Question 6 of 6: An Environmental Monitoring Program

Question 6: An Environmental Monitoring Program (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.

Program and objectives. The program monitored groundwater quality downgradient of a decommissioned fuel-storage facility in British Columbia, following a Phase II Environmental Site Assessment that identified a benzene, toluene, ethylbenzene and xylenes (BTEX) plume. The short-term objective was to delineate the current horizontal and vertical extent of the plume and confirm groundwater flow direction; the long-term objective was to track whether the plume was stable, expanding, or naturally attenuating over several years, to support a risk-based closure decision under the BC Contaminated Sites Regulation.

What was sampled, and how. Groundwater was sampled from a network of purpose-installed monitoring wells (2-inch PVC, screened across the water table) using a low-flow purge-and-sample technique with a peristaltic pump, minimizing drawdown-induced disturbance of the plume. Field parameters (pH, dissolved oxygen, conductivity, temperature, ORP) were logged with a flow-through cell until stabilized before each sample was drawn, following standard low-flow groundwater sampling protocol.

Sampling design and frequency. A combination of judgmental placement (wells sited from the known source location and inferred flow path, per the Phase II assessment) and systematic (grid) placement (additional wells on a regular spacing to bound the plume where its extent was still uncertain) was used – a purely random design was not practical because the population of interest (the plume) is spatially concentrated and its boundary, not the whole site, needed to be resolved. Fifteen wells were sampled quarterly for the first two years (to capture seasonal water-table fluctuation) and then semi-annually for three further years once the plume was shown to be stable, for a total of roughly $15\times(4\times2+2\times3)=210$ samples over the five-year program.

Hypotheses and statistical analyses. The primary hypothesis tested each round was $H_0:$ mean BTEX concentration in a given well has not changed from the previous sampling round (vs. $H_a:$ an increasing or decreasing trend exists), assessed with the Mann-Kendall trend test – appropriate because concentration data are typically non-normal and often contain non-detects, per Question 1(b) and Question 4. Spatial extent was mapped by ordinary kriging of the interpolated concentration surface at each round, and a two-sample t-test (or its non-parametric Mann-Whitney equivalent, given the skewness typical of this kind of data) compared upgradient (background) vs. downgradient well concentrations to confirm the plume was distinguishable from background.

Laboratory analyses and standards. Samples were analyzed by a CALA-accredited laboratory using EPA Method 8260 (purge-and-trap GC-MS) for BTEX, with full QA/QC (trip blanks for volatiles, field duplicates every tenth sample, matrix spikes). Results were compared against the applicable BC Contaminated Sites Regulation Schedule 6 water-use standards (drinking-water and aquatic-life protection generic numerical standards) rather than a US EPA MCL, consistent with the site's Canadian regulatory jurisdiction.

Other relevant issues. Seasonal water-table fluctuation was a significant confounder in the first two years' data, which is why the design stepped down from quarterly to semi-annual sampling only after two full annual cycles confirmed the seasonal pattern was consistent and could be accounted for rather than mistaken for a real trend. Well-network longevity (biofouling, silting-in of screens) required periodic redevelopment, and one well was replaced mid-program after its screen became compromised – a reminder that a monitoring network's own infrastructure needs its own QA program, separate from the sample data it produces.