Open-File Report 98-559
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Katz, B.G., and Collins, J.J., 1998, Evaluation of chemical data from
selected sites in the Surface-Water Ambient Monitoring Program (SWAMP) in
Florida: U.S. Geological Survey Open-File Report 98-559, 51 p.
A cooperative study between the Florida Department of Environmental
Protection (FDEP) and the U.S. Geological Survey was conducted to assess
the integrity of selected water-quality data collected at 150 sites in
the FDEP Surface-Water Ambient Monitoring Program (SWAMP) in Florida.
The assessment included determining the consistency of the water-quality
data collected statewide, including commonality of monitoring procedures
and analytes, screening of the gross validity of a chemical analysis, and
quality assurance and quality control (QA/QC) procedures. Four tests were
used to screen data at selected SWAMP sites to estimate the gross validity
of selected chemical data: (1) the ratio of dissolved solids (in milligrams
per liter) to specific conductance (in microsiemens per centimeter); (2) the
ratio of total cations (in milliequivalents per liter) multiplied by 100 to
specific conductance (in microsiemens per centimeter); (3) the ratio of total
anions (in milliequivalents per liter) multiplied by 100 to specific
conductance (in microsiemens per centimeter); and (4) the ionic charge-balance
error. Although the results of the four screening tests indicate that the
chemical data generally are quite reliable, the extremely small number of
samples (less than 5 percent of the total number of samples) with sufficient
chemical information to run the tests may not provide a representative
indication of the analytical accuracy of all laboratories in the program.
In addition to the four screening tests, unusually low or high values were
flagged for field and laboratory pH (less than 4.0 and greater than 9.0) and
specific conductance (less than 10 and greater than 10,000 microsiemens per
centimeter). The numbers of flagged data were less than 1 percent of the
19,937 water samples with pH values and less than 0.6 percent of the 16,553
water samples with specific conductance values.
Thirty-four agencies responded to a detailed questionnaire that was sent to more
than 60 agencies involved in the collection and analysis of surface-water-quality
data for SWAMP. The purpose of the survey was to evaluate quality assurance
methods and consistency of methods statewide. Information was compiled and
summarized on monitoring network design, data review and upload procedures,
laboratory and field sampling methods, and data practices. Currently, most
agencies that responded to the survey follow FDEP-approved QA/QC protocol
for sampling and have quality assurance practices for recording and reporting
data. Also, most agencies responded that calibration procedures were followed
in the laboratory for analysis of data, but no responses were given about the
specific procedures. Approximately 50 percent of the respondents indicated
that laboratory analysis methods have changed over time. With so many
laboratories involved in analyzing samples for SWAMP, it is difficult to
compare water quality from one site to another due to different reporting
conventions for chemical constituents and different analytical methods over
time. Most agencies responded that calibration methods are followed in the
field, but no specific details were provided. Grab samples are the most
common method of collection.
Other data screening procedures are necessary to further evaluate the
validity of chemical data collected at SWAMP sites. High variability in
the concentration of targeted constituents may signal analytical problems,
but more likely changes in concentration are related to hydrologic conditions.
This underscores the need for accurate measurements of discharge, lake stage,
tidal stage at the time of sampling so that changes in constituent
concentrations can be properly evaluated and fluxes (loads) of nutrients or
metals, for example, can be calculated and compared over time.
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