摘要:

The primary objectives of Phase I of the National Surface Water Survey were to determine the number of acidic or potentially acidic lakes and streams, their location, and their physical and chemical characteristics. To meet these objectives, a statistically designed survey was implemented. For both lakes and streams, probability samples were drawn in order to make population estimates within known confidence bounds. The National Lake Survey utilized a stratified design in which the population consisted of lakes identified on 1:250,000 maps in regions of interest. Strata were defined on the basis of region, subregion, and mapped alkalinity classes. A systematic random sample was taken within each stratum. The National Stream Survey was also stratified by region and subregion, but because a list of the extensive number of streams was not feasible, a two-stage sampling scheme utilizing a point frame was employed within strata. Both designs are flexible in that inferences about any subpopulation of interest can be made. However, because sampling intensities varied among strata, stratum-specific weights must be used when making estimates for those subpopulations that combine or cut across strata.

ISSN:1040-2381    

DOI:10.1080/07438148709354805

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The primary objective of the National Surface Water Survey was to determine the percentages of acidic and low acid neutralizing capacity surface waters in regions of the United States thought to be potentially susceptible to acid deposition effects. The Southern Blue Ridge Province is a region of the United States known to receive wet deposition with a pH less than 4.6 and was expected to contain predominantly low acid neutralizing capacity waters. Both the Eastern Lake Survey and the National Stream Survey sampled surface waters in the Southern Blue Ridge, thereby providing a unique opportunity to compare and contrast the “index” chemistry of lakes and streams in the region. Population descriptions for such important chemical variables as acid neutralizing capacity, calcium, and sulfate are remarkably similar, despite the fact that lakes were sampled at fall turnover, while streams were sampled in the spring and early summer. The similarity of lake and stream chemistries reflects the large number of drainage lakes and reservoirs in the region which typically have short retention times. Of the variables compared in this paper, only the pH distributions were appreciably different.

ISSN:1040-2381    

DOI:10.1080/07438148709354806

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

An effective, economical quality assurance program applied to lake monitoring uses a minimum number of quality assurance samples to provide maximum information about overall data quality. The resulting estimates of data quality can be expressed quantitatively as measures of precision, accuracy, representativeness, and comparability, The completeness of a data set can be assessed during the verification process and by an independent review (data audit). For a particular survey of lakes, the number and the types of quality assurance samples needed to evaluate measurement precision and accuracy can be calculated by identifying the components of potential variability in the data during preliminary field investigations (pilot studies) can be used to estimate the level of confidence associated with the routine sample measurements from the survey. The U.S. Environmental Protection Agency's National Surface Water Survey used laboratory performance evaluations, pilot studies, and measurements of quality assurance and quality control samples. This paper outlines the methods used for determining the confidence limits of data from the Eastern Lake Survey–Phase I and from the Phase II pilot study and spring sampling, including calculation of the mean and the variance of the observed measurement values that the analytical laboratories reported for audit samples. By assigning error limits to the confidence interval of the observed values, an equation was solved for the unknown number of replications (audit samples) needed to achieve the specified data quality objectives. Results of these calculations were used to determine the numbers of quality assurance samples needed for the Phase II summer and fall sampling. Recommendations for applying this approach to other monitoring projects include considerations of the size and complexity of the survey, the budgetary and logistical constraints, and the amount of information available regarding laboratory performance.

ISSN:1040-2381    

DOI:10.1080/07438148709354807

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The National Surface Water Survey is a three-phase project conducted by the U.S. Environmental Protection Agency and designed to document the chemical and biological condition of surface waters considered susceptible to acidic deposition. During the Eastern Lake Survey – Phase l, 1,929 lake samples, including 125 field duplicates, and 245 field blanks were collected. Duplicate and blank samples were used to estimate system precision and quantitation limits for each of 19 chemical parameters. Precision estimates are typically calculated as a single value, based upon data at or above the quantitation limit, and are not applicable below that limit. Where sufficient data are available, a relationship may be developed between concentration and precision. The resulting model may be used to estimate measurement uncertainty at any given concentration, thus supplying precision information for very low concentration samples. Examples of the development of measurement uncertainty models for 19 selected analytes are presented and discussed.

ISSN:1040-2381    

DOI:10.1080/07438148709354808

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor