EVALUATION OF DUPLICATE SAMPLES REGIONAL SEDIMENT SURVEYS , BRITISH COLUMBIA
INTRODUCTION The ability to discriminate real trends related to geological and geochemical causes from those that result from spurious factors such as sampling and analytical errors is of paramount importance in the suI:cess of geochemical data interpretation. Since the estimate of reproducibility (precision) allows us t o quantify the amount of variation due t o sampling and laboratory analysis it is an integral part of the evaluetion of geochemical data and should be conducted prior to carrying out any detailed interpreation. As part of our continuing study of rapid but thorough evaluation procedures for multi-element stream-sediment data (for example, Matysek. et a/., 1981) we designed a systematic computer-oriented method of evaluating the quality of geochemical survey data based on field-site duplicates. It irlcorporates a bias test, an analysis of variance technique, and the Thompson and Howarth (1976) approach to qua'ytifying precision. This detailed procedure utilizes the type and quality of data incorporated in various regional geochemical programs undertaken by the British Columbia Ministry of Energy, Mines and Petloleum Resources (the Ministry), but can be adapted easily for other programs, GENERAL METHODOLOGY steps: In brief, our general approach t o evaluation of the quality of geochemical data sets involves the following (1) Extraction of a t least 50 independent, field-site duplicate pairs from a geochemical data set f cmr use in subsequent data analysis. (2) Determination of the degree of systematic bias between duplicate pairs based upon the number of cases in which the first observation is greater than the duplicate. (3) Evaluation of the duplicated geochemical data set in terms of metal variability a t the regional, between-sample site and a t the local, within-sample site, sampling, and analvtical levels by a two-factor analysis of variance technique. (4) Quantification of the within-sample site variability by estimating precision utilizing the Thsmpson and Howarth (19761 method. D A T A Data obtained from the three most recent regional geochemical surveys undertaken Iby the Ministry (1981 I were evaluated using this method. The base data consist o f analyses of stream sediments and water samples collected a t an average density of one sample per 17.3 square kilometres over NTS map-sheet areas 92H, 921, and 92J. Silt samples were field dried and the minus 80-mesh (177 microns) fraction retained for subsequent analyses. The samples were analyzed for zinc, copper, lead, nickel, cobalt, silver, manganese, iron, molybdenum, arsenic, and antimony by the atomic absorption …
Seasonal, sampling and analytical variations in stream sediment surveys
Abstract Sampling and analytical variations in stream sediments from a background stream and from an anomalous stream in northwestern Maine are discussed. Samples were collected according to a carefully planned hierarchical sampling design in which six stations were selected along each stream; eleven samples were collected from each station, and three subsamples were prepared from each sample for determination of the hot HNO 3 -soluble Cu, Pb, Zn, Co, Ni, Mn, Fe, Na, and K by atomic absorption spectrophotometry. The variance component at each level of the hierarchy was estimated by a three-level nested analysis of variance model. Seasonal variation of Cu, Pb, Zn, Co, Ni, Mn, and Fe in stream sediments derived from a background stream and an anomalous stream were investigated by repeat sampling of two streams in northwestern Maine and three streams in northern New Brunswick. Data obtained were treated using a two-way analysis of variance to determine whether there is significant variation in metal contents between sampling times and between sample stations. The study shows that errors for Cu, Pb, Zn, Co, Ni, Mn and Fe resulting from sampling, sample preparation and analysis are small compared to the regional geochemical variation. For these elements the sampling error, which constitutes the main source of procedural error, is greatly in excess of the laboratory error. Time variations for Cu, Pb, Zn, Co, Ni, Mn and Fe in stream sediments in the survey areas are slight and insufficient to cause serious problems in routine application of geochemical sediment surveys.
Dupan 3, a subroutine for the interpretation of duplicated data in geochemical analysis
Duplicated analytical results, properly obtained, can be used to estimate analytical standard deviation as a function of concentration of the analyte, with distinct advantage to other methods. A computer program is given which performs the necessary calculations and also enables the investigator to detect serious deviations from the theoretical model assumed.