USGS Compares the CLAM to Discrete Water Samples and POCIS Passive Sampler

A study funded and conducted by the USGS compared; Standard Discrete Methods, POCIS Passive Samplers, and CLAM Continuous Samplers. We are pleased to share with you the peer reviewed study recently published in the journal of Science of the Total Environment.

Study Location and Design

The three sampling methods were tested at two sites on the Santa Cruz River in southeastern Arizona where base flow is almost completely supported by treated effluent discharge from the Nogales International Wastewater Treatment Plant. Sample types collected at each field site included: 1) CLAM samplers deployed for a period of about 24 hours during each of the 4 sample-collection weeks (sample volumes ranging from 24 to 68 liters); 2) POCIS samplers deployed for a period of four weeks; and 3) one, 1-liter, discrete water samples collected at each site during each of the four weeks at the time of CLAM deployment.

Water samples collected using continuous low-level aquatic monitoring (C.L.A.M.) samplers were compared to those collected using polar organic chemical integrative samplers (POCISs; Alvarez, 2010) and using standard discrete methods (Lewis and Zaugg, 2003). For all three sampling methods, lab analysis for organic waste-indicator compounds was performed by the U.S. Geological Survey’s National Water Quality Laboratory (NWQL).

Discussion and Summary of Results

  • CLAM technology provided enhanced sensitivity to micro pollutants often orders of magnitude lower than the discrete grab water samples, and equal or greater sensitivity than provided by the POCIS passive sampler for most target analytes.
  • The results could be easily quantitated into wt/unit volume because the total volume of water extracted was a known.
  • There were 13 compounds detected by all three methods, 4 by the CLAM and Grab samples only, 6 detected just by the CLAM and POCIS, and 9 compounds detected by just the CLAM and 3 detected by the POCIS only. The discrete grab samples did not detect any compounds not detected by either the CLAM or POCIS methods.
  • Laboratory control spikes and duplicate were added to the disks and extracted to demonstrate elution recovery efficacy on 73 compounds. There were 4 compounds which recovered below 25%. 5 between 25%-50% and 64 compounds which recovered between 50%-152%.
  • Blank data and field data showed evidence of blank contamination with high levels of low molecular weight PAH’s and other volatiles. This is usually caused by excessive drying of the disk by drawing air through the media bed until the residual water is removed. Avoiding this step using methanolic dewatering instead has since been established, and greatly improves blank cleanliness.
  • This initial USGS study evaluated the CLAM for analyte recovery efficacy, and precision, as well as method blank contamination. The performance of the CLAM SPE field in situ extractions can be compared to in house laboratory QA/QC acceptance criteria for the first time.
  • Three stage disks were employed, the first a pre-filter, the other two HLB media disks to establish dissolved values and breakthrough values.
  • The breakthrough observed was not due to media saturation, but to Kow values of the compounds reflecting polarity effects where the media could not retain the target analyte. Different media selection would be suggested for these highly polar compounds.


In our view this study validates that the C.L.A.M technology can provide a large volume time integrative extraction event. This event captured on standard laboratory SPE media can be extracted under routine procedures, and analysis methodologies to produce both qualitative and quantitative defensible data.


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