Transport of Natural Lead and Cadmium in Rivers: Global Flux Implications

Lead and cadmium concentrations in marine and terrestrial ecosystems, in surfaces of soils, and in the atmosphere have been highly elevated on a global scale due to industrial pollution. In order to ascertain the natural (rock-derived) levels of lead and cadmium in streams, pristine mountain watersheds in the Sierra Nevada, California were studied for their lead and cadmium contents, and the transport of lead and cadmium was related to metal relatively uninfluenced by pollution that share similar transport patterns. In addition, rock and unpolluted water samples from granodiorite, basalt and carbonate terrains were analyzed for the concentrations of lead, iron and other elements. Wholerock samples as well as biotite and feldspar mineral separates were used for laboratory leaching and adsorption-desorption experiments to investigate the relationship between lead and iron chemistry under controlled conditions. The concentrations of lead and cadmium in the late-summer drainage are shown to be close to the natural levels that are controlled by the weathering of bedrock and soil. This is demonstrated by measurements of 1) lead isotopic composition, and Fe/Pb ratios in stream water, ground water, soil and bedrock, and 2) the removal rate of excess atmospheric lead and cadmium from the water as it flows downstream. After the spring snow-melt runoff, most of the lead in alpine streams originates from ground water, and has isotopic ratios that are consistent with values expected from bedrock and soil sources, indicating that this lead is not anthropogenic in origin. The lead found in unpolluted ground waters is more radiogenic than the lead in the bedrock drained by these waters. A preferential release of radiogenic lead into waters and leached phases of rock and soil can be explained by the preferential weathering of radiogenic accessory minerals and to a lesser extent by a preferential release of U and Th decay products due to the recoil effect. The lead uptake mechanism is proposed to be adsorption on oxy-hydroxide surfaces. In contrast, the uptake of cadmium in the stream water is erratic and cannot be explained by the same mechanism. Adsorption-desorption experiments suggest that lead coprecipitates and is adsorbed on particle surfaces, mainly ferric iron hydroxides. Due to a similar transport mechanism and comparable rate of release from common rock and soil minerals, the ratio between natural (rock-derived) lead and iron in rivers should be similar to their average upper continental crustal molar ratio of 1:6,500. Experiments and speciation models indicate that complexation of lead by manmade organic compounds decreases the fraction of lead bound to surface sites. Such an indirect pollution effect mobilizes lead and decreases the Fe/Pb ratio in rivers regardless of any direct addition of anthropogenic lead. Many trace metals maintain their average upper continental crustal ratio with iron in unpolluted river water, river sediments and soils; However, large excesses of most trace metals relative to iron are found in deep-ocean water. At the transition from fresh water to saline ocean water, two processes take place: 1) rapid removal of iron (and other particle-forming elements) from the water column due to coagulation and settling; and 2) partial desorption of trace metals from particle surfaces. While more than 99% of the riverborne iron settles to the sediment within the continental shelf, some of the trace metals are released to solution as dissolved chloro-complexes and are further transported to the open sea. In addition, some of the trace metals attached to airborne and recycled sea-floor particles may desorb when these particles are in contact with sea water. The adsorption/desorption process in sea water account for the relative abundances of many trace metals in deep-sea water (not including REE). Furthermore, it is suggested that the observed concentrations of these trace metals in deep-ocean water are relatively unaffected by pollution and are largely determined by natural processes.