Methods
NAWQA was designed to be a nationally uniform and consistent program, and therefore methods were prescribed to be followed by all study units. Due to regional variability around the United States, however, some methods had to be modified for local conditions. The methods used in the tissue-sampling program are discussed in the sections that follow.
Site Selection
A total of 824 sites were selected to be sampled for aquatic tissue. Typically, 20 to 30 sites were selected within each study unit. These study-unit sites included four to eight periodic surface- water monitoring sites and multiple synoptic sites. The periodic monitoring sites were selected to represent large stream basins (integrator sites) or principal land uses (indicator sites) (Gilliom and others, 1995). The most common land uses monitored were agriculture, cropland, pasture, urban, commercial/industrial, residential, forest, rangeland, point source, and mining. Usually one or two monitoring sites within each study unit were selected to represent natural background environments where concentrations of trace elements might be expected to represent background (reference) conditions with minimal anthropogenic effects; typically, these reference sites were located in forest or rangeland land uses. Synoptic sites were selected to fill in geographic gaps between monitoring sites within a study unit, or to represent less prevalent land uses.
Species Targeted for Collection
From 1993 to 1995 whole bodies of caddisfly larvae (Hydropsyche spp.) and soft bodies of asiatic clams (Corbicula fluminea) were
targeted as the two best indicators of trace elements in streams and rivers (Crawford and Luoma, 1993). From 1996 to
1999 liver tissue taken from adult common carp (Cyprinus carpio) and white sucker (Catostomus commersonii) served as the primary targeted tissue because these two
species are more widely distributed and could be obtained at a larger number of collection sites, thereby better serving the goal of achieving a nationally consistent
data base. Dace (Cyprinidae)
and sculpin (Cottidae) were targeted in the
Pacific Northwest
region because common carp and white sucker were not consistently available in this region. Largemouth bass (Micropterus salmoides), smallmouth bass
(Micropterus dolomieu), rock bass (Ambloplites rupestris), and asiatic clams (Corbicula spp.) were collected in the Southeast and at scattered
sites elsewhere, where they are common. In extreme cases where fish and invertebrates were not available, aquatic plants were collected for the trace-element
analysis. Collection sites for fish and invertebrate species are shown in figures 2 and 3, respectively. Although not
discussed in this report, samples of streambed sediment were collected along with the tissue samples and were analyzed for the same suite of trace elements.
Figure 2. Locations of fish-tissue collection sites, 1993-1999.
Figure 3. Locations of invertebrate-tissue collection
sites, 1993-1999.
Sample Collection and Compositing Methods
Samples were collected, processed, preserved, and
transported according to the recommendations of Crawford and Luoma (1993). For quality assurance purposes, two or three samples of the same species were collected at 1 or more sites in each study unit to provide replication at approximately 10 percent of the sites.
Fish were collected using various types of
electro fishing gear or nets. Asiatic clams were collected from the stream
bottom either by hand-picking or using a clam rake. Caddisfly larvae and other aquatic insects were
collected using a D-frame or similar type of kick-net sampler. Immediately
following collection, all invertebrate organisms, such as clams and aquatic
insects, were stored overnight in native water to evacuate gut contents and then
were immediately frozen.
Composite samples were taken for all tissue types. For
fish liver samples, composites consisted of liver tissue from five to eight
individuals of a given species. At a few
stations, extracting liver samples from small-sized fish species was not
practical, and a whole fish composite from 10 to 20 individuals was collected. The number of asiatic clams collected for a
single composite sample typically ranged from 20 to 30 individuals. The number
of caddisfly larvae composing a single composite sample generally ranged from
40 to 60 individuals.
Multiple samples were collected at some sites at time
scales varying from hourly to annually. Two
samples were collected at 122 sites, three samples were collected at 40 sites,
four samples at 8 sites, five samples at 3 sites, and six samples at 1
site. Multiple samples at a site commonly
consisted of different organisms or different tissue types, but in other cases
the same organism and tissue type were collected. The data presented here also
include the results of a special mercury synoptic study done in the NAWQA Red
River of the North study unit (REDN). For this study, 221 samples of channel
catfish and carp were collected. Information
regarding this special study is discussed in detail by Goldstein and others (1996).
Chemical Analysis
Trace-element concentrations were determined in tissues at
the USGS National Water Quality Laboratory using the methods described in
Hoffman (1996). A low temperature (85
degrees Celsius) nitric acid digestion, followed by the addition of hydrogen
peroxide (30-percent solution), was used to decompose the biological
material. The digested solutions were
evaporated to incipient dryness, reconstituted with 5 percent nitric acid, and
filtered. After filtration, the
solutions were diluted to a known volume and analyzed by inductively coupled
plasma–mass spectrometry (ICP–MS), inductively coupled plasma-atomic emission
spectrometry (ICP–AES), and cold vapor-atomic absorption spectrophotometry. Many of the trace elements were determined by
both ICP–MS and ICP–AES. Minimum
reporting levels for the trace-element concentrations were the following: mercury, 0.005 μg/g
(microgram per gram dry weight); tin, beryllium, lead, nickel and uranium, 0.01
μg/g; iron, 0.13 μg/g; boron, 0.15 μg/g; cadmium, 0.03 μg/g; cobalt and
molybdenum, 0.045 μg/g; barium, manganese, strontium, arsenic and selenium, 0.05
μg/g; silver, 0.10 μg/g; iron, 0.13 μg/g; chromium, 0.165 μg/g; and copper
and zinc, 0.025 μg/g. Samples spiked
with known concentrations of trace elements were used as a quality-control
method to evaluate the efficacy of the sample extraction and processing
technique; recoveries from spiked samples ranged from 92 to 110 percent. Percent moisture in the raw
tissue sample also was measured, so dry weight concentrations reported here can
be recalculated back to wet weight concentrations.
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