12VAC5-610-597: Fill systems

A. Fill systems are systems where the infiltrative surface and some portion of the treatment medium is comprised of fill material and not a naturally occurring undisturbed soil. Fill systems may be located in-ground, shallow-placed, or above ground. Fill systems addressed in these regulations are the Wisconsin Mound system, the noncarbonaceous mountain colluvium system, and the sand-on-sand system.

B. Elevated Sand Mounds. Septic tank effluent may be utilized with elevated sand mounds. Pretreatment shall be required when effluent strength exceeds residential strength wastewater and may be required where hydrologic conditions meet the minimum criteria contained in this chapter. For the purpose of siting an elevated sand mound, the criteria in Table 4.4 shall apply. For the purposes of establishing minimum setback distances between an elevated sand mound and various structures or topographic features, the mound shall be considered an absorption field and distances shown in Table 4.2 utilized.

C. Sand-on-sand systems. Sand-on-sand is a process of modifying a soil absorption system site using fill material which is similar in texture to the original, naturally occurring material. Filling is accomplished in accordance with 12VAC5-610-965.

1. Criteria for utilizing septic effluent. Septic tank effluent may be utilized with sand-on-sand systems. For the purpose of siting a sand-on-sand system, the criteria in Table 4.4 shall apply. Sand-on-sand systems may be utilized with septic tank effluent when the following criteria are met:

a. Soil texture. In order to assure effluent dispersal under adverse conditions, while maintaining adequate treatment capacity, shallow-placed systems are limited to Texture Group I and IIa soils. The use of Texture Group IIb, III and IV soils for sand-on-sand systems is prohibited.

b. Soil structure. Sand-on-sand is restricted to soils classified as entisols (i.e., a young soil with no horizon development) and which have a texture of sand, loamy sand, coarse sandy loam, or sandy loam texture.

c. Depth of soil. A minimum of 18 inches of naturally occurring undisturbed soil, measured from the ground surface, is required before encountering soils with bedrock, or a seasonal or permanent water table. Additionally, to assure adequate hydraulic dispersal capacity, no restrictive horizons may occur within 30 inches of the ground surface.

d. Separation distances. Table 4.2 contains the minimum setback distances between an absorption field and various structures or topographic features.

e. Estimated or measured infiltration rates. When siting a sand-on-sand system, the estimated or measured infiltration rate shall not exceed 30 minutes per inch within the sidewall area of the trench or within 18 inches of the infiltrative interface where effluent encounters undisturbed soil.

f. Slope. Sand-on-sand is prohibited where the slope of the original site exceeds 5%.

2. Criteria for utilizing secondary effluent.

a. Depth of soil. A minimum of 12 inches of soil, measured from the ground surface, is required before encountering bedrock, or a seasonal or permanent water table. Additionally, to assure adequate hydraulic dispersal capacity, no restrictive horizons may occur within 24 inches of the ground surface.

b. Separation distances. Table 4.2 contains the minimum setback distances between an absorption field and various structures or topographic features.

c. Estimated or measured infiltration rates. The estimated or measured infiltration rate shall not exceed 30 minutes per inch within the sidewall area of the trench, if any, or within 18 inches of the infiltrative interface where effluent encounters undisturbed soil.

D. Fill systems in mountain colluvium. The criteria for conventional, in-ground trench systems contained in Table 4.3 shall be complied with to the greatest extent possible. However, fill material consisting of colluvial soil derived from sandstone (noncarbonaceous) in the mountainous area may be considered on a case-by-case basis for placement of subsurface soil absorption systems.

Table 4.1.

Minimum Separation Distances for Pretreatment Units, Conveyance Lines, and Header Lines.

Structure or Topographic Features

Minimum Horizontal Distance

Property Lines

5

Building Foundations

10

Basements

20

Drinking Water Wells (all classes)

50

Cisterns Bottom Elevation Lower than Ground Surface in Area of Pretreatment Unit)

100

Shellfish Waters

70

Natural Lakes & Impounded Waters and Streams

50

Developed Springs (when the spring is down slope)

100

Drainage Ditches:

Ditch Bottoms above Seasonal Water Table

10

Ditch Bottom below Seasonal Water Table and Ditch Normally Contains Water

50

Lateral Ground Water Movement Interceptor

50

Low Point of Sink Holes When Placed within the Bowl of the Sink Hole

100

Utility Lines

10

Table 4.2.

Minimum Separation Distances.

Structure or Topographic Features

Soil Texture Group

Minimum Distance (Ft) from Bottom or Sidewall of Subsurface Soil Absorption System Trench

Vertical

Horizontal

Property Lines

I, II, III, IV

--

5

Building Foundations

I, II, III, IV

--

10

Basements

I, II, III, IV

--

20

Drinking Water Wells

Class IIIA or IIIB

I, II, III, IV

--

50

Class IIIC or IV

I, II, III, IV

--

100

Cisterns (Bottom Elevation Lower than Ground Surface in Area of Subsurface Soil Absorption System)

I, II, III, IV

--

100

Shellfish Waters

I, II, III, IV

--

70

Natural Lakes & Impounded Waters

I, II, III, IV

--

50

Streams

I, II, III, IV

--

50a

Developed Springs (when the spring is down slope)

I, II, III, IV

--

200e

Rock and Rock Outcropping

I

2

2

Rock and Rock Outcropping

II, III, IV

1.5

1.5

Pans and Impervious Strata

I, II, III, IV

1.5

1.5

Drainage Ditches:

Ditch Bottoms above Seasonal Water Table

I, II, III, IV

--

10

Ditch Bottom below Seasonal Water Table and Ditch Normally Contains Water

I

--

70a

II

--

70a

III

--

50a

IV

--

50a

Water Table Depressor System

I

6b

70

II

3b

70

III

2b

50

IV

2

50

Lateral Ground Water

I

--

70c 10d

Movement Interceptor

II

--

70c 10d

III

--

50c 10d

IV

--

50c 10d

Low Point of Sink Holes When Placed within the Bowl of the Sink Hole

I, II, III, IV

--

100

Utility Lines

I, II, III, IV

--

10

aThe set back distance may be reduced to 10 feet in Group III and IV soils and 20 feet in Group I and II soils if the subsurface soil absorption system is designed to produce unsaturated flow condition in the soil.

bVertical Distance to the invert of the drain tile in the water table depressor system.

cAbsorption trench up slope from interceptor.

dAbsorption trench down slope from interceptor.

eArc of 180 degree up slope of spring and 100 ft. down slope.

Table 4.3.

Summary of Separation Distances between Systems Using Naturally Occurring Undisturbed Soils and Limiting Site Factors.

Site Factor

In-Ground System1

Shallow-Placed System1

Septic Tank Effluent

Secondary Effluent

Septic Tank Effluent

Secondary Effluent

Bed Rock

18"

12"

n/a

18"

Restriction

18"

12"

n/a

18"

Shrink-Swell Soil

18"

12"

n/a

18"

Slope

50%

50%

n/a

50%

Perc Rate

5-120 mpi

5-120 mpi

n/a

5-45 mpi

Water Table

18"

12"

n/a

12"

1The separation distances for in-ground and shallow-placed systems are measured from the trench bottom or other infiltrative interface vertically down to listed site factor.

Table 4.4.

Summary of Separation Distances between Fill Systems and Limiting Site Factors.

Elevated Sand Mound

Sand-on-Sand System2

Noncarbonaceous Mountain Colluvium

Site Factor

Septic Tank Effluent

Secondary Effluent

Septic Tank Effluent

Secondary Effluent

Septic Tank Effluent

Secondary Effluent

Bed Rock

24"1

24"1

60"

60"

18"

12"

Restriction

24"

12"

30"

24"

18"

12"

Shrink-Swell Soil

24"

12"

40"

30"

18"

12"

Slope 25%

25%

25%

5%

5%

50%

50%

Perc Rate

5-120 mpi

5-120 mpi

5-30 mpi

5-30 mpi

5-120 mpi

5-120 mpi

Water Table

24"

10"

18"

12"

18"

12"

124 inches refers to creviced bedrock. This distance may be reduced to 12 inches when noncreviced bedrock is encountered. See the Wisconsin Mound Soil Absorption System Siting, Design, and Construction Manual, January 1990.

2The separation distance for sand-on-sand systems is measured from the ground surface vertically down to the listed site factor.