National Water-Quality Assessment (NAWQA) Program
Design of the National Water-Quality
Assessment Program:
Occurrence and Distribution of Water-Quality Conditions
United States Geological Survey Circular 1112
By Robert J. Gilliom, William M. Alley, and Martin E. Gurtz
Bed-Sediment and Tissue Studies
Bed-Sediment and Tissue Studies assess trace elements and hydrophobic
organic contaminants in two phases. The Occurrence Survey is designed to
provide an initial identification of important constituents in each Study
Unit based on data from few sites. The Spatial Distribution Survey
improves geographic coverage for priority constituents through broader
areal sampling and improved resolution in priority areas. The design of
the Spatial Distribution Survey is determined by results of the Occurrence
Survey. Data from both phases are combined for assessing distribution.
Occurrence Survey
The primary objective of the Occurrence Survey is to determine which target
constituents are important to water-quality conditions in each Study Unit.
Importance is determined by the magnitude of constituent levels and the
extent of their occurrence. The highest level of importance is assigned to
constituents found at elevated levels over a wide geographic area or within
many small areas, such that they are viewed as significant water-quality
issues for a substantial part of the Study Unit. Site selection and
sampling strategy are designed to maximize the probability of detecting
important constituents in the Study Unit.
Site Selection
The site-selection strategy for the 15-20 sites sampled for the Occurrence
Survey builds on the selection of Basic Fixed Sites (typically 7-13) for
water-column sampling. The choice of additional Indicator Sites is a
balance between locating sites where contamination is known to be probable
and dispersing sites so that streams that drain each major Environmental
Setting in the Study Unit are sampled. Integrator Sites are chosen to
provide a coarse downstream network of sites where large-scale contaminant
occurrences not detected at the Indicator Sites have a reasonable chance of
being detected.
The sampling design in each Study Unit thus includes all Basic and
Intensive Fixed Sites and additional sites selected specifically to meet
objectives for bed sediments and tissues. Usually one or two Indicator
Sites are selected to represent the broadest possible range of background
trace-element levels expected in the Study Unit, if such sites are not
already included as Intensive or Basic Fixed Sites. These reference
Indicator Sites also serve to assess the background occurrence of
hydrophobic organic contaminants.
Sampling Strategy
Sampling for bed sediment and tissues is done during summer or autumn low
flows to minimize seasonal variability. The basic sampling strategy is the
same at all sites. Each "site" refers to a reach of stream at that
location. Sampling strategies for bed sediment and tissues ideally involve
targeted sampling within a reach of stream near the location of
water-column sampling or streamflow measurement. The general approach is
to composite multiple samples in a manner that averages out anticipated
spatial variability within the reach.
Bed Sediment
The strategy for sampling bed sediment is designed to exploit the sorptive
properties of trace elements and hydrophobic organic chemicals by
collecting fine-grained particles, which are natural accumulators of these
chemicals, and to exploit the properties of stream environments by
targeting low-flow depositional zones as natural integrators of
fine-grained sediment. The general sampling strategy is to collect samples
of fine-grained surficial sediments from natural depositional zones during
low-flow conditions and to composite samples from several depositional
zones in a stream reach, which yields a reach average of fine-grained
surficial bed sediment.
The sampling procedures are described by Shelton and Capel (1994). The
first step at a site is to identify the sampling reach. The standard
sampling reach is the 100-m reach of stream upstream from the location for
water-column sampling or streamflow measurement. However, the diverse
nature of site locations, some of which are selected primarily for purposes
other than bed-sediment sampling, make it necessary to go variable
distances upstream or downstream from the water-column sampling or
streamflow measurement location to find suitable depositional zones. Care
is taken, however, to avoid including or excluding significant point
sources or tributaries that may affect the suitability of the reach in
representing the sub-basin. The sampling reach is colocated with the reach
where tissue samples are collected and within the sampling reach for
Ecological Studies.
The ideal sample-collection procedure within each sampling reach is to
identify 5-10 wadeable depositional zones that contain fine-grained
particulate matter. The depositional zones within the sampling reach are
broad areas at some sites and small pockets at other sites. Left- and
right-bank and center-channel depositional zones with different depths of
water are sampled to ensure that sediment that represents a range of
histories and sources important for the reach is sampled. Samples from the
surficial 2-3 cm of bed sediment in each depositional zone are collected at
several locations, in approximate proportion to its size, and composited
with samples from other depositional zones sampled at the site. The
composited bed-sediment sample is sieved and processed into several
subsamples for analyses of trace elements, hydrophobic organic chemicals,
total organic carbon, and particle size.
Tissue
The sampling strategy for tissues in the Occurrence Survey is comparable
with bed sediment in that it also is designed to minimize local scale
spatial variability and to maximize comparability of data within and among
Study Units. Generally, one taxon is collected at each site in the
Occurrence Survey; the same taxon is collected at as many sites as possible
within a Study Unit. National consistency is provided by a National Target
Taxa (NATT) List and decision trees that will guide, with local
flexibility, selection from that list (Crawford and Luoma, 1993). The NATT
List includes mollusks, insect larvae, fish, and vascular plants (table 8). Selection of target taxa for each
Study Unit is based on information from the Retrospective Analysis and
water-quality reconnaissance.
| Table 8. National Target
Taxa List for tissue bioassessment in National Water-Quality Assessment
[Listed in order of priority within each group] |
| Group |
Taxon
|
| Mollusks |
Asiatic clam (Corbicula fluminea)
|
| Insect larvae |
- Trichoptera (caddisflies):
- Hydropsyche sp.
- Brachycentrus sp.
- Limnephilus sp.
- Chironomidae (midges)
- Chironomus
sp.
- Plecoptera (stoneflies)
- Perlidae
- Perlodidae
- Pteronarcyidae
|
| Fish |
Carp (Cyprinus carpio)
White sucker (Catostomus commersoni)
Longnose sucker (Catostomus catostomus)
Largescale sucker (Catostomus macrocheilus)
Channel catfish (Ictalurus punctatus)
Largemouth bass (Micropterus salmoides)
Bluegill (Lepomis macrochirus)
Brook trout (Salvelinus fontinalis)
Brown trout (Salmo trutta) |
| Vascular plants |
Pondweed (Potamogeton sp.)
Hydrilla (Hydrilla verticillata sp.)
Waterweed (Elodea sp.)
|
Decision trees for selecting target taxa are different for hydrophobic
organic contaminants and trace elements because organisms concentrate these
contaminants differently (Crawford and Luoma, 1993). For hydrophobic
organic compounds, the first-priority taxon for collection is the bivalve
mollusk, Corbicula fluminea (the Asiatic clam). If this taxon is not
present at a site, then whole fish from a single species of fish on the
NATT List would be the second-priority taxon to collect. Of the fish
species on the NATT List, bottom-feeders (carp, suckers, channel catfish)
are higher priority than predators (largemouth bass, bluegill, brook and
brown trout). If adequate numbers of these species cannot be collected,
then no sample is submitted for organics analysis, except where a nontarget
taxon may be of local importance.
For trace elements, Corbicula fluminea is again the
first-priority taxon for collection. If a sufficient sample cannot be
collected at a site, then the second-priority target organisms are
aquatic insects from the NATT List. Stoneflies are lower priority
than the caddisflies on the list. If a sufficient sample of any one
of these target taxa cannot be found, then the third- and
fourth-priority target organisms from the NATT List are fish livers
and vascular plants.
Sample-collection procedures vary for different taxa and are described in
detail in Crawford and Luoma (1993). Samples for each taxon are
composited for a given site to average individual variability and to meet
a minimum mass requirement for the intended analyses.
Sample Analyses
The analytical strategy is uniform for all Occurrence Survey sites except
that chlorinated dioxins and furans are analyzed only at selected sites
(for example, near potential sources, such as pulp and paper mills, for
which existing data are not available) and that polynuclear aromatic
hydrocarbons (PAH's) are analyzed only in bed sediment. The analytical
strategy for bed sediments and tissues is listed in table 9. The list for tissues is a subset of
the list for bed sediments because compounds known to be metabolized by
target taxa have been eliminated from the target analyte list for
tissues.
Table 9. Analytical
strategy for bed-sediment and tissue Occurrence Survey.
[Bed sediments analyzed for all constituents; tissues analyzed only for
those indicated by *; some of the analytes may be deleted or qualified
depending on method performance for ambient samples.]
|
| Trace
elements and major metals |
Aluminum*
Antimony*
 (Stibum)
Arsenic*
Barium*
Beryllium*
Bismuth
Boron (tissue only)
Cadmium*
Calcium |
Cerium
Chromium*
Cobalt*
Copper*
Europium
Gallium
Gold
Iron*
Holmium
Lanthanum |
Lead*
Lithium
Magnesium
Manganese*
Mercury*
Molybdenum*
Neodymium
Nickel*
Niobium
Phosphorus |
Potassium
Scandium
Selenium*
Silver*
Sodium
Strontium*
Sulfur
Tantalum
Thorium*
Tin |
Titanium
Uranium*
Vanadium*
Yttrium
Ytterblum
Zinc*
|
Organic
contaminants
Organochlorine insecticides and polychloringated biphenyls |
Aldrin*
cis-chlordane*
trans-chlordane*
Chloroneb
Dacthal*
o,p'-DDD*
p,p'-DDD*
o,p'-DDE*
p,p'-DDE*
|
o,p'-DDT*
p,p'-DDT*
Dieldrin*
Endosulfan
Endrin*
Heptachlor*
Heptachlor epoxide*
alpha-HCH*
beta-HCH*
|
delta-HCH
(in tissues only)
gamma-HCH
(Lindane)*
Isodrin
Methoxychlor, o,p'*
Methoxychlor, p,p'*
Mirex*
cis-Nonachlor*
|
trans-Nonachlor
Oxychlordane*
Polychlorinated
biphenyls
(PCBs-total)*
cis-Permethrin
trans-Permethrin
Pentachloroanisole
Toxaphene*
|
| Other semivolatile
organic contaminants |
Acenaphthene
Acenaphthylene
Acridine
C8-Alkylphenols
Anthracene
Anthraquinone
Azobenzene
Benzo(a)anthracene
Benzo(b)fluoranthene
Benzo(k)fluoranthene
Benzo(g,h,i)perylene
Benzo(a)pyrene
Benzo(c)quinoline
2,2'-Biquinoline
4-Bromophenylphenylether
Butylbenzyl Phthalate
9H-Carbazole
bis(2-Chloroethoxy) methane
bis(2-Chloroethyl) ether
bis(2-Chloroisopropyl) ether
4-Chloro-3-methylphenol
2-Chlorophenol
2-Chloronaphthalene
4-Chlorophenylphenylether
p-Cresol
Chrysene
|
Dibenzo(a,h)anthracene
Dibenzothiophene
1,2-Dichlorobenzene
1,3-Dichlorobenzene
1,4-Dichlorobenzene
2,4-Dichlorophenol
Diethyl Phthalate
3,5-Dimethylphenol
1,2-Dimethylnaphthalene
1,6-Dimethylnaphthalene
2,6-Dimethylnaphthalene
Dimethyl Phthalate
Di-n-butyl Phthalate
2,4-Dinitrophenol
2,4-Dinitrotoluene
2,6-Dinitrotoluene
Di-n-octyl Phthalate
bis(2-Ethylhexyl) Phthalate
2-Ethylnaphthalene
Fluoranthene
9H-Fluorene
Hexachlorobenzene*
Hexachloroethane
Indeno(1,2,3-cd) pyrene
Isophorone
Isoquinoline
|
2-Methylanthracene
2-Methyl-4,6-Dinitrophenol
4,5-Methylenephenanthrene
1-Methyl-9H-Fluorene
1-Methylphenanthrene
1-Methylpyrene
Naphthalene
Nitrobenzene
2-Nitrophenol
4-Nitrophenol
N-Nitroso-Diphenylamine
N-Nitroso-Di-n-Propyl Amine
Phenanthrene
Pyrene
Pentachloronitrobenzene
Pentachlorophenol
Phenanthridine
Phenol
Quinoline
2,3,5,6-Tetramethylphenol
1,2,4-Trichlorobenzene
2,4,6-Trichlorophenol
2,4,6-Trimethylphenol
2,3,6-Trimethylnaphthalene
|
| Carbon |
| Carbonate |
Organic carbon |
Total carbon
|
|
Spatial Distribution Survey
The Spatial Distribution Survey adds improved geographic coverage with
particular emphasis on the assessment of priority constituents identified
in the Occurrence Survey. Occurrence-Survey results affect the analytical
strategy and the geographic distribution of sampling sites. The combined
data from the two phases of sampling provide a basic description of spatial
distribution for each Study Unit, with emphasis on priority constituents,
and support initial evaluation of sources and biological availability for
priority constituents.
Site Selection
Typically, 20-30 sites are sampled and include selected sites also sampled
during the Occurrence Survey. The general goals in site selection for the
Spatial Distribution Survey are to attain improved representation of the
most important Environmental Settings in the Study Unit by appropriate
addition of Indicator Sites and to attain adequate spatial resolution in
priority main-stem channels and major tributaries by addition of Integrator
Sites. Large areas with low contaminant levels and low variance require
few sites. Compared to the Occurrence Survey, some parts of a Study Unit
may require a significant increase in site density to assess priority
contaminants.
Sampling Strategy
The sampling strategy for bed sediment in the Spatial Distribution Survey
is the same as that for the Occurrence Survey. For tissue studies, samples
of more than one species (preferably a fish and an invertebrate species)
are collected at as many as 50 percent of the Spatial Distribution Survey
sites. This collection ensures that some fish data are obtained for
comparison with data from other programs and maximizes the comparability of
data among sites within a Study Unit (where a single taxon may not be
present at all sites).
Sample Analyses
Sample analyses are the same as those for the Occurrence Survey (table 9) except that the scope is reduced on the
basis of results of the Occurrence Survey. To the extent that analytical
methods allow, costly analyses are limited to contaminants that are most
significant in the Study Unit or for National Synthesis.
Continue to Ecological
Studies, Glossary, or return to Contents
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