils to install enhanced treatment measures which will reduce nitrogen discharge by at least 50%. (Implemented August 1995; existing systems to be upgraded at the time of major remodels (originally projected rate of 1.2% (20 systems) per year is actually 0.3%, or 5.25 systems per year over the past 12 years).)
      • Encourage the use of nitrogen removal methods for any onsite disposal system which will use a nonstandard system. (Since 1995, 245 alternative systems with capability for nitrogen removal have been approved for use in the San Lorenzo Watershed: 15 sand filters, 63 Advantex Systems, and 167 FAST systems. The 61 systems installed in sandy soils will reduce the summer nitrate load from sandy areas by 6%.)
      • Continue to evaluate new onsite wastewater disposal technology for nitrogen reduction to identify more cost-effective measures. Require higher levels of nitrogen removal if measures become available that are more cost-effective than sand filters. (Some new technology is becoming available, but the cost continues to be high.)
      • Apply for State revolving funds and other funds to develop a funding source to assist property owners in repairing their systems to provide enhanced treatment. (Revised program is now being implemented, beginning June 2005. This could fund 100 upgrades over the next five years, although only 7 loans have been applied for in the past 2 years.)
      • When more cost-effective technology and/or funding assistance becomes available, require all onsite system repairs in sandy areas to utilize enhanced treatment for nitrogen removal. (Implementation deferred, pending more inexpensive technology and documented need for further nitrogen reduction.)
      • Require all large onsite disposal systems which serve more than 5 residential units or dispose more than an average of 2000 gallons per day to utilize enhanced treatment to reduce nitrate discharge by at least 50%. Installation of such measures for existing systems shall be required at the time of system repair or upgrade. (Estimated 1-2 upgrades involving approximately 5000 gallons per day per year, but only about 8 upgrades have occurred in the past 12 years.)
      • Require all new or revised waste discharge permits and all new development projects in the San Lorenzo Watershed to include nitrogen control measures consistent with this Nitrate Management Plan. (County staff has worked with staff at the Regional Board to include nitrogen reduction requirements in new or amended waste discharge permits. This was included in the permits for expansion of the Mount Hermon Association system, the Boulder Creek Country Club system, the San Lorenzo Valley High School system, Brookdale Lodge, Pasatiempo Inn, and Bear Creek Estates.)

Future research

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  1. CCRWQCB. 2000. Staff report for regular meeting for September 15, 2000: San Lorenzo River nitrate Total Maximum Daily Load. State of California: 1-32.
  2. CCRWQCB. 2000. Nitrate Total Maximum Daily Load for San Lorenzo River, Carbonera Creek, Shingle Mill Creek, and Lompico Creek. State of California: 1-44.


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