Question 3 of 7: Soil Remediation and Measurement Quality

Problem 3: Soil Remediation and Measurement Quality (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) In-situ remediation of toxic organic waste in soil

In-situ soil vapour extraction (SVE) with bioventing is the representative engineering method for a volatile/semi-volatile toxic-organic-contaminated site: it treats the soil in place, avoiding the excavation, transport and disposal liabilities of an ex-situ approach.

1. Site characterization and well network design. Delineate the contaminant plume (soil gas surveys, borings) and design an array of vertical/horizontal vapour-extraction and air-injection wells spaced to achieve an effective radius of influence throughout the impacted zone, verified by a pilot vacuum/pressure test.
2. Extraction with bioventing. Apply vacuum at the extraction wells to volatilize and draw off the more volatile fraction of the contaminant (SVE), while injecting air/oxygen at a low flow rate at separate wells to sustain aerobic biodegradation of the less-volatile fraction in place (bioventing) — the two mechanisms run concurrently and target different volatility fractions of the same contaminant mass.
3. Off-gas treatment and confirmatory monitoring. Route extracted vapour through granular activated carbon (or a thermal/catalytic oxidizer for high concentrations) before atmospheric release, and confirm progress with periodic soil-gas and groundwater monitoring against the site's cleanup criteria until site closure.

(ii) Measurement variability and instrument sensitivity in compliance sampling

Variability is the spread of repeated measurements of the same nominally constant condition — it comes from both true process variation (diurnal/seasonal loading swings in the actual effluent) and measurement (analytical) variation, and it is what a compliance sampling plan's frequency and statistical averaging period (e.g., a 30-day rolling average permit limit) is designed to average out without masking a genuine, sustained exceedance. Instrument sensitivity is the smallest concentration change the method can reliably resolve (the calibration-curve slope); if the method detection limit is not well below the permit limit, small true exceedances near the limit cannot be distinguished from noise/variability, undermining enforceability.

Designing a defensible compliance sampling plan therefore means: (1) choosing a sampling frequency and compositing strategy (grab vs. flow-proportional composite) matched to the process's own variability timescale so the reported value represents the true average condition, not a lucky/unlucky grab sample; and (2) selecting an analytical method whose sensitivity (and detection limit) is fine enough, relative to the permit limit, that the plan can actually distinguish compliance from non-compliance rather than reporting values that are statistically indistinguishable from the limit.