Evaluation of preservation techniques for trace metals and major cations for surface waters collected from the U.S. Geological Survey's National Water Quality Network Sites

The USGS National Field Manual (NFM) states "A representative sample is one that typifies in time and space that part of the aqueous system to be studied" and "obtaining representative samples is of primary importance for a relevant description of the environment." The NFM contains detailed chapters on the best practices for representative and contamination free sampling of major and trace metals (U.S. Geological Survey 2006). However, as analytical capabilities have advanced and new preservation methods developed, there is a need to study the effectiveness of various amounts of nitric acid preservation for trace metals and major cations in surface water and groundwater. The amount of nitric acid prescribed by the NFM for the preservation of cations and trace metals (0.4% v/v concentrated nitric acid, see attachment 1; U.S. Geological Survey 2002) is lower than some other commonly used methods (e.g., 1% v/v concentrated nitric acid). Furthermore, the NFM calls for the final sample pH to be less than 2, which may not always be measured or may change as the sample equilibrates (e.g. groundwater). The additional acid volume will likely prevent the formation of precipitates and may simplify field sampling protocols (i.e. measuring the final pH of the preserved sample may not be necessary). A total of eighty-eight replicate samples from sixty-nine different sites were collected from the U.S. Geological Survey's National Water Quality Network (NWQN). One replicate sample was preserved using the current method prescribed by the NFM (0.4% v/v concentrated nitric acid) and additional nitric was added to the second replicate sample (total of 1% v/v concentrated nitric acid). The paired samples were analyzed for trace metals by inductively coupled plasma-mass spectrometry and major cations by inductively coupled plasma-optical emission spectrometry.