The following sections briefly discuss the various types of sampling mechanisms, and

their relative advantages and disadvantages. More detailed discussions of the various types of

sampling devices are provided in Nielsen and Yeates (1985), Pohlmann and Hess (1988),

USEPA (1991a), and USEPA (1991b). Because new sampling devices will become available

on a periodic basis, the Agency recommends that the manufacturer's performance testing data

and results be reviewed before selecting a ground water sampling device. It is expected that

through design improvements, general operating ranges also will improve; therefore, some of

the information contained in the following discussions may become outdated.

7.3.1

Grab Samplers

There are two types of grab samplers available: bailers and syringe devices. The

following sections provide a general description of these devices.

7.3.1.1

Double and Single Check Valve Bailers

Bailers are among the simplest of ground water sampling devices. A bailer is a rigid

tube that fills with water when lowered into the well; when raised back out of the well, it is

sealed on one or both ends, typically by a ball and seat mechanism. Bailers that seal only at

the bottom are called single check valve bailers, bailers that seal at both ends are called

double check valve bailers or point source bailers. The ground water sample is transferred

into sample containers from the bailer. Bailers are relatively inexpensive to purchase or

fabricate, easy to clean, portable, simple to operate, and require no external power source

(USEPA, 1983).

Disadvantages of bailers are that their use can be time consuming and labor intensive

and that the transfer of water to a sample container may significantly alter the chemistry of

ground water samples due to degassing, volatilization, or aeration (oxidation). Recent

research focusing on the comparison of different types of ground water sampling equipment

demonstrates that significant loss of volatile organic compounds may occur when bailers are

used to sample ground water (Pearsall and Eckhardt, 1987; Yeskis et al., 1988; Tai et al.,

1991; Pohlmann et al., undated). Researchers also believe that the action of lowering and

raising the bailer in the well may mobilize naturally immobile particulates, and that the

velocity of ground water entrance into the device may actually approach that of high rate

pumping methods (Puls and Powell, 1992; Barcelona et al., 1990; Puls and Barcelona, 1989a;

Puls and Barcelona 1989b).

Studies have suggested that considerable imprecision is introduced into samples

collected with bailers, possibly as a result of differences in operator technique (USEPA,

1991a; Tai et al., 1991; Pohlmann et al., undated). In addition, it is difficult to determine the

exact location in the water column from which a bailed sample has been collected; inadequate

sealing of the check valves often increases this imprecision (USEPA, 1991a). In a study

comparing concentrations of volatile organic compounds detected using various sampling

November 1992

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