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189 5. POTENTIAL FOR HUMAN EXPOSURE The amount of lead that remains in solution in surface waters depends upon the pH of the water and the dissolved salt content. Equilibrium calculations show that at pH >5.4, the total solubility of lead is approximately 30 µg/L in hard water and approximately 500 ug/L in soft water. Sulfate ions, if present in soft water, limit the lead concentration in solution through the formation of lead sulfate. Above pH 11 5.4, the lead carbonates, PbCO.t and Pb2(OH)2CO31 limit the concentration. The carbonate concentration Al is in turn dependent upon the partial pressure of carbon dioxide, pH, and temperature (EPA 1986a). In ;n most surface waters and groundwaters, the concentration of dissolved lead is low because the lead will form �f compounds with anions in the water such as hydroxides, carbonates, sulfates, and phosphates that have low ,s water solubilities and will precipitate out of the water column (Mundell et al. 1989). if Ad A significant fraction of lead carried by river water is expected to be in an undissolved form, which can Ad consist of colloidal particles or larger undissolved particles of lead carbonate, lead oxide, lead hydroxide, et or other lead compounds incorporated in other components of surface particulate matters from runoff. Lead may occur either as sorbed ions or surface coatings on sediment mineral particles, or it may be carried as a part of suspended living or nonliving organic matter in water. The ratio of lead in suspended -ic solids to lead in dissolved form has been found to vary from 4:1 in rural streams to 27:1 in urban streams ,m (Getz et al. 1977). The fate of lead in soil is affected by the specific or exchange adsorption at mineral interfaces, the precipitation of sparingly soluble solid forms of the compound, and the formation of relatively stable organic-metal complexes or chelates with soil organic matter. These processes are dependent on such factors as soil pH, and organic matter content of soil, the presence of inorganic colloids and iron oxides, ion-exchange characteristics, and the amount of lead in soil (NSF 1977). The accumulation of lead in most ad soils is primarily a function of the rate of deposition from the atmosphere. Most lead is retained strongly ,)n. is soil, and very little is transported into surface water or groundwater (EPA 1986a; NSF 1977). Lead is )ns strongly sorbed to organic matter in soil, and although not subject to leaching, it may enter surface waters ;ric as a result of erosion of lead-containing soil particulates. Lead may be converted to lead sulfate at the ers soil surface which is relatively soluble when compared with lead carbonate or phosphate. Lead may also ces be immobilized by ion exchange with hydrous oxides or clays or by chelation with humic or fulvic acids -iay m the soil (Olson and Skogerboe 1975). In soils with pH of 2:5 and with at least 5% organic matter -nic content, atmospheric lead is retained in the upper 2-5 cm of undisturbed soil. Inorganic lead may be in bound into crystalline matrices of rocks and remain essentially immobile. Lead complexes and precipitates al. io soil and their transformation depend on the soil type. In soil with a high organic matter content and age ?pH of 6-8, lead may form insoluble organic lead complexes; if the soil has less organic matter at the AS same pH, hydrous lead oxide complexes may form or lead may precipitate out with carbonate or phosphate ion bns. At a pH of 4-6, the organic lead complexes become soluble and leach out or may be taken up by unt Punts (EPA 1986a). Entrainment of soil particles is another route of lead transport (EPA 1982a). This to 11�!er process may be important in contributing to the atmospheric burden of lead around some lead ven �taelting facilities and NPL sites that contain elevated levels of lead in soil. 1 is han 1�e downward movement of lead from soil to groundwater by leaching is very slow under most natural V Lied �nditions except for highly acidic situations (NSF 1977). The conditions that induce leaching are the han Prsence of lead in soil at concentrations that either approach or exceed the cation exchange capacity can , C) of the soil, the presence of materials in soil that are capable of forming soluble chelates with lead, kes d a decrease in the pH of the leaching solution (for example, acid rain) (NSF 1977). Partial favorable ghat editions for leaching may be present in some soils near lead smelting and NPL sites. Leaching of bluble lead from contaminated soils into groundwater may be minimized by the presence of lead carbonate the soil and maintaining a soil pH of 8-10 (Mundell et al. 1989).