15.1 Sampling, Preservation & Chain of Custody
Key Takeaways
- Representative samples require the correct location, timing, flush/technique, and avoidance of contamination—bad samples invalidate good instruments.
- Grab samples capture instantaneous quality (residual, pH, DO, micro); composites (time- or flow-proportional) represent average composition for loads such as BOD and TSS.
- Use method-specified containers: plastics often for metals/nutrients, glass for many organics, sterile bottles with Na₂S₂O₃ for chlorinated bacteriological samples.
- Common preservatives include H₂SO₄ (nutrients/COD pathways), HNO₃ (metals), cooling, and Na₂S₂O₃ dechlorination; never rinse out pre-dosed preservative.
- Florida operators own compliance sampling responsibilities under FDEP schedules/permits: right point, right bottle, certified lab when required, timely reporting.
15.1 Sampling, Preservation & Chain of Custody
Quick Answer: Lab results are only as good as the sample. Operators must collect a representative sample (correct location, timing, and method), use the right container and preservative, respect holding times, document chain of custody (COC), and run field blanks/duplicates when required. In Florida, licensed operators own compliance sampling responsibilities under their plant’s monitoring schedule and FDEP rules—bad sampling can create false violations or hide real ones.
A perfectly calibrated spectrophotometer cannot fix a sample taken from the wrong tap, left unpreserved in a hot truck, or mislabeled on the chain-of-custody form. On FDEP water and wastewater exams, sampling questions test whether you understand that data quality begins in the field, not only in the laboratory.
1. Representative Sampling
A representative sample reflects the true condition of the water or wastewater stream for the parameter of interest at the time (or over the period) the sample is meant to describe. Non-representative samples waste money and can drive wrong process decisions or compliance actions.
Field practices that support representativeness:
| Practice | Why it matters |
|---|---|
| Sample at the approved/compliance point | Permits and monitoring plans specify exact locations (e.g., finished-water entry point, effluent weir, distribution zone) |
| Flush sample taps (when appropriate) | Stagnant line water is not the process stream |
| Avoid contamination from dirty bottles, hands, or hoses | Especially critical for bacteriological and low-level metals |
| Match timing to the objective | Grab for instantaneous residual; composite for average load |
| Mix or sample mid-stream correctly | Surface scum or bottom sludge bias solids and BOD samples |
| Record field data (time, temp, residual, sampler) | Supports interpretation and legal defensibility |
For distribution bacteriological samples, never sample from a hose, fire hydrant (unless approved procedure), or a tap that is rarely used without proper flushing. For wastewater influent, avoid the wet well surface film when you need typical plant load; use a location the permit identifies as representative.
2. Grab Samples vs Composite Samples
| Type | Definition | Typical uses | Exam cue |
|---|---|---|---|
| Grab | Single sample collected at one time and place | Chlorine residual, pH, temperature, DO, bacteriological, many process spot checks | Instantaneous “right now” quality |
| Composite | Series of grabs combined (flow- or time-proportional) over a period (often 24 h) | Influent/effluent BOD, TSS, many NPDES average-load parameters | Average composition over a shift/day |
Time-proportional composite: equal volumes at equal time intervals (e.g., every hour for 24 hours).
Flow-proportional composite: volumes vary with flow so high-flow periods contribute more sample—preferred when flow swings and the goal is true mass-load average.
Do not composite parameters that change rapidly or must be measured immediately (free chlorine, pH, dissolved oxygen, temperature, residual chlorine for some compliance contexts). Those stay as grabs with field measurement or very short hold times.
3. Sample Containers: Plastic vs Glass
Container choice is not preference—it is chemistry and method rule.
| Container type | Typical parameters | Notes |
|---|---|---|
| Plastic (HDPE, PP, etc.) | Many metals (with acid), nutrients, some general chemistry | Lighter, less breakage; follow method for specific analytes |
| Glass | Organics, oil & grease, many volatiles (often with septa), some pesticides | Organics can adsorb to or leach from plastics; amber glass for light-sensitive analytes |
| Sterile plastic/glass | Total coliform / E. coli / fecal coliform | Pre-sterilized; often with sodium thiosulfate tablet for chlorinated waters |
| BOD bottles | BOD₅ (glass, ground-glass stopper) | Special bottle design; no air space after fill when sealed correctly |
Always use lab-supplied or method-specified bottles when possible. Never rinse a pre-preserved bottle with sample water—you will wash out the preservative.
4. Preservatives and Dechlorination
Preservation stops or slows biological activity, chemical change, or adsorption so the lab measures what was present at collection (within holding-time limits).
| Preservative / treatment | Common use | Operator note |
|---|---|---|
| H₂SO₄ (sulfuric acid) to pH ≤2 | Nutrients (e.g., ammonia, TKN, some phosphorus methods), COD in many protocols | Corrosive; add carefully; never mix incompatible preservatives |
| HNO₃ (nitric acid) to pH ≤2 | Metals | Prevents precipitation/adsorption of metals on container walls |
| Na₂S₂O₃ (sodium thiosulfate) | Dechlorination of bacteriological samples and some organic samples | Neutralizes residual chlorine that would continue killing microbes or reacting with organics after sampling |
| Cooling to ≤6 °C (not frozen unless allowed) | Most chemistry and micro samples | Ice chests; avoid ice melt contaminating bottles |
| NaOH (alkaline) | Some cyanide methods (pH high) | Method-specific—do not invent preservatives |
Dechlorination with sodium thiosulfate is critical for chlorinated drinking water bacteriological samples. Without it, residual chlorine keeps killing coliforms in the bottle and can produce false negatives—a public-health risk. Wastewater effluent samples for fecal coliform/E. coli after chlorination also typically require dechlorination tablets in sterile bottles.
Never combine preservatives unless the method says so. Never use a bottle preserved for metals for a bacteriological sample.
5. Holding Times
Holding time is the maximum time from collection to analysis (or to extraction for some organics) under proper preservation. Exceeding holding time can invalidate the result for compliance.
Common operator-level awareness (exact times are method- and parameter-specific—follow your lab’s chain-of-custody and approved methods):
| Parameter class | Typical holding-time theme |
|---|---|
| pH, residual chlorine, temperature, DO | Analyze immediately in the field when possible |
| Bacteriological (coliform) | Short hold (commonly within 30 hours for many drinking-water rules; often “as soon as possible,” ideally same day) |
| BOD₅ | Cool; start incubation within method holding time (commonly 48 hours max to start) |
| TSS | Cool; hold time on order of 7 days in many methods |
| Nutrients (acid-preserved) | Longer holds possible once acidified and cooled—still follow lab schedule |
| Metals (HNO₃) | Often longer holds (e.g., 6 months for many dissolved/total metals once preserved) |
Exam principle: short-lived parameters first; cold and preserved samples next; never invent a longer hold because the cooler “looked fine.”
6. Chain of Custody (COC)
Chain of custody is the documented trail of sample possession from collection through analysis. It supports legal and regulatory defensibility.
A proper COC form typically includes:
- Sample ID / location / date / time
- Sampler name and signature
- Parameters requested and preservatives used
- Relinquished-by / received-by signatures and times for every transfer
- Cooler temperature on receipt (lab)
- Special notes (composite start/end, grab, unusual conditions)
Broken chain of custody (missing signatures, unlabeled bottles, mismatched IDs) can make results non-defensible even if the number looks reasonable. Treat COC like evidence, not paperwork to fill later at the desk.
7. Field Blanks, Trip Blanks, and Duplicates
Quality control samples prove that contamination or random error is under control.
| QC sample | Purpose |
|---|---|
| Field blank | Clean water exposed to field handling/environment; detects contamination from air, hands, or equipment |
| Trip blank | Often for VOCs; travels with samples but is not opened in the field; detects contamination during transport |
| Equipment blank | Rinsate from sampling equipment; detects dirty pumps, bailers, or hoses |
| Field duplicate / split | Two samples from same place/time; checks sampling + analysis precision |
| Matrix spike (lab) | Known analyte added to sample; checks recovery/interference |
If a field blank shows the target contaminant, investigate contamination, not just plant process. If duplicates disagree widely, question technique or heterogeneity before changing chemical feed rates.
8. Florida Compliance Sampling Responsibilities
Florida public water systems and domestic wastewater facilities operate under FDEP primacy and facility-specific monitoring schedules / permits. Licensed operators are responsible for ensuring required samples are:
- Collected at correct locations and frequencies
- Handled with proper containers, preservatives, and hold times
- Submitted to a certified laboratory (where certification is required)
- Reported on time with correct identifiers and COC
- Triggering corrective action and public notice when results require it (e.g., coliform positive pathways—see Section 15.4)
Process-control samples (in-house residual, settleometer, MLSS) guide daily operation and may use plant labs or field kits. Compliance samples (MCL monitoring, permit limits, bacteriological RTCR-type programs, effluent limits) must follow regulatory methods, certified labs where required, and official reporting. Confusing the two is a common exam trap: a good plant kit residual check is not automatically a substitute for the compliance residual or micro sample protocol.
Operator exam takeaway: You may not personally pipette every metal analysis, but you own whether the right bottle left the right tap, with the right preservative, on time, with a complete COC.
9. Practical Field Sequence (Exam-Ready Checklist)
- Confirm parameter list and bottle kit from the lab (preservatives pre-dosed when required).
- Confirm sample point matches permit/monitoring plan.
- Prepare cooler with ice; protect labels from water damage.
- Flush/tap prepare; collect grab or start composite correctly.
- Fill bottles without overfilling headspace rules for volatiles; do not rinse pre-preserved bottles.
- Cap, label immediately, complete COC, place on ice.
- Include blanks/duplicates per program.
- Deliver within holding times; obtain lab receipt signature.
Master this sequence and you protect both public health and your license.
What is the primary purpose of adding sodium thiosulfate (Na₂S₂O₃) to a bacteriological sample bottle for chlorinated drinking water?
Which parameter is most appropriately collected as a grab sample rather than a 24-hour composite?
A chain-of-custody (COC) form is primarily used to:
Nitric acid (HNO₃) preservation to pH ≤2 is most commonly associated with which sample type?