PCI Insights : Septic Systems

SITE EVALUATION FACTORS

Effectively evaluating the site and optimally placing the onsite wastewater treatment system will maximize the lifespan and reduce operational and functional failures. The following factors are key design constraints when determining the feasibility of any site.

Soil Properties

Not too fast... Not too slow.

Soil properties critical to septic system performance include texture, permeability, and structure. Sandy and loamy soils typically allow for good wastewater infiltration and treatment, while clayey or compacted soils can impede drainage and lead to system failure. Key factors like percolation rate, depth to groundwater or bedrock, and presence of restrictive layers determine system suitability and design requirements.

Setbacks and Isolation Distances

Setbacks and isolation distances are required to protect public health and environmental resources by keeping septic components away from water sources, buildings, and property lines. In Utah, typical minimum distances include 10 feet from buildings, 50 feet from wells, and 100 feet from surface water bodies, though these can vary by system type and site conditions. Maintaining proper setbacks helps prevent contamination of drinking water and ensures access for maintenance.

Replacement Area

A replacement area is a designated section of land set aside for future use if the original drainfield fails or reaches the end of its lifespan. It must meet the same soil and setback requirements as the initial system and remain undeveloped and unaltered. Including a replacement area in the septic system design ensures long-term functionality and compliance with health regulations.

Slope

Slope is an important factor in septic system design, affecting how wastewater flows and disperses into the soil. Systems must be installed on slopes that allow for proper drainage without causing effluent surfacing or erosion—typically less than 25% grade for most systems. On sloped sites, special design considerations like step-downs, serial distribution, or pressure dosing may be required to ensure even distribution and safe operation.

Anticipated Ground Water & Surface Drainage

Anticipated groundwater and surface drainage must be evaluated to ensure the septic system remains effective and does not contaminate water sources. The system must maintain 24-48 inches of separation from the high seasonal groundwater table, and surface water must be directed away from the drainfield to prevent flooding or saturation. Proper grading, diversion ditches, and drainage features help protect the system and promote long-term function.

Bedrock or Impermeable Soils

Bedrock and impermeable soils, such as dense clay, limit the ability of wastewater to infiltrate and be treated naturally in the soil. Septic systems must maintain a minimum 48-inch vertical separation between the bottom of the drainfield and these restrictive layers. If adequate separation isn't possible, alternative or elevated systems like mounds or sand filters may be required.

VARIABLES THAT INCREASE/DECREASE COST

Soil properties and the percolation rate drive the loading factor used in sizing and designing the systems and have a direct impact to the cost. Other factors that may increase or decrease the cost include the maximum anticipated water table elevation, the type of septic tank selected, and other site specific factors.

CAUSES OF SYSTEM FAILURES

Onsite wastewater septic systems can fail as a result of several causes. These are typically either operational or functional failures.

Operational Failures are typically a result of wastewater moving too slow through the system. These failures most often directly impact the homeowner. Operational failures usually create an urgent problem for the homeowner since the sewage is no longer flowing.

Functional Failures are typically a result of wastewater moving too fast through the system. These failures typically affect the community through degraded water quality. Functional failures can exist unnoticed for extended periods of time.

Failures are most often a result of: