Laserfiche WebLink
191 <br /> 5. POTENTIAL FOR HUMAN EXPOSURE <br /> eventually to inorganic lead by a combination of direct photolysis, reaction with hydroxyl radicals, and <br /> reaction with ozone. The half-life of tetraethyl lead in summer atmospheres is approximately 2 hours, <br /> whereas, the half-life for tetramethyl lead will be on the order of 9 hours. In the winter, both compounds <br /> have half-lives of up to several days (DeJonghe and Adams 1986). Trialkyl compounds occur almost <br /> entirely in the vapor phase, whereas dialkyl compounds occur almost entirely in particulate form. Because <br /> of the relatively high water solubility of trialkyl and dialkyl lead compounds, washout in wet deposition is <br /> probably a major process for these compounds. In addition, the dialkyl lead compounds may be <br /> significantly removed by dry deposition. Adsorption of tetraethyl and tetramethyl lead to atmospheric <br /> particles does not appear to be an important fate process (DeJonghe and Adams 1986; EPA 1985a). <br /> 5.3.2.2 Water <br /> The chemistry of lead in aqueous solution is highly complex because this element can be found in a <br /> multiplicity of forms. Lead has a tendency to form compounds of low solubility with the major anions <br /> found in natural waters. The amount of lead in surface waters is dependent on the pH and the dissolved <br /> salt content of the water. The dissolved salt content, in turn, is dependent on the pH and the partial <br /> pressure of CO2 as well as the water temperature. In the environment, the divalent form (Pb2+) is the <br /> stable ionic species of lead. Hydroxide, carbonate, sulfide, and, more rarely, sulfate may act as solubility <br /> controls in precipitating lead from water. At a pH <5.4, lead sulfate limits the concentration of lead in <br /> solution, while at a pH >5.4, lead carbonates limit the lead concentrations (EPA 1979d). The relatively <br /> volatile organolead compound, tetramethyl lead, may form as a result of biological alkylation of organic <br /> and inorganic lead compounds by microorganisms in anaerobic lake sediments; however, if the water over <br /> the sediments is aerobic, volatilization of tetramethyl lead from the sediments is not considered to be <br /> important because the tetramethyl lead will be oxidized (EPA 1979d). <br /> In water, tetraalkyl lead compounds are subject to photolysis and volatilization with the more volatile <br /> compounds being lost by evaporation. Degradation proceeds from trialkyl lead to dialkyl lead to inorganic <br /> lead. Tetraethyl lead is susceptible to photolytic decomposition in water. Triethyl and trimethyl lead are <br /> more water-soluble and therefore more persistent in the aquatic environment than tetraethyl or tetramethyl <br /> lead. The degradation of trialkyl lead compounds yields small amounts of dialkyl lead compounds. <br /> Removal of tetraalkyl lead compounds from seawater occurs at rates that provide half-lives measurable in <br /> days (DeJonghe and Adams 1986). <br /> 5.3.2.3 Soil <br /> There is evidence that atmospheric lead enters the soil as lead sulfate, or it is converted rapidly to lead <br /> 'sulfate at the soil surface. Many plants commonly take up lead from soil and lead will eventually be <br /> Jeturned to soil when these plants decay unless they are harvested or removed (EPA 1986a). <br /> Limited data indicate that tetraethyl and tetramethyl lead are converted into water-soluble lead compounds <br /> din soil. Although tetraethyl and tetramethyl lead are not expected to leach significantly through soil, their <br /> ghly water-soluble metabolites, the trialkyl lead oxides, may be subject to leaching (EPA 1985a). <br /> f';�i <br />