A Guide To Septic Systems in Sonoma County

One of the biggest challenges for people looking to buy a piece of land outside of city services or for people purchasing a property that either requires an extensive remodel or for the purpose of vacation renting, is to understand what is involved in either putting in a septic system or upgrading a septic system. I sat down with Ben Monroe of Norcal Civil to talk about septic systems and poop! What are the different types of systems? What is the process for putting in a septic system? How long does it take to put a septic system in? How much does a septic system cost?

Because it is so important for buyers to understand the status of a septic system, it is also incredibly important for sellers to understand the implications for a property that has a failing septic system or a septic system that isn’t sized appropriately for the house. I start off by asking Ben what is a septic system?

What is a septic system?

A septic system is a soil-based method of treating and disposing of sewage without the use of a sewer collection system which you will find within the city limits of places like Sonoma and Healdsburg. However, just because a property isn’t within a city limit doesn’t mean it will definitely not be connected to a sewer system. For example, there are numerous properties in the Russian River, around Guerneville, Forestville and Graton that are in Sonoma unincorporated but are on sewer systems. There are also plenty of properties just outside of Santa Rosa city limits that have been connected into the city sewer system. Sometimes you will even find a property within City limits that is still on septic, such as in the Larkfield area of Santa Rosa.

“Septic systems vary based on when they were installed, site conditions, and what the requirements of the county were at the time were. In general, septic systems will include a septic tank (for solids removal), and a form of leach line (the sponge for liquid) to disperse the liquids out of the tank into the ground. In more advanced systems, there is a treatment system (like a mini-Municipal wastewater plant), which cleans up the sewage prior to discharge into the soil,” explain Ben.

What Are The Different Types of Septic Systems?

There are numerous different types of systems because over time septic system technology has improved and evolved. The oldest systems comprise little more than a hole in the ground and the waste just percolates into the soil. For example a Cesspool is exactly this. It has no septic tank and no leach lines. This is what the old redwood boxes along the Russian River are. The soil is so sandy they “work” but after time, they fill up with solids, or the redwood rots and they collapse. Originally, a lot of those homes were summer use only and therefore didn’t need much else. With today's rules, these do not qualify as a legal non-conforming septic system. A seepage pit is not much more sophisticated. It is a basic septic tank adjacent to a basin gravel that the clarified sewage seeps into or “percs” into (short for percolates) This is the bare minimum for legal non-conforming but it is not Compliant with todays code.

The septic system that most people are familiar with is a “Standard septic system”. This includes a tank and gravity fed leach lines or in instances where the only option for leach lines are above the tank, the liquids are pumped up from the tank to the leach lines from where the liquids seep into the soil. Leach Lines, themselves operate on gravity and consist of a 3”-4” perforated pipe with 12”-24” gravel beneath the pipe.

A variation on a standard septic system is a “Filled Land” system which uses 12-18” of imported soil through which the leach lines run to elevate the leach lines above dirt that otherwise would not support standard leach lines.

Another type of system actually operates under pressure forcing the water out of small holes in the leach lines to ensure that the water is distributed evenly along the entire trench. This is called a Pressure Distribution (PD) Leachfield. In these systems rather than use 3”to 4” of gravity perforated pipe, it uses 1” to 2” pipe with 1/8” to 3/16” holes drilled in it that squirt the water out. These are usually code compliant if it meets current criteria.

There are some types of septic systems that actually sit above the grade of the property. For example an At-Grade system, is similar to a PD, but instead of in-ground trenches, boards are used to form a gravel bed on the ground surface or “at-grade.” This trench is then capped with 12” soil cover which typically raises the grade by about 2 feet. Then there are mound systems which are easier to spot because they are similar to an at-grade system but between the gravel and natural soil is 12” of special sand that treats the sewage before getting to the natural soil layer. It is also capped with 12” soil and raises ground surface by 3’ +/-. These systems are usually code compliant.

Finally there is Subsurface Drip Irrigation System which includes a package treatment system component to clean the wastewater prior to discharging into 5/8” drip irrigation tubing (similar to that in your garden) for dispersal.

In the Russian River Flood Plain Area there are some systems which have a bottomless sand filter. This is a wood box with sides, filled with sand similar to a mound, but has no bottom on the box. Effluent from the septic tank is pumped over the sand and it percolates through sand prior to draining into native soil thereby providing a level of treatment before entering nature.

What Is the Process For Putting In a Septic System?

There are a number of different steps to the process which will vary depending on the type of soil on the lot where the system is being installed. The different steps can be summarized as follows:

1. Pre-Perc Test

This test is designed to learn about the depth of soil, soil types, potential permeability of soil, and presence of a limiting condition which will inhibit the ability for liquid to soak away. This is typically soil that contains clay, bedrock, over 50% concentration of gravel/rock or signs of seasonal saturation or perched groundwater It is called a pre-perc because the depth of percolation test is dictated by the depth of limiting condition identified and therefore will also dictate what type of system you can use to be code-compliant. As part of this test, the civil engineer is looking for a minimum depth of 24”of suitable soil before coming across a limiting condition.

Timeline: 2-6 weeks to apply to the County and schedule plus receive a report.

Cost: Approximately $4000 - $6000 depending on the size of the lot with a County fee of approx $750

2. Perc Test

This is a test that is completed in coordination with the county. The county must be notified of the testing date, but generally do not inspect the test. This requires digging 4”-6” holes with a post-hole digger or hand auger to the required depths. Depth varies by system type as do the number of holes by capacity of the proposed system. A perforated pipe is placed in the hole and backfilled up to 12” from bottom around the pipe with pea gravel. Water is then poured in the pipe up to the 12” mark. An hour later, the tester will take a measurement and note how far water has drained. This is required to be done every hour for six hours. Sandy soils will drain faster and need measurements every 10 minutes but only need testing for 2 hours after it has been determined that the water is draining too quickly for an hourly test. It is worth noting that a Perc test may not be required for an existing developed lot that is replacing a non-conforming system with a code-compliant system.

Timeline: 2-4 weeks to apply to the County and perform tests. The test results report must be submitted 90 days after performing.

Cost: Approximately $5000 with a county fee of $750

3. Wet Weather Perc Test (if needed)

When a site has clay or silt soils it may be necessary to conduct a wet weather percolation test. This is because soils will swell when saturated and drain slower. The purpose of this test is to create a worst case result for the test. One of the challenges for systems that require this test is that these tests can only be performed when the location has received 50% of the annual average rainfall. For example in Healdsburg, these tests can only be performed once the annual rainfall has reached 20”. Fortunately in 2022, 22” of water fell so this year the testing window did open. The County has a table to track the rainfall and ability to test a project site. https://permitsonoma.org/wetweathertesting

Timeline: As above but with the requirement for sufficient rain to have fallen

Cost: Approximately $5000 with a county fee of $750

4. Groundwater test (if needed)

A Groundwater test is required for septic areas with a ground slope of less than 5%. This is because water will perch in these flatter low areas. The procedure involves digging a hole and placing a perforated pipe in the hole at least 24 hours before taking your measurement. This pipe is used as an observation tube to review groundwater depths with measurements being taken from top of pipe to groundwater and top of pipe to soil surface to determine depth of groundwater. This dictates a limit to the depth of suitable soil before sewage will commingle with groundwater.

As with the wet weather per test, groundwater observations can only occur when the County has received 50% of average annual rainfall but it also requires 0.8” of rain falls in a 48-hour period after which you then have 10 days to call the county out for an inspection.

As I talk to Ben, you can tell this part of the job is a little stressful. “This time of year is kind of crazy, especially when you go a full year without groundwater opening in the whole of Sonoma County (2020-2021 winter) and then have to make up two years worth of project work when the testing window opens (Spring 2022). There is a lot of weather forecast watching when we get close to 50%.”

Timeline: Once the required rainfall has fallen it takes two days with right conditions

Cost: Approximately $1000 - 2000 with a County fee of Approx $300

5. Topographic Survey

Once you have completed Pre-perc, Perc, and a Groundwater test (if applicable) you are almost ready to design a system. You just need a topographic survey of the site to identify setbacks and available area for leachfields.

Timeline: The topo survey typically takes 2-6 weeks to schedule, perform field work and draw topo but this can be completed alongside other steps

Cost: Approximately $4000 to $6000 depending on the size of the lot and the system

6. Septic System Design

Once the topo is completed the design team will start the process of designing the system using all of the information gathered to date.

Timeline: This will take from 1 to 3 months depending on time of year, how busy the engineer is and other site factors such as working out where the house and driveway are going to go

Cost: $5000 to $6000 depending on the system

7. County Plan Check

Once all of the fieldwork has been completed and the septic design plans drawn up the plans need to be submitted to the County for approval.

Timeline: This can take 3 to 9 months depending on the number of revisions and the county backlog.

Cost: Approximately $3000 in permit fees

8. Septic System Installation

Once the system has been approved by the county, the engineers will lay out the septic tanks and leachfield area regardless of what type of system it is and perform calculations for sizing and prepare all of the construction details that are prepared for the installation team.

If a septic system is being installed on a blank lot with no existing, or previous house, the tanks are usually set first and then tested to make sure they are water tight. Then the plumbing to the tanks is done, the electric pumps are then installed and tested then finally the leachfield is installed. Finally the system is ready to be hooked up to the house and activated ready for you to take a bath!

Timeline: The installation will usually take from 4 to 8 weeks depending on the specific conditions on site, not to mention supply chain issues.

Cost: It really depends on the type of system. It might be as low as $30k for a standard gravity septic tank and leach line and over $100k for a 6 - 8 bed subsurface drip irrigation system. A 4 bed drip system is probably going to be approximately $60k

What is the Total Cost and Time Of Putting In A Septic System?

According to Ben, when asked how long does it take from beginning to end, “I tell people to plan a year from when you start the process, but it can take 2 years, or longer if you don’t get rain and you need it. If someone is trying to get an estimate of the type of process they might need to go through some quick file research combined with the parcel location can usually tell us pretty quickly what we are looking at.”

“When it comes to costs, if you are in West Sonoma County it will almost certainly cost you less than if you are putting a system in place in Napa. It is important to get a few bids as the costs can vary hugely based on the contractor and the market they are serving,” added Ben.

What is the process for expanding a system?

Similar to that above if you need a code compliant system. You might also add in verifying the location of the existing system with a magnetic pipe locator and backhoe. If it is a voluntary repair to expand the system, then that is much simpler and quicker. You need to draw up the expansion and apply for a permit. A “repair” permit doesn’t allow for additions of bedrooms, but may allow clearance on a new building permit.

What is the process for repairing a system? Do all repairs require a permit?

There is some level of work which is allowed without permits as “maintenance” that may include replacing a sewer line or distribution box, adding risers or effluent filters, etc. but work affecting the leach lines and replacing septic tanks generally require permits.

That said, there are several contractors which will “repair” septic systems without permits. You just want to make sure you at least have your own documentation for location and scope of work. If you do want a permit, I would recommend contacting a licensed sewage contractor to apply for the permit. There are a lot of quality contractors in our area. Some ask engineers to draw up the plan for the permit, as a to scale site plan showing the repair work and site features required.

Do you do design and installation?

No. We are engineers so will focus on steps 1 to 5 above. Any code compliant system will require engineers like us to prepare the plans and also submit a final letter at completion of construction saying it was done according to the plan. Generally, installation and repairs are done by licensed sewage contractors or General Engineering Contractors.

What is the difference between a Standard and Non-Standard System?

The main difference is that a Non-standard system, such as a mound system for example, requires an annual county operating permit as well as a recorded easement for County staff to access the site for monitoring every 1 to 3 years. Every quarter, the homeowner (or service provider they contract with) needs to inspect the monitoring wells for presence of subsurface liquid around the leachfield. Also, the control panel has a “dose counter” which logs pump activations and documents the average flow between readings. These items are submitted to the County once a year. After the homeowner has complied with this for 2 years, they can apply for a 50% operating permit fee.

Standard Systems do not have to be monitored by anyone and can be permitted by contractors if all the proper soil criteria are met although usually an engineer is involved

If you are Doing a Big Remodel will Your Plans Be Checked by The Septic Department of the County?

This is a complex question because there are so many variables with regard to the type of project. Any project that doesn’t need a building permit will not be reviewed by the septic department. Similarly any building permit that does not require a plan check will also not attract the attention of the septic department. For example putting in a new furnace, electrical repairs, HVAC, deck repairs, new roof require a permit but not a plan check.

For a building permit that requires a plan review the Septic code requires that the property is served by an existing non-conforming septic system consisting of a septic tank and dispersal system and not by a cesspool.

For any building improvement that increases the wastewater the County will require an existing code compliant system that can handle the additional wastewater as well as a code compliant reserve field. For a building improvement that does not increase the wastewater an existing non-confirming septic system is required. If there isn’t the necessary documentation to show the exact type of system, the county may require that a findings report is conducted to document the type of system, location of leach lines, system performance, size and all the relevant specification of the system.

Here is a detailed document provided by the county that outlines the septic requirements for various building improvements as well as for new build homes on existing and new lots.

How often do you need to empty a Septic Tank?

According to the county you should pump a tank every 3-5 years at a residence. It really comes down to what goes in the tank. If it's biodegradable, then you can really go 7-10 years. If you pump the tank routinely, there is less chance that you will pass solids or grease into the leachfield which will clog soil. Clogged soil doesn’t drain and leads to a leachfield surface failure. Even at $600-750 per pump for a septic tank, it’s a lot cheaper than a new leachfield or annual sewer charges. I also recommend everyone install an effluent filter on the outlet pipe inside the 2nd chamber of their tank. This is a cost-effective second line of defense for a full tank not passing solids downstream. 

In Summary

There is a lot of information in here, probably more than you will ever need to know about septic systems but if you are looking to buy or sell a house with a septic system or a piece of land with the goal of building, it is really important to understand what it takes to get a code compliant septic system. There are a number of instances where understanding the current septic system will make a huge difference to your decisions to buy or sell a piece of property. These cases are as follows:

• Buying a lot with the goal of building a home
• Buying a property with the aim of adding a guest house or ADU
• Buying a property with the goal of adding an additional bedroom
• Buying a lot that had a previous house on it that was lost in the fires
• Getting a vacation rental permit for a property with a septic system
• Selling a piece of land that has no septic system or existing Perc or Groundwater test

A Huge Thank You To Ben Menroe, Founder and CEO of Norcal Civil, for all his input into this blog post. Ben is contactable on Ben@norcalcivil.com

Norcal Civil provides general civil engineering consulting from project and site planning, entitlement, topographic survey, grading, drainage, decentralized wastewater, septic, and water system inspections, design, and permitting, stormwater management and construction phase oversight and assistance.

Frequently Asked Questions:

1. What is a septic system?

A septic system is a soil-based method for treating and disposing of sewage without a sewer collection system, common in areas outside city limits like Sonoma County. It typically includes a septic tank for solids removal and leach lines for liquid dispersion.

2. What are the different types of septic systems?

Several types include Standard septic systems, Filled Land systems, Pressure Distribution Leachfields, At-Grade systems, Mound systems, and Subsurface Drip Irrigation Systems, each with unique designs and purposes.

3. What is the process for putting in a septic system?

The process involves a Pre-Perc Test, Perc Test, Wet Weather Perc Test (if needed), Groundwater test (if needed), Topographic Survey, Septic System Design, County Plan Check, and finally, Septic System Installation. The timeline and costs vary based on specific conditions.

4. How long does it take to put in a septic system?

The process typically takes around a year, but factors like weather, soil conditions, and county approvals can extend it to two years or more.

5. What is the total cost of putting in a septic system?

Costs vary based on factors such as location and system type. For example, a standard gravity septic tank and leach line might cost around $30k, while a 6-8 bed subsurface drip irrigation system could exceed $100k.