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Home My WebLink About2016-02-23 OPH Ltr to MPCA Re Infiltration Basin er f Ts fi. . City of Oak Park Heights 14168 Oak Park Blvd. N•Box 2007.Oak Park Heights,MN 55082•Phone(651)439-4439•Fax(651)439-0574 February 23,2016 Ms.Michelle Beeman Deputy Comm Mt innesota Pollution Control Agency 520 Lafayette Road St Paul,MN 55155-4194 RE: SdiNuater Areas Schools—Infiltration basin Dear Ms.Beeman: At this time the City is anticipating the issuance of a permit or written authority to the Stillwater Area School District to install an infiltration baser upon it's lands that will serve to address storm water from its site as part of its planned High School Expansion. However, Minn R.4720.5100.13 expressly prohibits this nitration basin being located wither the City's DWSMA unless such basin is approved by a local unit of government with a current MS4 permit As the City is not an MS4 we cannot issue such permit In addition,the Valley Branch Watershed District(VBWD)does not control the City's DWSMA as the City is the party that has been compelled by the State of Minnesota via the(MPCA/DOH)to monitor and or manage the DWSMA through our well-head protection protocols.Moreover,the VBWD would not have any meaningful corrective action authority should an incident occur. Accordingly, the City concludes that the term local unit of govemment'cannot refer to the VBYVD and thus would have no authority to independently Issue this infiltration permit Moving forward however,the City is prepared to concede to the issuance of a permit for this infiltration based on your letter dated January 25w',2016 as the Valley Branch has concluded that this is allowable;your letter states: MbyteWhy cocludes the kart**,at this loads to allowsbio,t Construction ►Penalties may choose to •MIisafi ibrA Nr or implement other stotinwaterb•sfm tpathos's. Enclosed herewith is the Valley Branch review of the proposal as submitted by Barr Engineering to the City. The City would request that the MPCA promptly review tin information and indicate if you have any questions or concerns with this infiltration pond at this location and such permit issuance by the City which is a non-MS4.Please also affirm what party is the'Construction Stormwater•: '• ,;';we would believe this would be the School District Si 11,'' Johnson,AICP 00 Administrator CC: Lee Mann,STANTEC—via email only Tony Willger,School District—via email only Mike Trojan,MPCA—via email only John Hanson,VBWD/BARR Engineering—via email only Minnesota Pollution Control Agency 520 Lafayette Road North I St.Paul,Minnesota 55155-4194 1651.296.6300 I i EC0,15`I_If;Oon January 25,2016 Dr.Eric Johnson,AICP City Administrator City of Oak Park Heights 14168 Oak Park Boulevard North PO Box 2007 Oak Park Heights,MN 55082 Mr.Tony Winger Manager of Facilities/Site Operations Stillwater Area Public Schools 1875 Greeley Street South Stillwater,MN 55082 Dear Dr.Johnson and Mr.Willger: The Minnesota Pollution Control Agency(MPCA)has reviewed the available information concerning infiltration of stormwater at Stillwater High School in Oak Park Heights. In completing this review,we have considered the following: • The site is located in a Drinking Water Supply Management Area(DWSMA).The Construction Stormwater Permit states infiltration is prohibited in a DWSMA,unless allowed by a local unit of government with a current Municipal Separate Storm Sewer System(MS4)permit. • Valley Branch Watershed District(Valley Branch)is a regulated MS4.As an MS4 permittee, Valley Branch must not allow infiltration within a DWSMA,unless they perform a higher level of engineering review. If Valley Branch conducts a higher level engineering review of this project,the following scenarios exist: • If Valley Branch concludes that infiltration at this location is allowable,the Construction Stormwater Permittee may choose to infiltrate stormwater or implement other stormwater best management practices. • If Valley Branch concludes that infiltration at this location is not allowable,the Construction Stormwater Permittee is prohibited from infiltrating stormwater at this location. • If no higher level engineering review is conducted,the Construction Stormwater Permittee is prohibited from infiltrating stormwater. Dr.Eric Johnson Mr.Tony Willger Page 2 January 25,2016 The MPCA cannot release any entity,including the City of Oak Park Heights,from liability for actions related to management of stormwater. If you have any further questions,please contact Michael Trojan at 651-757-2790 or by email mike.troian@state.mn.us. Sincerely, Michelle Beeman Deputy Commissioner MTJRA:rm cc: Greg Buchal, Larson Engineering,Inc. Amal Djerrari,Minnesota Department of Health John Freitag,Minnesota Department of Health Ryan Anderson,MPCA Michael Trojan,MPCA resourceful, naturally. BARR engineering and environmental consultants -� Memorandum To: Dr.Eric Johnson,City Administrator,City of Oak Park Heights From: Ray Wuolo,PE,PG and John Hanson,PE Subject: Findings of Review of Proposed Stormwater Infiltration Facilities at Stillwater Area High School Date: February 22,2016 Project: 23820016.03 6320 007 c: VBWD Managers,VBWD Attorney,Amal Djerrari (MDH),Mike Trojan (MPCA), Greg Buchal (Larson Engineering),Heidi Hamilton (Kraus-Anderson),Tony Willger,and Kristen Hoheisel (SAHS) This memorandum summarizes Barr Engineering Co.'s review of the potential for adverse impacts to groundwater quality resulting from infiltrating stormwater at Stillwater Area High School in Oak Park Heights,Minnesota. The proposed site is within the Drinking Water Supply Management Area(DWSMA) of Oak Park Heights. According to the Minnesota NPDES/SDS Construction Stormwater General Permit (Part III D.1 j.vii),infiltration is prohibited when the infiltration system will be constructed in a DWSMA as defined in Minn.R.4720.5100,subp.13.,unless allowed by a local unit of government with a current Municipal Separate Storm Sewer System(MS4)permit.Valley Branch Watershed District(VBWD)is a regulated MS4. In its January 25,2016,letter to Dr.Eric Johnson,City Administrator of Oak Park Heights,and Mr.Tony Willger,Manager of Facilities/Site Operations for Stillwater Area Public Schools(attached),the Minnesota Pollution Control Agency(MPCA)reiterated that as a regulated MS4 permittee,the VBWD must not allow infiltration within a DWSMA unless it performs a higher level of engineering review. At its February 11,2016,meeting,the VBWD Board of Managers directed Barr Engineering Co.to formally summarize the methodology and results of its higher level of engineering review in this memorandum and provide it to the City of Oak Park Heights. Location with Respect to DWSMA and WHPA DWSMAs are geographically definable areas that encompass Wellhead Protection Areas(WHPAs)and, therefore,are always larger than WHPAs.Public roads are typically used as DWSMA boundaries to facilitate management and inventory of potential contaminant sources. WHPAs are defined solely on the basis of hydrogeologic processes that control the capture zone of the well and the groundwater time-of- travel to the well in the aquifer unit being pumped. In the case of Oak Park Heights,the aquifer pumped is the Jordan Sandstone. Some of the wells are also open to the St.Lawrence Formation,which underlies the Jordan Sandstone. The 10-year time-of-travel area within the wells'capture zones are determined through groundwater modeling and projected to the ground surface to define the WHPAs. The DWSMA for Oak Park Heights is shown on Figure 1. Stillwater Area High School is within the DWSMA,but outside of the WHPA,also shown on Figure 1. Barr Engineering Co. 4300 MarketPointe Dve ,Suite 200,Minneapolis,MN 55435 952.832.2600 www.barr.com To: Dr.Eric Johnson,City Administrator,City of Oak Park Heights From: Ray Wuolo,PE,PG and John Hanson,PE Subject: Findings of Review of Proposed Stormwater Infiltration Facilities at Stillwater Area High School Dare: February 22,2016 Page: 2 Thickness of Deposits Overlying Jordan Sandstone Barr Engineering Co.(Barr) reviewed logs of wells on and adjacent to Stillwater Area High School to determine(1)the depth to the water table,(2)the thickness and textural characteristics of unconsolidated deposits(also known as Quatemary deposits),and(3)the thickness of bedrock units above the Jordan Sandstone. This review disclosed the following: • Depth to static water in the wells is between 63 and 90 feet below ground surface. • Depth to bedrock is between 85 and 98 feet below ground surface. • The unconsolidated deposits above bedrock are described as clay to sand with gravel. • There is approximately 90 feet of Prairie du Chien Group dolomite(bedrock)that separates the unconsolidated deposits from the Jordan Sandstone. Bob Tipping of the Minnesota Geological Survey performed a three-dimensional interpolation of unconsolidated materials and Barr used these data to assign permeability values to the unconsolidated deposits in the Metropolitan Council's Metro Model 3 groundwater flow model. The range of horizontal permeability values of the unconsolidated deposits at Stillwater Area High School is estimated to be 2.7 to 23 meters per day,which is in the range expected for clayey to sanding materials. Water that infiltrates must migrate downward to the water table(a distance of about 60-90 feet)through unsaturated deposits. Vertical migration in unsaturated deposits is much slower than horizontal flow through saturated deposits because of variably saturated hydraulic conductivity values and vertical anisotropy(which results in lower permeability values in the vertical direction,compared to the horizontal direction). Therefore,infiltrating water would likely take several months to years to reach the water table. Groundwater Modeling Evaluation of Flow Paths Barr performed a steady-state groundwater flow path simulation(particle tracking)using Metro Model 3 to evaluate(1)where water infiltrating from Stillwater Area High School will travel(when it reaches the water table)and(2)how deep in the groundwater system it will travel. For this simulation,all wells in the region(including the Oak Park Heights wells)are set to pump at average rates for the period 2003-2011. The modeling results(shown on Figure 2)indicate that infiltrated stormwater,once it reaches the water table in the Quatemary deposits,will flow northeast into the Oak Park Heights WHPA and then east toward the St Croix River,where it will discharge. Over most of this flow path,the water will be in the Quaternary deposits. As the water migrates eastward,the model predicted that water will flow into the upper part of the Prairie du Chien Group. The model predicts that infiltrating water from Stillwater Area High School will not enter the Jordan Sandstone(the unit from which the Oak Park Heights wells pump). Role of Stormwater Infiltration in Regional Water Sustainability Stormwater is typically collected and routed to infiltration basins or other stormwater infiltration practices to promote surface water quality goals and to potentially reduce flooding. However,promoting infiltration of stormwater is vitally important to maintaining regional groundwater systems because infiltrating stormwater(through the root zone,the bottom of infiltration systems,or lakes and wetlands)is the ONLY source of water to the groundwater system. As regions develop roads,commercial facilities, To: Dr.Eric Johnson,City Administrator,City of Oak Park Heights From: Ray Wuolo,PE,PG and John Hanson,PE Subject: Findings of Review of Proposed Stormwater Infiltration Faa'6ties at Stilwater Area High School Date: February 22,2016 Page: 3 and other impervious infrastructure,the amount of available open space for infiltration of stormwater is reduced. To maintain the water balance of the groundwater systems,precipitation that falls on impervious areas must be routed to infiltration systems where they can recharge the groundwater system. Ideally,to sustain the groundwater supply to an existing well,this infiltration should be performed within WHPAs and well-capture zones—not outside of them. A cursory count of ponds already within Oak Park Height's WHPA reveals at least 30 such natural and constructed ponds east of Highway 5. Summary and Conclusions In addition to the Minnesota Department of Health,Barr performed a technical evaluation of the potential for adverse impacts to groundwater quality resulting from a proposed infiltration of stormwater at Stillwater Area High School in Oak Park Heights,Minnesota. Barr used well logs from the Minnesota Well Index to evaluate site-specific hydrogeologic conditions and the Metro Model 3 groundwater flow model to simulate the path of migration of infiltrated stormwater that would reach the water table. The modeling indicates that infiltrated stormwater would not migrate down into the Jordan Sandstone—the groundwater unit from which the Oak Park Heights wells pump. Therefore,the VBWD will allow infiltration on the site and the VBWD Managers issued a VBWD permit at their February 11,2016 meeting that induded on-site infiltration. The proposed stormwater infiltration basin appears to be similar to others already located in Oak Park Height's WHPA. These infiltration basins play a vital role in maintaining a sustainable groundwater supply now and in the future. As indicated in the MPCA's January 25,2016 letter,because VBWD(a regulated MS4) has concluded that 'A infiltration is allowable,the Minnesota NPDES/SDS Construction Stormwater General Permittee may choose to infiltrate stormwater or implement other stormwater best management practices. II To: Dr.Eric Johnson,City Administrate,City of Oak Park Heights From: Ray Wuolo,PE,PG and John Hanson,PE Subjech Findings of Review of Proposed Stamwater Infiltration Facilities at Stillwater Area High School Date: February 22,2016 Page: 4 F4, '' X/ r + w 4 1 'd ';0,'''''- r r ,i1.;,•—":7',..,'") j r A. sf 0 ,rept-, . .... . 't''''': { 't s''',./ 2 !t - . i'311 ' T �' ';`,* .` , ,,,o''' '1" , --,-,* '1. r .^ ''a ' ,� �b� ,r,A y�y"'.}��'#,#��: ' t +� } j�Y^t / ir,-..›...?, v.l "'�9F ,- ,o."‘',',....," '- -r's,.•' '. `f. '-,r F''.\„�' Mkt�`-,A '' r 6' 'x % • Highway 36 �` 7. R qi '� l°' • Yy-.. } r � ';rpJ[ ii.u .; t";15-'''-, F�., ,l,„.,.,..,,j„,-...,,,,,,,.:..fd C,t j j t f:efes. f', --7- ^#r ,- y ,v ✓i r :f li ^''$y Ir 1.41.''..,:!:,;:16/44',4.-,c* d." •c' }�[ i ;4`'''.19t. a .:Y• i' +Y'4 I:-,,,,Xst? - - 'lc-' /-.e-j.'r - A A 104,''''' ' *4.:,"'�€ �•' *, i ' I p,�+ F .. . n _ � 9tlihvaterArea / High School j � `' Legend � ti; � ice' V I1head Protection Area ra '�_ i° DWSMA toerabliitY ,,,,Se" Ina Wry High cax1 Moderate 0 250 125 0 0 25 Q5 0,7Mas Low _ Very Low Figure1 Location of Oak Park Heights WHPAs and Stillwater Area High School To: Dr.Eric Johnson,City Administrator,City of Oak Park Heights From: Ray Wuolo,PE,PG and John Hanson,PE Subject Findings of Review of Proposed Stormwater Infiltration Facilities at Stillwater Area High School Date: February 22,2016 Page: 5 . 0 s 4-40: } • Figure 2 Groundwater Flow Paths from Stillwater Area High School Using Metro Model 3 Note: Black lines depict flow paths in unconsolidated(Quaternary deposits) Blue lines depict flow paths in Prairie du Chien Group(upper bedrock)