18 AAC 72.260. Community and alternate soil absorption systems

(a) The department will approve the plans for construction of a community or alternate soil absorption system if

(1) the submittal requirements of 18 AAC 72.205 are met;

(2) the design or design criteria for the following parameters meet the requirements of 18 AAC 72.220:

(A) design flows and waste loads;

(B) primary treatment and treatment area configuration and design;

(D) methods to control operational variables;

(E) methods and location of disposal of sludges, septage, and other facility residuals;

(F) thermal protection considerations;

(3) the provisions of 18 AAC 72.245(3) - (9) are met;

(4) a sufficient usable wastewater disposal area exists with characteristics for both an initial and a replacement soil absorption system as follows:

(A) the soils report under 18 AAC 72.265 shows that the soil types, percolation rates, and depths to seasonal high water table and impermeable strata are suitable for the type of soil absorption system selected;

(B) subject to (C) of this paragraph, the restrictions at 18 AAC 72.035(d) (8) and (9) are met;

(C) for an alternate soil absorption system, the ground surface slope, distance to escarpments, and the size of the infiltrative surface conform to standard sanitary engineering principles and practices; and

(D) for a conventional soil absorption system, the size of the minimum infiltrative area is calculated from the wastewater application rate in Table C of this subparagraph that corresponds to the observed soil texture or the observed percolation rate;

_____________________________________________________________________________ TABLE C WASTEWATER APPLICATION RATES FROM PERCOLATION TEST RESULTS _____________________________________________________________________________ Application Rate in gal/day/ft 2 Application Rate Observed for design flow in gal/day/ft 2 Percolation less than or for design flow Rate a equal to greater than minutes/inch Soil Texture 2,500 gal/day 2,500 gal/day Faster than 1 Gravel, coarse sand Not suitable b Not suitable b 1-5 Coarse to medium sand 1.2 0.79 - 0.98 6-15 Fine sand, loamy sand 0.8 0.61 - 0.74 16-30 Sandy loam, loam 0.6 0.52 - 0.61 31-60 c Loam, porous silt loam 0.45 0.25 - 0.52 61-120 d Silty clay loam, clay loam e Not suitable d Not suitable d

a. Percolation tests must be performed in accordance with 18 AAC 72.265(9) . Submitted plans may prorate the soil application rate to match observed percolation rates; soils faster than five minutes per inch may be visually rated.

b. Soils with percolation rates faster than one minute per inch may be used if a shallow absorption trench or bed system is used, if native soils are replaced by a layer of medium-graded sands, as defined in Note 1 below, that is no less than two feet deep, and if application rates less than or equal to 1.2 gallons/day/ft 2 are used; these systems are subject to department approval on a case-by-case basis if protective of public health, public and private water systems, and the environment; the department will waive the requirement for a sand liner in the manner set out in 18 AAC 72.060.

c. Soils with percolation rates slower than 30 minutes per inch are unsuitable for seepage pits.

d. Suitable for elevated mounds or conventional systems, with prior department approval.

e. Soils without expandable clays.

Note 1. Subject to a waiver under 18 AAC 72.060, sands for sand liners installed in soils with percolation rates faster than one minute per inch must meet all of the criteria in at least one of the two following groups of minimum specifications, using sieves with standard U.S. sieve numbers:

Group A: 85-100 percent passing a #10 sieve (less than 2.0 mm); 60-90 percent passing a #20 sieve (less than 0.850 mm); 25-50 percent passing a #40 sieve (less than 0.425 mm); less than or equal to 15 percent passing a #60 sieve (less than 0.250 mm); less than 5 percent passing a #200 sieve (less than 0.075 mm); and the sand must not have more than 45 percent of the total passing any one sieve and retained on the next consecutive sieve, of those listed.

Group B: The Coefficient of Uniformity (C u ) must be less than 4; the Coefficient of Curvature (C c ) must be equal to or less than 1; the amount passing the #10 sieve (less than 2.0 mm) must be greater than or equal to 85 percent of the total; the amount passing the #200 sieve (less than 0.075 mm) must be less than 5 percent of the total; and the sand must not have more than 45 percent of the total passing any one sieve and retained on the next consecutive sieve, of those listed in Group A.

(5) for a system with design flows equal to or greater than 2,500 gallons per day, calculations prepared in accordance with standard sanitary engineering principles and practices and sealed by a registered engineer demonstrate that nitrate concentrations in the groundwater aquifer most likely to be affected by the proposed system will not exceed five milligrams per liter, as nitrogen, beyond a distance measured from the edge of the absorption system to the property line of the system site, or to a point that the department identifies as necessary to protect public health, public and private water systems, and the environment;

(6) the soils report complies with 18 AAC 72.265;

(7) areas to be used for initial and replacement soil absorption systems are reserved for that purpose and are shown on the site plan; and

(8) there is adequate depth of burial, mounding above grade, or insulation to protect against frost penetration, with insulation equivalent to that listed for the applicable geographical area in Table A in 18 AAC 72.035(c) .

(b) In areas known or suspected to contain permafrost, and as necessary to protect public health, public and private water systems, and the environment, the department will require that plans for a community soil absorption system be sealed by a registered engineer.

History: Eff. 4/1/99, Register 149

Authority: AS 44.46.020

Editor's note: Examples of ground surface slope, distance to escarpments, and the size of infiltrative surface, as addressed in 18 AAC 72.260(a) (4)(C) and that conform to standard sanitary engineering principles and practices, can be found in the references listed at 18 AAC 72.070(c) . To prepare the calculations required under 18 AAC 72.260(a) (5), there are many groundwater modeling references to choose from, including the three listed at 18 AAC 72.070(c) (9), (11), and (14).

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