The Home Stretch for a New Dissolved Gas Method
The dissolved methane analytical method studies highlighted in our previous newsletters have reached the final stage (Phase 7), which involves submission of the method to the United States Environmental Protection Agency (US EPA) SW-846 compendium of methods. The Marcellus Shale Coalition (MSC) commissioned Environmental Standards to perform the seven phases of the study, which began in 2014. In Phase 6 of the study, Environmental Standards initiated discussions with the US EPA in which the first five phases of the study were presented. The US EPA requested an additional inter-laboratory study (ILS) using several natural groundwaters with differing geochemistry. In addition, the US EPA requested that ethane, ethene and n-propane be added analytes to the proposed method.
Phase 7 involved preparation of a Work Plan for US EPA approval prior to initiating the final ILS. In preparation for this Phase 7 ILS, eight laboratories were contacted that demonstrated expertise in prior phases. Each laboratory agreed to participate for this groundwater validation phase and was instructed to follow the proposed method and to perform its analysis exactly as written.
The eight laboratories who participated in the ILS were provided with replicate synthetically prepared dissolved light gas standards containing methane, ethane, ethene, and n-propane at two concentrations in two different groundwater matrices sourced from northeastern Ohio and southeastern Ohio. The synthetic groundwater standards were prepared at nominal analyte concentrations of 80 and 800 microgram per liter (µg/L) in each groundwater sample. A single set of Certified Reference Materials (CRMs) was prepared by an ISO-accredited body at concentrations of approximately 5,000 µg/L for each analyte.
This study and the resulting data successfully validated the MSC’s Determination of Dissolved Light Gases Method as an acceptable method to achieve scientifically defensible data of known quality. Specifically, laboratory precision for the triplicate analyses of the reference standards was less than 10% relative standard deviation (RSD), with across-laboratory precision results less than 30% RSD. Pooled average percent recoveries of reference standards in the two synthetic groundwater standards ranged from 85-96%, with pooled average percent recovery of the CRMs ranging from 89-94% for each of the four analytes.
We believe the exceptional quality of the data from this study and those collected in Phase 5 as well as all steps and studies used to derive the proposed method exceed the US EPA requirements for new method development and formal validation. Based upon the specifications in the proposed method and the work conducted by the MSC Dissolved Methane Method Workgroup, this proposed method is deemed applicable to both drinking water and groundwater of less than 10,000 parts per million (ppm) total dissolved solids (TDS).