EPA Comments on Picacho Final Closure/Post

- Closure Maintenance Plan dated 10/27/10

2/10/2011

Page 1 of 3 The following comment reflect US EPA Region 9’s comments on the revised Picacho Final Closure/Post

Closure Maintenance Plan dated 10/27/10 . These comments ca n be incorporated into the Closure Plan or included as an addendum to the Closure Plan.

1. O verall cover thickness of 3

- ft vs. 4.5

- ft proposed in the Closure Plan : EPA recommends an overall cover thickness of 3.0

- ft vs. the 4.5

- ft cover thickness proposed in the Closure Plan for the following

reasons:
a. Modeling described in the Plan shows that a 3

- ft cover would perform better than a 4.5

- ft

cover. b. Given the extreme arid nature of the site, a thinner cover (3

- ft) is adequate to control infiltration versus the suggested 4.5

- ft thick cover.

c. As discussed below, our analysis shows that a 3.0

- ft thick cover is stable on 3:1 side slopes while a 4.5

- ft thick cover is not.

The suggested final cover profile is as follows:

2. Side Slope Stability: As discussed in the EPA analysis below , the steep 2.25:1 side slopes are not stable under static conditions
, however, are stable with 3:1 side slopes and a 3.0

- ft thick cover. T herefore, EPA recommends reg rading the side slopes during final closure to a maximum 3:1 slope and using a cover thickness of 3

- ft.

EPA analys i s shows that under static conditions the existing slopes are stable, using the input parameters utilized in the Plan

. However, the input param eters used in the Plan were not all conservative. The Plan’s write

- up for the soils states that the borrow soil is “predominantly cohesion l ess”, yet soil cohesion values were utilized in the Plan’s stability analysis . Cohesion is a sensitive input parameter and has a significant impact on calculated factors of safety. Cover Soil From approved borrow source

Existing Interim Cover (free of waste/debris) Verify thickness, Scarify and recompact

Waste Material Surface Treatme nt

– Rock/Soil Admixture 6”, min

.

3’

- 0”, min

.EPA Comments on Picacho Final Closure/Post

- Closure Maintenance Plan dated 10/27/10

2/10/2011

Page 2 of 3 E valuation of the cover side slope stability was performed for veneer stability. Values from the Closure Plan for soil weight, friction angle, slope, cover thickn ess, and cohesion were initially utilized. Values for cohesion, cover thickness, and slope were then varied based on more conservative possibilities. The following table shows calculated factor of safety results along with accompanying input parameters

.

With a cover thickness of 3

- ft and conservative cohesion value of zero, a factor of safety of 1.7 was estimated for static conditions. This is greater than the minimum factor of safety of 1.5. We therefore recommend that the side slopes be regraded during final closure to a maximum slope of 3:1 with a cover thickness of 3

- ft.

3. Erosion Layer Admixture and Gradations for 3% slopes and 3:1 slopes : EPA calculated viable erosion layer mixtures and gradation s for the slop es at the Picacho landfill

. 1

3% slopes: The c alculations for the 3% slopes show that the surface admixture should be composed of pea gravel ( 3/8

- in

– 1/2

- in diameter) mixed with cover soil at the ratio of 25% rock to soil by volume with a minimum 6

- in erosion layer

.
3:1 slopes : C alculations for the 3:1 slopes show that the surface admixture should be composed of gravel/rock ( 3⁄4

- in to 1

- in diameter ) mixed with cover soil at the ratio of 50 % rock to soil by volume and result in a minimum 6

- in erosion l ayer

. Other rock size and percent content ratios can be used depending on the availability of gravel and its size in the area , however, increasing rock sizes results in an increased overall admixture erosion layer thickness

. As an example using gravel/ro ck (3⁄4

- in to 2

- in diameter ) mixed with cover soil at the ratio of 50 % rock to soil by volume and results in a minimum 12

- in erosion layer. Viable erosion layer options are shown in the table below.

Viable Options Available to Design Erosion Layer Admixtur e Gradations for 3% slopes and 3:1 slopes

1 B ased on the method described in Dwyer et al 2007

.EPA Comments on Picacho Final Closure/Post

- Closure Maintenance Plan dated 10/27/10

2/10/2011

Page 3 of 3 A spreadsheet with the utilized parameters is available and can be used to evaluate different gravel size and quantities.

4. Perimeter B erm L ining : EPA continues to recommend lining the perimeter berms discussed in Section 3.7.1.3 of the Closure Plan in a way that will minimize soil lose and infiltration. BAS argues that the proposed rock mixed into the perimeter berm’s cover soil will mitigate any erosion. We disagree. Soil/rock admixtures are appr opriate to control sheet flow erosion
, however, are not appropriate for concentrated flow such as conveying storm flow along berms or in channels. We recommend asphaltic lining for the perimeter berms.