Environmental Sampling Techniques - Fundamentals of Environmental Sampling and Analysis - page 73

Even with the proper preservation, no samples can be stored for an extended
period of time without significant degradation of the analyte. The maximum holding
times (MHTs) are the length of time a sample can be stored after collection and prior
to analysis (or pretreatment) without significantly affecting the analytical results.
MHTs start with the moment of sampling and end with the beginning of the
analytical procedure. Samples that have exceeded their MHTs should be discarded
in order to not jeopardize data quality.
MHTs vary with analyte, sample matrix, and analytical methodology used to
quantify the analyte's concentration. Considering that multiple agencies
designate MHTs, they can be confusing when several matrices or multiple
projects are involved. The most frequently cited MHTs are those promulgated
under the CWA (40 CFR 136) for the analyses of wastewater relating to NPDES
permits (refer to Chapter 2). Other programs have established the MHTs as well,
such as SW-846 used to comply RCRA regulations, and the Contract Lab
Program for laboratories performing work under contract with the U.S. EPA in
Superfund site investigations. Other agencies that set holding time requirements
are the ASTM, USGS, APHA, AWWA, and WEF. For the most part, the holding
times established in these various programs are consistent. Some minor
inconsistencies do exist that can create confusion as to the selection of
appropriate holding times. For instance, holding times for VOCs range from
5 days from sample receipt for Superfund work, to 7 days from sampling for
NPDESpermits,to14daysfromsamplingforRCRAgroundwateranalyses
(Keith 1988; Popek, 2003).
Figure 4.2 depicts the general MHTs requirements recommended by APHA
(1998) for common water quality parameters. Several parameters must be measured
immediately in the field, such as pH, temperature, salinity, and DO. Many other
parameters must be measured within 1-2 days after sample collection. For
these parameters, a careful pre-planning is needed to avoid sampling on Friday,
Saturday, or near Holiday weekends. Many of the organic compounds (purgeable

ASAP 7-28 days 6 months

6-48 h

pH
Salinity
Cl 2 , ClO 2
CO 2 , I 2 , O 3
DO (by electrode)
Temperature

Color (48 h)
PO 4 3 , NO 3 (48 h)
Surfactant MBAs (48 h)
Chlorophyll (24-48 h)
Acidity/alkalinity (24 h)
CN , Cr 6+ (24 h)
Turbidity (24 h)
DO (Winkler) (8 h)
BOD (6 h)
Odor (6 h)

Oil and grease (28 d)
Total P (28 d)
F - , S 2 , SO 4 (28 d)
B, Si, Hg (28 d)
Conductance (28 d)
Solids (7 d)
Base/neutral/acid organics (7 d)
Pesticide (7 d)
Purgeable hydrocarbons (7 d)
NH 3 , TKN, COD, TOC (7 d)

Metals
Hardness

- -

2- 2-

, I

- +

MHTs

Figure 4.2 Maximum holding times (MHTs) for common water quality parameters. DO: dissolved
oxygen; BOD: biochemical oxygen demand; TKN: total Kjeldahl nitrogen; COD: chemical oxygen
demand; TOC: total organic carbon