Читать книгу Global Approaches to Environmental Management on Military Training Ranges - Tracey Temple - Страница 45

The global estimation error can be minimized (but not eliminated) using practices derived from sampling theory. GSE can be addressed by taking many increments from the area being sampled (DU) and combining them into a single sample. Each increment should represent a similar amount of area within the DU to best characterize the complete area being sampled. The increments should also be uniform in mass. FSE can be addressed by sampling a sufficient mass to overcome the differences in composition in the particles collected in the samples. It is important that each increment mass, depth and surface area sampled be as close and uniform as possible to ensure that each increment can be considered equal.

Research at CRREL and Envirostat over the last 30 years has demonstrated what is required to minimize error associated with characterizing surface soils for energetics’ residues. We have developed the Multi-Increment^® sampling (MIS) method for the environmental characterization of surface soils for heterogeneously-distributed munitions constituents. Sample increments are collected in a systematic fashion starting from a random location in one corner or location of the DU (figure 2.5). Reproducibility between samples has increased dramatically since MIS was implemented. Whereas prior sampling efforts resulted in proximate discrete samples varying by three orders of magnitude or more in a DU, using MIS has reduced variability to less than one order of magnitude, sometimes less than a factor of two between replicates. Whole population sample collection of high-order detonation residues on ice was compared to MIS of post-detonation residues on snow for the same operation and munition with no significant difference between the data sets [25].

Figure 2.5. A replicate random-systematic sampling pattern in a square decision unit (left). Map of non-square detonation DUs from field research on munitions (right).

Using specially designed tools, DU surface soils can be characterized quickly, efficiently, reproducibly and economically. These tools enable the collection of near-uniform increments in a variety of soils. In the case of the tool shown in figure 2.6, the diameter and depth of the increment can be varied in accordance with the desired final sample mass and number of increments [21]. The tool also enables the sampler to collect the increments from an upright position, greatly facilitating the process. Sampling bias is also reduced as the coring bit is pushed into the soil by the sampler’s foot, aiding in the penetration of the coring bit through surface vegetation, a situation that will cause a sampler using a spoon or trowel to seek an easier location to sample. The coring tool will help minimize ME during sampling, reducing the total sampling error.

Figure 2.6. An example of a sampling tool for obtaining uniform soil increments.

Sample handling is a very important aspect of sampling that is often overlooked, especially in the field. Most effort is focused on a chain of custody procedures, which are necessary but do not address sample integrity. Each sample collected in the field represents not only the DU from which it was obtained but a significant investment on the part of the entity tasked with sample collection. Sample labeling and documentation are thus a very important step in the field sampling process. Most MI samples should be collected in lab-grade clean polyethylene bags. The samples are labeled on the outside of the bag with an ink marker and recorded in a field notebook. Before sealing the bag with a plastic wire tie, we fill out a plastic tag with a sealable cover as a secondary label for each sample (figure 2.7). If the notation on the bag rubs off or becomes illegible, the tag can be used to identify the sample. If the tag is lost, the bag may retain enough information to identify the sample. The tag is also be used to ‘track’ the sample if it is rebagged for any reason. By transferring the tag to the new bag, all the original information is retained. During a field campaign at a closed military range, a contractor would not allow the employees to fill out and attach a tag to each sample. During shipping, the nomenclature rubbed off three sample bags, one of which was for a triplicate sample. The samples had to be reacquired at a cost of over USD 150 000 to the project.

Figure 2.7. Field labeling of samples: bags with tags (left) and close-up of tag (right).

Many soil samples will contain very low concentrations of munitions constituents. Cross contamination between samples from potentially high concentration DUs with samples known to be from areas of low potential contamination is very possible. A sample must be double-bagged in the field after documentation and stored separately during transportation and when in the lab. This precaution significantly improves the sample quality by reducing the variance between sample replicates.

Using the correct sample preparation techniques is also critical in maintaining sample quality [26, 27]. Subsampling of the DU sample must be done carefully. A large source of error occurs during comminution, the process used to reduce the variance in particle sizes in the sample, which allows more uniform distribution of the analytes within the total mass of the sample. Diminution typically is conducted with a grinder. Puck grinders, used in the mining industry, have been found to achieve the best results based on lab, field and project studies. It is important to follow the correct grinding procedures, as the analyte can be lost during grinding, as was found when using a ball mill. Subsampling should be done in the lab following grinding, never in the field using sample splitting. The process must ensure that the subsamples represent the sample, much as the sample must represent the DU from which it was taken. If not, the subsample will be no better than a grab sample. In section 2.7 we will go into detail on field splitting, sample preparation in the processing lab, grinding to reduce sample particles to a more consistent size (comminution) and subsampling.