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Groundwater Table Determinations

Return to Wet Weather Percolation and Groundwater Testing page

Revised February 2017

For additional information see Site Evaluation Methods and Investigation Requirements (Section 7, page 51) of the Permit Sonoma Onsite Wastewater Treatment System (OWTS) Manual (PDF: 9.7 MB).


In general, lands with slopes of 0-5% in a basin area require site inspection by the district specialist for the purpose of “wet-weather” groundwater table determinations typically between January 1 and March 1* after receiving one half the average rainfall, for each defined geographic area,** of the seasonal rainfall and within 10 days of receipt of 0.8 inches or more rainfall within a 48-hour period.


All groundwater requests are to be submitted by 12:00 noon, one business day prior to the proposed test date and must include the following:

  1. A Request for Service form with the current application fee. Clearly note ‘groundwater tests’ on the form.
  2. A vicinity map including the parcel, or written instructions for locating the parcel. Include gate code if applicable.
  3. A legible site map which indicates all areas to be read(if more than one test site is being evaluated). Number or letter the test holes so that the readings between the consultant and county personnel will be consistent.
  4. A maximum of 6 groundwater holes will be read for each fee. For additional holes, pay 1 fee for each 6 holes.

Inspection Request

An inspection request must be received no later than 12 midnight the day before the test day. You will need the WSR number, which starts with "WSR" followed by six numbers: WSR##-####

The inspection code for "wet weather groundwater test": 386

Visit our Inspection Scheduling page to make or cancel inspection requests.
View inspection codes for the Well & Septic section.

Cancellation of a groundwater test needs to take place by 9:00 AM on the day of the proposed test.


A) Hole Constructions Standards

  1. Preferred method of monitoring well 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 pipe should be three inches of perforated pipe. The type of pipe typically used in standard leach field pipe.
  3. An open backhoe pit, which has been properly secured, or a pipe set within a backhoe pit are acceptable methods of construction; however, they are typically less desirable because of degree of accuracy (due to bent pipes, harder to gauge where standing water exists, etc.).
  4. Holes should be well marked with a lathe and ribbons to increase visibility. A sketch should show the approximate location and number of wells. If pipes are installed, markings on the side of the pipe(indicating hole # and depth)would be helpful.
  5. A minimum of two holes is recommended, per test area. Site conditions and type of projected system will determine the optimum number of holes for the tested area.

It is better to collect more data to best locate and size the system. Attempts should be made to locate monitoring wells down slope of the system.

B) Hole Setup and Placement

  1. Under ideal circumstances, discussion between consultant and county staff will occur at the pre-percolation site evaluation stage regarding the groundwater test hole setup. In some instances, the site evaluation will occur after the groundwater evaluation. In these cases, it may be beneficial for the consultant to confer with the district specialist prior to the setup and placement of the groundwater tests. If this does not occur, the consultant may take a risk in having to provide supplemental test sites at a later point.
  2. The setup of the holes must occur a minimum of 24 hours prior to the reading by the consultant and the district specialist. If the holes are prepared too far in advance of the reading, accuracy of the readings may be impacted by silting effect, damage to the pipes, and other factors.
  3. All sites that have 5% slope of less will typically require groundwater testings; however, some sites with slopes in excess of 5% will require testing due to site conditions such as vegetation types, evidence of spring activity, indications of soil mottling, etc.

C) Groundwater Readings

  1. Due to the workload, the Well & Septic staff are not able to make appointments with consultants to read groundwater tests. The groundwater tests will likely be read at different times on the same day, by the consultant and the district specialist. In cases when the readings substantially differ, the consultant may want to confer with the district specialist after the results have been received from the County.
  2. The consultant should submit the groundwater test results as soon as possible, so that potential conflicts in readings can be resolved. After reviewing the consultant’s groundwater readings, the district specialist will send a copy of their groundwater readings to the consultant. If conflicting readings cannot be resolved immediately, the consultant should consider a formal waiver application with justifying data.
  3. 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 wells used in concert with one another, over multiple readings.
  4. In situations where a site has failed groundwater readings 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. Changing locations of the test site may make a difference, but the consultant should confer with the district specialist regarding the appropriateness of the proposed changes. This may require supplemental profile test holes to verify soil conditions in the alternate area.
  5. Plans for french (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). Grading permits are currently required for the installation of an intercept drain that is not part of a septic permit.

D) Data Submittal/Groundwater Reporting

  1. The consultant is to provide a report which includes the following information for groundwater certification: date and time of readings, the name of individual taking groundwater readings, address and assessor’s parcel number of the site, and the signature of the consultant. This can be in conjunction with the perc data submittal.
  2. Time limit for submittal of groundwater readings is 90 days of the readings, or in conjunction with the perc data submittal.

*Extensions beyond the time limits of the above criteria may be made by the Well & Septic Division Manager based on rainfall and groundwater monitoring.

**As reported by the official recording station for the National Weather Service located in Santa Rosa.
As reported by the officially recognized reporting stations as published in the Santa Rosa Press Democrat.

Contact Information

Contact Well & Septic by Phone
2550 Ventura Avenue
Santa Rosa, CA 95403
38.465074, -122.723705

Well & Septic Districts Map

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Well and Septic Districts Map

Map of well and septic environmental health districts within Sonoma County.