Coal tar, creosote, and similar viscous non-aqueous phase liquids (NAPLs) behave in alluvial soils in a manner significantly different from that of less viscous NAPLs, such as gasoline and chlorinated solvents. Their unique behavior is due to the interaction of their physical-chemical parameters: a density often greater than water, a viscosity significantly greater than water, and an interfacial tension that yields a positive initial spreading coefficient at airwater-NAPL interfaces. This results in slow, creeping flow that causes long-term contamination at former manufactured gas plants and wood-preserving sites and of their adjacent surface waters. Multiphase simulations of this creeping flow are shown for a site along the lower Fraser River near Vancouver, British Columbia, and the long-term consequences of the migration of viscous NAPLs in alluvium are discussed from the perspective of site characterization and brownfields redevelopment.Résumé : Le goudron, la créosote et autres liquides visqueux similaires (NAPLs) se comportent dans les sols alluvionnaires de façon appréciablement différente des NAPLs moins visqueux tels que l'essence et les solvants chlorés. Leur comportement unique est dû à l'interaction de leurs paramètres physico-chimiques : une densité souvent plus grande que celle de l'eau, une viscosité appréciablement plus grande que celle de l'eau, et une tension superficielle qui donne un coefficient d'étalement initial positif aux interfaces air-eau-NAPL. Il s'ensuit un écoulement lent en fluage qui entraîne une contamination à long terme sur les anciens sites de production d'essence et de bois traité, et dans les eaux de surface environnantees. Nous présentons des simulations miltiphases de cet écoulement en fluage pour un site longeant la partie basse du fleuve Fraser près de Vancouver, Colombie-Britanique, et nous discutons des conséquences à long terme de la migration des NAPLs visqueux dans les alluvions, dans la perspective de la caractérisation des sites et du développement des terrains décontaminés.
We present a contaminant treatment system (CTS) package for MODFLOW 6 that facilitates the simulation of pump-and-treat systems for groundwater remediation. Using the "nonintrusive" MODFLOW 6 application programming interface (API) capability, the CTS package can balance flows between extraction and injection wells within the outer flow solution loop and applies blended concentration/mass treatment efficiency within the outer transport solution loop. The former can be important when the requested extraction rate cannot be satisfied by the current simulated groundwater system conditions, while the latter can be important for simulating incomplete/imperfect treatment schemes. Furthermore, the CTS package allows users to temporally vary all aspects of a simulated CTS system, including the configuration and location of injection and extraction wells, and the CTS efficiency. This flexibility combined with the API-based implementation provide a generic and general CTS package that can be applied across the wide range of MODFLOW 6 simulation options and that evolves in step with MODFLOW 6 code modifications and advancements without needing to update the CTS package itself.
The geosystem approach to source-zone characterization was used at a dry cleaners contaminated with tetrachloroethene (PCE) as a dense nonaqueousphase liquid (DNAPL) within a shallow, low-permeability aquifer. This comprehensive approach to source-zone characterization provided a design-basis level of knowledge of the site geosystem, which includes aquifer heterogeneity, and the volume and spatial distribution of DNAPL. DNAPL was found to occur beneath and adjacent to the dry cleaning building, with both residual and free-phase DNAPL in the bottom 4 ft (1.2 m) of the shallow aquifer. The bottom 2.5 ft (0.76 m) of the aquifer consists of a fining downward sequence that grades from fine sand to clayey silt, with a resulting decrease in permeability in the DNAPL zone by at least one order of magnitude. The DNAPL zone is bounded below by a clay aquitard that serves as an effective capillary barrier to downward DNAPL migration. A partitioning interwell tracer test (PITT) measured approximately 81 6 7 gallons (280-330 L) of DNAPL in the test zone, with the majority of DNAPL located near the building and decreasing northward away from the building. Soil samples analyzed for PCE show that the DNAPL saturations are greatest vertically from 17 to 18.5 ft (5.2 to 5.6 m) below ground surface (bgs), in the fine-sand to sandy-silt portion of the DNAPL zone. DNAPL saturations generally decrease at greater depths, from approximately 18.5 to 20 ft (5.6 to 6.1 m) bgs, in a silt to clayey silt (i.e., lower permeability) zone, with little or no DNAPL penetration into the underlying clay aquitard. Postremediation soil core analyses indicate that low permeability at depth was a limiting factor in the detection of all the DNAPL within the targeted zone using the PITT. It is concluded that the partitioning interwell tracer test successfully detected DNAPL in the more sandy portions of the aquifer, while not detecting an estimated 23 6 5 gallons (87 6 19 L) of DNAPL in the clayey silts below 17.8 ft (5.4 m) bgs.
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