5.1 Soil Evaluation & Percolation Testing

5.1 Soil Evaluation & Percolation Testing

Soil evaluation is the foundational step in designing any onsite wastewater treatment system (OWTS). The soil serves as the ultimate physical, chemical, and biological filter for septic effluent before it reaches groundwater. An Environmental Health Specialist (EHS) must thoroughly understand soil science fundamentals and site evaluation techniques to prevent groundwater contamination and system failure.

Soil Science Fundamentals

Soil is a complex matrix of minerals, organic matter, water, and air. Its ability to absorb and treat wastewater depends heavily on its profile, texture, and structure.

Soil Horizons

A vertical cross-section of soil reveals distinct layers called horizons, created by weathering and biological activity over thousands of years. The standard horizons from the surface downward include:

• O Horizon (Organic): The topmost layer consisting of decomposing leaves, plant material, and organic matter. This layer is biologically active but usually too thin to play a role in wastewater treatment.
• A Horizon (Topsoil): A mixture of mineral soil and organic matter. It is typically dark and highly porous, making it excellent for biological activity, though it is often shallow.
• E Horizon (Eluviated): A heavily leached layer, typically lighter in color, where minerals and clays have been washed downward by percolating water.
• B Horizon (Subsoil): The zone of accumulation (illuviation). Clays, iron, and other minerals from upper layers gather here. This is often the most critical horizon for evaluating an absorption field, as it heavily dictates how well effluent will move and be treated.
• C Horizon (Parent Material): Partially weathered rock and minerals lacking the organic matter and structure of the layers above. It transitions into bedrock.
• R Horizon (Bedrock): Solid, unweathered rock. Effluent cannot readily pass through this layer without potentially moving untreated through fractures directly into aquifers.

Soil Texture

Soil texture refers to the relative proportions of sand, silt, and clay particles in a soil mass. These mineral particles vary significantly in size, determining the soil's porosity and surface area.

• Sand (0.05 - 2.0 mm): Large, gritty particles that create large pore spaces (macropores). Sandy soils drain rapidly and provide excellent oxygen transfer but offer minimal physical filtering and lower treatment capacity.
• Silt (0.002 - 0.05 mm): Medium-sized particles that feel smooth or floury. Silty soils have smaller pores, slowing water movement and increasing treatment contact time.
• Clay (< 0.002 mm): Microscopic, plate-like particles that are sticky when wet. Clay has a massive surface area and high chemical reactivity, but its tiny pore spaces (micropores) restrict water movement, often making pure clay unsuitable for conventional septic fields.

The USDA Soil Texture Triangle classifies soils into 12 textural classes (e.g., sandy loam, silty clay) based on these percentages. For onsite systems, loamy soils (a balanced mix of sand, silt, and clay) generally offer the best combination of drainage and treatment.

Soil Structure

While texture describes particle size, soil structure describes how those particles clump together into aggregates (peds). Structure heavily influences water movement through the soil profile.

1. Granular: Small, spherical peds, commonly found in the A horizon. Excellent for water movement.
2. Platy: Flat, horizontal, plate-like peds that impede the downward flow of water. Often found in compacted soils or the E horizon.
3. Blocky: Irregular, cube-like peds (angular or subangular) common in the B horizon. Good for moderate water movement.
4. Prismatic/Columnar: Tall, vertical pillars. Prismatic structures can allow rapid downward water movement along the cracks between peds, but the peds themselves may be dense clay.
5. Massive/Structureless: Solid mass with no distinct peds, such as dense clay or solid bedrock. Poor drainage.
6. Single Grain: Loose, non-cohesive particles, like beach sand. Very rapid drainage.

Soil Color and Drainage Indicators

Soil color is one of the most reliable field indicators of seasonal high water tables and drainage characteristics. Oxygen availability in the soil dictates the chemical state of iron and manganese.

• Well-Aerated Soils: Soils that drain well and contain ample oxygen allow iron to oxidize (rust). This results in vibrant, uniform red, yellow, or bright brown colors.
• Poorly Aerated Soils: In soils saturated by a seasonal high water table, oxygen is depleted by anaerobic bacteria. Iron is reduced (loses oxygen) and becomes water-soluble, washing away or turning the soil a dull gray, bluish, or greenish color.
• Redoximorphic Features (Mottling): Areas with fluctuating water tables exhibit a mix of oxidized and reduced conditions. This creates spots or blotches of contrasting colors, typically red or bright orange splotches against a dull gray background. The depth at which these redoximorphic features first appear indicates the seasonal high water table, a critical constraint for septic system design. Most codes require a minimum vertical separation (e.g., 2 to 4 feet) between the bottom of the septic trench and the seasonal high water table to ensure adequate treatment.

Percolation Testing and Permeability

While a soil profile evaluation visually assesses site suitability, a percolation test (perc test) measures the actual rate at which water drains through the soil.

Permeability vs. Percolation

It is important to distinguish between two related concepts:

• Permeability: A theoretical, quantitative measure of how easily a fluid can flow through a porous medium under standard conditions.
• Percolation: A field measurement of the rate at which water moves downward into the soil under specific test conditions.

The Percolation Test Procedure

The standard percolation test involves several controlled steps to mimic how an absorption field will behave when receiving effluent.

1. Preparation: Dig multiple test holes (typically 4 to 6 inches in diameter) across the proposed absorption field area to the depth of the planned trenches. The sides and bottom of the holes are scratched to remove any smeared soil surfaces that could artificially seal the pores. A layer of gravel is added to the bottom to prevent scouring.
2. Pre-soaking: The holes are filled with water and kept filled for a designated period (often 4 to 24 hours, depending on local code and soil type). Pre-soaking is essential because it allows the soil to swell and reach a state of field capacity, replicating the saturated conditions a septic system will face during wet seasons.
3. Measurement: After pre-soaking, the water level is adjusted to a specific depth (e.g., 6 inches above the gravel). The drop in water level is measured over strict time intervals (e.g., every 30 minutes).
4. Calculation: The final drop rates are used to calculate the percolation rate, expressed in Minutes Per Inch (MPI). For example, if the water drops 1 inch in 45 minutes, the rate is 45 MPI.

Determining Loading Rates

The percolation rate dictates the Soil Loading Rate, which is the maximum amount of effluent that can be applied to one square foot of soil per day, expressed in gallons per square foot per day (gpd/ft²).

• Fast Perc Rates (e.g., 10-15 MPI): Sandy loams with rapid drainage have high loading rates (e.g., 0.8 to 1.2 gpd/ft²). The absorption field can be smaller.
• Slow Perc Rates (e.g., 60-90 MPI): Clayey soils have slow drainage and low loading rates (e.g., 0.2 to 0.4 gpd/ft²). The absorption field must be significantly larger.
• Unsuitable Rates: If the rate is too fast (e.g., < 5 MPI), effluent may reach groundwater before being fully treated. If the rate is too slow (e.g., > 120 MPI), the system will likely fail hydraulically and surface. In either extreme, an alternative system or site modification is required.