Study Results

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Study Summary Ring Diagrams Tabular Summary Measurement Record

Study 70 Summary Latest

This study was completed in 2024.

These tables summarize the measurements reported to the QA-SAC for Inter-laboratory Comparison Study 70. A total of 55 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 May 23, 2025.

Download study summaries for Sample 1, Sample 2, or Sample 3 as a CSV.

Measurement Units N Median Sigmaest.

Good

25th to 75th percentiles

Satisfactory

Median ± Sigma

Marginal

Median ± 2 Sigma

Meets Data Quality Objectives
pH pH units 50 4.73 0.08(2) 4.69 4.80 4.65 4.81 4.57 4.89 4.66 4.80
Conductivity μS/cm 44 21.6 0.7(8) 21.2 22.1 20.9 22.3 20.1 23.1 20.1 23.1
Acidity μeq/L 4 18.4 9.4(7) 14.8 27.5 8.9 27.8 0.0 37.2 13.8 22.9
Sulfate mg/L 46 1.919 0.028(9) 1.904 1.942 1.891 1.947 1.862 1.976 1.824 2.014
Nitrate mg/L 47 0.968 0.036(3) 0.942 0.991 0.932 1.004 0.896 1.040 0.920 1.016
Ammonium mg/L 48 0.381 0.019(5) 0.371 0.397 0.362 0.399 0.342 0.419 0.354 0.407
Fluoride mg/L 14 0.050 0.006(1) 0.045 0.053 0.044 0.056 0.038 0.062 0.040 0.060
Chloride mg/L 47 2.018 0.049(7) 1.984 2.051 1.969 2.067 1.919 2.117 1.817 2.219
Sodium mg/L 47 1.111 0.037(1) 1.085 1.135 1.074 1.148 1.037 1.185 1.000 1.222
Potassium mg/L 46 0.214 0.009(3) 0.205 0.217 0.205 0.223 0.196 0.232 0.172 0.256
Calcium mg/L 46 0.265 0.013(0) 0.253 0.270 0.252 0.277 0.239 0.290 0.225 0.304
Magnesium mg/L 46 0.116 0.007(4) 0.110 0.120 0.109 0.123 0.102 0.130 0.105 0.127
Measurement Units N Median Sigmaest.

Good

25th to 75th percentiles

Satisfactory

Median ± Sigma

Marginal

Median ± 2 Sigma

Meets Data Quality Objectives
pH pH units 50 5.18 0.10(2) 5.12 5.25 5.08 5.28 4.98 5.38 5.08 5.28
Conductivity μS/cm 44 22.4 0.7(2) 22.0 22.9 21.7 23.1 21.0 23.8 20.9 23.9
Acidity μeq/L 4 6.8 5.8(6) 5.3 13.1 1.0 12.6 0.0 18.5 5.1 8.5
Sulfate mg/L 47 2.680 0.042(3) 2.653 2.710 2.638 2.722 2.596 2.764 2.546 2.814
Nitrate mg/L 47 1.680 0.059(3) 1.638 1.718 1.621 1.739 1.562 1.798 1.596 1.764
Ammonium mg/L 48 0.695 0.031(1) 0.672 0.713 0.664 0.725 0.633 0.756 0.646 0.743
Fluoride mg/L 14 0.029 0.004(1) 0.026 0.031 0.025 0.033 0.021 0.037 0.024 0.034
Chloride mg/L 47 2.240 0.054(9) 2.205 2.279 2.186 2.294 2.131 2.349 2.016 2.464
Sodium mg/L 47 1.401 0.053(4) 1.355 1.427 1.348 1.454 1.295 1.507 1.261 1.541
Potassium mg/L 46 0.282 0.015(0) 0.268 0.288 0.267 0.296 0.252 0.311 0.226 0.337
Calcium mg/L 46 0.436 0.024(1) 0.419 0.451 0.412 0.459 0.388 0.483 0.371 0.500
Magnesium mg/L 46 0.198 0.010(2) 0.189 0.202 0.188 0.208 0.178 0.218 0.179 0.217
Measurement Units N Median Sigmaest.

Good

25th to 75th percentiles

Satisfactory

Median ± Sigma

Marginal

Median ± 2 Sigma

Meets Data Quality Objectives
pH pH units 50 5.17 0.10(7) 5.10 5.24 5.06 5.27 4.96 5.37 5.07 5.26
Conductivity μS/cm 44 11.3 0.3(5) 11.2 11.6 11.0 11.6 10.6 12.0 10.6 12.0
Acidity μeq/L 4 6.3 6.1(0) 4.9 13.0 0.3 12.3 0.0 18.4 4.8 7.8
Sulfate mg/L 47 1.327 0.025(9) 1.310 1.345 1.302 1.352 1.276 1.378 1.261 1.393
Nitrate mg/L 47 1.480 0.033(4) 1.457 1.502 1.447 1.513 1.414 1.546 1.406 1.554
Ammonium mg/L 48 0.484 0.022(8) 0.467 0.497 0.461 0.506 0.438 0.529 0.450 0.517
Fluoride mg/L 14 0.055 0.004(6) 0.052 0.058 0.051 0.059 0.046 0.064 0.044 0.066
Chloride mg/L 47 0.353 0.014(8) 0.343 0.363 0.339 0.367 0.324 0.382 0.318 0.388
Sodium mg/L 46 0.215 0.009(1) 0.208 0.220 0.206 0.224 0.197 0.233 0.194 0.236
Potassium mg/L 45 0.061 0.007(0) 0.057 0.066 0.054 0.068 0.047 0.075 0.049 0.073
Calcium mg/L 46 0.308 0.021(1) 0.293 0.321 0.287 0.328 0.266 0.349 0.262 0.353
Magnesium mg/L 46 0.071 0.005(6) 0.066 0.073 0.066 0.076 0.060 0.082 0.064 0.078