HomeMy WebLinkAboutEngineer's Design Revision 07.2.25
Proposed Single-Family OWTS System
Onsite Wastewater Treatment System Design
3-BR Single-Family Residence
TBD County Road 320
Garfield County, CO 81620
Parcel No. 2175-274-01-002
Prepared for:
Clayton Homes
671 23 Road
Grand Junction, CO 81505
April, 2025
(Revised July 2025)
Prepared by:
2516 Foresight Circle #1, Grand Junction, CO 81505 Telephone: (970)241-7076
Page 1
1.0 SCOPE
This report presents the findings of an On-Site Wastewater Treatment System (OWTS) design
for a proposed three bedroom (3-BR) single-family residence at Parcel # 2175-274-01-002
(County Road 320 – address TBD) in Garfield County, Colorado. The purpose of this report was
to establish design criteria for a new sewage disposal system. The report includes an assessment
of observed subsoil and groundwater conditions, percolation test results, and the design and
construction parameters for the OWTS. The findings are based on data gathered from field
percolation testing, and engineering evaluation informed by experience with similar site
conditions.
2.0 DESIGN SUMMARY
1. Two test pits were evaluated at the project location. Encountered subsurface
conditions consisted of 1 foot of topsoil underlain by silty sand with gravels to the
depths explored. The percolation result at the elevation of infiltration was an average
of 11 minutes per inch. We determined from Table 10-1 of Garfield County’s On-Site
Wastewater Treatment System Regulation (Regulations) Soil Type 1 and used “Long
Term Acceptance Rate” (LTAR) = 0.80 gallons per day per square foot.
2. The design system will utilize a new CDPHE approved 1,000 gallon septic tank,
which will discharge effluent to four new (4) Quick 4 Plus Infiltrator trenches, each
with nine (9) units, for a total minimum disturbed area of about 24 feet by 36 feet.
3. Design placement based on setbacks and comparison to minimum requirements,
which must be confirmed prior to installation by the licensed installation contractor.
3.0 EXISTING SITE CONDITIONS
The 2.23-acre parcel located at Parcel # 2175-274-01-002 County Road 320 was a lot with no
structures and sloped down to the west at approximately 1%. The address is to be determined.
4.0 PROPOSED SITE IMPROVEMENTS
A proposed 3-BR single-family residence is to be constructed with no below grade/basement
areas or additional structures. If proposed construction is different than what is described above,
WestWater Engineering should be notified to re-evaluate the recommendations made herein.
The new 3-BR dwelling will be served by a new 1,000-gallon septic tank (minimum) and a total
of 36 new Standard Plus Quick4 Infiltrator® units configured in four (4) trenches, each with nine
(9) Q4 Plus units for a minimum 24’ x 36’ footprint including 4 feet separation distance between
trenches and chamber end caps (Figure B2).
5.0 SOIL INVESTIGATION
Soil and percolation data was collected by others on October 8, 2023. WestWater Engineering
collected addition soil data on July 1, 2025. The soil profiles in the vicinity of the proposed Soil
Treatment Area (STA) showed topsoil of silty sand to 1 foot below ground surface underlain by
silty sand to the maximum depth explored of 8 feet below ground surface. It appears that no
Page 2
groundwater was encountered at the time of observation to the maximum depth explored
(Appendix A).
Percolation rates ranged from 13:03 to 10:06 minutes per inch (Appendix A). We reviewed the
Garfield County, On-Site Wastewater Treatment System Regulations effective April 20, 2023,
Table 10-1 and determined Soil Type 1, Long Term Acceptance Rate (LTAR) = 0.80 gallons per
day per square foot most nearly describes the condition for this site and proposed construction.
6.0 OWTS WASTEWATER DESIGN FLOW
As per guidelines, the occupancy is 6 persons, tabulated as 2 people per bedroom for the first 3
bedrooms. The design flow is 6 persons x 75 gallons per day per person = 450 gallons per day
(see Table 6-1 of the Regulations).
7.0 OWTS DESIGN
Septic Tank
Minimum septic tank capacity required to service a total of three (3) bedrooms is 1,000 gallons
in accordance with Table 9-1 of the Regulations. The required septic tank volume corresponds
with a 48-hour (2-day) particle detention time for the design of the wastewater flow capacity of
the proposed OWTS. The required capacity of 1,000 gallons exceeds the estimated daily
wastewater flow of 450 gallons per day (GPD) over a two-day period (450 GPD × 2 days = 900
gallons), ensuring compliance with the Regulations. The tank shall be equipped with locking or
secured access lids to prevent unintended access, as well as an effluent screen (or filter) at the
tank outlet.
Furthermore, the proposed 1,000-gallon septic tank for the OWTS shall be selected from the
current Colorado Department of Public Health and Environment (CDPHE) approved septic tank
list.
Distribution Box
For this design, use a new distribution box (D-box) approved for use in Garfield County. Ensure
that the flow is equal between laterals by using flow equalizers in D-box and access is available
for maintenance and inspection via an approved riser assembly with locking lid.
Soil Treatment Area (STA)
Soils occurring at the 4.0-foot depth interval will be used for the upper infiltrative surface for the
proposed STA (Figure B3). Maximum trench depth must not exceed 4 feet in accordance with
the Regulations. Trench excavation will remove approximately 4.0 feet of soil and upper root
mass to prepare the level surface for installing infiltrators on the receiving soil infiltrative
surface, providing secondary treatment of discharged wastewater. Soil should not be wet at the
time of excavation. Soil covering the STA must be crowned over proposed trenches or match
the existing grade so water runs off and is not ponded over any area of the STA. Prior to
backfilling the final soil cover, a pervious soil barrier of non-woven geotextile fabric (2 oz/yd 2
minimum) or similar pervious material shall be installed over the STA to prohibit soil intrusion.
Page 3
All setback distances in accordance with the Regulations are applicable to septic system
installation.
The following design calculations propose to discharge septic tank effluent to four (4) Quick 4
Plus Infiltrator trenches, each with nine (9) units, for a total minimum disturbed area of about 24
feet by 36 feet. A total of 36 Quick 4 Standard Plus Infiltrator units will be installed in the four-
trench configuration, which is generally oriented north to south.
𝑆𝑇𝐴=𝐷𝑒𝑠𝑖𝑔𝑛 𝐹𝑙𝑜𝑤
𝐿𝑇𝐴𝑅× 𝑀𝑒𝑡ℎ𝑜𝑑 × 𝑀𝑒𝑑𝑖𝑎(1)
Where:
STA = Total surface area required to effectively discharge sewage effluent
Design Flow = Estimated volume of wastewater per day
LTAR = Long-Term Acceptance Rate of soil treatment area
Method = Method of effluent application and type of soil treatment area
Media = Type of distribution media
𝑇𝑜𝑡𝑎𝑙 𝐶ℎ𝑎𝑚𝑏𝑒𝑟𝑠= 𝑆𝑇𝐴
𝐶ℎ𝑎𝑚𝑏𝑒𝑟 𝐴𝑟𝑒𝑎(2)
Where:
Chamber Area = The County and State approved bottom chamber area for Quick 4 Plus
Standard Infiltrator Unit, 12 ft2
If,
Per Regulations Table 6-1,
Design Flow = 450 Gallons/Day
Long Term Acceptance Rate (LTAR) review of Regulations Table 10-1,
LTAR = 0.80 Gallons/Day/Foot2
Type of application is chosen as gravity fed trench system, per Regulations Table 10-2,
Method = 1.0
Page 4
Distribution media is chosen as chambers, per Regulation Table 10-3,
Media = 0.7
Then,
𝑆𝑇𝐴=450
0.8 × 1.0 × 0.7
(1)
The minimum STA Area is 393.75 ft2.
𝑇𝑜𝑡𝑎𝑙 𝐶ℎ𝑎𝑚𝑏𝑒𝑟𝑠= 393.75
12 (2)
The minimum number of Quick 4 Plus Infiltrators is 32.8. As 32.8 Infiltrators cannot be evenly
divided between four trenches to ensure equal effluent flow, the design increases the number of
Quick 4 Plus Infiltrators to a total of 36, divided between four trenches with nine (9) units each.
8.0 SYSTEM INSTALLATION REQUIREMENTS
The installer should confirm the feasibility of gravity discharge of sewage effluent from the
proposed residence to the STA. If field measurements do not confirm the design assumptions for
gravity discharge of sewage effluent from the proposed residence to the STA, or if the installer
does not consider the above design assumptions practical, please contact the design engineer to
address any design modifications necessary for the effective operation of the OWTS.
The installer must also confirm the setbacks from property lines, building envelopes, and
existing easements are maintained (see Appendix C for Setbacks Table). Additionally, any
unknown utility lines, easements, or other adverse conditions disclosed during construction must
maintain the required minimum setbacks
All sewer pipe couplings must be completed with primed and glued or bell and spigot joints
using 4-inch SDR 35 PVC pipe. Schedule 40 PVC 4-inch pipe with a minimum 1-foot soil cover
must be used if pipe is installed in a vehicular traffic crossing. Drain-waste-vent (DWV) rated
Schedule 40 PVC pipe is called out for crossings but may also be used instead of SDR 35 PVC
pipe.
One water line is proposed from the south face of the proposed single-family dwelling to the
existing water well (CO Permit #85270-F). All septic system elements must be setback by a
minimum of 100 feet from the water well per the Regulations.
Building sewer pipe must be installed at a slope between ⅛-inch and ¼-inch per foot slope (1 to
2%), preferably using the ¼-inch per foot slope as recommended by the International Plumbing
Code (IPC) as administered by the County. Fittings shall be sweep 90-degree curves or two (2)
45-degree elbows at all influent pipe turns. Fittings must be DWV for schedule 40 PVC pipe.
Two-way clean-outs are required between dwelling and tank and beyond tank where pipe lengths
exceed 100-feet from tank to STA and at all pipe bends greater than 45 degrees. Clean outs shall
Page 5
be installed with screw caps at finished grade with a protective housing for easy access by
homeowner.
The installer must adhere to minimum and maximum depth requirements for the STA. System
components must be placed on native soils or re-stabilized soils, as specified by the engineered
OWTS reports, in excavated areas.
The owner will ultimately be responsible for septic system maintenance including pumping the
septic tank once every 2-3 years and cleaning the effluent filter every 6 months.
9.0 SYSTEM INSPECTION
WestWater Engineering is responsible for inspecting the OWTS to ensure construction conforms
to the design plan and meets all aspects of County Regulations, including:
Inspect and measure the system before backfilling, including piping and septic system
components.
Complete the as-built drawing of the system and submit it to Garfield County for
approval.
Reviewing and approving any submitted changes necessary for installation of the OWTS
as deemed necessary by the owner or their contractor. Any system changes installed by
the owner/contractor that were not approved by Engineer are the responsibility of the
owner
10.0 DESIGN LIMITATIONS
The OWTS design is based on site-specific soil conditions and percolation data collected by
others in the vicinity of the proposed STA and local experience of designing OWTSs in
Colorado. The design is only valid for the proposed new OWTS serving the proposed 3-BR
dwelling at the referenced location provided in this report. WestWater Engineering assumes no
liability for inaccurate information or conditions that vary from the design assumptions listed
above. Additionally, WestWater Engineering is not responsible for the quality of work for any
person(s) or entity performing or supporting construction activities relating to the project, or for
any contractor’s failure to furnish or perform its work in accordance with the plans,
specifications or the Regulations. The owner/contractor is to notify WestWater Engineering
and/or Garfield County immediately if actual site conditions differ from those described herein.
Please do not hesitate to contact us if you have any questions regarding this report.
APPENDIX A
SOILS AND PERCOLATION DATA
A1
SOILS EVALUATION
Date: July 2, 2025
Consultant: Curtis Beckel, EIT
WestWater Engineering
2516 Foresight Circle #1
Grand Junction, CO 81505
(970) 241-7076
Client: Clayton Homes
Land Parcel: TBD County Road 320, Rifle, CO 81505
2175-274-01-002
SECTION 1 – SOILS LOG
Two test pits were evaluated at the empty 2.23-acre parcel, for an onsite wastewater treatment
system (OWTS) that will serve a new proposed 3-BR single family dwelling located at Parcel
2175-274-01-002, Rifle, Colorado.
Test Pit 1 (TP-1) was located approximately 90 south of the north property line and 17 feet east
of the west property line respectively (see Figure B1 of the Design Report). Test Pit 2 (TP-2) was
located approximately 90 south of the north property line and 17 feet east of the west property
line respectively.
The visual description of the profile soils, with ASTM D 2488 “Standard Practice for
Description and Identification of Soils (Visual-Manual Procedures)” used for guidance, is
summarized below and in Table 1 – Log of Exploratory Test Pits. ASTM D 2488 identifies soils
based on visual examination and manual tests.
A2
Table 1 - Log of Exploratory Test Pits
TBD County Road 320
Rifle, Colorado
TP-1
Location: See Figure B-1 Date: 10-8-23
Depth
(ft) Description Interval Notes
0
Stiff, dry, tan and brown, low plasticity, silty
SAND 0-1’ Topsoil
Stiff, dry, tan, low plasticity, silty SAND
w.gravels
1-8’
5
Total Depth = 8 feet below ground surface
A3
TBD Rapids View Lane
New Castle, Colorado
TP-2
Location: See Figure B-1 Date: 7-1-25
Depth
(ft) Description Interval Notes
0
Stiff, dry, tan and brown, low plasticity, silty
SAND 0-1’ Topsoil
Stiff, dry, tan, low plasticity, silty SAND
w.gravels 1-4’
Total Depth = 4 feet below ground surface
Subsurface conditions encountered in the test pits consisted of one foot of silty sand topsoil
underlain by silty sand with gravels to the maximum depth explored of 8 feet below ground
surface. The silty sand topsoil was stiff, dry, tan and brown. The silty sand with gravels was stiff,
dry, and tan. No groundwater was encountered at the time of observation. TP-1 was evaluated by
others and similar material was encountered in TP-2 by WestWater Engineering.
APPENDIX B
FIGURES
2516 Foresight Circle, #1
Grand Junction, CO 81505
(970) 241-7076
www.westwaterco.com
2516 Foresight Circle, #1
Grand Junction, CO 81505
(970) 241-7076
www.westwaterco.com
2516 Foresight Circle, #1
Grand Junction, CO 81505
(970) 241-7076
www.westwaterco.com
2516 Foresight Circle, #1
Grand Junction, CO 81505
(970) 241-7076
www.westwaterco.com
APPENDIX C
GARFIELD COUNTY ON-SITE WASTE WATER
TREATMENT SYSTEM REGULATIONS, TABLE
7-1 FROM THE 2023 REGULATION DOCUMENT
CODE OF COLORADO REGULATION 5 CCR 1002-43
Water Quality Control Commission
43
Table 7-1 Minimum Horizontal Distances in Feet Between Components of an On -Site Wastewater Treatment System Installed After
November 15, 1973 and Water, Physical and Health Impact Features
Spring,
Well,1
Suction
Line,
Potable
Water
Supply
Cistern4
Potable
Water
Supply
Line 2
Structure
w/basement,
crawl space
or footing
drains
Structure
without
basement,
crawl
space or
footing
drains
Property
Lines,
Piped or
Lined
Irrigation
Ditch,
upslope
curtain
drain
Subsurface
Drain,
Intermittent
Irrigation
Lateral,
Drywell,
Stormwater
Structure
Lake,
Water
Course,
Irrigation
Ditch,
Stream,
Wetland
Dry
Gulch,
Cut
Bank,
Fill
Area
(from
Crest)
Septic
Tank,
Higher level
treatment
Unit,
Dosing
Tank, Vault
or Privy
Septic Tank,
Higher Level
Treatment Unit,
Dosing Tank,
Vault or Vault
Privy
50 2 10 2 5 5 10 10 50 10 --
Building Sewer or
Effluent Lines
50 2 5 6 0 0 10 2 10 2 50 2 10 2 --
STA Trench, STA
Bed, Unlined
Sand Filter, Sub-
surface Dispersal
System, Seepage
Pit
100 3 25 2 20 10 10 25 50 3 25 5
Lined Sand Filter 60 10 2 15 10 10 10 25 10 5
Lined Evapo-
transpiration Field
or Outside of
Berm of Lined
Wastewater Pond
60 10 2 15 15 10 10 25 10 5
FIG. C-1
CODE OF COLORADO REGULATION 5 CCR 1002-43
Water Quality Control Commission
44
Spring,
Well,1
Suction
Line,
Potable
Water
Supply
Cistern4
Potable
Water
Supply
Line 2
Structure
w/basement,
crawl space
or footing
drains
Structure
without
basement,
crawl
space or
footing
drains
Property
Lines,
Piped or
Lined
Irrigation
Ditch,
upslope
curtain
drain
Subsurface
Drain,
Intermittent
Irrigation
Lateral,
Drywell,
Stormwater
Structure
Lake,
Water
Course,
Irrigation
Ditch,
Stream,
Wetland
Dry
Gulch,
Cut
Bank,
Fill
Area
(from
Crest)
Septic
Tank,
Higher level
treatment
Unit,
Dosing
Tank, Vault
or Privy
Unlined Sand
Filter in Soil With
a Percolation Rate
Slower than 60
Minutes per Inch,
Unlined or
Partially Lined
Evapotrans-
piration System,
Outside of Berm
of Unlined
Wastewater Pond,
or System Not
Relying on STA
for Treatment
Other than
Aerosol
100 25 2 15 15 10 25 25 15 10
Slit Trench
Latrine, Pit Privy
100 50 2 25 25 25 25 100 25 N/A
System Not
Relying on STA
for Dispersal
100 3 10 2 125 1255 10 0 25 3 10 10
NOTE: The minimum distances shown above must be maintained between the OWTS components and the features described. Where soil , geological or other conditions warrant,
greater distances may be required by the local board of health or by the Water Quality Control Commission pursuant to section 25-8-206, C.R.S. and applicable regulations. For repair
or upgrading of existing OWTS where the size of lot precludes adherence to these distances, a repaired OWTS must not be close r to setback features than the existing OWTS, as
reviewed and approved by the local public health agency. Components that are not watertight should not extend into areas of t he root system of nearby trees.
1 Includes potable wells, irrigation wells and monitoring wells set within a potable aquifer and infiltration galleries permitted as wells by the Division of Water Resources.
FIG. C-2
CODE OF COLORADO REGULATION 5 CCR 1002-43
Water Quality Control Commission
45
2 Crossings or encroachments may be permitted at the points as noted above provided that the water or wastewater conveyance pip e is encased for the minimum setback
distance on each side of the crossing. A length of pipe with a minimum Schedule 40 rating [ASTM Standard D 3034-16 (2016 version)] of sufficient diameter to easily slide over and
completely encase the conveyance must be used. Rigid end caps of at least Schedule 40 rating [ASTM Standard D 3034-16 (2016 version)] must be glued or secured in a watertight
fashion to the ends of the encasement pipe. A hole of sufficient size to accommodate the pipe must be drilled in the lowest section of the rigid cap so that the conveyance pipe rests on
the bottom of the encasement pipe. The area in which the pipe passes through the end caps must be sealed with an approved und erground sealant compatible with the piping used.
Other methods of encasement that provide equal protection are allowed. These methods must be reviewed and approved by the local public health agency.
3 Add eight feet additional distance for each 100 gallons per day of design flows between 1,000 and 2,000 gallons per day, unless it can be demonstrated by a professional
engineer or geologist by a hydrologic analysis or the use of a barrier, consisting of a minimum 30 mil PVC liner or equivalent, that contamination will be minimized. If effluent meets
Treatment Level 3N and the local publ ic health agency has a maintenance oversight program in accordance with section 14.D. of this regulation, the distance additi on is not required.
Flows greater than 2,000 gallons per day must be hydrologically analyzed for flow, velocity, hydraulic head, an d other pertinent characteristics as means of estimating distances
required to minimize contamination as part of the Division site application and permitting process.
4 All horizontal setbacks to a potable water supply cistern must be met unless a variance by the Board of Examiners of Water Well Construction and Pump Installation
Contractors is granted per section 18.2 of the Water Well Construction Rules, 2 CCR 402 -2. Setback requirements which may necessitate a variance are found within section.10.2 or
11.4 of the Water Well Construction Rules, as applicable. The minimum horizontal setback that may be granted through a variance is to 25 feet.
5 If the structure is not used as a habitable unit, the isolation may be reduced by the local board of health to no less than 50 feet.
6. Building sewer installations shall meet the design requirements of the Colorado Plumbing Code.
FIG. C-3