Welcome to Bonser & Son

Do you need a new septic system design? Your septic system replacement could qualify under the In Kind Replacement Rule? Your failing leach field might not need a new design. Give us a call and find out.

Bonser & Son installs all types of septic systems and will fix any problem. If you are having a septic problem and want to find out the cost of a replacement system, or if you have a question don't hesitate to call:

679-2036 or email:

Septic System Design And Repair

When having a septic system designed, or a sewer system repaired or replaced there are several leach field choices approved for New hampshire by the Department of Enviromental services NHDES. Depending on your lot's percolation rate, seasonal high water table, wetland and other distance requirements you have the following choices:

  • Stone & Pipe Septic System
  • Chambers (cement or plastic) Septic System
  • Enviro-Septic System
  • Eljen In-Drains Septic System
  • Trenches Septic System
  • Dry Well Septic System

Each septic system choice has advantages and disadvantages. Bonser and Son has over 35 years designing and installing septic systems and will work with you to get the best system that meets your needs.

There are serveral terms and acronyms you should know to help you understand the designs process.

TEST PIT: Is a hole dug by a backhoe to locate how far below ground there is any ledge, impervious soil (clay), and locate the depth to the seasonal water table. SEASONAL HIGH WATER TABLE: SHWT is how far below the surface the heighest average water height is during the year.
PERCOLATION RATE: Is how long it takes for 1 inch of water to be absorbed by the soil in the test pit. The absorbtion rate can range from 2 to 60 min/inch IMPERVIOUS SOIL:The soil that is hardened or cemented and takes over 60 minutes to absorb 1" of water. BEDROCK OR LEDGE: Is the solid rock that underlies the soil or is somtimes seen on the surface.

All residential septic systems are designed on the number of bedrooms in a house. Listed below are several factors that determine the size and location of your septic system.

Septic Tank Size:

2Bedrooms = 1250 Gallon tank
3 Bedrooms = 1250 Gallon tank
4Bedrooms = 1250 Gallon tank
Add 250 Gallons for each additional bedroom.

Bed Size:

The leach bed area for a conventional stone & pipe system would be as follows:


Percolation rate in minutes per inch 2 bedroom (300 GPD) 3 bedroom (450 GPD ) 4 bedroom (600 GPD )
2 minutes 400 sq.ft. 560 sq.ft. 750sq.ft.
30 minutes 875 sq. ft. 1310 sq. ft. 1750 sq. ft.
60 minutes 1500 sq.ft. 2250 sq.ft. 3000 sq. ft.

The above chart is an abbreviated version of the design size requirements in NH. The percolation rates start at 2 mins/inch and goes up in 2 mins/inch increments up to 60 mins/inch.

To determine the length of your system you choose a width from 10, 15, 20, 25, 30 feet wide and devide the area by the width to get the length that will best suit your site.

Chart-3 shows the different length of a 20 foot wide leach field depending on percolation rate and the number of bedrooms


Percolation rate in minutes per inch 2 bedroom (300 GPD) 3 bedroom (450 GPD ) 4 bedroom (600 GPD )
2 minutes 20 ft. 28 ft. 38 ft.
30 minutes 44 ft. 66 ft. 88 ft.
60 minutes 75 ft. 113 ft. 150 ft.
Reduced Leach Bed Sizes The Area in Chart-2 can be reduced by the percentage below: Example with 30 minute percolation rate

3 Bedrooms 450 GPD

0% Reduction for Stone & Pipe Stone & Pipe 1310 sq. ft.(0%)
40% Reduction for a Chamber System Chambers 786 sq. ft.(60%)
50% Reduction for Enviro-Septic Enviro-Septic 655 sq. ft. (50%)
75% Reduction for Eljen In-Drains Eljen In-Drains 328 sq. ft. (25%)

Why are some leach field big mounds ?

"Vertical Seperation", the distance above Seasonal High Water Table (SHWT):

The Estimated Seasonal High Water Table (ESHWT) from your test pit determines if your disposal field is in ground or raised. The bottom of the bed must be 4 ft. above the ESHWT except Enviro-Septic bed bottom can be 30 inchs. above the ESHWT. Lots with high water table will have large mounded raised systems.

Depending on your systems location it will be either a gravity or a pump system. Gravity systems are best, however due to lot restrictions and/or "Horizontal seperation", the system has to be located higher then the house and needs to be pumped.

Why are some leach field so far from the house?

"Horizontal Seperation" is the minimum distances the septic tank and leach field must be from objects listed in Chart 4.

The following distances are a condensed version of over 24 requirements in Table 1008-2 of NHDES rules. All distance are in feet.


Septic Tank Feet Leach Field    Feet
Surface Water 75 75
Poorly Drained Soil 50 50
Very Poorly Drained Soils (wetlands) 75 75
Private well on and off site 75 75
Foundation ,any type,With foundation drains 5 15
Foundation ,any type,Without foundation drains 5 10
Slab Without foundation Drains 5 5
Lot Line 10 10

These are NH minimum requirement, some town have greater distance requirements. Based on the above distances your septic system will not always end up where you originally wanted it.

Serving: Dover NH, Rochester NH, Nottingham NH, Barrington NH, Lee NH, Epping NH, Raymond NH, Deerfield NH, Northwood NH, Madbury NH, Newmarket NH, Newfields NH, Exeter NH, Durham NH, Brentwood NH, Fremont NH, Stratham NH, Somersworth NH, Greenland NH, Candia NH, Auburn NH