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Annual Report of Site Surveillance and Maintenance Activities at the Rocky Flats Site, Colorado Calendar Year 2018 Operations and Maintenance April 2019

LMS/RFS/S23330

Legacy Management

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U.S. Department of Energy Operations & Maintenance—2018 Annual Report, Rocky Flats Site, Colorado April 2019 Doc. No. S23330 Page i

Contents Abbreviations .................................................................................................................................. ii 1.0 2018 Highlights ......................................................................................................................1 2.0 Annual Site Inspection ...........................................................................................................3 3.0 Landfills .................................................................................................................................4

Present Landfill ...........................................................................................................4 Inspection Results ........................................................................................4 Settlement Monuments ................................................................................5

Original Landfill ..........................................................................................................5 Inspection Results and Maintenance Activities ...........................................9 Settlement Monuments ..............................................................................10 Seeps ..........................................................................................................10 Precipitation Response Repairs..................................................................11 Geotechnical Investigation and Revised Slope Stability Analysis ............11 East Subsurface Drain ................................................................................12 Temporary Groundwater Intercept System ................................................13 Seep 10 Sump and Siphon .........................................................................15

4.0 COU And Former Building Area Inspections ......................................................................16 5.0 North Walnut Creek Slump ..................................................................................................17 6.0 Site Road Maintenance .........................................................................................................23 7.0 Sign Inspections ...................................................................................................................23 8.0 Erosion Control and Revegetation .......................................................................................23 9.0 References ............................................................................................................................24

Figures Figure 1. Original Landfill, Rocky Flats Site, Colorado................................................................. 7 Figure 2. Piezometers 85517 and 85617 near ESSD .................................................................... 13 Figure 3. Piezometers 83916 and 84616 near 460GWI ................................................................ 14 Figure 4. Piezometers 85117 and 85217 near 440GWI ................................................................ 15 Figure 5. Piezometer 85717 Near Seep 10 Siphon ....................................................................... 16 Figure 6. North Walnut Creek Slump, Rocky Flats Site, Colorado .............................................. 19 Figure 7. NWCS Upgradient Piezometers .................................................................................... 21 Figure 8. NWCS Midgradient Piezometers .................................................................................. 22 Figure 9. NWCS Downgradient Piezometers ............................................................................... 22

Appendixes Appendix A Hydrologic Data Appendix B Water Quality Data Appendix C Landfill Inspection Forms, Fourth Quarter CY 2018 Appendix D Data Evaluation Flowcharts Reproduced from RFLMA Appendix E 2018 RFLMA Contact Records and Written Correspondence

3.1

3.2

3.1.1 3.1.2

3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 3.2.7 3.2.8

U.S. Department of Energy Operations & Maintenance—2018 Annual Report, Rocky Flats Site, Colorado April 2019 Doc. No. S23330 Page ii

Abbreviations ATV all-terrain vehicle BMP best management practice CDPHE Colorado Department of Public Health and Environment COU Central Operable Unit CR Contact Record CY calendar year EPA U.S. Environmental Protection Agency EPC East Perimeter Channel ESSD East Subsurface Drain ft feet gpm gallons per minute GWI groundwater intercept ICs institutional controls ITS Interceptor Trench System ITSS Interceptor Trench System Sump NWCS North Walnut Creek Slump OLF Original Landfill OLF M&M Rocky Flats Site Original Landfill Monitoring and Maintenance Plan O&M operations and maintenance PLF Present Landfill PLF M&M Present Landfill Monitoring and Maintenance Plan and Post-Closure Plan PLFTS Present Landfill Treatment System RCRA Resource Conservation and Recovery Act RFLMA Rocky Flats Legacy Management Agreement SPPTS Solar Ponds Plume Treatment System TP Test Pit USFWS U.S. Fish and Wildlife Service

U.S. Department of Energy Operations & Maintenance—2018 Annual Report, Rocky Flats Site, Colorado April 2019 Doc. No. S23330 Page 1

1.0 2018 Highlights This report addresses operation and maintenance (O&M) activities conducted at the Rocky Flats Site, Colorado (Site) during calendar year (CY) 2018 (January 1 through December 31, 2018). O&M highlights for 2018 include: • The annual verification that the Restrictive Notice (formerly the Environmental Covenant)

remains in the Administrative Record and on file in Jefferson County records was conducted on March 30, 2018. In addition, it was verified that physical controls (i.e., signs placed along the Central Operable Unit [COU] fence) were in place.

• The annual site inspection was conducted on April 5, 2018. No evidence of violations of institutional or physical controls was observed, and no adverse biological conditions were noted during the inspection.

• The Present Landfill (PLF) was inspected quarterly during CY 2018, on March 2, May 7, August 16, and November 14. The inspection conducted on May 7 was also a weather-related inspection. An additional weather-related inspection was conducted on September 6. No significant issues were observed during these inspections.

• The Original Landfill (OLF) was inspected monthly during CY 2018. In addition, weather-related inspections were conducted on May 7 and September 6. The OLF was also monitored weekly as a best management practice (BMP).

There was no slumping at the OLF hillside during CY 2018. The waste footprint area was stable, with the exception of some minor cracking above Berm 4. The area outside the waste footprint was stable with the exception of minor cracking behind Berm 5. Soil from the 2017 slump that blocked the East Perimeter Channel (EPC) remains in place as a temporary stabilizing measure.

Minor maintenance of the OLF cover was conducted throughout the year as needed to fill small cracks and erosion gullies as they were identified, using hand tools such as shovels and rakes.

Additional geotechnical data were collected in the eastern and western supporting hillsides of the OLF for the purpose of completing slope stability design. A total of 14 boreholes were drilled; 5 of these were completed as inclinometers with nested piezometers, and the remaining 9 were completed as piezometers. In addition, 4 test pits were excavated in key locations. Soil samples were collected from the boreholes and test pits for the purpose of geotechnical testing.

The OLF geotechnical evaluation was updated with the new geotechnical and groundwater elevation data, and a final slope stabilization method was selected.

The OLF slope stability design commenced. The design includes controlling infiltration, tieback anchors, and diverting groundwater by installing additional subsurface groundwater collection trenches similar to the East Subsurface Drain (ESSD).

In 2017, additional actions to divert groundwater from the eastern supporting hillside of the OLF included the repair and upgrade of the ESSD in the northeast corner of the OLF, and the installation of two temporary groundwater intercept (GWI) wells to the north. The temporary GWI wells utilize above-ground piping, and as such, are not operated during the winter months. The temporary GWI system was restarted on April 3, 2018.


Subsequently, the system was shut off on May 28, 2018, due to mechanical problems with the generator. The system was restarted on August 10 and was operated until October 11. The system was winterized and is on standby for use in 2019.

In April 2018, a siphon was installed at Seep 10. The siphon started operations on April 26 and was operated until October 29. The siphon utilizes above-ground piping, and as such, is not operated during the winter months. The siphon was winterized and is on standby for use in 2019.

• The former building areas (buildings 371, 771, 881, and 991) were inspected each quarter in 2018 and after significant precipitation events in May and September. There were no significant changes identified with respect to depressions or subsidence, nor were other issues of concern noted.

• NWCS slump monitoring points have been surveyed monthly since the baseline survey in September 2017. Visual observations and monitoring data indicate that slope creep continues through 2018 on a continuous basis, with greater movements occurring during wetter periods.

Total displacements indicated by the slump monitoring points show that, as of the end of 2018 (15 months of monitoring), total horizontal movements ranged from approximately 0.72–1.61 ft and total vertical movements ranged from 0.85–1.85 ft.

The North Walnut Creek Slump (NWCS) scarp line was regraded in February and August of 2018.

• Measurement of groundwater elevations in the piezometers installed at NWCS continued in 2018. This information was used to complete a geotechnical slope stability evaluation and preliminary cost estimate to stabilize the hillside; additional data collection is required to finalize the recommendation for stabilization. Due to the ongoing slope creep, by the end of 2018, some of the piezometers were no longer functional and all of the inclinometers were no longer functional.

• Site road maintenance was completed in 2018. In November and December, the roads were regraded, road base was added as needed, and dust suppressant was applied on the primary routes to aid in dust control.

• The signs posted on the COU boundary fence were inspected quarterly during CY 2018. No significant maintenance was required. Signs were reinstalled or replaced as needed (usually because they were knocked off by deer or elk or because old wires broke).

• Maintenance, repair, replacement, and monitoring of Site erosion control features continued through 2018. Two monitored locations, Mound System Plume Treatment System and East Trenches Plume Treatment System, met the success criteria for vegetation establishment; erosion control monitoring has been discontinued at these locations.

• Various O&M activities were conducted at the groundwater collection and treatment systems, the details of which are provided in the Groundwater Monitoring volume of this annual report.

• The U.S. Fish and Wildlife Service (USFWS) opened the wildlife refuge surrounding the COU in 2018. There were no observed impacts to the COU, the boundary fence, or COU access gates.


A detailed summary of the O&M activities conducted during 2018 is provided in this volume, which is one of the five volumes that comprise the Annual Report of Site Surveillance and Maintenance Activities at the Rocky Flats Site, Colorado, Calendar Year 2018. The other volumes of the 2018 annual report are: • Overview • Groundwater Monitoring • Surface Water Monitoring • Ecological Monitoring

2.0 Annual Site Inspection The Site must be inspected annually for evidence of significant erosion and institutional control (IC) violations, in accordance with Sections 5.3.4 and 5.3.6 in Attachment 2 of the Rocky Flats Legacy Management Agreement (DOE et al. 2007) (RFLMA). The RFLMA was executed on March 14, 2007. Attachment 2 to the RFLMA has been modified since then, most recently in 2018. The 2018 inspection was conducted on April 5 and reported in the Rocky Flats Site Quarterly Report of Site Surveillance and Maintenance Activities, Second Quarter Calendar Year 2018 (DOE 2018). The inspection forms and maps are included in the quarterly report for the second quarter of CY 2018 (DOE 2018). In accordance with RFLMA, the annual site inspection focuses on the following: • Evidence of significant erosion in the COU and evaluation of the proximity of

significant erosion to subsurface features. This monitoring includes visual inspection for precursor evidence of significant erosion (e.g., cracks, rills, slumping, subsidence, and sediment deposition).

• The effectiveness of ICs, as determined by any evidence of violation. • Evidence of adverse biological conditions, such as unexpected morbidity or mortality,

observed during the inspection and monitoring activities. A couple of areas showed minor depressions around former building areas. Marker flags were placed at these areas to assist follow-up by Site subject matter experts. Site field operations subject matter experts evaluated those areas, and none appeared to be significant. No evidence of violations of institutional or physical controls was observed, and no adverse biological conditions were noted. As part of the IC inspection, the presence of the Restrictive Notice in the Administrative Record and in Jefferson County records must be verified annually. The annual verification that the Restrictive Notice (formerly the Environmental Covenant) remains in the Administrative Record and on file in Jefferson County records was conducted on March 30, 2018. In addition, it was verified that physical controls (i.e., signs placed along the COU fence) were in place.


Most inspection observations were related to debris or trash on the surface that was either picked up at that time or marked for subsequent removal and pickup. While debris and trash removal is not one of the RFLMA required inspection items, it is conducted at the same time as the annual inspection.

3.0 Landfills The results of 2018 inspections, monitoring data, and maintenance activities for the PLF and OLF are provided below. 3.1 Present Landfill The PLF consists of an approximately 20-acre engineered cover that is compliant with Resource Conservation and Recovery Act (RCRA) Subtitle C and was placed over a former landfill. The landfill was operational from 1968 until 1998 and received solid wastes, such as construction debris and hazardous wastes, such as paints and solvents. A diversion channel surrounds the landfill and diverts storm water runoff away from the landfill to No Name Gulch. The landfill has a passive seep interception and treatment system (the Present Landfill Treatment System [PLFTS]) installed to treat landfill seep water and groundwater intercept system water that discharges into the former Landfill Pond area. A passive gas extraction system is also built into the landfill to allow gases to vent to the atmosphere. Subsidence and consolidation at the PLF is monitored during the quarterly inspections by visually examining the surface of the landfill cover for cracks, depressions, heaving, and sinkholes. Inspections and monitoring tasks follow the format and protocol established in the Present Landfill Monitoring and Maintenance Plan and Post-Closure Plan, U.S. Department of Energy Rocky Flats, Colorado, Site (DOE 2014) (PLF M&M Plan) and include the monitoring of groundwater and surface water, subsidence and consolidation, slope stability, soil cover, storm water management structures, and erosion in surrounding features.

Inspection Results During CY 2018, quarterly inspections were performed at the PLF on March 2, May 7, August 16, and November 19. The May 7 inspection also served as a weather-related inspection; one additional weather-related inspection was conducted on September 6. In accordance with the PLF M&M Plan, weather-related inspections are conducted after a storm event of 1 inch or more rain in a 24-hour period, or after a significant melt of a snowstorm of 10 inches or more (assuming 10 inches of snow is equivalent to 1 inch of water). See Section 3.2.1 for the quarterly precipitation data. The inspection process followed the format and protocol established in the PLF M&M Plan (DOE 2014). No significant issues were observed during these inspections. During the fourth quarter, microbial buildup was observed in the north and south manhole outlet pipes of the PLFTS. The microbial buildup is a routine occurrence and was removed using a mechanical pipe cleaner on November 19. In addition, minor cracking was observed in the grout around the

3.1.1


PLFTS manhole covers during the fourth quarter inspection. This grout is not structural in nature, but acts as a sealant around the base of the manhole covers. Maintenance of the grout will be conducted in the spring of 2019 when overnight temperatures are above freezing. This cracking does not constitute a remedy-related concern. Landfill inspection forms for the fourth quarter of CY 2018 are provided in Appendix C; earlier 2018 inspection forms are included in the applicable quarterly reports.

Settlement Monuments The 15 landfill settlement monuments are surveyed annually. Nine settlement monuments are located across the top of the landfill cap, and an additional six monuments are located on the east face of the landfill. The monument survey data are compared to the expected settlement calculated in the final design to determine the degree of subsidence at the landfill. The annual settlement monument survey was completed on December 3, 2018. Results of the survey indicate that settling at each monument does not exceed expected settlement calculated in the final design and therefore does not trigger any maintenance activity under the PLF M&M Plan (DOE 2014). 3.2 Original Landfill The OLF consists of approximately 20 acres of a former solid waste and construction debris landfill. The landfill was operated from 1952 until 1968 and received primarily solid sanitary and construction debris wastes. Other wastes received include solvents, paints, oils, pesticides, and items contaminated with beryllium and uranium. As part of landfill closure in 2005, the original surface was regraded to a consistent slope. A 20-foot-high, 1000-foot-long soil buttress was placed at the toe of the landfill. The final cover consists of a 2-foot-thick Rocky Flats Alluvium soil cover that was constructed over the regraded surface and the buttress fill. The area was then revegetated. Erosion of the soil cover is controlled by a series of diversion berms that carry storm water runoff away from the cover to channels on the east and west perimeter of the cover. The OLF is inspected monthly in accordance with the requirements in the Rocky Flats Site Original Landfill Monitoring and Maintenance Plan (DOE 2009) (OLF M&M Plan). As discussed in the CY 2017 second quarter report, the toe of the supporting hillside southeast of the landfill slumped and the boundaries of the slump were nearly identical to the boundaries of the spring 2016 slump. As a temporary stabilizing measure, the soil that slumped into the lower portion of the EPC was not removed as part of the CY 2017 maintenance. Instead, that soil was allowed to remain in the EPC through CY 2018 because it helps stabilize that portion of the hillside. It should be noted that the presence of the soil impedes flow in the EPC and has resulted in a new flow path to Woman Creek along a well-vegetated area. A geotechnical investigation project was completed in April 2018 and involved drilling 14 boreholes and excavating 4 test pits. These are shown on Figure 1 and include the following: • Boreholes completed as piezometers, East: 80318, 80418, 80518, 80618, 80718, 80818 • Boreholes completed as piezometers, West: 80918, 81018, 81118 • Boreholes completed as inclinometers, East: IN0418, IN0518, IN0618

3.1.2


• Boreholes completed as inclinometers, West: IN0718, IN0818 • Test Pits, East: TP-10, TP-11, TP-12 • Test Pit, West: TP-13 Test Pit 11 (TP-11) was excavated in the Seep 2/3 area, and an improved gravity-flow drainage line was installed. Another test pit, TP-10, was excavated near Seep 10 in an attempt to locate the end of a former storm drain line that had been severed and plugged during closure. The line was located during the excavation of the test pit, and a temporary drainage sump and 1-inch siphon line were installed as an interim measure. The sump and siphon line convey groundwater transported by the storm drain remnants from an area just above the slump area on the east side to a more stable location beyond the toe.

U.S. Department of Energy Operations & Maintenance—2018 Annual Report, Rocky Flats Site, Colorado April 2019 Doc. No. S23330

Page 7

Figure 1. Original Landfill, Rocky Flats Site, Colorado

OLF Slump Areas 2015- 2017

LEGEND

O Apparent Seep Sou rce (July 17, 2017)

© Piezometer

• RFLMAWell

fl Inclinometer

0 Test Pit

© Groundwater Intercept Well

• Settlement Monument

- 2015 Slump Boundary ... -, , _.,, 2017 Slump Boundary

- Approximate OLF Waste Footprint

-- Grading Area (2016 Movement)

Culverts, Storm Drains, and --- Associated Features

--- Appro x_Line

- - - East Subsurface Dra in

,v\.r-- Streams

Aerial Photo Date: June 26, 2018

N

! SCALE IN FEET

200 100 0

DATE PR EPARED FILENAME·

Apri l 4, 2019 S2408500

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Page 8



Inspection Results and Maintenance Activities Monthly inspections were performed at the OLF throughout CY 2018. Additional weather-related inspections were conducted on May 7 and September 6. In accordance with the OLF M&M Plan (DOE 2009), weather-related inspections are conducted within 2 days after a storm event of 1 inch or more of rain in a 24-hour period or after significant melt of a snowstorm of 10 inches or more. The OLF was monitored weekly throughout CY 2018 as a BMP initiated in 2015. The inspection process followed the format and protocol established in the OLF M&M Plan (DOE 2009). In February 2018, the use of a modified OLF landfill inspection form (Appendix A to the OLF M&M Plan) was approved by the Colorado Department of Public Health and Environment (CDPHE) and the U.S. Environmental Protection Agency (EPA). Minor modifications to the current OLF inspection form were necessary to (1) capture changes to water management controls resulting from routine maintenance and repairs conducted since 2009, (2) delete the vegetation inspection page, and (3) allow for the use of an electronic inspection form. Modification of the form is discussed in Contact Record (CR) 2018-02, which was approved on February 20, 2018. Use of the modified inspection form began with the March 2018 OLF inspection. The OLF M&M Plan will be revised to adopt the modified form after the long-term hillside stabilization measures are implemented. Landfill inspection forms for the fourth quarter of CY 2018 are included in Appendix C. Inspection forms for the previous quarters are provided in the respective quarterly reports. During the first quarter of CY 2018, the Site received 1.06 inches of precipitation as measured at the Site meteorological station. This precipitation gage is not heated, and therefore may not accurately record moisture received as snowfall. No significant erosion or slumping was observed during the first quarter inspections. The Site received 4.70 inches of precipitation for the second quarter of 2018, as measured at the Site meteorological station. Erosion was identified in the 2017 slump area and the EPC, and maintenance was conducted to remove sediment from behind wattles and Georidges, place an additional row of wattles on the hillside, and place additional rows of Georidges in the EPC. Erosion rills were repaired by raking, and disturbed areas were reseeded and mulched. Sediment on the hillside south of the EPC was observed during the May 7 weather-related inspection. While some of the sediment reached the wetlands north of Woman Creek, it did not reach the banks of Woman Creek. This sediment was raked out and reseeded. During the third quarter of CY 2018, the Site received 2.89 inches of precipitation as measured at the Site meteorological station. A small amount of sedimentation was identified in the EPC, and maintenance was conducted to remove sediment from behind wattles and Georidges. During the fourth quarter of CY 2018, the Site received 1.82 inches of precipitation as measured at the Site meteorological station. Some very minor erosion in west Berm 5 channel surrounding rocks in the channel bottom was filled in.

3.2.1


Settlement Monuments The OLF settlement monuments were surveyed quarterly on March 14, June 7, September 5, and December 3, 2018. Figure 1 includes the locations of the settlement monuments. Survey data indicate that vertical settling at each monument is within the acceptable limits specified in Figure 3-1 of the OLF M&M Plan (DOE 2009). The survey results for fourth quarter of CY 2018 are included in Appendix C. Survey data for other quarters are included in the respective quarterly reports.

Seeps Seeps at the OLF (Figure 1) were evaluated during the monthly inspections, which are summarized below. Additional seep information can be found in the OLF monthly inspection reports. Additional precipitation information can be found in the Surface Water Monitoring volume of this annual report. Seep 1—Moisture was noted after precipitation events or sudden snowmelts.

Seep 2/3—An average flow of approximately 2–3 gallons per minute (gpm) in January. This flow varied from 1–2 gpm to less than 1 gpm for the remainder of 2018.

Seep 4—Slightly saturated soil throughout the year, except September when it was dry.

Seep 5—Some soil saturation in March, July, and November (related to ongoing snowmelt), but relatively dry otherwise.

Seep 6—Dry in April, May, June, August, September, and October. Soil saturated in July, November (snowmelt), and December. Flow varied from less than 1 gpm to 1–2 gpm in January and March.

Seep 7—Produced a flow of less than 1 gpm in June, July, and August, and saturation with no flow the remainder of the year.

Seep 8—Saturated soil in January, February, April, and December. Flow of approximately 1–2 gpm was observed in March. Flow of less than 1 gpm the remainder of the year.

Seep 8A—Saturated soil through May, then dry the remainder of the year except for November (snowmelt).

Seep 8B—Dry in April, September, October, and December. Soil saturated the remainder of the year (November due to snowmelt).

Seep 8C—Dry in June, August, September, October, and December. Soil saturated the remainder of the year (November due to snowmelt).

Seep 9—Saturated in January, April, November, and December, with minimal flows of less than 1 gpm the remainder of the year.

Seep 10—Dry the entire year, except for November (due to snowmelt). A 15-inch-diameter riser pipe sump was installed at Seep 10 during the geotechnical investigation on April 18, 2018. A siphon line was installed in the sump and began conveying water on April 26. See Section 3.2.8 for additional information.

3.2.2

3.2.3


Temporary corrugated plastic drain lines are used at some of the seeps to convey flow off the cover and to minimize infiltration and erosion. These drain lines are inspected during the monthly OLF inspections and maintained as necessary.

Precipitation Response Repairs The OLF hillside has been prone to slumping, settling, and seeps since before the OLF began operation in 1952. Although smoothed over, these natural geologic features and behaviors remained after improvements were made to the OLF hillside in September of 2005 as part of site closure. In the spring of 2007, localized cover slumping and settling occurred in the northwest area of the OLF. This slumping and settling was repaired in 2008, the West Perimeter Channel was reconfigured to stabilize that portion of the hillside (which is not supported by the buttress at the base of the central portion of the constructed hillside; see Figure 1), and additional drainage features were installed. The reconfiguration and additional drainage features were generally successful with only minor cracking in this area during the following years. However, in the fall of 2013 after a significant week-long rain event, localized surface cracking and differential settlement in the northeastern portion of the OLF hillside (another area not supported by the buttress) were noted. Work to repair and regrade this area was scheduled for 2014; however, before the work could begin, there was additional cracking and slumping in the first and second quarters of 2014 on the east side of the EPC. The regrading work was reevaluated and redesigned and was ultimately completed in January 2015. From March through July of 2015, the Site received well above normal precipitation, which caused extensive movement of the hillside that supports the eastern edge of the OLF. This movement appeared as surficial cracks, settling (depressions), and rotational slumping. Most of this movement occurred outside the waste footprint. The repairs were completed in the fall of 2015. Geotechnical consultants reviewed the amount of soil movement in 2015 and concluded that the magnitude of movement was sufficient to decrease the shear strength of the soils in the slump area. The amount of precipitation measured at the Site in 2016 was less than normal; however, residual moisture remained from 2015 and sufficient precipitation occurred in the early part of 2016 to cause movement at the toe of the eastern supporting hillside. The movement area in 2016 was substantially smaller than it was in 2015. It was repaired in the fall of 2016. In 2017, precipitation in the early part of the year was again sufficient to trigger slump movement. The boundaries and extents of the 2017 slump were nearly identical to the 2016 slump. This was repaired in fall 2017. The 2017 repair included a more substantial compaction effort than in previous years in an effort to increase soil density and reduce infiltration. The toe of the 2017 slump also blocked the EPC. This material in the EPC has a buttressing effect that contributes to the stability of the slumped area. Due to this buttressing effect, the slumped materials were not removed but were graded and compacted along with the rest of the slumped area. This work was conducted outside the waste footprint in 2017. Water that reaches the EPC north and east of this blockage bypasses the EPC when it reaches the slump infill area and flows down a well-vegetated hillside north of Woman Creek. This area is inspected weekly. The EPC outfall will be reconfigured as part of the OLF hillside stabilization project scheduled for 2019–2020.

Geotechnical Investigation and Revised Slope Stability Analysis Following two independent geotechnical engineering reviews completed in 2016, a hillside slope stability analysis was conducted in 2017. The analysis provided specific recommendations for

3.2.4

3.2.5


stabilizing the hillside. One of those recommendations was to conduct additional geotechnical investigation and to monitor hillside movements and groundwater levels in the slump area. A geotechnical investigation was completed in April 2018 and included drilling 14 boreholes and excavating 4 test pits (Figure 1). One of the test pits was excavated in the Seep 2/3 area, and an improved gravity-flow drainage collection trench and collector line was installed. Another test pit was excavated near Seep 10 in an attempt to locate the end of the remaining portion of a former storm drain line that had been severed and plugged during closure. The line was located during the excavation of the test pit, and a temporary drainage sump and 1-inch siphon line (see Section 3.2.8) were installed as an interim measure. Five of the borings were completed as both inclinometers and vibrating wire piezometers. The remaining nine borings were completed as piezometers. Inclinometer data was collected, and groundwater level readings in the OLF piezometers continued during 2018 to provide data for the final design of the OLF hillside stabilization project. The inclinometer readings for the OLF during 2018 showed less than 1/8-inch movement at all locations except for IN0618, which showed about 1/2-inch total movement at the ground surface, tapering to less than 1/16-inch movement about 8 feet below ground surface. No visible slumping of the OLF hillside was observed during 2018. Groundwater levels at or near pertinent features such as the ESSD, temporary GWI system, and the Seep 10 siphon are presented in the following sections. A revised geotechnical evaluation using the site-specific geotechnical and groundwater data was performed for final design purposes. The evaluation confirmed that there are three primary causal factors to the OLF hillside instability: surface water infiltration, groundwater entering the OLF from the north, and a weak geologic layer made weaker by historic movements. A final recommendation for hillside stabilization was made, and the stabilization design commenced.

East Subsurface Drain As described in the Annual Report of Site Surveillance and Maintenance Activities at the Rocky Flats Site, Colorado, Calendar Year 2016 (DOE 2017), additional actions to improve the diversion of groundwater away from the OLF included maintenance and upgrade of the ESSD in the northeast corner of the OLF. In anticipation of the temporary GWI system project described in the next section, a solid wall pipe was paired with the perforated drain to provide a conduit for the intercepted groundwater. This work was conducted under CR 2016-04. Subsequently, piezometers were installed upgradient (85517) and downgradient (85617) of the ESSD to monitor the performance related to lowering groundwater levels in the area of the ESSD (Figure 1 and Figure 2). The piezometers confirmed that, during the spring of 2017, the ESSD initially lowered the groundwater level approximately 13 ft on the downgradient side of the drain relative to the upgradient side. As the ESSD continued to function, groundwater stabilized to an elevation difference of approximately 3.9 ft in August 2017, and subsequently dropped slowly on both sides due to continued drainage and relatively dry weather. By the end

3.2.6


of 2017, groundwater continued to drop while maintaining a groundwater elevation difference of approximately 4.2–4.6 ft. Monitoring of these piezometers continued in 2018. Both piezometers show a small rise in groundwater elevation after spring 2018 precipitation of approximately 2.5 ft. Subsequently, the groundwater elevation dropped below the minimum elevation observed before the spring rise, with continuing downward trends at the close of 2018. As such, the ESSD appears to be functioning properly, as it held groundwater rise to 2.5 ft during spring precipitation and continues to dewater this area of the hillside to elevations below those previously achieved.

Note: Data gap from July 2, 2018, to August 1, 2018, for 85517 data logger due to user error. Data logger was not restarted after the July 2, 2018, download.

Figure 2. Piezometers 85517 and 85617 near ESSD

Temporary Groundwater Intercept System In March 2017, two temporary GWI wells were installed along what were believed to be preferential groundwater flow paths to the OLF (460GWI and 440GWI, Figure 1). The objective of these wells is to divert groundwater away from the landfill during high water conditions (typically spring and early summer). Intercepted water is pumped through corrugated drain pipes on the ground surface to the solid wall pipe installed at the ESSD. The temporary GWI system wells remained in operation until October 26, 2017, when they were shut down and winterized. The system was restarted on April 3, 2018. Subsequently, the system was shut off on

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May 28, 2018, due to maintenance needs associated with the generator. The system was restarted on July 12, 2018, and was operated until October 11, 2018. The system was winterized and is on standby for use in 2019. These wells will resume operation during the spring and summer of 2019 to collect additional data before the OLF stabilization project is implemented. During 2018, a total of 198,000 gallons of groundwater were diverted by the system. Piezometers 84616 and 83916 monitor groundwater levels north and south of 460GWI, respectively (Figure 1). Piezometer 83916 shows a nearly immediate response to the generator shutdown on May 28, 2018, with the groundwater rising and leveling off to an increase of approximately 2.7 ft in 2 weeks (Figure 3). The groundwater elevation dropped very quickly in early July, immediately after the generator was brought back online. Piezometer 84616 shows a very different response (Figure 3). There was a small spike in groundwater elevation in early May due to spring precipitation, followed by a return to the prior elevation in about two weeks (the GWI system was operational during that time). The groundwater elevation at this location remained essentially constant for the remainder of the year, showing no effects from the generator shutdown in late May and startup in early July.

Figure 3. Piezometers 83916 and 84616 near 460GWI Piezometers 85117 and 85217 monitor groundwater levels north and south of 440GWI, respectively (Figure 1). Both of these piezometers show a nearly immediate response to the generator shutdown on May 28, leveling off in about 2 weeks (Figure 4). Similar to 83916 at

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460GWI, the groundwater elevation in both of these piezometers dropped very quickly in early July, more than a month before the generator was brought back online. These wells are also likely influenced by the 36-inch line that runs diagonally across the top of the OLF (Figure 1), and presumably the groundwater level here was affected by the operation of the siphon at Seep 10. See Section 3.2.8.

Figure 4. Piezometers 85117 and 85217 near 440GWI

Seep 10 Sump and Siphon In April, 2018, a geotechnical investigation was completed at the OLF (see Section 3.2.6). As part of this geotechnical investigation, a sump was installed in the test pit that was excavated near Seep 10. The sump was located near the downgradient terminus of the remnant 36-inch-diameter corrugated metal pipe that was left in place at closure and cut off just west of Seep 10. Prior to 2018, Seep 10 had a discrete discharge point at the ground surface in the spring and early summer. The water would flow down the hillside and be captured by Berm 5, which would route the water to the EPC. Because of this long flow path, some of this water would have infiltrated into the slump prone area to the east. A 1-inch siphon line was set up to divert the water from the Seep 10 area to its natural surface drainage collection point south of the slump area. This was done to minimize the infiltration of water from Seep 10 into the slump area. As of the end of 2018, the Seep 10 siphon had diverted approximately 725,000 gallons of water around

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the slump area. Piezometer 85717 is installed near the Seep 10 siphon (Figure 1) and shows the groundwater elevation response to the installation of the siphon compared to the groundwater elevations in 2017 (Figure 5). The siphon began operations on April 26, 2018, and effectively lowered the groundwater more quickly, earlier in the year, and to a greater total depth compared to the groundwater elevations measured in 2017, which were the result of natural drainage only. The siphon was shut down on October 29 due to freezing temperatures and an insufficient amount of water within the sump.

Figure 5. Piezometer 85717 Near Seep 10 Siphon

4.0 COU And Former Building Area Inspections In accordance with RFLMA, the COU is monitored for significant erosion annually and following major precipitation events. Particular attention is paid to areas proximate to remaining subsurface features (e.g., former buildings, trenches). In CY 2018, in addition to weather-related inspections, the former building areas were inspected quarterly as a BMP. The areas inspected include former buildings 371, 771, 881, and 991. No notable changes in depressions and no new areas of subsidence were observed during the quarterly and weather-related inspections in these areas.

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5.0 North Walnut Creek Slump Slumping on the hillside east of the Solar Ponds Plume Treatment System (SPPTS) (known as the North Walnut Creek Slump, or NWCS) continued to develop after the heavy precipitation events of 2015. The toe of the slump blocked the road leading to the SPPTS Discharge Gallery and the Interceptor Trench System Sump (ITSS) and approached RFLMA monitoring well B210489. During the spring precipitation of 2016, the slumping became more pronounced. Because of the magnitude of movement that occurred, it is possible that portions of the subsurface Interceptor Trench System (ITS) drain lines which are located downgradient of the SPPTS groundwater intercept trench have been affected by the slump movement. If significant additional movement occurs in the future, the monitoring well, the ITSS, or the SPPTS groundwater intercept trench may be impacted. In 2017, the slump area was evaluated using available geotechnical data from nearby wells. The geotechnical evaluation indicated that continued movement is likely and that further movement would likely impact SPPTS components. The evaluation also included several preliminary recommendations. In the spring of 2017, the NWCS was graded and tension cracks were repaired by excavating to the bottom of the crack and backfilling the excavation. This effort was undertaken to minimize rapid infiltration of surface water (e.g., precipitation, runoff) into the slump area soils, which is a major factor contributing to instability of the area. This grading effort was a temporary fix; a more extensive evaluation is underway. Subsequent to the NWCS grading project, a crack redeveloped along the alignment of the mid-slope scarp that was taken out during the regrading activities. Initial observations of the crack indicated horizontal movements of 1/16 inch to 3 inches with little to no vertical movement. Monitoring points were subsequently installed in September 2017 and have been surveyed monthly: 11 north of the scarp and 11 south of the scarp, for a total of 22. These monitoring points are located approximately 10 ft horizontally from the scarp. Visual observations and monitoring results indicate that the slump area is continuing to move slowly down the hillside (this is described as “slope creep”). Total displacements indicated by the settlement monitoring points show that as of the end of 2018 (15 months of monitoring), horizontal movements ranged from approximately 0.72–1.61 ft and vertical movements ranged from 0.85–1.85 ft. In the winter of 2017–2018, a geotechnical investigation project was completed; it included drilling 18 geotechnical borings, 15 borings completed as piezometers, and 3 borings completed as inclinometers. These are shown in Figure 6 and include the following: • Boreholes completed as piezometers: 72517, 72617, 72717, 72817, 72917, 73517, 73617,

73717, 73817, 73917, 74017, 74117, 74217, 74317, 74417 • Boreholes completed as inclinometers: IN0118, IN0218, IN0318 Soil samples were collected and sent to a lab for geotechnical analyses. CR 2017-03 describes these activities (Appendix F). The 14 geotechnical borings completed as groundwater piezometers were developed, and initial groundwater readings were taken. Groundwater readings


for the piezometers continued throughout 2018, with the exception of the two piezometers at the toe of the slump, 73517 and 74317. These two piezometers failed in July 2018 due to continued movement of the slump (Figure 8). By May of 2018, two of the inclinometers could no longer be read due to the magnitude of hillside movement. By June, all three of the inclinometers had sufficient movement to preclude further measurements.


Page 19

Figure 6. North Walnut Creek Slump, Rocky Flats Site, Colorado

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Page 20



The collected data was used to complete a geotechnical slope stability evaluation and to evaluate specific alternatives for hillside stabilization. After reviewing the slope stability evaluation, groundwater analyses, and cost estimates, it was determined that a streamlined approach to hillside stabilization may be possible with additional data and evaluation. The path forward for defining these data needs will be undertaken in 2019. Figure 7 through Figure 9 show the measured groundwater levels in the upgradient, midgradient, and downgradient piezometers, respectively. Most of the piezometers demonstrate a rise in groundwater elevations during the higher precipitation spring months. In some cases, the groundwater elevations are slowly returning to the seasonal low observed during March and April 2018. Others show little downward trend after the spring rise. The downgradient piezometers 74317 and 73917 show little seasonal variation, likely due to their proximity to North Walnut Creek. Downgradient piezometer 73517 never achieved a stable groundwater elevation during the early months due to very slow recovery and multiple sampling events.

Figure 7. NWCS Upgradient Piezometers

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Figure 8. NWCS Midgradient Piezometers

Figure 9. NWCS Downgradient Piezometers

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6.0 Site Road Maintenance Access to different areas of the Site is provided by established gravel roadways and all-terrain vehicle (ATV) paths. These are typically maintained on an annual basis, with smaller, localized improvements as required. Maintenance includes grading and filling ruts on the primary routes and trimming vegetation as needed on the ATV paths. In November and December 2018, maintenance was performed and a dust suppressant was applied on the primary routes to aid in dust control. Site roads are continually monitored throughout the year and after significant weather events to ensure safe passage of Site personnel for routine operations.

7.0 Sign Inspections RFLMA requires that signs be posted at intervals around the perimeter of the COU to notify persons that they are at the COU boundary. In addition, signs that list the use restrictions known as ICs and provide contact information are posted at access points to the COU. These signs are an important component of the required physical controls of the remedy. Signs are inspected quarterly and are maintained through repair or replacement as needed. The signs were inspected quarterly during CY 2018 as required (March 1, May 21, August 1, and December 12). Signs were reinstalled or replaced as needed (usually because they were knocked off by deer or elk or because old wires broke).

8.0 Erosion Control and Revegetation The existing erosion controls are maintained and repaired to protect bare soil areas until vegetation stabilizes the soil. Areas lacking sufficient vegetation cover are assessed and typically reseeded. In some cases, soil amendments are added to help establish the native vegetation. Additional information on the revegetation activities conducted at the Site during 2018 is provided in the Ecological Monitoring volume of this 2018 annual report. Maintenance, repair, replacement, and monitoring of the Site erosion control features continued as needed through 2018. Guidance in the Erosion Control Plan for the Rocky Flats Property Central Operable Unit (DOE 2007) was followed for various projects conducted in 2018. The plan addresses what regulatory requirements must be met, including monitoring inspections, and the scope of erosion control activities at the Site. It outlines the responsibilities, BMPs, and implementation aspects of erosion control activities before, during, and after projects. Erosion controls were inspected on a regular basis (typically weekly or monthly depending on location), following high-wind events and after significant precipitation events. Repairs in 2018 included installing, replacing, or securing wattles (by restaking or weighting with rocks) or securing erosion control blankets that had loosened. Woodstraw was added to some locations and GeoRidges were also installed, where needed, to provide added protection. Flexterra growth medium was reapplied at one location where elk had destroyed it. Elk are also beginning to become a nuisance with respect to restoration activities at the Site. At two recent revegetation locations, the OLF and NWCS project areas, the elk destroyed some of


the revegetation and erosion controls that were put in place after project completion. When the entire herd crosses these revegetation areas, their hooves destroy the coconut erosion matting, straw wattles, and Flexterra flexible growth media. This creates additional costs to constantly repair or replace the seeding and erosion controls needed to protect these areas. Some of these areas have been reseeded and replaced 2 or 3 times now as a result. The elk appear to be drawn to the bare ground areas. Future revegetation efforts may require the installation of fencing or the use of more permanent turf reinforcement matting and compost or woodchip-filled wattles, which cost significantly more than the options currently used.

9.0 References DOE (U.S. Department of Energy), 2007. Erosion Control Plan for the Rocky Flats Property Central Operable Unit, Office of Legacy Management, July. DOE (U.S. Department of Energy), 2009. U.S. Department of Energy Rocky Flats Site, Original Landfill Monitoring and Maintenance Plan, LMS/RFS/S05516, Office of Legacy Management, September. DOE (U.S. Department of Energy), 2014. Present Landfill Monitoring and Maintenance Plan and Post-Closure Plan, U.S. Department of Energy Rocky Flats, Colorado, Site, LMS/RFS/S03965, Office of Legacy Management, December. DOE (U.S. Department of Energy), 2017. Annual Report of Site Surveillance and Maintenance Activities at the Rocky Flats Site, Colorado, Calendar Year 2016, LMS/RFS/S15402, Office of Legacy Management, April. DOE (U.S. Department of Energy), 2018. Rocky Flats Site, Colorado, Quarterly Report of Site Surveillance and Maintenance Activities, Second Quarter Calendar Year 2018, Office of Legacy Management, LMS/RFS/S20699, October. DOE (U.S. Department of Energy), U.S. Environmental Protection Agency (EPA), and Colorado Department of Public Health and Environment (CDPHE), 2007. Rocky Flats Legacy Management Agreement, executed on March 14. Attachment 2 updated December 2012.

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