Environmental Monitoring, Sampling, and Analysis
What this domain tests
Sampling design, quality control, and standard-method arithmetic. Most marks sit in the calculation and in knowing which QC statistic answers which question.
Sampling design
- Water: locate control, impact and attenuation sections; sample at the specified depth and distance from bank; preserve with acid for metals, acid for COD and ammonia, and cool to about 4 C; measure pH, temperature, dissolved oxygen and residual chlorine on site.
- Air: ambient stations clear of obstructions with the inlet in the breathing zone; source sampling sections placed at least 6 diameters downstream and 3 upstream of a bend or transition; particle sampling must be isokinetic, and stack temperature, pressure, moisture, oxygen and velocity are recorded.
- Noise: boundary points 1 m outside the boundary, at least 1.2 m above ground and away from reflecting surfaces.
Quality control
- Calibration curve with at least five points and a correlation coefficient of 0.999 or better.
- Method blanks, duplicate samples at roughly 10 percent, and spiked recovery, typically accepted in the 80 to 120 percent band.
- Detection limit derived from three times the standard deviation of low-level replicates; the determination limit is about four times the detection limit. Report below-limit results as less than the detection limit, never as zero.
Standard methods
COD by potassium dichromate (HJ 828, mercury sulfate to mask chloride); BOD5 by dilution and seeding (HJ 505, 20 C, 5 days, dark); ammonia by Nessler reagent (HJ 535) or salicylic acid (HJ 536); total phosphorus by ammonium molybdate spectrophotometry (GB 11893); total nitrogen by alkaline potassium persulfate (HJ 636); hexavalent chromium by diphenylcarbazide; metals by flame or graphite furnace atomic absorption and ICP.
Worked example
COD by the dichromate method: ferrous ammonium sulfate titrant 0.2500 mol/L; blank 24.50 mL; sample 18.20 mL; sample volume 20.00 mL.
- COD = (V0 − V1) x C x 8000 / V = (24.50 − 18.20) x 0.2500 x 8000 / 20.00.
- 24.50 − 18.20 = 6.30 mL; 6.30 x 0.2500 = 1.575 mmol; x 8000/20.00 gives 630 mg/L.
- The 8000 factor is 8 mg O2 per millimole of electrons times 1000 to convert to litres; a student who uses 8000/V but forgets the sample volume will report 12,600 mg/L.
Traps
- Confusing detection limit with determination limit and with the regulatory limit.
- Using duplicates to judge accuracy; duplicates show precision, spiked recovery shows bias.
- A BOD5 dilution that leaves less than 1 to 2 mg/L residual dissolved oxygen gives a falsely low result.
- Non-isokinetic stack sampling under-represents coarse particles.
- Reporting a value below the detection limit as a number.