The Agency recognizes that there are certain circumstances where it is necessary to

filter or centrifuge a sample under controlled laboratory conditions prior to analysis to prevent

instrument damage. Sample filtration in the laboratory is permissible if insoluble materials

(e.g., silicates) remain after acid digestion of the sample, which could damage laboratory

equipment. If this step is necessary, the filter and the filtering apparatus should be thoroughly

cleaned and pre rinsed with dilute nitric acid. Laboratory personnel should refer to Chapter

Three of the EPA publication "Test Methods for Evaluating Solid Waste" (SW 846) for

information concerning these procedures.

The Agency also realizes that data generated from unfiltered samples may result in

higher concentrations of metals detected in ground water samples at some facilities. Because

background samples also will be unfiltered, however, false indications of contamination

should be minimized. In all cases, owners and operators should ensure that all samples used

in a statistical test are collected using the same procedures.

Ground water sampling that is conducted to perform ion balance calculations or to

classify ground water according to the amount of dissolved ions is not addressed in this

Chapter, because these analyses are not part of the Subpart F requirements. Scientific studies

that are performed to estimate aqueous concentrations of dissolved geochemical parameters

have different data objectives than Subpart F ground water monitoring, and commonly utilize

other techniques and procedures to achieve the desired research goals.

7.4

In Situ or Field Analyses

Physically or chemically unstable analytes should be measured in the field, rather than

in the laboratory. Examples of unstable parameters include pH, redox potential, dissolved

oxygen, and temperature. Although the specific conductance (i.e., electrical conductance) of a

sample should be relatively stable, the Agency recommends that this analyte also be measured

in the field. The Agency suggests that dissolved oxygen, turbidity, and specific conductance

be determined in the field as soon as practicable after the well has been purged. Most

conductivity instruments require temperature compensation; therefore, the temperature of the

samples should be measured at the time conductivity is determined unless the monitoring

equipment automatically makes this compensation.

Three methods are generally employed for measuring unstable parameters. The two

preferred methods are to use either an in line flow cell or specially designed analytical

equipment that has probes that may be lowered down into the well. These methods provide

results that typically are more precise than those obtained using the third method, collecting

discrete samples and analyzing them at land surface. Specifically, the third method involves

collecting a sample in a clean bottle or beaker in the same manner that a sample for

laboratory analysis would be collected, and then analyzing the sample using a field test kit or

meter at land surface. If down hole probes (e.g., pH electrode, thermistor) are used to

measure unstable parameters, the probes should be decontaminated in a manner that prevents

November 1992

7 22