Transcript Semi-Permeable Membrane Devices (SPMD’s) for Sampling

James River at Richmond, VA
Semi-Permeable Membrane Devices
(SPMD’s)
for Sampling Dissolved Organic
Substances from Surface Waters
Don Smith
Virginia Department of Environmental Quality
Dissolved Organic Substances
• The U.S. EPA has defined criteria and the States have
developed water quality standards (WQS) for numerous
organic substances which are either toxic or are
otherwise of environmental concern.
• However, these standards, as well as the concentrations
typically observed in ambient surface waters, have
generally been well below the minimum levels of
detection permitted by established analytical methods!
Biological Magnification
• The uptake of low-level dissolved organic substances
by living organisms often results in the accumulation
of much higher concentrations in animal tissues “bioconcentration”! (Organic compounds are much
more soluble in natural fats and oils than in water!)
• Fish-tissue analyses and “screening values” for
human health concerns (related to fish consumption)
have consequently been used to evaluate water
quality, but...
• Fish and other aquatic organisms are mobile… source
of contamination often in question!
• Different organisms may uptake and depurate
(metabolize or excrete) various organic compounds at
quite different rates…
• Whether fish-tissue concentrations are representative
of local water quality conditions is uncertain…
• Thus, true concentrations in the water column remain
unknown…cannot be ‘back-calculated’!
• and assessment of the water body for other designated
uses would still be questionable!
Semi-Permeable Membrane Devices
• Research scientists from the United States Geological
Survey (USGS), at the Columbia Environmental
Research Center (CERC), Columbia, Missouri,
developed an artificial sampling device that mimics
the parts of animals that cause “bioconcentration”!
(mid 1980’s through early 1990’s)
• More recent studies have improved this technology
and provided a powerful tool for measuring the
average concentrations of dissolved organic
compounds in ambient surface waters over extended
periods of time.
Semi-Permeable Membrane Devices
• SPMDs have been described as “Virtual Fish”
• Consist of purified ‘triolein’ oil (a natural fat found in
aquatic organisms) sealed within a flat, tubular
membrane of thin, low-density polyethylene (LDPE)
• The LDPE mimics a biological membrane by
allowing selective diffusion of organic compounds
from the water into the triolein .
• The organic compounds are highly soluble in triolein
and become concentrated there!
• SPMDs are sensitive - they can sample from air, water
or soil!
Examples of Contaminants that are Concentrated in TrioleinContaining SPMDs (not all-inclusive)
SPMD
Elizabeth
River
Uptake Rates of Organic Compounds are Initially
Linear
< Linear uptake for up to 30 days or more!
• When exposed over a reasonable time period
(up to 30 days in the field), the concentrations of
organic substances within the triolein become
high enough to be measured directly, and are in
direct proportion to the average concentrations
within the environment ( in this case ambient
surface water)!
• Subsequent extraction and analysis of the
organic compounds yields a direct measure of
the average environmental concentrations, and
a reliable assessment may be performed using
well-established Water Quality Standards!
< Transported frozen to
USGS/CERC, Columbia, MO
Very Sensitive, Very Accurate,
Very Expensive!
• Field enclosure(s) – USGS - loaned, but if you lose them,
– you buy them!
– (3 x $550 per site, if lost!)
$1650
– (DEQ - 1 x $20, disposable)
$
• Unit Cost, USGS contract
20
$4500
– (including SPMD, sample extraction and analysis)
• Total Cost (50 Units)
~$225,000
Further Information on SPMDs
http://wwwaux.cerc.cr.usgs.gov/SPMD/
Basic information, Glossary, Overview, Tutorial, Frequently asked questions,
References, USGS SPMD research activities, etc.