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NAPL Contamination Evaluation Using GIS
By Jeffrey A. Johnson and Pramod R. Thota, Environmental Systems and Technologies

GIS is commonly used in the groundwater industry for the storage, retrieval, and display of data but its analytical capabilities are only occasionally tapped. However, GIS is an excellent analysis tool to characterize subsurface contamination. Specifically, using GIS shows the relationship between spatially variable soil, groundwater, and contaminant conditions.

GIS not only facilitates delineating quantitative values of each of these different physical and chemical media at a specific location but can also be used to determine conditions that are a function of the properties of the media. For example, groundwater velocity is a function of the hydraulic conductivity of the aquifer and the groundwater gradient. Because both of these parameters vary spatially, the velocity can be readily calculated at any point using GIS.

More time-consuming and sophisticated analyses better demonstrate the applicability of GIS in the groundwater industry. For example, the occurrence of nonaqueous phase liquid (NAPL), a distinct oil phase in the subsurface, is controlled by the volume of oil and the characteristics of the soil, hydrocarbon product, and groundwater conditions. Characterizing NAPL involves defining two key factors that determine the potential migration of the oil phase. These factors are the inherent NAPL mobility and stability.

Figure 1
Figure 1: NAPL mobility quantitatively expressed where ø is porosity, Sof is the oil saturation, Z is the product elevation, nro is the oil viscosity ratio, ksw is the saturated hydraulic conductivity of water, kro is the oil relative permeability, and Zu and Z1 are the upper and lower elevations where free oil occurs.

Inherent mobility describes the capability of separate phase hydrocarbon to migrate and is a function of numerous spatial variables. These variables include the soil capillary properties; the soil porosity; the water saturation; the oil saturation thickness, and the density, viscosity; and surface tension of the hydrocarbon. The inherent mobility is quantitatively expressed as in the formula shown in Figure 1.

Stability defines the actual movement of the NAPL plume and is the product of inherent mobility and the oil gradient. Due to the complexity and the spatial heterogeneity of the variables defining the hydrocarbon migration, inherent mobility and stability of NAPL plumes at a release site have been difficult to thoroughly characterize using nonspatial methods.

A GIS platform is an excellent analysis tool for determining these important conditions at hydrocarbon-impacted sites. By defining the spatial distribution in the properties of the soil, hydrocarbon fluid conditions, oil gradient, and oil saturation volume as layers within a GIS platform, analyses can be performed to evaluate the spatial distribution in hydrocarbon migration.

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Figure 2: The project site shown with the focus area highlighted in yellow. Red dots represent wells where NAPL was detected, and blue dots represent the wells where NAPL was not detected.

A case study illustrates the applicability of ArcGIS, with the ArcGIS Spatial Analyst extension, to determine the spatial distribution of potential hydrocarbon migration of NAPL at a former petroleum refinery and processing facility where known releases had produced five distinct oil plumes. Site characterization activities at the facility involved the installation of more than 50 soil borings and 35 groundwater wells as shown in Figure 2. Data from these locations indicated that several NAPLs were present at the site, and the plumes varied in composition from fuel oil to diesel fuel. Measured oil thicknesses ranged from less than 0.25 feet to more than 8 feet. In addition, the soils across the site varied from clay to medium sand. To further complicate the situation, the water table at the site varied daily due to tidal fluctuations in an adjacent inlet.

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Figure 3: Delineation of soils across the site

To define the spatially varying complex site conditions, thematic data layers were developed in ArcGIS to define the soil properties as shown in Figure 3 and viscosity distributions as shown in Figure 4. Data layers for product density distributions, free oil saturation distributions, temporally variable water tables, derived gradients from water tables, basemaps, and well locations with relevant attribute data were also developed.

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Figure 4: Viscosity distributions across the site

Using these data layers, spatial analyses were performed to determine the inherent NAPL mobility and stability across the site. A grid composed of 699,000, one-square-foot cells was defined across the site. For each cell, the inherent NAPL mobility and stability were calculated using various parameters as defined in the thematic layers. This analysis included surface/raster generations, slope and aspect derivations, reclassification, projection conversions, feature-to-raster conversions, clipping, spatial joins between various thematic datasets, and map and data calculations.

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