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different hydrocarbon compounds and small quantities of metals, <br /> biodegrade at different rates Biodegradation rarely proceeds to <br /> uniform chemical end-products in microbial breakdown processes <br /> Larger molecules may break down slowly In minuscule quantities, <br /> biodegradation can be inhibited by a variety of factors, including the <br /> amount of methyl group branching on an aliphatic hydrocarbon chain <br /> Mineral oil, a subset of petroleum, is composed primarily of paraffinic <br /> hydrocarbons, which are thus more resistant to biodegradation than <br /> vegetable oils. <br /> C. TOXICITY <br /> A computer database search of the literature revealed no reported <br /> human toxicity associated with the ingestion of petroleum or <br /> vegetable base oils. SWRCB staff assumed that the most probable <br /> form of human exposure to hydraulic fluid in ground water is ingestion <br /> by means of drinking water Given the lack of volatility of these base <br /> oils, airborne exposure pathways are unlikely, so this potential route of <br /> exposure was not examined. It should be noted that no information <br /> on these possible routes of exposure was found in the literature <br /> search <br /> Staff also assumed that releases to soil and groundwater were unlikely <br /> to cause surface water discharges. As in the case of routes of human <br /> exposure, the literature search did not yield more information on this <br /> subject. Nonhuman exposures to hydraulic fluid base oils were fudged <br /> to be unlikely. Similarly, the database search revealed no studies or <br /> other information on this item <br /> D. CONCLUSIONS <br /> Based on the limited information in the literature, the following <br /> summary describes the probable environmental fate of hydraulic lift <br /> tank releases involving vegetable and petroleum-derived oils used as <br /> hydraulic fluids. <br /> (1) The base oils are relatively insoluble in water <br /> 121 The base oils are less dense than water, so any release to <br /> ground water will tend to float on top of the aquifer <br /> l3? The base oils have low volatility, tend to adhere to soil <br /> particles, and are relatively immobile in a subsurface <br />