Wells and Water Systems

This section has been prepared to answer questions about contaminated wells and to help rural homeowners to restore their wells to safe operating condition. If you need additional information email our department for advice.

Click on a question to see the answer, or Open All Questions.

• Should a Well be Disinfected?

  If a well has been contaminated, it should be thoroughly disinfected before the water is used for drinking or in the kitchen. For example, a well should be disinfected as soon as possible after a flood is over, or after a source of contamination has been removed. Of course, disinfection will kill only the disease germs that are already in the well, not any that come in after the disinfection.

  If the well was not properly constructed originally, or was damaged by a flood, it may require repair or rebuilding in addition to disinfection.

• Why Should I Disinfect a Well That Has Been Flooded?

  Flood waters are dirty waters. They carry disease germs from privies, septic tanks, and sewers, as well as filth and silt.

• Can I Disinfect the Well Myself?

  Certainly. You can disinfect your own well, if you have a few tools, some household bleach solution, and a knack for do-it-yourself work.

• How do I Disinfect a Well?

  These are the steps you should follow:

  1. Pump water from the well to waste until it looks relatively clean.
  2. After the water flowing from the well looks clear, stop the pump.
  3. Household bleach may now be added to the well. It may be necessary to lift the pump, but some wells have openings that can be used for this purpose. The bleach should be added between the casing and the suction pipe of the pump. See the table for amount of bleach to be used.
  4. Do not operate the pump for 30 minutes. After the 30 minute period, with the taps,  faucets and hydrants open or closed, surge the well by starting and stopping the pump several times.
  5. Open every tap, faucet or hydrant in the water piping system, start the pump and let water flow until clean water with a strong smell of chlorine comes out.
  6. Stop the pump and close all the taps, faucets and hydrants. Do not operate the pump for 8-12 hours.
  7. After a period of 8-12 hours (overnight is usually the most convenient period), open ALL taps, faucets and hydrants start the pump, allowing water to circulate to waste until no chlorine odor is present. (NOTE: if household waste is disposed of to a septic tank, DO NOT open inside taps at this time. Before starting pump open only outside taps, so that waste can be circulated to ground surface. Too large an amount of chlorine such as this my be harmful to the beneficial bacterial processes of the septic tank. In addition, the large volume of water would be a heavy load on the leaching field.)
• In Step 3 of your instructions for disinfecting a well, what kind of bleach should I use?

  You can use any ordinary liquid household bleach, if you make sure that it contains chlorine. Read the label and see that it says the bleach contains 5.25 percent sodium hypochlorite. Clorox, Purex, White Magic, and Sani-Clor are some of the trade names for liquid bleaches sold in grocery stores.

• How Much Bleach Should I Use?

  The amount of bleach you need depends on the size and depth of the well.
  If your well is 100 feet deep or less, you can use the following table to find the amount of bleach needed.

  Diameter of Well Casing	Amount of Household Bleach Needed
  4 inches	1 pint
  6 inches	1 quart
  12 inches	1/2 gallon
  16 inches	1 gallon

  If your well is 100 to 200 feet deep, you will need twice as much as for a well 100 feet deep or less. It won't matter if you use more than the amounts shown above, but be sure you use enough.

• Where Can I Get Help if I Have an Unusual Problem in Disinfecting my Well?

  Please call our office at (415) 473-6907 and we will assist you if you have questions about disinfection of wells that are not answered here.

A full description of the rules and regulations for domestic water systems in Marin County can be found in the Application to Operate a Domestic Water Supply. This PDF also contains blank forms to record test results and additional information that is needed to be submitted with an application. Additional information can be found at these links:

• Marin County Code Chapter 7.28
• Title 22 - Maximum Contaminant Levels for Private Drinking Water Wells; Required Testing

1. Source Yield and Storage Capacity

All sources shall be perennial. The minimum sustained yield of water from a source or sources and minimum water storage capacity shall not be less than the following:

Minimum Source Yield and Storage Capacity

	Sustained Yield			Storage
	Pumped Source		Gravity Source
	Gal/Min	Gal/3-day	Gal/Min
One Residence	1	2160	0.50	1,000
Two Residences	2	4320	1.00	2,500
Three Residences	3	6480	1.50	4,000
Four Residences	4	8640	2.00	5,000

For uses other than that specified above the minimum yield and storage requirements shall be based on estimated consumption as approved by the "Health Officer."

2. Test Method

A. Gravity Fed Source

Flow from the source shall be measured by recording the time it takes to fill a container of known volume (minimum size (2) two gallons). The average of three such measurements taken within (1) one hour shall comprise a test. At least (3) three tests shall be taken each spaced at least 6-days apart. Testing of a horizontal well shall not be performed until at least 30 days after the well has been drilled.

B. Pumped Source

Step 1. Commence the test no sooner than 7-days after development of the well. Record the time and the initial level of the water in the well prior to start of pumping.

Step 2. Start pumping at a rate of 10 GPM, or more, until a 3-day minimum yield is pumped from the well, or until the well is "pumped dry" (this is the point at which the pump starts pumping air, thus reducing the pumping rate).

• If a 3-day yield is obtained without "pumping dry" skip to Step 5.
• If the well is "pumped dry" continue to Step 3.

Step 3. Record the time at which the well is "pumped dry" and the new pumping rate at which the pump continues to operate.

Step 4. Continue pumping and record the pumping rate at one hour intervals commencing at the time in Step 3, until the same reduced pumping rate is obtained on 3 consecutive hours.

• If the 3 consecutive hourly readings are less than the minimum yield required, the yield is inadequate.
• If the 3 consecutive hourly readings are equal to or greater than the minimum allowable yield and a 3-day yield was pumped from the well, continue to Step 5 .
• If the consecutive hourly readings are equal to or greater than the minimum allowable yield, but a 3-day yield was not pumped from the well, continue pumping until this total 3-day yield is obtained, then continue to Step 5.

Step 5. End pumping and record the time and the elevation of the water level at the completion of pumping.

Step 6. 72 hours after the time recorded in Step 5, measure and record the water level in the well.

1. If the initial water level recovers 100% after 72 hours, the yield of the well is adequate.
2. If the above recovery is not obtained for the well, the yield is inadequate.

C. Test Results

The results of the yield tests shall be recorded on the forms provided by the "Health Officer." All applicable blanks on the forms shall be filled out. The yield of a gravity fed source shall be the lowest of the (3) three tests. The yield of a pumped source shall be the pumping rate established by the "3-day yield pumping tests."

3. Seasonal Timing of Tests

The yield testing shall be performed during the dry season (July 15 to October 1, unless the "Health Officer" expands this time limit due to lack of precipitation). Testing outside of the dry season may be allowed if sufficient data as approved by the "Health Officer" is submitted which correlates the actual test results with dry season results to establish actual sustained yield at the driest season.

4. Qualifications of Persons Making Tests

Yield tests shall be made only by State Licensed Well Drilling Contractors, General (Class A License) Contractors, Civil Engineers, or Geologists, except where a gravity fed source will supply (1) one single family dwelling, the test may be made by the property owner. Verification by a representative of the "Health Officer" may be required.

5. Source Setbacks

The source shall have minimum setbacks as specified below:

• From Property Line — 5 feet
• From Subsurface Disposal Field — 100 feet
• From Septic Tank or Other Subsurface Storage tanks (except water tanks) — 100 feet
• From Other Public or Private Sewer Pipe Line — 50 feet

6. Water Quality

The water shall meet the physical, chemical and bacteriological standards of the State Department of Public Health and the US Environmental Protection Agency. Where an analysis indicates that the source initially would not meet such standards, the applicant shall provide a proposed method of water treatment, or condition of, such as chlorinating, filtration, or chemical adjustment to meet such standards.

Additional Information

The following information should be presented in addition to the test results:

• Test Method
• Pumped Source
• Address of Well
• Date Well Was Developed
• Date of Pump Test
• Minimum Sustained Yield From Rules & Regulations
  • Gal/Min
  • Gal/3-day

For the well to be acceptable, the final pumping rate AND the total volume of water drawn from the well must exceed the minimum sustained yields AND a 100% recovery in the water level must be realized.

Minimum Source Yield and Storage Capacity by Number of Residences

	Sustained Yield			Storage
	Pumped Source		Gravity Source
	Gal/Min	Gal/3-day	Gal/Min
One	1	2160	0.50	1,000
Two	2	4320	1.00	2,500
Three	3	6480	1.50	4,000
Four	4	8640	2.00	5,000

• Drilling Instructions and Application - to construct monitoring well, repairs/upgrades, cathodic protection wells, soil bore/test holes.
• Domestic Water Supply Permit To Operate Application requirements and information that is needed to be submitted with application
• Well Destruction Application and Instructions 
• Report of Destruction of Test Holes or Geotechnical Borings 

• Local Authorities and Laws