Question 2 of 7: Phase II Investigation and Remediation of a Closed Chromium Plating Facility

Question A-2: Phase II Investigation and Remediation of a Closed Chromium Plating Facility (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.

Three facts from the Phase I fix the Phase II scope: the confirmed leak source is the outdoor waste-storage tanks/pits (not the building), the receiving media are an unlined concrete containment structure whose joints and cracks are the likely release point, and the site has three distinct sensitive receptors on three sides — a creek (south, ecological/surface-water receptor), housing (north and west, human-health receptor for both soil-vapour and any shallow groundwater use), and agricultural land (east, both an ecological and a potential food-chain receptor).

Phase II sampling. Soil borings immediately around and beneath the outdoor pits and tank containment, focused on the concrete wall/floor joints and any visible cracking, since that is the documented leak point; samples at multiple depths to establish vertical extent beneath the containment. A radial/transect network of soil borings and groundwater monitoring wells extending toward each of the three receptors — south toward the creek (both soil and groundwater, plus creek sediment and surface water as confirmation), north and west toward the housing (soil, soil-vapour, and groundwater if a drinking-water well or shallow utility corridor could be affected), and east toward the agricultural land (soil, given plant-uptake and grazing-animal exposure pathways are unique to this receptor). Because tests already confirmed leakage, at least one nested well pair (shallow/deep) near the tanks characterizes vertical groundwater impact, and a downgradient well establishes plume extent. Although no leakage was found from the building, a limited number of interior floor/sub-slab samples are still warranted, because Phase I evidence is not proof of absence — only proof no leak has yet been detected.

Analytical parameters: total and hexavalent chromium (Cr(VI) must be speciated separately from total Cr, since it is far more toxic and, being an anion, far more mobile in groundwater than the trivalent form); pH and redox potential (Eh) at each groundwater location, since Cr(VI)/Cr(III) speciation and mobility are strongly pH- and redox-dependent; and other plating-bath constituents typically co-present (cyanide if cyanide plating baths were used, nickel, copper, or cadmium depending on the specific plating process) even though the question specifies chromium.

Remediation technology. Because Cr(VI) cannot be destroyed — only reduced to the far less soluble, less toxic, and less mobile Cr(III), which precipitates as chromium hydroxide — the appropriate technology set is: (1) source removal — excavate and dispose of/stabilize the concentrated soil immediately beneath and around the leaking tank containment, the highest-concentration and most accessible zone; (2) in-situ chemical reduction of the residual dissolved and sorbed Cr(VI) in soil and groundwater using an injected reductant (ferrous sulphate, calcium polysulphide, or zero-valent iron), which converts the mobile Cr(VI) plume in place to immobile Cr(III); and (3), only if Phase II groundwater monitoring confirms the plume is migrating toward the creek, a pump-and-treat system with a reduction/precipitation or ion-exchange treatment train as a hydraulic-containment measure while the in-situ reduction takes effect. Excavation alone is preferred at the source because it gives immediate, verifiable removal of the worst-case material; in-situ reduction is preferred over excavation across the wider, more dilute plume area because excavating toward three separate receptor boundaries is far more disruptive and costly than treating the chromium in place.

Site clean-up standards. Because the site borders three different land uses simultaneously — residential (N/W), agricultural (E), and a surface-water/aquatic-life receptor (S) — a single "industrial" standard reflecting the site's own former use is not protective. The applicable numerical standards should be the most conservative of: the residential/parkland soil and groundwater standard along the north and west boundary (protective of the adjacent housing), the agricultural standard along the east boundary, and a surface-water/aquatic-life guideline (e.g., CCME Canadian Environmental Quality Guidelines for chromium) applied at the point of potential discharge to the creek, following the standard Canadian risk-based (e.g., Ontario Reg. 153/04 or the equivalent provincial framework) practice of setting the remediation target from the most sensitive land use actually adjoining the property, not from the property's own historical industrial classification.