HomeMy WebLinkAboutOWTS Installation Observation 03.08.2026March 18, 2026
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704 -0311 • Fax (970) 704 -0313
S OPRIS E NGINEERING • LLC civil consultants
Gregg Mackey
Po Box 398,
Basalt, CO, 81621
"gmack pers" gmackey3@gmail.com
Cc: Jorden Voskuil <jordenvoskuil@rocketmail.com>
RE: AS-Constructed-Onsite Wastewater Treatment System (OWTS), Mackey Commercial Shop
Facility, 0050 C.R. 110, Garfield County, Colorado
SE Job No. 18197.02 Parcel Number: 2393-074-00-012
New County OWTS County OWTS Permit SEPT-9487.
Dear Gregg:
Pursuant to County requirements, this letter provides documentation that the ugraded Onsite Wastewater Treatment
Systems (OWTS) recently installed at the above referenced site is in compliance with the permitted design. Sopris
Engineering inspected the excavations, soils, materials and the installation of the system components at various
times during construction, prior to final backfill and after all installations were completed.
Inspections were coordinated with the property owner and the Contractor, Voskuil Operations, that installed the new
system components. The as-constructed conditions and installation of the new OWTS components is in compliance
with Garfield County Regulations; the recommendations and specifications delineated on OWTS
permit/construction plan “C1-OWTS-Plan”, dated 05-01-2025, prepared by Sopris Engineering. The design criteria
and system sizing information is summarized below. The As-Constructed OWTS conditions, design criteria and
components installed are delineated on the attached AC OWTS Record Drawing, dated 03-18-26.
Summary Design Criteria
The upgraded OWTS is designed with a maximum treatment capacity of 160 gallons per day based on the estimated
maximum daily wastewater flow per population usage of up to 8 employees, 1 small washing machine and 5 person
transients in accordance with Table 6-2 values, Section 43.6, A.4. The OWTS maintains the previously installed
1000 gallon 2-compartment concrete septic tank with PVC risers and lids installed at the surface. A gravity sewer
pipe with a cleanout was installed from the facility structure to the tank. The tank was installed with an inlet sanitary
tee and outlet effluent filter sanitary tee. A new Orenco 4" effluent filter assembly was installed.
The upgrades soil treatment area (STA) was expanded to consist of a single, two row bed (40’ x 6’) with leaching
chambers that provide 270 S.F. of total absorption area. The septic tank effluent is distributed by gravity via the
existing 4” effluent pipe connected to a distribution box with risers and lid set to surface grade. Individual
distribution pipes connect from the D-Box outlet ports to the head end cap of each chamber row. Observation ports
with caps are installed on each end chamber unit.
Sub Surface Conditions and observation
Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering on
October 25, 2018. The soils in the area of the proposed absorption field were sampled and characterized by
application of the USDA soil texture method analysis. The soils were tested to determine the hydraulic loading rate
and to confirm the appropriate absorptive surface suitable for conventional absorption systems in native soils. Two
excavated profile pits 1 & 2 were observed in the East central portion of the site adjacent to the proposed field
location. The soils below 4 to 9 inches of mixed surface fill & topsoil consist of medium dense sandy silt loam with
moderate granular to slightly blocky structure with some cobbles and gravels. The percentage of cobbles and gravels
below 24 inches was less than 20% in the sandy silt loam to the 7-foot depth explored in both pits. The soil appears
to be consistent across the site.
Gregg Mackey, Upgrade/ As-Constructed OWTS
050 C.R. 110
SE Job No. 18197.02
March 18, 2026
The native soils sampled from 2-3 feet below the surface are characterized as a soil type 2 consisting of silt loam
texture with moderate granular structure. This soil has an effective loading rate for conventional soil treatment of
0.60 gal/S.F./day for a level 1 conventional treatment system. Sandy silt loam with cobbles and gravels < 30% are
encountered at shallow depths within 2-feet of the existing surface grade. No free water was encountered in the
excavations on site. Groundwater levels are expected to be well below 10 feet from the existing surface grades. The
soils ares suitable for a conventional shallow absorption field consisting of gravelless infiltration chamber units.
An additional shallow excavation was observed during a recent evaluation of the existing OWTS conditions
performed by Voskuil Operations in coordination with Sopris Engineering on October 8, 2025. Voskuil performed
an initial OWTS inspection on 08-06-25 observing a full tank with inflow from a leaking toilet and saturated
conditions in the existing STA. The tank and chamber were pumped out the following evening on 08-07-25 and the
leaking toilet was repaired. The follow up inspection the next day indicated that the wetted soils in the existing
chamber trench were allowing infiltration as no standing water was observed.
Observation of the shallow excavation at the proximal end of the trench indicated that previously saturated water
was infiltrating into the native sandy silt loam soils. The area was excavated to a depth of approximately 3.25' below
the surface grade, exposing the 4" effluent pipe, first chamber, inspection port and part of the second chamber. No
ponding water was observed in the shallow excavation or the STA at the time of observation. The existing
absorptive surface is 2 to 2.5 feet below the existing grades along the existing trench. The proposed expanded STA
replacement will be excavated with the absorptive surface established 3 to 3.5 feet below the finish surface grade.
Additional soil evaluations were performed by the engineer during construction to verify the soils in the entire
replacement STA field excavation prior to installation of the OWTS components. The observed soils in the bed
excavation were consistent with the descriptions above and consists of sandy silt loam with scattered cobbles and
gravels < 20% encountered at shallow depths within 3-feet of the existing surface grades.
Design Flow
The proposed upgraded onsite wastewater treatment system (OWTS) design flow is calculated as follows for low
usage commercial shop facility that serves two mechanical repair shops each with a single restroom (toilet and
lavatory) & 1 small washer/dryer in 1 shop.
From the Regulation 43 usage Table 6-2 :
Max. Design flow (Qd) = # of people x (avg.per capita flow)
Qd=daily worker office: 8 persons @ 15 gpd = 10 x 15 = 120 gpd
Qd=transient Vistor: 4 persons @ 5 gpd = 4 x 5 = 20 gpd
Qd=single small washing machine: 1 @ 20 gpd
Total Design flow Qd = 120+20+20 =160 gpd
Septic Tank Design
Continue use of the existing 1000 gallon, two compartment concrete septic tank with a new effluent filter installed
on the existing 4" outlet pipe end that extends into the tank and accessible below the riser lid in in the second
chamber. A new filter handle was installed to within 6" of lid. Tank was installed with sealed PVC risers and plastic
lids set at the finish grade surface that has been maintained.
Gregg Mackey, Upgrade/ As-Constructed OWTS
050 C.R. 110
SE Job No. 18197.02
March 18, 2026
Soil Treatment Area Design
The upgrade treatment/absorption field was designed based on utilizing the effective Long Term Acceptance Rate
(LTAR) for the native soils and the application of appropriate loading factors for a soil treatment unit system
utilizing gravity distribution to a gravelless chamber bed. The installed treatment system and soil treatment area
consists of gravity distribution of septic tank effluent via a distribution box to individual chamber rows with
treatment across the scarified native soil absorptive surface of the expanded bed excavation.
Long Term Acceptance Rate (LTAR) Wastewater Treatment Level 1;
A(sf) = Qd x LF 1 X LF 2.: A = Area;
LTAR LTAR = 0.60 Gal/S.F./day for silt loam
Qd = flow (gal/day) = 160 gpd
L.F.1=1.2 Gravity Distribution bed
L.F.2=0.7 Leaching Chambers
A(sf) = 160 gpd x 1.2 x 0.7 = 224 S.F.
0.60 Gal/S.F./day
Number of Arc-36 (5' long ) chambers:
224 S.F = 14.9 chambers, Install 16 ADS Arc-36 Biodiffuser chambers
15 S.F./Chamber Total Absorption area provided is 240 S.F.
Installed 16 Arc-36 Biodiffuser chambers with end caps in two rows 40' long by 6' wide as delineated on the plan.
The septic tank effluent is gravity distributed to the head of each chamber row via a 4" distribution pipe from two of
the new distribution box outlet ports. The D-box outlet to be equipped with flow equalizers and set level on
compacted subgrade with riser and lid set at the surface.
OWTS Operation and Maintenance
The Owner is to retain the services of qualified personnel to inspect the OWTS and to perform all maintenance and
repairs necessary to ensure that the system components are maintained in good operating condition and the STA is
protected from vehicular traffic and staging of materials. The septic tanks should be pumped every 2-3years. The
effluent filter should be cleaned every six months and at the time of pumping. Positive surface drainage away from
the absorption field should be maintained.
If you have any question or need any additional information, please call.
Sincerely,
SOPRIS ENGINEERING, LLC
Paul E. Rutledge Yancy Nichol, PE
Design Engineer Principal
3/18/2026