CSM Data Collection & Analysis 3 — Questions and Answers
Question 1: A Parshall flume is used in stormwater monitoring primarily to:
- Collect water quality grab samples automatically
- Measure open-channel flow rate based on water depth (Correct answer)
- Filter suspended sediments before analysis
- Determine groundwater infiltration rates
Correct answer: Measure open-channel flow rate based on water depth
A Parshall flume is a calibrated flow measurement device that converts a depth reading (via stage-discharge relationship) into volumetric flow rate.
Question 2: When using a rain gauge network to estimate watershed-average rainfall, the Thiessen polygon method accounts for:
- Rainfall intensity only, not duration
- The spatial area of influence attributed to each gauge (Correct answer)
- Elevation-based orographic enhancement
- Wind-driven gauge undercatch correction
Correct answer: The spatial area of influence attributed to each gauge
Thiessen polygons assign each gauge a weight proportional to the watershed area closer to that gauge than any other, giving a spatially weighted average rainfall.
Question 3: In stormwater quality monitoring, the 'first flush' phenomenon refers to:
- The highest discharge peak during a storm hydrograph
- The disproportionately high pollutant concentrations occurring early in a runoff event (Correct answer)
- The initial calibration flush performed on ISCO samplers
- Flushing sediment accumulated in detention basin forebays
Correct answer: The disproportionately high pollutant concentrations occurring early in a runoff event
First flush describes the tendency for pollutant concentrations to peak early in a storm as accumulated surface pollutants are mobilized before dilution by continued rainfall.
Question 4: Which QA/QC sample type is used to assess potential contamination introduced during field sampling and transport?
- Matrix spike
- Equipment blank (Correct answer)
- Duplicate sample
- Calibration check standard
Correct answer: Equipment blank
An equipment blank (or field blank) is collected using reagent-grade water passed through all sampling equipment and containers to detect contamination introduced during collection, transport, or processing.
Question 5: A stormwater manager wants to determine whether total phosphorus concentrations differ significantly between an agricultural sub-watershed and an urban sub-watershed. The appropriate statistical test for two independent groups with non-normal data is:
- Student's t-test
- Mann-Whitney U test (Correct answer)
- Pearson correlation
- One-way ANOVA
Correct answer: Mann-Whitney U test
The Mann-Whitney U test is the non-parametric alternative to the t-test, appropriate when data are not normally distributed and two independent groups are being compared.
Question 6: Dry-weather field screening of storm drain outfalls is conducted primarily to:
- Measure peak runoff rates during design storms
- Detect illicit discharges of sanitary sewage or industrial effluent (Correct answer)
- Calibrate ultrasonic flow meters at low flow
- Collect sediment cores for heavy metal analysis
Correct answer: Detect illicit discharges of sanitary sewage or industrial effluent
Dry-weather screening identifies outfalls with non-stormwater flow (sewage, wash water, industrial waste) by inspecting for flow, odor, color, or elevated indicator parameters during dry conditions.
Question 7: The event mean concentration (EMC) of a pollutant is calculated as:
- The arithmetic average of grab sample concentrations during a storm
- Total pollutant mass discharged divided by total runoff volume for the event (Correct answer)
- The concentration measured at peak discharge
- The geometric mean of all storm samples collected during a year
Correct answer: Total pollutant mass discharged divided by total runoff volume for the event
EMC equals total mass load divided by total runoff volume, effectively a flow-weighted average concentration that represents the storm event's overall pollutant contribution.
CSM Data Collection & Analysis 3
A Parshall flume is used in stormwater monitoring primarily to: