Increasingly, ground water supplies are being found to be contaminated with organic substances. Depending on the nature of these substances and the method by which they were introduced into the environment, they may form a second, dense liquid phase. Several such cases have been reported in the recent literature. In other cases, such a second phase might have actually been present but was not found because the principles governing its distribution were not understood and no effort was made to look for it. In this case, the undetected material could continue to migrate and act as a source of dissolved organics. This article reviews the general principles governing the behavior of dense, immiscible second‐phase organic liquids in both the unsaturated and saturated zones, based primarily on the petroleum industry literature for primary hydrocarbon migration and on the oil spill literature. This specialized knowledge can be used to conduct a meaningful and efficient investigation of such contamination sites and can also be used to select and implement successful remedial measures. Several actual case studies applying the principles of multi‐phase flow are also presented.

Current site assessment techniques do not always generate adequate information regarding the presence, type, or distribution of nonaqueous phase liquids (NAPLs) at sites with ground water contamination. Without this information, however, the design of remediation methods is uncertain, often resulting in costly and/or failed attempts to minimize risks associated with the contamination. In this work, it is proposed that a thorough multivariate analysis of data from ground water sampling efforts could improve our overall understanding of these sites. Multivariate analyses can provide considerable insight into the contaminant source characteristics by elucidating correlations in ground water concentrations that identify recurring chemical patterns or “signatures.” These correlations are related to the type of NAPL and proximity to the contaminant source. Multivariate plots and a principal components analysis (PCA) are used to interpret ground water data from a manufactured gas plant site in Iowa contaminated with both gasoline and coal tar. Conclusions from these analyses regarding the distribution of NAPL contaminants were generally consistent with those derived from direct physical evidence of the NAPL sources. The multivariate analyses, however, provide an additional level of interpretation regarding the distribution of coal tar in the subsurface that was not possible with the standard evaluation techniques used during the remedial investigation (RI). Comparison of chemical signatures of ground water samples among wells identified two distinct regions of coal tar contamination, and suggests which wells are impacted by each source providing greater confidence in the location of the DNAPL sources.

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The Pennsylvania Power & Light Company, in cooperation with the Electric Power Research Institute, is studying leachate generation at a large outdoor test cell of compacted coal combustion fly ash constructed at the Montour Steam Electric Station in north-central Pennsylvania. The test cell design is identical to that of the plant's active dry fly ash disposal facility. Leachate initially generated at the test cell contained concentrations of chromium and selenium approximately ten times above EPA's assigned Maximum Contaminant Levels (MCL's) for drinking water. Concentrations of these two parameters have since decreased approximately five-fold. Similar reductions have occurred in the concentrations of several of the other trace metals measured in the leachate. The dominant reaction mechanisms controlling leachate composition were examined in an attempt to explain these and other observations. Deterministic chemical modeling was performed to aid in the interpretation of the data. The dynamics of leachate generation were also examined through an evaluation of the infiltration and redistribution of moisture.

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