HomeMy WebLinkAboutEngineer's OWTS Design PlanProposed Single-Family OWTS System
Onsite Wastewater Treatment System Design
3-BR Single-Family Residence
TBD County Road 320
Garfield County, CO 81620
Parcel No. 2 I 7 5-27 4-01-002
Prepared for:
Clayton Homes
671 23 Road
Grand Junction, CO 81505
Digitally signed by
Gilliam
April 2025 2025.04.28
09:24i26-06'00'
Prepared by:
9$1**tmH[,,mgk""rins
2516 Foresight Circle #1, Grand.Itmction, CO 81505 Telephone: (970)241-7076
1.0 SCOPE
This report presents the findings of an On-Site Wastewater Treatment System (OWTS) design
for a proposed 3 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
l. The percolation result at the elevation of infiltration was an average of I 1 minutes per
inch. We determined from Table 10-l 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 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 about24 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 # 217 5-27 4-0 I -002 County Road 320 was a lot with no
structures and sloped down to the west at approximately lo/o. 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 1,0O0-gallon septic tank (rninimum) 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 minimum24'x 36' footprint including 4 feet separation distance between
trenches and chamber end caps (FigureB2).
s.0 soIL GATION
Soil and percolation data was collected by others on October 8,2023. The soil profile in the
vicinity of the proposed Soil Treatment Area (STA) appears to show topsoil of silty sand to 1
foot below ground surface underlain by silty sand to the maximum depth explored of 8 feet
Page I
below ground surface. It appears that no groundwater was encountered at the time of observation
to the maximum depth explored (Appendix A).
Percolation rates ranged from 13:03 to l0:06 minutes per inch (Appendix A). We reviewed the
Garfield County, On-Site Wastewater Treatment System Regulations effective Apri120,2023,
Table l0-l 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-l 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 x 2 days:900
gallons), ensuring compliance with the Regulations. Access lids must be locking or secured to
prevent unintended access.
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.O-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 ozlydz
minimum) or similar pervious material shall be installed over the STA to prohibit soil intrusion.
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 about24
Page2
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.
Desian Flow
STA =
-LLf AR x Method x Media (1)
(2)
Where:
STA:
Design Flow:
LTAR:
Method:
Media:
Total surface area required to effectively discharge sewage effluent
Estimated volume of wastewater per day
Long-Term Acceptance Rate of soil treatment area
Method of effluent application and type of soil treatment area
Type of distribution media
Total Chambers =
STA
Clr" bq A'rt"
Where
Chamber Area: The County and State approved bottom chamber area for Quick 4 Plus
Standard Infiltrator tJnit, 12 ftz
If,
Per Regulations Table 6-1,
Design Flow : 450 Gallons/Day
Long Term Acceptance Rate (LTAR) review of Regulations Table l0-1,
LTAR : 0.80 Gallons/DaylFoot2
Type of application is chosen as gravity fed trench system, per Regulations Table 10-2,
Method: 1.0
Page 3
Distribution media is chosen as chambers, per Regulation Table 10-3,
Media:0.7
Then,
The minimum STA Area is 393.75 ftz
450STA=0Sr1.0x0.7
393.75Total Chambers = T
(1)
(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 INSTALLA ON REOI]IREMENTS
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 above
design assumptions are not deemed practical by the installer, please contact the design engineer
to address any design modifications necessary for 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 frorn 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 t/"-inch and lq-inch per foot slope (l to
2o/o), preferably using the %-inch per foot slope as recommended by the International Plumbing
Code (IPC) as administered by the County. Fittings shall be sweep 9O-degree curves or two (2)
45-degree elbows at all influent pipe turns. Fittings must be DWV for schedule 40 PVC pipe.
Page 4
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
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 (if applicable) 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.
a
10.0
Reviewing and approving any submitted changes necessary for installation of the OWTS
as deemed necessary by the owner or their contractor.
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 seruing 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 notifu 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.
Page 5
APPENDIXA
SOILS AND PERCOLATION DATA
Perc Test Dotson Minor Subdivision
Kirk and Pat Dotson
5473 County Road 320
Rifle, CO 81650
Kirk9TO-274-4995
Pat 970-456-7887
Overview of Test (pictures attached)
L. Dug Profile Hole to 10 ft. All dry with uniform soit conditions.
2. Dug Perc Test Holes with backhoe on a 20' triangular spacing to 3'. Dug L2,, x L2', hole in the
bottom of all 3 holes by hand.
3. Pre-soaked L2" xL2" hole for eight hours prior to perc test onLolg/23at 9:00 am.
lf you have any questions we can be reached at the contact numbers above
Thank you!
The Dotson's
DatetolSlz3 Start End Time Rate
Hole # 1 5:14 pm 7:59 pm 2:45165 min)13:03 mln/inch
Hole # 2 5:15 pm 7:26pm 2:11(131min)10:09 min/inch
Hole # 3 5:16 pm 7:24 pm 2:08 (128 min)10:06 min/inch
APPENDIX B
FIGURES
APPENDIX C
GARFIELD COUNTY ON.SITE WASTE WATER
TREATMENT SYSTEM REGULATIONSO TABLE
7-1 FROM THE 2023 REGULATION DOCUMENT
,J'
n i>,'
i
At{rlest)fuhter
= Engineering
2516 Foresight Circle, #1
Grand Junction, CO 8 I 505
(970) 241'7076
www.westwaterco.com
FIGURE 81
TBD COUNTY ROAD 320
EXISTING OWTS SITE PI.AN
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t
OXIMATE' EXISTING,
GARFIELD COUNTY
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PROPERTY LI 2.23
G I S,MARCH.,/-o2s)
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EASMENT
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"r",r,. 'APP ROX IMATE l_ocA
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o F
{,:,
EXI STI N G IWATER,.;WELL
100 50 0
SCALE: 1 ":1 OO'
100
Ar$hstllrhtete Engineerlng
2516 Foresight Circle, #l
Grand Junction. CO 8 I 505
(970)241-7076
www.westwaterco.com
FIGURE 82
TBD COUNTY ROAD 320
PROPOSED OWTS I.AY-OI.JT
:.
I
, "''=I5
t,,\
-_l
NEW INFILTRATOR TRENCH
TRENCHES.g UNITS EACH
CHAMBEH UNITS COVER W
GEOTEXTILE FABRIC (SEE -' ,o')
AQ
6r"APPROXIMATE LOCATION
OF TELECTRIC MTTER
20' UTILITY EASEM
;APPRO
FROPO
XIMATE LOCATI
SED NEW 3-B
ES,
cuRE 83)
NEW DISTRIBUTION BO
PROPOSED ICAL
_-,ait -
NEW 2-WAi;!
CLEANOUT
\/ ,
------i=---:-
-+,- -'' i,
1t
I
I
4
I
t^
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at.\\ '\
ll,'\
PROPOSED
i
l-)6' MIry.'r
NE PRPPOSED NEW 1,
gALLON SEPTIC TANK ', t
,t )
I
t.I'
I
FUTURE STA,..,hE
AREA 24,X36.
PAIR ./
t
'a-t"1fi)/-
lRppnoxIMATE LocATI o. EXISTING WATER WELL\
NOF
(8527O'TF).,/
APPROXIMATE EXISTING P
LrNE - 2.23 AqRES
couNTY GIS, MARCH
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a'
'/, ;,
i'
\:
\:_
1OO' MINIMUM S
WATER WELL TO
FROM
STA LD
2025 )
60300 60
SCALE 'l "=60'
OWTS CROSS SECTION
FIGURE B3
allNestl/ULbter
= Engineering
2516 Foresight Circle, # I
Grand Junction. CO 81505
(970) 241-7076
www.westwaterco.com
NEW 1O'' ROUND IRRIGATION
VALVE BOX AT EACH PORT
MOUNTED FLUSH WITH GROUN
NEW 4,' SCH 40 PVC OBSERVATION
PORT WITH CAP
APPROXIMATE EXISTING G
SILry SANDY
TOPSOIL
10.0'
V GROUNDWATER- GREATER THAN 1O.O'
TRENCH NOTE:
MAXIMUM DEPTH 4.0 FEET
MINIMUM DEPTH 3.0 FEET
INSTALLATION NOTES:
1) FOLLOW MANUFACTURERS INSTRUCTION
www.i nfiltrotorsystems.com
2) SCREW pIpE iop poRT CHAMBER FACEP|-ATE
3) SCREW FACEPI-ATES AND UNITS TOG$HER4) INSTALL MONITORING PORTS TRENCH ENDS
ALL FOUR BED CORNERS
PLACE GEOTEXTILE OVER INFILTRATORS (TYP)
CROWN BACKFILL OVER TRENCHES
PROVIDE LEVEL SCARIFIED SURFACE
ACROSS LENGTH OF TRENCH(S), NOTTFY
ENGINEER IF TRENCH BOTTOM(S) ARE Wff
o+
oo 3.0'
36 Q4 INFILTRATORS
(4) TRENoHES
(s) uNrrs EAcH TRENcH
SCALE 1":5,
4.0'4.O'4.0'
O
z.
a
FJ
a
1,OOO GALLON SEP"NC TAI.IK
FIGURE 84
alll{estll/lliltet
= Engineering
2516 Foresight Circle, #l
Grand Junction. CO 81505
(970)241-7076
www.westwaterco.com
NEW 24"A ACCESS HATCHES WITH
LOCKING/SECURED TANK LIDS
FINISHED GRADE
NATIVE BACKFILL
COMPATED TO 95%
MSHTO T_99 T2%
OPTIMUM SOIL MOISTURE OPTIONAL ORENCO
(oR EOUTVALENT)
4" EFFLUENT FILTER
GRAVITY FLOW +
FROM DWELLING TO STA
NEW 4" SCH 40
PVC SEWER LINE
NEW 4" SCH 40
DISTRIBUTION BOX
PVC WYE/TEE
NEW 1,OOO GALLON
SEPTIC TANK
NEW 7+" MINUS WASHED
ROCK (a" truCn DEPTH)
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N
CODE OF COLORADO REGULATION
Water Quality Control Commission
5 CCR 1002-43
Table 7-1 Minimum Horizontal Distances in Feet Between Components of an On-Site Wastewater Treatment System lnstalled After
November 1 1973 and Water P icaland Health I Features
5
Septic
Tank,
Higher level
treatment
Unit,
Dosing
Tank, Vault
or Privy
5
5
Dry
Gulch,
Cut
Bank,
Fiil
Area
(from
Crest)
01
102
25
10
10
502
503
25
Lake,
Water
Course,
lrrigation
Ditch,
Stream,
Wetland
50
25
102
25
10
10
Subsurface
Drain,
lntermittent
lrrigation
Lateral,
Drywell,
Stormwater
Structure
1010
102
10
10
10
Property
Lines,
Piped or
Lined
lrrigation
Ditch,
upslope
curtain
drain
0
10
10
15
Structure
without
basement,
crawl
space or
footing
drains
55
0
20
51
51
Structure
Mbasement,
crawlspace
or footing
drains
56
252
102
102
Potable
Water
Supply
Line 2
102502
502
100 3
60
60
Spring,
Well,1
Suction
Line,
Potable
Water
Supply
Cistern4
Septic Tank,
Higher Level
Treatment Unit,
Dosing Tank,
Vault or Vault
Privy
Building Sewer or
Effluent Lines
STA Trench, STA
Bed, Unlined
Sand Filter, Sub-
surface Dispersal
System, Seepage
Pit
Lined Sand Filter
Lined Evapo-
transpiration Field
or Outside of
Berm of Lined
Wastewater Pond
43
FtG. C-1
CODE OF COLORADO REGULATION
Water Quality Control Commission
5 CCR 100243
Septic
Tank,
Higher level
treatment
Unit,
Dosing
Tank, Vault
or Privy
10
N/A
10
Dry
Gulch,
Cut
Bank,
Fiil
Area
(from
Crest)
15
25
1 0
Lake,
Water
Course,
lrrigation
Ditch,
Stream,
Wetland
25
100
253
Subsurface
Drain,
lntermittent
lrrigation
Lateral,
Drywell,
Stormwater
Structure
25
25
0
Property
Lines,
Piped or
Lined
lrrigation
Ditch,
upslope
curtain
drain
1 0
25
10
Structure
without
basement,
crawl
space or
footing
drains
1 5
25
1255
Structure
dbasement,
crawlspace
or footing
drains
1 5
25
125
Potable
Water
Supply
Line 2
252
502
102
Spring,
Well,l
Suction
Line,
Potable
Water
Supply
Cisterna
100
100
100 3
Unlined Sand
Filter in SoilWith
a Percolation Rate
Slower than 60
Minutes per lnch,
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
Slit Trench
Latrine, Pit Privy
System Not
Relying on STA
for Dispersal
NOTE: The minimum distances shown above must be maintained between the OWTS components and the features described. Where soil, geological or other conditions warrant,
or upgrading of existing OWTS where the size of lot precludes adherence to these distances, a repaired OWTS must not be closer 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 the root system of nearby trees.
1 lncludes potable wells, irrigation wells and monitoring wells set within a potable aquifer and infiltration galleries permitted as wells by the Division of Water Resources.
4
FtG. C-2
CODE OF COLORADO REGULATION
Water Quality Control Commission
5 CCR 1002-43
2 Crossings or encroachments may be permitted at the points as noted above provided that the water or wastewater conveyance pipe 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
the bottom of the encasement pipe. The area in which the pipe passes through the end caps must be sealed with an approved underground 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
required to minimize contamination as part of the Division site application and permitting process.
4 All horizontal setbacks to a potable water supply cistem must be met unless a variance by the Board of Examiners of Water Well Construction and Pump lnstallation
1 1 .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 lf the struc{ure 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.
45
FtG. C-3