2.3 Environmental Site Constraints, Receiving Waters & Sensitive Areas

2.3 Environmental Site Constraints, Receiving Waters & Sensitive Areas

A comprehensive site assessment extends beyond grading and soil texture to inventory surrounding ecological, geological, and regulatory constraints. Construction projects do not exist in isolation; they interact directly with receiving waters, wetlands, vulnerable aquifers, and native vegetation. Identifying environmental constraints during the pre-planning phase allows the CPESC practitioner to avoid catastrophic off-site contamination, regulatory enforcement actions, and costly project shut-downs.

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Impaired Receiving Waters, CWA 303(d) Lists & TMDL Requirements

Under Section 303(d) of the Federal Clean Water Act (CWA), states, territories, and authorized tribes are required to compile a biennial list of surface waters that do not meet established Water Quality Standards (WQS) for their designated uses (e.g., aquatic life support, public drinking water supply, contact recreation).

Total Maximum Daily Loads (TMDLs)

For each waterbody listed on the 303(d) list, the regulatory authority must develop a Total Maximum Daily Load (TMDL). A TMDL defines the maximum assimilative capacity of a waterbody for a specific pollutant while still meeting water quality standards, mathematically expressed as:

$\text{TMDL} = \sum \text{WLA} + \sum \text{LA} + \text{MOS}$

• Waste Load Allocations (WLA): The portion of the pollutant load allocated to existing and future point sources, which legally includes construction stormwater discharges regulated under NPDES permits.
• Load Allocations (LA): The portion allocated to nonpoint sources and natural background levels.
• Margin of Safety (MOS): An accounting factor reflecting scientific uncertainty regarding pollutant loading and water quality response.

Compliance Obligations for Sediment- and Nutrient-Impaired Waters

When a construction site discharges stormwater into a waterbody listed as impaired for sediment, suspended solids, turbidity, or nutrients (nitrogen and phosphorus), or where an approved TMDL applies, the EPA Construction General Permit (CGP) and state NPDES general permits impose heightened regulatory mandates:

1. Accelerated Soil Stabilization: Operators must initiate stabilization immediately whenever earth-disturbing activities have permanently ceased or will be inactive 14 or more calendar days, and complete installation within 7 calendar days rather than the 14-day window that would otherwise apply. This impairment-based trigger runs in parallel with the acreage-based trigger — more than five acres disturbed at any one time also forces the 7-day completion deadline — so a large site discharging to an impaired water is on the 7-day clock twice over.
2. Increased Inspection Frequencies: Routine SWPPP inspections must be conducted at least once every 7 calendar days AND within 24 hours of the occurrence of a storm event generating $0.25\text{ inches}$ (or $0.50\text{ inches}$ depending on state permit) or greater of rainfall within a 24-hour period.
3. Enhanced Perimeter & Basin Controls: Mandatory use of redundant sediment barriers (e.g., double silt fence with wire backing, or compost filter sock placed behind silt fence) and advanced basin features, such as surface skimmers and floating baffle curtains.
4. Discharge Turbidity Monitoring: Under EPA CGP Part 3.3, sites dewatering to sensitive waters sample turbidity daily and compare the weekly average against the standard 50 NTU benchmark. Many state jurisdictions add their own numeric limits (commonly no more than 10% or 25 NTU above background turbidity).

Outstanding National Resource Waters (Tier 3 / ONRW) & Coldwater Fisheries

Under federal Antidegradation Policy (40 CFR 131.12), waters designated as Outstanding National Resource Waters (Tier 3 / ONRW)—such as waters of National Parks, wildlife refuges, and state wild and scenic rivers—receive the highest degree of protection. No permanent degradation of ambient water quality is permitted under any circumstances.

Similarly, coldwater trout and salmonid fisheries are extremely vulnerable to construction impacts:

• Thermal Pollution: Stormwater runoff heated by unshaded construction surfaces or held in shallow, unshaded sediment basins increases receiving stream temperatures, depleting dissolved oxygen levels critical to coldwater species.
• Intergravel Siltation: Silt and fine sediments settle into gravel spawning beds (redds), blanketing salmonid eggs and suffocating benthic macroinvertebrates that form the base of the aquatic food chain.

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Wetland Identification, Jurisdictional Delineation & Buffer Zones

Wetlands are transitional environments between terrestrial ecosystems and open waters that perform critical ecological functions, including flood attenuation, water filtration, groundwater recharge, and wildlife habitat. Discharging dredged or fill material into jurisdictional Waters of the United States (WOTUS), including wetlands, is strictly prohibited without a permit issued by the U.S. Army Corps of Engineers (USACE) under Section 404 of the Clean Water Act and associated state Section 401 Water Quality Certifications.

The Three-Parameter Jurisdictional Test

Per the 1987 USACE Wetland Delineation Manual and Regional Supplements, a site must satisfy all three mandatory diagnostic parameters simultaneously to be classified as a jurisdictional wetland:

Hydrophytic Vegetation  +  Hydric Soils  +  Wetland Hydrology  =  Jurisdictional Wetland

1. Hydrophytic Vegetation: The plant community is dominated by species that are physiologically adapted to survive and reproduce in anaerobic, water-saturated soils. Plant species are categorized into wetland indicator statuses:
   • Obligate Wetland (OBL): Occurs in wetlands $> 99%$ of the time under natural conditions.
   • Facultative Wetland (FACW): Usually occurs in wetlands ($67%\text{–}99%$ frequency).
   • Facultative (FAC): Equally likely to occur in wetlands or non-wetlands ($33%\text{–}67%$ frequency).
   • Facultative Upland (FACU): Usually occurs in uplands ($1%\text{–}33%$ in wetlands).
   • Upland (UPL): Rarely occurs in wetlands ($< 1%$ frequency). A wetland community satisfies the vegetative criterion if more than $50%$ of the dominant species are classified as OBL, FACW, or FAC (the 50/20 Dominance Rule).
2. Hydric Soils: Soils that formed under conditions of saturation, ponding, or flooding long enough during the active growing season to develop anaerobic conditions in the upper root zone. Hydric soil indicators include:
   • Low-Chroma Matrix: Soil matrix colors having a Munsell chroma of 2 or less (or 1 or less if mottles are absent).
   • Gleying: Bluish-gray, greenish-gray, or neutral gray colors resulting from the chemical reduction of iron from ferric ($ ext{Fe}^{3+}$) to ferrous ($ ext{Fe}^{2+}$) states.
   • Redoximorphic Features: Distinct orange, red, or brown iron/manganese concentrations occurring alongside depleted gray matrices.
   • Organic Soils (Histosols): Thick muck or peat layers saturated for long durations.
   • Sulfidic Odor: Distinctive "rotten egg" odor caused by hydrogen sulfide ($ ext{H}_2\text{S}$) gas produced by anaerobic sulfate-reducing bacteria.
3. Wetland Hydrology: The land is inundated or saturated to the surface by water at a frequency and duration sufficient to support hydric vegetation. Field indicators are classified into Primary and Secondary categories:
   • Primary Indicators: Visual observation of surface inundation, high water table within 12 inches of the surface, water-stained leaves, sediment deposits, drift deposits (wrack lines), oxidized rhizospheres along living plant roots, and water marks on trees.
   • Secondary Indicators: Drainage patterns, surface soil cracking, moss trim lines, and crayfish burrows.

Mandatory Vegetative Buffer Requirements

Under the EPA CGP, for any land disturbance occurring within 50 feet of a surface water or wetland, the operator must implement one of three compliance alternatives:

1. Provide and maintain an undisturbed 50-foot natural vegetative buffer.
2. Provide an undisturbed buffer of less than 50 feet complemented by additional structural BMPs (e.g., dual silt fences, compost filter socks, check dams) that achieve an equivalent sediment load reduction.
3. If maintaining any buffer is completely infeasible (e.g., bridge abutment construction, stream crossing installation), implement advanced sediment and turbidity controls designed to treat the entire volume of runoff discharging into the waterbody.

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Karst Topography & Sinkhole Hazards

Karst topography is a distinctive geological landscape formed by the dissolution of soluble carbonate bedrock—primarily limestone ($ ext{CaCO}_3$), dolostone ($ ext{CaMg(CO}_3)_2$), and gypsum ($ ext{CaSO}_4 \cdot 2\text{H}_2\text{O}$)—by carbonic acid present in natural groundwater.

Structural and Hydrologic Vulnerabilities

Karst terrain features underground caves, caverns, vertical solution conduits, losing (sinking) streams, and sinkholes (dolines). Karst environments present severe hazards to construction and environmental protection:

• Rapid Groundwater Conduit Flow: Unlike granular sand aquifers where groundwater moves at velocities of feet per year through tortuous pore spaces, groundwater in karst conduits moves at open-channel velocities of miles per day without any mechanical soil filtration.
• Direct Aquifer Contamination: Runoff entering a sinkhole or sinking stream discharges directly into regional potable aquifers within minutes or hours. Contaminants such as sediment, petroleum fuels, polycyclic aromatic hydrocarbons (PAHs), concrete washout slurry, and fertilizers bypass natural attenuation and pollute public drinking water supplies.
• Catastrophic Sinkhole Collapse: Concentrating stormwater runoff into unlined basins or trenches in karst areas accelerates subsurface soil piping. Downward percolating water washes overlying soil into bedrock solution voids, creating an underground cavity. As the cavity migrates upward toward the ground surface, the structural roof collapses catastrophically, destroying structures and draining basins.

Engineering and Siting Prohibitions in Karst Areas

• Prohibition of Infiltration BMPs: Unlined stormwater infiltration basins, infiltration trenches, and permeable pavements are strictly prohibited in karst zones.
• Impermeable Basin Liners: Any temporary sediment basin or sediment trap constructed in a karst area must be lined with an impermeable barrier, such as a Geosynthetic Clay Liner (GCL) or a minimum 60-mil HDPE geomembrane, backed by a non-woven geotextile cushion to prevent downward seepage.
• Setback Distances: Maintain a mandatory undisturbed vegetative setback buffer (typically 100 feet minimum) from the rim of any active or inactive sinkhole.
• No Direct Discharges: Runoff from construction activity must never be routed directly into sinkholes, caves, or swallets.

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Steep Slopes, Highly Erodible Land & Floodplains

Steep Slopes and Unstable Geological Formations

Slopes exceeding $15%\text{ to } 20%$ (approx. 5:1 to 3:1) are classified as critical erosion hazard areas. Disturbed steep slopes are prone to mass wasting, including rotational slumps, translational slides, and debris flows, especially when saturated. CPESC practitioners must require:

• Phased grading to minimize slope exposure.
• Immediate surface protection using bonded fiber matrices (BFMs) or turf reinforcement mats (TRMs) anchored with heavy-duty staples.
• Installation of interceptor dikes and slope drains at the top of slopes to prevent up-slope stormwater from cascading over bare cut/fill faces.
• Continuous monitoring of cut slopes for groundwater seepage (daylighting water tables), which triggers subsurface slope destabilization.

Highly Erodible Land (HEL)

Under USDA NRCS criteria, Highly Erodible Land (HEL) is defined as land capable of eroding at excessive rates based on the Erodibility Index (EI), calculated from the Universal Soil Loss Equation ($EI = R \times K \times LS / T \ge 8$). Construction on HEL soils requires enhanced BMP redundancy, shortened stabilization timeframes, and strict limits on concurrent acreage disturbance.

Acid-Producing Soils and Pyritic Geologies

Certain geological strata (e.g., black shales, coal seams, coastal marine clays) contain iron disulfide minerals, predominantly pyrite ($ ext{FeS}_2$). When exposed to atmospheric oxygen and water during construction excavation, pyrite oxidizes rapidly, generating sulfuric acid:

$2\text{FeS}_2 + 7\text{O}_2 + 2\text{H}_2\text{O} \longrightarrow 2\text{Fe}^{2+} + 4\text{SO}_4^{2-} + 4\text{H}^+$

This reaction drives soil and runoff pH down to $< 3.5$, leaching toxic heavy metals (aluminum, iron, manganese), preventing vegetative growth, and killing aquatic life in downstream waters. Identifying acid-producing soils during site assessment requires deep soil borings, specialized capping with impermeable clays, and extensive agricultural lime ($ ext{CaCO}_3$) buffering.

Floodplain Restrictions and FEMA 100-Year Base Flood Elevations

The Federal Emergency Management Agency (FEMA) delineates 100-year floodplains (Special Flood Hazard Areas - SFHA, Zone A or AE), which represent areas inundated by a flood having a $1%$ annual exceedance probability, defined by the Base Flood Elevation (BFE):

• Regulatory Floodway: The stream channel and adjacent floodplain that must remain unencumbered to discharge the 100-year flood without increasing flood heights by more than a designated height (typically 1.0 foot or 0.0 feet under "no-rise" rules). Land disturbance in the floodway is heavily restricted.
• Storage Prohibitions: Soil stockpiles, temporary sediment basins, construction staging yards, hazardous material tanks (diesel fuel, hydraulic fluid), and concrete washout pits must never be sited below the 100-year BFE where floodwaters can inundate them and wash pollutants downstream.

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Existing Vegetation Inventory & Root Protection Zones

Preserving established, healthy mature trees and native plant communities is the most cost-effective erosion control practice available. Undisturbed root systems provide high shear strength to hold soil together, while tree canopies intercept rainfall and reduce kinetic impact energy.

The Critical Root Zone (CRZ) and Dripline Geometry

Tree roots do not mirror tree canopies as taproots deep in the earth; instead, over $85%\text{ to } 90%$ of active feeder roots reside within the upper 12 to 18 inches of soil, extending horizontally well beyond the visible foliage canopy (dripline).

• Critical Root Zone (CRZ) Radius: Calculated as $1.0\text{ to } 1.5\text{ feet}$ of radial distance for every $1.0\text{ inch}$ of trunk Diameter at Breast Height (DBH), measured at 4.5 feet above the ground.
• Example: A specimen white oak with a 24-inch DBH possesses a CRZ radius of $24 \times 1.25 = 30\text{ feet}$ extending radially in all directions from the trunk ($60\text{ feet}$ total diameter).

Tree Protection Zones (TPZ) and Construction Restrictions

To preserve designated trees and natural vegetation buffers, CPESC plans must establish a physical Tree Protection Zone (TPZ):

1. Protective Fencing: Erect rigid, high-visibility barrier fencing (minimum 4-foot-tall blaze-orange poly fencing or chain-link fencing) along the outer perimeter of the CRZ prior to initiating any site clearing or equipment mobilization.
2. Prohibited Activities within the TPZ:
   • No vehicular traffic, parking, or equipment turnaround (prevents lethal soil compaction that destroys soil macropores and suffocates roots).
   • No trenching, grade cuts, or root severing.
   • No placement of fill dirt (as little as 2 inches of clay fill over a root zone can suffocate a mature tree).
   • No chemical storage, fuel staging, or concrete washout dumping.