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Appeal Exhibits 001~~ .,,,~ November 1q, 2003 E~C~U~L~O COY (Updated September 2005) GENERAL USE LEVEL DESIGNATION FOR PRETREATMENT (TSS) PILOT USE LEVEL DESIGNATION FOR OIL TREATMENT For Vortechnics Inc.'s Vortechs System Ecoloev's Decision: Based on the applicant's submissions and the Technical Review Committee's (TRC) Recommendations (see below), Ecology hereby issues the following use designations for the Vortechs technology: 1. General Use Level Designation (GULD) for pretreatment use, as defined in the Ecology Manuel Volume I, (a) ahead of infiltration treatment, or (b) to protect and eztend the maintenance cycle of a Basic or Enhanced Treatment device (e.g., sand or media filter). This GULD applies to Vortechs units sized at an operating rate of no more than 35 gpm/st at the Water Quality design Bow rate as determined using the Western Washington Hydrology Model (W WHM). 2. Pilot Use Level Designation (PULD) for oil and grease treatment. This applies to Vortechs units sized at an operating rate of no more than 13 gpm/sf at the Water Quality design Bow rate as determined using the W WHM. This PULD expires on August 1, 2006 unless extended by Ecology. 3. Properly designed and operated Vortechs systems may also have applicability in other situations (example: low-head situations such as bridges or terry docks), for TSS and oiUgrease removal where, on a case-by-case basis, it is found to be infeasible or impracticable to use any other approved practice. Local jurisdictions should follow established variance or exception procedures in approving such applications. 4. Ecology finds that the Vortechs, sized at an operating rate of 13 GPM/sf, could also provide: o Water quality benefits in retrofit situations. o The first component in a treatment train. o EB'ective removal of deicing grit/sand. Ecoloev's Conditions of Use: 1. Vortechs Systems must be designed, assembled, installed, operated, and maintained in accordance with Vortechnics, Inc.'s applicable manuals and documents and the Ecology Decision. 2. On or before December 31, 2003, Vortechnics shall submit a QAPP that meets the TAPE requirements for attaining a GULD for oil and grease removal. 3. Local jurisdictions must file a "Pilot Level Technologies Notice of Intent" form with Ecology prior to authorizing Vortechs for a PULD application. All facilities installed under a PULD must monitor oil and grease in accordance with the Ecology-approved QAPP. 4. Vortechnics, Inc. shall complete all required testing and submit a TEER for TRC and Ecology review by December 2005. 5. Vortechnics, Inc. may request Ecology to grant deadline or expiration date ezteosions, upon showing cause for such extensions. 6. Discharges from the Vortechs System shall not cause or contribute to water quality standards violations in receiving waters. Applicant: Vortechnics. Inc. .Applicant's Address: 200 Enterprise Drive Scarborough, ME 04074 Application Documents: Vortechs System Conditional Use Approval Application Letter [o [he Washington Slate Department of Ecology (June 25, 2003) • Vortechs S[ormwater Treatment System Technology Report. June 2003 Technical Appendices 1 through 16 Applicant's Use Level Request: Conditional Use Designation as a Basic Treatment device in accordance with Ecology's 2001 s[ortnwa[er manual. Applicant's Performance Claims: Based on laboratory trials, the Vortechs System will achieve an 80% TSS removal efficiency for sediment particles ranging from 38 [0 75 microns at an operating rate of 13 gallons per minute per square foot (GPM/st) at the peak Ilow Tor the Ecology design treatment storm. The system is recommended only for sites likely to produce relatively high TSS concentrations (above 100 mg/L), where TSS is primarily composed of 50 microns and larger. Potentially appropriate sites include parking lots, highways and urban streets, material transfer sites. hydrocarbon transfer sites, retrofits, steep/erosive sites, and space-limited sites. Technical Review Committee's Recommendation: The TRC finds that: • The Vortechs system, sized at 13 GPM/sf, should provide, at a minimum, equivalent performance [o a presettling basin as defined in the most recent Stormrra[er Managemem Manua] for Western Washington), Volume V, Chapter 6. • Vortechnics Inc. should he given the opportunity to demonstrate, through additional laboratory and field testing, whether the Vortechs System can attain Ecology's Basic Treatment performance goal for TSS removal. • Vortechnics Inc. should be given the opportunity to demonstrate, through additional laboratory and field testing, whether the Vortechs System can attain Ecology's Oil Treatment performance goal. Findings of Fact: I. Laboratory testing was completed by Vortechnics, Inc. for sieved sand using a Vortechs Model 2000. Laboratory results for the "50 micron" particle range (included particles ranging from 38 [0 75 microns) showed 80% removal al 13 GPM/sf operating rate. 2. Laboratory testing was completed by Vortechnics, Inc. for 1 OW40 motor oil at I S to 90 mg/L using a Vortechs Model 2000. The system provided about 90% removal a[ 13 GPM/sf operating rate. 3. Abbreviated laboratory testing was completed by Vortechnics, Inc. for Sil-Co-Sit 106, a ground silica product with a mean particle size of about 20 microns. Removal rates at 5 to 10 GPM/sf were around 40%. 4. Various field studies were completed by independent parties in the eastern and northeastern United States (Lake George, NY; South Windsor, CT; Yarmouth, ME; Harding Township. NJ; Lexington, MA; Burlington, VT; and Charlottesville, VA). Study details are provided in the technical appendices. These studies generally show above 80%TSS removal rates. However, the results from a particle size distribution analysis on sediment captured in the Lake George Vortechs System indicate that mainly coarse particles were present. Because the influent particle size distribution was not measured removal efficiency of specific panicle sizes could not be determined. 5. Three field studies were completed by independent parties in the Pacific Northwest (WSDOT SR-405; Buffalo Slough/City of Portland; Unified Sewerage Agency, Oregon). Study details were not included in Vortechnics submissions. These studies generally show TSS removal rates significantly below Ecology's Basic Treatment goals. Vortechnics questions the studies' apparent inconsistencies and sampling errors. 6. The system is easily maintained using a vacuum truck 7. There are over 3500 and 100 Vortechs systems installed nationwide and in the Pacific Northwest, respectively. Other Vortechs-Related Issues to be Addressed By the Company: I. Design of future facilities must verify that effective vertical hydraulic oil rise rates (the generally accepted rate is 0.033 feet per minute) can be achieved within the separation chamber. 2. Field testing to date suggests that the Vortechs System cannot reliably attain 80% removal of the finer particles comprising TSS found on local highways, parking lots, and other high-use areas. Design of future facilities should consider: a. a flow diverter, b. reducing the design velocity for small particle applications, c. testing the system in conjunction with a filter as part of a treatment train. d. deepening or modifying the sediment sump, or other design features as needed to prevent resuspension of settled solids. e. laboratory re-testing with Sil-Co-Sil 106, after modif}~ing the system to reduce resuspension (for example, retrofitting with a deeper sump). Technology Description: CD-ROM of Vortechs System Technology Report, dated June 2003, may be requested from Vortechnics, Inc. Design Manual and technical bulletins can be do+rnloaded from company's +veb site. Contact Information Applicant: Vaikko Allen Stormwater360 (877) 907-8676 callcn ii storm+cater?60.com Applicant websile: httn:"~+++++storm++ated60.com Ecology web link: ht[p -; ++++++ ee+ ~+a_go+.progrumsScq'storm++utnr'ne++tech%inde~.hunl Ecology: Mieke Hoppin l*.'ater Quality Program (360)407-6435 mhopa6l2?ccc.++a.aoc TRC: Dave Tucker. P.E. Ki[sap County (360) 337-7292 dtucker(a)co.k itsap. wa. u s -~ ~- The following are responses to the Mitigation Measure # 3 of the Mitigated Determination of Non-signiTicance, issued August 12, 2005, File Number SPR- OS-0160-YL and BSP-OS-0197-V L. 3. Civil plan submission shall include an updated preliminary stormwater plan that meet or exceeds the standards in the 1992 Stormwater Manual as adapted by the City of Yelm. The stotmwater plans shall include the following elements: a. Alternative technologies are no[ acceptable. The staff report, in the Smrmwater Quality and Quantity section, stares "..emergent technologies have been apprared are large entities such as the School District with the ability to maintain the system and complete the reporting reyuirements. " The City of Yelm has approved the emergent technologies as Genera/ Use Level Designation (GULD) in School District projects The GULD does not require any additional reporting requirements as stated. The GULD requires the treatment device must be "designed, assembled, installed, operated and maintained in accordance with !/nrtechnics, Inc. " In order for the City to ensure that the manufacturer required maintenance is performed the City could require a maintenance plan contract with the manufacturer or comparry that performs this type of work. This would satisfy the requirements of the Department of Ecology GULD. Limiting large entities only [o emergent technologies is unfair !o smaller developers, limiting the use of the property, and reducing the property owners ability to maximize development per the zoning [he City has in place. b. The elevation of the bottom of the infiltration gallery shall be a minimum of six feet above the elevation of the azea as identified by Thurston County (approximate elevation 335), and must be 50 feet horizontally, or 2 feet vertically away from the High Ground water area. which ever distance is less. Only 150 feet ojthe five acre site is within the 300 buffer of the High Groundwater Hazard Designated Area as mapped by Thurston County Geodata. Therefore the proposed development site is greater than JO feel horizaniatlyfrom the high ground water area Is the gallery bottom supposed to be six or two feet vertically from the High Ground water area? The 1992 DOE slarmwater manual indicates that three (3) feet of separation is required beM~een the bottom of the infiltration facility and dte seasonal high ground water as determined from soil logs. J.W. Mnrri.rsette and Associates conducted 3 separate soil logs on site (see Preliminrvy Drainage & Erosion Control Report) and did not ohserre arty evidence that x~ould suggest That the ground water was any higher in elevation than 326. This e(eratinn was at dte baaom of the soil log, located at the lowest portion of the site. Finding and Fact, item N$ states that "... in the lnlertm Development Standards for A'ew Development in Salmon Creek Basin... " Har the City forma//y adopted Salmon Creek Standards wilhtn the }'elm Creek area? The Salmon Leek Basin standards allowed monitoring (he ground water [eve! jor a period of a months (December Through March). Does the City have monitoring wells with sufficient data that has determined this elevation of 335'? How was the 335' elevation determined, and what datum was This' elevation derived from? !n my professional opinion, this project should monitor the site for a period of A months (December 2005 to March 2006), to determine the actual groundwater elevation. This will provide accurate depths of the groundwater on the project site. c. Sepazate water quality treatment for roof runoff shall be provided. The 1992 Drainage Manual, adopted by the City, does not require that roof runoff be separated and treated prior to infiltration. Precious projects within the Ciry of Yelm have not been required to separate or treat roof runoff b. The stormwater system shall be designed to accommodate s[ormwater generated by required frontage improvements. Currently there is a stormwaler collection system adjacent to West Road. We unders(and That [his system was designed to handle the stormwater requirements for area located within the street right-oj way of West Road. Ijtha[ is the case, and it was designed for the entire West Road SlormN~ater, then [he proponent of (his project requests the frontage improvements required by (his project be designed to direct all run off to this svstem? sr~w Robert Tauscher, PE Jerome W. Morrissette & Associates, Inc. / ~ ~~`~-~ ~\ v ~ ~ 0 o QJ N ~ 7 Y L U U i, /? ~f " ~' ~~~~ ~ ~ ~' 0 a ~~ ~1 ~, y (/~ ~ ~zq,a ~T v .Q 3 0 a 0 j IL O ~ ~ } L ~ ~ O I N COU~T1' CO\IMI5510\EFS C ;K: , Dietricr One " Diane Oberquell - .Q [Aucicc Tan _ tecin J. O'<ullivan District Three THURSTON COUNTY sr..a ~~a~ DEPARTMENT OF tiVATER AND WASTE MANAGED•LENT RichurJ D. Flinn, F.E Direcurr NOTICE OF ADOPTION Revised !~terim Stormwater Desien Standards for New Development in Salmon C*eek Basin FROM: Richard Blinn, P.E ~!~~~ Thurston County Drainage btanual Administrator DATE: October 30, 2000 Section I. SUMMARY Effective immediately, revised interim storm\vater standards for new dzvelopment are imposed for the Salmon Creek Basin (the Basin). These standards replace interim standards imposed on the Basin pursuant to the Administrator's action dated Febmary 9, 2000. These interim standards avill remain in effect until ftrrther notice pending conclusion of the County's consultant work evaluating the extent and possiblz mitigations to groundwater flooding throughout the Basin. The completion date for this work is expected sometime during the fall of 2002. Basin boundary maps maintained by the Thurston County Storm and Surface Water Utility describe the boundaries for the Basin. Interim stormwater design minimum standards for new development in the Basin is presented below under Section 3. Section 2. BACKGROUND In response to recurrent groundwater floodine \\ithin thz Basin. the Thurston County' Board of Counn~ Commissionzrs (BoCC) expanded the zxisting Stonn and Surface Water Utility Ratt Boundan~ in Ateaust 1999. Dunne late summer 1999. the BoCC imposed a moratorium on ne~c dtvetanment in groundwater floodine areas. During early tall 1999. staff prepared a work plan and solicited for consultant support. During October 1999. a contract was signed kith URS Greiner Woodward Chde to product a calibrated ground and surface water model for the Basin. Follo\eine thz successful drlivery of [hese models. specific alternatives will be evaluatnd to dztzrmine loon-term flood alleviation stmteeizs for the Basin. Responding to cuncems irom counp~ residents. the BoCC votzd to approve Critical Area Ordinance amendments addressing groundwater-flooding arias on February 7.2000 (Ordinance`-12li?). These amendments providz additional requirzments for nzw deczlopmem in areas idtntitied on the "Resource flap" for groundwater flooding as maintained by Thurston County' Development Szrvices Dzpartment. On Febman~ 7. 2000. the BoCC took action zxtending the building moramrium for tour months for the Basin (Ordinancz =I'_I>6): this moratorium has since been lifted. - S,Ii ! CC.m;, \I~.uu,.:ue^:. ? :•nn \ hmL;,• R'rc.r 11.:rva-n.:v. U'.rte a P.~u~c Crilino a~eovi,vss. LnGr: I\•.el.~ne.~nr ~fenuf im: i:.. PitLinc Y,~.i.F.wa IS.•CIL::ieLh.l\r:.~>R', .?k:nrr.i,\\'{a:;~_ J Section 3. INTERIM DEVELOPMENT STAPPD_4RD5 A. Purpose: The County seeks to limit the adverse poten«al impact from new development within the Basin To this end. the County u providing interim stormwater design standards for new development within the Basin. These standazds will remain in place until such time that the County's consultant completes the modelin¢ and alternative evaluation for flood alleviation strategies for the Basin. Upon review of the consultant's final basin report and recommendations, the interim standards will be reconsidered for ensuring consistency with the basin report. B. Interim Standards: These interim standards are contained within "Interim Site Development Standards for New Development in Salmon Creek Basin", URS Greiner Woodwazd Clyde, October 6, 2000. These standards establish screening criteria for impact, new groundwater monitoring requiremenu, alter the manner in which new developments are modeled (hydrologic) and require groundwater-mounding analysis where appropriate. This additional guidance does not guarantee that new development can successfully complete the review process prior to the County completing the consultant modeling work. These interim standards are available upon request. Interested parties should contact Mark R. Cook Storm and Surface Water Program Manager, at 360-7i~3-4681 or visit 921 Lakeridge Drive S W, Building 4, Room 100, Olympia, WA 98102. C. Authority: In taking this ac«on, the Thurston County Drainage Manual Administrator is exercising the Adrninistra«ve Authority of Sec«on 1 ~ of the Draine¢e Design and Erosion Control Manual for Thurston Counri Washinmoa 1994 (the Manual). Developmrnt proponenu are encouraged to review "Interim Site Development Guidelines for New Development in Salmon Creek Basin' prior to submittintt any drainage plan For review. ~7he following list is not intended to bean-inclusive but does provide some direr«on on key chapters and sections of the Manual affected by the interim guidelines: D. Relationship to Manual Standards: • New' screening criteria are established to determine preliminary impact thresholds for nny development. The County has crrntzd a "Depth to Water' (DTW) map for use within the Basin. This map provides gross guidance on the probable water table elevation for Basin properties during exveme rechaz¢e events. Providing a minimum vertical separation of six feet from the bottom of proposed drainage facilities is main[ained, design methods as demilzd in the 1994 edi[ion of the Manual may be used fur new development. • Providing that the screrning c«[eria suggest that less than six fez[ of renical separation existi. new monitoring requirements apply. it(oni[orine shall be for a period of one year. Proponents may alto to minimally monimr for a period of four months. two of which must be from the pe«od desc«bed by December to ivla¢h. Providing that this reduced monitoring period is elected be the proponent, the monitorin¢ shall continue until final stormwater plan preparation. Prior to final stormwater plan prepamtion, the proponent will provide all monitoring data with a statistical correlation to County reference wells. If this final analysis alters previous determinations regarding the preliminary stormwnter plan, then additional mitigation of the stormwater plan shall be required. • Section 3.1.1, Section 3, is amended to include the additional bore analysis requirement • Section 3.1.1, Section 9; is amended to include input and output files from continuous simulation modeling and water balance analysis • Section 4.1.1 is amended to reflect the requirement that any increase in off-site groundwater flooding or septic system failures due to rechar¢e be prevented. Maximum release rates are amended to reflect the predeveloped runoff hydrograph as described by the continuous simulation model. • Sections 4.12 is amended such that sizing is based on the results of the amended modelins requirements. • Section 4.2 is amended to reflect the sizing as defined by the amended modeling requirements. • Section 42 ~ is amended to reflect the sizin¢ as defined by the amended modeline requirements. • Section 4.3 is amended to refer to Chapter 8, Section 3.5.3. • Chap[er 5 is replaced by the continuous simulation-modeling requirement. Existing condition is as described by aerial photoemphy as captured by the County's 1996 fli¢ht. • Section 8.5.3 is amended to require six feet of vertical separation. Unless otherwise amended by "Interim Site Development Guidelines for New Development in Salmon Creek Basiri'_. URS Greiner Woodward CI}'de. October 6, 2000, all other Ntanual provisions apply. Any questions regarding this administrative action. please contact Mark R. Cook, Storm and Surface Water Proemm btanaeer, at 360-754-463 L ct: Board of County Commissioners Linda Hoffman URS Greiner Woodward Clyde DPA Jeff Fancher Fred Knotsman Don Krupp Mark R. Cook llC:rp\intarimmem_I WO.doc FINAL INTERIM SITE DEVELOPMENT STANDARDS FOR NEW DEVELOPMENT IN SALMON CREEK BASIN Prepared for Thurston County Dep:viment of Water and Wus[e Manasement Storm and Surface W-ater Proeram October 6. 2000 ~~~_S-l`am' 1500 Century Square 1501 Fourth Avenue Seattle. Washington 98101 (206) 343-7933 9900046-00.00601 TAB1E OF ~ONTEHTS Section 1 Intrdduction ......................................................._................ .._........................._............ 1.1 1.1 Back?round ....................................................... .......................................1-1 Section 2 Interim Standards ...................................._.......................... ...........................................2.7 2.1 Screening Evafuation ............................-........... .......................-..............2-1 ?? Performance Standards ..................._.._............_ ........._...........................2-3 Figures Fiwre la Interim Screening Evaluation Process for Salmon Creek Basin Appendices Appendix A Use of On-site Pizzometen and Reference Wells for Estimation of Winter-1999 Groundwater Levels - Appendix B Groundwater Ntounding Analysis Guidelines VSV7 ¢~waa~ecc~r.rn ro~aavvsnuucr~o~~c a.~sirv-xttco-cer~m i SFCTI©NONE ~mroduciion 1.1 BACKGROUND The Salmon Creek basin has experienced si onificant flooding problems during the past several years. Hish groundwater conditions appear to be the primary cause of the recent Flooding. In Ausus[ 1999, Thurston County imposed asix-month moratorium on new development in the basin to avoid increasing the tloodins problems. Thursron County is now conducting a study of groundwater and surface water conditions in the basin to (1) evaluate [he causes and estima2d recurrence frequency of the recent fleodine; (2) estimate (using groundwater analysis) [ht approximate extent of high groundwater condi Lions ou¢ide the flooded areas that could Flood basements or impair septic system drainfields; and (3) identify and assess alternative measurzs to mitigate the existing problem areas and avoid future problems. In tune 1993,-Thurston County besan insmllins a network of monitoring wells and stream gages ro collect the data needed to develop groundwater and surface water models of the basin. Calibrated models should be available in tune of ?001. As an interim measure to ensure chat new development within the basin does not exacerbate the extent of existing floodins, the County is requiring that new development meet additional drainage review criteria, under the authodtyof Section 12 of the Drainage Desi sn and Erosion Control Manual for Thurston County, L994. The interim standards for new development in the basin are described below. The inters m standards define the procedure that project proponents must follow to obtain approval for nzw development in the Salmon Creek basin. --+ o~wcu~svaaa+ssxnroHa~¢K anaiu.r.occe.:c.aa~ 1-l SECTIONTWO Imerim Standaniss The interim standards outlined below are intended to guide new development in the Salmon Creek basin until the basin plan has been completed. The interim standards have two basic components: • Screening Evaluation. Because of the past history of groundwaer flooding in the Salmon Creek basin, Thurston County has established abasin-specific screening criterioo regarding the veaical separation between the bottom of an infiltration facility and [he high (winter 1999) groundwater elevation at the site. Statistical analyses indicate that the 1999 groundwater elevations are likely the highest that have occurred during the last 50 years. According to the basin-specific screening criterion, the maximum groundwater elevators must be at least 6 feet below the bottom of any infiltration facility at the site. Each project proponent must conduct asite-specific evaluation to determine whether their proposed development would meet this basin-specific screening criterion. • Performance Standards. If the site-specific evaluation shows that the proposed project is unlikely to flood or exacerbate existing groundwater flooding problems, the project proponent may proceed with design. However, the design must meet basin-specific performance standards intended to minimize potential impacts on basin hydrology. In addition, continuous simulation modeling will be required to design stormwater facilities for some projects. The screening evaluation and basin-specific performance standards are described below. 2.1 SCREENING EVALUATION The screenins evaluation involves the steps outlined below. Figure I a provides an overview of the screening process. Step 1 -Estimate depth to water under winter 1999 conditions. The project proponent must estimate the depth to water at their site under winter 1999 conditions, using the Depth-to-Water map recently prepared by Thurston County. If the Depth-ro-Water map indicates more than 6 feet of separation between the 1999 groundwater elevation and the bottom of any infiltration facility at the site, the project proponent may proceed with design and permit[inv. The project must be designed to comply with the most current version of the County's Drainage Design and Erosion Control Manual. ` - If the Depth-to-Water map indicates less than 6 feet of separation at the project site, the project proponent can either defer the project until the Salmon Creek Basin enoineerino analysis and plan have been completed, or perfoanxite-specific groundwater measurements as described in Step 3. Step 2- Measure groundwater elevations and estimate the winter 1999 groundwater elevations at the project site. The project proponent must install and monitor piezometers to obtain on-site groundwater elevations at the project site. The project proponent must also obtain groundwater elevation data for several "reference wells" that are monitored by the County. The project proponent must then perform a regression analysis to correlate the on-site water level data to the reference well data, and use the resuhin~ regression equation to estimate the winter 1999 water levels at the project site. The required pracedu[es for piezometer installation, water __W O:\WpAlg3O1p,SGO:O~hU$AL\qN CPEER 9A51N }.ppCB-0CT.]T\ ?-t interim standards SECTIONTW O level measurement, reference well data acquisition, regression analysis, and estimation of on-site water levels are specified in Use of On-site 6Vells and Reference iVells ra fstimare 4Vimer 1999 Grotmrhvarer Levels in the Salmon Creek Basin (Appendix Al. The key requirements are outlined below. • Piezometer Insmllation & Surveying. For sites less than 5 acres, three piezometers will be required, unless the County Drainage Manual Administrator determines that fewer piezometers will be acceptable. For sites greater than ~ acres. [he County Drainage Manual Administrator will specify the number of piezometers required. Piezometers must be installed at or near [he topographic low point of the site and at planned locations of stoanwater infiltmeion facilities. Piezometer locations should also allow for broad coverace of site conditions, including triangulation for groundwater Flow direction determinations. The borinss must be advanced to contact [he uppermost Tower-permeability unit (e.g., till). If no low-permeability unit is encountered within 50 feet of ground surface, the drilling can be terminated and a piezometer installed. Piezometer screen lengths shall be 20 feet and screens shall extend downward from the highest anticipated water table depth unless geologic Geld conditions indicate a shorter screen. Piezometers should screen only [hose geologic materials generally considered to be the Vachon recessional deposits (Qvr) and should not span substantial low permeability layers. Piezometer diameter shall be at least 1 inch. The elevation of the [op of the piezometer (measuring point) must be surveyed to within 0.01 foot, based on the NGVD 29 vertical datum. The heieht of the measuring point above the mean natural wound level within a radius of 5 feet of the piezometer must be reported to O.l- feetprecision • Piezometer ~lonitorin~. The County recommends monthly groundwater level monitoring for one year. However, for the purposes of this initial screening. a project proponent may elect to monitor weekly for as little as four months, provided the monitoring period includes at least two months within the December to March timeframe. The on-site croundwater elevations must be measured to within 0.01 too[ usins methods standard for the industry. • Reference FYell Duty Acquisition. Thurston County has installed automated groundwater elevation measurine devices in several reference wells in the Salmon Creek buin. These reference wills have groundwater records extending back [a at least [he fall of 1995. Therefore, these. wells provide a record of water level chances durinc the worst of [he sroundwater flooding in the winter of 1999. Curtent daily Groundwater elevation data are also available for each reference well The proponent must contact the County and acquire water level elevafions from all reference cells for those dates with on-site water level measurements. If more than one measurement was collected for a particular well, the mean daily depth-to-voter shall be calculated and u>ed throughout. County data shall be used nt 0.01-foot precision. • EsSmation of ~4'inter 1999 Groundwater Elevations a[ Project Site. The project proponent must perform a recression analysis using the on-site water level data and [he reference welt water level data for the some dates. The proponent must then use the resultin_e regression equation to estimate the winter 1999 water levels at the project site. - If the reference well eepWation indicates that the site meets the screening criterion (i.e., at least 6 feet of separation between the winter 1999 groundwater elevation and thz=bo)ttfo]m{• ~-~ 0:1WCUlpi1N+S3P4XgY C•FFn d>LN-2.OC(:S GCi-pbn ~~~ v~{i~ SECT101lTW O interim siandanis of any infiltration facility a[ the site), [he project proponent may proceed with design and prepamtion of the requisite permit applications. The project must be designed to comply with the most current version of [he County"s Drainage Design and Erosion Control Manual and the Performance Standards described below. - If the reference well evaluation indicates that the site does no[ meet the screenins criterion (i.e„ less than 6 feet of separation 6enveen the winter 1999 groundwatzr elevation and the bottom of any infiltration £aciliry az the ;ire), the proponent can either defer the project until the Salmon Creek Basin engineering analysis and plan have been completed, or conduct a site specific sroundwater moundins analysis as desm bed in Step 3. Step 3 -Conduct site-specific groundwater mounding analysis- The project proponent may perform asite-specific mounding analysis to assess the potential impacts of the proposed project on neighboring properties. An HSPF continuous simulation model must be prepared for the project sitz to estimate pre- and post-development recharse rates. The HSPF model must be prepared using the parameter values and precipitation data provided by the County. The proponent must estimate the maximum water level that would occur liven the same precipitation conditions that led to the winter 1999 sroundwater levels, and considering discharge of impored potable water to drainfields. The moundins analysis must be conducted in accordance with the "Groundwater Mounding Analysis Guidelines" (,Appendix R). Appendix C provides guidelines for County review of mounting analyses. - If the site-speci Fic groundwater mounding analysis shows that the proposed project will not increase groundwater elevations at the project site property line, the project proponent may proceed with the design and preparation of requisite permit applications. Stormwa[er facilities must be sized using the HSPF model developed for the project site, and the design must comply with the Performance Standards described below. - If the sroundwa[er moundins analysis indicates that the proposed project would cause an increase in sroundwater levels at the progeny boundary, the project proponent must revise the proposzd project and provide site-specific mitigation as needzd to avoid such impacts. 2.2 PERFORMANCE STANDARDS Proposed projects that pass the screenins zvaluation must be dzsisned to meet all o the applicable requirements of the most recent version of the Countys Drainage Design and Erosion Control Manual for control of surface water runoff. All nzw developments in the Salmon Creel basin must be desisned to prevent on-sit tloodins for antzcedent precipimtion equivalent to that precedins thz 1999 tloodin~, and prevent any increase in off->ite groundwater Floodin_v or szp[ia systzm failures due to increased recharse (or runoff) from the site. As' noted above, the HSPF continuous simulation model must be used to desisn stornwatzr facilities for projects that reyuire a groundwater mounding analysis (Step 3 above), and these projects must be designzd so that they wll nut increase groundwater elevations at the prope¢y line. - -A 0~~1vC:A~S9p:CapS.LL,yy:N CTE-eY 9ayw2 CCCti.CC"-0P~ 7-J Figure la. Interim Screening Evaluation Process for Salmon Creek Basin Step 1: Estimate winter 1999 depth to water at project site using County Depth- to-Water map. Is there at least 6 feet of Proceed with design and separation between the,r yes permitting. Design winter 1999 groundwater stormwa[er facilities In elevation and the bottom accordance with most ^ of any infilaation facility no proposed for the site? recent version of Thurston County Drainage Desi n g and Erosion Control. Step 2: Measure groundwater elevations at Manual and basin-specific the proposed project site. Obtain Performance Standards. groundwater data for same time period from nearest County reference well. Use these data to estimate winter 1999 depth to water at the project site. Is there at least 6 feet of separation between [he winter 1999 groundwater y~ elevation and the bottom of any inFhration facility proposed for the site? no Step 3: Prepare HSPF model to estimatepre- and post-development recharge rates. Use the recharge rates to perform site specific groundwater mounding analysis, in accordance with County guidance. Does mounding analysis indicate [hat the Proceed with design.and proposed project will not yes permitting. Use HSPF cause anv increase in model [o design sroundwa[zr elevations s[ormwater facilities in at the prooetty accordance with basin- boundary? specific Pertormance no Standards. Revise proposed project and/or incorporate site-specific mitigation measures to ensure project meets basin-specific Performance Smndurds. Coordinate with the County Drainage Manual Adminis[mtor. AppendlMA Use Of On-Sfte Piezomeiers and Aeterence Wells for Estimatien of Winter 1999 6roundwaierleueis /~ Pacilic Groundwater Graup ?37i Eastlake Ave. c'. Seatne. Wasnmgtan 9G?G2 NIEiVIORANDUM t o: rbiark Cook. Thursron Counn~ From: Charter T. Ellingson. Paci£c Groundwater Group Re: tiSE OF ON-SITE PIEZO~IETERS AYD REFERENCE WELLS FOR ESTIbIATION OF WINTER 1999 GROLRQDWATER LEVELS Date: October 6. ?000 Introduction If predicted depth-to-groundwater below a proposed stonmvater infiltration facility is less than 6 feet based on the County's depth-to-water map for winter 1999, the proponent may collect new on-site depth-to-water data. correlate the new on-site data to new data from a reference well, then use the correlation to estimate on-site depth-to-water in the winter of 1999. The purpose of this document is to specify [he requirements for on-site data collection and correlation anah~sis. The general procedure is also discussed in the County's Interim Sire Derelopmertt Srcndcrrds jor ,\'eu Derelopment in Snlmon Creek Bassin. $tzo 1 -Install On-Site Pizzometers The project proponent must install three or more piezomerers on the project site. Fur sires less than i acres, three pi¢ometers will bt required, unless the County Dminase ,Manual Administrator de[errnines [ha[ fester piezometers will be acceptable. For sites greater than ?acre;. the Counn' Drainage Manual Administrator will sptcil}~ the number of pitzomettts required. Piezome[ers' must bt installed at or near the topographic low point o[ [he site and at planned locations of stormaa[er intimation facilities. Piezometer locations should also allow for broad cocemee of site conditions. including triansutation for groundwater tlow direction determinations. Proponents should discuss piezomatr locations with the Count}' prior to installation. Tht borings mu;[ ht adcanad to mntaa [ht uppermost ;ubstan[ial lower- permeabilin-unit Itzpectad ro bt till user most of the basin) or w a depth of =0 fret- which ever is hrs. Pieznmt!tr scrrn lengths shall be ?0 feet and screens shall extend downward from the higher[ anticipated svattr table depth unless geologic find condition; indictor a shorter screen. Pitzomaers should onls~ screen only the Vachon rectssionnl deposits (Qrrl and sr, erns and sand packs should not span substantial low permeability lavers. Piezometer diameter shall be at feast 1 inch. The elevation of the top of the piezometer (meastuine point) must be surveyed to within 0.01 foot. based on the tiGVD 29 vertical datum. The height of the measurins point above the mean natural ground level within a radius of 5 feet of the piezometer must bereported ro 0.1-Feet precision. Detailed logs of piezometers shall be generated and include at least the following information: ` • eeologic log • drillin¢ method • sampling methods and intervals • construction lo¢ showdng piezometer and annular-space materials and dimensions (referenced to ground surface). • elevation of the measuring point (top of piezometer) to O.DI-foot precision and referenced to the NGVD29 vertical datum • State-plane north and east coordinates • height of the measuring point above the mean ground level within a radius of ~ feet azound the well • drilling company name • date of completion Steo 2 - iv[onitor On-Site Water Le ~el- The project proponent must monitor groundwater ele~ztions in their on-site piezometers. The County recommends mon[hh~ groundwater level monitoring for one year. Ho«ecer. for the purposes of this screening. a prof zc[ proponent may elett to monitor weekly for as little as four months. proaided the monioring period includes at has[ nvd months within [he December-to-~lamh timefrome. Depth-to-water in [he piezometers must be measured to within O.DI-foot precision using methods standard for the industn-. ~Ieasuremen[s must bt referenced to the surczczd measuring paint (tap of piezometer) and eoresponding water-table elew[ions must be calculated. Steo i -Identify Most Aooroori~te Cuuntc Rtferen~e tVzll and Gene to Lineaz R,gmssron Relarionshios Thurston Count}' has insmlled automated groundwater elevation measuring devices in several reference wills in the Snlmon Creek basin (Figure 1). These reference wells have groundwater records ectending back w the fall of 1993 (Figure ?l. Therefore these swells provide a record of wamr local changes during the ~round~carer tloodine period in the winter of 1999. Current daily graundtca[er elevation dam are alw available for each ref-erenee scell. PaciFc Grourdwate: The proponent must contact the County and acquire water Irvel elevations trom all reference wells for those dates with on-site water level measurements. if more than one measurement was collected for the reference well on the required day, [he mean daily depth-to-water shall be calculated and used throughout. County data shall be used at 0.01-foot precision. ' The proponent shall identity the reference well that will provide the best approximation of data from each on-site piezometer by calculating lineaz correlation parameters for each on-site piezometer/reference-well pair Thr reference well with the highest correlation coefficient shall bt selected.t A table showing the relationship between data from a hypothetical on-site piezometer and reference wells is shown in Figure 3. The reference well with the highest cortela[ion coefficient for each piezometer shall be identified and used for further evaluations, as shown in Figure 3. The proponent shall prepare a table and graph similar to those on Figure 3 for each on-site piezometer. Each figure shall show the correlation coefficients for each piezometerlrefzrence-well pair, the best-fit line for the selected piezometer/reference-well pair, and the equation for the Tine. If the linear correlation is poor using all of the data pairs (maximum r'' < 0.7), or if the best-fit line through all the data pairs deviates from the data trend more than 2 feet a[ the hi__hest recorded water ievel, a modified approach should be attempted. Tht analysis is most critical at high elevation because the equation for doe best-fit line will he used to predict groundwater elevations that are higher than any measurzd on site. In the case of a poor match to high elevation data, the proponrnt should first revizw thr scatter-crams for other reference wells. If [he best-fit line for an altemative ref-zrence well matches hi¢h-elevation data pairs and the cortela[ion coefficient is only marginally below that of the maximum, thz altemative reftrence well should be selzctzd. [[ altemative reterenct wells do not improve the match to high zhva[ion data pairs. [hz proponent should rzmovz low-elevation data pairs from the correlation and generate a nrw best-Fit line. Bzst-fit lines using all the data pairs and a ttuncamd data se[ are shown on Figure 3. As indicated on Figure 3. removing 6 data pairs decreased r' but improved the match brat zrn the line and the highest- zhvation data pair. Rhtthrr or not such a modi73ca[ion is lilak to improve the prtdicticz capabilin~ o[ the recurring brs[-ti[ lint a[ high tlewation will depend on the drerzz of confidrnce in the tirld data and Iht number of high-tlzvation data pairs upon which w judgr the match. Thesz art project-specific faeton that will reyuire consideration b}' dtz proponrnt and Counn~. In the txantplr of Figure 3 only one high-rhvation data pair gists and thz moditird approach is probably not jus[itied. ~ See standard statistical 2a 6ools for definition o[ the carrduion mtfiident. r. A convenient ma[hod of calculating cotfFcien(s and plotting best-tit lines is to use a eommnciai soiocar eackaer such as Ylicrosott Ecccl. 7aci0c Groundvrdfer Non-linear correlation approaches are discouraeed because they can result in physically unrealistic rela[ionships. particularly outside the field-data range. Nonetheless. the County will consider non-lineaz approaches Utat result in physically realistic predictions if the linear approaches described herein do not result in physically realis[ic predictions. The proponent must present andjustify any non-lineaz approaches in a manner similar to that specified for the lineaz approaches herein. , Steo 4 -Estimate Winter 1999 Deoth to Water The proponent must estimate awinter-1999 groundwater elevation in each on-site piezometer by using historical reference well data and the vaziables A and B from the best-fit line. The proponent shall calculate the maCimum average elevation of groundwater in the selected reference wells for any 10-day period between January li, 1999 and May 1 i, 1999 -rounded to the nearest O.l-fool. A ] Oday running average of groundwater elevation bettveen those dates is therefore required. b. The proponent shall then estimate winter-1999 groundwater elevations in each on- site piezometer to 0.1-foot precision usine the equations for the best-fit lines: The linear equations will have [he form: Eun-sire = h'1 ' Ercteanc<+ B where: E„~-,~sc = elevation of on-site eroundwater Erc~ercnc. = maximum 10-dav average groundwater elevation in reference well during [he winter of 1999 hl = slope of best-fit line B = intercept ofbest-tit line The variablzs ~( and B will be eenem[ed by the best-tit correlation between each piezometer and reference well as shown in Figure 3. Non-linear rela[ionships would hao~e ditYaent variablzs but the approach is the same. Finalh•, depth-to-water shnll be calculated to 0.1 ;-oot precision by subtrattine thz masimum average ~cinter-1999 elevation of groundwater in each piezometu from the local ground surface elevation tiom S[ep I. In some cases the proponent may wish to create a depth-to-watzr map in addition to the piezometer-specific calculations. The map could be generated by contouring the groundwater data tiom on-site piezomemrs and subtracting dte elevation contours from land surface elevation contours. Pac7(ic Croundxater Steo i -Calculate Groundwater Flow Direction The groundwater zlevation data for [he maximum- and minimum elevation measurement rounds shall bz contoured (separatzly). Thz contour maps shall indicate groundwater flow direction. Steo 6 - Reoart to Counn~ Art On-Site Deprh-ro-4G'arer Report shall bz subminzd to the County and shall indudz at least the following: • vicinity map shoeving the site and surroundin_e properties, buildings, roads, parcels, and hydrography • site map showing piezometer locations and land surface elevation contows (two-foot contours aze available from the County for all of the Salmon Creek basin) • brief interpretation of on-site shallow geology • geology/piezometer logs • table of piezometer survey data • table of on-site w$tzr level measurements • table of reference well water Izvel mzasurements • table of piezometerlreference +vell correlation parameters • scattergram (graph) ofpiezometer/referenez well data pairs. showing best-fit line(s) and zquation(s) • tablz showing on-site maximum 10-dap groundwater elevations and minimum depths-to-watzr from winter 1999 • groundwater contow maps of maximum and minimum measwed water level elevations Steo 7 - Coun[~ Intemretation The Coung will review thz mpurt for consistzncy with these requirements. If the work is found to hacz been performzd in rzasonablz conformance with these requirements and in general conformance with acczptzd hydroezoloeic practices. the County wilt ecahratz the depth-W-water criteria. !f [hz zstintated (winter 1999) depth-to-water beto+v a proposed s[onnwater infiltration facility is- grzatzr than 6 fzzt. thz Coung will inform thz proponent that the site has passed the scrzzning zcaluation identified in thz tnrerinr ,Site Decelopmm~r.Staadnrds for :\~ew Derelopmenr in SaLrtmr C)~eek G~i.cin. [f the depth N +vater is 6 fzzt or less_ the County will in[orm the proponent That they can zithzr dzfer [he project until the Salown Creak Bassin engineering analysis and pion hacz bzen compktzd. or perform a sitz spzcific groundm atzr mounding anaksis v dzscribed in Groundwater ;lfounriing .-tnah'.rir Guid~~lirtrs. Gaufit 5 Grauntlwarn- s__ r...,.,~ 3 s s ti _ ~ o -- '~ ~ .~ A ~~ ~~ ~ ~ ~:~ T ~: - \ •- ,~/ `" "~ \ ! ~ __ ~~~ iii s ~ ~ ~ ~ r~ i ~ ~'_. e - ~V ~ ~ ,.- @. __ .-~ ~ _ ~' ~4 - \ ., - , ~ , ` , `~ ~ - ... ~ ~ ~ s -, ,..,_ , ~ ,nnza -----~. 4q ' ~ j- /N£2i -,. d 3 m U C d d O L a W O N c~ LL _ _~o ~ --- ----.. .__ 4 -----f ~ m ~~y <° ~P m m - ~ _ o 8 ° o° °c o Q _ _-_-"'---__- o N Nl rA N N C \~ N ___ O _ _ L. __ .' _ O ~z ~ ~i '~ °a _' _ _ o ' ~ i`~a ~_- _- _ ~ d j ~ m ~ O __- I E N n -_____ ____. j m 3 ~ N 6yd ~ ul c _._ ~'1L'___.- -_._ -__ __-_ __' ~ N 1 " i ~ I H •~ ~ ~ Y ; GI ____ _ _ _____ __ _ ~ _ _____ _ __ _ _ _ r 3-s j _ _ l'1 C' O . __.__.. O .OO C. _.O O ~ N W N N N o fG ~ K 'L' C J J J J ~ 0 0 0 o c o 'r O U7 q t~ .rV- N (6ZanJN laa;) uopena~~:a;eMpuno:~ d 3 iz e •~~g @ o my _._rn _ ____ - y `° a? r A ~ N a + e - x °~ m ~+ o P~ ~ y O II T S a a. n ~ M m m m r 4 H LL> j m e- j d Cl 3 m m M1 L~ m C7 O N O W (O < N C c~ c~ c~ rn c m m m m (uo-sa~l ozald leoyay;od6H'nala lM ~~1 Appendix9 6roandwaier Mounding Analysis Gnldelines Pacilic Grpuntlwafer Group 2377 Eestlak2.4ve. E Seattle. Wasflmpron 93[02 IN 2J6.3297 U! -, 329.o9e3 MEMORANDUM To: Mark Cook. Thurston County From: Charles T. Ellingson. Pacific Groundwaer Group Re: GROUNDWATER MOUNDING ANALYSIS GUIDELINES Date: October 6. 2000 Introduction If predicted depth to groundwater is less than 6 feet based on the County's /nterim Site Development Standards for Netv Development in Salmon Creek Basin, [he proponent may perform a a_rotmdwater moundine analysis to try to demonstrate conformance with basin- specific Performance Standards as defined in the Interim Standards. The purpose of this moundine analysis guide is to specify the sofrware, input data, calibration requirements. and output format for the referenced groundwater analyses. In several, the guidelines result in an estimate of the effects of site development on groundwater levels in the unconfined aquifer (a. k. a., water table) during the wimer and possible exacerbation of groundwater flooding. Processes that must be considered are: • changes to averase recharge quantity o~-er the sit as a result of changes in evapotranspiration. • distribution of recharge on site (pavement and stormwater infiltration plans). and • quantity and distribution of imported eater supplies that ~.vill be disposed to dminfields. hak from pipes. or infiltrate as a result of excess irritation. Summan~ of Standard aoorouch The proponent must develop a>imolitlM. '_-dimensional. transient, finite difference ero undwater model [o simulate groundma[a moundine under [he afrten[ and built conditions. The County anticipates moundine as a result of increased total recharge caused by reduced use of reatar by plants (land clearing) and discharge oFimported potable [tarot throuvh septic draintields. The current and built-condition models shall be the same. except far rev haree yuantin and distribution. Hzads (groundwater Izcelsl under [he torten[ condition shall be sub[r:toted. on a cell-bv-cell basis. from heads under the built condition. The recharut conditions wed [o cvalua[e Che chanee shall be avzra_sz recharge for each month (12 values) of [catar }ears 1997. 1993. and 1999 (October 1. 1996 through September 30. 1999) as calculated by an HSPF model also venerated by the proponent using standardized propartiesdehned by the County. The County will not approve projects that are predicted by this analysis to cause increased winter or spring heads at [he proponent's propern~ boundary. The County will apply this criterion using a precision of whole feet - in other words. results shall be rounded [o the nearest foot. Projects will not be approved unless the predicted chance at the property boundary in winter and spring is 0 feet. Sources of Hvdro~eologic and Hvdroloeic Information for the Area Several sources of background information about the hydrogeologic environment in and near the Salmon Creek drninage basin are readily available. The most up-todate aze the Hydrologw, and Quality ojGround YVater in \-brfhern Thurston Caunry, W¢rhingmn (Oros[ and others, 1995), the report on 3-dimensional groundwater modeling of Thursmn CounTy' (Drost and others, 1999), and the Safmon Creek Drainage Basin Conceptual Hydrologic ;L(octe! and Dnra Collectiott Plan (Paeihe Growdwater Group, 2000). Detailed information on local conditions also may be found in well logs on file with Dept. of Ecology (Southwest Regional Office: Lacey. WA) and in hydrogeologic reports on drilling and testing of water-supply wells and monitoring wells in the area. Manv of the latter are listed in the bibliography of the Pacific Groundwater Group (?000) report. SoBware and Computer Requirements The project proponent must use either the MODFLOW or PLASM Fnite-difference modeling code to estimate changes due to proposed development. The simple conceptual model to be simulated shall be two-dimensional and consist of a single-layer. unconfined aquifer with an impertneable base. Cell sizes shall be commensurate kith the project size and details of site layout. Because of the need to account for mounding near infiltration ponds, ceh sizes a[ [he ponds must be small. vet the model boundaries must be sufficiently distant to not unaeceptabty-influence model results. Therefore. although all modus will be simple, most will have large numbers of cells and users are cautioned a_ainst using a slow computer, or one with insufficient memon~. Also, model versions with advanced pre- and post-processors are highly recommended. Flesibilin These guidelines are designed to reduce work required ofa proponent 67 specifying aset of acceptable. ya simplified. requirements. A proponent may modify these guidelines if themoditicu[ium are approved by the County Dmanage Manual Adminisvator and result in a morn realistic model. Additional simplih~ing assumptions are unlikely to be approved. - Although conccpmally~ reasonable, these model mquiremants have not been ~~tested-~ and therefore moditieatiotu map be necessary duritw modeling to achieee reasonable resuiti. For instance. tht current-condition model should no[ predict surface flooding. it such conditions were nut obsen ed or e~peaed based on field conditions. Aiso. the gradient of Paci9c Grountlwater the uniform flow- field may need to be altered to approximate the average measured h7dmulic gradient -given that arzal recharge also will be applied to the model. N[odel Plan The proponent shall review these guidelines and site data and then prepare a brief plan for modeling the sitz. The plan should be submitted to thz Coun[7 Drainage Manual Administrator for comment. The memo should include env proposed deviations from the standard approach that are deemed necessary by the proponent at that tarty stage. The County will comment on the plan; however, given unknowns that may arise during modelins, thz County cannot assure that the plan will result in an acceptablz model nor overall approval of the project. Standard Model Domain and Grid Design The model domain shall extend to ten times the project-site dimensions in all dimc[ions. from the project boundary (with allowance for square cells approximating an imegulaz property boundary), unless the proponent demonstrates that a model with a smaller domain is equally insensitive to boundary conditions. bfode] cells shall be sufficiently small to simulate the influence of stotanwatzr infiltration ponds; however. because the model is numerical, the maximum groundwater-mound height will not be calculated by the mode(, and the model should not be~solely relied upon for design purposes. The pond design must also be based on the County Drainage Manual Stormwater ponds shall be modeled usin¢ no fewer than 4 model cells unless the pond is smatter than 400 squaze feet, in which case a sin¢le model cell may be used. The distribution of impervious surfaces dots not havz to be esplicith• simulated by arranging thz model cells. However. the modeler must attempt to rzplicate thz distribution of recharge giczn normal grid-dzsisn consuaints. Also. thz site-wide water balance must be maintainzd by any aczragin_ process used rn definz rechar_sz in cells with mired land-surfaez covem~e. Given the small model cells rzquired for stormwatzr-pond simulation. L;e numb::r oY cells used to simulate the project sitz (parcels) will likely be hi~_h and dictated by the following standard limitation on cell-sizz rates-of-change: the [rngtlr o.' <<i;a~ enr re[!s shci! not differ hr mUr¢ r:xra n fdtc'ror aJ lS. The project aria should :-~r •icc=h appnnima[zd by cell boundaries. Standard Batndarc Conditions The simplified model shall consist of a unifornt gradient equal to the a~~erarzz ¢mdizn[ as indicatzd by mapping the synoptic water-level data collzc[ed b}' Thurston County on March'0. 2000 (Groundwatzr-Basin Boundary and Synopric \7"atz r-Lzvet Sun~ev for Salmon Crzek area. Pacific Groundwater Group. 2000). Made; boundaries shall consist ofa ronstant-head boundary up-gradient. eithzr a corutant-head or general-head down- ' Pan'fic cmdient, and no-flow boundaries on the sides of the model to create the uniform flow field. The superposition of areal recharge on the uniform flow field will alter the uniform flow field, and the modeler may need to adjust the heads at constant head or general-head boundaries in order [o maintain reasonable saturated thicknesses and gradients in the project vicinity. Pre-Calibration Aquifer Prooetties The single Layer shall be modeled as art unconfined aquifer (transmissivity shall be sensitive to head). Recommended pre-calibration aquifer prgperties are: h7dtaulic conductivity of ] 50 fUd, based on Drost and othets (1999), and a specific yield (Sy) of 025, based on mean values for fine to medium sand (Johnson, 1967). The laver thickness shall be site-specific, if known: otherwise the values from Drost and others (1999) may be substituted. Drost and others indicate that upper aquifer (Qvr) thickness is between 25 and 50 feet over most of the basin. Current-Condition and Built Condition Models Current and built-condition models shall differ only in recharge quantity and distribution. The differences in recharge quantity shall be calculated by HSPF modeling usin¢ standard parameter and precipitation data provided by the County plus calculated discharges from septic drainfields. Septic drainfield discharges shall be bazed on existine Thurston County guidelines. Differences in recharge distribution shall be dependent on [he development proposal and must consider locations of stormwater infiltration end the area, and approximate distribution, of impervious surfaces. Standard Time De=cretization Both the current-condition and built-condition models shall have stress periods of ont month and time sups established using the default (Modflow or PLASM) method. Both the torten[-condition and built-condition models must simulate transient conditions in order to estimate averace head for each month. However. because the modeling ooal is to simulate lone-term chances in head. a c}•c1ic, quasi-steady-state condition shall be achieved by simulating twelve one-month stress periods in a repetitive fashion for as mane peals (evcles) as necessan' to reach approximate steady-stare. Cyclic s[:adv-state conditions shalt be assumed ~chen the head in all cells chance by less than 0.05-feet from one czar [o the nest for each monthly simulation period. Standard ~,lonthk Recharca Input w the current-condition groundwater model shall consist of thz aeerage recharse rate for each month (L valucsl as caladated be a site-specific. current-condition HSPF cominuous-simulation model considerin_ enter years 1997. 1993. and 1999. The HSPF modal must be prepared usine the parameter values and przcipita[iun data provided by Pacrfie ..... Gmundwakr the County. Attachment A to this mzmorandum contains more detailed guidance on genemtine recharge from the HSPF model. The built-condition model must be exactly the same as the current-condition model except that recharee quantin~ and distribution shall be based on site-development plans (including septic discharge) and the output fiom a site-specific, built-condition HSPF continuous-simulation model. Average recharge shall be calculatzd for each month (12 values) considering HSPF modeling results for wares years 1997. 1995, and 1999. Assumed septic discharee quantity shall be based on existing Thurston County guidelinzs. ' Standard Conversence Criterion The volumetric water budeet for both models must balance to less than 1% in order to demonstrate conver¢ence of the mathematical processing. Current-Condition Model Calibration A truly calibrated model is not required or appropriate givzn the simplified approach. However, since seasonal water-level flucruations are the focal point of the analysis, somz calibration to seasonal water-level Fluctuation is required. Typical inter-season head changzs were 7 to 12 feet at three in-basin wells. as summarized in the Snlmon Creek Drainage Basin Preliminary Conceptual Hvdrolo~ic ;bladel and Data Collection Plan (Figures I0, 11, and L3; Pacific Groundwater Group, 2000). The proponent shall use measurements, from on-sitz piezometers or representative off-site data to calibrate the current-condition model to seasonal water-level fluctuation. If less than onz-year of on-site data are available, the proponent shall predict seasonal water level Flucruations by correlatine on-sitz data to County reference well data (see "Use of On-Site mefLr and Reference 4Veffs ro Estimate 6'inrer 1999 Groundwater Lerels in the .Salmon Creek Basin', Pacific Groundrearer Group, October 2000j. The aquifrr s hydraulic conduc[iviry. thickness. and specific yield may be modified within generally acceptzd ranges for on-site material types to achierz calibmtion. Recharge shalt not be altzred. Enact replication of measured ware: Iz~ eIs from specific years should not bz expectzd (and is not required) unlzss HSPF rechar2z data from those specific years is used in calibration (not thz time-avzmsed HSPF data spzcifled as the standard approach). Standard Data Rzduction and Presentation Models shalt be documentzd complztel} in a repon to the County. usim_ standard modzl- reportine practices. Thz documentation shall include maps and tables dztinine: • grid design superimposzd on sitz-dzvzlooment plans and regional tzatures • aquikr hydraulic properties • boundan~ definitions Paafic w GnwMwaM • recharge quantities • head output • other model features, iY implemented • documentation of cyclic steads-state. model convergence, and calibration In addition, specific output shall be generated to allow efficient evaluation of the Counri criteria. This output shalt consist of hydrographs {head versus time) of the cyclic steady- state heads generated by [he current- and built-condition models (two lines on one graph). The heads shall be from the last time step of each stress period. A third plot of the difference between the current- and built-condition heads over time shall also be provided, along with [abulaz data for each plot shall These hydrographs shall be provided for dte following key model cells: • the cell with the higher[ head below each stotmwater i~ltration pond • the cell just outside the property boundary downgradient of each stormwater pond • the cell just outside the property boundary closest to each stormvvater pond • one cell just outside the propeny boundary along each segment of the property boundary (in other words -cells to represent typical conditions along each segment of the property line) References Drwt and others, 1995, Hi-dralo,~ and quality ojgraundwater in northern 77nvston Coanty, dVashington. LC S. Geological Survey, Water-Resources Investi¢ation Report 92- 4109 (revised), Tacoma, WA. Drost and others. 1999, Conceptual mode! and numerical simulation ojthe grounrl- tvater flou• system in the uncorzsolidntedrediments ofThtrrsron Caunry, Washington. U. S. Geological Survey, Water-Resources Invesigation Report 99--416:. Tacoma. LL,A, Johnson. A. 1.. l96 i. Specific }Meld -compilation ojspecific yields for vcnious materials. U. S. Geological Sun~zy. \\'attr-Supple Papzr l66?-D, 7d p. Pacific Groundwater Group. ?000, Sulnzun Creek elrainage basin, prefiminnry concepnraf hedrolog!c model and darn collrrrion plan. pmpared for Thursmn County Water and Waste iVlanasemznt. Prickett. T. A. and Lonnquist. C G.. 19 i I. Selected digital computer tec/miques for ~rourxhrnter'resource rraluution. Illinois S[a[e LL"ater Survey Bulletin lYO. ~~. Champaign. [L. 6'_ p. (later nicknamed PLASM for Prickett-LOnnyuist Aquifer Simulation b(odell- Pacific Groundwater ATTACHMENT A TO GROUNDWATER MOUNDING ANALYSIS GUIDELINES Precipitation recharge shell be calculated for each groundwater model cell using HSPF. An additional component will be included to represem discharve of septic effluent. The following saps will be involved: Outside the orooo d d I ed boundarv~ Since there are no chances proposed for this area. recham_e will be tht same between existing and built conditions. Simulate historical precipitation records with long-term PET dma between I995/h1 and 1999/1280 for the combined coral area outside [he proposed developed boundary. Simulating storm events prior [o water year 1997 is required to establish the correct initial soil condition prior m 1997. Recharge shall be composed of three components in HSPF: Surface outFlOw {SURO), interflow outflow ((F WO) and groundwater outflow (AGWO). The monthly sum of these three components between October 1, 1996 and September 30, 1999 (water years 1997, 1998, and 1999) shall be output Calculate the recharge rate (length per time) Car each off-site model cell using [he HSPF recharge [orals and HSPF areas. Within the oro sad d 1 ed ar I A detailed model is necessarv'to evaluam the impazt to groundwater due to the development. Far existino conditions, the procedure is same as [he one described for the area outside the proposed developed boundary. The Following steps summarize procedures for the built condition: The surface outFlow (SURO) from the proposed developetl parcels will be assigned m recharge in the cells associated with rhr storm water infiltration faciliro. assuming all the storm water runoff wil I be cartied to the facility by a convevanw system. If more than one infiltration facility is proposed. land area attributable to rash Facility should be delineated and sepamtt calculations for txh (acilin-should hr made. In addition to the abuse. inmrtlorv tlFtt O) and grdundwaer outtlow (AGR'O) should he caleulaced For the total da'tloped site and br disrihuted as recharge throughout the groundwater mold ttlls within tht proposed developtd parctls. Recharge resulting from disharer oFseptic effluent shall also be distributed [o groundw acre model ttlls. A uniform dis[ribmion across [he developed area is am acceptable approximation of actual septic dischar¢e unless u communiq drain}celd is propusM. in which case the actual lucmion of the dminlirld shall br simulamd. Tht [able below summarizes tix may the differca rechargt terns shall hr di~tnbnrN in [ht ground~~'amr model: Built COndiuun to stomncattr dis[ribum distribute dismibu[t unl¢s acommunin pond drainfceld is propostd asemc 6rotmdwater