&EPA United States Environmental Protection Agency Environmental Monitoring Systems Laboratory P.O. Box 93478 Las Vegas NV 89193-3478 February 1992 OFFICE OF RESEARCH AND DEVELOPMENT lL r\ TECHNOLOGY SUPPORT PROJECT Correct Sampling Using the Theories of Pierre Gy INTRODUCTION The Environmental Monitor- ing Systems Laboratory in Las Vegas is interested in the optimization of sampling protocol, sampling tools, subsampling techniques, and sample analysis. The importance of obtaining representative samples in the field and retaining their integrity throughout the analytical procedures is fundamental to the genera- tion of meaningful data. Because sampling correct- ness and representativeness is critical to the collection and handling of environmental samples, the EMSL-LV has hosted short courses pre- sented by M. Francis Pitard to explain and enforce the theories of Pierre Gy relating to sampling practice. The inherent heterogeneity of soils presents a particular challenge to field personnel who are responsible for sampling hazardous waste sites. This heterogeneity is also a factor that must be addressed by statisticians, geostatisticians, and chemo- metricians as they develop sampling plans for the location and frequency of sampling. It affects the manner in which analytical chemists subsample in the laboratory. Finally, heteroge- neity influences the interpre- tation of data and the deci- sions made about the actions taken to remediate contami- nation at a site. The theories of Pierre Gy present practical sampling and subsampling methods that can be applied for little or no added expense. Careful attention to these techniques can result in samples that better represent the site and data that more truly represent the sample. True and complete homoge- neity is impossible to achieve because many factors, including gravity, work against it. But the extent of heterogeneity and its effect on environmental sampling can be minimized. Estab- lished methods from the mining industry are appli- cable to the sampling of soils. The work of George Matheron, father of geostatistics, and Pierre Gy, sampling expert, can provide useful insights for environ- mental scientists who are faced with sampling a complex matrix for trace contaminants. TYPES OF ERROR Pierre Gy's theory addresses seven types of sampling error and offers proven techniques for their minimization. The seven major categories of sampling error cover differ- ences within samples. Other differences can exist, such as, within space (covered by geostatistics) and within time (covered by chronostatistics.) The internal sample errors are: Fundamental Error: This is loss of precision inherent in the sample and includes particle size distribution. It is circumstantial error. It can be reduced by decreasing the diameter of the largest par- ticles or by increasing the sample volume. Grouping and Segregation Error: Error due to non- random distribution of particles, usually by gravity. It can be minimized by compositing an analytical sample from many randomly selected increments or by properly homogenizing and splitting the sample. Long-range Heterogeneity Error: This is fluctuating and non-random. It is spatial and may be identified by variographic experiments and can be reduced by taking many increments to form the sample. Periodic Heterogeneity Error: This fluctuation error is temporal in character and can be minimized by compositing samples correctly. Increment Delimitation Error: Error tied to inappropri- ate sampling design and the wrong choice of equipment. Increment Extraction Error: This error occurs when the sampling procedure fails to precisely extract the intended increment. Well-designed sampling equipment and good protocols are crucial. Preparation Error: This error is the expression of loss, contamination, and alteration of a sample or subsample. Field and laboratory tech- niques exist to address this problem. 1M1EX920DC ------- SAMPLE INTEGRITY To truly represent a tot (or a hazardous waste site) a sample must be both accu- rate and precise. Obviously, 100% accuracy and precision cannot be obtained. It is important to minimize the error that is introduced in that sample-taking and in the subsequent handling, subsampling. and prepara- tion. If large-scale heteroge- neity is ignored in a sampling design, data generated from the preferentially sampled material will never truly reflect the character of the site. Some sampling devices and protocols preselect fines or coarses. This error is very serious in environmental work where concentration is fundamental to decision making. For example, if the action level for compound X is 100 micrograms/ kilogram, a sample containing very fine particles coated with com- pound X would exceed action levels but a large rock of the same sample weight would not. But both samples came from the same site, in fact, from the same cubic meter of soil. I! samples spanning all particle sizes are sent to the analytical laboratory, a very confusing picture of the site will emerge. When decisions are made based on the ensuing data, they will be incorrectly made (or made correctly by accident!) DEVICES SUMMARY Correct sampling devices are essential to good sampling protocol and to good labora- tory practice. Pierre Gy recommends scoops and spatulas that are flat, not spoon-shaped, to avoid the preferential sampling of coarse particles. Additional care must be taken at the analytical laboratory, where error can be introduced by poorly designed riffle split- ters, spatulas, and vibrating tools. It is recommended that the sample be subsampied using a system of alternate shovelling wherein a large sample is "dealt out" into several smaller piles, One of these subsamples is chosen for the analysis. This method avoids preferential sampling by saving the subsampie selection until last. Methods developed for the mining industry can provide environmental scientists with guidance for the correct sampling and subsampling of soils. The sampling theories of Pierre Gy are applicable to most sampling events at hazardous waste sites and to the successful subsampling of those samples at the analytical laboratory. Greater sample volume yields data that better represent the site. Careful use of practices suggested by Pierre Gy will result in higher quality data for little or no added expense. REFERENCES Prtard, F. F., Pierre Gy's Sampling Theory and Sampling Practice, 2 Volumes, 1989, CRC Press. Inc., Boca Raton, Florida. FOR FURTHER INFORMATION O O echnology % jupport roject Z © ^%OGY For more information about the application of Pierre Gy's theories to environmental sampling, contact: Dr. George Flatman Environmental Monitoring Systems Laboratory P.O. Box 93478 Las Vegas, NV 89193-3478 (702) 798-2628 FTS 545-2628 For information about the Technology Support Center at the EMSL-LV, contact: Mr. Ken Brown, Manager Technology Support Center EMSL-LV P.O. Box 93478 Las Vegas, NV 89193-3478 (702) 798-2270 FTS 545-2270 The Technology Support Center fact sheet series is developed and written by Clare L Geriach. Lockheed Engineering & Sciences Company, Las Vegas. ------- |