A quality-assurance assessment for constituents reported by the National Atmospheric Deposition Program and the National Trends Network

A continuing quality-assurance program has been operated by the U.S. Geological Survey to evaluate any bias introduced by routine handling, shipping, and laboratory analyses of wet-deposition samples collected in the National Atmospheric Deposition Program (NADP) and National Trends Network (NTN). Blind-audit samples having a variety of constituent concentrations and values were selected. Only blind-audit samples with constituent concentrations and values less than the 95th-percentile concentration for natural wet-deposition samples were included in the analysis. Of the major ions, there was a significant increase of Ca²⁺, Mg²⁺, Na²⁺, K⁺, SO₄²⁻ and Cl⁻¹ in samples handled according to standard protocols and shipped in NADP/NTN sample-collection buckets. For 1979–1987, graphs of smoothed data showing the estimated contamination in blind-audit samples indicate a decrease in the median concentration and ranges of Ca²⁺, Mg²⁺ and SO₄²⁻ contamination of blind-audit samples shipped in sample-collection buckets. Part of the contamination detected in blind-audit samples can be attributed to contact with the sample-collection bucket and lid; however, additional sources also seem to contaminate the blind-audit sample. Apparent decreases in the magnitude and range of sample contamination may be caused by differences in sample-collection bucket- and lid-washing procedures by the NADP/NTN Central Analytical Laboratory. Although the degree of bias is minimal for most constituents, summaries of the NADP/NTN data base may contain overestimates of Ca²⁺, Mg²⁺, Na⁻, K⁺ and SO₄²⁻ and Cl⁻ concentrations, and underestimates of H⁺ concentrations.