Statutes, codes, and regulations
Rhode Island Administrative Code
Title 250 - Department of Environmental ManagementChapter 150 - Water ResourcesSubchapter 10 - Wastewater And Storm Water
Part 6 - Rules Establishing Minimum Standards Relating to Location, Design, Construction and Maintenance of Onsite Wastewater Treatment Systems
Section 250-RICR-150-10-6.33 - Leachfields

250 R.I. Code R. 250-RICR-150-10-6.33

Download
PDF
Current through December 26, 2024
Section 250-RICR-150-10-6.33 - Leachfields
A. Applicability - This Rule applies to leachfields with dispersal trenches (§ 6.34 of this Part), leachfields with concrete chambers in a trench configuration (§ 6.35 of this Part), and to Alternative or Experimental leachfield systems approved pursuant to § 6.41 of this Part except for specifically approved design elements that are not consistent with this § 6.33 of this Part.
B. Minimum Leaching Area - The minimum leachfield area necessary for dispersal trench and concrete chamber leachfields shall be determined by dividing the maximum daily wastewater flow (design flow) for the facility, as determined from § 6.22 of this Part, by the loading rate established in § 6.33(C) of this Part. Use the lowest loading rate obtained in the manner described below:
1. If the bottom of the stone is above the original grade, use the soil horizon with the lowest loading rate within five feet (5') of the original ground surface, excluding any A horizons;
2. If the bottom of the stone is below the original grade, use the soil horizon with the lowest loading rate within five feet (5') below the elevation of the distribution pipe invert, including the soil horizons receiving side wall effluent;
3. If no natural soil exists within the five feet (5') referenced in § 6.33(B)(1) and (2) of this Part above because of gravel fill, use the loading rate of the first naturally occurring soil horizon below that depth.
C. Loading Rates Determined by Soil Category

Soil Category

Loading Rate (gals/sq ft/day)

1 (Note 1)

.70

1m (Note 2)

.61

2

.61

3

.70

4

.61

4m

.70

5

.52

6

.61

6m

.70

7

.52

7m

.61

8

.46

8m

.48

9

.40

9m

.43

10

Not Allowed (Impervious)

Notes:

(1) For OWTS Applications for Repair, a loading rate of .93 may be used for Soil Category 1.

(2) "m" Means soil has gravelly or channery coarse fragment modifiers.

D. Effective Leaching Area - The effective leaching area of OWTSs shall be determined in accordance with § 6.34 of this Part for dispersal trench OWTSs and § 6.35 of this Part for concrete chamber OWTSs.
E. Depth to Groundwater from Original Ground Surface - Using the test holes required in § 6.16(I) of this Part, the seasonal high groundwater table determination that is closest to the original ground surface shall be used for OWTS design. The leachfield shall be located in an area where the seasonal high groundwater table is a minimum of two feet (2') below the original ground surface. All test holes within twenty-five feet (25') of the leachfield shall meet the minimum depth to groundwater from original ground surface. Where the seasonal high groundwater table is less than four feet (4') below the original ground surface, the Department shall require additional soil testing to ensure the minimum depth to groundwater meets the minimum standards within twenty-five feet (25') of the leachfield. On lots twenty thousand (20,000) square feet or larger that are not located in the Salt Pond or Narrow River Critical Resource Areas as defined in § 6.42 of this Part, the leachfield may be located in an area where the seasonal high groundwater table is less than twenty-four inches (24") but greater than or equal to eighteen inches (18") from the original ground surface if the OWTS utilizes a bottomless sand filter in accordance § 6.37 of this Part, the applicant has no variance requests pursuant to § 6.51 of this Part, and the design flow is less than or equal to six hundred ninety (690) gallons per day of residential strength wastewater.
F. Depth to Restrictive Layer or Bedrock from Original Ground Surface - The leachfield shall be located in an area where a restrictive layer or bedrock is a minimum of four feet (4') below the original ground surface. The minimum depth to a restrictive layer or bedrock shall be met within twenty-five feet (25') of all sides of the leachfield.
G. Leachfield Design Point - Where the seasonal high groundwater table is greater than or equal to four feet (4') below the original ground surface, the leachfield shall be designed using the original ground surface elevation at the center of the leachfield. Where the seasonal high groundwater table is less than four feet (4') below the original ground surface, the leachfield shall be designed using the highest original ground surface elevation within the leachfield.
H. OWTS Separation Distance to Groundwater - The bottom of the stone underlying the leachfield (or surface upon which the biomat develops) shall be at least three feet (3') above the seasonal high groundwater table.
I. OWTS Separation Distance to a Restrictive Layer or Bedrock - The bottom of the stone underlying the leachfield (or surface upon which the biomat develops) shall be at least five feet (5') above a restrictive layer or bedrock. This five foot (5') vertical separation shall be maintained to a distance of twenty-five feet (25') from the side wall of the leachfield. In the upgradient direction, the five foot (5') vertical separation requirement may be waived as long as a restrictive layer or bedrock is no higher than the bottom of the stone within twenty-five feet (25') of the leachfield (§ 6.59 of this Part, Figure 1). Excavating into a restrictive layer or bedrock is not permitted unless otherwise approved by the Director.
J. Excavation - All trees, brush and stumps within the area of the leachfield and within ten feet (10') of the leachfield shall be removed. Care must be taken to assure that the soil at the bottom and sides of the excavation for the leachfield is not compacted or smeared. The bottom of the excavation shall be level and the bottom and sides of the excavation shall be scarified. In no case shall exposed boulders in the walls or bottom of the excavation be left in place. Voids created by the removal of boulders shall be filled with gravel meeting the requirements in § 6.33(M) of this Part. Exposed roots within the excavation shall be cut back to the walls of the excavation. No part of the excavation for the leachfield shall be into groundwater. All storm deposited sand in the backdune environment and human transported material existing in the proposed leachfield and five feet (5') around and below shall be removed prior to OWTS installation unless the material is deemed to be acceptable to the Director.
K. Stone - The stone used in the leachfield shall consist of clean, washed stone ranging from three quarter inch (3/4") to not more than two inches (2") in size and free of fines, soils, stone dust or debris. The stone shall be covered with a layer of synthetic filter fabric that meets the requirements of § 6.33(L) of this Part.
L. Filter Fabric - A layer of non-woven synthetic filter fabric shall be placed over all stone used in the OWTS construction before backfilling. The filter fabric shall have adequate tensile strength to prevent ripping during installation and backfilling, adequate air permeability to allow free passage of gases, and adequate particle retention to prevent downward migration of soil particles.
M. Gravel
1. The gravel base material and, where applicable, the gravel between the trenches shall consist of clean sand and gravel free of organic matter and foreign substances. The gravel shall not contain any material larger than three inches (3") and up to ten percent (10%) may be sized between three quarters of an inch (3/4") and three inches (3"). Gravel shall meet the following criteria:

Sieve Size

Percent Passing

3/4"

90% - 100%

#4

55% - 100%

#10

40% - 100%

#40

10% - 50%

#100

0% - 20%

#200

0% - 5%

2. The gravel shall be placed in shallow lifts and properly compacted. The surface of the gravel upon which the stone will be laid shall be level and scarified.
N. Depth of Cover - The minimum cover over the invert of the distribution lines shall be one and one half feet (1 1/2') and the maximum cover shall be two and one half feet (2 1/2'). Minimum cover elevations shall be maintained over the full area of the leachfield, including area excavated pursuant to § 6.34(F)(1) of this Part.
O. Backfill - All backfill shall be free of boulders and stones greater than six inches (6") in diameter, frozen clumps of earth, rubbish, masonry, stumps or waste construction materials. Backfill shall be placed carefully to avoid displacement and damage to piping and chambers. Heavy machinery shall not be permitted to pass over the leachfield.
P. Fill Easements - Where human transported material ("fill") is required and where it is necessary to fill beyond the boundary of the subject property to meet the requirements of these Rules, no approval will be granted unless the adjoining property owner(s) has given a permanent legal release (easement, etc.) filed in the land evidence records of the municipality granting such right to the owner of the applicant property. A copy of such right of access and use shall be attached to the application.
Q. Adjacent Side Slope - A minimum ten foot (10') horizontal separation distance shall be provided between the outer edge of the stone in the outer dispersal trench and any ground surface elevation less than the elevation of the invert of the distribution line. The adjacent side slope shall not be steeper than 3:1 (horizontal: vertical) for a twenty-five foot (25') minimum distance from the edge of the stone in the dispersal trench or until the toe of the slope returns to the elevation of the original grade. The toe of the 3:1 slope shall be a minimum of five feet (5') from any property line.
R. Structural Retaining Walls - A minimum of twenty-five feet (25') shall be maintained from the outer edge of the stone in the outer dispersal trench to any structural retaining wall. If the structural retaining wall is above the seasonal high groundwater table a lesser setback distance may be allowed. A greater setback distance may be required for OWTSs with a design flow of one thousand (1,000) gallons per day or more. The Department may require additional information, including but not limited to an analysis of the hydrogeological conditions of the site. Structural retaining walls shall be a minimum of two feet (2') from the property line unless the adjacent property owner grants the applicant written permission or a construction easement to provide access to install the wall. The wall design shall ensure adequate control of surface water runoff. The Director may require that the structural retaining wall be designed by a Professional Engineer registered in Rhode Island.
S. Surface Water Drainage - OWTSs shall be designed to prevent the flow of surface water from the surrounding area onto the leachfield. The OWTS design shall provide for diversion of surface water runoff so as not to cause or increase the severity of drainage problems to adjacent properties.
T. Leachfield Protection
1. Curbing - OWTSs serving other than individual dwellings shall be adequately curbed or fenced so as to exclude vehicular traffic, unless the OWTS is a concrete chamber OWTS constructed in accordance with § 6.35(L) of this Part. Parking areas adjacent to a leachfield shall be graded or curbed to divert runoff from the leaching area.
2. The boundary of the leachfield shall be staked and flagged to protect the leachfield from vehicle traffic and excessive weight loads before and during construction of the OWTS and the structure. Flagging shall remain in place until all construction activities at the site are complete.
U. Finish Grade Stabilization - Immediately after completion of final grading, the area of disturbance due to installation of the OWTS shall be stabilized by mulching and seeding, or sodding, to establish a permanent vegetative cover to prevent erosion.

250 R.I. Code R. 250-RICR-150-10-6.33

Amended effective 11/25/2018
Amended effective 12/28/2021
Amended effective 7/1/2022