CES Environmental Sampling & Data Analysis 2 — Questions and Answers
Question 1: Which type of blank is used to identify contamination introduced during sample transport and field handling?
- Method blank
- Field blank
- Trip blank (Correct answer)
- Equipment blank
Correct answer: Trip blank
A trip blank accompanies samples during transport to detect VOC contamination introduced after collection, not during analysis.
Question 2: What is the purpose of calculating the Relative Percent Difference (RPD) in duplicate sample analysis?
- To assess bias between a sample and a spike recovery
- To measure precision between two replicate measurements (Correct answer)
- To determine the detection limit of an analytical method
- To evaluate the accuracy of a calibration curve
Correct answer: To measure precision between two replicate measurements
RPD measures the variability (precision) between duplicate or replicate samples by expressing the difference as a percentage of the mean.
Question 3: In a soil gas survey, which sampling method is most appropriate for collecting volatile organic compound (VOC) vapor samples?
- Grab sampling with Summa canisters (Correct answer)
- Passive diffusion tubes over 24-hour periods
- Sorbent tube sampling with active pump
- Tedlar bag collection via syringe
Correct answer: Grab sampling with Summa canisters
Summa canisters are electropolished stainless steel vessels used to collect instantaneous (grab) VOC vapor samples under evacuated conditions.
Question 4: Under EPA SW-846, what minimum purge volume is typically required before collecting a groundwater sample from a monitoring well?
- 1 well volume
- 3 well casing volumes (Correct answer)
- 5 well volumes
- 10 gallons regardless of casing size
Correct answer: 3 well casing volumes
Standard low-flow or traditional purging protocols require removing at least three well casing volumes to ensure the sample represents formation water.
Question 5: Which statistical test is appropriate to determine whether concentrations at a monitoring well are significantly higher than background concentrations?
- Chi-square goodness-of-fit test
- Paired t-test
- One-sided Welch's t-test (Correct answer)
- Kolmogorov-Smirnov test
Correct answer: One-sided Welch's t-test
A one-sided Welch's t-test (which does not assume equal variances) is used to test whether downgradient concentrations exceed upgradient background concentrations.
Question 6: A laboratory reports a result as '<2 µg/L' for a compound with a practical quantitation limit (PQL) of 2 µg/L. How should this non-detect be treated for statistical analysis?
- Exclude it from all calculations
- Substitute zero for the non-detect value
- Use the PQL value as a surrogate
- Apply a substitution method such as PQL/2 or maximum likelihood estimation (Correct answer)
Correct answer: Apply a substitution method such as PQL/2 or maximum likelihood estimation
Non-detects should be treated using established censored-data methods such as substituting half the detection limit or using maximum likelihood estimation (MLE), not simply excluded or zeroed.
Question 7: What does a surrogate spike recovery outside the acceptable window (e.g., 70–130%) indicate in an environmental sample?
- The target analytes are present above the reporting limit
- Matrix interference or extraction/analytical error may have affected results (Correct answer)
- The calibration curve must be recalculated
- The sample should be diluted and re-analyzed
Correct answer: Matrix interference or extraction/analytical error may have affected results
Poor surrogate recovery signals that matrix effects, extraction inefficiency, or instrument error may have compromised the accuracy of target analyte results.
CES Environmental Sampling & Data Analysis 2
Which type of blank is used to identify contamination introduced during sample transport and field handling?