Water, bed sediment, and invertebrate tissue were sampled in streams from Butte to near Missoula, Montana, as part of a monitoring program in the Clark Fork Basin. The sampling program was completed by the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, to characterize aquatic resources in the Clark Fork Basin and monitor trace elements associated with historical mining and smelting activities. Sampling sites were on the Clark Fork River and a subset of its tributaries. Water samples were collected periodically at 22 sites from October 2019 through September 2020. Bed-sediment and tissue samples were collected once at 12 sites during July 2020.
Water-quality data included concentrations of major ions, dissolved organic carbon, nitrogen (nitrate plus nitrite), trace elements, and suspended sediment. Daily values of turbidity were determined at four sites. Bed-sediment data included trace-element concentrations in the fine-grained (less than 0.063 millimeter) fraction. Biological data included trace-element concentrations in whole-body tissue of selected aquatic benthic invertebrates. Statistical summaries of water-quality, bed-sediment, and invertebrate tissue trace-element data for sites in the Clark Fork Basin were provided for the period of record: March 1985–September 2020.
Clark, G.D., Hornberger, M.I., Hepler, E.J., and Heinert, T.L., 2022, Results of water-quality, bed-sediment, and invertebrate tissue trace-element concentrations for tributaries in the Clark Fork Basin, Montana, October 2019–September 2020: U.S. Geological Survey data release,
U.S. Geological Survey, 2022, USGS water data for the Nation: U.S. Geological Survey National Water Information System database,
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Multiply | By | To obtain |
Length | ||
---|---|---|
mile (mi) | 1.609 | kilometer (km) |
Area | ||
square mile (mi2) | 2.590 | square kilometer (km2) |
Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:
°F=(1.8×°C)+32
Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).
Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or micrograms per liter (µg/L).
Pore size is given in micrometers (µm).
Suspended-sediment sizes are given in millimeters (mm) and weights are given in grams (g) of dried material.
Bottle capacities or liquid measurements are given in milliliters (mL).
Liquid-phase trace-element concentrations are given in micrograms per milliliter (µg/mL), and solid-phase concentrations are given in micrograms per gram (µg/g).
A water year is the 12-month period from October 1 through September 30 and is designated by the calendar year in which it ends. For example, water year 2020 is the period from October 1, 2019, through September 30, 2020.
certified reference material
inductively coupled plasma-mass spectrometry
laboratory reporting level
minimum reporting level
National Institute of Standards and Technology
National Water Information System
National Water Quality Laboratory
relative standard deviation
species
standard reference material
U.S. Geological Survey
The Clark Fork originates near the town of Warm Springs in western Montana at the confluence of Silver Bow and Warm Springs Creeks (
Location of the study area in the Clark Fork Basin, Montana.
Figure 1. Map showing location of the study area in the Clark Fork Basin, Montana.
Copper, gold, silver, and lead ores were extensively mined, milled, and smelted in the drainages of Silver Bow and Warm Springs Creeks from about the 1860s to the 1980s. Moderate- and small-scale mining also took place in the basins of most of the major tributaries to the upper Clark Fork Basin. Tailings produced during past mineral processing commonly contained large quantities of trace elements such as arsenic, cadmium, copper, lead, and zinc. Eroded tailings mixed with stream sediment and deposited downstream in stream channels, on flood plains, in the Warm Springs Ponds, and at the former Milltown Reservoir (
Concern about the toxicity of trace elements to the aquatic ecosystem and human health has resulted in a comprehensive effort by State, Federal, Tribal, and private entities to characterize the aquatic resources in the Clark Fork Basin. This effort was designed to guide and monitor remedial activities and to evaluate the effectiveness of remediation and cleanup. Water-quality data have been collected by the U.S. Geological Survey (USGS) in the Clark Fork Basin since 1985 (
The purpose of this report is to present water-quality data from samples collected at 22 sites and bed-sediment and biological data from samples collected at 12 sites in the Clark Fork Basin from October 2019 through September 2020 (
Sampling sites for the monitoring program in the Clark Fork Basin are from Butte to near Missoula (
Table 1. Type and period of data collection at sampling sites in the Clark Fork Basin, Montana, through September 2020.
[Dates are listed in month/year format. USGS, U.S. Geological Survey; --, no data; P, present (September 2020); D, discontinued]
USGS site number ( |
USGS site name | Continuous-record streamflow | Periodic |
Daily suspended sediment | Daily turbidity | Fine-grained bed sediment2 | Tissue2 |
12323230 | Blacktail Creek at Harrison Avenue, at Butte | -- | 03/1993–08/1995, 12/1996–08/2003, 12/2004–06/2020 | -- | -- | -- | -- |
12323233 | Blacktail Creek above Grove Gulch, at Butte | 06/2020–P | 06/2020–P | -- | -- | -- | -- |
12323242 | Silver Bow Creek at Montana Street, at Butte | 06/2020–P | 06/2020–P | -- | -- | -- | -- |
12323250 | Silver Bow Creek below Blacktail Creek, at Butte | 10/1983–P | 03/1993–08/1995, |
-- | -- | -- | -- |
12323600 | Silver Bow Creek at Opportunity | 07/1988–P | 03/1993–08/1995, |
03/1993–09/1995, D | -- | 07/1992–P | 07/1992, 08/1994–08/1995, 08/1997–P |
12323670 | Mill Creek near Anaconda | 10/2004–P | 12/2004–P | -- | 06/2006–09/2012, D | -- | -- |
12323700 | Mill Creek at Opportunity | 04/2003–P | 03/2003–P | -- | 04/2013–P | -- | -- |
12323710 | Willow Creek near Anaconda | 03/2005–P | 12/2004–P | -- | 06/2006–09/2012, D | -- | -- |
12323720 | Willow Creek at Opportunity | 04/2003–P | 03/2003–P | -- | 04/2013–P | -- | -- |
12323750 | Silver Bow Creek at Warm Springs | 03/1972–09/1979, |
03/1993–P | 04/1993–09/1995, D | -- | 07/1992–P | 07/1992–P |
12323760 | Warm Springs Creek near Anaconda | 10/1997–P | 10/2005–P | -- | 05/2006–09/2012, D | -- | -- |
12323770 | Warm Springs Creek at Warm Springs | 10/1983–P | 03/1993–P | -- | 04/2013–P | 08/1995, 08/1997, 08/1999, 08/2002, 08/2005, 08/2008, 08/2011, 08/2014, 08/2017, 07/2020 | 08/1995, 08/1997, 08/1999, 08/2002, 08/2005, 08/2008, 08/2011, 08/2014, 08/2017, 07/2020 |
12323800 | Clark Fork near Galen | 07/1988–P | 07/1988–P | -- | -- | 08/1987, 08/1991–P | 08/1987, 08/1991–P |
12323840 | Lost Creek near Anaconda | 10/2004–P | 12/2004–P | -- | 05/2006–P | -- | -- |
12323850 | Lost Creek near Galen | 04/2003–P | 03/2003–P | -- | -- | -- | -- |
461415112450801 | Clark Fork below Lost Creek, near Galen | -- | -- | -- | -- | 08/1996–P | 08/1996–P |
461559112443301 | Clark Fork at county bridge, near Racetrack | -- | -- | -- | -- | 08/1996–P | 08/1996–P |
461903112440701 | Clark Fork at Dempsey Creek diversion, near Racetrack | -- | -- | -- | -- | 08/1996–P | 08/1996–P |
12324200 | Clark Fork at Deer Lodge | 10/1978–P | 03/1985–P | 03/1985–08/1986, 04/1987–03/2003, 08/2003–2014, D | 03/2016–09/2016, D | 08/1986–08/1987, |
08/1986–08/1987, |
12324400 | Clark Fork above Little Blackfoot River, near Garrison | 02/2009–P | 03/2009–P | -- | -- | 08/2009–08/2019 | 08/2009–08/2019 |
12324680 | Clark Fork at Goldcreek | 10/1977–P | 03/1993–P | -- | -- | 07/1992–P | 07/1992–P |
12331800 | Clark Fork near Drummond | 04/1993–P | 03/1993–P | -- | -- | 08/1986, 08/1987, |
08/1986, 08/1991–08/2019 |
12334550 | Clark Fork at Turah Bridge, near Bonner | 03/1985–P | 03/1985–P | 03/1985–03/2003, |
-- | 08/1986, 08/1991–P | 08/1986, 08/1991–P |
12340000 | Blackfoot River near Bonner | 10/1939–P | 03/1985–P | 07/1986–04/1987, |
-- | 08/1986, 8/1987, 08/1991, 08/1993–08/1996, 08/1998–08/2001, 09/2003, |
08/1986, 08/1987, 08/1991, 08/1993, 08/1996, 08/1998, 09/2000, 09/2003, |
12340500 | Clark Fork above Missoula | 03/1929–P | 07/1986–P3 | 07/1986–04/1987, 06/1988–01/1996, 03/1996–03/2003, 08/2003–09/216, D | 04/2007–09/2007, D | 08/1997–P | 08/1997–P |
Onsite measurements of physical properties and laboratory analyses for selected major ions, trace elements, and suspended sediment. Before March 1993, laboratory analyses included only trace elements and suspended sediment. Beginning in 2012, dissolved organic carbon and turbidity analyses were included at selected sites. Beginning in 2013, nutrient sample analyses were included for two sites (12323230 and 12323250) near Butte, Montana.
Once annual laboratory analyses of fine-grained bed sediment and aquatic benthic insect tissue for trace elements.
Before October 1989, water-quality data for Clark Fork above Missoula included only suspended-sediment data.
Properties measured onsite and constituents for which water, bed-sediment, and biota samples were analyzed are listed in
Table 2. Properties and constituents measured onsite or analyzed in water, bed-sediment, and biota samples from the Clark Fork Basin, Montana.
Water | Bed sediment | Tissue | |
Property | Constituent | Constituent | Constituent |
Streamflow | Hardness (calculated) | Arsenic | Arsenic |
pH | Calcium | Cadmium | Cadmium |
Specific conductance | Magnesium | Chromium | Chromium |
Temperature | Potassium | Copper | Copper |
Turbidity | Sodium | Iron | Iron |
Alkalinity | Lead | Lead | |
Chloride | Manganese | Manganese | |
Fluoride | Nickel | Nickel | |
Silica | Zinc | Zinc | |
Sulfate | |||
Nitrate plus nitrite | |||
Cadmium | |||
Copper | |||
Iron | |||
Lead | |||
Manganese | |||
Zinc | |||
Arsenic | |||
Dissolved organic carbon | |||
Suspended sediment |
Table 3. Data-quality objectives for analyses of water samples collected in the Clark Fork Basin, Montana.
[lab, laboratory; NTRU, nephelometric turbidity ratio unit; --, not determined; mg/L, milligram per liter; µg/L, microgram per liter; mm, millimeter]
Constituent | Data-quality objectives | ||
Detectability | Precision | Bias | |
Laboratory reporting |
Maximum relative standard deviation of replicate analyses (percent) | Maximum deviation of spike recovery (percent) | |
Turbidity, unfiltered, lab | 2.0 NTRU | 20 | -- |
Calcium, filtered | 0.04 mg/L | 20 | -- |
Magnesium, filtered | 0.02 mg/L | 20 | -- |
Potassium, filtered | 0.6 mg/L | 20 | -- |
Sodium, filtered | 0.8 mg/L | 20 | -- |
Alkalinity, filtered, lab | 8.0 mg/L | 20 | -- |
Chloride, filtered | 0.04 mg/L | 20 | -- |
Fluoride, filtered | 0.02 mg/L | 20 | -- |
Silica, filtered | 0.1 mg/L | 20 | -- |
Sulfate, filtered | 0.04 mg/L | 20 | -- |
Nitrate plus nitrite, filtered | 0.02 mg/L | 20 | -- |
Cadmium, filtered | 0.06 µg/L | 20 | 25 |
Cadmium, unfiltered recoverable | 0.06 µg/L | 20 | 25 |
Copper, filtered | 0.8 µg/L | 20 | 25 |
Copper, unfiltered recoverable | 0.8 µg/L | 20 | 25 |
Iron, filtered | 10 µg/L | 20 | 25 |
Iron, unfiltered recoverable | 10 µg/L | 20 | 25 |
Lead, filtered | 0.04 µg/L | 20 | 25 |
Lead, unfiltered recoverable | 0.12 µg/L | 20 | 25 |
Manganese, filtered | 0.8 µg/L | 20 | 25 |
Manganese, unfiltered recoverable | 0.8 µg/L | 20 | 25 |
Zinc, filtered | 4 µg/L | 20 | 25 |
Zinc, unfiltered recoverable | 4 µg/L | 20 | 25 |
Arsenic, filtered | 0.2 µg/L | 20 | 25 |
Arsenic, unfiltered recoverable | 0.2 µg/L | 20 | 25 |
Organic carbon, filtered | 0.46 mg/L | 20 | -- |
Sediment, suspended, percentage finer than 0.062 mm | 1 percent | 20 | -- |
Sediment, suspended | 1 mg/L | 20 | -- |
Quality assurance of data was maintained using documented procedures described in the following sections. These quality-assurance data were designed to provide environmentally representative data. Acceptable results of the procedures were verified with quality-assurance samples that were collected systematically to provide a measure of the accuracy, precision, and bias of the environmental data and to identify variability associated with sampling, processing, or analysis.
Water-quality data consist of onsite stream properties and laboratory determination of concentrations of chemical and physical constituents (listed in
Water samples were collected and composited from vertical transits throughout the stream depth at multiple locations along the channel cross section using depth- and width-integration methods described by
Instantaneous streamflow was determined at the time of water sampling either by direct measurement or from stage-discharge rating tables (
Concentrations of arsenic, cadmium, copper, lead, manganese, and zinc in filtered samples (0.45-micrometer [µm] pore size) were measured using inductively coupled plasma-mass spectrometry (ICP–MS;
Water samples for analysis of suspended sediment were collected from multiple vertical transits when periodic water samples were collected. Water samples were analyzed for suspended-sediment concentration and the percentage of suspended-sediment mass finer than a 0.062-millimeter (mm) diameter (silt size and smaller) by the USGS Wyoming-Montana Water Science Center Sediment Laboratory (hereinafter referred to as the “Wyoming-Montana Sediment Laboratory”) in Helena, Mont., according to methods described by
Continuous turbidity was measured from early spring (after ice breakup) to early fall (before stream freezeup) using model 6136 turbidity sensors (YSI, Inc, Yellow Springs, Ohio) at four tributary sites in the upper Clark Fork Basin near Anaconda (
Water-quality data from samples collected periodically during water year 2020 (October 1, 2019, through September 30, 2020) are listed in the accompanying data release (“table_4_water_quality_data_Clark_Fork_Basin.xlsx” in
Quality-assurance procedures used for the collection and field processing of water samples are described by
The quality of analytical results reported for water samples was evaluated using quality-control samples that were sampled and analyzed concurrently with primary environmental samples. These quality-control samples consisted of replicates, spikes, and blanks that provided quantitative information on the precision and bias of the overall field and laboratory processes. The number of quality-control samples represented about 15 percent of the total number of water samples.
Replicate data provided an assessment of the precision (reproducibility) of analytical results and collection variability (
Precision of analytical results for field replicates can be affected by numerous sources of variability within the field and laboratory environments, including sample collection, processing, and analysis. Overall precision for samples exposed to field and laboratory sources of variability were provided by obtaining replicate stream samples for chemical analysis by splitting a composite stream sample. Replicate stream samples for suspended-sediment analysis were obtained in the field by collecting two independent cross-sectional samples. Analyses of field replicate samples indicated the reproducibility of environmental data that were affected by the combined potential variability introduced by field and laboratory processes.
In addition to analyzing quality-control samples submitted from the field, internal quality-assurance practices are completed systematically by the NWQL to provide quality control of analytical procedures (D.L. Stevenson, U.S. Geological Survey, written commun., 2012). These internal practices include analyses of quality-control samples such as calibration standard samples, standard reference water samples, replicate samples, deionized-water blank samples, or spiked samples at a proportion equivalent to at least 10 percent of the samples loaded. The NWQL participates in a blind-sample program in which standard reference water samples prepared by the USGS Quality Systems Branch are routinely inserted into the sample line for each analytical method at a frequency proportional to the sample load. The laboratory also participates in external evaluation studies and audits with the National Environmental Laboratory Accreditation Program, the U.S. Environmental Protection Agency, Environment and Climate Change Canada, and the USGS Quality Systems Branch to assess analytical performance.
Precision of analytical results for laboratory replicates, which exclude field sources of variability, was determined using two independent chemical analyses of aliquots from a single sample selected from the group of samples constituting each analytical run. A separate analysis of the sample was completed at the beginning and end of each analytical run to provide information on the reproducibility of laboratory analytical results independent of variability caused by field sample collection and processing. Laboratory replicates of suspended-sediment samples were not obtainable because the samples were consumed during the analysis.
Spiked samples were used to evaluate bias, which measures the ability of an analytical method to accurately quantify a known amount of analyte added to a sample. In the laboratory, deionized-water blank samples and aliquots of stream samples were spiked with known amounts of the same trace elements for which water samples were being analyzed. Analyses of spiked blanks indicated if the spiking procedure and analytical method are within control for water that is presumably free of chemical interferences. Analyses of spiked aliquots of stream samples indicated if the chemical matrix of the stream water interferes with the analytical measurement and if these interferences could contribute substantial bias to reported trace-element concentrations for stream samples.
Field blank samples were submitted for every field trip and analyzed to identify the presence and magnitude of contamination that could bias analytical results. Field blanks consisted of deionized water that is certified as constituent free and is processed in the field through clean sampling equipment used to collect stream samples. These blanks were subjected to the same processing (sample splitting, filtration, preservation, transportation, and laboratory handling) as stream samples. Blank samples were analyzed for the same constituents as stream samples to detect contamination.
All water samples were handled in accordance with chain-of-custody procedures that provide documentation of sample identity, shipment, receipt, and laboratory handling (
Data-quality objectives (
The precision of analytical results was determined by calculating the standard deviation of the differences in concentrations between replicate analyses. These replicate analyses consisted of pairs of field replicate samples and laboratory replicates. Standard deviations were calculated according to the following equation (
is the standard deviation of the difference in concentration between replicate analyses,
is the difference in concentration between each pair of replicate analyses, and
is the number of pairs of replicate analyses.
Precision also was expressed as a relative standard deviation (RSD), in percent, which was computed from the standard deviation and the mean concentration for all the replicate analyses. Expressing precision relative to a mean concentration standardized the comparison of precision among individual constituents. The RSD was calculated according to the following equation (
is the relative standard deviation,
is the standard deviation, and
is the mean concentration for all replicate analyses.
Sample results and the corresponding field replicate data are listed in the accompanying data release (“table_9_results_field_replicates_Clark_Fork_Basin.xlsx” in
Recovery efficiencies for constituent analyses were determined by comparing a sample and a spiked aliquot of the same sample. The data-quality objective for acceptable spike recoveries of trace elements in water samples was a maximum deviation of 25 percent from a theoretical 100-percent recovery of an added constituent. At the NWQL, a spiked deionized-water blank sample and a spiked aliquot of a stream sample were prepared and analyzed along with the original unspiked sample. The differences between the spiked and unspiked sample concentrations were determined and used to compute recovery, in percent, according to
is the spike recovery, in percent;
is the difference between the spiked and unspiked sample concentrations; and
is the concentration of material used to spike the sample.
If the spike recovery of a trace element was outside a range of 75 to 125 percent, the instrument was recalibrated, and the sample set and all spiked samples were reanalyzed for that element until recoveries were improved to the extent possible. Recovery efficiency for individual trace elements in spiked blank samples and in spiked stream samples is listed in the accompanying data release (“table_12_recovery_efficiency_deionized_water_blank_samples.xlsx” and “table_13_recovery_efficiency_stream_samples.xlsx” in
High or low bias is indicated if the 95-percent confidence interval does not include 100-percent recovery, thereby indicating a consistent deviation or bias, either high or low. Confidence intervals for percentage recovery include 100 percent for all laboratory-spiked blank samples (“table_12_recovery_efficiency_deionized_water_blank_samples.xlsx” in
Analytical results for field blanks are listed in the accompanying data release (“table_14_analyses_of_field_blanks_for_water_samples.xlsx” in
Constituent concentrations in field blanks almost always were less than the LRL. Two sample concentrations of chloride, filtered (0.04 microgram per liter [µg/L] and 0.06 µg/L), exceeded the LRL of 0.02 µg/L. Two sample concentrations of silica, filtered (0.10 mg/L and 0.08 mg/L), exceeded the LRL of 0.05 mg/L. One sample concentration of copper, filtered (1.4 µg/L), exceeded the LRL of 0.4 µg/L. Two sample concentrations of copper, unfiltered recoverable (1.1 µg/L, and 2.6 µg/L), exceeded the LRL of 0.4 µg/L. One sample concentration of iron, unfiltered recoverable (5.2 µg/L), exceeded the LRL of 5.0 µg/L. Two sample concentrations of organic carbon, filtered (0.26 mg/L and 0.30 mg/L), exceeded the LRL of 0.23 mg/L. No adjustments were made to water-quality sample data based on a review of these results.
Bed-sediment data for the long-term monitoring program in the Clark Fork Basin consisted of trace-element concentrations in the fine-grained (less than 0.063 mm) fraction of bed-sediment samples. Bed-sediment samples were collected once annually at 12 sites (
Fine-grained bed-sediment samples were collected in July 2020 using protocols described by
Individual samples of bed sediment were collected by scooping material from the surfaces of three to five randomly selected deposits along pools or low-velocity areas. The three to five individual samples were combined to form a single composite sample. This collection process was repeated three times to obtain three composite samples. Each composite sample was wet sieved onsite through a 0.063-mm polyester-mesh sieve using ambient stream water. The fraction of bed sediment in each composite sample that was finer than 0.063 mm was collected in an acid-washed 500-milliliter (mL) polyethylene bottle and transported to the laboratory on ice.
Bed-sediment samples were processed and analyzed at the USGS Metal Bioavailability Laboratory in Menlo Park, California. Bed-sediment samples were oven dried at 60 degrees Celsius and ground into smaller particle sizes using an acid-washed, ceramic mortar and pestle. Single aliquots of about 0.5–0.6 gram of sediment from each of the three composite bed-sediment samples were digested using a hot, concentrated nitric acid reflux according to methods described by
Solid-phase concentrations of trace elements measured in samples of fine-grained bed sediment collected during July 2020 are listed in the accompanying data release (“table_15_bed_sediment_data_Clark_Fork_Basin.xlsx” in
is micrograms of trace element per gram of sediment, by dry weight;
is micrograms per milliliter of liquid-phase trace element; and
is milliliters.
The reported solid-phase concentrations (
The USGS protocols for field collection and processing of bed-sediment samples are designed to prevent contamination from metal sources. Nonmetallic sampling and processing equipment (white plastic scoop, funnel-frame apparatus, and 500-mL sample bottles) was acid washed and rinsed with deionized water before the collection of the first sample. Polyester-mesh sieves were washed in laboratory-grade detergent and rinsed with deionized water. All equipment received a field rinse onsite with native water. Sampling equipment used at more than one site was rinsed thoroughly between sites with site-specific stream water. Separate sieves were used at each site and, therefore, did not require decontamination between sites.
Quality assurance of analytical results for bed-sediment samples included laboratory-instrument calibration with standard solutions and analysis of quality-control samples designed to identify the presence and magnitude of bias (Ellen V. Axtmann, U.S. Geological Survey, written commun., 1994). Quality-control samples consisted of standard reference materials (SRMs) issued by the National Institute of Standards and Technology (NIST) and procedural blanks. In total, 12 low-concentration SRMs, 12 high-concentration SRMs, and 13 procedural blanks were analyzed.
SRMs are commercially prepared materials that have certified concentrations of trace elements. Analyses of SRMs were used to indicate the ability of the method to accurately measure a known quantity of a constituent. Multiple analyses of SRMs were made to derive a mean and 95-percent confidence interval for recovery. Recovery efficiency for trace-element analyses of SRMs for bed sediment is listed in the accompanying data release (“table_16_recovery_efficiency_bed_sediment_samples.xlsx” in
Mean recoveries of arsenic, cadmium, chromium, copper, iron, lead, manganese, nickel, and zinc from SRM sample 2709 ranged from 49.0 to 92.1 percent (“table_16_recovery_efficiency_bed_sediment_samples.xlsx” in
Procedural blanks for bed-sediment samples consisted of the analysis of the same reagents used for sample digestion and reconstitution. Concentrated nitric acid used for sample digestion was heated and evaporated to dryness. After evaporation, 0.6 N hydrochloric acid was added to reconstitute the dry residue. Analytical results of procedural blanks for bed sediment were reported as a liquid-phase concentration, in micrograms per milliliter. A procedural blank was prepared and analyzed concurrently with bed-sediment samples for each site. Concentrations of trace elements in all procedural blanks were less than the MRL for all elements (“table_17_analyses_of_procedural_blanks_for_bed_sediment_samples.xlsx” in
Tissue data for the long-term monitoring program in the Clark Fork Basin consist of trace-element concentrations in the whole-body tissue of aquatic benthic invertebrates. Invertebrate samples were collected once annually at the same 12 sites and on the same dates as bed-sediment samples (
Insect samples were collected using protocols described in
Samples of each taxon were sorted by genus in the field and placed in acid-washed plastic containers. Samples were frozen in a small amount of ambient stream water on dry ice within 30 minutes of collection. Between 1986 and 1998, macroinvertebrate containers were kept on ice to allow the insects to evacuate their gut contents (depurate) for 6 to 8 hours. Excess water was drained, and insects were frozen for transport to the laboratory. Since 1999, samples were immediately frozen on dry ice in the field to reduce the possibility of metal loss through intracellular breakdown during depuration. A comparison of immediately frozen to depurated samples indicated that although no substantial difference was detected for most metals, concentrations of copper were about 20 percent lower in the depurated samples than in the samples that were immediately frozen. The data were not adjusted for this difference.
Invertebrate samples were processed and analyzed at the USGS Metal Bioavailability Laboratory in Menlo Park, Calif. Insects were thawed, rinsed with ultrapure deionized water to remove particulate matter, and then sorted to their lowest possible taxonomic level. If large numbers of specimens were collected at a site, similar-sized individuals were composited into replicate subsamples. Subsamples were placed in tared scintillation vials and oven dried at 70 degrees Celsius. Subsamples were weighed to obtain a final dry weight and digested by reflux using concentrated nitric acid (
Concentrations of trace elements in whole-body tissue of aquatic invertebrates collected during July 2020 are listed in the accompanying data release (“table_18_biological_data_Clark_Fork_Basin.xlsx” in
The USGS protocols for field collection and processing of tissue samples were designed to prevent contamination from metal sources. Nonmetallic nets, sampling equipment, and processing equipment were used in all sample collection. Equipment was acid washed and rinsed in ultrapure deionized water before the first sample collection. Nets and equipment were thoroughly rinsed in stream water at each main-stem site. Clean nets were used at each tributary site.
Quality control of analytical results for tissue samples included laboratory-instrument calibration with standard solutions and analyses of quality-control samples designed to quantify precision and to identify the presence and magnitude of bias. Quality-control samples consisted of 12 replicates of the certified reference material (CRM) TORT–3 (lobster hepatopancreas), which was purchased from the National Research Council Canada. Quality-control samples were analyzed in a proportion equivalent to about 20 percent of the total number of biota samples. Recovery efficiencies for trace-element analyses of the TORT–3 CRM are listed in the accompanying data release (“table_19_recovery_efficiency_biota_samples.xlsx” in
Mean CRM recoveries for TORT–3 ranged from 85.3 to 112 percent for all constituents except lead (“table_19_recovery_efficiency_biota_samples.xlsx” in
Procedural blanks for biota consisted of undiluted aliquots of the same reagents used to digest and reconstitute tissue of aquatic insects. Analytical results of procedural blanks for biota (“table_20_analyses_of_procedural_blanks_for_tissue_samples.xlsx” in
Statistical summaries of long-term water-quality, bed-sediment, and biological data for the Clark Fork Basin are provided in an accompanying USGS data release (“table_21_long_term_water_quality_data_Clark_Fork_Basin.xlsx,” “table_22_long_term_bed_sediment_data_Clark_Fork_Basin.xlsx,” and “table_23_long_term_biological_data_Clark_Fork_Basin.xlsx” in
The summaries do not include data for supplemental samples collected at selected sites that targeted high-flow conditions or maintenance drawdowns of Milltown Reservoir, which might disproportionately skew the long-term statistics relative to the other sites in the network. Sample results at sites that have been sampled for other projects can be accessed in the USGS NWIS database at
Statistics for long-term bed-sediment data (“table_22_long_term_bed_sediment_data_Clark_Fork_Basin.xlsx” in
In contrast, statistics for long-term tissue data (“table_23_long_term_biological_data_Clark_Fork_Basin.xlsx” in
The presence or absence of insect species at a given site can vary among years and may result in different taxa being analyzed in the long-term period of record. Because
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Helena, MT 59601
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