7.2 Sampling Preparation and Analytical Parameters

Advanced Planning for Stormwater Sampling Events

Stormwater monitoring cannot be improvised after dark clouds gather over a jobsite. Because qualifying storm events are transient, unpredictable, and frequently occur outside normal weekday business hours, advanced planning the approved practice detail is essential to collect legally defensible samples.

Developing a Written Sampling and Analysis Plan (SAP) / QAPP

Before earth disturbance or monitoring begins, the project team should establish a written Sampling and Analysis Plan (SAP)—often integrated into the SWPPP or accompanied by a formal Quality Assurance Project Plan (QAPP). A complete SAP specifies:

1. Exact Monitoring Station Locations: Mapped GPS coordinates and physical descriptions of every permitted outfall (Outfall-001, Outfall-002), dewatering treatment discharge point, and upstream/downstream receiving water reference stations.
2. Qualifying Storm Event Criteria: The exact precipitation depth or runoff condition that triggers sampling (for example, a storm producing actual discharge from the outfall, or $\ge 0.25\text{ inch}$ / $\ge 0.50\text{ inch}$ of rainfall with at least a $48\text{- to }72\text{-hour}$ antecedent dry period, depending on the applicable state permit).
3. Target Analytes and Approved Analytical Methods: Every parameter to be measured in the field or analyzed in the laboratory, citing approved 40 CFR Part 136 EPA or Standard Methods for the Examination of Water and Wastewater procedure numbers.
4. Field Personnel Assignments and Safety Protocols: Named primary and backup CESSWI samplers, 24-hour weather tracking assignments, lightning and high-flow abort criteria, and personal protective equipment (PPE).
5. Quality Assurance / Quality Control (QA/QC) Schedule: Required frequency of field duplicates, equipment rinsate blanks, trip blanks, and daily instrument calibrations.

Pre-Storm Laboratory Coordination and Field Kit Staging

A CESSWI preparing for a forecasted storm event must complete four critical logistical steps before rainfall begins:

• Certified Laboratory Coordination & Container Selection: Confirm that the receiving laboratory holds current state or NELAP / TNI accreditation for every requested Clean Water Act parameter. Obtain pre-cleaned, analyte-specific bottle kits directly from the laboratory:
  • Borosilicate Glass (Amber or Clear Wide-Mouth with PTFE-lined caps): Required for organic compounds, Oil and Grease, and Total Petroleum Hydrocarbons ($\text{TPH}$) because hydrocarbons and organic solvents adsorb onto or leach plasticizers from plastic bottles.
  • High-Density Polyethylene (HDPE) or Polypropylene: Used for Total Suspended Solids ($\text{TSS}$), nutrients, general minerals, and trace metals (acid-washed HDPE ensures trace metals do not leach from glass silicates).
  • Sterile Plastic or Glass with Sodium Thiosulfate ($\text{Na}_2\text{S}_2\text{O}_3$): Required for microbiological bacteria (E. coli, fecal coliform) to dechlorinate the sample immediately.
• Preservatives, Labels, and Custody Supplies: Verify that pre-measured chemical preservative ampoules (such as nitric acid $\text{HNO}_3$ or sulfuric acid $\text{H}_2\text{SO}_4$) match the bottle labels; stage waterproof labels, indelible ink pens, blank Chain of Custody (COC) forms, tamper-evident Custody Seals, heavy-duty Ziploc bags, bubble wrap, insulated coolers, a temperature blank vial, and a dedicated supply of wet ice.
• Holding Time & Weekend/Holiday Receiving Verification: Cross-check the maximum regulatory holding time of every parameter against the laboratory's operating schedule. If a storm occurs on a Friday afternoon and the permit requires a parameter with a 6-hour, 24-hour, or 48-hour holding time (such as bacteria, unpreserved nitrate, or laboratory turbidity), the sample will expire before Monday morning unless the inspector has pre-arranged Friday evening delivery and weekend laboratory reception!
• Safe Physical Access to Outfalls: Inspect outfall paths during dry weather to clear brush, install stable stepping stones or safety lines, mount staff gauges, and confirm that samplers will never have to wade into deep, fast-moving channels or climb slick riprap slopes during a storm.

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Target Water Quality Constituents and Analytical Parameters

1. Turbidity ($\text{NTU}$) vs. Total Suspended Solids ($\text{TSS}$, $\text{mg/L}$) vs. Settleable Solids ($\text{mL/L}$)

Sediment is the single largest pollutant by volume generated on construction sites, and permits evaluate sediment in water through three distinct analytical measurements that candidates must never confuse:

• Turbidity ($\text{NTU}$ — Nephelometric Turbidity Units): Turbidity is an optical measurement of water clarity (cloudiness or haziness). A nephelometer (portable field turbidimeter) directs a focused beam of light through a glass sample vial and measures the intensity of light scattered at a $90^\circ$ angle by suspended particles in the water column—including fine clay, colloidal silt, finely divided organic and inorganic matter, and microscopic plankton. The higher the concentration of fine light-scattering particles, the higher the $\text{NTU}$ reading.
• Total Suspended Solids ($\text{TSS}$, $\text{mg/L}$): $\text{TSS}$ is a gravimetric (mass-per-volume) laboratory measurement (Standard Methods 2540 D / EPA Method 160.2). In the laboratory, a precisely measured volume of well-mixed stormwater is vacuum-filtered through a pre-weighed standard $1.5\text{ }\mu\text{m}$ glass-fiber filter. The filter and retained solid residue are dried in an oven at $103\text{ to }105^\circ\text{C}$ until they reach constant weight, cooled in a desiccator, and weighed on an analytical balance. The mass increase divided by the sample volume yields $\text{TSS}$ in milligrams per liter ($\text{mg/L}$).
• Settleable Solids ($\text{mL/L}$): Settleable solids measure the volume of heavier coarse sand, silt, and flocculated aggregates that settle out of suspension under gravity within 1 hour. A well-mixed $1\text{-liter}$ sample is poured into a graduated cone-shaped glass vessel called an Imhoff cone (Standard Methods 2540 F). The sample settles quiescently for $45\text{ minutes}$, the inspector gently stirs or spins the cone sides with a rod to dislodge clinging particles, and after $15\text{ additional minutes}$ ($60\text{ minutes}$ total), the volume of settled sediment at the cone tip is read directly in milliliters per liter ($\text{mL/L}$).

2. pH ($0\text{ to }14$ Logarithmic Scale)

pH measures the hydrogen ion activity ($\text{pH} = -\log_{10}[\text{H}^+]$) of water on a logarithmic scale from $0.0$ (extremely acidic) to $14.0$ (extremely alkaline/basic), with $7.0$ representing neutral. Because the scale is logarithmic (base 10), a 1.0-unit change in pH represents a 10-fold ($10\times$) change in acidity or alkalinity, and a 2.0-unit change represents a 100-fold ($100\times$) change (for example, water at $\text{pH } 10.0$ is $100\text{ times}$ more alkaline than water at $\text{pH } 8.0$).

• Acceptable Permit Range: Most federal and state NPDES stormwater permits require discharges to remain between $\text{pH } 6.0\text{ and }9.0$ standard units (s.u.), while stricter state permits (such as California's CGP Risk Level 2/3 NAL) narrow the acceptable range to $\text{pH } 6.5\text{ to }8.5$.
• Construction Sources of High (Alkaline) pH ($> 9.0\text{ to }12.5+$): High pH on construction sites is overwhelmingly caused by calcium hydroxide ($\text{Ca(OH)}_2$) and calcium oxide ($\text{CaO}$) leaching from uncured concrete, mortar, grout, stucco, or shotcrete washout, concrete saw-cutting slurry, lime or cement soil stabilization, fly ash / cement kiln dust, or stormwater percolating through stockpiles and base courses of recycled concrete aggregate (RCA).
• Construction Sources of Low (Acidic) pH ($< 6.0$): Low pH is typically caused by excavating and exposing acid sulfate soils or pyritic rock (iron sulfide, $\text{FeS}_2$, such as marine clays or coal-bearing shales)—which oxidize upon exposure to air and rainwater to form sulfuric acid (acid rock drainage)—or by spills of masonry muriatic acid ($\text{HCl}$) and vehicle battery acid ($\text{H}_2\text{SO}_4$).
• Immediate Field Analysis Mandate: Under 40 CFR Part 136, pH must be measured immediately in the field (within 15 minutes of sample collection) because carbon dioxide ($\text{CO}_2$) degassing, temperature shifts, and chemical reactions inside a closed bottle rapidly alter sample pH.

3. Additional Physical, Chemical, and Biological Parameters

• Temperature ($^\circ\text{C}$) and Dissolved Oxygen ($\text{DO}$, $\text{mg/L}$): Both must be measured in-situ or within 15 minutes in the field. Sheet runoff flowing across sun-heated asphalt and unshaded graded pads experiences thermal loading; warm water holds physically less dissolved oxygen, and suspended organic sediment exerts biochemical/chemical oxygen demand, depleting $\text{DO}$ below the $5.0\text{ to }7.0\text{ mg/L}$ minimum needed by cold-water salmonids and benthic macroinvertebrates.
• Specific Conductance ($\mu\text{S/cm}$) and Total Dissolved Solids ($\text{TDS}$, $\text{mg/L}$): Specific conductance measures the water's ability to conduct an electrical current (standardized to $25^\circ\text{C}$), serving as a rapid field proxy for dissolved ionic salts ($\text{TDS}$), road deicing chlorides, fertilizers, or concrete leachate.
• Nutrients (Total Phosphorus, Orthophosphate, Nitrate/Nitrite, Ammonia): Sourced from hydroseeding/turf fertilizers, topsoil stripping, and sanitary waste leaks; excess nitrogen and phosphorus drive cultural eutrophication and toxic algal blooms in receiving lakes and estuaries.
• Oil and Grease / Total Petroleum Hydrocarbons ($\text{TPH}$, $\text{mg/L}$): Sourced from leaking equipment hydraulic lines, diesel refueling spills, form-release oils, and asphalt curing/tack coats. Because hydrocarbons float on water and cling tenaciously to plastics, oil-and-grease samples must always be collected as a direct grab in a wide-mouth glass jar—never composited and never pumped through plastic peristaltic tubing.
• Total and Dissolved Heavy Metals ($\text{Cu, Zn, Pb, Cd, Cr, Ni}$): Sourced from galvanized culverts/fencing (zinc), vehicle brake pads (copper), industrial roofs, and contaminated fill soils. Toxicity to fish gills generally increases at lower water hardness and extreme pH.

Summary Comparison of Target Stormwater Monitoring Constituents (40 CFR Part 136)

Apply the analytical method and sampling plan

40 CFR 136.3 Table II and its footnotes, checked October 13, 2026, govern preservation and maximum holding times for the applicable Clean Water Act methods. “Nutrients” is not one preservation category: nitrite alone and orthophosphate generally require analysis within 48 hours; preserved combined nitrate/nitrite, ammonia, and total phosphorus commonly allow 28 days. Routine mercury CVAA allows 28 days, whereas CVAFS has method-specific acid preservation and 90 days. Six months is a calendar interval, not always 180 days. Dissolved-oxygen probe readings are immediate, but the Winkler method has different preservation and holding provisions. Microbiological holding limits depend on the approved method and footnotes; do not apply a drinking-water 24-hour allowance to every stormwater test.

Oil-and-grease extraction is not synonymous with total petroleum hydrocarbons; TPH methods and target fractions have their own bottle, preservation, and holding instructions. Use laboratory-supplied bottles and the approved sampling plan. Downstream-to-upstream movement avoids contaminating later stations when wading, but does not override safety, permit locations, timing, or an approved sampling design. Use clean equipment between locations. Calibrate meters at the frequency and number of standards required by the method, instrument, and quality plan; a pH calibration normally brackets the expected sample pH rather than universally requiring all three buffers. A field-quality issue must be documented and qualified with the lab/regulator, rather than declared automatically legally void.