scope:

1.0 EXECUTIVE SUMMARY

1.1 Introduction

The presence of gasoline in surface waters and groundwater has been an environmental concern receiving increasing consideration. Surface waters (lakes, ponds, rivers), subsurface waters, soils, and estuarine and coastal waters are all possible hydrocarbon sinks, primarily due to accidental spills and leaks both above and below the soil surface and in water systems. The treatment or removal of such contamination has been mainly via energy-intensive processes. For example, waters containing gasoline or gasoline-derived hydrocarbons have frequently been treated using activated carbon adsorption, air stripping, etc., depending on hydrocarbon concentrations. However, several natural mechanisms have been identified as acting to remove hydrocarbons. In soil systems, such mechanisms have included adsorption onto soil particles, microbial degradation, and volatilization from the soil surface. In water systems, photolysis may also be a primary removal mechanism, in addition to those mechanisms acting in the soil environment. While such mechanisms may affect the fate of hydrocarbons following accidental contamination, they may also act to remove hydrocarbons in natural treatment systems.

The assessment of the natural mechanisms acting in soil and water systems and the extrapolation of such data to possible natural treatment techniques are key in the area of gasoline contamination in surface and subsurface waters. Factors which must be considered in such an assessment include type and concentration of the hydrocarbon, removal mechanisms present, environmental conditions affecting or influencing removal, and possible applications to natural treatment techniques.

In an earlier TRC report to API ("Literature Survey: Hydrocarbon Solubilities and Attenuation Mechanisms"), components of gasoline and their solubilities in pure water were identified. In addition, natural removal mechanisms occurring in the subsurface (soil/ground water) were considered, such as adsorption, biodegradation, and volatilization.

During this phase of the project, the literature was searched for information on the natural removal/attenuation mechanisms present in aquatic systems. Six chemicals, previously agreed upon by API and TRC, were considered: benzene, toluene, ethylbenzene, xylene (o, p, m), methyl-t-butyl ether (MTBE), and t-butyl alcohol (TBA). Evaluation of these chemicals showed that: they were poorly soluble in water. The aromatics as a group, however, were more soluble than the other major hydrocarbon groups which comprise gasoline, e.g., the alkanes.

The evaluation of the removal/persistence of hydrocarbons in gasoline-csntaminated waters considered several mechanisms including biodegradation, adsorption, volatilization, and photolysis. Factors which were considered in the evaluation of biodegradation included indigenous microbial populations, temperature, seasonal effects, nutrient availability, and seeding. Adsorption of these aromatics was evaluated, as well as the effects of sediment organic carbon content and sediment concentration. Volatilization was examined as a function of vapor pressure and temperature. Finally, photolysis in water systems was evaluated.

The incorporation of these mechanisms into natural treatment processes was searched in the literature. Processes considered included lagooning, land farming, and channeling. Unfortunately, only a limited amount of data was located on such processes,

1.2 Summary of Results

This literature search was performed using several computerized data bases. Review of the primary references obtained from the computerized search produced a list of secondary references. Once these were reviewed and evaluated, further information was obtained through telephone interviews with major researchers. Information from each reference was then summarized on a review form. A summary of the findings follows.

The literature search involved four major aromatic components of gasoline (benzene, toluene, ethylbenzene, and the xylenes) and two additives (methyl-t-butyl ether (MTBE) and t-butyl alcohol (TBA)). However, it appears that the attenuation of these additives in aquatic systems has either not been researched or has not yet been reported in published sources. Benzene and its derivatives are of particular importance in aquatic systems. Benzene has the highest solubility of those aromatics considered here (1780 ± 45 mg/l at 25°C) (McAuliffe, 1966). The solubility of MTBE in water at 2O°C is approximately 48,000 mg/l (Csikos et al., 1976).

The natural removal mechanisms identified in this search included biodegradation, adsorption, volatilization, and photolysis. The aromatics considered here appeared relatively susceptible to microbial attack and subsequent degradation. However, numerous factors were identified which may affect hydrocarbon degradation, including temperature, nutrient availability, and sediment characteristics. It has been suggested by several researchers that petroleum products or mixtures of components may be degraded differently than individual hydrocarbons in solution. Under certain environmental conditions, biodegradation increases when hydrocarbons are sorbed to suspended solids. Laboratory conditions, however, do not always simulate natural conditions, thus care must be taken while extrapolating laboratory data to natural environmental conditions. Unfortunately, few full-scale studies which quantify or qualify gasoline contamination in aquatic systems and subsequent biodegradation have been published.

The information in the literature on monocyclic aromatic adsorption generally minimizes the importance of this mechanism as a primary environmental fate. Contradictions in various authors' conclusions regarding sorption (e.g., the influences of pH, sediment:water ratio, and organic carbon content) prohibit any definite conclusions regarding sorption. However, sorption doer; appear to vary inversely with solubility.

Volatilization of the aromatics was identified as a major removal mechanism from aquatic systems, and the effects sf vapor pressure result in high volatilization rates. However, the amount of data generated from natural systems containing dissolved gasoline components and subsequent decay due to volatilization is not substantial.

Data on the photolysis of the aromatics is also limited, and the data which has been generated has involved component volatilization from a synthetic jet fuel. Generally photolysis sf the monocyclic aromatics was found to be low. Environmental conditions such as sunlight intensity and suspended solids would most likely influence photolysis rates, but quantification of such factors is lacking in the published literature.

Having covered natural removal mechanisms, the emphasis of this study became the evaluation of natural treatment alternatives. Numerous treatment alternatives for the removal of organics from "wastewaters" are available. The majority of these treatment processes used in the past are not "natural" and are often highly energy consumptive, such as activated carbon adsorption. Several natural treatment alternatives which may be used for hydrocarbon removal include wastewater lagoons/holding ponds and land application/land farming. Because volatilization is the major removal mechanism in aquatic systems for the aromatics, lagoons may be a feasible method. Obviously, the choice of such a method would depend on land availability, concentration of aromatics in solution, etc, Air emission rates must be considered for the highly volatile components. In addition, the anaerobicity of such a pond (without aeration) should be investigated. Land spreading and overland flow land treatment may also be alternatives. Due to the biodegradability of the hydrocarbon components studied in soils, removal may occur to reasonable or acceptable degrees. In overland flow systems, volatilization may also contribute significantly to removal. Application rates, source strengths, land availability, and soil characteristics are factors which have not been researched with the aromatics. Significant amounts of additional information are needed.

It is apparent that many gaps are present in this literature. Data on MTBE and TBA and their effects on gasoline and hydrocarbon solubility are necessary. In addition, the fates of these components and their susceptibility or resistance to sorption, biodegradation, etc., should be established.

It appears that major emphasis in research is needed in the area of natural treatment techniques. The two treatment processes tentatively identified here (i.e., lagooning and land treatment) have not been examined for the treatment of gasoline-contaminated water. Bench-scale studies would be necessary to determine the feasibility of these methods. Land availability and air/odor emissions are two possible factors which may limit these methods' applications.