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.     

Representativity of wide-spaced lower-layer overbank sediment geochemical sampling

To develop a technique of implementing global ultra-low density geochemical sampling and as a contribution to the International Geochemical Mapping Program (IGCP Project No. 259), an orientation study in the use of deep (lower-layer) overbank sediments was conducted in Jiangxi Province of Southeast China in 1989–1990. Ninety-four samples were collected at depths of 50 to 120 cm from overbank terraces at an average density of 1 site per 1800 km². The total area of sampled catchment basins is approximately equivalent to 18% of Jiangxi Province. Most of the samples were collected at outflow sites of catchment basins with areas of 100 to 800 km². The samples were analyzed for 39 elements.The representativity of wide-spaced lower-layer overbank sediment sampling is discussed from various perspectives; the following features have been observed: (1) Widespaced lower-layer overbank sediment data and the data from China's national geochemical mapping (RGNR) project show similar geochemical patterns for W, Sn, Pb, Cu and Zn. (2) The results of wide-spaced lower-layer overbank sediment sampling demonstrate that catchment basins with areas of 100 to 800 km² are suitable sample site locations for the global geochemical reference network. (3) Wide-spaced lower-layer overbank sediment sampling is a fast and cost-effective way to identify geochemical provinces and has strategic significance in mineral exploration. (4) There is a significant correlation between the W content of wide-spaced lower-layer overbank sediment samples and the presence of W mineralizations within the catchment basins. (5) The distributions of Ni, Cr and V in wide-spaced lower-layer overbank sediment samples distinctly reveals the boundary between the Yangtze sedimentary platform and the South China Caledonian fold system in Jiangxi Province. (6) Distributions of Rb and Be coincide with the Yanshan granites, which are closely related to the major ore-forming episodes in Jiangxi Province.     

超低密度河漫滩沉积物地球化学采样的代表性

为了响应国际地球化学旗图计划(ICCP259)中的全球地球化学采样子计划的实施,研制超低密度地球化学采样的最佳方法,1989～1990年在江西省开展了一项试点研究.在约15.7万平方公里的可采面积内采集了121件河漫滩沉积物样品.平均采样密度约为五个点/1300km².采样覆盖度为24%.采样点主要布置在100-500km²的汇水盆地口上,其中有33个采样点控制的汇水盆地面积在400～2000km²之间.河漫滩沉积物样品来自深度在0.5m以下到1.2m的柱状剖面.样品定量分析了38种元素.文章从多方面讨论了超低密度河漫滩沉积物地球化学术样的代表性,并得出以下主要结论:1.在400-2000km²的汇水盆地口上采集河漫滩沉积物样品能较好地反映上游汇水盆地中的主要矿化.2.超低密度采样和以2个点/km²采集水系沉积物样品所获得的W、Sn、Cu、Pb和Zn五个元素的地球化学趋势相似.3.超低密度样品中W、Ph、Be、Rb等元素的分布特征与省内花岗岩的分布相吻合.4.超低密来样品中“3Ni+Cr+V”的分布能较好地反映扬子准地台与华南加里东精皱系在省内的分界线.     

全球地球化学填图——历史发展与今后工作之建议

区域性与国家性的地球化学填图已取得进展,如何在可期待的未来以极低密度采样获得周期表内大多数元素在全球的分布,有赖于对填图理念的更新及采样介质、采样部署与采样方法研究的进展。英国Webb等发现在数平方千米至数十平方千米汇水盆地之河口采集水系沉积物样品,其分析结果可大致逼近其上游汇水盆地内土壤中元素之平均值。挪威及中国的研究工作表明在更大河流(其汇水盆地达数百、数千以至数万平方千米)的河口采样,这一规律依旧适用。看来,这种分形规律还可进一步延展至世界上一些汇水盆地达数十万至百万以上平方千米的主要入海河口,但这方面尚需作更多的研究。在这种新的填图理念指导下,提出了为实现全球地球化学填图的短期研究与试点计划和长期全面实现之规划。     

Elemental Concentrations of Stream Sediments in Southern China

Over 3 million samples were collected from southern China at a density of 1–2 stream sediment samples per square kilometre as part of the Regional Geochemistry National Reconnaissance (RGNR) programme of China initiated in the late 1970s. Approximately 5244 composite samples of stream sediments from twelve provinces of southern China were prepared from the original RGNR samples collected from a territory with an area of 2300000 km² at a density of one composite sample per 1:50000 map sheet (about 400 km²). Seventy‐six elements were determined by the geoanalysis laboratory of Henan Province (platinum‐group elements) and by the Institute of Geophysical and Geochemical Exploration (IGGE) laboratory for the remainder. Internal quality and external quality control methods were applied to ensure that the analytical data were comparable. Statistics were used to derive the mean and background values and indicate the average concentrations of the seventy‐six elements. Comparisons were made against the mean values obtained from stream sediments, floodplain sediments, overall sediments and overall soils of the entire territory of China. The concentrations of Hg, Cd, rare earth elements and 24 other elements were higher than their background for the whole of China. In contrast, the concentration of Na₂O, CaO, Ba and Sr were lower in the stream sediments in southern China than their whole China background.     

Geochemical mapping in China

China's National Geochemical Mapping Project (Regional Geochemistry-National Reconnaissance, RGNR project) was initiated in 1979. From 1978 to 1982, cooperative research projects were carried out for the preparation and distribution of standard reference samples and for the development of field sampling techniques, multi-element analytical methodology and a unified data quality monitoring procedure. Large pilot surveys were also commenced in several provinces. After five years of technical preparation, the project came into its full implementation. More than 5 million km² of Chinese land surface has been covered by this project. During 1993–1995, another national geochemical project, under the name of ‘Environmental geochemical monitoring network and dynamic geochemical maps in China’ as a pilot survey to choose the suitable sampling medium for the global geochemical mapping application, was carried out in China. The remarkable achievements of China's geochemical mapping projects are widely recognized. Nearly 66% of new discoveries of economic mineralization by MGMR were attributed to the RGNR project. New concepts and new methodologies have emerged through these projects. They also made a great contribution to the international activity toward standardization of geochemical mapping methodology and the possible realization of wide-spaced global geochemical mapping.     

National-Scale Geochemical Mapping Projects in China

Regional, national and global scale geochemical mapping projects have been carried out in China since the late 1970s, due to the development of cost‐effective, low detection limit analytical methods. These projects have provided a huge mass of high‐quality, informative and comparable data for mineral resource exploration and are now making contributions to environmental assessment. In this paper, four national‐scale geochemical mapping projects are described. (1) The Regional Geochemistry‐National Reconnaissance Project (RGNR project), which is China's largest national geochemical mapping project, has covered 6 million km² of upland regions since 1978. Generally, stream sediment samples were collected at a density of 1/km² and four samples were composited into one sample and analysed for thirty‐nine elements. (2) The deep‐penetrating geochemical mapping project (DEEPMAP Project) has been conducted since 1994 in covered terrains, including sedimentary basins, at a density of 1 sample per 100 km² with thirty to seventy elements determined per sample. In the past 10 years, an area of approximately 800 000 km² has been covered and this project has played an important role in finding sandstone‐type uranium deposits in basins. (3) The seventy‐six geochemical element mapping project (76 GEM project) has been carried out since 1999 and involved the collection of stream sediment samples from the RGNR project targets which were analysed for seventy‐six elements. Samples from each 1:50 000 map sheet were composited into one analytical sample (approximately one composite sample per 400 km²). Approximately 1 million km² have been surveyed to date. (4) The multi‐purpose eco‐geochemical mapping project has been conducted since 1999 in Quaternary plain areas for environmental and agricultural applications. Surface soils (depths from 0–20 cm) were collected at a density of one sample per km², and four samples were composited into one for analysis. Deep soils (from a depth of 150 to 200 cm) were collected at a density of one sample per 4 km² and four samples were composited into one analytical sample. All the composite samples were analysed for fifty‐four elements.     

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 km² to 1 per 3500 km². 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.     

化探数据处理的新技术

本文论述了一种新的空间点数据处理系统（ＮＡＳＳＤ）在化探数据处理扣的应用效果，以ＮＡＳＳＤ处理平均采样密度为每１５０００ｋｍ＾２一个，样品总数为５２９个泛滥平原沉积物数据，制作 的中国铜和银地球化学图，可以较清晰地指示出中国已知大型，超大型铜和银矿床的分布，以传统的数据处理方法无论是处理上述的极低密度采样数据处理还是每１ｋｍ＾２～５０ｋｍ＾２一个的区域水系沉积物测量数据，在制作（以相同含量间隔表示     

Estimating specific surface area of fine stream bed sediments from geochemistry

Specific surface area (SSA) of headwater stream bed sediments is a fundamental property which determines the nature of sediment surface reactions and influences ecosystem-level, biological processes. Measurements of SSA – commonly undertaken by BET nitrogen adsorption – are relatively costly in terms of instrumentation and operator time. A novel approach is presented for estimating fine (<150 μm) stream bed sediment SSA from their geochemistry – after removal of organic matter – for agricultural headwater catchments across 15,400 km² of central England, UK. From a regional set of 1972 stream bed sediment sites with common characteristics for which geochemical data were available, 60 samples were selected – based on maximising their variation in Al concentrations – and their BET SSA measured by N₂ adsorption. After careful selection of potential regression predictors following a principal component analysis and removal of a subset of samples with the largest Mo concentrations (>2.5 mg kg⁻¹), four elements were identified as significant predictors of SSA (ordered by decreasing predictive power): V > Ca > Al > Rb. The optimum model from these four elements accounted for 73% of the variation in bed sediment SSA (range 6–46 m² g⁻¹) with a root mean squared error of prediction – based on leave-one-out cross-validation – of 6.3 m² g⁻¹. It is believed that V is the most significant predictor because its concentration is strongly correlated both with the quantity of Fe-oxides and clay minerals in the stream bed sediments, which dominate sediment SSA. Sample heterogeneity in SSA – based on triplicate measurements of sub-samples – was a substantial source of variation (standard error = 2.2 m² g⁻¹) which cannot be accounted for in the regression model.     

Zn, Pb, Cu and As distribution patterns in overbank and medium-order stream sediment samples: their use in exploration and environmental geochemistry

Overbank and medium-order stream sediment samples were collected in Belgium and Luxembourg from 66 sampling locations (area of about 33,000 km²) and analysed for major and trace elements among which Zn, Pb, Cu and As. At each sampling location large bulk samples were taken, namely in the lower (normally at ≥1.5 m depth, over an interval of about 20–40 cm) and upper (normally upper 5–25 cm) parts of the overbank profiles and from the stream sediments. Furthermore, at a number of these sites, a detailed geochemical analysis of vertical overbank sediment profiles (sampling intervals of 10–20 cm) was subsequently carried out to unravel element variations through time and to help in the overall evaluation. For most sampled sections evidences such as ¹⁴C-dating and the absence of anthropogenic particles point towards a pre-industrial and often pristine origin of the lower overbank sediment samples. From the latter bulk samples, mean background concentrations were deduced. They reveal the existence of significant differences between the northern and southern part of Belgium (incl. Luxembourg) which relate to the difference in geological substrate. In the north dominantly non-lithified Quaternary and Tertiary sands, marls and clays occur while in the south Palaeozoic sandstones, shales and carbonate rocks outcrop. Consequently separate mean background values were calculated for the two areas. In the southern study area, some anomalous metal concentrations have been recorded in pre-industrial sediments. They are derived from mineralised Palaeozoic rocks, a feature which could be of interest for base metal exploration. In the upper overbank and stream sediments, in general, higher heavy metal and As contents were recorded with highest values in areas with metal mining, metal melting and cokes treatment industries. By comparing the trace element concentrations of the upper overbank or stream sediment samples with the concentrations detected in the lower overbank samples at each of the sampling locations, and by evaluating the vertical distribution patterns where available, the degree of pollution of the alluvial plain and the present-day stream sediments can be assessed. From this exercise, it is clear that highest pollution occurs in the northern part of Belgium, which relates to its high population density and industrial development.     

Temporal and spatial variations in water discharge and sediment load in the Alaknanda and Bhagirathi Rivers in Himalaya, India

The Alaknanda and Bhagirathi Rivers originate in the mountainous regions of the Himalayas (Garhwal) and result in high sediment yields causing flood hazards downstream of the Ganga River and high sediment flux to the Bay of Bengal. The rivers are perennial, since runoff in these rivers is controlled by both precipitation and glacial melt. In the present study, three locations in the upper reaches of the Ganga River were monitored for 1 yr (daily observations of, more than >1000 samples) for suspended sediment concentrations. In addition, more than one hundred samples were collected from various locations of the Alaknanda and Bhagirathi Rivers at different periods to observe spatial and temporal variations in river suspensions. Further, multi-annual data (up to 40 yrs) of water flow and sediment concentrations were used for inferring the variations in water flow and sediment loads on longer time scales. In most previous studies of Himalayan Rivers, there has been a general lack of long term water flow and sediment load data. In the present study, we carried out high frequency sampling, considered long term discharge data and based on these information, discussed the temporal and spatial variations in water discharge and sediment loads in the rivers in the Himalayan region. The results show that, >75% of annual sediment loads are transported during the monsoon season (June through September). The annual physical weathering rates in the Alaknanda and Bhagirathi River basins at Devprayag are estimated to be 863 tons km⁻² yr⁻¹ (3.25 mm yr⁻¹) and 907 tons km⁻² yr⁻¹ (3.42 mm yr⁻¹) respectively, which are far in excess of the global average of 156 tons km⁻² yr⁻¹ (0.58 mm yr⁻¹).     

Comparison of stream sediment and soil sampling for regional exploration in the eastern Alps,Austria

Tests in three different areas of the Austrian Alps showed that in mountainous areas, soil samples taken at the break of slope are an alternative sampling medium to stream sediments for regional geochemical exploration even if taken at the unusually low density of 1 sample/km². B-horizon soil samples from the nearest break of slope were collected in conjunction with stream sediment orientation studies. The minus 0.2 mm fraction of all samples was routinely analyzed for the total contents of 36 elements. The results for the indicator elements in the three test areas (Area 1: Cu; Area 2: Ag, As, Cu, Pb, Sb, Sn, Zn; Area 3: As, Cu) showed that break-of-slope soil samples more clearly indicate known mineralization than drainage geochemistry. In the soils, almost all elements showed a higher natural variability of the data with considerably higher metal contents. In mountaineous regions, each soil sample will have a clearly defined area of influence. The break of slope represents a site where clastically and hydromorphically dispersed elements are concentrated. It is concluded that soil samples can be used for regional exploration using low sampling densities in mountaineous areas. At the same time they will provide valuable information for other fields of science (e.g. agriculture, forestry, environmental sciences) and are thus superior to stream sediments.     

Use of GEMAS data for risk assessment of cadmium in European agricultural and grazing land soil under the REACH Regulation

Over 4000 soil samples were collected for the “Geochemical Mapping of Agricultural and Grazing Land Soil of Europe” (GEMAS) project carried out by the EuroGeoSurveys Geochemistry Expert Group. Cadmium concentrations are reported for the <2 mm fraction of soil samples from regularly ploughed fields (agricultural soil, Ap, 0–20 cm, N = 2218) and grazing land soil (Gr, 0–10 cm, N = 2127). The samples were collected in 33 European countries, covering 5.6 million km² at a sample density of 1 sample each per 2500 km² and were analysed in an aqua regia extraction followed by an ICP-MS finish. The median Cd value is 0.181 mg/kg for the Ap and 0.202 mg/kg for the Gr soil samples. The data allow a directly comparable country-specific regional exposure and risk characterisation for all EU countries covered. Direct risks of Cd for terrestrial organisms are only predicted for a few isolated sample sites: 2.3% of the Ap and 4.5% of the Gr sites, respectively.     

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 km² 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 km²/sample for hilly areas, and a few hundred km²/sample for lowland areas.     

全球地球化学填图

作指出了1973年至今世界上50余项地球化学填图计划中普遍存在的缺陷大都涉及分析问题。1988－1992年实施的国际地质对比计划IGCP259项目旨在使全世界地球化学填图方法标准化。在此项目中对分析问题提出了若干规定，主要是要求今后的填图计划应统一分析71种元素，痕量及超痕量元素的检出限必须低于相应的地壳丰度值及采用中国的GSD和加拿大的STSD标样系列，以使全球数据可以对比，在其后开始延续至今的全球地球化学填图计划IGCP360，旨在用极低密度采样早日覆盖全球大陆，讨论了正在实行的两种极低密度采样方案，并提出通过极低密度采集地极少量样品示范性实现IGCP259项目对分析要求的具体建议。     

Geochemical exploration in China

Geochemical exploration in China was commenced in the early 1950's. In 1951, the first experimental work was carried out in Yeshan, and a geochemical exploration section was set up in the Ministry of Geology in 1953.Regional geochemical reconnaissance (metallometric surveying) was initiated in 1956 on a nation-wide scale. Soil samples have been collected, and analyzed by semiquantitative spectrography. The results were heavily biased and were not adequately processed and utilized. Renewed efforts have been made to reprocess the vast amount of data accumulated and to utilize them more fully in mineral exploration.Meanwhile, another nation-wide project of regional geochemistry using more refined techniques is in its preparatory stage. It is the Regional Geochemistry-National Reconnaissance Project. In this project stream sediment sampling with a density of one per km² will be used in China Proper, and low-density sampling of various kinds of media in different environments will be used in remote areas. Pilot surveys covering areas of several thousand square kilometers are being undertaken in several provinces.Beside regional reconnaissance, geochemical prospecting has been carried out at virtually all phases of mineral prospecting in China.A brief summary of current research in exploration geochemistry taken by research institutes and universities is given, including studies on the methodology of regional geochemical surveys, primary halos around various types of ore deposits, mercury vapor survey techniques, refinement of analytical methods and instrumentation, and computerized data processing and plotting techniques.Several case histories are described where geochemical exploration techniques have led to successful ore discoveries in China.     

Geochemistry of overbank and high-order stream sediments in Belgium and Luxembourg: a way to assess environmental pollution

A geochemical survey of Belgium and Luxembourg was carried out as part of an international research project entitled ‘Regional geochemical mapping of Western Europe towards the year 2000'. The aim of this research was to map regional background geochemical patterns based on pristine or at least pre-industrial overbank samples and to deduce regional information on the degree of environmental pollution of floodplain and present-day river sediments. Over the entire study area (about 33,000 km²), 66 overbank sites have been sampled. Catchment areas range between 60 and 600 km². At each site an overbank profile has been dug out in the immediate vicinity of the river and described in detail. A first composite sample was taken 5–25 cm below the surface. This sample is supposed to represent deposition over the last centuries. Human interferences in this interval are often inferable based on changed sedimentary characteristics and the presence of anthropogenic particles such as charcoal, slags and brick fragments. A second composite sample was taken at depth, usually >1.5 m below the surface over an interval of about 20 cm. In most sites, the profile characteristics allowed to assume pre-industrial or even pristine conditions for this lower overbank sample. In some profiles this was confirmed by ¹⁴C-dating and/or by the absence of anthropogenic particles. Finally, a present-day stream sediment was sampled on the site to infer the actual pollution status. After drying at 80°C, disaggregation and sieving, the <125 μm fractions of the three sediment samples were analysed by XRF for major elements and several trace elements. Lower overbank samples generally show a direct link with the geological substrate and allow to assess natural background concentrations. Results from the mapping exercise as well as from the statistical analyses display a clear contrast between the northern part of Belgium where Cenozoic unconsolidated sandy and silty formations dominate which are especially vulnerable for erosion, and the southern part of Belgium and Luxembourg where Paleozoic and Mesozoic sandstones, carbonates, marls and shales are the prevalent lithologies. Here the shales are the most intensively eroded lithologies. This is especially reflected in the element patterns of Al₂O₃, MgO, K₂O, Ga, Ni, Rb, Sc and V which negatively correlate with SiO₂. Despite the human related pollution, the geological contrast between north and south Belgium is still recognisable in the geochemical pattern of the upper overbank and present-day stream sediment samples for the above-mentioned elements. Furthermore there is a clear increase in heavy metal contents (Zn, Cd, Pb, Cu), As and in certain locations in Ba from the lower to the upper overbank sediment, as well as to the present-day stream sediment. The relative increase in element content allows to assess the degree of pollution and helps to define those drainage areas where more detailed research is needed.     

High resolution regional hydrogeochemical baseline mapping of stream water of Wales,the Welsh borders and West Midlands region

The stream water hydrogeochemical database, produced by the Geochemical Baseline Survey (GBASE) of the British Geological Survey, has recently been enhanced in the light of experimental pilot studies in North Wales, to meet a wider range of economic and environmental objectives which require modern, integrated and strategic geochemical surveys for their implementation. Hydrogeochemical data are therefore now collected, in conjunction with geochemical data for stream sediments, soil samples and mineral concentrates. The density of sampling, based on the collection of stream water at near-baseflow conditions during the late summer period each year, has been increased to one site per 1.75 km² and a broader spectrum of geochemical determinants introduced. Provisional regional datasets are being prepared for an extensive region covering Wales, the Welsh Borders, and part of the West Midlands representing over 17 000 sample sites. Bedrock geology and base metal sulphide mineralisation are particularly well reflected in the stream water chemistry at the regional scale. The influence of secondary factors, such as geomorphology, atmospheric controls, and anthropogenic contamination due to agriculture, urban, industrial and military developments, can also be readily distinguished.The data obtained by systematic high resolution sampling of first and second order streams, vary in concentration over three or four orders of magnitude for many of the analytes studied here. This compares with a range of only one or two orders of magnitude for many of the analytes in stream sediment samples. The extended range in values for stream water is an important factor in the gridding, plotting and production of relatively stable maps. They are relatively unaffected either by short temporal changes in stream water flow, which are attributable either to storm events noted during the sampling campaign, or by annual differences between wet and dry summers in different years. This has enabled a series of robust surface hydrogeochemical maps to be prepared for analytical data collected during the summer sampling campaigns conducted annually from 1989 to 1994. These maps provide a unique source of synoptic baseline information for a wide range of economic and environmental applications especially, when combined with other geoscience datasets in a GIS environment.     

Sediment supply from the Betic-Rif orogen to basins through Neogene

We present a quantification of total and partial (divided by time slices) sedimentary volumes in the Neogene basins of the Betic-Rif orogen. These basins include the Alboran Sea, the intramontane basins, the Guadalquivir and Rharb foreland basins and the Atlantic Margin of the Gibraltar Arc. The total volume of Neogene sediments deposited in these basins is ~ 209,000 km³ and is equally distributed between the internal (Alboran Basin and intramontane basins) and the external basins (foreland basins and Atlantic Margin). The largest volumes are recorded by the Alboran Basin (89,600 km³) and the Atlantic Margin (81,600 km³). The Guadalquivir and Rharb basins amount 14,000 km³ and 14,550 km³, respectively whereas the intramontane basins record 9235 km³. Calculated mean sediment accumulation rates for the early-middle Miocene show an outstanding asymmetry between the Alboran basin (0.24 mm/yr) and the foreland basins (0.06-0.07 mm/yr) and the Atlantic Margin (0.03 mm/yr). During the late Miocene, sedimentation rates range between 0.17 and 0.18 mm/yr recorded in the Alboran Basin and 0.04 mm/yr in the intramontane basins. In the Pliocene-Quaternary, the highest sedimentation rates are recorded in the Atlantic Margin reaching 0.22 mm/yr. Sedimentary contribution shows similar values for the inner and outer basins with a generalized increase from late Miocene to present (from 3500 to 6500 km³/My). Interestingly, the Alboran Basin records the maximum sedimentary contribution during the late Miocene (5500 km³/My), whereas the Atlantic Margin does during the Pliocene-Quaternary (6600 km³/My). The spatial and time variability of the sediment supply from the Betic-Rif orogen to basins is closely related to the morphotectonic evolution of the region. The high sedimentation rates obtained in the Alboran Basin during the early-middle Miocene are related to active extensional tectonics, which produced narrow and deep basins in its western domain. The highest sedimentary contribution in this basin, as well as in the foreland and intramontane basins, is recorded during the late Miocene due to the uplift of wide areas of the Betics and Rif chains. The analysis of the sedimentary supply also evidences strong relationships with the post-Tortonian crustal thickening and coeval topographic amplification that occurred in the central Betics and Rif with the concomitant evolution of the drainage network showing the fluvial capture of some internal basins by rivers draining to the Atlantic Ocean (the ancestral Guadalquivir).