Global Geochemical Baselines Research Articles

A synthesis of the National-scale Geochemical Survey in the last 50 years in Brazil

At the end of the 19th century, classical geochemistry arrived in Brazil through Henri Gorceix, being primarily used in his studies of mineralogy and petrology. Meanwhile, exploration geochemical arrives later, in the mid-20th century. But it was only in the late 1960s that systematic survey at the national level began, starting with the creation of CPRM - Geological Service of Brazil, culminating in over 50 years of uninterrupted activities, providing coverage of the entire country at various scales: global (7.3%), national (6.2%), regional (22.7%), and local (2.3%). Throughout this period, 681 projects were carried out and a variety of sample matrices were collected, such as stream sediment (226,792), soil (122,202), heavy-mineral pan concentrate (107,959), rock (56,807), among others (8,258), totaling 522,018 samples so far. All this collection has been progressively recovered, compiled, and organized into a robust public database (open access), developed by CPRM - Geological Service of Brazil. More recently, high-density geochemical surveys were conducted in partnership with the China Geological Survey with the aim of comparing analytical and sampling methodologies. For the future (until 2050), two main actions are planned: 1) an expectation of an increase in the coverage of high-density prospecting surveys, ranging from 951,386 km² (restrictive scenario: 11.2% of Brazilian territory) to 2,204,778 km² (favorable scenario: 25.9% of Brazilian territory); 2) a forecast of Brazil's participation in the Global Geochemical Baselines Program, where sampling will be subdivided by regions – North (736), Central-West (225), Northeast (305), and South-Southeast (343) – totaling 1,609 sampling cells.

Provenance, source weathering, and tectonics of the Yarlung Zangbo River overbank sediments in Tibetan Plateau, China, using major, trace, and rare earth elements

This paper studies the major, trace, and rare earth elements (REEs) in fluvial sediments from the Yarlung Zangbo River (YZR) on the Tibetan Plateau, China, with the aim of investigating their provenances, sediment maturity, weathering conditions, and tectonic settings. All samples and analytical techniques are based on Environmental Geochemical Monitoring Networks, which are a crucial part of IGCP259/360 (Global Geochemical Baseline). The results show that the YZR sediments consist of greywacke and litharenite with low‐moderate maturities of textures and minerals. The chemical index of alteration (CIA) (49–71), plagioclase index of alteration (49–77), and index of compositional variability (ICV) values (0.82–1.54) indicate that both mature and immature sediments are present in the YZR and have experienced weak to moderate weathering with little intense weathering in the source area, which is also confirmed by the ICV‐CIA, A‐CN‐K, A‐CNK‐FM, and MFW plots. Moreover, the provenance discrimination diagrams suggest that the YZR sediments are predominantly derived from felsic rocks that are close to the upper continental crust and from acid intermediates, and the tectonic settings of the YZR source rocks were associated with active continental margins and continental island arcs. The overall sediment types, weathering conditions, and tectonics of the YZR were consistent with those of the Brahmaputra River and Meghna River during the Himalayan–Tibetan orogeny, while the sediments of the Brahmaputra River are characterized by recycled‐quartzose sources when compared with the other two Himalayan rivers due to the contributions of quartzolithic composition.

Report on the Ad Hoc Review of the IUGS Commission on Global Geochemical Baselines (GGB)

Report on the Ad Hoc Review of the IUGS Commission on Global Geochemical Baselines (GGB)

Global Geochemical Baseline Mapping in India for Environmental Management Using Topsoil

Abstract As a part of International Union of Geological Sciences (IUGS)/ International Association of Geochemistry (IAGC) Task Group on Global Geochemical Baselines, CSIR-National Geophysical Research Institute (CSIR-NGRI), Hyderabad, India has carried out geochemical baseline mapping of India to provide high quality data for a wide range of environmental applications. A project based on IUGS global baselines project was initiated by NGRI to cover the whole of India which is having 122 Global Terrestrial Network (GTN)/Global Reference Network (GRNs) of 160 x 160 km. Soil samples were studied in 115 out of the 122 GRN cells in India. 544 topsoil samples were collected for these studies. The soil samples were collected as per the guidelines given in the IUGS/IAGC field manual. The samples were analyzed for 10 major and 12 trace elements (SiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, TiO2, P2O5, Ba, Co, Cr, Cu, Ni, Pb, Rb, Sr, V, Y, Zn and Zr) by X-ray fluorescence spectrometer at NGRI. The analytical data were computed to prepare geochemical maps showing the distribution of major oxides and trace elements all over India. These maps will help to know the distribution of the toxic and hazardous metals and their relation to human health. These maps also help to see the hot spots for the entire country and in suggesting remedial measures.

The history, progress, and future of global-scale geochemical mapping

Global-scale, or continental-scale, geochemical surveys cover millions of square kilometers of the Earth’s surface generally at a very low sample density (1 site per 1,000 to 10,000 km2). Geochemical patterns produced from these low-density surveys reflect processes that act at the broad scale of sampling. These processes are related to many factors including tectonics, climate, weathering, geochemical and mineralogical composition of the original soil parent material, continental-scale glaciation, topography, regional-scale alteration and mineralization, and in some cases, human activity. A multi-element geochemical atlas of the Earth’s land surface based on this type of survey has been a topic of discussion among applied geochemists since the 1980s. Over the past 15 years, several global-scale geochemical surveys have been conducted (Australia, China, Europe, India, Mexico, United States of America) and the data and maps are being used as a tool to aid in environmental and resource management. In 2016, the establishment of both the International Union of Geological Sciences (IUGS) Commission on Global Geochemical Baselines and the United Nations Educational, Scientific and Cultural Organization (UNESCO) International Centre for Global-Scale Geochemistry has provided a hopeful future for continuing global-scale geochemical mapping in other parts of the world, with the ultimate product being a global geochemical database and atlas derived from this data set.

General concepts of geochemical mapping at global, regional, and local scales for mineral exploration and environmental purposes

Planning an effective geochemical sampling campaign to meet survey objectives should be based on a phased or stepwise use of applied geochemical methods, starting from a desktop study that collects and evaluates all available data and information. This is followed by a field investigation, which should begin with an orientation survey, where a variety of sampling media, sample preparation methods, and analytical protocols are tested with the objective of finding the most cost-effective methodology to be applied in the succeeding steps of the follow-up and detailed investigations. Sampling is the most important part in any geochemical survey for mineral exploration or environmental purposes. Apart from sampling, another significant part of a survey is sample preparation. Mistakes in sampling and sample preparation are difficult to trace and affect the final results and success of a survey. Hence, these are the two parts of a geochemical survey that should be performed with the utmost care. Sampling procedures for stream sediment, overbank or floodplain sediment, stream water, ground water, rock, and residual soil are described, as well as sampling in the laboratory. The importance of randomisation of samples and implementation of strict quality control procedures are also discussed. Finally, it is emphasised that the success of any geochemical survey, including global geochemical baseline mapping, depends on the training of the applied geochemist.

Reprint of "Comparison of datasets obtained by global-scale geochemical sampling in Australia, China and Europe"

Catchment/floodplain sediment geochemical datasets from three continental-scale geochemical projects in Australia (AU), China (CH) and Europe (EU) were studied by comparing sample media, sample preparation, elements determined, analytical methods, detection limits and proportions of reportable values, and certified reference materials used. Twenty six elements (Ba, Ce, Co, Cr, Cu, Mo, Nb, Ni, Pb, Rb, Sr, Th, V, Y, Zn, Zr, Al2O3, CaO, K2O, MgO, MnO, Na2O, P2O5, SiO2, Fe2O3 (total), TiO2) were considered comparable among the 35 determined by all three projects, according to the criteria for global comparability of elements proposed in this paper. The median concentrations of all elements (except Co, Y, Zr, SiO2 and TiO2) in the three datasets increase in the following order: AU<EU≤CH, and the median CH/EU ratios for all elements range from 0.98 to 1.94, while the median CH/AU and EU/AU ratios for elements, such as CaO, Na2O, MgO and Zn are generally greater than 2 (and up to 7), which could be explained partly by protracted weathering in Australia, and partly by the smaller grain size fraction analysed of the CH dataset (<1mm) compared to AU and EU (<2mm). In conclusion, a unified sample medium should be collected and unified sample preparation techniques should be followed. Key elements related to mineral resources and the environment should be determined and international or exchanged internal standard materials inserted in new national or global geochemical mapping projects in order to generate globally comparable datasets for establishing global geochemical baselines.

Comparison of datasets obtained by global-scale geochemical sampling in Australia, China and Europe

Catchment/floodplain sediment geochemical datasets from three continental-scale geochemical projects in Australia (AU), China (CH) and Europe (EU) were studied by comparing sample media, sample preparation, elements determined, analytical methods, detection limits and proportions of reportable values, and certified reference materials used. Twenty six elements (Ba, Ce, Co, Cr, Cu, Mo, Nb, Ni, Pb, Rb, Sr, Th, V, Y, Zn, Zr, Al2O3, CaO, K2O, MgO, MnO, Na2O, P2O5, SiO2, Fe2O3 (total), TiO2) were considered comparable among the 35 determined by all three projects, according to the criteria for global comparability of elements proposed in this paper. The median concentrations of all elements (except Co, Y, Zr, SiO2 and TiO2) in the three datasets increase in the following order: AU<EU≤CH, and the median CH/EU ratios for all elements range from 0.98 to 1.94, while the median CH/AU and EU/AU ratios for elements, such as CaO, Na2O, MgO and Zn are generally greater than 2 (and up to 7), which could be explained partly by protracted weathering in Australia, and partly by the smaller grain size fraction analysed of the CH dataset (<1mm) compared to AU and EU (<2mm). In conclusion, a unified sample medium should be collected and unified sample preparation techniques should be followed. Key elements related to mineral resources and the environment should be determined and international or exchanged internal standard materials inserted in new national or global geochemical mapping projects in order to generate globally comparable datasets for establishing global geochemical baselines.

Outlines of New Global Geochemical Mapping Program

Abstract:Since 1988, great efforts and enthusiasm had been paid by applied geochemists in the implementation of global geochemical mapping through the International Geological Correlation Program's Projects 259 and 360, and the Task Group on ‘Global Geochemical Baselines’ established by the International Union of Geological Sciences (IUGS), in collaboration with the International Association of Geochemistry (IAGC). But how to use extremely low‐density sampling to obtain a global picture of the distribution of most elements in the periodic table in a reasonably short time is still a great challenge faced by the applied geochemistry community. It will depend on the continuous development of new mapping concept, and the advisable and courageous innovation of methodology for searching other suitable sample media and sampling layout. Based on the encouragement results obtained from the representativeness study of delta sediments conducted at the mouth of Yangtze River, and at the mouths of its four major tributary, it is expected to broadly apply the geochemical fractal self‐similarity nature to main rivers and their estuaries with catchments up to hundreds of thousands or over a million square kilometers in the world. With this new mapping concept, a new outlines of a Global Geochemical Mapping Program was advanced and the establishment of an International Research Center of Global Geochemical Mapping was also suggested to facilitate the programs implementations.

Overbank sediment sampling in Greece: a contribution to the evaluation of methods for the ‘Global Geochemical Baselines’ mapping project

The different steps in the development of the floodplain or overbank sediment sampling procedure for continental-scale geochemical mapping are described with examples from the Greek pilot project of the Western European Geological Surveys’ Working Group on ‘Regional Geochemical Mapping’. The Greek overbank and stream sediment results were compared statistically with those of the national regional stream sediment survey. The purpose was (1) to compare the geochemistry of overbank sediment, collected from a single site, with that of several tens or hundreds of regional stream sediment samples taken from the upstream drainage basin, and (2) to develop a cost-effective procedure to assess the geochemistry of a drainage basin. ‘ Composite’ samples taken over the whole thickness of the overbank sediment sequence showed that element contents were close to the median, or within the interquartile range, of the same elements in the regional stream sediment samples. Overbank sediment samples collected from pre-defined intervals were not considered satisfactory, for they were taken across overbank sediment layers. Therefore, the method was subsequently refined by collecting samples from different overbank sediment layers, with the recommendation to collect samples at 25-cm intervals when thick layers of over 1 m were encountered. In selected river basins, surficial overbank and active stream sediments reflect contamination, whereas overbank sediment layers at depth show natural element contents, or pristine conditions. Such data are required for a realistic assessment of the degree of contamination of individual drainage basins. After considering results from Greece and other countries, it is recommended to sample both the surface overbank sediment (0–10 cm), and the deepest possible layer in order to measure pristine or pre-industrial conditions. The Greek results demonstrate that overbank sediments are useful for mapping natural geochemical conditions and baseline concentrations, as well as assessing anthropogenic contamination. The cost-effectiveness of overbank sediment sampling for the ‘Global Geochemical Baselines’ project of the International Union of Geological Sciences (IUGS) and International Association of GeoChemistry (IAGC) is verified.

A New Round of Global Geochemical Mapping: Opportunity and Challenge to China

Since the projects of “International Geochemical Mapping (IGCP259)” and “Global Geochemical Baseline (IGCP360)” were met with approval by UNESCO in 1988, decisive contributions of working out the methodological guidance and the technical standard of global geochemical mapping have been made by China and Europe. Though demonstrations have been made by China and Europe through carrying out the projects of “Environmental Geochemical Monitoring Network and Dynamic Geochemical Mapping (EGMON)” and “FOREG geochemical baseline mapping”, respectively, the expectation of acquiring a general picture of global geochemistry in 10 years has not yet been realized. Geochemists from China and Norway are now deliberating, through International Association of Hydrological Sciences/International Commission on Continental Erosion (IAHS/ICCE), about a major international cooperative project on “Global Geochemical Mapping and Sediment-Bound Flux of Major World Rivers”, on carrying out a new round global geochemical mapping, and planning to start working first on the Arctic region under the project of the Year of Polar (IPY317). The practical plan for the global geochemical mapping is based on the “Draft of Sampling Plan of Floodplain Sediments” for the global geochemical mapping suggested by China and the “Draft of Sampling Plan of Overbank Sediments in Deltas” suggested by Norway, thereby further strengthening the advantages of Chinese methodology in geochemical mapping. After analyzing the opportunities and challenges to China, this article suggests that the competent authorities of science and technology should take the initiative of supporting the new round global geochemical mapping.

Geochemical Mapping: With Special Emphasis on Analytical Requirements

Abstract: More than 40 national and regional geochemical mapping projects in the world carried out from 1973 to 1988 do not conform to common standards. In particular they have many analytical deficiencies. In the period 1988 to 1992, the International Geochemical Mapping project (Project 259 of UNESCO's IGCP Program) prepared recommendations designed to standardize geochemical mapping methods. The analytical requirements are an essential component of the overall recommendations. They included the following: 71 elements should be analyzed in future mapping projects; the detection limits of trace and ultratrace elements must be lower than the corresponding crustal abundances; and the Chinese GSD and Canadian STSD standard sample series should be used for the correlation of global data. A proposal was also made to collect 5000 composite samples, at very low sampling densities to cover the whole Earth's land surface. In 1997 an IUGS Working Group on Global Geochemical Baselines was formed to continue the work which began with IGCP 259. From 1997 up to now, new progress has been made especially in China and FOREGS countries under the aegis of this working group, including the study of suitable sampling media, development of a multi‐element analytical system, new proficiency test for selection of competent laboratories and role of wide‐spaced mapping in mineral exploration. One of the major problems awaiting solution has been the inability of many laboratories to meet the IGCP recommendations to generate high quality geochemical maps. Fortunately several laboratories in China and Europe have demonstrated an ability to meet the requirements and they will be well placed to render technical assistance to other countries.

The distribution of uranium over Europe: geological and environmental significance

The variation of baseline levels of uranium in soil and stream sediments over Europe is described, based on new data prepared by the Forum of European Geological Surveys (FOREGS). The samples have been collected and analysed according to the protocols established for the International Union of Geological Sciences/International Association of Geochemistry and Cosmochemistry (IUGS/IAGC) Working Group on Global Geochemical Baselines. The baseline levels of U vary between 0·21 to 53 mg kg-1 in topsoils, 0·19 to 30 mg kg-1 in subsoils and < 1 to 59 mg kg-1 in stream sediments. There is generally good agreement between the levels of U in the three sample types, and the median concentration in all three media is approximately 2 mg kg-1. The most anomalous baseline levels occur over the Variscan orogen, especially areas into which late postorogenic radiothermal high heat production (HHP) granites were emplaced. Spiderdiagrams based on trace element levels and rare earth element (REE) plots, confirm the association between the highest U anomalies and evolved radiothermal granites. High values are also associated with parts of the Alpine terrain especially in Slovenia, where there are historical U workings, and Southern Italy, where high values of U reflect contemporary volcanism. In contrast, much of the Caledonides of North West Europe and the Precambrian of the Baltic Shield and East European craton and its overlying sedimentary cover have very low values, generally < 4 mg kg-1. The results suggest that the main concern for the environment and human health from U, and the Th and K with which it is generally associated, is the naturally occurring total gamma radiation and radon potential associated with radiothermal granites. This is likely to be especially important where the granites are mineralised and have been worked historically, for example in the North West of the Iberian Peninsula where U and its decay products are likely to be more dispersed in the surface environment. The study also indicates the value of multielement data in distinguishing between anthropogenic and naturally occurring anomalies.

Global geochemical mapping and its implementation in the Asia–Pacific region

“IGCP” Projects 259 (International Geochemical Mapping) and 360 (Global Geochemical Baseline) under the aegis of UNESCO's International Geochemical Correlation Program made recommendations for standardizing geochemical mapping protocols and for conducting a wide-spaced sampling of the Earth's land surface based on a Global Reference Network of approximately 5000 160×160 km cells. A pilot study has been conducted throughout China during which these recommendations were implemented. During the pilot study, 500 floodplain samples representing drainage basins ranging in area from 1000 to 6000 km2 were collected throughout China and analyzed. The widely spaced sampling used for the pilot study was compared to the very dense sampling (× million samplings of sediment for all of China) used for China's Regional Geochemical National Reconnaissance Program. The geochemical maps generated from the wide-spaced sampling are strikingly similar to those generated from data based on the detailed sampling. Such low density floodplain sampling presents a relatively low-cost way to get a quick overview of the geochemistry of a large area of the Earth's surface. The implementation of this project in the Asia–Pacific region and the problems encountered are discussed in the present paper.

Global Geochemical baselines for environmental management in the new millennium

It is a well-known fact that international interest, during the last few years, has been turned onto the possible hazardous consequences on the health of humans, animals and plants, because of environmental contamination by toxic chemical elements. The rapid rates of economic development and population increase on our planet have caused the aggravation of environmental problems, concerned with degradation from uncontrolled urban and industrial development, intensive agriculture and overexploitation of water resources. All these problems are affecting, on a global scale, the chemistry and sustainability of life support systems. The methodology applied to evaluate the intensity and area extent of these problems, as well as their effects, is concerned with the recognition of changes in the level of chemical elements on the earth's surface environment, which is the subject studied by the branch of science known as applied geochemistry. Consequently, to secure the sustainable development of our planet, it is required to create a high quality geographical information database of the chemistry of surface earth materials on which the well-balanced development of life is based. The Geological Survey of each country is participating in the project of "Global Geochemical Baselines", which aims to compile the multielement Global Geochemical Atlas, that is the chemical atlas of earth materials for 78 chemical elements by the year 2010. The European part of the project has started in 1998, and results will be published by the year 2004. Resulting information will be useful to agriculture, animal-rearing, epidemiology, urban planning, legislation and to policy- and decision-makers in general, which are all related to the sustainable development of our planet. Completion of this very significant project for effective management of the global surface environment in the new millennium, and for better quality of life of the present and all future generations, will depend on the availability of funds.

Low-density geochemical mapping in Hungary

Guidelines for a low-density geochemical survey were described in 1990 by the Western European Geological Surveys. A low-density geochemical survey of Hungary was carried out in 1991–1995. The results are useful for future surveys and for the IGCP 360 project ‘Global Geochemical Baseline’. In regions with well-developed drainage systems in Hungary, 196 catchment basins of approx. 400 km 2 were delineated and flood-plain deposits sampled at their outlets. The samples were taken from 0 to 10 cm and from 50 to 60 cm depths. Samples were analysed by ICP-AES and AAS techniques in two laboratories. A Geochemical Atlas of Hungary is in preparation that will show the distribution of 25 elements in the two sampled layers. Maps for the lower layer represent regional geochemical baseline values and a geochemical subdivision of the country (maps showing the distribution of element associations) was made on the basis of factor variables. Maps constructed from the data of the upper sampling level show us the present state of contamination of the surface. The results of this survey have contributed to the establishment of guidance values for soils prepared by the Hungarian Ministry of the Environment in 1995. Safe levels were established for As, Cd, Cr, Cu, Hg, Pb and Zn and regional environmental loads plotted. Differences between the median values of the two levels are generally small. However, the concentrations of certain elements like P, Pb and S are significantly greater in the upper layer reflecting contamination from agriculture. In certain regions, the rate of sedimentation was fairly fast such that the environmental effects of ore mining in Transylvania and southern Slovakia as well as those of heavy industry in northern Hungary can be observed in samples from the lower level. The main factor controlling the geochemical pattern in Hungary is the predominance of young (Pleistocene or Miocene) clastic sediments at the surface. Approx. 90% of the surface is covered by these young sediments. This kind of survey has the disadvantage of not providing enough contrast to differentiate geologically dissimilar areas but it has the advantage to provide regional surface background geochemical data and it helps to outline areas of possible surface contamination. Based on the results of this survey we conclude that it would be much better to sample smaller, but geologically homogeneous areas in mountainous terrain to obtain data characteristic of the geochemical background of lithologic units. This approach would mean a sampling density of a few tens of km 2/sample for hilly areas, and a few hundred km 2/sample for lowland areas.

The Forum of European Geological Surveys Geochemistry Task Group inventory 1994–1996

The Forum of European Geological Surveys (FOREGS) includes representatives from 33 European countries and is responsible for co-ordinating Geological Survey activities in Europe. The FOREGS Geochemistry Task Group was established in 1994 to develop a strategy for the preparation of European geochemical maps following the recommendations of the International Geological Correlation Programme (IGCP) Project 259 ‘International Geochemical Mapping’ (now the International Union of Geological Sciences (IUGS) /International Association of Geochemistry and Cosmochemistry (IAGC) Working Group on Global Geochemical Baselines). The FOREGS geochemistry programme is aimed at preparing a standardised European geochemical baseline to IGCP-259 standards. The principal aims of this dataset will be for environmental purposes, as a baseline for the assessment of the extent and distribution of contaminated land in the context of variations in the natural geochemical background, but it will also have applications in resource assessment and for the development of policy for the sustainable management of metalliferous mineral and other resources. The first phase of the programme was the compilation of an inventory of geochemical data based on the results of a questionnaire completed by Geological Surveys and related organisations throughout the FOREGS community. The results show that the sample types which have been used most extensively are stream sediment (26% coverage), surface water (19% coverage) and soil (11% coverage). Stream sediments have been collected using a narrow range of mesh sizes (< 150–< 200 μm), but soil samples have been collected according to two different conventions: some surveys used a similar mesh size range to that used for stream sediments while others employed the < 1000 or < 2000 μm fractions traditionally used by soil surveys. Sample densities range from 1 sample per 0.5 km 2 to 1 per 3500 km 2. Various analytical methods have been used, but most of the available data have been calibrated using international reference materials, and data for the most important of the potentially harmful elements (PHEs) are available for most datasets. Systematic radiometric data are available for only a small proportion of Europe, a situation which compares very unfavourably with that in Australia, North America, the former Soviet Union and many developing countries. Recommendations are made for increasing the compatibility of geochemical methods between national geochemical surveys as a basis for the preparation of a series of European geochemical maps. The next stage of the FOREGS Geochemistry Task Group will be the collection of the Global Reference Network of samples against which to standardise national datasets according to the methods recommended in the final report of the IGCP 259 programme.

Applicability of overbank sediment for environmental assessment according to wide-spaced sampling in Fennoscandia

IGCP Project 259, International Geochemical Mapping, and its successor IGCP Project 360, Global Geochemical Baselines, aim at a global geochemical map, which is becoming increasingly an environmental concern. As a pilot survey for Project 259 overbank sediment, till, humus and river water were sampled from 49 large catchment basins in Fennoscandia. The deeper layers of overbank sediment were deposited hundreds of years ago and reflect natural pre-industrial geochemical background. In this survey the geochemistry of one sample of overbank sediment was found to represent catchment basins up to 7000 km 2 in size, and to correspond to that of composites of 6–20 random till samples taken from the same basin. The surface layers were deposited recently and reflect pollution in the basin. Distinct pollution was detected in some basins with mining activities and industries. A large province with high Pb contents in the southern parts of Sweden and Norway is caused by atmospheric deposition. In some basins metal contents from both anthropogenic and natural sources and exceeding values considered (phyto)toxic were measured. According to other surveys 30% of the river plains in Europe are severely polluted, and many of these plains have toxic contents of heavy metals. Therefore, and because population is concentrated to river valleys, it is of great importance to determine the degree of, and future threats from this pollution. Overbank sediment is a most suitable medium for this purpose, because it can be used in basins ranging from a few up to thousands of km 2 to determine the geochemical background (deep layers), pollution history (profile sample), degree of pollution (deep vs surface sample), and for monitoring change.

Some notes on the importance of airborne gamma-ray spectrometry in International Geochemical Mapping

Abstract Airborne gamma-ray spectrometry (AGRS) is an important component of the International Geochemical Mapping (IGM) project. It permits geochemical mapping of radioactive elements (naturally occurring and man-made) by remote sensing. Data can be reported in terms of radioelement abundance and nuclide dose-rate. The technique offers continent-wide coherency because it may be applied across any land surface, with 90% of the response coming from the top 25 cm (termed the A25 horizon). Quantitative correlation of ground and airborne data can be assured within definable limits because airborne equipment is calibrated by relating airborne to upper regolith ground measurements, under controlled conditions, over test ranges several km in length. Evidence ranging from local to continental scales suggests that the reverse of the calibration process enables AGRS to provide an important supplementary function for global mapping. In compiling geochemical maps for large regions, based on conventional surficial geochemical sampling methods, significant differences are commonly observed in element base levels for individual survey blocks, especially where methods have not been standardized. Uncertainty then exists as to the extent to which apparent changes in base level are due to real geochemical differences, or to the methods employed. Continuous profiles of surface Th and K abundance, derived from airborne data, provide an independent reference level against which estimates of Th and K, derived from various direct sampling techniques, can be compared. This comparison can identify inconsistencies in Th and K data, which may in turn raise questions concerning the consistency of other data. This is important in the context of establishing reliable global geochemical baselines for all elements.