Study Results

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Study Summary Ring Diagrams Tabular Summary Measurement Record

Study 71 Summary Latest

This study was completed in 2025.

These tables summarize the measurements reported to the QA-SAC for Inter-laboratory Comparison Study 71. A total of 62 laboratories reported measurements. Some measurements were missing or below the analytical detection limit (DL). N designates the number of reported values above the DL. At the center of each set of N measurements is the Median, which is the accepted target value of the set. Non-parametric Sigmas were estimated from the interquartile ranges and were used to calculate Z-scores. Go to Definitions and DQOs for a description of Good, Satisfactory, and Marginal, and for a list of the Data Quality Objectives.

Study last updated on Aug 12, 2025.

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Measurement Units N Median Sigmaest.

Good

25th to 75th percentiles

Satisfactory

Median ± Sigma

Marginal

Median ± 2 Sigma

Meets Data Quality Objectives
pH pH units 55 4.62 0.05(2) 4.59 4.66 4.57 4.67 4.52 4.72 4.55 4.69
Conductivity μS/cm 49 21.9 1.0(0) 21.1 22.4 20.9 22.9 19.9 23.9 20.4 23.4
Acidity μeq/L 4 26.7 5.3(0) 21.7 28.7 21.4 32.0 16.1 37.3 20.1 33.3
Sulfate mg/L 53 2.460 0.054(1) 2.427 2.500 2.406 2.514 2.352 2.568 2.337 2.583
Nitrate mg/L 52 1.809 0.039(7) 1.790 1.843 1.769 1.848 1.730 1.887 1.719 1.898
Ammonium mg/L 55 0.598 0.023(7) 0.583 0.615 0.575 0.621 0.551 0.645 0.557 0.639
Fluoride mg/L 17 0.037 0.003(0) 0.035 0.038 0.035 0.039 0.032 0.042 0.030 0.044
Chloride mg/L 53 0.761 0.018(5) 0.750 0.775 0.743 0.779 0.724 0.798 0.685 0.837
Sodium mg/L 53 0.407 0.017(0) 0.392 0.415 0.390 0.424 0.373 0.441 0.367 0.447
Potassium mg/L 53 0.122 0.010(4) 0.116 0.129 0.112 0.132 0.102 0.142 0.098 0.146
Calcium mg/L 54 0.330 0.015(2) 0.318 0.338 0.315 0.345 0.300 0.360 0.281 0.379
Magnesium mg/L 54 0.095 0.008(7) 0.089 0.100 0.087 0.103 0.078 0.112 0.086 0.104
Measurement Units N Median Sigmaest.

Good

25th to 75th percentiles

Satisfactory

Median ± Sigma

Marginal

Median ± 2 Sigma

Meets Data Quality Objectives
pH pH units 55 5.14 0.06(7) 5.10 5.19 5.08 5.20 5.01 5.27 5.04 5.24
Conductivity μS/cm 49 8.8 0.3(3) 8.7 9.1 8.5 9.1 8.2 9.4 8.2 9.4
Acidity μeq/L 4 7.3 0.5(9) 7.0 7.8 6.7 7.8 6.1 8.4 5.5 9.0
Sulfate mg/L 53 0.908 0.027(4) 0.886 0.923 0.881 0.935 0.854 0.962 0.863 0.953
Nitrate mg/L 52 1.019 0.024(1) 1.001 1.033 0.995 1.043 0.971 1.067 0.969 1.069
Ammonium mg/L 55 0.380 0.021(5) 0.368 0.397 0.359 0.401 0.338 0.422 0.354 0.406
Fluoride mg/L 17 0.055 0.004(8) 0.051 0.057 0.051 0.059 0.046 0.064 0.044 0.066
Chloride mg/L 52 0.187 0.009(5) 0.179 0.191 0.178 0.195 0.168 0.205 0.168 0.205
Sodium mg/L 52 0.099 0.009(8) 0.091 0.104 0.090 0.108 0.080 0.118 0.090 0.108
Potassium mg/L 47 0.034 0.005(9) 0.030 0.038 0.029 0.039 0.023 0.045 0.028 0.040
Calcium mg/L 53 0.207 0.017(4) 0.194 0.217 0.190 0.224 0.173 0.241 0.176 0.238
Magnesium mg/L 50 0.040 0.003(2) 0.038 0.042 0.037 0.043 0.034 0.046 0.036 0.044
Measurement Units N Median Sigmaest.

Good

25th to 75th percentiles

Satisfactory

Median ± Sigma

Marginal

Median ± 2 Sigma

Meets Data Quality Objectives
pH pH units 55 5.01 0.05(2) 4.98 5.05 4.96 5.06 4.91 5.11 4.91 5.11
Conductivity μS/cm 49 9.4 0.4(1) 9.2 9.7 9.0 9.8 8.6 10.2 8.8 10.0
Acidity μeq/L 4 9.8 1.1(9) 9.1 10.7 8.6 10.9 7.4 12.1 7.4 12.1
Sulfate mg/L 53 1.097 0.034(1) 1.071 1.117 1.063 1.131 1.029 1.165 1.043 1.151
Nitrate mg/L 52 0.695 0.028(2) 0.675 0.712 0.667 0.723 0.639 0.751 0.661 0.729
Ammonium mg/L 55 0.282 0.016(3) 0.272 0.294 0.266 0.298 0.250 0.314 0.263 0.301
Fluoride mg/L 16 0.023 0.002(0) 0.022 0.024 0.021 0.025 0.019 0.027 0.019 0.027
Chloride mg/L 52 0.287 0.011(1) 0.279 0.294 0.276 0.298 0.265 0.309 0.259 0.315
Sodium mg/L 53 0.140 0.011(5) 0.131 0.146 0.129 0.151 0.118 0.162 0.126 0.154
Potassium mg/L 53 0.078 0.009(3) 0.072 0.084 0.069 0.087 0.060 0.096 0.063 0.093
Calcium mg/L 53 0.144 0.016(3) 0.131 0.152 0.128 0.160 0.112 0.176 0.123 0.165
Magnesium mg/L 53 0.052 0.006(3) 0.047 0.055 0.046 0.058 0.040 0.064 0.047 0.057