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Groundwater Table Determinations

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For additional information on groundwater table determinations, see Site Evaluation Methods and Investigation Requirements (Section 7) of the Sonoma County Onsite Wastewater Treatment System (OWTS) Manual.


In order to protect the County's groundwater from pollutants from septic systems, wet weather groundwater table determinations must be conducted in the following situations:


When installing a new or replacement septic system, groundwater table determinations depend on the slope of the property.

  • Lands with slopes of 0 to 5 percent: Groundwater table determinations are required.
  • Lands with slopes greater than 5 percent: Groundwater table determinations may be required if high seasonal groundwater is suspected or known to exist.


Direct groundwater table determinations (also known as wet weather groundwater observations) shall be conducted under the following circumstances:

  • Between January 1 and March 1,
  • After having received 50 percent of the average seasonal rainfall for each defined geographical area,
  • Within 10 days of receiving 0.8 inches or more of rainfall within a 48-hour period as reported by the officially recognized reporting stations published in the Press Democrat.

Time extensions for direct groundwater table determinations may be authorized by Permit Sonoma based on extended periods of rainfall before January 1 and/or after March 1.

Application and Notification

All applications for direct groundwater table determinations shall be submitted to Permit Sonoma no later than 12:00 PM (noon) the business day prior to the proposed observation date and shall include the following:

  1. A Request for Service form with “Groundwater Observation” clearly noted on the form.
  2. Payment of current application fee.
  3. A site plan in accordance with CSS-019 Minimum Standard Site Plan Requirements for Building & Engineering Applications and includes the following information:
    1. A location and vicinity map that clearly identifies how to access the property and the site with written directions to guide Permit Sonoma staff and disclosure of any application gate codes.
    2. All groundwater observation holes clearly and accurately shown and identified with numbers or letters, so the readings performed by the consultant and county staff are consistent.

Each application for direct groundwater table determinations shall include up to six observation holes. Additional holes shall be observed under a separate application and fee. The address must be clearly posted at the entrance to the property. Direct groundwater table observations shall be performed by a qualified consultant on the same day as performed by Permit Sonoma staff. If a closing date falls on a weekend or holiday then the last day to perform direct groundwater table observations shall be the last previous business day relative to the closing date. Any revisions to site plans must be submitted to no later than 8:00 AM on the day of the requested observation date.

Inspection Requests

All requests for direct groundwater table observations shall be made no later than 12:00 AM (midnight) the day before the desired observation date. Visit our Inspection Scheduling page to make or cancel inspection requests for direct groundwater table observations. Please have your activity number available when making an inspection request. Activity numbers for direct groundwater table observations start with “WSR” and are followed by six numbers, for example: WSR12-3456.

Inspection code for “wet weather groundwater test”: 386

To cancel inspection requests after 12:00 AM (midnight) on the day of the requested observation date, a cancelation notification must be sent to no later than 8:00 AM on the day of the requested observation date.

Observation Hole Construction and Measurements

Observation hole construction and measurements shall be performed in compliance with Section 7.5 of the Sonoma County Onsite Wastewater Treatment System (OWTS) Manual.

Construction and Placement of Observation Holes

  1. Preferred method of observation hole construction is by hand auguring, however, in some situations the depth of the hole may require the use of a backhoe or a drill rig to construct.
  2. The minimum diameter of the perforated pipe is three inches.
  3. An open backhoe pit, which has been properly secured, or a pipe set within a backhoe pit are alternate methods of construction.
  4. Holes should be well marked with a lath and ribbons to increase visibility. All holes must be clearly marked and numbered or lettered and match the most current site plan.
  5. A minimum of three holes (more may be required depending on anticipated OWTS dispersal area size) supporting a consistent design type is required, spaced to encompass the entire future design areas (both primary and reserve). Site conditions and type of anticipated septic system will determine the required number of holes for the tested area. Each groundwater hole is considered representative of a 50 feet radius except when an adjacent hole shows conditions dissimilar enough to warrant a different system design. Then additional holes may be required to demonstrate suitable separation to groundwater. For example, when the intended design is a 12 inches subsurface drip system and two holes yield 36 inches to groundwater and one hole yields 28 inches, then an additional hole with a groundwater depth of at least 36 inches will be needed between the dissimilar holes.
  6. Test holes must be constructed at depths, to determine the highest anticipated groundwater level below the proposed trench bottom and /or absorption surface area. If the proposed system design is to be above an impermeable soil layer, the test holes must be placed above the impermeable soils layer. If the system design is proposed below an impermeable soil layer, then the test hole must be placed below the impermeable soil layer. The testing shall correlate with the soils evaluation and proposed system design.
  7. The setup of the holes must occur a minimum of 24 hours prior to the requested observation date. If the holes are prepared too far in advance of the observation, accuracy of the readings may be impacted by silting effect, damage to the pipes, and other factors. All holes that are setup 24 hours prior to the requested observation date must be left in place to be observed by Permit Sonoma staff.

Groundwater Measurements and Reporting

  1. Permit Sonoma staff may not be able to make appointments with consultants to measure groundwater depths at the same time. The groundwater measurements will likely be performed by the consultant and Permit Sonoma staff at different times on the same day.
  2. Within 90 days of performing groundwater measurements, the consultant shall submit to Permit Sonoma a report which includes the following information for groundwater certification: date and time of groundwater measurements, the name of individual who performed the groundwater measurements, address and assessor’s parcel number of the site, and the signature of the consultant.
  3. If the consultant has questions about Permit Sonoma staff groundwater measurements, they work with the District Specialist to resolve any issues. If conflicting groundwater measurements cannot be resolved immediately, the consultant should contact the Well and Septic Section Supervisor for further assistance.
  4. Groundwater averaging is not considered appropriate for one day readings; however, in some situations where nitrate plumes and groundwater mounding is being evaluated, groundwater gradient and groundwater contours are usually established by three or more observation holes used in conjunction with one another, over multiple measurements.
  5. In situations where a site has failed groundwater measurements in the past, alterations made to the site such as an in interceptor drain or surface water diversion may allow for retesting of the site at an alternate time. This may require supplemental soil profile test holes to verify soil conditions in the alternate area.
  6. Plans for interceptor drains or other surface drainage improvements should be submitted to the Well & Septic section, if the improvements are to be made in advance of the septic installation (i.e., for the groundwater determinations). A separate permit may be required for the installation of an interceptor drain that is not part of a septic permit.