This section includes the standards for wastewater retention systems for feedlot runoff. The basic concept of retention systems is to intercept and collect runoff and wastes from the animal feeding area until it can be disposed of via land application. Although retention systems are usually the most economical method of treatment, other systems will be evaluated on a case-by-case basis.
(a) Location. - (i) Groundwater shall be excluded from entering the wastewater pond or the wastewater flow into the pond.
- (ii) Ponds shall not be located within the ordinary highwater mark of perennial rivers, streams, or creeks. Ponds not containing hazardous or toxic wastes may be located within the ordinary high water mark of intermit tent rivers, streams, creeks, draws, coulees, or other natural drainages provided a by-pass ditch is installed capable of passing the 24-hour - 100 -year precipitation event.
- (iii) The wastewater retention system shall be as near to the animal feeding operation as possible to keep construction to a minimum. The retention ponds shall be located outside the pen area for safety and maintenance purposes. Sufficient space must be left between streams or drainage areas to allow construction of the necessary collection ditches and retention ponds.
(b) Basis of design. All livestock confinement areas, alleyways, etc., shall be graded to prevent accumulation of surface waters and to drain all contaminated water to the retention system. Collection ditches shall be provided when necessary to intercept contaminated water. The wastewater retention system shall be designed to contain the 25-year, 24-hour precipitation event. Wastewater in the retention pond shall be removed and disposed of as soon as possible after a precipitation event. The applicant shall demonstrate that equipment is available for removing the wastewater. - (i) Diversion ditches. The animal feeding area shall be protected with diversion ditches that will direct uncontaminated runoff from areas above and adjacent to the site away from the ponds and shall be capable of diverting the 25-year, 24-hour precipitation event.
- (ii) Collection ditches. Collection ditches shall be constructed around the feeding area to intercept the contaminated runoff and transport it to the settling and/or retention pond. The depth shall be adequate to handle the design flow and shall have a bottom slope sufficient to produce a velocity of not less than two (2) feet per second. Side slopes shall not be steeper than eight (8) horizontal to one (1) vertical.
- (iii) Settling pond. A settling pond ahead of the retention pond is recommended to accumulate the solids in the waste flow and to simplify their removal and final disposal. The surface area shall be sized to reduce the flow velocity below one (1) foot per second to allow settling of solids. The pond shall be between three (3) to six (6) feet deep to allow sufficient capacity for holding the solids and yet allow easy removal of the solids. The outlet structure shall minimize the overflow of solids into the retention pond.
- (iv) Retention pond. The retention pond shall be capable of containing all runoff from the feeding area for the design storm until the contaminated runoff can be disposed. If a settling pond is not provided before the retention pond, the design volume shall be increased by 10 percent to accommodate collection of solids.
(c) Retention pond layout. - (i) The shape and depth shall facilitate ease of cleaning and maintenance. A minimum freeboard of 1.5 feet shall be required above the high water level of the spillway.
- (ii) Spillways shall be provided on all retention ponds to pass flows in excess of the 25 year, 24-hour precipitation event. The spillway shall be placed above the design high water level.
(d) Retention pond construction. The retention pond construction shall meet the following requirements: - (i) Soils used in constructing the pond bottom and dike cores (not including the liner) shall be relatively incompressible, have a low permeability, and be free from organic material or trash. The soil shall be compacted at a water content that will insure structural stability, minimize hydraulic seepage, and minimize settling. The soil shall provide an adequate foundation for the liner, if used.
- (ii) On ponds that are not specified to receive an artificial liner, no rocks larger than six inches in length shall be permitted in any of the designated embankments.
On ponds that are specified to be lined with an artificial liner, rocks larger than six (6) inches in length shall not be placed within five (5) feet surface of the interior slope of any pond embankment. Material containing by volume less than 25 percent of rock larger than six (6) inches and less than twelve (12) inches in length may be placed in the remainder of the embankment.
- (iv) The minimum top dike width shall be eight (8) feet to permit access of maintenance vehicles. Top dikes wider than eight feet (8) shall be required when necessary to ensure structural stability.
- (v) The pond bottom may be sloped to facilitate pumping but shall not exceed a 0.5 percent slope.
(e) Liners. - (i) Seepage limits. The seepage through the pond bottom and side walls shall not cause a violation of the groundwater standards as described in Chapter 8 (Quality Standards for Wyoming Groundwaters) of the Wyoming Department of Environmental Quality, Water Quality Rules and Regulations. Liners shall be required if the wastewater characteristics or site conditions will not insure the protection of the groundwater for which it is classified.
If the applicant cannot document that the facility poses no threat to groundwater and elects not to perform a subsurface study in accordance with Chapter 3, Section 15(a) and (b), then the groundwater shall be protected from contamination by the wastewater with a liner equivalent to three (3) feet of soil having a permeability of 10-7 cm/sec or less. When an applicant performs a subsurface study, the requirement for the liner shall be determined based on the results of the study and the groundwater protection required. In no instance shall the maximum seepage rate exceed 1/8 inch per day in the primary pond(s).
- (ii) Soil and bentonite liners. The specifications for a soil or bentonite liner shall be based upon the results of a preliminary testing program and shall contain at a minimum the type of material, optimum and acceptable range in water content, acceptable range for compaction, and maximum allowable particle size.
Soil or bentonite liners used to protect groundwater quality shall meet the following criteria: Written certification that the soil liner was constructed in accordance with specifications shall be provided by a Wyoming registered professional engineer or an independent soils laboratory. Tests for water content and density shall be taken during application of each lift. Additionally, either permeability testing of undisturbed core samples from the in-place seal, or detailed tests such as particle size distribution and Atterburg limits confirming that the soil used in the liner construction was the same soil initially tested, shall be provided. In all cases, at least one test shall be provided per acre per lift, except for core sampling of the in-place liner, where one core of the completed liner shall be tested per acre.
- (iii) Synthetic liners. The thickness requirements for synthetic liners shall be determined on a case-by-case basis but shall not be less than 30 mils. The type of liner shall be compatible with the wastewater characteristics. The synthetic liner shall have a permeability equivalent to that of Section 29(e)(i).
Synthetic liners shall be anchored to prevent movement, slippage, and flotation. The synthetic liner shall be protected from degradation by ultraviolet light, ice damage and settling of underdrain trenches. An air venting system may be required beneath the synthetic liner to expel gases trapped during installation, produced by decomposing organic material, or produced by a fluctuating water table.
- (iv) Exfiltration evaluation. All ponds designated with a maximum exfiltration rate shall be tested for exfiltration. A maximum exfiltration rate not in excess of the design rate shall be deemed acceptable. If the exfiltration rate is deemed excessive, the seal shall be repaired, and the test procedure repeated. This procedure shall be repeated until the maximum exfiltration rate criteria is met. Results of all testing shall be submitted to the Department of Environmental Quality.
020-11 Wyo. Code R. § 11-29