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Measurement uncertainty in drinking water analysis

Problems with measurement uncertainty

According to the German drinking water regulation alle drinking water laboratories have to be accredited according to ISO/IEC 17025. This also implies that they have to know their measurement uncertainty and have to report under certain circumstances.

The laboratories are therefore urged to carry out uncertainty estimations. In order to propose a feasible procedure the NORDTEST approach on the estimation of measurement uncertainties was transformed into a German guideline (Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung A0-4 "Leitfaden zur Abschätzung der Messunsicherheit aus Validierungsdaten"). For more details about the uncertainty estimation see here. You will find more documents on this topic here.

From the customer's view the question comes up, how to handle this uncertainty statement and how to use it in conformity assessment. You may find our point of view here.

One could think that the measurement uncertainty reflects the quality of a laboratory. This might lead to the situation that customers are tempted to place contracts for laboratories on the basis of reported measurement uncertainties. This again would step by step lead to lower, unrealistic measurement uncertainties to be reported. Finally then there is no benefit from reported uncertainties. Dies would also contradict the fundamental idea of quality. The quality of a measurement result is sufficient, if the customer's requirements are fulfilled. We don't need more.

But how can customers, in the case of drinking water analyses the public health authorities, define the requirements? One could think that for drinking water analyses this is covered by the EU drinking water directive which specifies requirements for the analysis. Unfortunately the requirements laid down on precision, trueness and limit of detection cannot readily be converted in measurement uncertainties.

It would be benefitial to all involved if the requirements of the EU directive could uniformly be interpreted as custonmer's requirement on measurement uncertainty.

To support this, variance functions were calculated from the data of drinking water proficiency tests in Germany of the recent years. The support from the following PT providers is gratefully acknowledged:

  • Landesinstitut für den öffentlichen Gesundheitsdienst Nordrhein-Westfalen
  • Behörde für Wissenschaft und Gesundheit der Freien und Hansestadt Hamburg
  • AQS Baden-Württemberg at the University of Stuttgart

For the calculation of the variance function the relative reproducibility standard deviations (variation coefficients VCR) are depicted versus the concentration of the analyte. As described in ISO/TS 20612 a regression function of the form

Formel Varianzfunktion

is calculated. This enables to state an average reproducibility standard deviation at the limit as well as the presentation of the concentartrion dependence of this standard deviation. In the following table the data of all parameters analysed in the PTs are collected. To enable a comparison with the Horwitz function the concentration were converted into kg/l prior to the calculation of the coefficients Θ0 und Θ1. For elektrical conductivity, colour and turbidity the stated units were used. By clicking on the parameter name you will get a graphical display. (We apologize for not translating the parameter names into English language.)

Parameter Concentration range θ0 θ1 Limit VCR, GW NRV Ndata sets
Aluminium 0.0345 - 9.8575 mg/l -6.8586 0.6966 0.2 11.31%  28  130 
Antimon 2.3 - 423.8 µg/l -5.15811 0.81681 5 19.08%  9  85 
Arsen 2.5957 - 278.7 µg/l -5.6563 0.80318 10 13.12%  7  66 
Blei 4.8 - 90.5 µg/l -6.5659 0.7505 10 13.94%  7  58 
Cadmium 1.0506 - 51.7 µg/l -5.3503 0.8428 5 9.58%  7  59 
Chrom 6 - 102.613 µg/l -5.1562 0.8452 50 7.78%  7  58 
Chrom(VI) 8.5758 - 88 µg/l -7.5634 0.7157   5  36 
Kupfer 21.0938 - 4452.1 µg/l -4.6818 0.8757 2000 4.73%  12  70 
Nickel 5.9 - 89.8 µg/l -6.84 0.7487 20 9.21%  7  58 
Quecksilber 0.5299 - 11.58 µg/l -6.09724 0.7908 1 17.17%  5  44 
Selen 3 - 449.8 µg/l -5.44477 0.79922 10 17.44%  7  66 
Ammonium 0.0841 - 9.8575 mg/l -5.3664 0.8113 0.5 7.22%  22  93 
Bor 0.0794 - 1.9415 mg/l -7.4855 0.6572 1 6.4%  8  78 
Calcium 20.821 - 201.8 mg/l -3.84707 0.94011   6  57 
Eisen 0.0311 - 2.788 mg/l -6.1342 0.7676 0.2 7.81%  27  141 
Kalium 1.549 - 37.536 mg/l -5.30488 0.7905   9  86 
Magnesium 6.205 - 74.768 mg/l -4.09984 0.90838   5  50 
Mangan 0.0161 - 0.664 mg/l -5.7787 0.8026 0.05 8.54%  15  100 
Natrium 8.836 - 185.682 mg/l -4.1309 0.9017 200 3.71%  10  94 
Bromat 0.0074 - 0.07 mg/l -5.2276 0.7718 0.01 35.93%  7  61 
Chlorid 12.2483 - 289.636 mg/l -4.851 0.8523 250 2.66%  10  97 
Cyanid 0.009 - 0.1339 mg/l -5.0481 0.7958 0.05 19.9%  7  61 
Fluorid 0.3948 - 4.801 mg/l -6.26 0.7298 1.5 7.16%  10  97 
Nitrat 4.09 - 189.898 mg/l -5.3693 0.7914 50 3.68%  16  115 
Nitrit 0.049 - 1.4447 mg/l -7.4718 0.6969 0.5 4.62%  13  78 
Phosphat 0.381 - 11.2502 mg/l -3.29661 0.9038   4  28 
Phosphor 1.455 - 5.354 mg/l -2.2883 1.0091   3  33 
Silikat 3.4657 - 50.9648 mg/l -3.7257 0.9028 5 7.89%  5  40 
Sulfat 22.861 - 296.024 mg/l -4.6726 0.8549 240 3.13%  8  76 
El. Leitfähigkeit 274.89 - 5387 µS/cm -4.64017 1.04643 2500 1.39%  12  66 
Färbung 0.1469 - 1.849 1/m -2.4878 0.4031 0.5 12.57%  12  61 
Oxidierbarkeit 1.0262 - 8.6316 mg/l -10.795 0.31288 5 9%  11  63 
TOC 0.8678 - 10.4434 mg/l -8.6437 0.48134   10  50 
Trübung 0.3048 - 8.609 NTU -1.9464 0.5433 1 14.28%  13  72 
Benzo(a)pyren 0.005 - 0.2303 µg/l -4.9113 0.8537 0.01 29.96%  9  76 
Benzo(b)fluoranthen 0.008 - 0.2332 µg/l -3.5836 0.9175 0.025 20.82%  9  76 
Benzo(ghi)perylen 0.016 - 0.24 µg/l -1.7197 0.98153 0.025 28.12%  9  76 
Benzo(k)fluoranthen 0.011 - 0.2056 µg/l -4.1913 0.8892 0.025 22.62%  9  76 
Indeno(1,2,3-cd)pyren 0.013 - 0.2576 µg/l -0.9698 1.0145 0.025 26.85%  8  69 
Bromdichlormethan 0.647 - 22.6431 µg/l -3.0463 0.9376 12.5 14.79%  8  70 
Bromoform 0.591 - 22.7734 µg/l -4.1853 0.873 12.5 15.35%  8  71 
Chloroform 0.623 - 22.3871 µg/l -4.8607 0.8373 12.5 14.96%  8  71 
Dibromchlormethan 0.73 - 24.1129 µg/l -2.9363 0.941 12.5 15.51%  8  71 
1,2-Dichlorethan 1.59 - 15.188 µg/l -4.0175 0.873 3 21.76%  7  64 
Benzol 0.57 - 9.0613 µg/l -4.9755 0.8253 1 25.76%  6  51 
Tetrachlorethen 0.606 - 18.4875 µg/l -3.4225 0.9053 5 19.94%  8  71 
Trichlorethen 0.574 - 17.5987 µg/l -3.0901 0.9236 5 19.6%  8  71 
Epichlorhydrin 0.1264 - 0.9866 µg/l -0.13097 1.02212 0.1 52.71%  2  6 
2,4-D 0.0853 - 0.7597 µg/l -4.0961 0.8757 0.1 29.12%  4  31 
2,4-DB 0.0605 - 0.7553 µg/l -6.4653 0.7653 0.1 34.65%  4  31 
2,4,5-T 0.0705 - 0.7353 µg/l -4.5326 0.858 0.1 28.28%  4  31 
Aldrin 0.0225 - 0.6761 µg/l -3.28614 0.91498 0.1 26.49%  4  32 
alpha-HCH 0.034 - 0.087 µg/l -9.99156 0.63372 0.1 21.06%  1  9 
Atrazin 0.0599 - 0.833 µg/l -3.48323 0.92525 0.1 17.17%  7  53 
Bentazon 0.0658 - 0.8591 µg/l -3.7629 0.8932 0.1 27.15%  6  45 
Bromoxynil 0.0609 - 0.8164 µg/l -2.4649 0.9466 0.1 29.11%  6  45 
Chlortoluron 0.069 - 0.8126 µg/l -3.81891 0.90846 0.1 18.06%  5  38 
Desethylatrazin 0.0866 - 0.855 µg/l -2.1916 0.97172 0.1 21.43%  5  38 
Desisopropylatrazin 0.1553 - 0.7865 µg/l -2.69911 0.93552 0.1 29.69%  3  18 
delta-HCH 0.0256 - 0.4469 µg/l -4.00329 0.88012 0.1 28.85%  2  15 
Dichlorprop 0.0741 - 0.8747 µg/l -3.3584 0.9173 0.1 23.36%  6  45 
Dieldrin 0.0267 - 0.5646 µg/l -3.00324 0.9341 0.1 22.63%  5  38 
Dimethoat 0.077 - 0.212 µg/l -2.64345 0.9228 0.1 42.07%  1  9 
Diuron 0.0569 - 0.8087 µg/l -5.51077 0.82789 0.1 21.27%  7  53 
Endrin 0.0259 - 0.461 µg/l -4.36967 0.86542 0.1 28.06%  2  15 
Fenoprop 0.074 - 0.7851 µg/l -3.7987 0.8984 0.1 23.26%  4  31 
gamma-HCH 0.0273 - 0.6235 µg/l -5.21303 0.83158 0.1 26.31%  4  31 
HCB 0.0184 - 0.5225 µg/l -2.06738 0.96752 0.1 26.73%  5  38 
Heptachlor 0.0417 - 0.488 µg/l -1.98714 0.96202 0.1 32.87%  2  14 
Ioxynil 0.091 - 0.732 µg/l -3.97476 0.87137 0.1 36.32%  2  14 
Isoproturon 0.0598 - 0.7742 µg/l -4.95714 0.85153 0.1 21.47%  7  53 
Linuron 0.145 - 0.6875 µg/l -7.26296 0.74236 0.1 26.43%  2  15 
MCPA 0.0804 - 0.8899 µg/l -2.4505 0.9503 0.1 27.11%  6  45 
MCPB 0.0628 - 0.8078 µg/l -3.445 0.9095 0.1 25.62%  4  31 
Mecoprop 0.0585 - 0.819 µg/l -5.4155 0.8228 0.1 26.28%  6  45 
Metazachlor 0.1197 - 0.827 µg/l -5.98407 0.78484 0.1 35.7%  2  15 
Metobromuron 0.0539 - 0.8813 µg/l -3.72355 0.91059 0.1 18.92%  3  23 
Metolachlor 0.074 - 0.7559 µg/l -6.121 0.79292 0.1 25.85%  5  33 
Metribuzin 0.068 - 1.0234 µg/l -4.51048 0.86506 0.1 24.58%  3  23 
p,p'-DDD 0.0245 - 0.4058 µg/l -4.99597 0.84248 0.1 25.44%  3  24 
p,p'-DDE 0.0583 - 0.599 µg/l -4.36128 0.86861 0.1 26.29%  2  14 
p,p'-DDT 0.0233 - 0.4026 µg/l -2.7112 0.93463 0.1 29.94%  3  24 
Propazin 0.067 - 0.8203 µg/l -6.84051 0.77033 0.1 21.18%  3  24 
Simazin 0.07 - 0.875 µg/l -2.39728 0.966 0.1 19.9%  5  39 
Terbutylazin 0.0577 - 0.8117 µg/l -3.71702 0.90942 0.1 19.57%  7  53 
alpha-Endosulfan 0.0839 - 0.5961 µg/l -3.996 0.89174 0.1 22.24%  1  7 
beta-Endosulfan 0.0666 - 0.5536 µg/l -2.70254 0.95094 0.1 20.75%  1  7