Summary of the U.S. Geological Survey National Field Quality Assurance Program from 1979 through 1989

Abstract

Almost 38,000 pH and specific conductance measurements and almost 5,200 alkalinity measurements have been evaluated in the U.S. Geological Survey's National Field Quality Assurance Program since its inception in 1979. The measurement values were analyzed to determine the relative fourth-spread (F-spread), a measure of the width of the middle half of the data, and the fourth-pseudosigma (F-pseudosigma), a robust replacement for the standard deviation, for each of seven different specific measurement ranges. The results of the statistical analysis indicate that the vast majority of values for all three field measurements were within acceptable ranges. Levels of precision were best for pH measurements in the range between 6.00 to 6.99 units. The relative F-spread percentage was 0.4, and the F-pseudosigma was 0.02 pH units. For specific conductance and alkalinity, the percentage of the relative F-spread remains relatively constant for the different measurement ranges. The level of precision was best for specific conductance in the range between 250 to 499 microsiemens per centimeter. The relative F-spread was 2.3 percent, and the F-pseudosigma was 8.9 microsiemens per centimeter. The level of precision was best for alkalinity ranges between 150 to 199 milligrams per liter as calcium carbonate. The relative F-spread percentage was 3.1 and the F-pseudosigma was 4.6 milligrams per liter. INTRODUCTION Part of the mission of the Water Resources Division of the U.S. Geological Survey (USGS) is to collect water-quality information on the water resources of the Nation. Implementation of this part of the mission requires both the collection of water-quality samples for laboratory analyses and field measurements of alkalinity, pH, and specific conductance. An estimated 45,000 field measurements are made annually by USGS field analysts (hydrologists and hydrologic technicians). All USGS personnel who perform field determinations are required to participate in the National Field Quality Assurance Program (NFQA). Contract and cooperator personnel who collect these types of field data to be used in USGS reports or stored in the USGS National Water Data Storage and Retrieval System (WATSTORE) are encouraged to participate in this quality assurance program. The formal quality assurance program to monitor the accuracy and precision of pH and specific conductance field measurements made by USGS personnel was initiated in March 1979. Initially, a set of two pH and two specific conductance proficiency samples was distributed about every 6 months to participating personnel in the USGS. The measurement of alkalinity was added as a test parameter in 1985. The frequency of distribution of the subsequent rounds of proficiency samples was changed in 1985 to two sets of proficiency samples to each participant about every 15 months. The frequency of distribution was changed to facilitate the addition of