Fla. Admin. Code Ann. R. 62-672.760 - Procedures for Decanting Process Water from Top of Phosphogypsum Stack

The owner of a phosphogypsum stack system, including inactive stacks or temporarily inactive stacks, shall comply with the following requirements for decanting process water from the top of phosphogypsum stacks.

(1) One or more of the following three methods may be used to decant water from the top of a phosphogypsum stack:

(a) Overflow broad crested weir dug in gypsum, such as controlled flow through an open cut;

(b) Decant pipe placed in a backfilled cut, or

(c) Siphon line or positive pressure line that does not penetrate the gypsum dike.

Any exception to the above shall be specifically recommended, on a case-by-case basis, by a third-party engineer and approved by the department upon a demonstration that such exception will maintain the integrity of the impoundment. Any exception to the requirements specified in subsections 62-672.760(2) -(14), F.A.C., shall be specifically recommended, on a case-by-case basis, by a third-party engineer and the department shall be notified of the exception in a timely manner.

(2) The maximum depth of any open cut used to decant water and any cut made to place or remove a decant pipe shall be limited to no more than 10 feet. The depth of cut shall be measured from the top of the fluid level elevation in the rim ditch at the decant location, i.e., from the maximum elevation of the slurry flowing in the rim ditch at any time prior to making the cut. The depth shall be measured to the bottom invert elevation of the decant pipe or open cut beneath the centerline of the gypsum dike.

(3) Each facility shall select a range of bottom widths and side slopes for any cut to be made that are consistent with the site-specific decanting and backfilling procedures adopted by that facility.

(4) The minimum distance from the decant location (within the settling compartment on top of the stack) to the outer edge of any cut on the exterior slope of the stack, measured along the invert of the cut, shall be no less than 40 feet. Moreover, if the distance from the decant location to the outer edge of the cut on the exterior slope is less than fifty-five (55') feet, either one of the following additional precautionary measures shall be implemented:

(a) The cut made across the inner levee (upgradient from the rim ditch) shall be offset at least fifteen (15') feet relative to the cut made across the gypsum dike crest, or

(b) A gypsum "beach" or delta shall be placed or built into the inner settling compartment at the decant location prior to making the cut.

(5) The invert of any cut through a gypsum dike shall be located in material that has been allowed to consolidate and age for no less than 2 weeks.

(6) The centerline of a new decant cut shall be offset a minimum distance of fifty (50') feet from the location of the most recently backfilled cut (i.e., older cut which is no longer being used to decant water).

(7) If an open cut is used to decant water, the depth of water over the broad-crested weir opening shall be controlled at less than two (2') feet. If a decant pipe is used, the diameter of the pipe shall be no greater than thirty (30'') inches, and the pressure rating of the pipe shall be no less than 50 psi (e.g., for High Density Polyethylene (HDPE) pipes, the Standard Dimension Ratio (SDR) shall be no greater than 32.5; and for Polyvinyl Chloride (PVC) pipes, the pipe Schedule shall be equal to or greater than 40). Moreover, the horizontal section of any decant pipe placed in a backfilled trench shall be extended no less than two (2') feet and no more than five (5') feet beyond the edge of the cut on the exterior slope of the stack, or the discharge end of the pipe shall be laid along the exterior slope of the stack.

(8) Additional measures for decant pipe.

(a) If a decant pipe is used, and a cut is made to place or remove the pipe, the following precautionary measures shall be implemented prior to making the cut:

1. Place a gypsum "beach" or delta into the inner settling compartment extending no less than thirty (30') feet from the inside edge of crest of the inner levee, then lower the water level in the settling compartment below the bottom invert elevation of the decant pipe, and construct a temporary cofferdam on the gypsum beach as an added safety measure; or alternatively,

2. Place a gypsum "beach" or delta into the inner settling compartment extending no less than one hundred (100') feet from the inside edge of crest of the inner levee, temporarily isolate the compartment where the decant is located to prevent the introduction of additional water or slurry, and construct a temporary gypsum cofferdam as needed to isolate the decant location. The cofferdam shall have a minimum crest width of twenty (20') feet. The excavation shall not be allowed to extend across an imaginary 3.0 Horizontal: 1.0 Vertical line projected from the outside toe of the cofferdam towards the exterior slope of the stack; or alternatively,

3. Place a gypsum "beach" or delta into the inner settling compartment extending no less than five hundred (500') feet from the inside edge of crest of the inner levee, and temporarily isolate the compartment where the decant is located to prevent the introduction of additional water or slurry.

(b) Once the excavation has progressed below the water level elevation in the settling compartment, the cut shall be completed and the excavation backfilled as expeditiously as possible but no later than within 48 hours.

(9) Prior to backfilling a cut, the exposed gypsum surface shall be scarified (e.g., with the dozer tracks or with the backhoe bucket) as needed to break up and remove any cemented surface crust, if present.

(10) Only moist or wet gypsum may be used in backfilling operations. Dry gypsum shall not be used unless it is moisture-conditioned prior to or during placement. Moreover, gypsum used in backfilling a decant cut shall have an equivalent texture and consistency to freshly sedimented gypsum excavated from the rim ditch.

(11) Backfilling operations shall incorporate one or more of the following construction steps or procedures, as applicable, or other methods certified as equivalent by a third-party engineer.

(a) Any open cut through the gypsum dike shall be backfilled with wet or moist gypsum placed in lifts not exceeding eighteen (18'') inches in thickness, as needed to ensure that the gypsum backfill is in intimate and complete contact with the sides of the cut and with the external surface of the decant pipe, when present.

(b) Either tracked equipment (e.g., dozer) shall be used to roll the surface and compact each lift of moist to wet gypsum, scarifying between lifts as needed; or the bucket of a hydraulic excavator (backhoe) shall be used to place and tamp wet to very wet (e.g., "sluiced" or flowable) gypsum, having a saturated paste consistency, in lifts, scarifying between lifts as needed. The latter method is suited for use in filling all around a decant pipe, when present, provided the pipe is restrained and prevented from being uplifted during any such filling operation.

(c) Construction equipment shall not be allowed to travel directly over any buried decant pipe until a gypsum cover thickness sufficient to prevent damage to the pipe has been placed over the pipe (as approved by a registered professional engineer).

(d) If saturated gypsum has been used in backfilling a cut through the gypsum dike (i.e., wet to very wet gypsum placed and tamped with the bucket of a hydraulic excavator), then the freshly backfilled plug shall be allowed to set for at least 48 hours before the remainder of the cut inboard of the restored outer dike is backfilled with gypsum slurry via the rim ditch, and before water is allowed to flow in the rim ditch across the backfilled cut.

(12) Backfilling of any decant cut through the gypsum dike shall be done during daylight hours only (unless the entire work area is well lighted); and shall be inspected and monitored by a qualified company employee familiar with the specified backfilling procedures.

(13) The placement in service and initial operation of the rim ditch adjacent to any backfilled cut shall be inspected and monitored by a qualified company employee, with periodic monitoring to continue at least once every 12 hours during the first 36 hours after re-activating the area adjacent to the cut. Any of the following items shall be considered as indicative of a potentially critical condition requiring immediate notification of supervisory personnel and performance of more frequent inspections until the situation has stabilized or remedial action has been implemented: concentrated seepage on the outer face of the backfilled cut, any sign of sediment transport, cracking or subsidence of the exposed surface on the crest and downstream face, and concentrated seepage or boils in the vicinity of a decant pipe.

(14) All inspections shall be documented in writing and the findings shall be recorded, signed by the qualified company employee who conducted the inspection and maintained at the facility for a period of not less than three years.

Notes

Fla. Admin. Code Ann. R. 62-672.760

Rulemaking Authority 403.4155 FS. Law Implemented 403.4155 FS.

New 6-28-99.

New 6-28-99.