On-site Wastewater Treatment (Septic) in a Subdivision - PUB2226

A common question the Missouri Department of Natural Resources (MoDNR) receives is “What size lots do I need in a subdivision so I can use septic systems?” This publication is to help developers and other interested parties understand the Residential Housing Development Rule, found in Code of State Regulations 10 CSR 20-6.030, how to calculate minimum lot size (acres) for the use of individual onsite wastewater treatment (septic¹) systems in a subdivision and how a subdivision receives written approval for the method of wastewater treatment. 

As described in the Residential Housing Development Rule, new subdivisions need to receive written approval from MoDNR for the method of wastewater treatment when a developer proposes seven or more lots, each less than five acres and use individual on-site wastewater treatment systems on the individual lots. MoDNR written approval for the method of wastewater treatment is also required for any expansion of an existing subdivision by three or more lots, where each of those lots is less than five acres and use individual on-site wastewater treatment systems on those individual lots. As described in 10 CSR 20-6.030(7)(B), when a subdivision is required to receive written approval from MoDNR for the method of wastewater treatment it shall be received prior to the sale, lease or the commencement of construction on any lot by any developer(s) or owner(s).

Approval for the method of wastewater treatment under the Residential Housing Development Rule should not be confused with permitting any individual on-site wastewater treatment system on any individual lot within a subdivision. Once a subdivision receives written approval from MoDNR for the method of wastewater treatment, the developer(s) or owner(s) should contact the Missouri Department of Health and Senior Service or local on-site wastewater treatment authority, commonly the local health department, for assistance concerning their laws and regulations with respect to the construction, operation and maintenance of any individual on-site wastewater treatment system on any individual lot

The following is a brief explanation of the four steps a developer should follow to receive written approval from MoDNR for the method of wastewater treatment in a subdivision.

Step 1 - Geohydrological Evaluation

As described in 10 CSR 20-6.030(2) developers required to receive written approval for the method of wastewater treatment shall apply for a geohydrological evaluation conducted by MoDNR’s, Missouri Geological Survey, Environmental Geology Section to determine a minimum lot size (acreage) based on groundwater contamination potential only. This evaluation is performed as a service to a developer at no fee and while any individual can request an evaluation it must be signed by the owner of the property to allow MoDNR access for the evaluation. 

While the geohydrological evaluation will give a minimum acreage based on groundwater contamination potential only is not an approval it is just one step in your approval process. MoDNR suggest requesting and receiving the geohydrological evaluation first given the fact that no lots within a subdivision can be less than what is concluded in it unless a centralized wastewater collection and treatment system is proposed.

If the minimum lot size (acres) stated in the geological evolution is acceptable then the developer should contact a qualified soil scientist, as defined in Missouri State Statute 701.040(2)(e), RSMo² and have them conduct a detailed soils investigation, as described in 10 CSR 20-6.030(3). If the minimum lot size determined in the geohydrological evaluation is not acceptable then the developer should contact MoDNR or a registered professional engineer to discuss alternatives to individual on-site wastewater treatment systems³ as the method of wastewater treatment in the subdivision.

Step 2 - Soils Report

If the geohydrological evaluation indicates a minimum lot size (acres) acceptable to the developer then as described in 10 CSR 20-6.030(3) they should contact a soil scientist, as defined in 701.040(2)(e), RSMo to prepare a detailed soils report for the subdivision. The soils report can only be generated after a thorough, systematic investigation of the soil properties and landscapes in the proposed subdivision. The report must indicate if the proposed systems to be used on the individual lots are either a subsurface soil dispersal (absorption) system or individual lagoon that will serve no more than a single family residence.

The soil scientist can only perform the soil and site investigation by excavating soil observation pits (backhoe or hand dug) dug to a depth to reveal the major soil horizons. The minimum number of soil observation pits shall be one every 10 acres for lots proposed to be between two to five acres or in subdivisions where the majority of lots could be less than two acres, the minimum number of soil observation pits shall be one every five acres. Please remember these are just the minimum number of soil observation pits more may be needed depending upon the complexity of the soil - landscape model. Please note that while the geohydrological evaluation gives a minimum lot size based on groundwater contamination potential only, the soils investigation will assist in determining minimum lot size based on the soil and landscapes ability to treat and hydrologically control effluent within the subdivision.

At minimum soil profile descriptions for each soil observation pit excavated shall include the following:

• Describers name
• Date described
• Slope: aspect, gradient, shape and position
• Horizon: nomenclature, depth (i.e., thickness), boundary and matrix color
• Mottling: quantity, orientation and size
• Redoximorphic features: kind, quantity and size
• Texture: percentage clay and sand (if applicable)
• Structure: type, size and grade and consistence
• Rock fragment: size and percentage
• Water table: depth and kind
• Drainage class
• Flooding: frequency and duration
• Ponding: frequency, depth and duration
• Vegetative cover
• Other pertinent features related to the treatment and control of the effluent within the soil

As described in 10 CSR 20-6.030(3), the soils report shall also contain the following maps prepared by the soil scientist.

• A topographic map clearly delineating the following slope categories: 0 to 2%, 3 to 14%, 15 to 30% and 31% and greater.
• A map clearly delineating the depth of acceptable soil into the following categories: less than 18 inches, 18 to 30 inches over bedrock, 18 to 30 inches over a limiting layer and greater than 30 inches.

Once all of the relevant documentation is compiled, use Table 1: Minimum Lot Size (acres) for Soil Absorption Systems Based on Soil Depth and Slope, Acceptable Soil (inches), to determine the minimum lot size based on the soil and landscape investigation.

Step 3 - Plat Map

As described in 10 CSR 20-6.030(4), the developer shall provide a preliminary plat map drawn to a scale of one inch equals from 50 to 200 feet showing the location of the individual lots, roads, existing wells, and known or proposed easements. The number of lots, lot sizes as calculated from the geohydrological and soils investigation and type of water supply shall also be provided. The rule only requires a preliminary plat drawn as described in 10 CSR 20-6.030(4), it does require that any field surveying be conducted nor is it requiring that the plat is sealed by a professional land survey or a registered engineer. Any individual can produce the preliminary plat as long as it is legible and contains the minimum documentation described in rule.

As described in 10 CSR 20-6.030(7), the minimum lot size is calculated from the larger value of the geohydrologic evaluation or the soils report. However, there is flexibility as described in 10 CSR 20-6.030(3) when calculating lot size as determined in the soils report. For instance, if a lot can be configured in a manner where greater than 50% is of the lower acreage based on Table 1 then that lot can be the lower acreage category as long as it is not less than the minimum stated in the geohydrological evaluation. It should be noted that the lower acreage must be assessable and usable to treat and hydrologically control the effluent from an individual on-site wastewater treatment system on that lot. If that cannot be demonstrated then the minimum lot size would be the large value.

The following is an example on how to calculate minimum lot size for the use an individual on-site wastewater treatment system with subsurface soil (absorption) dispersal on individual lots. Please remember this is only an example; lot size will vary throughout the state depending upon the geology, soils and topography within any proposed subdivision.

Example:
A proposed development receives a geohydrological evaluation of 1.4 minimum acres based on groundwater contamination potential only. As previously described the geohydrological evaluation is not approval for the method of wastewater treatment but should be used as a baseline realizing that no lots within the subdivision will be less than it when proposing to use individual onsite wastewater treatment systems on individual lots.

Following the methodology described 10 CSR 20-6.030(3), for this example the soils report gave a 1.0 acre minimum on the ridge tops and shoulder slopes with the side slopes receiving a 2.0 acre minimum based on Table 1.

Therefore, as described in 10 CSR 20-6.030(3) and (7)(A) if a lot can be configured in a manner where greater than 50% is of the lower acreage as determined in Table 1 then that lower acreage can used for that lot as long as it is not less than the minimum acreage given in the geohydrologic evaluation. If that cannot be demonstrated then the minimum lot size would be the large value. Therefore, lot sizes for this example proposing subsurface soil dispersal system can range from 1.4 to 2 acres⁴. As previously stated lot size will vary throughout the state depending upon the geology, soils and the topography.

As described in 10 CSR 20-6.030(4) a copy of a US Geological Survey topographic map along with a copy USDA, Natural Resources Conservation Services, County Soil Survey map and legend with the area of the proposed subdivision clearly outlined must be provided along with the preliminary plat.

Step 4 - MoDNR Approval

After all of the necessary documentation, the geohydrologic evaluation, the soils report and the preliminary plat is collected it should then be submitted for approval. MoDNR performs the approval process as a service for the developer with no fee associated with it. To ensure the approval process proceeds in a timely manner and reduce any delays please submit all of the required documentation at one time. 

As described in 10 CSR 20-6.030(7)(B), when a subdivision receives written approval for the method for wastewater treatment it shall comply with all conditions set-forth in writing by MoDNR as contained in the Missouri Clean Water Law and corresponding regulations, prior to the sale or lease of any lot or the commencement of construction on any lot by any developer(s) or owner(s).

As described in 10 CSR 20-6.030(7)(C), once written approval is received there shall be no deviation or change that may adversely affect the geohydrologic evaluation, lot sizes, number of lots or the proposed water supply for the subdivision without first securing written approval of the proposed changes from MoDNR.

As described in 10 CSR 20-6.030(7)(G), noting in this rule shall excuse any person from complying with or from liability for violations of the Missouri Clean Water Law and its regulations or any other laws or Missouri.

The individual lots are to be used for single family residences only. If multiple family residences are proposed then the developer should contact MoDNR for assistance in receiving written approval for the method of wastewater treatment for them as described in 10 CSR 20-6.030(6) Multiple Family Housing Units.

Once written approval for the method of wastewater treatment is received for a subdivision, the developer(s) or other owner(s) should contact the Department of Health and Senior Services, Onsite Wastewater Treatment Program or the local on-site wastewater treatment authority for assistance concerning their laws and regulations with respect to the construction, operation and maintenance of any individual on-site wastewater treatment system on any individual lot⁵.

Summary

As described in 10 CSR 20-6.030(7)(B), when written approval from MoDNR for the method of wastewater treatment within a subdivision is required it shall be received prior to the sale, lease or the commencement of construction on any lot by any developer(s) or owner(s).

MoDNR understands there is an infinite number of “What if” scenarios that this publication cannot answer. If you have additional questions, please contact us for assistance before proceeding with your project. Do not let a misunderstanding or misinterpretation of any rule cause delays with your project.

Other developments regulated by the residential housing rule, 10 CSR 20-6.030 include recreational developments and multiple family housing units please contact MoDNR to determine if any of your projects may require approval.

¹For the purpose of this publication the term “septic” will not be used except as a component of an individual on-site wastewater treatment systems, i.e., septic tank, anaerobic treatment, etc.

²Please note that not all individuals listed with the Department of Health and Senior Services, On-site Wastewater Treatment Program as an On-site Soil Evaluator are qualified as a soil scientist, as defined in 701.040(2)(e), RSMo.

³Lot size cannot be mitigated by proposing a specific type of individual on-site wastewater treatment system on any specific lot.

⁴If local regulations do not prohibit it a lagoon (wastewater stabilization pond) serving no more than a single family residence on any individual lot can be used but the minimum lot size will be at least 2.5 acres. Lot sizes may be large depending upon the geohydrologic evaluation and/or soils report.

⁵ As described in 10 CSR 20-6.030(7)(A), approval under this rule does not guarantee that an individual on-site wastewater treatment system will be approved on each lot in a subdivision.