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Alistair May 15, 2020 11:45 AM
Hi, I need some advice on your WISE Water Framework Directive data. We are working on EU wide project checking the presence of CAS_319-84-6 In ground and surface waters. In the lists we have generated CAS_319-84-6 is present surface and groundwater in some parts of Europe. However the analytical data seems a bit odd as in we are mostly looking at low rounded figures (i.e. 1 µg/l or 5 µg/l) rather than very variable data what would be normal (i.e. 0,0024 or 1,6801 µg/l). Would it be possible that we are looking at the detection limits of the laboratories (i.e. < 1 µg/l or < 5 µg/l) and the < has been left out for the data to be processable? If that is the case, is there any way we can reset these detection limit values to 0?
Replies (2)
EEA May 18, 2020 04:13 PM
Dear Alistair,

Thank you for contacting the European Environment Agency (EEA). Your enquiry has been forwarded to our relevant experts. We will get back to you as soon as possible. Thank you for your patience.
Kind regards,

EEA Enquiry Service

EEA May 20, 2020 09:42 AM
Dear Alistair,

We assume the data you are interested in come from the WISE water quality database (rather than WFD, which reports status information, not concentrations).
http://dd.eionet.europa.eu/datasets/3163

Via the exports tab, the technical specification for the dataset sets out that:
Countries are required to report the LoQ
“5. Values below the limit of quantification in disaggregated data
If, for a specific sample value, the measured concentration is below the limit of quantification, then report the limit of quantification value in the "Observed Value" field and report 'True' (or '1') in the boolean "Observed value below LOQ" field.
6. Values below the limit of quantification in annually aggregated data
When preparing annually aggregated data, sample concentrations below the limits of quantification must be treated in accordance to the QA/QC Directive (Commission directive 2009/90/EC of 31 July 2009 laying down, pursuant to Directive 2000/60/EC of the European Parliament and of the Council, technical specifications for chemical analysis and monitoring of water status): - If a sample concentration value is below the limit of quantification, then, for the mean value calculation only, it must be replaced with a value equivalent to HALF THE LIMIT OF QUANTIFICATION value;
 If the calculated annual mean value of the measurements results is below the limit of quantification value, then report the limit of quantification value in the "resultMeanValue" field and set 'True' (or '1') in the boolean "resultQualityMeanBelowLOQ" field. Use the same rule when reporting minimum, maximum and median values. If the samples in the aggregation had different limits of quantification, then the highest of the different LOQ values must be used in these cases.”
When the observation does not exceed LoQ it is set to the value of the LoQ and therefore the rounded figure. The LoQ field would not have had the < character, so that is not the issue.
Analytically, it is not correct to convert a <LoQ (or LoD) value to zero. The LoQ value is the lowest concentration that can be measured with confidence. Chemical analysis may have detected the substance being present, but was insufficiently sensitive to quantify how much was there.
Commonly, people use “half LoQ” as an estimation of the concentration, though that can create other issues.
 
Handling LoQ for hazardous substances for a European dataset is not straightforward. We would be interested to hear how the researchers tackle this issue.

Kind regards,
EEA Enquiry Service
 
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