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HomeMy WebLinkAboutKENNEDY WELL SUSCEPTIBILITY - WEL Letters / Memos - 2/26/2024 1015 East 4th Avenue, Olympia, Washington 98506 Phone: 360.754.2128 Fax: 360.754.9299 February 26, 2024 Sadie Kennedy 51 Lynnwood Beach Road Belfair, Washington 98528 Report Well Susceptibility 51 Lynnwood Beach Road Belfair, Washington Project No. 2126-002-02 INTRODUCTION Insight Geologic, Inc. is pleased to provide our report regarding the susceptibility of your onsite water supply well to possible contamination from septic sources. We understand that you are considering constructing an additional dwelling unit (ADU) on the property which will be served by its own septic system. Mason County Environmental health in their letter dated January 22, 2024, indicates that the existing water supply well for the property is less than the required 100-foot setback distance from portions of the existing on-site septic system. In order to connect the ADU to the existing well and operate as a two-party system, the County is requesting this Well Susceptibility Evaluation prior to issuance of a variance for the reduced setback distance. SCOPE OF SERVICES The purpose of our Well Susceptibility Evaluation was to assess local hydrogeology and the potential for downward migration of septic effluent through the hydrogeologic regime. The scope of our services consisted of the following tasks. 1. Review relevant and available well logs and geologic reports of the site and surrounding area as accessed through the Washington State Department of Ecology and the Washington State Geological Survey. 2. Develop our opinion regarding the susceptibility of the water supply well to possible contamination from on-site sources based on the hydrogeology of the area. 3. Evaluate the results of our computer modeling efforts with respect to groundwater elevations beneath the proposed infiltration galleries and at the property boundaries. FINDINGS The property is located at 51 Lynnwood Beach Road in unincorporated Mason County, Washington (Mason County tax parcel no. 22201-23-00140). The location of the site is shown relative to surrounding physical features in the Vicinity Map, Figure 1. The property comprises approximately 0.27 acres and is about 229 by 50 feet in plan dimension, oriented in a north-south direction adjacent to Lynnwood Beach Road. The property is currently occupied by a newer manufactured Sadie Kennedy Well Susceptibility Evaluation February 26, 2024 File No. 2126−002−02 3 Insight Geologic, Inc. home which is served by an existing water supply well. Sanitary wastes are disposed of through an on-site septic system and drainfield. The existing septic system drainfield is located within 50 feet of the water supply well. The proposed ADU will be served by a new septic system and drainfield located on the southern portion of the property. No modifications are planned for the water supply well or the existing septic system. The planned layout of the site is shown in Figure 2, Site Plan. The general area is developed as a moderate density neighborhood with lots varying in size from about 0.2 to 1.0 acres. Lots are served with potable water either by Group A water systems (Cherokee Strip and Lynnwood Beach) or by individual permit-exempt wells. All lots have on-site septic systems for disposal of sanitary wastes. Geology The property is located on an alluvial fan or delta which has been deposited southward into Hood Canal by Mission Creek and Little Mission Creek to the north. Surficial materials consist of alluvial sands and gravels. Underlying this surficial material are a series of glacial and non-glacial deposits consisting of dense glacial till as well as sands and gravels. The glacial till deposits are composed of a dense mixture of gravel, sand and silt which has been overridden and compacted by the weight of glacial ice. Hydrogeology Potable water for the properties in the area is provided by either single-family wells or by Group A water systems. We were not able to identify a well log for the well serving the subject property. Most of the well logs identified for the area in Ecology’s files do not contain accurate location information. The wells with locations able to be identified are summarized in Table 1, with the locations shown in Figure 3. Well logs for these wells are contained in Attachment A. Well logs for other wells in the area whose location could not be accurately identified are contained in Attachment B. A comparison of well logs identified in the area indicates the hydrogeologic regime to be somewhat consistent in the general area of the site. As discussed above, the surficial deposits consist of sand and gravel alluvial deposits from Mission Creek. Beneath these are a series of interbedded glacial till and sand/gravel layers. The dense glacial till serves as an aquitard, reducing the downward groundwater movement between sand and gravel layers. For the 23 well logs reviewed for this evaluation, we noted that the well depths fell into 3 general ranges; 40 to 50 feet deep, 70 to 90 feet deep, and 100 to 130 feet deep. Each general range is separated by a unit described by the driller as “hardpan”, “clay bound gravel” or “sand with clay binder”. Each of these descriptions can be considered as glacial till and each would serve as an aquitard for downward movement of groundwater. A conversation with Nicholson drilling who recently serviced the well on the subject property indicated that the pump is set at a depth of about 75 feet below ground surface which would indicate the well is approximately 80 feet deep (or deeper). Sadie Kennedy Well Susceptibility Evaluation February 26, 2024 File No. 2126−002−02 4 Insight Geologic, Inc. Recharge and Groundwater Flow Recharge to the groundwater system is through direct infiltration of precipitation and lateral flow from areas within the aquifer having higher groundwater potential to the north of the property. Groundwater flow in each of the aquifer zones is likely from north to south, discharging into Hood Canal. Wellhead Protection Areas The subject property is located within the 10-year time of travel boundaries for several Group A water systems as shown in Figure 4. The closest Group A wells are for the Cherokee Strip Water Company and the Lynnwood Beach Community system. We evaluated published nitrate information for the two closest water systems to assess the susceptibility and vulnerability of the aquifers to nitrate contamination from on-site septic systems in the area. We selected nitrate as the indicator analyte rather than coliform because nitrate has a low potential for interference from the sampling method as opposed to total coliform. The data indicates that the aquifer tapped by the Lynnwood Beach Community well (52 feet deep) appears to be somewhat affected by nitrate with concentrations ranging between 0.92 and 1.39 milligrams per liter (mg/l) for the past five years. The Maximum Contaminant Level (MCL) for nitrate in drinking water is 10.0 mg/l. The Cherokee Strip Water Co. well (126 feet deep) contained nitrate concentrations ranging from 0.61 to 1.2 mg/l. These data are slightly lower than the shallower Lynnwood Beach well which is to be expected since the Cherokee well is deeper. However, the numbers don’t appear to be significantly different between the two wells. The data are summarized in Table 2. OPINION Based on our review of the proposed project and hydrogeologic conditions in the area of the subject property, it is our opinion that the existing septic drainfield at the site is not likely currently affecting overall groundwater quality of the on-site well. The proposed additions to the property including a new residential structure and septic system are not likely to affect the groundwater quality since the new septic system will be both hydraulically downgradient from the well and will be outside the standard 100-foot sanitary radius as shown on the site plan. The hydrogeology of the area generally provides protection from the numerous septic drainfields because of one or more intervening layers of glacial till between the upper alluvial deposits and the deeper sand and gravel aquifers in the area. While detectable concentrations of nitrate exist in groundwater from the upper and deep aquifers, there does not be a discernable increasing or decreasing trend to the data from local water systems. Additionally, the source of the nitrate is assumed to be only from local septic systems and it is possible that another unidentified source exists. Sadie Kennedy Well Susceptibility Evaluation February 26, 2024 File No. 2126−002−02 5 Insight Geologic, Inc. We believe that the hydrogeology of the area along with the depth of the onsite well combine to make the proposed two-party water system safe from potential contamination. We believe that a variance for a reduced sanitary radius around the well can be justified and should be granted. LIMITATIONS We have prepared this well susceptibility evaluation for use by Sadie Kennedy. This evaluation may be made available to regulatory agencies. Our analysis, interpretations and opinion should not be construed as a warranty of subsurface conditions beneath the site. We have relied on information prepared and supplied by others in developing our opinion. Insight Geologic, Inc. makes no guarantee as to the accuracy or reliability of these data, and therefore does not guarantee the results of our analysis. Within the limitations of scope, schedule and budget, our services have been performed in accordance with generally accepted practices in this area at the time our report was prepared. No other conditions, express or implied, should be understood. __________________________ We appreciate the opportunity to be of service to you on this project. Please contact us if you have questions or require additional information. Respectfully Submitted, Insight Geologic, Inc. William E. Halbert, L.E.G., L.HG. Principal Attachments Insight Geologic, Inc. TABLES Table 1 Wells Within 1,000-foot Radius of Site Well Number (Table) Well Depth (feet) Casing Diameter (inches) Owner Name Well Address State Plane X Coordinate State Plane Y Coordinate A 131 6 CHEROKEE STRIP WATER CO CHEROKEE BEACH RD 1050346 772504 B 57 6 LYNNWOOD COMM. BEACH ASSOC. 1051689 772491 C 48 6 RONALD RYDER 700 NE Beck Rd 1050417 773809 D 67 6 ALICE HEATH NE 3554 NORTH SHORE RD, BELFAIR 1050417 773809 E 84 6 MARVIN PETITE NE 3552 N SHORE RD, BELFAIR 1050417 773809 F 49 6 GROVER FLUERY NE 680 BECK RD, BELFAIR, 98528 1050417 773809 G 75 10 WA STATE PARKS 1050346 772504 Table 2 Nitrate Concentrations in Groundwater Date Lynnwood Community Water System 1 Cherokee Strip Water System 2 Nitrate Source 3 Nitrate Source 4 2023 1.23 2 0.08 3 2022 1.39 2 1.20 3 2021 1.20 2 0.76 3 2020 0.92 2 0.61 3 2019 1.33 2 NA -- Notes Data collected from the Washington Department of Health website for the most recent 5 years. Data presented as milligrams per liter (mg/l) 1 Group A community system, ID 492508 2 Group A community system, ID 12450Y 3 Source 2 is listed as 57 feet deep 4 Source 3 is listed as 126 feet deep Insight Geologic, Inc. FIGURES SITESITEFigure 1Vicinity MapLAKE WOOTEN QUADRANGLEWASHINGTON - MASON COUNTY7.5-MINUTE SERIESYear 2023BELFAIR, WASHINGTON51 NE LYNNWOOD BEACH RDSource: USGS (c) 2023 Lynnwood Beach Rd Lynnwood RdExisting 2 Bedroom ST ST Existing Shed Proposed 2 Bedroom DF Reserve DF Reserve DF Primary DF 50' Well Radius 100' Well Radius D/W LEGEND: APPROXIMATE PROJECT BOUNDARY Source: APDdesigns Figure 2 SITE PLAN BELFAIR, WASHINGTON 51 NE LYNNWOOD BEACH RD APPROXIMATE WELL LOCATION A B C D E F G LEGEND: APPROXIMATE PROJECT BOUNDARY APPROXIMATE WELL LOCATION APPROXIMATE 1,000 FT WELL RADIUS Source: Google Earth (c) 2024 Figure 3 Radius Well Location Map BELFAIR, WASHINGTON 51 NE LYNNWOOD BEACH RD WELL DATA TABLE Well Number Well Depth (feet) A 84' B 48'C D E 57' 131' 67' F G Casing Diamater (inches) 49' 75' 6 6 6 6 6 6 10 Owner Name CHEROKEE STRIP WATER CO LYNNWOOD COMM. BEACH ASSOC RONALD RYDER ALICE HEATH MARVIN PETITE GROVER FLUERY WA STATE PARKS Well Address CHEROKEE BEACH RD NE 680 BECK RD, BELFAIR, 98528 NE 3552 N SHORE RD, BELFAIR NE 3554 N SHORE RD, BELFAIR 700 NE BECK RD State Plane X Coordinate State Plane Y Coordinate 1050346 1051689 1050417 1050417 1050417 1050417 1050346 772504 772491 773809 773809 773809 773809 772504 SITESITE LEGEND: APPROXIMATE PROJECT LOCATION Source: Source Water Assessment Program (SWAP): Washington State Department of Health (c) 2023 Figure 4 Wellhead Protection Area Map BELFAIR, WASHINGTON 51 NE LYNNWOOD BEACH RD 5 YEAR TIME OF TRAVEL 10 YEAR TIME OF TRAVEL 1 YEAR TIME OF TRAVEL 6 MONTH TIME OF TRAVEL ASSIGNED TIME OF TRAVEL Insight Geologic, Inc. ATTACHMENT A The reported location data of this well was incorrect and has been changed in Ecology’s well report database. In most cases, the original T/R/S data plotted this well offshore. The Well Construction and Licensing Office used address, tax parcel, and other information to more accurately map this well report in the Well Report Viewer database. This well report location was updated in April-2018. Insight Geologic, Inc. ATTACHMENT B The reported location data of this well was incorrect and has been changed in Ecology’s well report database. In most cases, the original T/R/S data plotted this well offshore. The Well Construction and Licensing Office used address, tax parcel, and other information to more accurately map this well report in the Well Report Viewer database. This well report location was updated in April-2018.