Geochemical Data for Hong Kong Stream Sediments
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GEOCHEMICAL DATA FOR HONG KONG STREAM SEDIMENTS GEO REPORT No. 230 R.J. SEWELL GEOTECHNICAL ENGINEERING OFFICE CIVIL ENGINEERING AND DEVELOPMENT DEPARTMENT THE GOVERNMENT OF THE HONG KONG SPECIAL ADMINISTRATIVE REGION GEOCHEMICAL DATA FOR HONG KONG STREAM SEDIMENTS GEO REPORT No. 230 R.J. SEWELL This report is largely based on GEO Geological Report No. GR 1/2007 produced in July 2007 - 2 - © The Government of the Hong Kong Special Administrative Region First published, July 2008 Prepared by: Geotechnical Engineering Office, Civil Engineering and Development Department, Civil Engineering and Development Building, 101 Princess Margaret Road, Homantin, Kowloon, Hong Kong. - 3 - PREFACE In keeping with our policy of releasing information which may be of general interest to the geotechnical profession and the public, we make available selected internal reports in a series of publications termed the GEO Report series. The GEO Reports can be downloaded from the website of the Civil Engineering and Development Department (http://www.cedd.gov.hk) on the Internet. Printed copies are also available for some GEO Reports. For printed copies, a charge is made to cover the cost of printing. The Geotechnical Engineering Office also produces documents specifically for publication. These include guidance documents and results of comprehensive reviews. These publications and the printed GEO Reports may be obtained from the Government’s Information Services Department. Information on how to purchase these documents is given on the second last page of this report. R.K.S. Chan Head, Geotechnical Engineering Office July 2008 - 4 - FOREWORD This report is a compilation of all stream sediment geochemical data for Hong Kong held within the Hong Kong Geological Survey archive. It comprises the raw data upon which geochemical maps and interpretations in the Geochemical Atlas of Hong Kong have been based. The report has been written and compiled by Dr R.J. Sewell of the Hong Kong Geological Survey in the Planning Division. (K.C. Ng) Ag. Chief Geotechnical Engineer/Planning - 5 - CONTENTS Page No. Title Page 1 PREFACE 3 FOREWORD 4 CONTENTS 5 1. INTRODUCTION 6 2. SAMPLING 6 3. METHODS USED IN STREAM SEDIMENT GEOCHEMICAL 6 ANALYSIS 3.1 Introduction 6 3.2 Inductively Coupled Plasma Atomic Emission Spectrometry 7 3.3 X-ray Fluorescence Spectrometry 8 4. PRESENTATION OF RESULTS 8 5. DIGITAL DATA SETS 8 6. CONCLUSIONS 9 7. REFERENCES 9 LIST OF TABLES 10 APPENDIX A: DESCRIPTION AND LOCATION OF ANALYSED 14 SAMPLES APPENDIX B: GEOCHEMICAL DATA 78 - 6 - 1. INTRODUCTION During the mid-1990’s, the Hong Kong Geological Survey carried out a comprehensive drainage-based regional geochemical survey by systematically sampling and analysing stream sediments from all of the major catchments within the Hong Kong Special Administrative Region (SAR). The main aim of the survey was to establish the overall natural background levels of a wide range of elements for use in determining contamination criteria for offshore contaminated sediments. The basic data give valuable information on the distribution of trace elements in the weathering zone environment and also indicate areas of metallic mineralisation. The panned concentrates help in distinguishing between the natural distribution patterns of elements controlled by the underlying geology and those related to superimposed man-made contamination. Potential applications of the geochemical data include land use planning, environmental monitoring and pollution control, waste management, the recognition of the effects of trace element deficiency or toxicity on crops, and various aspects of human and animal health. So far, the basic data have been used primarily by the Hong Kong Geological Survey to assist with modelling of geological processes (e.g. Sewell, 1997), and with the compilation of a geochemical atlas of Hong Kong (Sewell, 1999). Except for a handful of representative analyses, most of this information remains unpublished. This report provides a comprehensive listing of all raw stream sediment analytical geochemical analyses currently held in the Hong Kong Geological Survey archive, and should be read in conjunction with the “Geochemical Atlas of Hong Kong” (Sewell, 1999). 2. SAMPLING The bulk of the sampling for the analytical data set, which comprises 2246 stream sediment analyses, took place during the winters of 1993-94, 1994-95 and 1995-96 at an average sampling density of one sample per 0.5 km2. Methods used for collection of samples followed those recommended in Ball (1993). Samples for geochemical analysis were collected, as far as possible, upstream of any potential source of contamination, such as concrete weirs, human habitation, tracks or roads. A sieve set comprising a wooden dulang type pan, a coarse (2 mm) sieve and a 100 mesh (#) fine sieve were employed. Sediment obtained from the middle part of the stream bed was sifted through the coarse sieve. Approximately 50 g of material finer than 150 µm was collected in the pan and transferred to a Kraft® paper bag. A heavy mineral concentrate was then obtained by panning 1 - 2 kg of stream sediment screened through the 2 mm sieve. 3. METHODS `USED IN STREAM SEDIMENT GEOCHEMICAL ANALYSIS 3.1 Introduction Two main analytical methods have generally been employed in obtaining the stream sediment geochemical data: Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and X-ray fluorescence spectrometry (XRF). Analytical work was undertaken at in two phases at two laboratories operating identical machines and software. Each sample - 7 - was analyzed for thirty-six elements with major elements expressed in weight percent oxide (wt%) and trace elements in parts per million (ppm). The first 625 stream sediment samples were analyzed at the University of Nottingham, U.K. Except for Ag, Cd, Cu, Co, Mo, and Se, which were carried out by ICP-AES, all the elements concentrations reported for these samples were determined by XRF. Operating conditions were 40 kV and 50 mA. Limits of detection for XRF and ICP analyses used at the University of Nottingham are shown in Table 1. The remaining 1,621 samples were analyzed at the University of Leicester, U.K. With the exception of Ag, Cd, and Se, which were measured by ICP, all the element data for these samples were determined by XRF. Limits of detection for XRF and ICP analyses used at the University of Leicester are shown in Table 2. The main purpose of keeping with XRF as the main analytical tool was to minimize errors caused by differences in machine precision and accuracy. The bulk of the analytical work was completed in 1997. Brief descriptions of the analytical methods are given below. 3.2 Inductively Coupled Plasma Atomic Emission Spectrometry ICP-AES is a ‘flame’ technique involving a flame temperature in the range 6000 - 10 000ºK. At the time the first batch of sediment samples was analyzed, a monochromator was used for detection which made it necessary for all elements to be analyzed sequentially. The analytical process was slow and laborious, resulting in a total time of at least one hour per sample. Owing to the high cost involved in obtaining accurate and precise data, the method was later abandoned in favour of the more cost-efficient XRF technique. In ICP-AES, rock samples are first dissolved using standard silicate dissolution methods, and the solution is then passed as an aerosol from a nebulizer into an argon plasma. The inductively coupled plasma is a stream of argon atoms, heated by the inductive heating of a radio-frequency coiled and ignited by a high frequency Tesla spark. The sample dissociates in the argon plasma and a large number of atomic and ionic spectral lines are excited. The spectral lines are detected by a range of photomultipliers, compared with calibration lines, and their intensities converted into concentrations. Stream sediment solutions of Hong Kong samples were prepared using a lithium metaborate fusion procedure. Solutions were then presented to the plasma alternately with a single element standard. The system comprised a Plasma-Therm 2.5 kW ICP Generator and torch-box with an automatic power control and matching network. The plasma torch used a radio frequency of 27.12 MHz and was coupled to a Spex 1802 scanning spectrometer with a x2 image of the torch focused on the slit. The detection system comprised a Spex DPC-2 amplifier whch integrated the analytical signal for 10 s. Instrumental parameters used were as follows: Plasma Power 1-1.5kW, coolant argon 12 l/min, Nebulizer Argon c. 1 l/min, auxilliary Argon 0.4 l/min, Spectrometer entrance slit 20 µm, Spectrometer exit slit 20 µm. - 8 - 3.3 X-ray Fluorescence Spectrometry XRF is a technique which provides rapid, accurate and precise analysis for a wide range of geological materials. The material is generally supplied as a homogenous fine- grained powder and prepared either as pressed powder pellets or fusion beads for analysis. When both pressed powder pellets and fusion beads are used, a wide range of elements can be routinely analyzed from parts per million to near 100%. The general principle of XRF spectrometry is that a sample is irradiated with primary X-rays, which excites secondary X-rays with wavelengths characteristic of the elements present in the sample. The intensity of the secondary X-rays is used to determine the concentration of elements present by reference to calibration standards with appropriate corrections made for instrumental errors. The sediment samples were prepared for analysis using the methods outlined by Harvey (1989) and Brewer et al. (1998). Six to eight grams of sediment powder was required to make pressed powder pellets following the addition of a few millilitres of an inert PVP-methylcellulose binder. Glass fusion beads were used for major element analysis, and these were made by combining c. 0.4 g of sediment powder with ca.