Time scales of organic contaminant dissolution from complex source zones: coal tar pools vs. blobs

https://doi.org/10.1016/S0169-7722(02)00075-XGet rights and content

Abstract

Groundwater contamination due to complex organic mixtures such as coal tar, creosote and fuels is a widespread problem in industrialized regions. Although most compounds in these mixtures are biodegradable, the contaminant sources are very persistent for many decades after the contamination occurred (e.g., more than 100 years ago at gasworks sites). This limited bioavailability is due to slow dissolution processes. This study presents results from a large scale tank experiment (8 m long) on the long-term (354 days) dissolution kinetics of BTEX and PAHs from a 2.5 m long coal tar pool and 0.5 m long (smear) zone containing coal tar blobs distributed in a coarse sand. The results indicate (1) that Raoult's law holds for estimation of the saturation aqueous concentrations of the coal tar constituents, (2) that for the dissolution of smear zones longer than approximately 0.1 m and with more than 3–5% residual saturation, the local equilibrium assumption is valid and (3) that although very small (<0.1 mm), the transverse vertical dispersivity dominates the pool dissolution processes. Typical time scales for removal of the pollutants from the blob zone and the pool are in the order of a few weeks to more than 10,000 years, respectively.

Introduction

Organic compounds such as chlorinated solvents, aromatic (benzene, toluene, ethylbenzene, xylene=BTEX) and polycyclic aromatic hydrocarbons (PAHs) are the most frequently occurring contaminants in groundwater. BTEX compounds and PAHs usually occur in mixtures such as fuels, petroleum products, coal tar or wood preservatives (creosote). The constituents of such mixtures are very persistent in the subsurface environment, i.e. they are still present in high concentrations many decades or even centuries after the contamination occurred, and they cannot be removed from the source zone within a reasonable time period by pump-and-treat or in-situ biodegradation. This persistence is mainly caused by slow dissolution kinetics of the compounds from the non-aqueous phase liquids (NAPLs) and slow diffusion of the contaminants from low permeability domains (which have accumulated the pollutants over decades). Such slow processes also reduce the bioavailability of the organic compounds, leading to long-term persistence of easily biodegradable compounds in the source zone.

This study presents results from a large scale experiment for the determination of the long-term dissolution behavior of contaminants from two typical scenarios: distribution of NAPLs as blobs (representing “smear zones”, e.g. in groundwater table fluctuation zones) and pools, which form if infiltrating NAPLs encounter the groundwater table or low permeability layers. DNAPLs (NAPLs denser than water) can sink into the aquifer and pool on top of the aquitard. As an example of a complex organic mixture, an industrially produced “fresh” coal tar (Rütgers VFT AG) was selected with a density of 1.198 g cm−3, which includes numerous contaminants covering a large range of physical, chemical and toxicological properties (highly soluble aromatic compounds such as the BTEX and PAHs of lower solubility). Of the many coal tar constituents, only a fraction can be identified and quantified by common analytical methods (i.e. gas chromatography). This study includes volatile aromatic compounds (BTEX plus propylbenzene, three trimethylbenzenes, benzofuran, indane, indene) and the 17 US-EPA priority pollutant PAHs plus 1-methylnaphthalene (Table 1).

The specific objectives of the study were (1) to determine the contribution of the transverse vertical dispersion on dissolution of BTEX and PAHs from a coal tar pool by measuring the concentration profiles which develop downgradient of the 2.5 m long pool as well as by quantifying the overall dissolution fluxes, (2) to evaluate Raoult's law for prediction of the saturation aqueous concentration and (3) to validate the local equilibrium assumption for dissolution of the coal tar constituents from extended blob zones. Overall the study aimed to predict the time periods necessary for removal of coal tar constituents from residual phases.

Section snippets

Equilibrium aqueous concentrations of constituents of complex mixtures

In complex mixtures of organic compounds (e.g. coal tar, gasoline or diesel fuel), the saturation aqueous concentration of the individual components depends on the composition of the mixture Banerjee, 1984, Mackay et al., 1991, Lane and Loehr, 1992, Lee et al., 1992. It is always less than the solubility of the pure substances in water and can be determined in liquid/liquid mixtures following Raoult's law with adequate accuracy:Ci,sat=χi,oγi,oSiwhere Ci,sat, γi,o, χi,o, and Si denote the

Coal tar composition and aqueous solubility of the constituents

To determine the composition of the coal tar, 0.5 and 1 g of coal tar was extracted with 20 ml cyclohexane or 10 ml of pentane. Coal tar samples from two of in total seven coal tar vessels were taken and each was extracted in triplicate. The monoaromatic compounds, benzofuran, indane and indene were analyzed from the pentane extract in a dilution 1:100, whereas PAHs were analyzed in the cyclohexane extract after further dilution to 1:40. The composition of the coal tar and the properties of the

Saturation concentrations—Raoult's law

The composition of the coal tar and the properties of the analyzed compounds are summarized in Table 1. Fig. 2 shows the comparison of Ci,sat measured in the long-term dialyses equilibration experiments and Ci,sat calculated with Raoult's law assuming an activity coefficient of 1. In general the agreement is very good. No systematic deviation of the BTEX-compounds nor PAHs from the 1:1 line is observed indicating that the activity coefficient for all compounds is the same. The mean molecular

Conclusions—time scales

Raoult's law predicts the observed saturation concentrations in water reasonably well despite the complex composition of the coal tar; so far the activity coefficients of the compounds considered have not significantly deviated from 1 (Fig. 2).

The minimum time scale of dissolution of the coal tar constituents from the blob zone can be estimated in a first approximation from the retardation of the dissolution front for the individual compounds. As shown in Fig. 6, the time necessary to remove

Acknowledgements

This work has been supported partially by the German Federal Government through the research program PTWT (02WT9714) and by GRACOS (Groundwater Risk Assessment at Contaminated Sites) funded by the EU in the 5th research framework program.

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