2.2 Soil Identification, NRCS Soil Surveys & Hydrologic Soil Groups

2.2 Soil Identification, NRCS Soil Surveys & Hydrologic Soil Groups

Soil is the fundamental medium in erosion and sediment control. The physical, chemical, and hydrologic properties of on-site soils determine their susceptibility to detachment (erodibility), their ability to infiltrate rainfall, their vegetative establishment potential, and the ease with which suspended particles settle out in sediment basins. A thorough pre-construction soil inventory enables the CPESC practitioner to select appropriate BMPs and predict site runoff volumes accurately.

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Soil Particle Size Fractions & The USDA Textural Classification

Mineral soil is categorized into three primary particle size fractions (soil separates) according to the United States Department of Agriculture (USDA) classification system:

1. Sand (0.05 mm to 2.0 mm): Sand grains are visible to the naked eye, feel distinctly gritty when rubbed between the fingers, and lack cohesion when moist. Sand particles are further divided into very coarse ($1.0\text{–}2.0\text{ mm}$), coarse ($0.5\text{–}1.0\text{ mm}$), medium ($0.25\text{–}0.5\text{ mm}$), fine ($0.10\text{–}0.25\text{ mm}$), and very fine ($0.05\text{–}0.10\text{ mm}$). Soils high in sand exhibit rapid infiltration rates, low moisture retention, and high structural friction.
2. Silt (0.002 mm to 0.05 mm): Silt particles range from 2 to 50 microns in diameter. Silt feels smooth, floury, and velvety when dry, and slippery like wet talc when moistened, lacking true stickiness. Silt is the most erodible of all soil fractions. Because silt particles have low cohesive bonding yet small individual mass, raindrop splash detaches them with ease, and overland runoff readily transports them down-slope.
3. Clay (< 0.002 mm / < 2 microns): Clay consists of microscopic, plate-like secondary aluminosilicate minerals. Moist clay is sticky, highly plastic, and easily molded into ribbons. When dry, clay hardens into cohesive clods that resist mechanical detachment.

Specific Surface Area and Colloidal Behavior of Clays

As particle size decreases, the ratio of surface area to mass increases exponentially:

• Sand: Specific surface area is negligible, typically $0.01\text{ to } 0.1\text{ m}^2\text{/g}$.
• Silt: Specific surface area ranges from $1\text{ to } 10\text{ m}^2\text{/g}$.
• Clay: Specific surface area ranges from $10\text{ to } 800\text{ m}^2\text{/g}$ (e.g., kaolinite $\approx 10\text{–}20\text{ m}^2\text{/g}$; illite $\approx 80\text{–}100\text{ m}^2\text{/g}$; smectite/montmorillonite $\approx 600\text{–}800\text{ m}^2\text{/g}$).

Clay particles behave as colloids. Due to isomorphic substitution within their crystalline lattices, clay surfaces carry permanent negative electrical charges. In aqueous suspension, these negative charges attract positive counter-ions (cations such as $\text{Ca}^{2+}$, $\text{Mg}^{2+}$, $\text{Na}^+$), creating a diffuse electrical double layer. When suspended clay particles approach one another, their electrostatic fields repel them, preventing natural agglomeration.

According to Stokes' Law, the settling velocity of a spherical particle in water is directly proportional to the square of its particle diameter ($v_s \propto d^2$). While coarse sand ($0.5\text{ mm}$) settles through 1 meter of still water in approximately 5 seconds, and coarse silt ($0.02\text{ mm}$) settles in roughly 40 minutes, fine colloidal clay ($0.001\text{ mm}$) requires over 40 to 100 days to settle through the same depth under gravity alone. In active construction sediment basins, colloidal clays will remain in suspension indefinitely, producing turbid effluent that violates discharge limits unless treated with chemical coagulants or flocculants (e.g., polyacrylamide [PAM] or chitosan).

The USDA Soil Texture Triangle

The USDA Soil Texture Triangle graphically defines 12 standard textural classes based on the relative percentages of sand, silt, and clay. A soil composed of $40%$ sand, $40%$ silt, and $20%$ clay is classified as a loam—an ideal balance of moisture retention, nutrient capacity, and drainage.

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Soil Profile Horizons & Topsoil Preservation

A natural soil profile develops distinct horizontal layers termed soil horizons, shaped over millennia by climate, biological activity, topography, parent material, and time:

• O Horizon (Organic Layer): Surface layer dominated by partially decomposed organic matter, leaf litter, and humus. Provides natural mulch that buffers raindrop impact.
• A Horizon (Topsoil): The uppermost mineral horizon, darkened by enriched organic matter. Characterized by granular structure, high biological activity (microorganisms, mycorrhizal fungi, earthworms), high available water capacity, and the natural seedbank. This is the primary vegetative rooting medium.
• E Horizon (Eluvial / Leached Layer): Light-colored horizon beneath the A horizon where silicate clays, iron, and aluminum oxides have been leached down-slope by percolating water. Often sandy or silty with weak structure.
• B Horizon (Subsoil / Illuvial Layer): Zone of accumulation where clays, iron, and carbonates washed down from overlying layers concentrate. Typically has blocky or prismatic structure and lower saturated hydraulic conductivity ($K_{sat}$) than the A horizon.
• C Horizon (Substratum / Parent Material): Unconsolidated weathered parent material (weathered bedrock, alluvium, or glacial till) with minimal biological development. Low organic content and infertile.
• R Horizon (Bedrock): Consolidated hard bedrock (granite, limestone, sandstone, basalt) that cannot be excavated with standard earthmoving equipment.

Topsoil Stripping, Stockpiling, and Preservation Protocols

Subsoils (B and C horizons) exposed during bulk earthwork are deficient in organic matter and beneficial soil biota, making vegetative re-establishment exceptionally difficult. CPESC specifications must require:

1. Selective Topsoil Stripping: Strip the entire native A horizon (typically 4 to 12 inches) prior to rough cut/fill grading.
2. Stockpile Siting: Locate topsoil stockpiles outside active construction traffic zones, away from steep grades, and at least 50 feet away from natural waterways, wetlands, and drainage swales.
3. Stockpile Geometry & Stabilization: Maintain stockpile side slopes at $2:1$ (H:V) or flatter (preferably $3:1$). Apply temporary seeding and hydraulic mulch or rolled erosion control products within 14 calendar days (or 7 days in sensitive watersheds) if stockpiles will remain dormant for more than 14 days.
4. Perimeter Containment: Install compost filter socks or silt fence around the entire base perimeter of the stockpile to capture sloughed material.
5. Preserving Biological Viability: Avoid operating heavy machinery over topsoil stockpiles when wet to prevent severe compaction that destroys mycorrhizal fungi and creates anaerobic, sterile conditions.

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NRCS Web Soil Survey Interpretation

The Natural Resources Conservation Service (NRCS) provides digital soil geographic data through the Web Soil Survey (WSS). CPESC professionals utilize soil survey reports to identify:

• Map Units and Map Unit Symbols (MUSYM): Geographic delineations representing a specific soil series or soil complex (e.g., "CsB - Cecil sandy loam, 2 to 6 percent slopes").
• Depth to Restrictive Layers: Distance from the surface to impermeable layers such as lithic bedrock, dense basal till, or a fragipan (a dense, brittle subsurface horizon that perches water and blocks plant roots).
• Depth to Seasonal High Water Table (SHWT): The shallowest depth at which the soil is saturated for significant periods during wet seasons, identified in soil pits by redoximorphic features (mottles of bright red/orange iron concentrations and low-chroma gray/gleyed depletions). Siting infiltration basins or sediment basins below the SHWT causes continuous baseflow inundation, basin failure, and groundwater contamination.
• Soil Erodibility Factor (K): An empirical measure of soil susceptibility to particle detachment and transport by rainfall and runoff, ranging from 0.02 (least erodible) to 0.69 (most erodible).

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NRCS Hydrologic Soil Groups (HSGs)

The NRCS classifies soils into four major Hydrologic Soil Groups (HSGs)—A, B, C, and D—based on their minimum saturated infiltration rate after prolonged wetting, which directly determines their runoff potential.

Group A: Low Runoff Potential / High Infiltration

• Infiltration Rate: Greater than $0.30\text{ in/hr}$ ($> 8.0\text{ mm/hr}$) when thoroughly wetted.
• Soil Characteristics: Deep, well-drained to excessively drained sands, gravels, and loamy sands with high permeability ($K_{sat} > 5.0\text{ in/hr}$) and low runoff coefficients. Water is transmitted freely through the profile.
• CPESC Application: Exceptional suitability for post-construction stormwater infiltration basins and bio-retention cells. However, unstabilized coarse sands are prone to severe wind erosion and excessive gullying in concentrated flow channels.

Group B: Moderately Low Runoff Potential / Moderate Infiltration

• Infiltration Rate: $0.15\text{ to } 0.30\text{ in/hr}$ ($4.0\text{ to } 8.0\text{ mm/hr}$) when thoroughly wetted.
• Soil Characteristics: Moderately deep to deep, moderately well-drained to well-drained soils with moderately fine to moderately coarse textures (loam, silt loam, sandy clay loam). Moderate permeability and runoff generation.
• CPESC Application: Readily vegetated; responsive to standard erosion control and infiltration practices.

Group C: Moderately High Runoff Potential / Slow Infiltration

• Infiltration Rate: $0.05\text{ to } 0.15\text{ in/hr}$ ($1.0\text{ to } 4.0\text{ mm/hr}$) when thoroughly wetted.
• Soil Characteristics: Soils with an impeding layer that impedes the downward movement of water, or soils with moderately fine to fine textures (clay loam, silty clay loam). Saturated hydraulic conductivity is slow.
• CPESC Application: Generates substantial runoff volumes during moderate storm events. Requires enlarged sediment traps and sediment basins.

Group D: High Runoff Potential / Very Slow Infiltration

• Infiltration Rate: $0.00\text{ to } 0.05\text{ in/hr}$ ($0.0\text{ to } 1.0\text{ mm/hr}$) when thoroughly wetted.
• Soil Characteristics: Heavy clays with high shrink-swell potential, soils with a permanent or semi-permanent high water table, soils with a shallow claypan or hardpan near the surface, and shallow soils over nearly impervious bedrock.
• CPESC Application: Generates the highest peak runoff discharges and runoff volumes. Infiltration BMPs are strictly unfeasible. Turbid colloidal clay runoff requires extended detention, surface skimmers, and chemical flocculant treatment.

Dual Hydrologic Groups: A/D, B/D, and C/D

Certain wet soils are assigned dual hydrologic groups (e.g., B/D). The first letter represents the soil's hydrologic performance if artificially drained (via agricultural subsurface tile lines or perimeter drainage ditches), while the second letter (D) represents its performance in its natural, undrained condition. If a construction project destroys or removes existing agricultural subsurface drainage tiles, the soil immediately reverts to Group D hydrologic behavior, producing massive spikes in surface runoff volume.

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Practical Soil Texture Field Tests

When laboratory sieve and hydrometer data are unavailable, CPESC inspectors verify soil classifications using standard field methodology:

1. The Feel Test

Moisten a walnut-sized soil sample and rub it vigorously between the thumb and forefinger:

• Gritty: If the sample feels scratchy and makes an audible grating sound, sand dominates (> 50%).
• Floury / Velvety: If the sample feels exceptionally smooth and silky like cornstarch when moist, silt dominates.
• Sticky / Plastic: If the sample feels cohesive, adheres stubbornly to the skin, and shines when smeared, clay dominates.

2. The Ribbon Test

Knead a soil sample moistened to the consistency of putty. Roll the sample into a uniform cylinder approximately 0.5 to 0.75 inches in diameter. Gradually push the cylinder upward over the forefinger using the thumb, squeezing it into a flat ribbon of uniform thickness (approx. 1/8 inch), letting it extend over the edge until it breaks under its own weight:

• Less than 1.0 inch (< 2.5 cm) Ribbon: Coarse texture (sand, loamy sand, or sandy loam). Low clay content (< 15–20%).
• 1.0 to 2.0 inches (2.5 to 5.0 cm) Ribbon: Medium texture (loam, silt loam, sandy clay loam, or clay loam). Moderate clay content (20–40%).
• Greater than 2.0 inches (> 5.0 cm) Ribbon: Fine texture (clay, silty clay, or sandy clay). High clay content (> 40%). Shows high plasticity and produces a shiny surface when burnished with a fingernail.

3. The Sedimentation Jar Test

Place a representative 1-cup soil sample into a clear, straight-sided quart glass jar. Add water until the jar is 3/4 full, along with 1 teaspoon of liquid dishwashing detergent (which serves as a surfactant/dispersant to break clay clods into individual particles). Shake vigorously for 5 minutes and place the jar on a level surface away from vibrations:

• At 1 minute: All sand particles settle out to the bottom. Measure the thickness of the bottom sand layer.
• At 2 hours: All silt particles settle out, forming a distinct layer above the sand. Measure the combined sand + silt thickness.
• At 24 to 48 hours: Coarse clay particles settle, forming a fine, darker layer on top. Measure the clay thickness.
• Calculate the relative percentage of each layer to identify the soil textural class on the USDA Soil Texture Triangle.