This article was downloaded by: [University Library Utrecht] On: 29 August 2013, At: 12:39 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Human and Ecological Risk Assessment: An International Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bher20 Natural Sources of Metals to the Environment Robert G. Garrett a a Geological Survey of Canada, 601 Booth St., Ottawa, Ontario K1A 0E8, Canada Published online: 03 Jun 2010. To cite this article: Robert G. Garrett (2000) Natural Sources of Metals to the Environment, Human and Ecological Risk Assessment: An International Journal, 6:6, 945-963, DOI: 10.1080/10807030091124383 To link to this article: http://dx.doi.org/10.1080/10807030091124383 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the âContentâ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions Human and Ecological Risk Assessment: Vol. 6, No. 6, pp. 945-963 (2000) 1080-7039/00/$.50 © 2000 by ASP Natural Sources of Metals to the Environment Robert G. Garrett1 Geological Survey of Canada, 601 Booth St., Ottawa, Ontario K1A 0E8, Canada ABSTRACT Almost all metals present in the environment have been biogeochemically cycled since the formation of the Earth. Human activity has introduced additional pro- cesses that have increased the rate of redistribution of metals between environmen- tal compartments, particularly since the industrial revolution. However, over most of the Earthâs land surface the primary control on the distribution of metals is the geochemistry of the underlying and local rocks except in all but the worst cases of industrial contamination and some particular geological situations. Fundamental links between chemistry and mineralogy lead to characteristic geochemical signa- tures for different rock types. As rocks erode and weather to form soils and sedi- ments, chemistry and mineralogy again influence how much metal remains close to the source, how much is translocated greater distances, and how much is trans- ported in solutions that replenish ground and surface water supplies. In addition, direct processes such as the escape of gases and fluids along major fractures in the Earthâs crust, and volcanic related activity, locally can provide significant sources of metals to surface environments, including the atmosphere and sea floor. As a result of these processes the Earthâs surface is geochemically inhomogeneous. Regional scale processes lead to large areas with enhanced or depressed metal levels that can cause biological effects due to either toxicity or deficiency if the metals are, or are not, transformed to bioavailable chemical species. Key Words: metals, geochemistry, biogeochemical cycle, natural sources, anthropogenic impacts, bioavailability. INTRODUCTION All metals present in the environment, except those small quantities that have arrived extraterrestrially or have been created in nuclear reactions, have been present in and on the Earth since its formation some 4.5 billion years in the past (Dalzeil, 1999). As part of the natural biogeochemical cycle, metals are released from rocks by weathering processes, are cycled through various environmental 1 Telephone: 613-995-4517; Fax: 613-996-3726; E-Mail:
[email protected] 200317.pgs 12/12/00, 5:36 PM945 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 946 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 Garrett compartments by biotic and abiotic processes, and ultimately find their fate in the oceans as sediments. Given sufficient geological time they become ârocksâ again, tectonic processes form new land, and the cycle continues. Since the advent of man, and in particular the industrial revolution, additional processes have be- come involved in the redistribution of metals between environmental compart- ments; and the rate of transfer between many compartments has increased signifi- cantly (Fyfe, 1998). However, for most of the Earthâs land surface the primary control on the distribution of metals remains the geochemistry of the underlying rocks, either directly or through the influence they have on locally derived glacial and alluvial sediments. Exceptions occur where there has been significant indus- trial contamination, and situations where surface materials, e.g., aeolian sands and glacial and alluvial sediments, have been transported long distances and cover local bedrock. THE FUNDAMENTAL LINK BETWEEN CHEMISTRY AND MINERALOGY Trace metal background levels in rocks are controlled by the abundance of the common rock-forming minerals. It is the availability of substitution sites (e.g., Al, Ca, Mg, Fe, Na, K) in mineral structures that permit trace element incorporation. The key controls of replacement are ionic charge together with radius and electronega- tivity (Figures 1 and 2). Thus Rb with similar ionic radius and electronegativity as K replaces it in K-rich feldspars, and levels of Rb are highest in K-feldspar-rich rocks. Similarly, Ba and Pb with ionic radii not too different from K will replace it in K-rich feldspars. However, in the presence of S, these two elements behave very differently, Pb due to its high electronegativity combines with S to form galena, lead sulfide, and a wide variety of other sulfide and sulfur-salt minerals. In contrast, barium due to its low electronegativity does not form sulfides, but forms sulfates, carbonates, oxides and is present in silicates. Barium has only been observed as a constituent of one extremely rare sulfide mineral, owensite, where it is associated with Pb in a Fe-Ni- Cu sulfide. The similarity of ionic charge and electronegativity for S and Se explains their like behavior, which is evidenced in both the mineral and biological âking- doms.â Of particular interest are elements that pair differently, e.g., Cd with a similar ionic radius to Ca, but a similar electronegativity to Zn. This explains phenomena of both mineral and biological importance. In geology Cd commonly is regarded as a chalcophile element, it is associated with Zn in sulfide minerals due to its elec- tronegativity. However, due to its similar ionic radius to Ca it is present in calcium phosphate, apatite; particularly those apatites that form certain sedimentary rock- phosphates mined for the production of agricultural fertilizers. In fertilizer manu- facture, most of the Cd remains in the product, and thus, undesirably, Cd may be added to agricultural lands with the P required to foster plant growth. In the biological realm, Cd can replace Ca in carboxyl-based, and Zn in sulfurhydryl-based, compounds, thus providing a pathway for Cd to enter biota in one form but then cause an effect through a different mechanism. Ionic and electronegativity relationships permit the explanation of many of the associations observed between elements in both rocks (Table 1) and their weather- ing products, i.e., soils, waters, and newly deposited sediments, as will be demon- strated below. 200317.pgs 12/12/00, 5:36 PM946 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 947 Natural Sources of Metals to the Environment BACKGROUND LEVELS IN ROCKS AND THEIR WEATHERING TO FORM SOIL AND SEDIMENTS Table 2 provides examples of the ranges, some up to two orders of magnitude, that occur for some metals of biological concern between rock types. The actual ranges of individual values from these, and other, rocks are even greater. In the geochemical study of an area containing a wide variety of rock types it is common to observe individual trace element levels spanning upwards of three orders of magnitude. Where ore deposits are found, the range may exceed five orders of magnitude for metals that normally only occur in trace amounts; it is an interesting fact that for these metals average ore grade is generally about 104 times average background abundance (Garrett, 1986). Figure 1. Plot of ionic radius versus ionic charge. 200317.pgs 12/12/00, 5:36 PM947 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 948 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 Garrett The erosion and weathering of bedrock, or bedrock derived sediments, provide the materials at the Earthâs surface that support life. Interactions of water with rocks and their weathering products influence its chemistry, and that water is essential for lifeâs continued existence. A key factor influencing the formation of weathering products at the Earthâs surface is the pH of the fluids in contact with the weathering materials. In most instances there is available oxygen at weathering sites, however, the redox state of the environment becomes important once weathering products commence moving through different environmental compartments on their way to the ultimate sink in the oceans. The time to reach that sink can be extremely variable, almost immediate in areas close to the ocean, to hundreds of millions of years where weathering products are retained in âsinksâ within continental land masses. Figure 2. Plot of electronegativity versus ionic charge. 200317.pgs 12/12/00, 5:36 PM948 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 949 Natural Sources of Metals to the Environment The influence of pH and redox state on the mobility of trace elements in the surface environment is illustrated in Table 3. The development of soil from rock or underlying glacial and fluvial sediments is a complex process of interaction between the source material, climatic and biological factors. Of these, temperature, rainfall and the position of the land surface relative to that of the water table are critical. In hot or cold desertic regions, immature soils develop by the mechanical breakdown of the parent material. In temperate regions, a wide variety of soils can develop, some highly differentiated (layered), while in tropical wet conditions intensely leached soils may develop where the base cations (Ca, Mg, Na, K) have been Table 1. Common geochemical associations (after Rose et al., 1979). Group Associations Generally associated elements K-Rb Ca-Sr Al-Ga Si-Ge Zr-Hf Nb-Ta Rare Earths (REEs), La, Y Pt-Ru-Rh-Pd-Os-Ir Plutonic Rocks Generally associated elements Si-Al-Fe-Mg-Ca-Na-K-Ti-Mn-Cr-V Zr-Hf-REEs-Th-U-Sr-Ba-P B-Be-Li-Sn-Ga-Nb-Ta-W-Halogens Specific associations Felsic igneous rocks Si-K-Na Alkaline igneous rocks Al-Na-Zr-Ti-Nb-Ta-F-P-Ba-Sr-REEs Mafic igneous rocks Fe-Mg-Ti-V Ultramafic igneous rocks Mg-Fe-Cr-Ni-Co Some pegmatites Li-Be-B-Rb-Cs-REEs-Nb-Ta-U-Th Some contact metasomatic deposits Mo-W-Sn Potash feldspars K-Rb-Ba-Pb Many other potash-rich minerals K-Na-Rb-Cs-Tl Ferromagnesian minerals Fe-Mg-Mn-Ni-Co-Cu-Zn Sedimentary Rocks Fe-oxides Fe-As-Co-Ni-Se Mn-oxides Mn-As-Ba-Co-Mo-Ni-V-Zn Phosphatic limestones P-F-U-Cd-Ag-Pb-Mo Black shales Al-As-Sb-Se-Bi-Zn-Cd-Ag-Pb Au-Mo-Ni-V 200317.pgs 12/12/00, 5:36 PM949 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 950 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 Garrett removed from the soil. In wetland areas organic materials, e.g., peat, may accumu- late where their immersion in water inhibits the natural oxidation processes that normally lead to the breakdown of vegetable matter. The result of these processes is that the levels of trace elements in soils vary over very large ranges as a result of the interaction of the biogeochemical cycle with an already diverse bedrock geochem- istry. Table 4 provides estimates of the average global content of some trace ele- ments in soils. However, it must be stressed that actual ranges may vary over many orders of magnitude depending upon the geochemical diversities of the soil parent materials present and the soil forming processes acting in the region. Particular soil forming processes can lead to the accumulation or depletion of elements in different layers of the soil. Where soils have developed over very long periods of time, tens or hundreds of millions of years in some instances, they may be very deep (â 100 m) and be leached of available base cations and trace elements, including micronutrients. In some instances, trace elements concentrated in resistate minerals that do not weather may become mechanically concentrated in the upper levels of these ancient chemically leached soils. However, because of the presence of these elements in resistitate minerals they are prevented from entering the biological cycle. Two important processes that lead to the accumulation of metals at specific soil horizons are, firstly, their sequestration in the upper organic rich layers of soils, and secondly, deeper in zones of iron, manganese, and aluminum sesquioxide, and clay mineral, accumulation. The importance of accumulation of metals in the organic rich surface horizons as a natural process was recognized by Goldschmidt (1937), where he coined the term âgeochemical barrier.â Metals are held there for considerable periods of time, so accumulating, before degradation of the organic materials permits continued transfer through the biogeochemical cycle. Sequestration deeper in the profile with sesquioxides can lead to elevated metal levels as observed in temperate climate podsols and tropical latosols. Depletion of metals in soils may be the result of general mineral breakdown and leaching, or in some soils, e.g., podsols, the downwards percolation of organic acids from the decay of near-surface organic matter, can leach the soils so effectively that all that remains Table 2. Some average trace element compositions of common rocks (after 1 - Lee and Yao, 1970; 2 - Turekian and Wedepohl, 1961; 3 - McLennan, 1992; 4 - Faust and Aly, 1981; and 5 - Dunn, 1990). 200317.pgs 12/12/00, 5:36 PM950 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 951 Natural Sources of Metals to the Environment T ab le 3 . M ob ili ty o f el em en ts i n th e su rf ac e en vi ro nm en t (a ft er R os e et a l., 1 97 9) . O xi di si ng O xi di si ng R el at iv e m ob ili ty (p H 5 -8 ) (p H < 4) R ed uc in g H ig h ly m ob ile C l, B r, I , S , R n , H e C l, B r, I , S , R n , H e C l, B r, I , R n , H e C , N , M o, B , S e, T e C , N , B M od er at el y m ob ile C a, N a, M g, L i, F, Z n C a, N a, M g, S r, L i, F C a, N a, M g, L i, Sr A g, U , V , A s, S b, S r, H g Z n , C d, H g, C u, A g B a, R a, F , M n C o, N i, U , V , A s, M n , P Sl ig h tl y m ob ile K , R b, B a, M n , S i, G e, P K , R b, B a, S i, G e, R a K , R b, S i, P, F e Pb , C u, N i, C o, C d, I n R a, B e, W Im m ob ile Fe , A l, G a, S c, T i, Z r, H f Fe , A l, G a, S c, T i, Z r, H f Fe , A l, G a, T i, Z r, H f T h , S n , R E E s, P t m et al s T h , S n , R E E s, P t m et al s T h , S n , R E E s, A u, C u A u, C r, N b, T a, B i, C s A u, A s, M o, S e Pt m et al s, A g, P b, Z n C d, H g, N i, C o, A s Sb , B i, U , V , S e, T e M o, I n , C r, N b, T a 200317.pgs 12/12/00, 5:36 PM951 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 952 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 Garrett is a white or gray sand. As a result, soils are not only spatially variable as discussed earlier, but they are vertically variable as a result of specific soil forming processes. In fluvial and marine environments further differentiation takes place. Coarse grain sediments, which are depleted in the clay mineral and sesquioxide fractions that often contain high metal levels, accumulate in high energy environments where the fine grained often-metal-rich material will not sediment. These fine-grained sediments become deposited in low energy environments where they are mechani- cally stable. In addition, changes in pH, redox potential, and anion composition may either lead to precipitation, or dissolution of bedload materials. For example, pH may be increased at stream confluences where stream flows from different surficial environments mix, with a resulting deposition of iron, aluminum and manganese sesquioxides together with trace metals that co-precipitate with them. Streams flowing through wetlands and marshes may become metal depleted due to sequestration of metals with organic matter, a process that is used for remediation in âconstructed wetlands,â and in sufficiently sulfur-rich anoxic environments sul- fides will be formed. It is in such environments that mercury can become methylated by sulfate-reducing bacteria, facilitating entry of mercury into the biological cycle and its bioaccumulation up the food chain in a lipophilic form. On leaving wetland areas the level of dissolved organic carbon in the stream waters is higher, which permits the formation of stable metal-organic complexes with free metal ions. The partial pressure of carbon dioxide plays an important role in determining the solubility of uranium, with it remaining in solution in alkaline oxygen-rich environ- ments, but precipitating in acid and reducing environments. Finally, on entering brackish and marine environments many of the metal-bearing particulates floccu- late and become incorporated in newly formed marine sediments. All these pro- cesses lead to a broadening diversity of geochemical compositions that will influence the local biogeochemical cycles where they are found. OTHER PROCESSES OF METAL TRANSFER TO THE SURFACE ENVIRONMENT In addition to soil-forming processes, metals may be added to surface environ- ments directly by volcanic activity through eruption, gaseous fumaroles and hot- spring activity, and through degassing along fractures and faults that provide suit- able pathways from depth. For example, on June 14 to 15, 1991, it is estimated that Mt. Pinatubo ejected 3 to 5 km3 of rock, equivalent to 104 M tonnes that contained Table 4. Some ranges and averages for trace element compositions of soils (sources: 1 - Kabata-Pendia and Pendias, 1984; 2 - Koljonen, 1992; and 3 - Alloway, 1990). 200317.pgs 12/12/00, 5:36 PM952 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 953 Natural Sources of Metals to the Environment some 100,000 tonnes of lead and 800 tonnes of mercury (Table 5). Although most of this mass of material fell within 100 km or so of Mount Pinatubo, a significant amount reached the stratosphere and circled the globe within several weeks, affect- ing the global climate for the following 2 years. That metals can be transported in this manner has been confirmed by Reitmeijer (1993), who has reported the presence of plattnerite (a lead oxide) in volcanic mineral fragments recovered from between 34 and 36 km altitude. While eruption events spectacularly introduce large amounts of metals to the surface environment, continued contributions from qui- escent volcanoes are important. Estimates of the annual rate of release of metals from Mt. Etna (Table 5) have been made by Buat-Menard and Arnold (1978), who note that the release rates for cadmium, mercury, selenium, copper and zinc are Table 5. Examples of sub-aerial volcanic releases. The Mt. Pinatubo estimate is for particulate matter only and is based on the estimate of 3 to 5 km3 of ejecta by Scott et al. (1991), which was accompanied by an estimated 2 ÃÃÃÃà 107 tonnes of SO2 (Bluth et al., 1992) over the same period of 2 days. Using the volumetric estimate and applying a specific gravity of 2.7 for intermediate rocks, an estimate for this generalization of 1010 tonnes was obtained to which average abundance estimates for intermediate rocks were applied. Mt. Pinatubo, June 14-15, 1991, Eruption Metal Tonnes Cu 600,000 Zn 800,000 Pb 100,000 Cd 1,000 Ni 300,000 Cr 550,000 As 10,000 Hg 800 Mt. Etna, 1975 Quiescent Eruption (Buat-Menard and Arnold, 1978) Metal Tonnes/Yr Cu 365 Zn 1095 Pb 131 As 40 200317.pgs 12/12/00, 5:36 PM953 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 954 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 Garrett comparable to those from anthropogenic sources in the Mediterranean region. In areas more remote from human industrial activity, volcanic sources, where present, may be major contributors to the biogeochemical cycle. Recent studies by Rasmussen et al. (1998a) have quantified mercury vapor re- leases from several different geological terranes, and have demonstrated that a linear relationship exists over some 4 to 5 orders of magnitude between the rate of mercury evasion from soil and rock surfaces and the mercury content of those materials. With respect to active fault structures that penetrate into the Earthâs crust (â15 km), a magnitude 3 or 4 earthquake can generate some 107 to 109 tonnes of fluid, which will be expelled upward toward the surface carrying metals during and after the event (Duke, 1997). When for brief periods during an earthquake contain- ing pressures are reduced to close to zero there is an opportunity for volatile elements, such as mercury, helium and radion to pass from the liquid to gas phase. Although an earthquake is a brief event, its effect on the chemistry of groundwaters can persist for months. Chinese researchers concerned with the prediction of earthquakes have reported increased levels of mercury in groundwaters prior to and during major earthquakes (Jin et al., 1989). The marine environment comprises in excess of 69% of the Earthâs surface. As has been noted above, the oceans are the ultimate sink for weathering products from the land mass prior to their being reincorporated as geological materials. In addition, submarine volcanism associated with mid-oceanic ridges, volcanic arcs, seamounts and islands is a major source of new geological materials to the marine environment. Although mixing times for deep ocean waters are long, some of the ânewâ metal introduced through hydrothermal springs is incorporated into the oceans, while the remainder contributes to new sediments. Estimates of the volcanic related flux of metals to the oceans are difficult to make as submarine mapping is incomplete and the majority of sources likely have not yet been identified. Rubin (1997) has estimated hydrothermal and gaseous releases from seamounts and mid- ocean ridges (Table 6) together with expected residence times for the released metals in seawater, and Hannington (see Duke, 1997) and Rasmussen (1994) have estimated hydrothermal fluxes (Table 7). There is considerable divergence between estimates for the same metal, largely due to the different knowledge bases the estimates are derived from. Rubinâs work is heavily influenced by seamount (Ha- waii) data, while that of Hannington is influenced by studies of âblack-smokerâ hydrothermal systems. That new sources are continuing to be identified is exempli- fied by the recent work of Stoffers et al. (1999), who have reported marine hydro- thermal vents off New Zealand, each emitting some 1 kg/yr of mercury. Such natural sources had been suspected on the basis of the geological record now on land, but had not been previously observed. The challenge, and it is a risky one, is to extrapolate from a single new observation to determine what its impact might be on a global scale, this has been attempted (Table 8). However, whereas there is reasonable confidence in estimating the output of a single hot-spring source, scaling up to the number of similar arc-type volcanic centers globally and the number of hot-springs per center is very conjectural. Still, the methodology is obvious in Table 8, and the impact of changes to the assumptions can easily be ascertained. What is important is that a new unaccounted-for source that contributes some 100 tonnes/ 200317.pgs 12/12/00, 5:36 PM954 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 955 Natural Sources of Metals to the Environment T ab le 6 . E st im at es o f gl ob al f lu x fr om m id -o ce an r id ge s in to t he o ce an s (a ft er R ub in , 19 97 ) 200317.pgs 12/12/00, 5:36 PM955 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 956 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 Garrett Table 7. Estimates of fluxes from high temperature vents on submarine ridge crests (after Hannington in Duke, 1997, and Rasmussen (Hg), 1994). Metal Tonnes/Year Hg 1860â14680 As 2110 Cd 430 Cu 8540 Pb 2170 Zn 95360 Table 8. Estimates of mercury release to the sea floor, Taupo Volcanic Zone, New Zealand, and scaling up for global estimate from arc volcanic sources. Hot Spring Flow 10 l/sec Likely Hg content 5 µg/l Hg released to sea 1.5 kg/yr Scaling Up Assumed Hg release rate 1 kg/spring/yr Global length of favorable Arc geology 75,000 km Average frequency of volcanic centers 1 per 75 km Number of volcanic centers 1000 Number of hot springs per center 100 Total number of hot springs 100,000 Estimated Hg releases 100,000 kg/yr 100 tonnes/yr 200317.pgs 12/12/00, 5:36 PM956 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 957 Natural Sources of Metals to the Environment yr into the surface environment has been identified which may have had a biological impact resulting in the abandonment of a tuna fishery (Hecht, 1999). Finally, extraterrestrial sources require a brief mention. Bevan et al. (1998) have reported rates of meteorite falls for varying periods in the past up to 50 ka BP in the range of 36 to 116 of size >10 g per 106 km2 per year. Assuming a global surface area of 510 million km2 results in minimum estimates for >10 g falls in the range of 180 to 600 kg per year. The majority of discovered falls appear to be in the range of 10 to 50 g; using 50 g as an average estimate infers falls of 0.9 to 3 tonnes per year. However, perhaps 5% of the mass entering the atmosphere reaches the Earthâs surface (Hughes, 1998). Assuming the ablated material is finally deposited, addi- tions to the global biogeochemical cycle will be of the order 18 to 60 tonnes per year. Evidence from the Earthâs surface and the geological record indicate a history of impact by large extraterrestrial bodies. Hughes (1998) has estimated that for bodies in the 0.1 to 104 tonne range a 500-tonne body impacts Earth about once every 400 years. Apart from the fact that an impact can be catastrophic (Hallam, 1998) to life locally, or globally through mass extinctions, the metals contained in 60 tonnes of material, i.e., 60 g at 1 ppm, added annually are insignificant compared to those from the terrestrial processes of the biogeochemical cycle. GEOCHEMICAL MAPPING The Earthâs surface is chemically inhomogeneous due to the diversity of igneous, metamorphic and sedimentary rocks, with varying mineralogies, that were formed by a variety of different processes from different source materials. Superimposed on this are the processes of weathering that have further led to metal accumulation or depletion due to natural processes of soil and sediment formation. Geochemical mapping as an endeavor has come to the fore in the last 50 years, fostered by advances in analytical chemical methodologies and field logistics. Geochemical maps depicting the distribution of trace elements in a variety of surficial materials at a wide range of scales, from local to continental, are prepared by industry, private and government institutions to support decision making for both economic devel- opment and public policy. As an example of a geochemical map one for the Yukon Territory, Canada, based on the analysis for zinc ( 958 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 Garrett fundamental inhomogeneity in the Earthâs crust. In the central part of the Yukon, between the Tintina fault to the northeast and the Denali fault to the southwest, crystalline schists are common that exhibit lower background levels. Other areas reflect intermediate zinc levels. The Ecodistrict medians are unaffected by individual high values related to mineral occurrences or anthropogenic factors, and the map clearly demonstrates the scale of variations in background geochemical levels that result due to geological differences occurring across a region. Figure 3. Zinc in ecodistricts of the Yukon Territory, Canada, as defined by the analysis of the Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 959 Natural Sources of Metals to the Environment The role of geochemical maps and data have been recognized internationally as an important tool for environmental and resource management (Darnley et al., 1995, where examples of geochemical maps on up to national scales are presented). Many geological survey organizations have geochemical mapping programs. Those in developed countries are driven by both mineral resource and environmental considerations, while those in developing countries are mostly driven by the search for new mineral resources to be developed as instruments of national growth. The Kola Ecogeochemistry Project (Reimann et al., 1998) is an excellent example of a recent environmentally driven multimedia mapping program where the distribu- tion of metals in an area surrounding major anthropogenic sources has been studied. Sampled media ranged vertically from surface moss, through several soil horizons to the glacial sediments on which soils have developed, and continue to develop. The project outlined the impact of metals from these major anthropogenic sources on various sample media. Analysis of mosses reflected the presence of wind- borne contaminants, metals levels falling off exponentially with distance for some 100 to 200 km from the sources, beyond which natural processes reassert them- selves. Mass balances indicate that the metal deposition within this impact area requires the total mass of metal emitted, indicating that only relatively small amounts travel longer distances from the sources (Caritat et al., 1997). Studies around base metal smelters in Canada at Flin Flon (McMartin et al., 1999), and Rouyn-Noranda and Trail, B.C., (Henderson, pers. comm. 2000), and in France (Sterckeman et al., 2000), have indicated smaller radiuses of impact for the deposition of releases from these facilities, and generally limited translocation of deposited metals into lower soil horizons. Thus, whilst deposition of airborne contaminants from major anthro- pogenic sources obscure the natural patterns of geochemical variation in the surface environment on local and some regional scales, their impact at greater distances (>100 to 200 km) is difficult to ascertain. The metal where global scale long-range transport is a possibility is mercury due to its volatility, being a liquid over normal surface temperature ranges. There is debate over the role of natural and anthropo- genic processes in the distribution of mercury, e.g., Rasmussen (1994); Fitzgerald et al. (1998) and Rasmussen et al. (1998b), and there is clear evidence of the transport of small amounts of base metals over long distances through the analysis of Greenland ice cores, e.g., Hong et al. (1994, 1996). However, when these metals are deposited to land surfaces remote from their anthropogenic sources they add incrementally very small amounts to what is present due to natural processes. This is borne out by regional geochemical mapping, where the dominant processes influencing the geochemistry of soils, stream and lake sediments and waters in areas distant from anthropogenic sources have been consistently observed to relate to the local sources of these sampling media. BIOLOGICAL IMPACTS For metals in the various compartments of the surficial environment to exert any biological effects they have to be in chemical forms that can enter biota, i.e., they have to be bioavailable. Again the environment exerts its effects through pH, redox potential, the presence of complexing anions for metals, and the availability of organic ligands. The interaction of these factors and the solid materials present 200317.pgs 12/12/00, 5:36 PM959 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 960 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 Garrett determine the form and levels of trace metals and other elements in solution, either in soil or sediment pore waters, or the aquatic environment (Table 3). Coupled with bioavailability are the processes of bioaccumulation and biomagnification. The latter are a particular concern with mercury in aquatic environments. The formation of methyl mercury is a biotic process, its synthesis being a process that sulfate-reducing bacteria use to rid themselves of mercury. Once mercury is in the methyl form it is bioavailable and enters the food chain at its lowest levels, e.g., zooplankton. From that point, as predator feeds on predator, levels in biota increase until they find their highest levels in fish, birds and some riparian and marine mammals. Mercury levels appear to be partly a function of length of food chain, fish of equal size that feed on lower levels in the chain generally exhibit lower mercury levels than those that are the apex of a more complex food web where larger fish have fed on smaller fish. The problem is exacerbated when the biota are long-lived, and it is in such cases that mercury levels in birds and mammals can reach toxic levels. Interestingly, while neurotoxic effects may be observed in birds and mammals, high levels of mercury do not seem to affect fish. Of direct human concern is the impact of bioavailability on the chemistry of plants and the composition of diet; it has been long recognized that particular areas of the world are characterized by endemic plant, animal or human health problems. Homeostastic processes in biota, including humans, permit the maintenance of appropriate levels of trace elements, either storing reserves or eliminating excesses. However, in some instances homeostasis cannot be maintained. Geologically asso- ciated human health problems, both nutritional deficiencies (e.g., Zn, I, Se) and toxicity issues (e.g., Hg, As, Se, Cu), and the role of geochemistry, are receiving increasing attention, as demonstrated by recent publications and reviews (Combs et al., 1996; Appleton et al., 1996; Dissanayake and Chandrajith, 1999). Thus a knowl- edge of the natural distribution of metals at the Earthâs surface, and the processes affecting their distribution and controlling the presence of bioavailable forms, is essential if the problems of micronutrient deficiency and endemic disease are to be addressed. Recent studies of selenium in China (Johnson et al., 1999; Fordyce et al., 2000) provide examples of the critical roles that soil pH and the availability of soil organic ligands play in determining bioavailability to crops and its consequences for micronutrient deficiency and toxicity in local populations. A CHALLENGE, NATURAL FLUX RATES A major challenge being addressed by geoscientists is the quantification of natural releases for comparison with anthropogenic releases. Whereas anthropo- genic releases can be estimated relatively easily, such is not the case for natural releases (Garrett, 1999). There are major uncertainties in estimates for the weath- ering rates and releases of trace elements and metals from rocks and soils that remain to be resolved. In addition, new sources, that help better constrain the addition of metals to surface environments, continue to be recognized, e.g., mercuriferous hot springs off the coast of New Zealand (Stoffers et al., 1999). However, just as important as the discovery of such point sources is the mapping and charting of the Earthâs solid surface in order to determine their location, number 200317.pgs 12/12/00, 5:36 PM960 D ow nl oa de d by [U niv ers ity L ibr ary U tre ch t] at 12 :39 29 A ug us t 2 01 3 Hum. Ecol. Risk Assess. Vol. 6, No. 6, 2000 961 Natural Sources of Metals to the Environment and spatial frequency. It is these data concerning the broad-scale diffuse fluxes of metals and natural point-sources that will permit improved estimates of the rate of transfer through the natural biogeochemical cycle. 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