PIAHSProceedings of the International Association of Hydrological SciencesPIAHSProc. IAHS2199-899XCopernicus PublicationsGöttingen, Germany10.5194/piahs-375-11-2017The use of bed sediments in water quality studies and monitoring programsHorowitzArthur J.horowitz1@mindspring.comElrickKent A.U.S. Geological Survey, Norcross, Georgia, 30093, USADept. of Geosciences, Georgia State University, Atlanta, Georgia, 30303, USAretiredArthur J. Horowitz (horowitz1@mindspring.com)3March20173751117This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://piahs.copernicus.org/articles/375/11/2017/piahs-375-11-2017.htmlThe full text article is available as a PDF file from https://piahs.copernicus.org/articles/375/11/2017/piahs-375-11-2017.pdf
In most water quality monitoring programs, either
filtered water (dissolved) or suspended sediment (either whole water or
separated suspended sediment) are the traditional sample media of choice.
This results both from regulatory requirements and a desire to maintain
consistency with long-standing data collection procedures. Despite the fact
that both bed sediments and/or flood plain deposits have been used to
identify substantial water quality issues, they rarely are used in
traditional water quality monitoring programs. The usual rationale is that
bed sediment chemistry does not provide the temporal immediacy that can be
obtained using more traditional sample media (e.g., suspended sediment,
water). However, despite the issue of temporal immediacy, bed sediments can
be used to address/identify certain types of water quality problems and
could be employed more frequently for that purpose. Examples where bed
sediments could be used include: (1) identifying potential long-term
monitoring sites/water quality “hot spots”, (2) establishing a water
quality/geochemical history for a particular site/area, and (3) as a
surrogate for establishing mean/median chemical values for suspended
sediment.
Summary of Utoy Creek data from the < 63 µm fraction of bed sediment samples (n=47).
In typical environmental/water quality-monitoring programs the most common
samples collected and analyzed are filtered and unfiltered (whole) water
(e.g., ASTM, 2011a, b; APHA, 2012; Horowitz, 2013; USGS, various). The
former are used to determine “dissolved” constituents whereas the latter are
used to determine suspended sediment concentrations (SSCs)/grain-size
distributions, and sediment-associated chemical constituents by subtraction.
Although bed sediments have been used in large-scale (e.g., national)
environmental surveys, as well as for geochemical reconnaissance (e.g.,
mineral exploration), they rarely are used in traditional water
quality-monitoring programs (e.g., Hawkes and Webb, 1962; Webb et al.,
1978; Fauth et al., 1985; Otteson et al., 2000; Gustavsson et al., 2001).
However, of all the potential sample media, bed sediments probably are the
easiest to collect and process, and are least likely to suffer from
contamination or insufficient sample mass/analytical detection issues.
Hence, their lack of use is somewhat surprising, but results from several
factors. First, since the publication of the Hawkes and Webb (1962) treatise
on geochemical exploration, as well as the publication of several national
geochemical/environmental atlases (e.g., Webb et al., 1978; Fauth et al.,
1985; Otteson et al., 2000), bed sediments have been used to detect
geochemical/environmental spatial differences, but rarely are viewed as
sensitive to short-term (geo)chemical variations; whereas the latter are the
usual goal of most monitoring programs. Second, bed sediment surveys
typically employ grain-size limited aliquots (e.g., < 180 µm)
to facilitate spatial comparisons. That traditional grain-size range has
little connection with suspended sediment grain-size distributions; hence,
bed sediments have never been viewed as surrogates for suspended sediment.
However, recent surveys using the < 63 µm fraction of the
upper 1 cm of bed sediments, collected after low-flow periods (e.g.,
Horowitz et al., 1999, 2001a, b, 2012, 2014), or surficial bed sediments collected from exposed
surfaces after storms or floods (e.g., Van Metre et al., 2006; Horowitz et al., 2014),
do appear to reasonably reflect suspended sediment chemical
composition. Hence, grain-size limited bed sediment aliquots may represent a
useful media for non-traditional monitoring programs intended to detect
event-related effects, or long-term environmental/water quality-changes
(e.g., decadal climate/land use-changes), provided samples are collected and
analyzed every 5 to 10 years, or to determine mean/median suspended sediment
associated chemical concentrations as a means of establishing first order
approximations of fluvial fluxes of suspended sediment-associated chemical
constituents (e.g., Horowitz et al., 2012, 2014).
Additionally, the chemical analysis of cored bed sediment samples, in
conjunction with absolute age dating, can be used to reconstruct the water
quality/geochemical history of an impoundment (e.g., Horowitz et al., 1988, 1995; Grosbois et al., 2001;
Van Metre and Horowitz,
2013; Gray et al., 2015). Three examples of water quality studies that
employed bed sediments in lieu of more traditional sample media, as
a proof of concept, are described herein.
Utoy Creek, Atlanta, Georgia
Utoy Creek is a small urban stream that flows through the City of Atlanta,
Georgia as well as well as being a tributary of the Chattahoochee River (e.g., Horowitz
and Hughes, 2006; Horowitz, 2009; West Atlanta Watershed Alliance, 2016).
The creek is some 42 km long and the watershed encompasses some 89 km2
(West Atlanta Watershed Alliance, 2016). In 2015 representatives from Fulton
County contacted the authors with a request to evaluate water quality in the
watershed with a view to identifying potential contaminants and to recommend
long-term monitoring sites. The creek already had been listed as impaired as
a result of elevated dissolved Zn levels (Andrew Mycroft, personal communication, 2015).
Methods
Bed sediment samples were manually collected during a week in late 2014
after an extended period of low-flow at roughly equidistant intervals
covering the entire watershed; additionally, major tributaries to Utoy Creek
also were sampled. Sampling procedures were similar to those used by
Horowitz et al. (2012) and entailed collecting multiple equal volume
aliquots collected from the upper 1 cm of the creek bed and/or the still wet
floodplain at each site. The aliquots for each site were composited in the
field, stored in clear plastic bags, and refrigerated until they could be
returned to the laboratory. In total, 47 samples were collected, of which 5
were replicates to evaluate sampling precision.
All the samples were processed and analyzed following the procedures
outlined in Horowitz et al. (2012). Each sample was oven-dried at 105 ∘C.
Representative aliquots of each composite were obtained by coning and
quartering the dried sediment and then wet-sieved through a 63 µm
non-contaminating nylon and clear plastic sieve to obtain representative
subsamples for subsequent chemical analysis. A selected group of trace
elements (Ag, Al, Cd, Co, Cu, Fe, Pb, Ti, and Zn) were determined using AAS
after a HF/HClO4/aqua regia digestion in teflon beakers at
200 ∘C. TOC was determined by combustion using a Carlo-Erba C / N analyzer.
Results and discussion
With the exception of Cd, all the other constituents determined contained
elevated individual as well as median levels that exceeded national baseline
concentrations (Table 1; Horowitz and Stephens, 2008). Whilst the median
concentration for Cd did not exceed national baseline levels, individual
samples did.
Of particular interest to Fulton County was that whilst most of the samples
contained elevated major and trace element, as well as TOC concentrations,
the samples displaying the most elevated levels all clustered around one
particular location in the watershed. That location is near the mouth of
Utoy Creek where it discharges to the Chattahoochee River, and also is the
site of a number of medium to light industries, including a galvanizing
plant (Fig. 1). The survey also indicated that Utoy Creek is likely to
display impaired dissolved concentrations for a number of constituents other
than Zn (e.g., Cu, Co, TOC). Finally, based on this survey, potential new,
long-term monitoring also sites were identified.
Plots of chemical data from the < 63 µm fraction of
bed sediments from Utoy Creek, Georgia, USA. There are similar plots for
Zn/Fe, Zn/TOC, Zn/Al, and Zn/Co.
Plots of chemical data from two sediment cores collected in
impoundments along the Spokane River, Washington, USA.
Lake Coeur d'Alene, Idaho and the Spokane River
Lake Coeur d'Alene (CDA) is a natural (submerged river bed) lake in the
northern panhandle of Idaho that formed from the outwash from Lake Missoula
during the last interglacial (e.g., Hobbs et al., 1965). The lake lies
between the Selkirk and the CDA Mountains and extends northward from the St.
Joe River to the headwaters of the Spokane River near the city of CDA, Idaho
(Meckel Engineering et al., 1983; Bender, 1991). The main body of the lake
is about 3.2 km wide by 40 km long, and up to 150 m deep in the thalweg
(Meckel Engineering et al., 1983). The Spokane River flows downstream from
the lake, passes through a series of dams, and eventually discharges into
the Columbia River, some 180 km downstream.
The South Fork of the CDA River drains a substantial part of the CDA mining
district and the so-called “Silver Valley”. The mining district has been in
operation since the 1880's and was one of the major sources of Ag, Pb, and
Zn in the US (e.g., Bender, 1991). Most of the mining and ore-processing
wastes were discharged directly into the South Fork of the CDA River (e.g.,
Horowitz et al., 1993). As late as 1964, estimates indicated that some 2200
tonnes/day of mining and processing wastes still were entering the South
Fork (Reece et al., 1978). It also has been estimated that during the course
of mining, processing, and smelting operations in the area, some 115 million
tonnes of mine tailings were produced and that over 60 % of this material
probably entered the South Fork and the CDA River system (Javorka, 1991).
These materials were highly enriched in Ag, As, Cd, Cu, Fe, Hg, Mn, Pb, Sb,
and Zn (Rabe and Bauer, 1977; Bender, 1991). In 1968, tailings ponds were
established to limit sediment dispersion and downstream transport
(Horowitz et al., 1993). A series of studies conducted between 1989 and
1999 indicated that Lake CDA contained some 75 million tonnes of trace
element rich sediment, and that elevated sediment-associated constituents
could be traced all the way to the Columbia River in the state of Washington
(e.g., Horowitz et al., 1993, 1995; Grosbois et al.,
2001).
The local mining companies, as well as the Northwest Mining Association,
contended that the majority of the impacted sediment resulted from natural
weathering processes associated with exposed segments of the ore body. To
address this issue, a series of gravity cores were collected in Lake CDA and
in the Spokane River system (Horowitz et al., 1995; Grosbois et al., 2001).
The analytical data from the cores, in conjunction with absolute age dating
based on 137Cs and 210Pbex, clearly indicates that the onset
of sediment-associated trace element enrichment in Lake CDA and in the
Spokane River began somewhere around 1910±20 years, which is roughly
contemporaneous with the onset of mining and smelting operations in the
region (Fig. 2). Note that the onset of sediment associated-trace element
enrichment of the most downstream core that was collected in the Spokane
River arm of Lake Roosevelt began around 1930 (Fig. 2; Grosbois et al.,
2001). That is contemporaneous with the initial closing of the Grand Coulee
Dam, downstream from the Spokane River, on the Columbia River that also
caused the formation of Lake Roosevelt and the Spokane River arm (e.g.,
Grosbois et al., 2001). Also note that the sediment-associated trace element
concentrations, at least in the Spokane River, began to decline around 1970;
that is contemporaneous with the construction of the tailings ponds along
the South Fork of the CDA River. This would indicate that bed sediment
chemistry does appear to respond fairly quickly to land use changes and or
remediation measures. The CDA ore body has been dated as Proterozoic
(> 500 million years BP; Hobbs et al., 1965), whereas Lake CDA
has been dated as forming some 14 000 years BP (Wyman, 1993); as a result, it is
highly unlikely that the vast majority of the trace element-rich sediments
in Lake CDA, and in the Spokane River system, resulted from natural
weathering processes.
The U.S. National Coastal River Survey
Between 2010 and 2011, the U.S. Geological Survey (USGS) carried out a bed
sediment-associated chemical survey using material collected at or near the
mouths of 132 coastal rivers in the US. The primary objective of the study
was to use the chemical data generated from the samples to develop
first-order approximations of the chemical fluxes emanating from the US to
the coastal zone (Horowitz et al., 2012). The methods employed were
essentially the same as those used in the Utoy Creek survey described
earlier.
Comparison between long-term suspended sediment-associated chemical levels with the
< 63 µm fraction of bed sediment-associated chemical levels collected
during National Coastal Survey.
AlSbAsBaBeCdCrCoCuFePbLiMnHgMoNiTPSeAgSrVZnTiTOCTCTNSample Name(%)(mg kg-1) (%)(mg kg-1) (%) Mississippi River@ Belle Chasse, LA∗n99999999999999999919999699Min.2.40.45.63100.80.5406211.313248000.012308900.5< 0.516039910.121.41.90.20Med.5.80.89.55801.70.76513242.7243812000.0523412001.2NA210831200.352.02.90.45Max.6.70.9116202.12.27318273.4314616000.1154250002.40.7360981500.443.2131.9Bed Sediment6.10.8116201.70.96015233.0283713000.0823210000.7< 0.5150911100.441.41.80.16Wax Lake Outlet@ Calumet, LA∗n99999999999997999939999999Min.2.20.34.13600.60.3254111.17173700.041138000.4< 0.514037480.111.01.60.20Med.6.50.8105901.90.57114243.2244213000.0723611001.0NA200951200.382.22.80.40Max.7.10.9126202.20.67916373.5265414000.1534015001.40.94101101500.413.6120.61Bed Sediment6.40.8106301.90.56216233.3264115000.051359800.6< 0.5140971200.441.31.70.14Lower Atchafalaya River@ Morgan City, LA∗n99999999999999999929999999Min.2.30.24.73700.70.3264111.27185000.021147900.5< 0.515037540.110.91.60.20Med.6.50.8115801.90.46914233.4224313000.0523511001.1NA230981200.372.23.00.40Max.7.40.8126502.30.68217343.7275616000.1134213001.50.54201101500.433.0110.56Bed Sediment5.70.78.16501.50.35211162.520308900.021247600.4< 0.514077810.420.81.10.08Rio Grande nr Brownsville, TXn40404040393939404040404040387394040114040404034357Min.1.90.57.0850.50.116380.910226500.042128000.4< 0.534027490.091.63.10.21Med.5.01.6104001.40.4377172.3254415000.1132212000.7NA660651100.232.85.00.30Max.6.89.8164902.03.416013415.81505141000.42218818002.62.61500902500.334.19.70.65Bed Sediment5.70.97.54801.70.3378142.719374900.051186400.3< 0.539074860.320.63.30.08Colorado River @ N.I.B. aboveMorelos Dam nr Andrade, CAn2525252525212525252525252516725252522525252514142Min.2.00.34.41600.60.136381.18214900.026204000.3< 0.531025390.120.40.90.32Med.4.21.57.15101.00.2658162.0183014000.048387000.9NA47054620.271.11.90.41Max.5.229266101.71.712012413.0435445000.26117614003.43.6910711400.362.23.90.50Bed Sediment4.31.2136801.30.44610382.5343741000.092259801.20.549065910.311.63.80.25Columbia River near BeaverArmy Terminal, ORn10110110110110199951011011011011011019331951011018101101101101899733Min.5.40.22.63801.00.1219342.87155900.021158600.1< 0.524063690.310.50.60.06Med.7.80.97.15401.40.75818504.3222611000.0723613000.4NA3201101600.502.22.30.32Max.9.550196702.01.5200221505.68510021000.30257526001.11.44501503400.666.16.21.1Bed Sediment8.01.09.66901.81.26114473.466264500.06< 1255800.3< 0.53201402200.651.51.60.14
NA – not available∗ The small numbers of samples for these sites are due to a change in the most
downstream sampling location for the Mississippi River from St. Francisville to Belle Chasse, and on the Atchafalaya
The basic premise of the study is that the chemical analyses of the
< 63 µm fraction of the upper 1 cm of riverbed sediments can
serve as a surrogate for the average chemical composition of recent
suspended sediments transiting each site. To evaluate the validity of that
premise, minimum, maximum, and median suspended sediment-associated chemical
concentrations determined between 1994 and 2006, as part of the revised
NASQAN Program, determined for separated suspended sediment and analyzed
utilizing the same procedures used in this study (Horowitz et al., 2001a,
b), were compared with the chemical concentrations generated from the
< 63 µm fraction of the collected coastal river samples
(Table 2; Horowitz et al., 2012). Based on that comparison, it appears that
the premise is valid for the majority of the trace/major elements determined
in the study. On the other hand, the data for carbon (TOC and TC), nitrogen
(TN), and phosphorus (TP) are more ambiguous (Table 2). The bed sediment
values for these constituents tend to fall either just below or just within
the minimum levels for suspended sediment. This difference may be a
reflection of the relatively small number of samples available for the
comparison (especially for TN, and the Mississippi River Basin sites).
However, the difference also may be the result of chemical/biological
post-depositional remobilization that could reduce bed sediment-associated
nutrient concentrations. As a result, regardless of the cause, the carbon
and nutrient concentrations/annual fluxes determined from the < 63 µm
fraction of the bed sediments collected and analyzed during
this study should be viewed as minimums.
Conclusions
Based on the three studies cited, it is possible to draw some conclusions
about the efficacy of using bed sediment-associated major/trace element and
nutrient concentrations in water quality studies and monitoring programs. It
does appear as if bed sediment chemical data, particularly that generated
from the < 63 µm fraction of the upper 1 cm, can be used for
a variety of water quality studies and monitoring programs provided that
short term variations and/or temporal immediacy are not of primary concern,
e.g.: for reconnaissance surveys to identify potential monitoring sites
and/or “hotspots” (Utoy Creek); to establish historical reconstructions of
the water quality/geochemical history of an area (Lake CDA and the Spokane
River); and as surrogates for determining the mean/median concentrations of
suspended sediment-associated trace/major elements and nutrients (National
Coastal Survey). In the latter case, bed sediment-surrogates may provide a
means of establishing long term-trends that could be associated with
decadal-long processes such as climate change, without having to resort to
much more resource intensive intra- and interannual sampling and analysis
programs.
Individual data are available from the author(s) on request.
The authors declare that they have no conflict of interest.
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