New Mexico Administrative Code
Title 20 - ENVIRONMENTAL PROTECTION
Chapter 7 - WASTEWATER AND WATER SUPPLY FACILITIES
Part 3 - LIQUID WASTE DISPOSAL AND TREATMENT
Section 20.7.3.703 - DESIGN; AREA OF DISPOSAL FIELD AND SEEPAGE PITS
Current through Register Vol. 35, No. 18, September 24, 2024
A. The minimum required absorption area in a disposal field in square feet, and in seepage pits in square feet of side wall, shall be predicated on the liquid waste design flow rate and shall be determined by utilizing the following Table 703.1 based on the soil classification found in the proposed location of the disposal field.
B. Two test holes, located at opposite ends of the proposed disposal area, may be required for obtaining the soil profile and as provided in Subsection A of 20.7.3.203 NMAC.
C. A detailed soil profile, in accordance with USDA soil classification methodology, shall be submitted with the liquid waste application for each hole, indicating soil horizons, horizon thickness as a function of depth, and soil texture.
D. USDA soil surveys may be used where available to help assess typical soils in the area of the proposed installation.
E. The required absorption area shall be sized on the most restrictive soil horizon located below and within four feet of the bottom of the absorption area.
F. Conventional treatment systems shall not be constructed in type Ia soils where the depth to groundwater is less than 30 feet. For these soils, refer to 20.7.3.605 NMAC.
G. Effluent distribution to type IV soils shall be accomplished with an appropriate disposal method as approved by the department such as timed low pressure dosed distribution or alternating drainfields.
H. The required absorption area shall be calculated by the following formula: ABSORPTION AREA = Q X AR, where: Q = the design flow rate in gallons per day; AR = application rate (from Table 703.1)
Table 703.1: Application Rates by Soil Types for Conventional Treatment Systems
|
Soil Type |
Soil Texture |
Application Rate (AR) (sq. ft./gal/day) |
|
Ia Ib |
Coarse Sand or up to 30% gravel Medium Sand, Loamy Sand |
1.25 (See Subsection F of 20.7.3.703 NMAC) 2.00 |
|
II |
Sandy Loam, Fine Sand , Loam |
2.00 |
|
III |
Silt, Silt Loam, Clay Loam, Silty Clay Loam, Sandy Clay Loam |
2.00 |
|
IV |
Sandy Clay, Silty Clay, Clay |
5.00 (See Subsection G of 20.7.3.703 NMAC) |
I. The gravel content of in-place natural soil shall not exceed 30%. The department may identify and map areas of the state where groundwater is not at risk from microbial contamination from on-site liquid waste disposal systems that discharge into gravel, and where gravel contents greater than 30% may be allowed. The following hydrogeologic conditions may be considered when determining if groundwater is not at risk:
J. Disposal trenches:
K. Absorption beds may be used in lieu of trenches. The absorption area of the bed shall be at least 50% greater than the minimum required absorption area for trenches with a minimum of 450 square feet of absorption area. The total absorption area shall be calculated utilizing the total bed bottom and sidewall area.
L. The minimum effective absorption area in any seepage pit shall be calculated as the excavated side wall area below the inlet pipe exclusive of any hardpan, caliche, rock, clay or other impervious formations and may be provided in one or more seepage pits.
M. For secondary and tertiary treated effluent, the minimum calculated absorption area required for conventional treatment may be reduced 30%. In no case shall the maximum reduction for the drainfield absorption area exceed 30%.
