Guidance for Verifying and Interpreting Field Blank Determinations that Indicate High Pesticide or Trace Element Concentrations or that Show Large Numbers of Detections In Reply Refer to: Mail Stop 412 November 14, 2000 Office of Water Quality Technical Memorandum 2001.02 Subject: Guidance for Verifying and Interpreting Field Blank Determinations that Indicate High Pesticide or Trace Element Concentrations or that Show Large Numbers of Detections BACKGROUND Good quality assurance (QA) for water-quality data typically involves collection of quality control (QC) samples (field and laboratory blank and spike samples) and the subsequent data analysis of these samples. Multiple sources of laboratory QC are available for both blank and spiked QC samples, and several sources of QC data are available for field spikes. However, only project-specific sources of data are available for field blanks because of unique features of each project, although national aggregations of project blank data have produced useful results. Occasionally, through errors in QC sample handling, field blanks or spikes may be misidentified, or environmental samples may be misidentified as QC samples. It is relatively straightforward to recognize misidentified spikes because the array of analytes in field samples rarely looks like a typical spiked sample. However, it is more difficult to identify an environmental sample mislabeled as a blank, and the consequences of such a misidentification can be substantial for subsequent data interpretation if the analytes of interest are present in the environmental sample. PURPOSE OF MEMORANDUM Guidance is provided to project hydrologists and others on when to question blank results for pesticide and trace element determinations for field samples. The National Water-Quality Assessment (NAWQA) Program, in conjunction with the Office of Water Quality (OWQ) and the National Water Quality Laboratory (NWQL), developed the pesticide blank criteria. If the criteria for questioning blanks are met, then a series of checks and reviews are recommended prior to accepting the questionable blank results as truly representative of blank conditions. GUIDANCE AND INTERPRETATION PESTICIDES For NWQL pesticide schedules 2001, 2010, 2050, and 2051, the NAWQA program has suggested that any field blank that contains an unusually large number of pesticide detections, or pesticide detections at an unusually high concentration, be carefully examined for possible misidentification errors. For blanks associated with ground water, 3 or more pesticide detections or concentrations of 0.015 micrograms per liter (ug/L) or greater are reasonable limits to investigate the sample for misidentification or other errors. For blanks associated with surface water, 4 or more pesticide detections or concentrations of 0.060 ug/L or greater are reasonable limits to investigate the sample for misidentification or other errors. These limits were derived from a statistical analysis of 363 ground-water field blanks and 458 surface-water field blanks. In all cases, the suggested limits correspond to the 95th percentile of ground- or surface-water blank numbers of detected pesticides or pesticide maximum concentrations. Thus, only 5 percent of the time should a true blank exceed these upper limits of numbers of detections or concentrations. For comparison, set blank results are available to USGS employees from the NWQL at: http://wwwnwql.cr.usgs.gov/USGS/Catalog/SearchQCSets.html. Enter the desired pesticide schedule number, select "Procedure/Schedule", search, and then select your appropriate NWQL quality control set number. The set blank and set spike information will be displayed, along with the reporting limits, for each analyte. The NWQL is developing two new low-level pesticide methods, which are currently known as Lab Codes 9002/9011 and 9060. When approved, these will be Schedules 2002/2011 and 2060. Although the same logic can be applied to blank determinations using these Lab Codes or Schedules, different numbers of detections or maximum concentrations may be appropriate because the analyte lists are different. At this time, there are insufficient field blanks to determine these limits. When published, the Open File Reports for these methods will provide laboratory blank information, which can provide a starting point for hydrologists to identify anomalous field blanks that may require verification. TRACE ELEMENTS For trace elements, conductivity is determined on all samples at the NWQL that undergo Inductively-Coupled Plasma/Mass Spectrometry (ICP/MS) and for Inductively-Coupled Plasma - Optical Emission Spectroscopy (ICP-OES). For these trace element determinations, if a blank shows an unexpected concentration or number of trace element detections, check the measured conductivity. Inorganic blank water should have a very low conductivity, while a misidentified blank should have a conductivity more representative of other samples collected at that site, typically much higher than blank water. For comparison, Inorganic Blank Water analysis results are available to USGS employees from the Quality-Water Service Unit in Ocala at: http://qwsu.er.usgs.gov/qwsu_ibw.html. If a simple switch has been made, then one of the environmental samples may have a low conductivity representative of blank water, and the blank data may be recovered from that sample, along with the sample data from the switched blank. SUGGESTED ACTIONS FOR EXCEEDANCES If any field blank samples exceed the above criteria for numbers of pesticide detections, maximum pesticide concentration, or higher-than-expected conductance values for trace element blanks, then all aspects of this sample handling need to be reviewed with care, such as: Ensure that the sample submitted for analysis was a field blank by verifying that all sample data submitted in the cooler containing the questioned blank are consistent with each assigned sample identification. This checks for most possible misidentification of samples and blanks in the field or at NWQL login. Contact the NWQL and request that the sample results be verified, along with the reason for the verification, such as a high number of pesticide detections or high concentration for a blank, or a high conductivity (or other anomalous analytical result). WHAT NEXT? If a clear picture emerges of a sample handling error that can be repaired, then the corrections should be made in the data base, along with appropriate written supporting documentation. If there is no clear picture, then the hydrologist should not make data base corrections, but should carefully consider the impacts of questionable blanks on data interpretation, such as in the following examples: Including a high blank will have the effect of calling into question all environmental analytical results near the concentration of the high blank. If these environmental concentrations are the basis of an important, and potentially controversial finding, then keeping the high blank concentration may be vital. Collection of additional field blanks and possibly field spikes near the concentrations of concern may be appropriate. If, on the other hand, the environmental results are more than an order of magnitude higher in concentration than the high blank result, then keeping the high blank result may be less important, and it could be rejected as an outlier without substantial impact to the data interpretation. These two examples lie at the extremes of accepting or rejecting the high blank as an anomaly in data interpretation. Most actual situations will be less clear, and will require the hydrologist to carefully examine actual data-quality objectives and evaluate the potential impacts of including an anomalously high blank, and the impacts of rejecting a high blank that may be real. CONTACTS AND FEEDBACK Pete Rogerson, Senior Chemist, OWQ (rogerson@usgs.gov) will be glad to discuss specific applications with project personnel and provide additional guidance, if possible ((303) 236-1836). Pete would also appreciate feedback on unusual blank occurrences and how you resolved them. Jeff Martin (jdmartin@usgs.gov) would appreciate feedback on how the guidance works for identifying questionable pesticide field blanks. Merle Shockey, NWQL, (mshockey@usgs.gov) would appreciate any information relating to sample misidentifications or other problems occurring at the NWQL. Janice R. Ward /s/ Acting Chief, Office of Water Quality This memorandum does not supersede any other OWQ Technical Memorandum. Distribution: All WRD Employees ********************************************* Janice Ward Acting Chief, Office of Water Quality US Geological Survey Reston, VA 703-648-6871 jward@usgs.gov