§ 39-176E — CONSTRUCTION REQUIREMENTS FOR NEW PHOSPHOGYPSUM STACKS — LATERAL EXPANSIONS OF EXISTING PHOSPHOGYPSUM STACKS

Any operator desiring to construct a new phosphogypsum stack, a material component thereof, or a lateral expansion shall submit to the department of environmental quality for review and approval prior to commencing construction a design and construction plan, including construction quality control, that includes minimum design and construction requirements to control and minimize the movement of waste and waste constituents into the environment. Plans and specifications submitted to satisfy the requirements of sections 39-176A through 39-176F, Idaho Code, shall be certified by a registered professional engineer. The minimum design requirements include the following features and standards:
(1) Run-on and runoff controls for the phosphogypsum stack systems for the collection, control, and treatment, as needed, of run-on and runoff from the systems. The controls shall be capable of managing a flow resulting from a twenty-four (24) hour, twenty-five (25) year rainfall event or from a combined peak precipitation and snowmelt event over a twenty-four (24) hour period using snowfall, precipitation, and other meteorological data from the historical record.
(2) Any new phosphogypsum stack or lateral expansion shall be designed with an overall factor of safety of 1.5 for any potential failure surface encompassing the impoundment on top of the stack and passing through the phosphogypsum slope or bottom liner interfaces or extending into earthen material in contact with the bottom liner.
(3) Liner and leachate control systems that achieve the following minimum design standards:
(a) Phosphogypsum stacks, collection ponds, decant ponds, and cooling ponds shall be constructed atop a composite liner or approved alternative of equivalent hydraulic conductivity and durability. Liners shall be constructed of materials that have appropriate physical, chemical, and mechanical properties to prevent failure. A composite liner will have both a synthetic and a nonsynthetic component.
(i) The synthetic component shall consist of a sixty (60) mil or thicker HDPE or equivalent geomembrane with a maximum water vapor transmission rate of twenty-four hundredths (0.24) of one gram per square meter per day as determined by the American society for testing and materials (ASTM) method E96-80, procedure BW.
(ii) The nonsynthetic component shall consist of either soil, phosphogypsum, or sedimented phosphogypsum or equivalent material.
1. Soil or equivalent material. A layer of compacted soil or other equivalent material at least eighteen (18) inches thick, placed below the geomembrane, with a maximum hydraulic conductivity of 1 x 10-7 centimeters per second, constructed in six (6) inch lifts.
2. Phosphogypsum. A layer of mechanically compacted phosphogypsum at least twenty-four (24) inches thick, placed above the geomembrane, with a maximum hydraulic conductivity of 1 x 10-4 centimeters per second. A layer of compacted phosphogypsum is not required for any vertical expansion or natural ground slopes steeper than 2.5H:1V abutting a vertical or horizontal expansion where phosphogypsum slurry is discharged in the expansion area.
(iii) The nonsynthetic layer of the composite liner is not required for vertical expansions where:
1. It has been demonstrated that a synthetic liner alone or in contact with sedimented phosphogypsum placed in slurry form will be equivalent or superior to a composite liner;
2. It has been demonstrated that a synthetic liner in contact with sedimented phosphogypsum placed in a slurry form is equivalent or superior to a composite liner with twenty-four (24) inches of compacted phosphogypsum placed above the geomembrane; or
3. For lateral expansions, it has been demonstrated and certified by a third-party professional engineer that a synthetic liner in contact with sedimented phosphogypsum placed in slurry form, and with consideration of the physical hydrogeological setting of the specific lateral expansion, provides an equivalent or superior degree of protection for human health and the environment.
(iv) The nonsynthetic layer of the composite liner will not be required for an alternative liner, such as a double synthetic liner, that has the equivalent hydraulic conductivity and durability to a composite liner.
(v) An approved alternative shall have the equivalent hydraulic conductivity and durability of a composite liner.
(vi) For an intermediate liner, a composite liner is not required.
(b) All liner and leachate control system components shall have appropriate quality control and quality assurance standards, specifications, and procedures for construction, including:
(i) Procedures and tests that will be used to monitor the installation of the liner system components shall be described in detail;
(ii) Description of sampling activities, sample size, sample locations, minimum frequency of testing, acceptance and rejection criteria, and plans for implementing corrective measures that may be necessary; and
(iii) Description of reporting for constructive quality assurance and quality control activities, including observation data sheets, problem identification, corrective measures, and final documentation.
(c) Phosphogypsum stacks shall have a leachate control system. Any leachate emanating from a phosphogypsum stack system shall be routed to a collection pond, such as a decant pond or similar water structure, to be contained within the system or recirculated to the production plant, or, if discharged, treated if required to meet applicable water quality and discharge requirements. All toe drain or leachate collection systems shall be constructed within the lined system. Leachate control systems shall:
(i) Have a perimeter underdrain system designed to stabilize the side slopes of the phosphogypsum stack that is installed above the geomembrane liner; and
(ii) Have perimeter drainage conveyances that either consist of covered or uncovered ditches that are lined continuously with sixty (60) mil or thicker HDPE or equivalent geomembrane, or of chemically compatible leachate collection pipes. Covered ditches shall have manholes or appropriate cleanout structures at appropriate intervals unless a third-party engineer certifies and identifies areas where manholes or cleanout structures in piped systems are not feasible.
(d) Auxiliary holding ponds shall be designed with a synthetic liner or an approved alternative of equivalent hydraulic conductivity and durability.
(e) Process wastewater conveyances shall be constructed with a liner or pipe.
(4) Perimeter dikes that shall incorporate minimum design standards for freeboard, safety, and slope stability design factors, construction methods, and other related parameters, including:
(a) Ground that will become the foundation of perimeter dikes shall be stripped of vegetation and organic detritus or residue, including muck, slimes, or other material that would flow or undergo excessive consolidation under heavy loading. All earth foundation surfaces on which fill is to be placed shall be scarified, or moistened and compacted, prior to spreading the first course of fill material.
(b) A program of soil sampling and testing adequate to determine the characteristics of the foundation material that will support the proposed perimeter dike and the material to be used to construct the perimeter dike.
(c) The crest of the perimeter dike shall be graded toward the inside slope or the outside slope. Both inside and outside slopes shall not be steeper than two and one-half (2.5) horizontal to one (1.0) vertical (2.5H:1V). A liner shall be constructed on the inside slope of the perimeter dike and be suitably connected to the remainder of the liner system to provide seepage control.
(d) The freeboard of an above-grade perimeter dike shall not be less than five (5) feet unless a freeboard of less than five (5) feet is justified based on results of seepage and stability analysis and wave run-up analysis. In no event shall the freeboard of an above-grade perimeter dike be less than three (3) feet.
(e) A stability analysis shall be performed taking into consideration the minimum fluid level as well as the fluid level at the freeboard on the upstream slope of the perimeter dike and possible fluctuations of the tailwater level. When applicable, the stability analysis will include a seepage or flow net analysis.
(f) The following minimum safety factors will be used for perimeter dikes: 1.75 for horizontal shear at base of fill; 1.5 for horizontal shear within the fill due to seepage through the outer face; 1.5 for horizontal shear or circular arc failure through the foundation soils; 1.5 for protection against shear failure of any circular arc in either inside or outside slope. In determining design safety factors, water pressure distribution must be addressed.
(g) A seismic stability analysis shall be performed.
(h) Appropriate quality control and quality assurance standards, specifications, and procedures for perimeter dike construction shall be implemented.
(5) Any lateral expansion must be constructed in accordance with the same requirements as a new phosphogypsum stack. Except for incidental deposits of phosphogypsum entrained in the process wastewater, or conditioned phosphogypsum used as a cushion layer against rock slope, placement of phosphogypsum outside the phosphogypsum stack footprint is considered a lateral expansion.
(6) The design and construction plan submitted to the department must contain a process for notification and department approval of deviations from the approved design and construction plan.

Legislative History

[39-176E, added 2020, ch. 51, sec. 5, p. 121; am. 2021, ch. 246, sec. 5, p. 764.]