Chemical contaminants and potential toxicity
of extracts from semipermeable membrane devices (SPMDs):
. Semipermeable membrane devices (SPMDs)
are passive samplers that concentrate trace levels of hydrophobic
(not capable of uniting with or absorbing water) organic contaminants
in aquatic systems. The samplers are designed to mimic the bioaccumulation
of organic contaminants in the fatty tissues of aquatic organisms.
The SPMDs were constructed from low-density, polyethylene (LDPE)
tubing filled with a thin film of purified lipid triolein (fat
found in most aquatic organisms) that simulates the exposure
to and passive uptake of highly lipid-soluble organic compounds
by biological membranes (Huckins and others, 1993). The LDPE
tubing only allows dissolved bioavailable organic contaminants
to diffuse or pass through the membrane to be concentrated in
the lipid. Because SPMDs integrate chemical conditions over an
extended period of time and flow conditions (low-flow and high-flow),
they may offer a more complete representation of potential chemical
exposure of organism to contaminants than water samples collected
at one point in time (Huckins and others, 1993). Among the organic
contaminants that may be concentrated by the SPMDs are polychlorinated
dioxins and furans, polycyclic aromatic hydrocarbons (PAHs),
polychlorinated biphenyls (PCBs), organochlorine insecticides,
and pyrethroid insecticides. The chemicals concentrated in the
SPMDs were analyzed for both concentration and potential toxicity.
In this study, SPMDs were deployed to compare chemistry and potential
toxicity of SPMD extracts at sites along a gradient of urban
intensity from low to high and to determine the relation to other
physical, chemical, and biological factors.
What we measured:
. SPMD extracts were analyzed for hydrophobic
organic contaminants, such as polychlorinated dioxins and furans,
polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls
(PCBs), organochlorine insecticides, and pyrethroid insecticides.
. SPMD extracts were analyzed for potential
toxicity using three bioassays, an ultraviolet (UV) fluorescence
scan to screen for PAHs, Microtox® bioassay, and P450RGS assay.
When we sampled:
. SPMDs were placed at each site during
low flow for a period of 4 to 6 weeks prior to collection of
invertebrate and algae samples for studies that sampled in 2003
. Six-inch (15 cm) SPMDs were housed inside a
protective cover for placement in the stream. Each SPMD was placed
in a shipping canister to prevent atmospheric contamination during
shipment. SPMDs were suspended in the stream by attaching the
protective cover to a metal rod that was pounded into the stream
. SPMDs were deployed for a period of 4
to 6 weeks.
. At the end of the deployment period, the
SPMDs were placed back in the shipping canister and sent to
Environmental Sampling Technologies (EST), in St. Joseph, Missouri
. At the end of the deployment period,
contaminants concentrated in the SPMDs were extracted (separated)
from the lipid by dialysis in an organic solvent at Environmental
Sampling Technologies (EST), in St. Joseph, Missouri, by using
methods described in Huckins and others (1990).
. Two assays were run on the extract at
the USGS Columbia Environmental Research Center (CERC) in Columbia,
Missouri - an ultraviolet (UV) fluorescence scan (Johnson and
others, 2004) and a Microtox® bioassay (Johnson, 1998).
. An additional assay, the P450RGS test, was
run by the U.S. Army Corp of Engineers Environmental Laboratory in
Vicksburg, Mississippi (Murk and others, 1996). The P450RGS assay
provides a rapid screen for aryl hydrocarbon receptor (AhR) type
compounds that include PCBs, PAHs, dioxins, and furans. All vertebrates
produce detoxifying enzymes upon exposure to AhR compounds; the amount
of enzymes produced is directly proportional to the concentration
of the compounds. Quantifying one of these enzymes (the gene CYP1A1)
serves as a measure of dioxin activity. The concentration of AhR
compounds in the SPMD extract that induce CYP1A1 production is expressed
as the amount of dioxin, in toxic equivalents (TEQs), that would
induce the same response.
o The UV fluorescence scan provided a semi-quantitative
screen for PAHs, which fluoresce under UV light. A standard curve
was developed by using various concentrations of pyrene (a specific
PAH compound) under a specific wavelength of UV light. The SPMD
extracts were then exposed to same conditions and the resulting
florescence was reported as a pyrene index in milligrams per
o The Microtox® bioassay measured the light
production of photo-luminescent bacteria when exposed to the
SPMD extracts; the biochemical pathway for light production
is lowered by a wide range of compounds concentrated by the
SPMDs. Results were reported as EC50, the concentration of
the SPMD extract that caused a 50 percent decrease in light
. A portion of each SPMD extract was sent
to the U.S. Geological Survey's National Water Quality Lab for
chemical analyses using gas chromatography/mass spectrometry
(GC/MS) analysis (Tom Leiker, U.S. Geological Survey, written
. Quality-control samples for the SPMDs
included dialysis, solvent, and trip blanks. During processing
in the laboratory, dialysis blanks and solvent blanks were collected
to monitor for possible manufacturing and laboratory contamination.
Trip blanks were collected in the field by exposing a SPMD to the
air for the amount of time it took to remove a SPMD from the canister
and place it in the stream, and then to remove the same SPMD from
the stream and place it back into the canister. With the trip blank,
however, the SPMD was left in the canister while the field-SPMDs
were deployed in the stream. In this way, the trip blank mimicked
exposure to airborne chemical contamination that field-deployed
SPMDs experienced during deployment and retrieval. In addition
to the trip blanks, replicate SPMDs were deployed at a subset of
What these samples represent:
. SPMDs integrate chemical conditions over
an extended period of time and flow conditions (low flow and high
flow) and represent the potential chemical exposure of organism
to contaminants better than water samples collected at one point
. Chemical analyses of SPMD extracts include
additional organic compounds not analyzed in water samples.
. Chemical Analyses of SPMD extracts provides
information on specific hydrophobic contaminants present in the
SPMD extract that complements the bioassay results.
. The bioassays represent the potential toxicity
of specific groups of organic contaminants concentrated in the
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