9.1 Drinking Water Laboratory Testing & Compliance Sampling

9.1 Drinking Water Laboratory Testing & Compliance Sampling

Accurate laboratory testing and compliance sampling provide the empirical foundation for drinking water process control and public health protection. Certified operators must understand not only how to execute analytical procedures with precision, but also how to properly collect, preserve, and document representative samples under Illinois EPA and federal Safe Drinking Water Act (SDWA) regulations.

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Sampling Fundamentals & Preservation Protocols

A laboratory analysis is only as valid as the sample on which it is performed. Water quality varies dynamically across treatment units and distribution networks, requiring operators to select appropriate sampling techniques and preservation methods.

Grab vs. Composite Samples

1. Grab Samples: An individual sample collected at a single specific location and time, representing the water composition only at that instantaneous moment. Grab samples are mandatory for parameters that degrade rapidly, undergo gas exchange, or alter physical state during storage:
   • Dissolved gases: Dissolved Oxygen (DO), Carbon Dioxide ($CO_2$)
   • Disinfectant residuals: Free and Total Chlorine
   • Physical parameters: Temperature, pH
   • Biological parameters: Total Coliforms, E. coli, Heterotrophic Plate Count (HPC)
   • Volatile compounds: Volatile Organic Compounds (VOCs), oil and grease
2. Composite Samples: A series of individual sample aliquots collected over a designated timeframe (typically 24 hours) and combined into a single container. Composite samples can be time-proportional (fixed aliquot volume taken at uniform time intervals) or flow-proportional (aliquot volume or frequency adjusted proportionally to plant flow rate). Flow-proportional composites are standard for tracking cumulative mass loadings and NPDES compliance in wastewater, but grab samples dominate drinking water compliance testing.

Container Selection & Chemical Preservation

Containers must prevent chemical leaching, analyte adsorption, and sample contamination. The table below outlines standard preservation protocols, approved container types, and regulatory holding times:

Chemical Mechanism of Sodium Thiosulfate ($Na_2S_2O_3$): Sodium thiosulfate neutralizes free and combined chlorine residuals in bacteriological and DBP sample bottles to prevent continued biocidal action or byproduct formation after collection: $\text{Na}_2\text{S}_2\text{O}_3 + 4\text{Cl}_2 + 5\text{H}_2\text{O} \rightarrow 2\text{NaHSO}_4 + 8\text{HCl}$

Chain of Custody (COC)

Compliance samples must be legally defensible. A complete Chain of Custody (COC) document must accompany every sample container from field collection to final reporting, detailing:

• Unique sample identification code and public water system (PWS) ID number
• Exact sampling site location description (physical address and tap designation)
• Date and exact time of collection (military format)
• Sample matrix (finished drinking water, raw groundwater, surface water)
• Specific analyses requested and analytical EPA test methods
• Bottle type, lot number, and chemical preservatives added
• Signatures of sample collector and every individual who relinquishes or accepts physical custody
• Temperature blank reading upon laboratory receipt (must verify $\le 6^\circ\text{C}$ without freezing).

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Microbiological Analysis: Revised Total Coliform Rule (RTCR)

The Revised Total Coliform Rule (RTCR) establishes an analytical and operational framework to prevent waterborne pathogens from reaching consumers.

The Indicator Organism Concept

Pathogens (Salmonella, Shigella, Hepatitis A, Cryptosporidium) occur in water intermittently and in very low concentrations, making direct routine isolation technically difficult and cost-prohibitive. Drinking water protection therefore relies on indicator organisms:

• Total Coliform Group: Aerobic and facultatively anaerobic, Gram-negative, non-spore-forming, rod-shaped bacteria that ferment lactose with gas and acid production within 48 hours at $35^\circ\text{C} \pm 0.5^\circ\text{C}$. Coliforms naturally inhabit the intestinal tract of warm-blooded animals, but many genera (Klebsiella, Enterobacter, Citrobacter) also persist in soil and decaying vegetation. Their presence indicates potential distribution system compromise, biofilm regrowth, or surface water ingress.
• Escherichia coli (E. coli): A specific species within the coliform group that exclusively inhabits the intestines of warm-blooded animals and birds. It cannot survive indefinitely in the environment. Detection of E. coli provides positive proof of recent fecal contamination and an acute risk of enteric pathogen exposure.

Analytical Methodologies

Illinois EPA certified laboratories analyze bacteriological compliance using two primary standard methodologies:

                                Bacteriological Testing Methodologies
                                                 │
               ┌─────────────────────────────────┴─────────────────────────────────┐
               ▼                                                                   ▼
    Membrane Filtration (MF)                                            Enzyme Substrate (Colilert / P-A)
    • 100 mL filtered through 0.45 µm membrane                         • 100 mL sample + dehydrated reagent
    • Placed on m-Endo broth / agar at 35.0°C ± 0.5°C                  • Incubated at 35.0°C ± 0.5°C for 24 hours
    • Incubated for 24 ± 2 hours                                        • ONPG + β-galactosidase → Yellow (Total Coliform)
    • Sheen colonies (golden-green metallic) counted                    • MUG + β-glucuronidase → Blue Fluorescence (E. coli)

1. Membrane Filtration (MF, Standard Methods 9222 B):

   • Exactly $100\text{ mL}$ of sample is pulled under vacuum through a sterile, $47\text{-mm}$ diameter, $0.45\text{ \mu m}$ pore-size cellulose ester grid membrane filter.
   • Bacteria are retained on the membrane surface. The filter is transferred grid-side-up to a sterile Petri dish containing m-Endo medium (broth or agar).
   • The plate is incubated inverted at $35.0^\circ\text{C} \pm 0.5^\circ\text{C}$ for $24 \pm 2\text{ hours}$.
   • Positive Identification: Total coliform colonies produce acetaldehyde, which reacts with sodium sulfite and basic fuchsin in the medium to produce a distinctive golden-green metallic sheen over dark pink/red colonies. Colonies are counted under a fluorescent lamp.

2. Enzyme Substrate Test (Colilert / Presence-Absence, Standard Methods 9223 B):

   • Utilizes hydrolyzable chromogenic and fluorogenic nutrient-substrates targeting specific bacterial enzymes, eliminating secondary confirmation steps:
     • ONPG (ortho-nitrophenyl-$\beta$-D-galactopyranoside): Total coliform bacteria produce the enzyme $\beta$-galactosidase, which cleaves ONPG to release free yellow ortho-nitrophenol. A color shift from clear to yellow (equal to or greater than the comparator standard) confirms Total Coliform positive.
     • MUG (4-methylumbelliferyl-$\beta$-D-glucuronide): E. coli strains uniquely produce the enzyme $\beta$-glucuronidase, which cleaves MUG to release 4-methylumbelliferone. When exposed to a $365\text{-nm}$ longwave ultraviolet (UV) light, the sample exhibits an intense bright blue fluorescence, confirming E. coli positive.
   • Incubation occurs at $35.0^\circ\text{C} \pm 0.5^\circ\text{C}$ for 24 hours (or 18 hours for Colilert-18).

Compliance Sampling Protocols & Repeat Triggering

• Sample Collection: Collected using sterile, non-rinsed $100\text{-mL}$ polypropylene bottles containing $Na_2S_2O_3$. The tap must be free of aerators, hose attachments, or swivel heads. The tap is flushed for 2 to 5 minutes until cold water temperature stabilizes, throttling flow to a steady stream without splashing, filling the bottle to the $100\text{-mL}$ fill line leaving a 1-inch headspace for mixing.
• Sample Siting Plan: Systems must collect monthly distribution samples based on population served (e.g., population 2,501–3,300 requires 3 samples/month; 25,001–33,000 requires 30 samples/month), distributed evenly across representative pressure zones and storage service areas.
• Repeat Sampling Requirements: If ANY routine distribution sample tests positive for total coliform:
  1. The laboratory must automatically test the positive culture for E. coli.
  2. The utility must collect a set of 3 repeat samples within 24 hours of notification:
     • One repeat sample from the original tap location.
     • One repeat sample at a tap within 5 service connections upstream of the original site.
     • One repeat sample at a tap within 5 service connections downstream of the original site.
  3. Groundwater systems must collect a triggered raw groundwater source sample from each operating active well under the Ground Water Rule.
• Violations & Public Notice:
  • E. coli Maximum Contaminant Level (MCL) Violation: Occurs if an E. coli-positive routine sample is followed by a total coliform-positive repeat, or a total coliform routine is followed by an E. coli repeat. Triggers a mandatory Tier 1 Public Notification (broadcast media, reverse 911, posting within 24 hours) and a Boil Water Order.
  • Treatment Technique Triggers (Level 1 & Level 2 Assessments): Exceeding 5.0% coliform-positive samples in a month (systems collecting $\ge 40$ samples) or $\ge 2$ positives (systems collecting $< 40$ samples) triggers a Level 1 Assessment. An E. coli MCL violation or failure of a Level 1 trigger twice in 12 months triggers a comprehensive Level 2 Assessment by an IEPA-approved certified inspector.

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Physical & Chemical Water Quality Testing

Daily process control requires field and benchtop physical-chemical analysis.

Turbidity (Nephelometric Method)

Turbidity quantifies optical water clarity by measuring light scattering caused by suspended colloids, silt, clay, and microorganisms.

• Nephelometric Principle (EPA Method 180.1): A stable light source shines through the water sample, and a photodetector positioned at a $90^\circ$ angle to the incident light beam measures scattered light intensity. Results are reported in Nephelometric Turbidity Units (NTU).
• Calibration & Standards: Primary standard calibration is performed using formazin polymer suspensions (4,000 NTU stock diluted gravimetrically) or certified secondary polymer standards (AMCO Clear). Stray light calibration must be verified quarterly.
• Sample Cell Protocol: Optical sample cuvettes must be constructed of optical-grade glass, indexed to align the manufacturer's indexing mark with the instrument geometry, wiped with a clean, lint-free wipe, and coated with a microscopic film of silicone oil to fill microscopic scratches that refract light.
• Compliance Limits: For conventional/direct surface water filtration, combined filter effluent (CFE) must remain $\le 0.3\text{ NTU}$ in at least 95% of monthly measurements and must never exceed $1.0\text{ NTU}$ at any time. Individual filter effluent (IFE) must be monitored continuously (every 15 minutes).

Chlorine Residual Testing (DPD Method)

Chlorine disinfection residual monitoring ensures persistent pathogen inactivation throughout the distribution system.

• Analytical Chemistry: The DPD (N,N-diethyl-p-phenylenediamine) colorimetric method (Standard Methods 4500-Cl G) measures light absorbance between $515\text{ nm}$ and $530\text{ nm}$:
  • Free Available Chlorine: In the absence of iodide ions, DPD reacts instantaneously with free chlorine ($HOCl$ and $OCl^-$) at buffered pH 6.2–6.5 to produce a magenta/pink Wurster dye.
  • Total Chlorine: Addition of potassium iodide ($KI$) catalyzes the oxidation of iodide to iodine by combined chloramines (monochloramine, dichloramine), which subsequently reacts with DPD to yield total color intensity.
  • Combined Chlorine: Calculated by subtraction: $\text{Combined Chlorine} = \text{Total Chlorine} - \text{Free Chlorine}$.
• Interferences & Troubleshooting:
  • Oxidized Manganese ($Mn^{4+}$): Manganese dioxide directly oxidizes DPD to pink dye, producing a false-positive chlorine reading. To correct, run a blank containing sodium arsenite to reduce chlorine without reducing manganese.
  • Bleaching Effect: When chlorine concentrations exceed $4\text{--}5\text{ mg/L}$, the excessive chlorine oxidizes the magenta Wurster dye into a colorless imine compound. The sample flashes momentary pink upon reagent addition and then turns completely clear or pale pink, falsely indicating low chlorine. The sample must be diluted with chlorine demand-free water and retested.
  • Monochloramine Bleed: Monochloramine slowly oxidizes DPD even without iodide; the free chlorine reading must be recorded strictly within 1 minute of reagent addition.

pH Measurement (Electrometric Method)

pH represents the negative logarithm of hydrogen ion activity ($-\log_{10}[H^+]$). Measurement is critical for coagulation efficacy, corrosion passivation, and chlorine speciation.

• Electrode Operation: Uses a combination electrode consisting of a thin-walled hydrogen-ion-sensitive glass membrane and an internal reference electrode ($Ag/AgCl$ in $3\text{ M } KCl$) immersed in an electrolyte bridge.
• Temperature Compensation: Electrode potential follows the Nernst Equation, where the millivolt response slope per pH unit shifts with temperature ($54.2\text{ mV/pH}$ at $0^\circ\text{C}$ vs. $59.16\text{ mV/pH}$ at $25^\circ\text{C}$). Meters must utilize an Automatic Temperature Compensation (ATC) probe.
• Two-Point Buffer Calibration: Daily calibration requires two certified reference buffers that bracket the anticipated sample pH. Standard practice begins with pH 7.00 buffer (the isopotential zero-millivolt point), followed by pH 4.01 buffer for acidic waters or pH 10.01 buffer for alkaline waters. Slope must fall between 95% and 105% (56.2 to 62.1 mV/pH at 25°C).

Alkalinity Titration

Alkalinity measures the acid-neutralizing capacity of water, primarily composed of bicarbonate ($HCO_3^-$), carbonate ($CO_3^{2-}$), and hydroxide ($OH^-$) anions.

• Analytical Titration: Exactly $100\text{ mL}$ of sample is titrated with standardized $0.0200\text{ N}$ sulfuric acid ($H_2SO_4$) using a calibrated burette or digital titrator:
  • Phenolphthalein Alkalinity ($P$): Titrated to pH 8.3 (color shift from pink to clear). Measures all hydroxide and half of carbonate alkalinity.
  • Total Alkalinity ($T$): Titration continued to pH 4.5 using bromcresol green-methyl red indicator (color shift from green to light pinkish-gray). Measures total buffering capacity.
• Calculation Formula: $\text{Alkalinity (mg/L as } \text{CaCO}_3\text{)} = \frac{A \times N \times 50,000}{V_{\text{sample}} \text{ (mL)}}$ where $A = \text{mL of standard acid used to pH 4.5}$, and $N = \text{normality of acid } (0.0200\text{ N})$. When $100\text{ mL}$ is titrated with $0.0200\text{ N } H_2SO_4$, $\text{Alkalinity (mg/L as } \text{CaCO}_3\text{)} = A \times 10$.
• Species Fractionation:
  • If $P = 0$: All alkalinity is Bicarbonate ($HCO_3^- = T$).
  • If $P = T$: All alkalinity is Hydroxide ($OH^- = T$).
  • If $P < 0.5T$: Carbonate ($CO_3^{2-} = 2P$) and Bicarbonate ($HCO_3^- = T - 2P$).
  • If $P = 0.5T$: Carbonate ($CO_3^{2-} = 2P = T$).
  • If $P > 0.5T$: Carbonate ($CO_3^{2-} = 2(T - P)$) and Hydroxide ($OH^- = 2P - T$).

Total Hardness (EDTA Titrimetric Method)

Hardness quantifies multivalent metallic cations, dominated by calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$).

• Analytical Chemistry: Ethylenediaminetetraacetic acid (EDTA) forms soluble, stable chelate complexes with divalent cations. The sample is buffered to pH 10.0 $\pm 0.1$ with an ammonium chloride-ammonium hydroxide buffer. Eriochrome Black T (EBT) indicator is added, which binds to magnesium ions to form a distinct wine-red complex.
• Endpoint: As standardized $0.01\text{ M}$ EDTA titrant is added, it binds free calcium, then free magnesium, and finally extracts magnesium from the EBT dye. The true endpoint is a sharp, unmistakable color transition from wine-red to pure sky blue (zero reddish tint).
• Calculation: $\text{Total Hardness (mg/L as } \text{CaCO}_3\text{)} = \frac{\text{mL EDTA titrant} \times 1,000}{V_{\text{sample}} \text{ (mL)}}$ When titrating a $50.0\text{-mL}$ aliquot with $0.01\text{ M}$ EDTA, each $1.0\text{ mL}$ of titrant equals $20.0\text{ mg/L as } CaCO_3$.

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Regulatory Compliance Monitoring: Lead, Copper & PFAS

Compliance testing enforces stringent National Primary Drinking Water Regulations across the distribution system.

Lead and Copper Rule (LCR / LCRI)

Corrosion monitoring targets metallic leaching from service lines, interior plumbing, and brass fixtures:

• Sampling Protocol: Tap samples must be first-draw 1-liter volumes collected from kitchen or bathroom cold water taps after a mandatory minimum stagnation period of at least 6 hours.
• Crucial Restrictions: Samples must NOT be collected from taps with point-of-use filters, outside hose bibbs, or commercial buildings. Pre-stagnation flushing and aerator removal are strictly prohibited prior to the stagnation period, as they artificially suppress lead concentrations.
• Action Levels (90th Percentile):
  • Lead Action Level: $0.015\text{ mg/L}$ ($15\text{ \mu g/L}$ / ppb).
  • Copper Action Level: $1.3\text{ mg/L}$ ($1,300\text{ \mu g/L}$ / ppb).
  • An exceedance occurs when greater than 10% of collected tier-site samples exceed the action level. Exceedances do not constitute immediate violations but trigger mandatory public education, source water monitoring, and corrosion control treatment (CCT) optimization (e.g., orthophosphate passivation).

Per- and Polyfluoroalkyl Substances (PFAS) Monitoring

PFAS compounds ("forever chemicals" such as PFOA, PFOS, PFNA, PFHxS, and HFPO-DA/GenX) are regulated under federal SDWA Maximum Contaminant Levels (e.g., $4.0\text{ ng/L}$ or parts per trillion for PFOA and PFOS).

• Analytical Methods: EPA Method 533 (isotope dilution anion exchange SPE LC-MS/MS) and EPA Method 537.1 (solid phase extraction and liquid chromatography/tandem mass spectrometry).
• Field Contamination Protocols: Because PFAS compounds are ubiquitous in commercial materials, sampling protocols enforce extreme hygiene:
  • Samplers must NOT wear clothing containing Gore-Tex, Teflon, water-repellent treatments, or coated fabric; no Tyvek suits; clothing must be well-laundered cotton without fabric softeners.
  • Sample bottles must be high-density polyethylene (HDPE) or polypropylene with unlined caps. Glass containers, Teflon/PTFE septa, and aluminum foil are strictly forbidden due to adsorption or fluoropolymer contamination.
  • No waterproof pens, sticky notes, or coated clipboards in the sampling zone.
  • Field Reagent Blank (FRB): A bottle of laboratory-certified PFAS-free reagent water is shipped to the sampling site, opened during sample collection, poured into a sterile empty container, capped, and processed alongside field samples to prove zero external contamination.