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

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Nitrate concentrations have increased in the San Lorenzo River watershed since the 1950's and are dangerously close to violating parts of the Central Coast Region Water Quality Control Plan, also known as the Basin Plan. The Basin Plan contains the following objective for taste and odor: “Waters shall not contain taste or odor-producing substances in concentrations that impart undesirable tastes or odors to fish flesh or other edible products of aquatic origin, that cause nuisance, or that adversely affect beneficial uses." [1]

Since high nitrate levels can stimulate biological growth of algae, molds, and fungi, it threatens drinking water supplies by releasing organic compounds. These compounds are known to cause noxious tastes and odors,but more importantly, they produce potentially carcinogenic disinfection byproducts when the water is treated.

File:TMDL SCCounty.jpg
[1] Mapof San Lorenzo water quality stations.

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


The San Lorenzo River Total Maximum Daily Load 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 Regional Water Quality Control Board (CCRWQCB)] approved an amendment to the original Basin Plan. The amendment addressed the issue of inadequate nitrate concentration reporting limits, which was 5-7 times above background levels, by switching to the target concentration required by the TMDL for the San Lorenzo River. The CCRWQCB approved the nitrate TMDL for the San Lorenzo River in September of 2000, and it was finally approved by the EPA in 2003 [2].

The 1995 Nitrate Management Plan determined that nearly 67% of the nitrate in the San Lorenzo River detected in the summer season came from areas close to the Santa Margarita Sandstone. Septic systems in these areas contribute between 10-15 times more nitrate to the river than other less permeable soils. Thus, reduction efforts are most effective in areas with sandy soils. 20% of contamination is directly caused by humans during the wet season. [3] The San Lorenzo River is still considered impaired and nitrate levels have not declined as rapidly as projected. Reductions in harmful nitrate levels in the San Lorenzo River can be achieved with better management of wastewater disposal methods (e.g. septic system upgrades), improved livestock management strategies, and various land-use practices which require less fertilizers. Continued monitoring of established sites is crucial to ongoing efforts that aim to identifying nitrogen hot spots and point sources.

Action Taken Since TMDL Approval

Observed decrease in nitrate in San Lorenzo river from 1990-2007.[4]

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 [5]. It was required that Santa Cruz County submit a Report on Nitrate Management Plan Implementation to the CCRWQCB in 2005, 2010, and 2020 [6].

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% from 1995-2005, 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. Ben Lomond 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".

Below is a list of planned actions and progress of implementation plan of nitrate TMDL in the San Lorenzo River. The following nitrate reduction measures have been directly taken from the 2008 program status report:[4].

Manage Wastewater Disposal for Nitrogen Reduction

  • Summary

There are 13,000 parcels with a total of 13,900 septic systems in the county’s database. In the past, the status of the 13,900 systems were evaluated via site inspections for failures, assessment of groundwater levels, water quality monitoring and septic tank pumping records [7]. Over 80 boreholes and 11,700 parcels were inspected by the time the 2002-2007 Draft Program Status Report was produced. Since 2008, roughly 10 wells continue to be monitored on an annual basis and periodic site inspections are ongoing but infrequent. New Development

  1. Parcel-size
    1. Planned:
  • Most of the suitable parcels have already been developed. According to the 2002-2007 Management Plan, 30% of new developments must use alternative systems [8], and any new developments must meet the existing requirement implemented in 1983 of a one acre minimum parcel size if the resulting septic system(s) are in the San Lorenzo Watershed. Additionally, in 2002, the county approved an exception which allows 20 commercial parcels to be developed in the village areas of the San Lorenzo Valley. However, the most common reason for bringing septic systems up to current standards is due to remodeling projects on existing homes in the watershed.
  • Land Use Regulations for Nitrogen Reduction

Current density restrictions require that owners maintain a 10 acres-per-parcel limit for new land divisions. Current regulations for erosion control, land clearing practices, and riparian corridor protection are required to be maintained and ongoing. No new land use projects within the Watershed can be approved if the project area will increase nitrate discharges to groundwater or surface water by more than 15 pounds per acre, per year [9]
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