20.3 Sampling Protocols, Chain of Custody & Laboratory QA/QC
Key Takeaways
- Grab sampling is legally mandatory for parameters subject to rapid physical, biological, or chemical alteration (pH, temperature, DO, chlorine residual, coliforms, VOCs, and oil/grease), whereas composite sampling captures daily mass loading for BOD, TSS, nutrients, and metals.
- Sample preservation requires immediate cooling to ≤ 6°C on wet ice and parameter-specific chemical fixatives: H2SO4 to pH < 2 for nutrients and COD, HNO3 to pH < 2 for trace metals, and NaOH to pH > 12 for cyanides.
- Maximum regulatory holding times range from 15 minutes for field parameters (pH, DO, residual chlorine) and 6 hours for drinking water coliforms, to 48 hours for BOD, 7 days for TSS, and 6 months for preserved trace metals.
- Chain of Custody (COC) documentation establishes legal evidentiary defensibility through an unbroken custody log detailing sample origin, collector signatures, preservatives, requested methods, and temperature verification upon laboratory receipt.
- Laboratory QA/QC verifies accuracy and precision through method blanks (< MDL), Laboratory Control Samples (85–115% recovery), Matrix Spikes evaluating matrix interference, and duplicate analyses quantified via Relative Percent Difference (RPD).
20.3 Sampling Protocols, Chain of Custody & Laboratory QA/QC
[!NOTE] Legal Defensibility & Arizona Standards: Under A.R.S. Title 49 (The Arizona Environmental Quality Act), laboratory data generated for compliance reporting under the Safe Drinking Water Act (SDWA) or Arizona Pollutant Discharge Elimination System (AZPDES) must be legally defensible. A laboratory analysis is only as valid as the integrity of the sample collected in the field. Non-compliance with approved EPA 40 CFR Part 136 preservation methods, holding times, or chain-of-custody documentation renders test results inadmissible and triggers civil enforcement penalties.
Water and wastewater operators routinely collect samples that represent millions of gallons of daily flow. Because physical and biological matrices change continuously through biological decay, chemical volatilization, and gas absorption, operators must implement standardized sampling protocols, chemical preservation routines, and rigorous quality assurance and quality control (QA/QC) programs.
Grab vs. Composite Sampling Methodologies
The choice between grab and composite sampling is determined by the physical-chemical stability of the target analyte and regulatory permit requirements.
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| Sampling Methodology Comparison |
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| 1. Grab Samples : Discrete single aliquot collected at a specific point in |
| time and space. Captures instantaneous conditions. |
| MANDATORY FOR: pH, Temp, DO, Residual Chlorine, Coliforms, |
| Volatile Organic Compounds (VOCs), Oil & Grease (HEM). |
| |
| 2. Composite Samples : Multiple discrete aliquots collected over 24 hours. |
| Captures average mass loading and diurnal variability. |
| MANDATORY FOR: BOD5, CBOD5, TSS, Total N, Total P, Metals. |
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Mandatory Grab Sampling Parameters
A grab sample represents the exact characteristics of the water matrix at the instant of collection. Regulations prohibit compositing for parameters that degrade, volatilize, or adhere to collection tubing:
- Dissolved Gases & Field Parameters: pH, temperature, and dissolved oxygen degrade within seconds to minutes due to carbon dioxide exchange, atmospheric oxygen absorption, and ambient heat transfer. Analysis must occur within 15 minutes.
- Disinfectant Residuals: Free and total chlorine decompose rapidly under solar ultraviolet radiation and through demand reactions with organic compounds.
- Microbiological Indicators: Total Coliforms, E. coli, and enterococci multiply or undergo die-off when held in composite collection jugs at fluctuating temperatures.
- Volatile Organic Compounds (VOCs): Benzene, toluene, trichloroethylene (TCE), and trihalomethanes volatilize into headspace if subjected to peristaltic pump vacuum or multi-aliquot pouring.
- Oil & Grease (Hexane Extractable Materials, HEM): Hydrocarbons adhere tenaciously to the plastic tubing, carboy walls, and glass pump chambers of automated composite samplers, resulting in severe negative bias. Oil and grease must always be collected as discrete grabs in clean, wide-mouth amber glass bottles with PTFE-lined caps.
Composite Sampling: Time-Proportional vs. Flow-Proportional
Composite samples consist of individual sample aliquots (sub-samples) collected over a standardized 24-hour cycle:
- Time-Proportional Compositing: Collects a fixed aliquot volume at regular, predetermined time intervals (e.g., 200 mL collected every 60 minutes for 24 hours). This approach is acceptable only where wastewater flow rates remain relatively uniform and unvarying throughout the day.
- Flow-Proportional Compositing: Required by AZPDES permits for municipal wastewater treatment facilities experiencing diurnal flow swings:
- Method A (Flow-Variable Volume): Aliquots are taken at constant time intervals (e.g., every 30 minutes), but the volume of each aliquot varies directly with instantaneous flow measured by a primary metering flume or weir.
- Method B (Constant Volume, Flow-Paced Time): A uniform aliquot volume (e.g., 200 mL) is collected at variable time intervals triggered every time a set volume of wastewater (e.g., 50,000 gallons) passes the totalizing flow meter.
- Mass Loading Calculation: Flow-proportional compositing is vital because regulatory discharge limits are expressed both as concentrations (mg/L) and mass discharge rates (pounds per day):
Sample Preservation, Chemical Fixatives & Maximum Holding Times
Preservation techniques retard biological action, chemical hydrolysis, precipitation of dissolved metals, and physical adsorption onto container surfaces during transit.
Regulatory Preservation and Storage Standards (EPA 40 CFR Part 136)
| Parameter / Analyte | Preservative / Chemical Fixative | Container Material | Maximum Holding Time |
|---|---|---|---|
| pH, Temperature, DO, Chlorine Residual | None allowed; analyze immediately in field | Glass or Plastic (HDPE) | 15 minutes |
| Coliform Bacteria (Drinking Water) | Sodium thiosulfate ($Na_2S_2O_3$, ~100 mg/L) + Cool on wet ice ≤ 10°C | Sterile Polystyrene / Polypropylene | 6 hours (ADEQ compliance) / 30 hrs absolute |
| Coliform Bacteria (Wastewater Effluent) | Sodium thiosulfate ($Na_2S_2O_3$) + Cool on wet ice ≤ 6°C | Sterile Plastic or Glass | 8 hours |
| Biochemical Oxygen Demand ($BOD_5 / CBOD_5$) | Cool on wet ice ≤ 6°C (never freeze) | Plastic or Glass | 48 hours |
| Total Suspended Solids (TSS / TVSS) | Cool on wet ice ≤ 6°C | Plastic or Glass | 7 days |
| Total Dissolved Solids (TDS) | Cool on wet ice ≤ 6°C | Plastic or Glass | 7 days |
| Nutrients: Ammonia, Nitrate+Nitrite, TKN, Total P | Sulfuric acid ($H_2SO_4$) to pH < 2.0 + Cool ≤ 6°C | Plastic or Glass | 28 days |
| Chemical Oxygen Demand (COD) | Sulfuric acid ($H_2SO_4$) to pH < 2.0 + Cool ≤ 6°C | Glass preferred | 28 days |
| Oil and Grease (HEM) | Sulfuric acid ($H_2SO_4$) or $HCl$ to pH < 2.0 + Cool ≤ 6°C | Wide-mouth Amber Glass only with PTFE-lined cap | 28 days |
| Total Metals (As, Cd, Cr, Cu, Pb, Ni, Zn) | Concentrated Nitric acid ($HNO_3$) to pH < 2.0 (no refrigeration required) | Polyethylene (HDPE) or Fluoropolymer | 6 months (180 days) |
| Mercury (Total) | Concentrated Nitric acid ($HNO_3$) to pH < 2.0 + Cool ≤ 6°C | Glass or PTFE | 28 days |
| Total Cyanide | Sodium hydroxide ($NaOH$) to pH > 12.0 + Cool ≤ 6°C (plus ascorbic acid if chlorine present) | Plastic or Glass | 14 days |
Chemical Fixative Mechanisms
- Refrigeration (Wet Ice at ≤ 6°C): Retards biological metabolic kinetics (halving microbial respiration for every 10°C drop) without rupturing cellular walls. Freezing is strictly prohibited for BOD, TSS, and bacteriological testing because ice crystals lyse bacterial cells and alter floc structures.
- Sodium Thiosulfate ($Na_2S_2O_3$): Neutralizes free and combined chlorine residuals ($Na_2S_2O_3 + 4 Cl_2 + 5 H_2O \rightarrow 2 NaHSO_4 + 8 HCl$), preventing continuing disinfection during transit.
- Sulfuric Acid ($H_2SO_4$ to pH < 2): Halts all cellular respiration and enzymatic hydrolysis in nutrient and COD samples, keeping ammonia, nitrate, and phosphates stabilized.
- Nitric Acid ($HNO_3$ to pH < 2): Maintains metallic elements as free, dissolved cations in solution, preventing hydrolysis into insoluble metal hydroxides and stopping metal ions from adsorbing onto the container walls.
- Sodium Hydroxide ($NaOH$ to pH > 12): Prevents toxic hydrogen cyanide ($HCN$) gas volatilization by keeping the species locked in the non-volatile cyanide ion ($CN^-$) state.
Chain of Custody (COC) Documentation
A Chain of Custody (COC) is a legally binding document that establishes an unbroken, documented trail of accountability for sample identification, collection, possession, transfer, and analytical processing.
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| Legal Criteria for Sample Custody |
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| A sample is officially in legal custody if it satisfies ANY of these conditions: |
| 1. It is in the collector's actual physical possession. |
| 2. It is in the collector's direct visual sight after being in physical possession.|
| 3. It is secured in a locked area, vehicle, or locker to prevent tampering. |
| 4. It is placed in a designated secure storage area with restricted access, or |
| sealed inside a transport cooler with intact, tamper-evident custody tape. |
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Mandatory Fields on a Legal COC Form
- Unique Sample Identification Number: Cross-referenced exactly to sample bottle labels.
- Date and Time of Collection: Recorded in military time (e.g., 14:25) or clearly designated AM/PM.
- Sampling Location Description: Explicit description (e.g., "Central WWTP Outfall 001 Final Effluent Weir").
- Sample Matrix: Clearly specified as potable water, non-potable surface water, untreated raw sewage, secondary effluent, or dewatered biosolids cake.
- Sample Type: Designated as discrete Grab or 24-hour Composite.
- Chemical Preservatives Used: Documenting added fixatives ($HNO_3$, $H_2SO_4$, $NaOH$, $Na_2S_2O_3$, ice) and recorded field pH checks.
- Requested Analytical Parameters & EPA Methods: Explicit test codes (e.g., EPA 180.1 Turbidity, SM 5210 B BOD5, EPA 200.7 Total Metals).
- Relinquishing and Receiving Signatures: Every physical transfer must include the printed name, original signature, organization, date, and exact time of both the party relinquishing and the party receiving the samples.
- Cooler Receipt Temperature: Upon delivery at the laboratory, receiving personnel must check and record the interior cooler temperature using a calibrated infrared thermometer or by measuring an included Temperature Blank bottle. If the temperature exceeds 6°C (or 10°C for drinking water coliforms), the laboratory must flag all analytical data as compromised or reject the shipment.
Laboratory Quality Assurance & Quality Control (QA/QC)
Quality Assurance (QA) is the overall management program guaranteeing data integrity (SOPs, technician training, instrumentation service contracts). Quality Control (QC) represents the specific analytical techniques used to measure and verify laboratory precision and accuracy.
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| Laboratory QC Toolset |
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| 1. Method Blank (LRB) : Deionized water processed through entire analytical run. |
| Evaluates reagent purity and glassware contamination. |
| 2. Lab Control Sample : Certified standard in clean matrix (85% to 115% recovery).|
| Evaluates analytical accuracy without matrix interference.|
| 3. Matrix Spike (MS) : Known analyte added to actual environmental sample. |
| Evaluates matrix interferences and recovery percentage. |
| 4. Duplicate (RPD) : Split sample analyzed in duplicate. |
| Evaluates analytical precision and repeatability. |
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1. Method Blanks (Laboratory Reagent Blank, LRB)
An aliquot of analyte-free reagent water is processed through all preparation, digestion, filtration, and analytical steps identically to real samples. A method blank detects background contamination from dirty glassware, impure chemical reagents, or laboratory atmospheric dust. The method blank result must remain below the Method Detection Limit (MDL). If analyte is detected in the blank, all associated sample results must be qualified or re-analyzed.
2. Laboratory Control Samples (LCS / Known Standard)
An LCS consists of a clean reagent water matrix spiked with a known concentration of target analyte obtained from an independent, secondary certified reference standard. The LCS verifies instrument calibration accuracy and analyst performance without interference from the sample matrix. Target recovery typically falls between 85% and 115%.
3. Matrix Spikes (MS) & Percent Recovery
A matrix spike assesses whether components within an environmental sample (high dissolved salts, industrial surfactants, suspended clays) interfere with analyte recovery:
- An actual field sample aliquot is split in two.
- One aliquot is analyzed unspiked ($C_{\text{unspiked}}$).
- A known quantity of target analyte ($C_{\text{added}}$) is added to the second aliquot, which is analyzed ($C_{\text{spiked}}$).
- Acceptable Criteria: Recovery generally must fall between 70% and 130% (or 80% to 120% depending on parameter). A recovery below 70% indicates negative matrix suppression; recovery above 130% indicates positive matrix enhancement.
4. Duplicate Samples & Relative Percent Difference (RPD)
Duplicate samples (field duplicates or laboratory sample duplicates) evaluate analytical precision and repeatability:
Where $D_1$ is the first sample concentration and $D_2$ is the duplicate concentration. Typical acceptance limits require an RPD < 10% to 20% for concentrations well above the detection limit.
5. Multi-Point Calibration Verification Curves
Instruments (spectrophotometers, ICP-MS, ion chromatographs) must be calibrated using a minimum of 3 to 5 calibration standards across the linear dynamic range plus a blank. The calculated linear regression line must demonstrate a correlation coefficient ($r$) $\ge 0.995$ (or coefficient of determination $r^2 \ge 0.990$). Calibration stability is verified during testing by analyzing a Continuing Calibration Verification (CCV) standard every 10 injections.
Toxicity Characteristic Leaching Procedure (TCLP, EPA Method 1311)
Wastewater treatment processes generate significant quantities of biological sludge (biosolids). Under ADEQ rules and the federal Resource Conservation and Recovery Act (RCRA, 40 CFR Part 261), treatment works must verify that dewatered biosolids are non-hazardous prior to disposal in municipal solid waste landfills or agricultural land application.
Analytical Extraction Protocol
The TCLP test (EPA Method 1311) simulates the harsh leaching conditions inside an unlined municipal sanitary landfill, where decomposing refuse generates acidic municipal leachate (acetic acid):
- Particle Size Reduction: Dewatered biosolids cake must pass through a 9.5 mm standard sieve.
- Extraction Fluid: Formulated with dilute acetic acid buffer adjusted to pH 4.93 ± 0.05 (Fluid #1) or pH 2.88 ± 0.05 (Fluid #2, for highly alkaline samples).
- Agitation: The solid sample is placed in a zero-headspace extractor or extraction vessel with a 20:1 liquid-to-solid ratio (20 mL extraction fluid per 1.0 gram of dry solid). The vessel is rotated end-over-end in a rotary agitation device at 30 ± 2 rpm for 18 ± 2 hours at 23°C ± 2°C.
- Filtration and Analysis: The liquid extract is separated from solids using a 0.6 to 0.8 µm borosilicate glass fiber filter and analyzed for hazardous contaminants.
The 8 RCRA Heavy Metals and Regulatory Action Thresholds
If the leachate concentration exceeds any of the following regulatory limits, the biosolids cake is legally classified as hazardous waste and cannot be accepted at a standard municipal landfill:
| RCRA Toxic Metal | EPA Waste Code | Maximum Regulatory TCLP Limit (mg/L) |
|---|---|---|
| Arsenic (As) | D004 | 5.0 mg/L |
| Barium (Ba) | D005 | 100.0 mg/L |
| Cadmium (Cd) | D006 | 1.0 mg/L |
| Chromium (Cr) | D007 | 5.0 mg/L |
| Lead (Pb) | D008 | 5.0 mg/L |
| Mercury (Hg) | D009 | 0.2 mg/L |
| Selenium (Se) | D010 | 1.0 mg/L |
| Silver (Ag) | D011 | 5.0 mg/L |
Which regulatory rule correctly pairs specific wastewater parameters with their legally mandated sampling methodology and operational rationale?
In accordance with EPA 40 CFR Part 136 and standard compliance monitoring rules, which chemical preservative and maximum holding time are correctly paired with the target parameter?
In laboratory quality control, what distinct functions do Matrix Spikes (MS) and Duplicate sample analyses fulfill when evaluating analytical data quality?