Collection And Pretreatment

Groundwater samples can be collected with portable or dedicated in situ sampling equipment. Portable equipment includes bailers, syringes, suction-lift pumps, submersible pumps, and gas-driven devices. In situ sampling equipment includes cone penetrometer samplers (e.g., Hydropunch, BAT, CPT, or DMLS), chemical-sensitive probes, ion-selective electrodes, fiber-optic chemical sensors, multilevel capsule samplers, and multiport casings. A description of these types of equipment as well as their advantages and disadvantages is in the U.S. EPA desk reference guide on subsurface characterization and monitoring techniques (1993).

Selecting sampling equipment should be based on the purpose of the sampling as well as the construction materials of the sampling equipment and the method of sample delivery. The construction materials of the sampling device could affect the integrity of the sample because constituents can leach from the materials into the water samples or contaminants from the water sample can adsorb onto the sampler materials (Barcelona, Gibb, and Miller 1983). Therefore, inert materials should be specified when necessary. The method of sample delivery is important because devices that cause aeration, degassing, or pressure changes of the sample may not preserve the chemical quality of the sample. For example, devices that introduce dissolved oxygen into the sample could cause oxidation of ferrous iron to ferric iron, which affects the speciation and concentration of many chemical constituents in the sample (Hrzog, Pennimo, and Nielson 1991). Turbulence and depressurization can affect the sample's original content of dissolved oxygen, carbon dioxide, and volatile organic compounds (Barcelona, Gibb, and Miller 1983).

Another decision environmental engineers should make before sampling is whether to filter the sample in the field. This decision should be based on the characteristics of the constituents and the purpose of the sampling program. For example, samples requiring analysis for dissolved metals, alkalinity, and anionic species should be filtered. In contrast, samples for dissolved gases or volatile organics should not be filtered since the handling required by filtration could lose these chemicals. Furthermore, filtration should be performed when the sampling program is concerned only with those constituents that are dissolved in groundwater, excluding all constituents which can be adsorbed onto particulate matter in suspension, such as PCBs or polynuclear aromatic hydrocarbons. However, when a drinking water source is studied, samples should not be filtered before analysis because water taken from private wells is generally not filtered before use. In some instances, engineers must run parallel sets of filtered and unfiltered samples to determine the dissolved and adsorbed portions of the constituent of interest.

Filtration is accomplished by vacuum, pressure, or inline filtration devices. Stolzenburg and Nichols (1986) de scribe a variety of filtration equipment and their effects on sampling. The preferred device is the inline filter because it reduces the aeration and degassing of the sample as well as the potential of sample cross contamination caused by improper equipment decontamination.

To prevent cross contamination, engineers should decontaminate the equipment used for sample collection or pretreatment prior to and after each use. The decontamination should involve a minimum of scraping or brushing to remove any soil or residue from the device, washing with potable or deionized water, washing with detergents or cleaning fluids such as acetone, and pressure cleaning with a high-pressure steam cleaner.

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