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.</p

Redox chemistry of iron in fog and stratus clouds

The redox chemistry of Fe in fog and cloudwater has been investigated at coastal and inland locations in the Los Angeles basin, in Bakersfield California, and in Delaware Bay. Samples were collected and analyzed for Fe (Fe(II)), Fe(III), total(Fe), sulfur (S(IV), S(VI)), organic ligands (formate, acetate, oxalate), total organic carbon (TOC), pH, major cations (sodium, calcium, magnesium, potassium, ammonium), chloride, sulfate, nitrate, peroxides, and aldehydes (HCHO); the amount of sunlight was also measured. The ratio Fe(II)/Fe(total) varied between 0.02 and 0.55. The concentration of Fe(II) varied between 0.1 and 5 micromole, and the concentration of total Fe varied between 2 and 27 micromole. The atmospheric redox cycle of Fe involves both dissolved and aerosol surface species and appears to be related to the presence of organic compounds which act as electron donors for the reduction of Fe(III). Fe(III) reduction is enhanced by light but significant Fe(II) levels were observed in the dark. We suggest that reduction of Fe(III) species by organic electron donors may be an important pathway that affects the speciation of Fe in both urban and rural atmospheres. It is possible that reactions involving Fe and organic compounds might be an important source of carboxylic acids in the troposphere

Lead in Archeological Human Bones Reflecting Historical Changes in Lead Production.

Forty years ago, in a seminal paper published in Science, Settle and Patterson used archeological and historical data to estimate the rates of worldwide lead production since the discovery of cupellation, approximately 5000 years ago. Here, we record actual lead exposure of a human population by direct measurements of the concentrations of lead in petrous bones of individuals representing approximately 12 000 years of inhabitation in Italy. This documentation of lead pollution throughout human history indicates that, remarkably, much of the estimated dynamics in lead production is replicated in human exposure. Thus, lead pollution in humans has closely followed anthropogenic lead production. This observation raises concerns that the forecasted increase in the production of lead and other metals might affect human health in the near future

Reply to the Comment by M. Bau on “Modeling of rare-earth element partitioning between particles and solution in aquatic environments”

In his comment on our paper (Erel and Stolper, 1993) on modeling of rare-earth element partitioning between particle surfaces and aqueous solutions, Bau (1994) raises a number of questions about the validity of our model as a tool to explain the REE distribution in Precambrian banded iron formations (BIFs)

Leakage of industrial lead into the hydrocycle

Quantitative knowledge concerning the contamination effect by industrial Pb migrating through soils into groundwaters has been delineated in a special study carried out in a remote, high altitude mountain valley. Approximately 0.5 ton of industrial Pb has been added in past decades from the atmosphere via precipitation and dry deposition to the 3 km^2 area of lightly forested and open meadow soil lying within the 13 km^2 area of the rocky valley. Industrial Pb could be distinguished and its amounts quantitatively determined by use of its unique isotopic composition, which was different from natural Pb in meadow soil. Industrial Pb introduced into the canyon within snow was interacting and exchanging with the larger reservoir of industrial Pb accumulated in canyon soil. Lead in the snow-melt runoff had become attached to soil-derived colloids, and a mixture of industrial and natural particulate Pb was released from the soil to stream water and groundwater. The flux of Pb leached from the accumulated reservoir of industrial Pb in soil could be measured as it flowed through soil pathways into stream runoff waters draining the valley. Such leached industrial Pb comprised about 75% of the total Pb in stream runoff of snow-melt and 20% of the total Pb in stream runoff of groundwater

A silicate weathering mechanism linking increases in marine 87Sr/ 86Sr with global glaciation

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155795/1/Blum_Erel_1995_Silicate_weathering.pd

Rb-Sr isotope systematics of a granitic soil chronosequence: The importance of biotite weathering

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155801/1/Blum_et_al_1997_Rb-Sr_isotope.pd

The coupled release of REE and Pb to the soil labile pool with time by weathering of accessory phases, Wind River Mountains, WY

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155685/1/Harlavan_et_al_2009_Coupled_release.pd