Environmental Samples Holding Time Studies with Alternative Preservation Strategy

Oral Presentation

Prepared by K. Kong1, J. Hardenstine2, R. Arold1, W. Gala1
1 - Chevron Energy Technology, 100 Chevron Way 50-1263, Richmond, CA, 94801, United States
2 - NewFields Environmental Forensics, 300 Ledgewood Place, Suite 305, Rockland, MA, 02370, United States

Contact Information: kitty.kong@chevron.com; 510-242-1654


Site investigations require the collection and analysis of representative environmental samples to delineate impacts, risk, and remediation options. For petroleum-related sites, BTEX and semi-volatile polycyclic aromatic hydrocarbons (PAH) are the primary contaminants of concern. When environmental samples such as water and soil/sediment are removed from the natural environment, the chemical components such as BTEX and the semi-volatile polycyclic aromatic hydrocarbons (PAH) begin to change due to physical processes such as evaporation, adsorption, precipitation, chemical processes such as photodegradation and microbial degradation. Preservation techniques are used to minimize these changes between collection and analysis of the environmental sample. The most common preservation methods for samples are refrigeration and freezing (solid samples) and acidification (water samples) at sub-ambient temperatures (e.g., 4ᴼC). The regulatory agencies have created consensus holding time requirements of seven (7) days for water and (14) fourteen days for soil samples. The technical basis of these requirements is not well defined, yet the cost and data usability impacts are substantial.

Laboratory-based studies were performed on water samples for BTEX and on water and sediment samples for PAH compounds to determine if acid (water) and sodium azide (sediment) preservation methods could be used to minimize sample degradation at ambient temperature. Statistical analysis of the data determined that there was no statistical difference from T=0 days for the PAH in the sediment samples for up to 60 days preserved with sodium azide and up to at least 14-days with no preservative. The largest impact to sample degradation was PAH phytolysis of samples for one time series of sample extracts that were exposed to ambient light for an excessive period of time (> 5 days). Corrective action to remedy this issue was later implemented by having the laboratory process the extracts immediately after sample extraction, and to store the extracts in a dark environment when they were not being processed. Finally, a study of BTEX holding times in water determined that these compounds were stable when preserved at ambient temperature for 21 days.

To further delineate accurate and defensible holding times for water and soil/sediment samples collected at petroleum impacted sites. Petroleum-impacted sediment samples were collected, homogenized and multiple replicates processed as preserved and unpreserved samples in the field. There, samples were shipped overnight to the laboratory at 4°C where they were separated into 4 groups: unpreserved at ambient temperature (Group 1) and 4°C (Group 2), sodium azide preserved at ambient temperature (Group 3) and sodium azide preserved at 4°C (Group 4). Replicates from the 4 groups were then analyzed over a period of 30 days for unpreserved samples and 60 days for the sodium azide preserved samples. Statistical analysis of the data determined that for sediment contaminated with petroleum, there was no statistical degradation of PAHs in the sediment samples for up to 60 days preserved with sodium azide at ambient or 4°C and up to at least 21 days with no preservative at 4°C. Unpreserved samples at ambient temperature showed significant losses of PAHs within the first 7 days. This pilot study has demonstrated that USEPA criteria are reasonable for unpreserved samples (7-14 days), however, when preservation methods are implemented within a field sampling program, sediment samples can be stored up to 60 days at ambient temperature without measurable degradation.