INTRODUCTION - TR02-120007

TR02-12
Page Navigation
6

Ideally, a groundwater-sampling device should not alter the chemistry of the

sample, be easy to use, and relatively inexpensive. However, a literature review

on the effects of groundwater-sampling devices on sample integrity (Parker 1994

a, b) found that most devices, including the more commonly used bailers, peri-

staltic pumps and bladder pumps, can alter the chemistry of some samples for

some analytes, especially analytes such as volatile organics (VOCs) and oxidi-

zable inorganic constituents that are subject to precipitation reactions. There are a

number of physical and chemical reasons why this is so. Change in temperature

will affect the solubility and volatility of constituents and can cause degassing. A

sample's temperature can change when it is brought to the surface, if either warm

or cold air is entrained in the sample by the device, by heat generated by a pump,

or from tubing exposed to hot or cold ambient air or intense solar radiation.

Samples taken from deep wells undergo changes in pressure that can cause

degassing of the sample. Air and other gases may be introduced into the well by

the action of the device or inadvertently by agitation. Tubing and pump materials

may sorb or leach analytes of concern. Once at the surface, volatiles and oxidi-

zable contaminants may be lost as a result of aeration as the sample is dispensed

into sample vials.

Theoretical and field studies over the last decade (Robbins 1989, Gibs and

Imbrigiotta 1990, MartinHayden and Robbins 1991, Robbins and Martin

Hayden 1991, Gibs et al. 1993, Puls and Paul 1997), using multi-level samplers

and other devices, have shown that with conventional sampling methods, con-

centrations in the well are typically integrated over the length of the screened

interval and often reflect concentrations in the most permeable part of the

screened interval. Therefore most methods that utilize large purge volumes or

rapid pumping (for purging or sampling) do not produce information about the

true concentration gradients within the aquifer. If the pumping rate or pumped