Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 11.2
SAMPLE PRESERVATION AND HOLD TIMES

11.2 SAMPLE PRESERVATION AND HOLD TIMES

This section presents the recommended sample preservation for soil and water sample collection. Always consult with the laboratory when planning fieldwork to ensure that the proper sample containers and preservatives are used. Tables 11-A and 11-B, provided in the Appendices, present the recommended preservation and hold times for soil and groundwater, respectively.

Sample preservation consists of methods to assure the samples analyzed in the laboratory are representative of the field conditions. Preservation methods may include maintaining sample temperatures, analyzing the samples within recommended hold times or using chemicals (such as HCl or nitric acid [HNO3]) to stabilize the target contaminants by altering the sample chemistry.

Several potential difficulties associated with field preservation techniques include: sampling gravely soil matrix (if using EnCore® type samplers), spillage or evaporation of pre-weighed chemical preservatives, shipping restrictions involving chemical preservatives, or potential chemical reactions between calcareous particles and HCl preservative.

11.2.1 Sample Temperature

Upon collection and sealing of sample containers, immediately begin the cooling process in the field by placing the sample containers in an insulated cooler containing water ice or frozen gel packs. Use of water ice is generally considered by the HEER Office to be more efficient to rapidly cool samples, and may be especially important for use with samples for volatile analyses, when feasible. Maintain the temperature of the sample containers at less than or equal to 6 degrees Celsius (°C) from the time of collection through the delivery to the analytical laboratory.

The National Environmental Laboratory Accreditation Conference (NELAC) states that samples which require thermal preservation shall be considered acceptable if the arrival temperature is within 2°C of the required temperature or the method specified range. For samples with a temperature requirement of 4°C, an arrival temperature from 0°C and 6°C meets specifications. Samples that are delivered to the laboratory on the same day that they are collected may not meet these criteria. In these cases, the samples are considered acceptable if there is evidence that the chilling process has begun, such as arrival at the analytical laboratory on ice.

11.2.2 Chemical Preservation

When employing chemical preservation, add the chemical reagents to the sample containers prior to mobilizing to the field, or request sample containers with pre-measured chemical reagents from the analytical laboratory. Carefully place the soil or groundwater samples into the sample containers to minimize loss of chemical preservative as well as volatile constituents in the sample (i.e., do not overfill water sample containers or leave the cap off a jar containing methanol for soil samples).

Some commonly used preservation chemicals may react with the sample media. For example, calcareous soil may react with sodium bisulfate; turbid groundwater collected from a coral aquifer formation may react with hydrochloric acid. If the sample media reacts with the chemical preservative, volatile organic constituents may be lost due to effervescence during sample collection, so an alternative preservation method or no chemical preservation may be needed.

The use of chemical preservatives for fieldwork in Hawai`i presents challenges when shipping preserved samples between islands or to the mainland (see Subsection 11.4 for additional details on shipping preservatives).

Some samples collected for specific analysis, such as dissolved metals in groundwater, may require pre-treatment prior to collection, as well as preservation (see Section 6.0).

11.2.3 Sample Hold Times

The soil and water samples should be analyzed as soon as possible after collection. Hold times are the maximum allowable times that a sample, or any subsample generated from the bulk sample, may be held before analysis. Several methods provide guidelines for both the hold time until extraction (denoted by "E" on Tables 11-A and 11-B in the Appendices) and the hold time after extraction until analysis (denoted by an "A" on the Tables).

Results from samples analyzed past the hold times may or may not be usable, depending upon the DQO set forth in the SAP. At a minimum, the results from analyses conducted past the recommending hold times should be interpreted as minimum concentrations.

Volatile Soil Samples

The use of Multi Increment sampling methods are recommended for testing of soil for VOCs (refer to Section 4.2.8.1). The use of discrete soil samples is discouraged, due to the small mass of soil represented by the laboratory data (e.g., five-grams). Sample containers, preservation, holding times and laboratory method for testing of VOCs in soil and water samples are described in EPA Method 5035 and 5035A and summarized in Appendices 11-A and 11-B, respectively (USEPA 1997g, 2002h).

The sample collection and preparation methods in EPA Method 5035 and 5035A include short hold times, ranging from 48 hours up to 14 days. The guidance recommends a number of potential methods including 1) preservation of soil samples in methanol, 2) freezing samples at low temperatures, 3) holding unpreserved samples for short time periods on ice, 4) preservation of soil with an acidic solution (sodium bisulfite), and 5) preservation of increments in reagent water. Preservation of calcareous, coralline soils with acidic solutions can cause the material to effervesce, resulting in a loss of VOCs. This could pose a problem in low lying coastal areas underlain by “caprock”, marine sediment, as well as fill material derived from these areas. The use of reagent-grade water has also come under question and is not recommended for use in Hawai’i, due to concern about extraction efficiency as well as the short hold time of 48 hours (refer to Section 5). Preservation of MI samples with methanol is preferred and considered most reliable, although freezing of increments (e.g. using dry ice or water ice with salt) and shipment to the laboratory for extraction in methanol is unavoidable in some cases. This is especially true for projects on islands other than Oʿahu, due to logistical and safety issues associated with the air transportation and storage of methanol.

MI soil sample preservation methods and associated hold times for VOC testing are presented below (in order of preference) and summarized in Appendix 11-A and Section 4.2.8.1:

  • Preserved soil samples collected with a sampling device capable of providing a pre-determined mass of soil and immediately extruded into a glass jar containing methanol must be analyzed within 14 days from the time of sample collection. This approach is recommended by the HEER Office when feasible in the field. Reporting limits achieved by the use of methanol might be elevated due to the need to dilute the solution for testing. If this is the case, the reporting limits can be used for general screening purposes in place of the applicable Environmental Action Levels (EALs; refer to HDOH, 2016). In this application, methanol acts as the preservative as well as the extraction solvent and a limited volume of methanol extract is introduced directly into the laboratory instrument. As such, the dilution and the reporting limits are higher than with some other preservation and extraction approaches. In some cases, the laboratory may be able achieve lower reporting limits for specific contaminants preserved in methanol by using additional analytical techniques, so consultation with the laboratory is recommended. The methanol preservation method may not be feasible due to the requirements or restrictions on hazardous material air shipments by the Department of Transportation (DOT) and/or the International Air Transportation Association (IATA). Consequently, applicable shipping regulations should be understood and carefully followed.
  • Unpreserved soil increments collected in sealable, airtight sampling devices and immediately frozen in the field to less than negative 7°C must be analyzed within 14 days from the time of sample collection. Note that sealable, airtight coring tools or vials should not be frozen to less than -20°C to prevent problems with the integrity of the seals. Dry ice in direct contact with the sample containers may freeze them below -20°C, so dry ice needs to be used appropriately to achieve the desired temperature range (for example use a layer of insulating material between the dry ice and sample containers). Alternately, bags of water ice mixed with table salt may be used to freeze samples to below -7°C in the field.
  • Unpreserved soil increments collected with a sealable, airtight coring device (e.g., EnCore® samplers or equivalent device) or in sealable, airtight vials and stored at 4°C must be extracted and analyzed within 48 hours from the time of sample collection. However, analysis time may be extended to 14 days of the sample collection date if the unpreserved soil increments are either placed in methanol or frozen to <-7°C by the lab within 48 hours from the time of sample collection in field.
  • Preserved soil samples collected with a sampling device capable of providing a pre-determined mass of soil and immediately extruded into a glass jar containing sodium bisulfate preservative in reagent water must be analyzed within 14 days from the time of sample collection. This approach is appropriate for non-calcareous soils. A field check to determine whether the soil is calcareous is recommended (calcareous formations are commonly encountered in coastal areas when performing fieldwork in Hawai`i). In addition, certain VOCs such as styrene, TCE, trichlorofluoromethane, cis- and trans 1,3-dichloropropene, 2-chloroethylvinyl ether, and vinyl chloride may be decomposed by the bisulfate leading to results biased low, so acid preservation should not be used for these contaminants of concern. The use of sodium bisulfate as a preservative should be discussed with the analytical laboratory since it may cause laboratory instrument problems requiring frequent recalibration.