Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

nutritional characteristics of the Pb dosed diets. In many field and laboratory studies, dietary
exposure is rarily considered, but food provided to biota in these studies adsorb metals from
water. Therefore, both dietary and waterborne exposure are occurring and both may be
considered to play roles in eliciting the measured effects.
Lead concentrations in the tissues of aquatic organisms are generally higher in algae and
benthic organisms and lower in higher trophic-level consumers (Eisler, 2000). Thus, trophic
transfer of Pb through food chains is not expected (Eisler, 2000). Metals are not metabolized;
therefore, they are good integrative indicators of exposure in aquatic biota (Luoma and Rainbow,
2005). Metal uptake is complex, being influenced by geochemistry, route of exposure (diet and
adsorption), depuration, and growth (Luoma and Rainbow, 2005). This section discusses the
factors affecting uptake of Pb by aquatic biota and the state of current research in this area.
As described in Section AX7.2.2.1, the solubility of Pb in water varies with pH,
temperature, and ion concentration (water hardness) (Weber, 1993). Lead becomes soluble and
bioavailable under conditions of low pH, organic carbon content, suspended sediment
concentrations, and ionic concentrations (i.e., low Cd, Ca, Fe, Mn, Zn) (Eisler, 2000). Lead
rapidly loses solubility above pH 6.5 (Rickard and Nriagu, 1978) and precipitates out as Pb(OH) +
and PbHCO3 + into bed sediments. However, at reduced pH levels or ionic concentrations,
sediment Pb can remobilize and potentially become bioavailable to aquatic organisms (Weber,
1993).
The most bioavailable inorganic form of Pb is divalent Pb (Pb 2+ ), which tends to be more
readily assimilated by organisms than complexed forms (Erten-Unal et al., 1998). On the other
hand, the low solubility of Pb salts restricts movement across cell membranes, resulting in less
accumulation of Pb in fish in comparison to other metals (e.g., Hg, Cu) (Baatrup, 1991).
The accumulation of Pb in aquatic organisms is, therefore, influenced by water pH, with
lower pHs favoring bioavailability and accumulation. For example, fish accumulated Pb at a
greater rate in acidic lakes (pH = 4.9 to 5.4) than in more neutral lakes (pH = 5.8 to 6.8) (Stripp
et al., 1990). Merlini and Pozzi (1977) found that pumpkinseed sunfish exposed to Pb at pH 6.0
accumulated three-times as much Pb as fish kept at pH 7.5. However, Albers and Camardese
(1993a,b) examined the effects of pH on Pb uptake in aquatic plants and invertebrates in acidic
(pH ~5.0) and nonacidic (pH ~6.5) constructed wetlands, ponds, and small lakes in Maine and
Maryland. Their results suggested that low pH had little effect on the accumulation of metals by
AX7-147