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HomeMy WebLinkAboutOWTS Report 06.26.2023June 26, 2023 502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704 -0311 • Fax (970) 704 -0313 S OPRIS E NGINEERING • LLC civil consultants Paul & Robyn Whaley 1776 County Road 241, New Castle CO 81647 robyn.whaley13@gmail.com RE: As-Constructed Report–Onsite Wastewater Treatment System (OWTS), Whaley Property, 1776 County Road 241, New Castle CO, Tr In E1/4 Of Lot 3 Lying Ely of New Harris Ditch, Garfield County SEJob No. 32219.01 Parcel No: 212318300001. Garfield County OWTS Permit # Sept-7985, Issued 3-160-2023 (2-Bedroomsonly-Adu) Dear Paul & Robyn: Pursuant to County requirements, this letter provides documentation of the usage and as -constructed conditions of the recently upgraded Onsite Wastewater Treatment System (OWTS) installed at the above referenced site to support post construction building and OWTS permit for the existing ADU. The original and recently upgraded OWTS installations were constructed to serve a detached guest bedroom suite on the 14 acre rural parcel. The existing bedroom suite contains 2 bedrooms, a ¾ bath and basin and does not have any kitchen or laundry fixtures. Regulatory usage is considered to be equivalent to additional bedrooms in addition to the existing 3-bedroom residence. The water source for the detached bedroom facility is from a water storage tank plumbed for use in the 775 S.F. accessory dwelling unit structure. It is reported by the owners that the average monthly water use withdraw from the water storage tank equates to approximately 25 gallon per day when occupied. Sopris Engineering performed site visits to inspect and document the as -constructed conditions of the OWTS. Inspections were coordinated with the property owner that had the system components installed approximately 2 years ago. Sopris Engineering inspected the new 2 chamber upgrade installation performed by Great Lakes Construction on April 7, 2023 prior to final backfill. The as -constructed conditions of the OWTS components are in compliance with Garfield County Regulations in regard to design criteria for usage based on 75 gpd per additional bedroom over the first three bedrooms. The limited usage of the single guest bedroom suite may be considered equivalent to a hotel room usage of 75 gpd per unit or a luxury resort room usage of 125 gpd per unit. The field observations and post upgrade installation system sizing/component information is summarized below. Summary Existing OWTS Conditions The installed OWTS has a regulatory treatment capacity of 150 gallons per day based on the design wastewater flow to serve two additional bedrooms over the first 3, for a total 5-bedroom residential complex in accordance with Table 6-2 values, Section 43.6, A.4. The OWTS utilizes a 1,060-gallon two compartment reinforced plastic septic tank and soil treatment area (STA) consisting of a single chamber trench installed west of the westerly corner of the accessory structure. The system is installed in line, along the edge of a protected gravel surfaced area adjacent to a large expansive lawn area. The tank was installed with sealed risers and lids to the finish grade surface, an inlet sanitary tee and an effluent filter outlet sanitary tee in the second compartment. The soil treatment unit (STA) consists of 9 Arc-36 leaching chamber trenches that provide 135 S.F. of total absorption area. The septic tank effluent is gravity discharged through a 4-inch assumed schedule-40 gravity sewer pipe installed to the head of the first trench chamber. Observation ports with threaded PVC caps are installed on the head, the seventh chamber and on the ninth and distal end chamber units in series. The design is in compliance with the current County regulations with design calculations based on assumed soil type, texture and structure long term acceptance rate (LTAR). The installed system meets all required setbacks and was installed within the general Whaley Property AC-OWTS, 1776 County Road 241 SE Job No. 32219.01 June 26, 2023 Page2 location indicated on the plan. The existing well, serving the primary residence, is located over 100 feet east of the existing STA. The back calculated OWTS design criteria, assumed soil characteristics and system components are further summarized below and delineated on the attached revised C-1 As-Constructed OWTS Record drawing, dated 06-26-2023 prepared by Sopris Engineering. Regulatory Flow The OWTS is regulatory design flow is 150 gallons per day based on the minimum design wastewater flow to serve 1 person per additional bedroom for a single family residential complex that has a total 5 bedrooms, in accordance with Table 6-2 values, Section 43.6, A.4. Regulation 43: Minimum population based on 1 person per bedroom for the additional 2 bedroom capacity = 2 persons. Gallons per day per person =75 gal/person/day, No peaking factor Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day. Design flow Qd = 2*75=150 gpd Septic Tank Design The existing septic tank has adequate capacity for minimum 48 hour retention time: V = 150 gal/day * 2 = 300 gallons. Installed a minimum 1,060 gallon, two compartment reinforced plastic septic tank with an inlet sanitary tee and effluent filter outlet sanitary tee in the second chamber. Tank was installed with sealed risers and lids to the finish grade surface. Sub Surface Conditions and Observation A site assessment was performed by Sopris Engineering in on October 3, 2022. The onsite soils near and adjacent to the existing STA were sampled and characterized by application of the USDA visual/tactile soil texture method analysis from samples obtained in a driveway excavation cut and a shallow hand dug excavation near the distal end of the trench. The soils below 3-6 inches of topsoil consist of medium dense loamy sand texture soil with slightly granular structure to a depth of 3 feet. The soils contain scattered gravels and cobbles assumed to be less than 30% in the loamy sand to sandy loam soil matrix. The moderately flat area is situated at higher elevations slightly above the upper ridgeline of the draw in an area referred to as the Ferrin Heights located at the base of a massive steep sandstone flat iron feature on the easterly mountainous slope of the East Elk Creek valley and the northerly side of the upper bench of the U shaped draw. No free water was encountered in the shallow excavations or observed in the vicinity. The native sandy soils in the immediate area appear to be well drained and our situated at elevation approximately 100 feet above the agricultural fields to the south located across the broad expanse of the terrain feature draw. Seasonal high Groundwater levels are expected to be below 20 feet from the existing surface grades on the moderately slope terrain. The native soils sampled at 3 feet below the surface grades, in the vicinity, are characterized as a soil type 1 consisting of loamy sand texture with slightly granular structure. This soil is assumed to have an effective loading rate for conventional soil treatment of 0.8 Gal/S.F./day for a level 1 conventional treatment system. The equivalent percolation rate is assumed to be 5-15 minutes per inch. The soils observed appear to be suitable for the conventional shallow absorption field consisting of gravelless infiltration chamber units. Septic tank & Treatment-Soil Treatment Area Existing Conditions The existing treatment system and absorption field consists of gravity distribution of septic tank effluent to a single chamber trench with treatment across the native soil absorptive surface. The STA trench was installed to a maximum depth at the absorptive surface approximately 36 inches from pre-construction existing surface grades and backfilled with 6 to 12-inches of native soils and imported gravel to allow positive drainage off the finish gravel surface parallel to the existing contours.