IGCP 259

Project development according to IGCP 259 Recommendations

Phase I - IGCP 259:  International Geochemical Mapping

• Summary of principal recommendations

• A Global Reference Network

• Site Location and Sampling Media

• Sample Quantities

• Analytical Arrangements

• Field Methods for Regional Surveys

• Sample Collection

• Sample Preparation

• Geoanalytical Requirements

• Radioelement Mapping

• Data Management

• Map Presentation Recommendations

• Implementation

• Conclusions

Summary of principal recommendations

The following were identified by IGCP 259 as basic prerequisites for a global geochemical database of permanent value:

• commonly available representative sample media, collected in a standardised and harmonised manner;

• continuity of data across different types of landscape;

• adequate quantities of the designed sample media for future reference and research requirements;

• analytical data for all elements of environmental or economic significance;

• the lowest possible detection limits for all elements, and

• tight quality control at every stage of the process.

A Global Reference Network

In order to begin systematic international geochemical mapping it is necessary to establish a primary Global geochemical Reference Network (GRN), analogous to a geodetic grid. Wide spaced sampling is required over the entire land surface, including regions already surveyed, and regions where there is low probability of any geochemical mapping being carried out in the near future.  The samples collected will serve as analytical reference materials, so strict procedures must be followed, and adequate quantities must be obtained and retained for future reference and research requirements.

Site Location and Sampling Media

The primary reference network will be based on approximately 5000 grid cells of 160 x 160 km distributed over the Earth's land surface (see map showing grid cell boundaries);

• Sample sites should be located in several (minimum of 5, 8 are preferred) 20 x 20 km, or 40 x 40 km, subcells within each 160 km cell to permit the application of statistical analysis of variance techniques; access and cost considerations will determine whether the subcells are randomly selected along one or two sampling profiles or road corridors, or randomly distributed in any part of a 160 km cell; in undeveloped regions helicopter-borne sampling profiles are recommended;

• Several materials are to be sampled from within each cell:

• 0-25 cm depth regolith on residual and/or alluvial and/or glacial surfaces,

• C-horizon regolith at a depth range between 50 and 200 cm,

• Surface humus, if present,

• Stream sediment or lake sediment,

• Stream water, if present,

• Surface alluvial sediment (0-25 cm) – floodplain or overbank sediment, and

• Deep alluvial sediment – floodplain or overbank sediment (depth range depending on site conditions).

• Stream sediment and stream water samples should be collected from drainage basins not exceeding 100 km2;

• Each stream sediment sample should be a composite of a minimum of 5 subsamples; this composite material must be thoroughly homogenised;

• Overbank sediment (top and bottom) should be taken from an adjacent site;

• Residual soil samples should be taken from a suitable site within the same 100 km2, and over the dominant rock type;

• Each residual soil sample should be a composite from three adjacent subsites at a maximum distance of 5 m;

• Floodplain sediment (top and bottom) should be taken from large basins, between 1000 km2 and 6000 km2 in area;

• Overbank and floodplain sediment samples should be taken from a single site, and

• Gamma ray spectrometer readings should be taken at each site for in-situ radioelement determination; the acquisition of carborne or airborne gamma ray spectrometry in transit between sample collection sites is recommended to establish a linked data network.

Sample Quantities

• Sufficient material should be collected to provide, from each cell, 10 litres of humus, if present, and 5 kg of <0.18 mm fraction of sediment and each type of regolith sample;

• A bulk 0-25 cm regolith sample, and <2 mm regolith size fraction should be retained separately (2 kg each) for radiometric and agricultural reference respectively;

• If surface water is present, three separate samples are required, about 300 ml in total:

• unacidified, unfiltered, for anion determination,

• unfiltered, acid digested for total cations, and

• filtered (45 micron), acidified, to provide "dissolved" cations.

Please note that the above recommendations are included in the FOREGS/EuroGeoSurveys Geochemical Mapping Field Manual, which should be consulted, and is presently at the process of being updated.
[URL: http://www.gsf.fi/foregs/geochem/fieldman.pdf].

Analytical Arrangements

Instructions will be provided by the project co-ordinators concerning which international reference analysis laboratories will receive the samples.  A cell-composite sample must be prepared (with equal weights from each sampling site) for each media type within each cell, retaining a minimum of 50 g from each individual site for separate analysis at a later date.  Fifty grams of each cell-composite should be sent immediately to the participating international reference laboratories.  The bulk of each composite sample should be retained in the country or region of origin in order to provide a range of secondary Standard Reference Materials to standardise future geochemical work in the area. Individual samples from within each cell of the reference network should be analysed at the earliest opportunity. 

This IGCP 259 recommendations were not followed fully in the FOREGS/EuroGeoSurveys Geochemical Atlas of Europe project.  All individual samples were analysed in the same laboratory for the same suite of elements (Refer to Geochemical Atlas of Europe Internet site, http://www.gtk.fi/publ/foregsatlas/).

Field Methods for Regional Surveys

The principal demand for systematic geochemical data collection is at the regional and/or national level.  In general, a much greater data density is required than can be provided as part of a Global Reference Network.  Thus, national or regional agencies, using methods selected by them, will continue to be the producers of most of the world's geochemical data.  It is strongly recommended that the detailed data they collect should be tied into, and be compatible with, the Global Reference Network data, so that data at any level of detail may be compiled and/or compared across organizational and political boundaries.  Data collected for detailed national or regional surveys should be compatible, regarding methods and spatial overlap, with those collected as part of the Global Reference Network.  Therefore, the IGCP 259 report (Darnley et al., 1995) provides recommendations relating to the conduct of these more detailed surveys, since these should provide the major part of the future global geochemical database.

Sample Collection

• Regional and national mapping should be performed according to internationally compatible and agreed standards;

• At least one sampling procedure should be applicable and used consistently in any given geographical area, from global to regional scales;

• Stream sediment samples from tributary drainages are the preferred sample medium, complemented by regolith samples;

• Stream water should be sampled in conjunction with stream sediments wherever possible;

• If stream sediments cannot be collected, acceptable substitutes are regolith, till or lake sediment;

• Where a change in landscape requires a change in sample media, sample media collected in neighbouring blocks must overlap to allow comparison of data;

• Stream sediment catchment basins should be not more than 100 km2;

• Analyses should be undertaken on composites of 5 or more subsamples;

• Duplicate samples should be obtained from at least 3 per cent of sites;

• Systematic labelling and documentation is essential.

It is noted that following pilot projects carried out in Europe, China and South America, the preferred sample medium for continental scale geochemical mapping is overbank or floodplain sediment, since this is the most cost-effective medium.

Sample Preparation

• Contamination must be avoided in sample collection, preparation and storage by appropriate choice of tools, equipment and containers;

• Wet or dry sieving may be employed;

• Upper limit of drainage sediment grain-size fraction analysed should be in the range of 0.18 to 0.10 mm;

• A minimum of 100 g of the sieved fraction is required;

• Non-stream sediment samples should be reduced to <0.1 mm prior to analysis;

• Sample material not required for immediate analysis should be archived for future use, in contaminant-free permanent containers;

• Systematic labelling and documentation is essential.

Geoanalytical Requirements

The analytical requirements for the purpose of producing internationally comparable geochemical baseline data are as follows:

• Given the many applications of baseline data, a comprehensive multi-element approach is essential.  Analytical requirements are considered in two categories, i.e.,

(a) for the Global geochemical Reference Network (GRN), and
(b) national or regional surveys conducted by national or international organisations.

• Samples collected as part of the GRN should ultimately provide abundance data for 78 elements, i.e., most elements in the periodic table, using methods with limits of detection significantly below presently estimated crustal abundances.  Abundance data are not required for the following:  H, O; inert gases other than Rn; Tc, Pm, Po, At, Fr, Ac and Pa.

• Analytical requirements for national or regional surveys must be fully compatible with those for the GRN.  Where possible identical methods should be used, but alternative multi-element schemes based on the techniques of XRF, ICP-AES, ICP-MS, NAA, etc. can be employed as long as the criteria for precision and accuracy are met.  In order to portray the spatial distribution of elements as completely as possible, detection limits must be as low as possible.  In the FOREGS/EuroGeoSurveys Geochemical Atlas of Europe project all samples of the same medium were analysed for the same suite of elements in the same laboratory.  This is the only practical way to produce harmonised and compatible data across political boundaries.

• A proposal is made for a standard list of elements to be determined; elements are classed into List 1 (51 elements) and List 2 (20 elements) (see Darnley et al., 1995); Ru, Rh, Re, Os and Ir require the establishment of satisfactory methods and detection limits; Ra and Rn require radiometric methods.

• For national surveys, if analytical facilities are insufficient to cover all List 1 elements initially, missing data should be added at a later date.  List 2 is of lower priority, but potentially important.

• For the purpose of establishing reproducible baseline data of permanent value, analytical methods should be employed, which provide the total concentration of each element present.  Sample decomposition, where required, must be total.

• Partial decomposition (partial extraction) methods have many variants in different laboratories and are difficult to standardise, so they are NOT recommended for any data that may be used for international compilation or correlation.

• With respect to national and regional surveys, it is recommended that if more than 20% of the reportable values for any element determined fall below the limit of detection, the results for that element should be considered as unsatisfactory and alternative methods considered.

• Strict quality control, through the use of appropriate primary and secondary Standard Reference Materials (SRMs) is essential, and the manner in which SRMs have been used, and the resulting quality control statistics, must be reported with each data set.

• The Chinese series GSD, GSS and the Canadian STSD standard samples are recommended to be used as primary SRMs in international geochemical mapping.  For national surveys, primary and secondary SRMs should be used to monitor the analytical accuracy:  (a) primary SRMs to monitor the international or interlaboratory bias, and (b) secondary SRMs to monitor the routine between-batch drift within a laboratory.

• Radioelements are to be determined by gamma ray spectrometry.

Radioelement Mapping

General

• Radioactive elements, both naturally occurring isotopes and anthropogenic products, cause particular public concern and their spatial distribution should, therefore, form part of a comprehensive geochemical database;

• Gamma ray spectrometry enables the abundance of natural and human-made radioactive elements to be determined in a laboratory, in situ, or from a vehicle or aircraft;

• An aircraft can provide a continuous quantitative profile of radioelement abundance over any type of land surface;

• Flight-line spacing can be varied according to the sampling density required;

• Airborne data may be used to provide an inter- and trans-continental Th reference datum to assist in levelling geochemical maps from geographically diverse regions;

• Methodologies published by the International Atomic Energy Agency (IAEA) should be followed.

Use of Existing Data

• Countries should make an inventory of all airborne and carborne gamma ray spectrometric surveys carried out since 1970;

• If necessary, arrangements should be made through the IAEA to establish radiometric calibration facilities;

• If necessary, a back-calibration exercise should be carried out to allow the preparation of quantitative maps;

• For environmental health radiation monitoring purposes, a map of air dose rate (Gy s-1) or effective dose equivalent (mSv a-1), should be produced;

• Where sufficient data are available, they should be used to prepare an appropriate grid, suitable for the production of a regional or national atlas and ultimately a radioelement map of the world.

Collection of New Data

• Many past radiometric surveys were undertaken with inadequate equipment and insufficient quality control; at best, they were qualitative rather than quantitative; such data are unsatisfactory for baseline purposes;

• In situ measurements with a field portable gamma ray spectrometer (GRS) should be made at each subsite in the primary 160 x 160 km reference grid wherever a regolith reference sample is collected;

• Additional information can be obtained by using a portable GRS to obtain a continuous profile whilst in transit in a road vehicle between sample sites (i.e., a carborne survey);

• Airborne Gamma Ray Spectrometry (AGRS) is the preferred method of obtaining comprehensive radioelement baseline data;

• To complement the 160 x 160 km primary reference grid, flight-line spacing should be at 80 km, or 40 km or 20 km in populated or contaminated areas;

• If funding permits, ground sample collection and AGRS may be undertaken as a single combined operation by using helicopter transportation along regularly spaced cross-country profiles;

• A helicopter-mounted combined operation would provide the fastest way of obtaining comprehensive radiometric global geochemical coverage, at the same time facilitating the linkage of geochemical with aerogeophysical data.

Data Management

Databases

Microcomputers, or work stations, are acceptable for data management; the following databases should be established;

• A Global Reference Database, which will contain new data consistent with the proposed GRN specifications;

• An index database for administrative and general information;

• A block database for averages of elements in large blocks (0.5 x 0.5 degrees spherical rectangles for old compiled data, and 160 x 160 km cells for global reference sampling data) for global presentation;

• A bibliographic database for related publications.

Quality Control Requirements for Sampling and Analysis

• Composited sampling to reduce sampling error;

• Replicate sampling and analysis in an unbalanced 2-fold design for evaluation of geochemical relief in relation to sampling and analytical variability;

• SRMs to monitor analytical drift, and randomisation to convert any residual systematic drift to non-spatially related analytical variability.

Data Storage

• Miscellaneous recommendations relate to analytical data reporting (maximum of 4 significant figures, and adequate resolution close to the detection limit), data formats, units of measurement, geographical coordinates, data transfer, and the availability of a standard query (command) language.

Levelling and Normalisation of Existing Data Sets

• Parametric, linear or non-linear levelling may be used, where it is possible to reanalyse some samples or recollect samples from some of the same sites, or if reference materials were analysed in both data sets;

• In other situations, it is necessary to apply non-parametric normalisation, using the fractile method or Clarke normalisation.

• Airborne gamma ray spectrometry can be used to overfly adjoining and isolated survey areas to provide an independent Th datum to assist in levelling ground radiometric geochemical data sets.

Map Presentation Recommendations

• Equal-area map projections for international and global maps (van der Grinten or Goode's Interrupted Homolographic preferred);

• Coloured surface maps, based on interpolated and smoothed data, are most effective for conveying information.

Implementation

In order to obtain a global multi-element geochemical database, the following practical considerations need to be addressed:

• The cost of establishing a complete geochemical reference network, as recommended, is estimated to be in the range of US$300-400 million (1993 estimate);

• Full data acquisition will require a minimum of a decade; given the data's immediate relevance to intensifying land-use problems, early completion (by 2005) is highly desirable;

• If a more rapid and relatively low cost global overview is required, a preliminary reconnaissance confined to floodplain sampling could be undertaken.  However, floodplain data alone are of somewhat restricted value.  The cost of the overall project would be increased by the necessity to re-visit every cell at a later date to collect the other recommended sample media, which would better indicate the ranges of element abundance, and provide the preferred means of correlating global and national datasets. 

• Irrespective of the rate or mode of progress, the quality and consistency of data must be controlled throughout the acquisition period;

• Standard reference materials must be provided and renewed as necessary;

• A mechanism is required for assessing and introducing new techniques as they become available;

• No useful purpose will be served unless global and regional data are readily accessible; and

• Globally interlinked geochemical data centres are required, which connect to population, environmental, natural resource and global change data centres.

Because the scientific and technical requirements of systematic geochemical mapping have practical significance for all countries:

• Countries should be encouraged to support and participate in the work, and extract maximum value from the information which is obtained;

• Appropriate training and technical assistance should be made available where needed, and

• Regional centres should be established to encourage cooperative research and the dissemination and application of geochemical data.

Conclusions

• Central coordination is necessary for the duration of the project;

• Progress must be expedited and facilitated by a small technical secretariat, funded and administered through a recognised international organisation.