1、HEALTH AND ENVI RONMENTAL SCI ENC ES DEPARTMENT API PUBLICATION NUMBER 4593 SEPTEMBER 1994 API PUBLX4593 94 0732290 054Lb43 934 Transport and Fate of Non- BTEX Petroleum Chemicals In. Soils and Groundwater I. E ad Pulurdrp American Petroleum Institute 1220 L Street, Northwest 11 Washington, D.C. 200
2、05 API PllRb*4593 94 0732290 0543644 70 % Envimnmer#al Partnmbip One ofthe most significant long-term trends affecting the future vitality of the petroleum industry is the publics concerns about the environment. Recognizing this trend, API member companies have developed a positive, forward Wing str
3、ategy called STEP Strategies for Todays Environmental Partnership. This program aims to address public concerns by improving our industrys environmental, health and safety petformance; documenting performance improvements; and communicating them to the public. The toUndat one based on mammalian non-
4、carcinogenic toxicity; and one based on a combination of abundance in petroleum products and properties favoring migration through soils. From these three lists, a set of 12 compounds that represented the range of environmental behavior and toxicity was selected. The subsequent study elements then i
5、ncluded a general review of the sources and fates of the 12 selected compounds in soil and groundwater (Section 4), a more detailed evaluation of subsurface transport (Section 5), a review of biological effects (toxicity) data (Section 6), and a review of available chemical analysis methods (Section
6、 7). The general fate and subsurface transport evaluations were based on chemical properties reported in the literature and commonly used methodologies for evaluating subsurface chemical fate and transport. Based ES-1 on the results of this work, recommendation for further work, including laboratory
7、 and field studies were developed (Section 8). Chemical Composition and Properties The chemical compositions of crude and refined oils are extremely complex and vary substantially. The most abundant and readily quantifiable non-BTEX hydrocarbons in all crude and refined products are the low molecula
8、r weight saturated and aromatic hydrocarbons. As a general rule, concentrations of individual hydrocarbons, and related oxygen-, sulfur-, and nitrogen-substituted hydrocarbons within an homologous series decrease as molecular weight increases. The major exception to this generalization is that Cl- t
9、hrough C,-alkyl aromatic and heterocyclic compounds often are more abundant than the unalkylated parent compounds. The most abundant non-BTEX aromatic hydrocarbons in most refined and crude oils include: trimethylbenzenes; tetralins/indans; tetramethylbenzenes; naphthalene; methylnaphthalenes; dimet
10、hylnaphthalenes; methylphenanthrenes; and dimethylphenanthrenes. Some used engine oils also contain relatively high concentrations of methylfluorenes/pyrenes, benz(a)anthracene, and methyl benzopyrenes. The most abundant saturated hydrocarbons are the normal alkanes (paraffins) from hexane through d
11、ecane; higher molecular weight n-alkanes also are abundant, but abundance tends to decrease with increasing molecular weight. Light refined products and light crude oils also contain high concentrations of cyclohexane, methylcyclohexane, isopentane, methylpentane, trimethy lpentanes, dimethylbutane,
12、 and methylcyclopentane. Gasoline may contain percent concentrations of pentenes and methylpentenes (olefins). The aqueous solubility decreases and octanol/water partition (Kow) increases as molecular weight increases for all classes of hydrocarbons. As a general rule, chemicals with an aqueous solu
13、bility less than about 0.1 mgL and a Log kw greater than about 5.0 will have limited mobility in soils and are unlikely to appear in groundwater at greater than ultratrace (low parts per billion) concentrations. By this definition, the most mobile hydrocarbons include monoaromatic hydrocarbons with
14、molecular weights less than that of n-hexylbenzene, polycyclic aromatic hydrocarbons with molecular weights less than that of pyrene, saturated hydrocarbons with molecular weights less than that of n-nonane, alkenes with molecular weights less than that of 1-nonene, and heterocyclic compounds with m
15、olecular weights less than that of methyldibenzothiophenes. If bulk transport of a petroleum product occurs through subsurface soils, then higher molecular weight hydrocarbons may ultimately migrate to groundwater. The 12 organic components of crude and refined petroleum chosen for more detailed eva
16、luation in this report include: ES-2 Compound Molecular Weight Solubility (mg/L) Log KO, Cyclohexane n-Hexane 2,3-Dimethylbutane 2,2-Dimethylpentane 1,2,4-Trimethylbenzene Naphthalene 2 -Methylnaphthalene Dibenzothiophene 1 -Methylphenanthrene Benz(a)anthracene 5-Methylchrysene Benzo( a)pyrene 84.2
17、86.2 86.2 100.2 120.2 128.2 142.2 184.3 192.3 228.3 242.1 252.3 57.5 13.0 19.1 20.0 60.0 34.4 25.5 1.47 0.27 0.01 0.007 0.004 3.44 3 .O0 3.85 3.10 3.65 3.37 4.1 1 5.08 5.14 5.91 6.42 6.83 Seven of these compounds are abundant in crude oils: n-hexane, cyclohexane, 1,2,4- trimethylbenzene, naphthalene
18、, 2-methylnaphthalene, 1 -methylphenanthrene, and dibenzothiophene. Three potentially carcinogenic polynuclear aromatic hydrocarbons (PAH), benz(a)anthracene, benzo(a)pyrene, and 5-methylchrysene, are present at trace concentrations in most crude oils and at higher concentrations in some residual pe
19、troleum products and used engine oils. These higher molecular weight PAH generally are present at only trace concentrations in gasoline and middle distillate fuels. Gasoline and light middle distillate fuels, such as kerosene and jet fuels, contain high concentrations of hexane, cyclohexane, dimethy
20、lbutane, dimethylpentane, and 1,2,4-trimethylbenzene. Middle distillate fuels, particularly the heavier fuels, such as diesel fuel and No. 2 home heating oil, as well as many of the residual fuels, contain moderate concentrations of naphthalene, phenanthrene, dibenzothiophene, and their alkyl homolo
21、gues. The values for physical/chemical properties for the 12 compounds span the range of physicdchemical properties for the major chemical components of crude, refined, residual, and used oils. Therefore, their behavior in soils and groundwater should be characteristic of most mobile nonpolar organi
22、c compounds in petroleum. Sources and Fate Not all the hydrocarbons and related hetero-substituted compounds in soils are derived from petroleum products. Other fossil fuels, such as peat and coal, contain a wide variety of saturated, aromatic, and heterocyclic hydrocarbons. The burning (pyrolysis)
23、of organic matter produces a wide variety of hydrocarbons, particularly high molecular weight PAH, similar to those in crude oil and refined or residual oil products. Normal and branched alkanes are synthesized by nearly all living organisms, particularly bacteria and plants. Some aromatic hydrocarb
24、ons or substituted aromatic hydrocarbons may be synthesized by living organisms. There probably are no mobile hydrocarbons or related heterocyclic compounds that are unique ES-3 API PUBLU4593 94 = 0732290 O543659 2TL to petroleum. However, hydrocarbon assemblages from different sources differ widely
25、 in composition and complexity. The most complex assemblages are from petroleum. There are very few reliable data on the concentrations of petroleum-derived hydrocarbons in soils and sediments. Because aliphatic and olefinic hydrocarbons are readily biosynthesized by many species of microbes, plants
26、, and animals, their concentrations in soils and sediments rich in organic matter often are high. Therefore, it is difficult to use saturated hydrocarbons as an indication of the level of contamination of soils with petroleum hydrocarbons. Concentrations of individual and total PAH in soils and sedi
27、ments vary widely primarily in relation to proximity to known sources. However, because of wide airborne dispersal of PAH- contaminated soot from anthropogenic and natural combustion sources, PAH are ubiquitous trace contaminants of all soils. Soils from remote areas may contain less than 1.0 mgkg (
28、ppm) total hydrocarbons; heavily-contaminated soils from industrial sites may contain more than 20 percent total hydrocarbons. Concentrations of total PAH range from less than about 0.1 mgkg in clean soils to at least 25 mgkg in some contaminated soils from industrial sites. Nearly all soils contain
29、 colonies of bacteria and fungi that are capable of biodegrading at least some petroleum hydrocarbons. Soil bacteria and fungi show a tremendous diversity and adaptability in utilizing different types of organic molecules as a sole or supplemental carbon source. Many groups of microorganisms are abl
30、e to oxidize saturated and, to a lesser extent, aromatic hydrocarbons and heterocyclic compounds completely to carbon dioxide and water and use them as a source of carbon for biomass accretion. In some cases, aromatic hydrocarbons and heterocyclics are metabolized only partially to a variety of pola
31、r, oxygenated metabolites. Rates of hydrocarbon degradation are much lower under anoxic than oxygen-rich conditions. Following a release of crude or refined petroleum to soils, different hydrocarbon classes are degraded simultaneously, but at widely different rates by indigenous microbiota. Low mole
32、cular weight n-alkanes with chain lengths of 10 to 22 carbons are metabolized most rapidly, followed by isoalkanes and higher molecular weight n-alkanes, olefins, monoaromatics, PAH, and finally, highly condensed cycloalkanes, resins and asphaltenes. Sulfur heterocyclics seem to be more resistant to
33、 microbial degradation than PAHs of similar molecular weight. Subsurface Transport Several transport processes control the physical motion of petroleum chemicals through and between the NAPL (non aqueous phase liquid), aqueous, and sorbed solid phases in soils. When released to the subsurface enviro
34、nment, petroleum hydrocarbons are usually initially in the NAPL (oil) phase. NAPL transport, like any subsurface liquid motion, is driven by head gradients, the rate being a direct function of the permeability of the soil, and the travel distance being limited by the amount of liquid available. In t
35、he unsaturated zone, the NAPL gradients are predominantly vertical, producing a downward flow direction. As the NAPL ES-4 - API PUBLU4593 94 m 0732290 0543660 TL3 m migrates downward, it leaves some material behind in the pores of the porous medium. The amount left behind is called the residual satu
36、ration and is held in place by capillary forces against the downward force of gravity. The NAPL will continue to migrate vertically through the unsaturated zone until one of the following three conditions is achieved: . The mass of NAPL necessary to maintain flow is depleted due to the residual satu
37、ration left behind. The permeability declines to a level at which the downward gravi-tational force can not overcome the capillary force. This can occur either abruptly, at a stratigraphic discontinuity, or gradually. The top of the saturated zone is reached. At this point, the NAPL will float on th
38、e water if the density of the NAPL is less than water or sink through the water column if the density is greater than water. Floating NAPL may accumulate in a mound on the groundwater surface and then travel radially in all directions outward from the mound center. The centroid of mass, however, wil
39、l move down gradient with respect to the groundwater. In general, the lower molecular weight hydrocarbons within the NAPL Will evaporate and diffuse upward, while the higher molecular weight compounds will remain within the NAPL pool. In addition, chemicals with low oil/water partition coefficients
40、will tend diffuse outward and dissolve into the aqueous phase. These processes will eventually cause depletion of the NAPL in low molecular weight chemicals, although at a slow rate. Some chemicals and chemical fractions (e.g., resins and asphaltenes) in petroleum NAPL have such low environmental mo
41、bilities that they will remain as NAPL essentially indefinitely, or solidi as the lighter fractions are removed. Compounds in petroleum products that boil at temperatures below about 250C, or have vapor pressures greater than about 0.1 mm Hg, will tend to evaporate from the surface of an oil deposit
42、 or from oil-contaminated unsaturated soils. Included in this category are alkanes up through n-dodecane (vapor pressure, 0.12 mm Hg) and aromatics up through naphthalene (vapor pressure, 0.09 mm Hg as a solid and 0.24 mm Hg as a liquid). The rates of evaporation of different hydrocarbons are direct
43、ly proportional to their vapor pressures. Estimates were made of the equilibrium distribution among air, soil water, and soil particles for the dissolved (aqueous) phase of the 12 hydrocarbons evaluated in detail, for soils containing 0.1 and 5 percent total organic carbon and for both saturated and
44、 unsaturated soils. In unsaturated soils, the twelve hydrocarbons will be distributed primarily in the air and soil phases. As the concentration of total organic carbon in the soil increases, the fraction of each hydrocarbon sorbed to soil particles increases. The high molecular weight PAH sorb near
45、ly completely to soil particles under all conditions. ES-5 API PUBLg4593 94 0732270 054LbbL 5T m A majority of the compounds of interest also sorb to soil particles if the soil is saturated. As in unsaturated soils, the fraction of sorbed hydrocarbons increases as the concentration of total organic
46、carbon in the soil increases. These results indicate that, once in the groundwater environment, the compounds of interest will remain primarily sorbed to soil particles. In the saturated zone, evaporation is negligible. Ail of the selected hydrocarbons, however, particularly the more soluble low mol
47、ecular weight ones, will migrate slowly through the soil, transported by groundwater. The rate of migration of hydrocarbons through soils in the aqueous phase is inversely proportional to the soilwater partition coefficient for the chemical and directly proportional to the permeability of the soil.
48、The compounds of interest can be divided into two groups based on mobility in the aqueous phase. The more mobile compounds include 1,2,4-trimethylbenzene, naphthalene, 2- methylnaphthalene, cyclohexane, n-hexane, 2,3-dimethylbutane, and 2,2-dimethylpentane. The relatively less mobile compounds inclu
49、de benz(a)anthracene, benzo(a)pyrene, 5- methylchrysene, 1 -methylphenanthrene, and dibenzothiophene. The single best criterion to use for assignment to a class is the value for log kW . Compounds with log kW less than about 5.0 are in the first class. Compounds with log KO, greater than about 5.0 are in the second class. Biological Effects Saturated and aromatic hydrocarbons have acute toxicities to aquatic organisms, expressed as the median effective dose (EC50), that span a range of 8,000 rnm01/m3 or more. As a general rule, there is an inverse correlation betw
