2017 Annual Surface Impoundment Inspection

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1 2017 Annual Surface Impoundment Inspection Cell 3 Prepared for Minnkota Power Cooperative, Inc. Milton R. Young Station January 2018

2 2017 Annual Surface Impoundment Inspection Cell 3 Prepared for Minnkota Power Cooperative, Inc. Milton R. Young Station January 2018 Minneapolis, MN, Bismarck, ND

3 2017 Annual Surface Impoundment Inspection January 2018 Contents 1.0 Introduction Review of Existing Information Design and Construction Previous Periodic Structural Assessments Results of Weekly Inspections Results of Monthly Instrumentation Monitoring Results of Previous Annual Inspections Structural Integrity Review Visual Inspection of CCR Unit Visual Inspection of Hydraulic Structures Other Changes Impoundment Storage Capacity... 7 List of Tables Table 3-1 Summary of Visual Inspection... 5 Table 4-1 Approximate Minimum, Maximum, and Present Depth and Elevation of Impounded Water and CCR... 7 P:\Mpls\34 ND\33\ Minnkota Ash Management\WorkFiles\CCR Annual Inspections\2017\MPC_Annual Inspection Report_SI_FINAL.docx i

5 1.0 Introduction Minnkota Power Cooperative, Inc. (MPC) operates the Milton R. Young Station (MRY), near Center, rth Dakota. MRY operates two lignite-fired cyclone boilers, resulting in production of coal combustion residuals (CCR). CCR management is subject to Federal Standards for Disposal of Coal Combustion Residuals in Landfills and Surface Impoundments per 40 CFR 257 Subpart D (CCR Rule). MPC currently sluices flue gas desulfurization (FGD) material via blow down lines to Cell 3 which is considered a CCR surface impoundment. Under 40 CFR CCR surface impoundments are subject to annual inspections by a qualified professional engineer (QPE). This report documents the third surface impoundment annual inspection performed by Seth W. Hueckman, P.E. on September 29, 2017, as required by the CCR Rule. Other annual inspection duties, including a review of the available information regarding the status and condition of the CCR unit and storage capacity evaluations, were performed prior and following the on-site inspection. 1

6 2.0 Review of Existing Information A review of existing information was performed to ensure that the design, construction, operation and maintenance of the surface impoundment is consistent with recognized and generally accepted good engineering standards. The existing information reviewed is described in the following subsections. 2.1 Design and Construction Cell 3 was designed by Barr Engineering Co. (Barr) and was constructed in four phases. Below is a summary of construction events for Cell 3: Phase IA (2011) Phase IB (2012) Phase 2 (2013) Phase 3 (initiated in 2015 and completed in 2016) The following construction documentation reports were reviewed in conjunction with the annual inspection: Construction Documentation Report Cell 3 Phase IA, Barr Engineering, December 2011 Construction Documentation Report Cell 3 Phase IB, Barr Engineering, December 2012 Construction Documentation Report Cell 3 Phase 2, Barr Engineering, December 2013 Construction Documentation Report Cell 3 Phase 3, Barr Engineering, September 2016 deficiencies were found. 2.2 Previous Periodic Structural Assessments The following initial structural assessments as required by 40 CFR were prepared by Barr and reviewed as part of this annual inspection: Initial Hazard Potential Classification for Cell 3, Barr Engineering, September 2016 Initial Structural Stability Assessment for Milton R. Young Station CCR Surface Impoundment Cell 3, Barr Engineering, October 2016 Initial Safety Factor Assessment for Cell 3, Barr Engineering, October 2016 deficiencies were found. 2

7 2.3 Results of Weekly Inspections Weekly inspection reports from vember 23, 2015 through December 30, 2016 were reviewed as part of the 2016 annual inspection. Weekly inspection reports from January 6, 2017 through December 25, 2017 were reviewed as part of this annual inspection. deficiencies were found. 2.4 Results of Monthly Instrumentation Monitoring The surface impoundment is a clay and geomembrane lined surface impoundment with relatively flat 4.5 horizontal-to-1.0 vertical (4.5h:1.0v) exterior slopes and 2.5h:1.0v interior slopes and, per previously performed slope stability evaluations, acceptable slope stability factors of safety. As a result, the surface impoundment does not have any instrumentation installed in the perimeter embankments and therefore, no results of monthly monitoring were reviewed. 2.5 Results of Previous Annual Inspections The annual inspection performed in September 2016 documented the following visual observations and associated remedial activities: Some rocks observed on top of the exposed geomembrane liner system; MPC removed the rocks following the inspection. Few small 2-4 dia. burrows with loose soil observed near toe of slope on west and south embankments; MPC monitored the burrows and brought in a professional rodent exterminator to assist in controlling them. Vegetation appeared well established and well maintained for embankments constructed in Phase IA, Phase IB, and Phase 2; Phase 3 vegetation was not yet fully established. Phase 3 vegetation, planted in summer of 2016, became better established in 2017; however, it is still not fully established. Erosion observed on recently constructed interior slope above the geomembrane liner on north embankment (Phase 3); MPC monitored the erosion and repaired as necessary. Erosion observed on recently constructed south embankment outer vegetated slope; MPC to repair rills once Phase 3 vegetation is fully established (Phase 3 vegetation has yet to become fully established and the south embankment outer slope was disturbed in Cell 4 construction commencing in fall of 2017; the entire south embankment outer slope will be regraded and clay lined during Cell 4 construction in 2018). Erosion observed near west embankment toe of slope adjacent to the former clay stockpile; MPC removed the former clay stockpile in early 2017 and flattened the steeper slopes causing the erosion. 3

8 3.0 Structural Integrity Review An on-site inspection was performed to visually identify signs of distress or malfunction of the CCR unit and appurtenant structures. The results of the inspection are included in the following subsections. 3.1 Visual Inspection of CCR Unit The surface impoundment was visually inspected for structural weakness. Inspection consisted of on-foot inspection of impoundment embankments, including toe-of-slope, mid-slope, and crest-of-slope of embankments. Visual inspection items and results are summarized in the following table: 4

9 Table 3-1 Summary of Visual Inspection Item Structural Weakness Description Excessive, turbid, or sediment-laden seepage Signs of piping and other internal erosion Transverse, longitudinal, and desiccation cracking Slides, bulges, boils, sloughs, scarps, sinkholes, or depressions Changes in geometry of impounding structure Visibly Present/Deficient (Yes/) tes seepage present piping present cracking present structural weaknesses identified geometry changes observed 6 Damage to liner systems damage to liner systems observed 7 Abnormally high or low pool levels 8 Animal burrows 9 Excessive or lacking vegetative cover 10 Slope erosion Except as noted Except as noted Except as noted rmal pool level at time of inspection burrows of significance; few small 2-4 dia. burrows with loose soil observed near toe of slope on west embankment and near the mid-to-upper slope on east embankment; MPC to monitor burrows and repair if they become more significant/frequent. Vegetation appeared well established and well maintained for embankments constructed in Phase IA, Phase IB, and Phase 2; Phase 3 vegetation planted in summer of 2016 did not fully establish in significant erosion present. Erosion observed on interior slope above the geomembrane liner on north embankment; MPC to continue to monitor and repair as necessary. Erosion observed on Phase 3 south embankment outer vegetated slope; entire south embankment outer slope will be regraded and clay lined during Cell 4 construction that commenced in 2017 and will be completed in Debris debris of significance present 3.2 Visual Inspection of Hydraulic Structures The surface impoundment includes subsurface drainage features to prevent hydraulic pressure build-up in a nearby hagel coal bed seam that if such pressure build-up occurred and went unchecked during initial liner construction, could potentially interfere with liner and embankment construction activities. One drainage feature on the interior slope of the impoundment, consisting of a 6-inch diameter high density polyethylene (HDPE) pipe embedded in drainage aggregate is of small enough diameter that, even if it failed, would not be expected to have a detrimental impact on performance of the overlying 8-foot thick clay liner and geomembrane liner. An additional drainage feature, consisting of a minimum 4-foot wide by 5

10 4-foot tall granular finger drain, was constructed through the embankment and day lighted at the toe of the exterior slope in areas where the hagel bed seam outcropped within the embankment fill. These drainage features were not designed to be inspected and inspection is not warranted. An area of cattails with some surface water present was observed on the east embankment near one of the granular finger drain outlets which is to be expected; no additional inspection or investigation was performed. The surface impoundment does not have any other hydraulic structures underlying the base or passing through the dike and therefore, no visual inspection of hydraulic structures was performed. 3.3 Other Changes Cell 4 construction was going on during the time of the Cell 3 annual inspection. The initial phase of Cell 4 construction started in fall of 2017 that included removing a portion of vegetation on the south embankment outer slope of Cell 3 and excavation down to the hagel coal bed seam. The second and final phase of Cell 4 construction will be performed in Cell 4 lies directly south of Cell 3 and they will share a common dike, e.g., the south embankment of Cell 3 will be the north embankment of Cell 4. Construction of Cell 4 requires excavation near the toe of the existing Cell 3 south embankment down to the Cell 4 floor. The splitter dike stability between Cell 3 and Cell 4 was examined by Barr for both shortterm and long-term conditions during the Cell 4 design process and met applicable factors of safety criteria. other changes to the surface impoundment design, maintenance, or operations were observed that could affect the stability or operation of the impounding structure. 6

11 4.0 Impoundment Storage Capacity A bathymetric survey of the surface impoundment was performed on May 16, 2017 and that data was used to linearly interpolate the approximate volume of impounded water and CCR in Cell 3 present during the September 29, 2017 inspection. Additional information was provided by MPC for the estimated minimum and maximum depths experienced this past year. The following table summarizes the storage capacity of Cell 3 over the last year. Table 4-1 Approximate Minimum, Maximum, and Present Depth and Elevation of Impounded Water and CCR Water Level Approx. Elevation Approx. Depth of Impounded CCR and Water Approx. Volume of Impounded CCR and Water Approx. Volume of CCR Design Volume of Cell 3 (assumes 5ft of freeboard at el. 2093) Approx. Percentage of Storage Capacity Remaining Minimum ft 1.65 MCY MCY 19% Maximum ft 1.78 MCY MCY 12% Inspection 2086* 66 ft 1.72 MCY 1.41 MCY 2.03 MCY 15% te: *Estimated pond elevation at time of inspection. 7

2017 Annual Landfill Inspection

2017 Annual Landfill Inspection Cell 2 Prepared for Minnkota Power Cooperative, Inc. Milton R. Young Station January 2018 2017 Annual Landfill Inspection Cell 2 Prepared for Minnkota Power Cooperative,