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

Pb, Cd, pollution, aquatic systems, trace metals

Erel, Yigal ; Patterson, Clair C. (advisor)

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Pb, Cd, pollution, aquatic systems, trace metals;   

Others  :  https://thesis.library.caltech.edu/6289/1/Erel_y_1991.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Lead and cadmium concentrations in marine and terrestrial ecosystems, insurfaces of soils, and in the atmosphere have been highly elevated on a global scale dueto industrial pollution. In order to ascertain the natural (rock-derived) levels of lead andcadmium in streams, pristine mountain watersheds in the Sierra Nevada, California werestudied for their lead and cadmium contents, and the transport of lead and cadmium wasrelated to metal relatively uninfluenced by pollution that share similar transport patterns.In addition, rock and unpolluted water samples from granodiorite, basalt and carbonateterrains were analyzed for the concentrations of lead, iron and other elements. Wholerocksamples as well as biotite and feldspar mineral separates were used for laboratoryleaching and adsorption-desorption experiments to investigate the relationship betweenlead and iron chemistry under controlled conditions.

The concentrations of lead and cadmium in the late-summer drainage are shownto 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 ratiosin stream water, ground water, soil and bedrock, and 2) the removal rate of excessatmospheric lead and cadmium from the water as it flows downstream.

After the spring snow-melt runoff, most of the lead in alpine streams originatesfrom ground water, and has isotopic ratios that are consistent with values expected frombedrock and soil sources, indicating that this lead is not anthropogenic in origin. Thelead found in unpolluted ground waters is more radiogenic than the lead in the bedrockdrained by these waters. A preferential release of radiogenic lead into waters andleached phases of rock and soil can be explained by the preferential weathering ofradiogenic accessory minerals and to a lesser extent by a preferential release of U and Thdecay products due to the recoil effect.

The lead uptake mechanism is proposed to be adsorption on oxy-hydroxidesurfaces. In contrast, the uptake of cadmium in the stream water is erratic and cannotbe explained by the same mechanism. Adsorption-desorption experiments suggest thatlead coprecipitates and is adsorbed on particle surfaces, mainly ferric iron hydroxides.Due to a similar transport mechanism and comparable rate of release from common rockand soil minerals, the ratio between natural (rock-derived) lead and iron in rivers shouldbe similar to their average upper continental crustal molar ratio of 1:6,500.

Experiments and speciation models indicate that complexation of lead by manmadeorganic compounds decreases the fraction of lead bound to surface sites. Such anindirect pollution effect mobilizes lead and decreases the Fe/Pb ratio in rivers regardlessof any direct addition of anthropogenic lead.

Many trace metals maintain their average upper continental crustal ratio with ironin unpolluted river water, river sediments and soils; However, large excesses of mosttrace metals relative to iron are found in deep-ocean water. At the transition from freshwater to saline ocean water, two processes take place: 1) rapid removal of iron (andother 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% ofthe riverborne iron settles to the sediment within the continental shelf, some of the tracemetals are released to solution as dissolved chloro-complexes and are further transportedto the open sea. In addition, some of the trace metals attached to airborne and recycledsea-floor particles may desorb when these particles are in contact with sea water.

The adsorption/desorption process in sea water account for the relativeabundances 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 waterare relatively unaffected by pollution and are largely determined by natural processes.