(a) Purpose. The purpose of this section is to describe the control facility design requirements that apply to concentrated animal feeding operations (CAFOs). Any CAFO operator that does not use a retention control structure (RCS) is not subject to subsections (e), (f), and (g) of this section.
(b) Well buffers. Except as provided by subsection (c) of this section, the control facility of an animal feeding operation (AFO) shall be separated from a well by ensuring a minimum buffer zone, as described in this subsection. An AFO shall not locate a new RCS or holding pen within the required well buffer zones:
  (1) public drinking water supply wells - 500 feet;
  (2) drinking water wells used for private water supply - 150 feet; or
  (3) water wells used exclusively for agriculture irrigation - 100 feet.
(c) Buffer variance. A CAFO operating under an existing authorization may continue the operation and use of any existing land management units (LMUs), holding pens and RCSs located within the required well buffer zones provided they are in accordance with the recharge feature evaluation and certification required under §321.34(f)(3) of this title (relating to Permit Applications). For new wells drilled after July 20, 2004, documentation supporting variances of the buffer zones that were previously authorized shall be kept on site and made available to agency personnel upon request.
(d) 100-year flood plain. All control facilities, including holding pens and RCSs, shall be located outside of the 100-year flood plain unless the facility is protected from inundation and damage that may occur during the 100-year flood event.
(e) RCS design capacity. The following design requirements apply to any CAFO.
  (1) The design of a control facility shall include measures that will be used to minimize entry of uncontaminated runoff into RCSs.
  (2) Any CAFO constructing a new or modifying an existing RCS shall ensure that the design specifications and completed construction specifications are certified by a licensed Texas professional engineer. The failure to obtain the certifications or to maintain records verifying the certifications is a violation of this subchapter.
  (3) Except as provided in this subsection, each RCS, at a minimum, shall be designed and constructed in accordance with the technical standards developed by the Natural Resources Conservation Service (NRCS), American Society of Agricultural and Biological Engineers, American Society of Civil Engineers, American Society of Testing Materials, or other technical standard approved by the executive director that are in effect at the time of construction. Where site-specific variations are warranted, a licensed Texas professional engineer shall document these variations and their appropriateness to the design.
  (4) Any existing RCS that has been properly maintained without any modifications and has no apparent structural problems or leakage is considered to be properly designed with respect to the RCS sizing, embankment design and construction, and liner requirements of this section, provided that any required documentation was completed in accordance with the requirements at the time of construction. If no documentation exists, the RCS must be certified by a licensed Texas professional engineer as providing protection equivalent to the requirements of this section.
  (5) Any RCS documented to have been built in accordance with site-specific NRCS plans and specifications is considered to be in compliance with the design and capacity requirements of this subchapter provided that:
    (A) the site-specific conditions are the same as those used by the NRCS to develop the plan (numbers of animals, runoff area, manure, sludge, and wastewater generated, etc.); and
    (B) the RCS is operated and maintained in accordance with NRCS requirements.
  (6) The production area of a new or expanding AFO shall not be constructed in any stream, river, lake, wetland, or playa, except as provided in §321.41 of this title (relating to Special Requirements for Discharges to a Playa).
  (7) The design plan must include documentation of the sources of information, assumptions, and calculations used in determining the appropriate volume capacity of the RCSs. Poultry (chickens and turkeys), swine, or veal calf CAFOs subject to the new source performance standards in subparagraph (B) of this paragraph shall be designed in accordance with subparagraphs (B) and (C) of this paragraph or subparagraphs (B) and (D) of this paragraph. For all other CAFOs, the volume must include design rainfall event runoff and normal operating capacity requirements in accordance with subparagraphs (A) and (C) of this paragraph or design rainfall event runoff and evaporation systems in accordance with subparagraphs (A) and (D) of this paragraph.
    (A) Design rainfall event runoff. All CAFOs, other than poultry (chickens and turkeys), swine, or veal calf CAFOs subject to the new source performance standards in subparagraph (B) of this paragraph, shall have an RCS designed and constructed to meet or exceed the capacity required to contain the runoff and direct precipitation from the 25-year, 24-hour rainfall event, except as required by §321.42(c) of this title (relating to Requirements Applicable to the Major Sole-Source Impairment Zone) or authorized under §321.37(d)(3) of this title (relating to Effluent Limitations for Concentrated Animal Feeding Operation (CAFO) Production Areas).
    (B) New source swine, veal, or poultry (chickens and turkeys) CAFOs. Any swine, veal, or poultry (chickens and turkeys) CAFO subject to the new source performance standards in 40 Code of Federal Regulations (CFR) §412.46 shall have an RCS designed and constructed such that no discharge will occur in accordance with the following:
      (i) Information used in the design of the RCS shall include, but is not limited to, the following: design rainfall event, additional minimum capacity for chronic rainfalls identified in the evaluation required by clause (ii) of this subparagraph, the requirements of subparagraph (C) or (D) of this paragraph, additional storage capacity for wastewater intended to be transferred to another recipient at a later time, and any other factors that would affect the sizing of the RCS.
      (ii) An evaluation of the adequacy of the designed RCS using the most recent version of the Soil Plant Air Water (SPAW) Hydrology Tool, or other tool approved by the executive director. The evaluation must include all inputs to SPAW including, but not limited to, daily precipitation, temperature, and evaporation data for the previous 100 years, user-specified soil profiles representative of the LMUs, planned crop rotations consistent with the nutrient management plan, and the final modeled result of no discharges from the designed RCS. For those CAFOs where 100 years of local weather data is not available, a simulation with a confidence interval analysis conducted over a period of 100 years may be used.
    (C) Design capacity requirements for systems using irrigation.
      (i) The RCS shall be designed for the authorized number of animals to include any storage volume required by a hydrologic needs analysis (water balance) that documents that the typical irrigation demands of the adopted crop and irrigated land area will not be exceeded.
      (ii) Precipitation inputs to the water balance shall be the average monthly precipitation reported in a National Weather Service current publication.
      (iii) The consumptive use requirements of the cropping system shall be developed on a monthly basis, and shall be calculated as a part of the water balance.
      (iv) The maximum required storage value calculated by the water balance shall not encroach on the storage volume required for the design rainfall event.
      (v) Wastewater application rates used in the water balance shall not induce uncontrolled runoff or create tailwater that causes a discharge.
      (vi) All process-generated wastewater produced during a 21-day or greater period.
      (vii) Any other relevant volume needed in the water balance, including any required under the air standard permit in §321.43 of this title (relating to Air Standard Permit for Animal Feeding Operations (AFOs)).
    (D) Design requirements for evaporation systems. Evaporation systems shall be designed:
      (i) to withstand a ten-year (consecutive) period of maximum recorded monthly rainfall (other than catastrophic). In any month in which a catastrophic rainfall event occurs, the water balance shall replace such an event with not less than the long-term average rainfall for that month as determined by a water balance; and
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