UNDERSTANDING SOIL TPH RESULTS Peter Robinson, Hill Laboratories, Hamilton, NZ

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1 UNDERSTANDING SOIL TP RESULTS Peter Robinson, ill Laboratories, amilton, NZ Abstract Total Petroleum ydrocarbon (TP) analyses are requested for a range of reasons, from petroleum industry site work through contaminated sites and spills to monitoring of trade wastes, tip leachates and groundwaters. This paper will briefly cover how a TP test is carried out in the laboratory, the variations of the test that exist, including historical methods, and the extra, and sometimes unexpected, information that can be obtained by looking at the TP chromatogram. C 4 What is a hydrocarbon? Methane Ethane n-butane Compound containing only hydrogen and carbon atoms n-decane 2,2,4-Trimethylpentane (isooctane) Sources ydrocarbons in the environment may come from two main sources; petroleum oil based hydrocarbons, such as natural gas, LPG, petrol, kerosene, jet fuel, diesel, fuel oils, bunker oils, lubricating oil, transformer oil, greases, asphalt, and bitumen natural living sources, such as terpenes (eg rubber, pinene, limonene, camphor), phytane, pristane, squalane and squalene. Uses ydrocarbons are used principally as either fuels (the petroleum based hydrocarbons) or industrial chemicals (both petroleum based and natural). Industrial chemicals may be used as solvents and degreasing agents (toluene, xylene, Stoddard s Solvent, petroleum spirits/ethers, mineral turpentine, limonene) or as precursors for the synthesis of a wide range of chemicals such as polymers (from styrene) and detergents (from alkyl benzenes). ydrocarbons and the environment ydrocarbons can enter the environment either naturally, from spills, by leakage from storage facilities or from deliberate application (oils spread on unsealed roads, diesel as a solvent for herbicide application).

2 Importance of hydrocarbons in the environment ydrocarbons can affect the environment in a number of ways. 1 They provide an energy source for microbiological activity and so can add to the oxygen demand loading ie they contribute to BOD. 2 They can add to an odour problem eg cyclopentadiene 3 They are flammable (explosive in confined spaces) and so increase the risk of fires 4 Some are toxic a Neurotoxic eg hexane b Carcinogenic eg benzene, benzo[a]pyrene 5 Most are insoluble in water and they are also less dense than water, so they float on water bodies and may coat earth, animals, birds and other surfaces. Chemical Classification ydrocarbons (Cs) are classified chemically as saturated Cs (alkanes, only single bonds between carbon and/or hydrogen atoms) unsaturated Cs (alkenes or olefins - which contain at least one carbon-carbon double bond - and alkynes, which have a carbon-carbon triple bond, acetylene is the only common alkyne) aromatic Cs (both monoaromatic such as benzene, toluene and xylenes, and polycyclic aromatic, the PAs, such as naphthalene, benzo[a]pyrene and fluorene). ydrocarbon properties Because hydrocarbon properties depend on the size of the molecules, it is often useful to refer to the number of carbon atoms contained in a hydrocarbon. Volatility (and flammability risk) decreases with increasing size eg C1-C4 are gases, C5-C16 are liquids above C16 are solids. ydrocarbons have a low solubility in water, with solubility decreasing as size increases. A rough idea of relative solubility is given by; monoaromatic (benzene>toluene>ethylbenzene,etc) > olefins > alkanes Analysis The three tests which are commonly requested in relation to hydrocarbons in the environment are; TP (Total Petroleum hydrocarbons BTEX (Benzene Toluene Ethylbenzene and the Xylenes) PA (Polycyclic aromatoc ydrocarbons)

3 Total Petroleum hydrocarbons (TP) istorically TP was measured using US EPA Method which involved extraction into freon and measurement using infrared spectroscopy. Freon is no longer available so this method cannot now be used, which presents problems where Resource Consents specifically state the method of analysis. Methods now in use are based on subsampling from the supplied container extracting the sample into an organic solvent and analysis using gas chromatography with a flame ionisation detector (GC-FID). It is useful to consider these steps separately so that any variations between results from different laboratories can be understood. Subsampling ydrocarbons range from the very volatile (gases) to involatile (bitumen). As soon as a container is opened the volatile hydrocarbons will start to diffuse away and be lost from the analytical procedure. Laboratories will try and minimise this by keeping samples cold (we use dry ice), subsampling quickly, eg with a cork borer, and immersing the sample in the organic solvent as soon as possible. This is the most difficult step of the analysis for samples containing volatile hydrocarbons such as petrol, and the step which will probably lead to the greatest variation between laboratories. Non-homogeneous samples containing stones or other non-soil matter can make getting a representative sample very difficult. The lab takes say g of sample for analysis. This may represent many cubic metres of soil on site! We have found that petrol samples on an impervious substrate such as sand can drop by more than 90% when two separate samples are taken from the same container even a few hours apart. Absorbent matrices such as peaty soils are more resistent to loss. Extraction The sample is mixed with the solvent using a variety of methods to ensure efficient extraction. These may include sonication shaking heat ASE (Accelerated Solvent Extraction heat and pressure) supercritical carbon dioxide (heat and pressure) Provided the conditions are optimised, these will all give similar extraction efficiencies.

4 Analysis A general idea of how gas chromatography (GC) works makes it easier to interpret the final result, or chromatogram. A very schematic GC is shown in the diagram. The GC consists of a heated (300 C) injection port, a long capillary column in a temperature controlled oven, and a flame ionisation detector (FID). elium eated injection port FID Air 2 The column, typically m long, 0.3 mm ID, has an thin internal coating of a silicone gum (the stationary phase). Column Temperature controlled oven A stream of carrier gas (hydrogen or helium, the mobile phase) passes through the injection port and the column to the detector. A small aliquot of sample (2 µl) is injected and vaporises. It is carried through the column by the carrier gas where separation occurs. The most volatile compounds reach the detector first. Less volatile compounds interact with the stationary phase in the column, the larger molecules generally being more soluble in this phase, and so take longer to reach the detector. The column oven is temperature programmed from C to assist separation of a wide range of hydrocarbons from around n-heptane () to tetracosane (C40) or higher TPSTD 100ppm It can be seen that there is a large peak just before the hydrocarbon. This is the solvent used for extraction, and it overlaps other volatile compounds such as hexane, benzene, etc.

5 There is always a trade-off in selecting a GC column and conditions which will give good separation of volatiles, and one which will elute the higher hydrocarbons. Because of the loss of volatiles during extraction, US EPA and other authorities do not accept that valid results for benzene, etc, can be obtained using solvent extraction methods. ill Laboratories follows the NZ Oil Industry Environmental Working Group standard by reporting the first band from to, not C6-. The more volatile hydrocarbons must be analysed using a Purge & Trap or eadspace technique. Examples of chromatograms Motor fuels and lube oil 5.00 Petrol ppm 1.00 Diesel 500ppm 4.17 Toluene Xylenes / Lube Oil 1000ppm Toluene Xylenes Petrol contaminated water In the chromatogram of the petrol contaminated water, note the smaller toluene/xylene ratio peak compared with the petrol above. Benzene and toluene are more soluble in water than the other aromatic hydrocarbons, so are washed away first. Groundwater from the front of a contamination plume may contain only benzene, with lesser levels of toluene.

6 Polycyclic aromatic hydrocarbons typical of a gas works site / Polycyclic Aromatic ydrocarbons ex Gasworks site Toluene and 2-butoxyethanol solvents / / ex Bitumen plant Toluene 2-Butoxyethanol Phthalate plasticisers. These are very widely found in the environment, and it has been suggested that they have oestrogenic properties / Benzylbutylphthalate Bis(2-ethylhexyl)phthalate Naturally occurring terpene hydrocarbons camphor alpha-pinene 7.00 beta-pinene 3.50

7 Specific tests (necessary for Risk Assessment calculations) Either after the results of screening tests are available, or for other reasons, it may be necessary to carry out more specific C testing. The specific tests provide quantitative information, not available from the screening tests, which can be used for risk assessment calculations for example. The specific tests include; BTEX (benzene, toluene, ethylbenzene and xylenes). Used mainly for petrol contamination, but also where solvents such as toluene and the xylenes have been used. Must be carried out using a purge and trap (P&T) or headspace technique, preferably with gas chromatography-mass spectrometry (GC-MS). PA (Polycyclic Aromatic ydrocarbons, also called PolyNuclear Aromatics, PNA) These come principally from diesel, heavy petroleum fractions and from coal sources. The term Total PA, which is sometimes used, should be discouraged as there is no method which measures this. All methods separate out the individual PAs and so any Total value reported will reflect however many individual compounds the method determines before they are added to give a Total. Different methods will, therefore, give different Totals. Analysis for PA can be either by GC-MS or by PLC with a fluorescence detector. Acknowledgements I would like to thank the ill Laboratories technicians (Susan Bowyer, elen McGowan and Marty Cowell) who analysed these samples, and helped convert the chromatograms for publication, and the Organics Lab Manager, Graham Corban, for ensuring they had the time to do it.