The U.S. Geological Survey (USGS) plays an instrumental role in operating and maintaining the only nationwide network of precipitation chemistry monitoring stations in the United States. This program is called the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). Precipitation samples are collected weekly for analysis at approximately 200 NADP/NTN sites across the country. The USGS provides about one-third of the funding for the operation of these sites as part of the National Trends Network initiative. The information gained from monitoring the chemistry of our Nation's precipitation is essential for evaluating the effectiveness of emission control efforts and in determining trends in precipitation quality. Precipitation chemistry data are also critical to integrated water quality assessments at the watershed, regional and national level. The USGS ensures the quality of NADP/NTN data through an integrated external quality assurance program which has operated since 1978.
Every analytical measurement inherently contains a certain amount of error. This error component is introduced when water samples are collected, transported, stored, and analyzed, and when the data are stored electronically. For low-ionic strength samples (such as precipitation samples), this error term commonly becomes very significant. Investigators from the USGS operate several quality assurance programs that document and enhance the NADP/NTN data quality from the point of sample collection to storage of the data in a centralized database. Without quality assurance efforts of the USGS, the error component of NADP/NTN data would be largely unknown and the usefulness of the network's data in environmental studies would be diminished.
Each week NADP/NTN site operators remove the precipitation sample from their collectors and determine the pH and specific conductance before shipping it to a central laboratory for further analysis. Since November 1978, the USGS has operated a quality assurance program for these on-site measurements. Investigators prepare and ship synthetic rain samples to all site operators twice per year and request they measure and report the pH and specific conductance. Operators receive additional help from the USGS if they fail to meet the measurement-accuracy goals. This help includes supplying the operator with additional check samples to measure. In addition, all procedures used to make the readings are carefully reviewed with the operator. If faulty equipment is determined to be the source of measurement difficulties, the USGS assists in arranging for its replacement. As a result of this program and other improvements in site operator training and equipment, the percentage of operators routinely meeting quality assurance goals has increased from approximately 50 percent to more than 90 percent in the past 14 years.
The USGS initiated an intensive program to quantify and reduce contamination in NADP/NTN collection methods because contamination can be specially significant for low-ionic strength samples. USGS investigators regularly send solutions with known ionic concentrations to all NADP/NTN sites. Operators split the sample into two portions and handle part of the sample as if it were an actual precipitation sample collected at their site. They are then instructed to pour this portion of sample into a shipping bottle, disguise it as the regular weekly sample from their site, and send both portions of the sample in for laboratory analysis. This approach yields a two part sample from each site where the one portion has been exposed to all sample handling, shipping, and laboratory procedures while the other portion remains in its original bottle and is therefore subject only to error associated primarily with laboratory analysis. By analyzing the differences in chemistry between many of these sample pairs, USGS investigators are able to isolate which chemical properties are being affected by sample shipping and handling steps and whether or not the effect is significant enough to warrant further action. Over the past 12 years, findings from this quality assurance study have resulted in a reduction in sample contamination due to sample handling, shipping and containers. The figure below illustrates the reduction in estimated sulfate contamination in NADP/NTN samples from field processing, which includes all shipping and handling steps.
All samples collected from the network are sent to a central laboratory for chemical analysis. The USGS operates an intensive laboratory comparison program to verify that laboratory results are accurate and are in agreement with other laboratories in North America that routinely analyze rain samples. Twenty-six times a year investigators send specially prepared rainwater samples to the central laboratory and the other participating laboratories for analysis. For a given mailing, the chemical makeup of the rain samples sent to each laboratory is identical; when the results are tallied, any significant differences between the laboratories can be identified. Some of the rainwater samples sent to the laboratories by the USGS have been certified by the National Institute of Standards and Technology(NIST); the analytical results for these samples, as determined by the laboratories, are compared to NIST values. Another estimate of laboratory performance is given by laboratory precision. Precision is defined by how closely a participating laboratory is able to reproduce repeated measurements on rain samples. All mailings to each laboratory include multiple bottles of each sample type so the precision of each laboratory can be determined. The precision estimates of the central laboratory are then compared to precision goals in the quality assurance plan and to the precision estimates derived for the other laboratories in the comparison program. In addition to annual publications of the interlaboratory comparison results, quarterly reports that detail each laboratory's performance in the comparison program are sent to each laboratory's manager so that problems can be quickly identified and corrected. Recent reports by USGS investigators demonstrate that the performance of the analytical laboratory used to analyze NADP/NTN samples exceeds network quality assurance standards.
The three quality assurance programs described previously isolate the influence of particular steps in the network's operation on the quality of the data. The fourth program estimates overall network precision from the point of sample collection through electronic data storage. The USGS accomplishes this by locating a second set of collection equipment next to four existing collectors each year. Since 1989 the USGS has obtained 24 site years of collocated sampling data and has recently been able to report on the overall quality of network data. In this program precision is estimated as the typical random error associated with network measurements. For sulfate and nitrate, two of the most important chemical measurements in precipitation chemistry studies, random error in network measurements is less than 5 percent. The random error for pH is found to vary widely depending upon the site location. In the western United States, random error for pH measurements is in the range of 25 to 30 percent, whereas in the eastern US, where pH of rainfall is generally lower, the error is less than 5 percent. Estimates of overall network data precision provide scientists investigating precipitation chemistry with confidence in their findings. These estimates also provide those working to improve network data quality with a baseline against which to measure the performance of improved collectors and sample collection procedures.
The USGS precipitation chemistry quality assurance program has been in operation for more than 15 years and has effectively documented the quality of data from the precipitation chemistry network. Recent trend analyses performed on quality assurance data by USGS investigators revealed that the network data quality has not remained constant over the entire period of record the network has been operating. Rather, error in the network's measurements have been reduced as improvements have been made in laboratory equipment, sample handling, field equipment and operator training. While continuous improvement in data quality is desirable, these changes can create problems for investigators attempting to use the network's data to evaluate the effect of reductions in power plant emissions, urban area expansion and other changes that may influence this nation's precipitation chemistry. An ongoing quality assurance program allows investigators to adjust for changes in the network's data and correctly attribute changes found in the data records to changes in external influences.
For further information, contact:
Greg Wetherbee, Project Chief, Precipitation Chemistry Quality Assurance
U.S. Geological Survey
Box 25046, MS-401
Denver, CO 80225
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