Variability of dissolved trace metals in the mississippi river

0016~7037/87/s3.00

Geochimica ei Cosmochimica Acta Vol. 51, pp. 3273-3217 B Pcrgamon Journals Ltd. 1987. Printed in U.S.A.

+ NJ

Variability of dissolved trace metals in the Mississippi River ALAN M. SHILLER*and EDWARDA. BOYLE Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technofogy, Cambridge, MA 02 139, U.S.A. (Received June 23, 1987; accepted in revisedform September 17, 1987) Abatraet-The concentrations of eight dissolved trace metals were measured in the lower Mississippi River seven times at various flow stages during a two-year interval. Using trace metal sampling and analysis techniques demonstrated to be reliable, these metals are shown to occur at levels lower than frequently reported. There are systematic relations between the metals and discharge which can serve as predictors of metal variability. An~ro~enic ~n~butions cannot be asses& accurately from these data but do not appear to cause the concentrations of most of these metals to be significantly higher than observed in less disturbed systems, with the possible exception of Ni and Cd. THE BASIN OF THE Mississippi River is the most highly developed and industrialized of the ten largest river systems in the world. The chemical ~rn~~tion and variability of this river are of interest for a variety of reasons, including (a) as a case study for furthering the understanding of fluvial geochemistry, (b) as a major source of dissolved and particulate substances to the Gulf of Mexico and the ocean, and (c) for an evaluation of anth~~ni~ captions of river ~rn~~on. There is a large data base on the variabiity of the major dissolved components obtained in the course of routine large-scale monitoring (BRIGGS and RCKE, 1978). However, less certain information is available on coneentration levels of dissolved trace metals owing to their s~ptib~ity to ~n~mination during sampling and analysis (BRULAND, 1983). In this study, a time series for eight metals in the lower Mississippi River was obtained which provides direct evidence for the lack of accurate dissolved metals data, and also provides preliminary evidence for the nature of metal variability in the Mississippi.

Seven dissolved trace metal samples were collected from the lower Mississinni River between Aoril 1982 and Amil 1984. The first two samples were collected near Venice, Louisiana; the other samples were collected at Baton Rouge, Louisiana. In all cases, these near-surface samples were taken from a boat in mid-channel. For the samples collected near Venice, water was pumped into the ship’s laboratory through tetlonlined tubing towed several meters off the side of the ship (BOYLE et d, 1982). The samples from Baton Rouge were collected by placing an acid-cleaned linear polyethylene bottle in a plastic holder attached to the end of a long non-metallic pole; the bottle was immersed upside-down below the river surface on the upstream side of the boat, inverted, and allowed to fill. At least two replicate sampkes were taken each time. Samples were fihered through 0.4 pm Nuclepore polycarbonate filters after cleaning and flushing, using a clean filtration system deseribed elsewhere (BOYLE et al., 1982); they were then acidified to pH 1.8 with double vycor-distihed hydrochloric acid with negligible blank concentrations. Dissolved metals were analyzed by graphite furnace atomic absorption * Present address: Center for Marine Science, University of Southern Mississippi, National Space Technology Laboratories, NSTL, MS 39529, U.S.A.

spectroscopy using a Perkin-Elmer 5OOO/HGA400combination. Samples were preconcentrated prior to analysis using the ant-p~lidine ~~~rnate method (BOYLEet d.,

1981). Metal recovery is dependent on the pH of the co-preeipitation; the co-precipitation was therefore carried out at pH 1.8 for Cu, Ni, and Cd, and at pH 4.5 for the other metals. Metal recovery by this separation procedure was determined by addition of stable metals to samples. Recoveries ranged ~m-S7%forCrto~l~forV.Si~~~o~~ that Cr(II1) and Cr(Vl) were recovered with equal efficiencies as were V(IV) and V(V). Pm&ion is estimated to be &lO% for Zn and Fe, and +5% for the other metals. In several instances, 0.4 gm filtered samples were rel&ered through 100,000 MW Nucleporeultralilte~. There were no an&&ally

&u&ant diKe.reneesbetween 0.4 pm filteredand ultrafiltered samples, which indicates that for this river the 0.4 lrnr data are representative of the truly dissolved eoneentration, with no s&&cant colloidal Fraction for these metals. Validation of data is of major importance in any study of trace metals in natural waters. A decade ago, concern about the inconsistency of available oceanic trace metal data caused marine geochemists to undertake more stringent precautions in sampling and analysis, leading to the realization that most older oceanic trace metal samples had been contaminated (BRULAND, 1983 and references therein). Based on this experience, it is likely that contamination also plagues freshwater trace element analysis. Indeed, it has recently been demonstmt~ that commonly quoted values for dissolved zinc con~n~tions in rivers are a factor of 100 too high (SHILLER and BOYLE, 1985). The validity of data from this study is ensured by four considerations. (1) The sampling and analytical methods have been used successfully in other studies of natural waters, including systems where the observed inanition levels are sub~nti~y lower than in the Mississippi (BOYLE et al., 1982; BOYLE et al., 1981; SHILLER and BOYLE, 1985). (2) Analysis of samples collected in replicate were almost always within the analytical precision. (3) River samples from April and November 1982 were part of estuarine transects; observed trace metaf ~n~nt~tions in these samples varied systematically with respect to salinity, as would be expected in a mixing regime, and the seawater end-

3273

3274

A. M.

1982

1983

Shiller and E. A. Boyle

1984

FIG. 1. Discharge of the Mississippi River, January 1982 to July 19w, m recordedat Tar&t Landing, hGsissippi(Data from USGS Water-Data Reports.Additional unpublished data courtesy U.S. Army Engineer District, New Orleans).

member concentrations observed for these transects are consistent with accepted values (SHILLER and BOYLE, 1983). (4) The metal &IX variations for these data are systematic. During the sampling period, the flow of the Mississippi was characterized by maximum discharge in spring with a secondary peak discharge in early winter (Fig. 1). Various stages of river flow were sampled, including the crest of a 50-year flood in May 1983. Concentrations of eight dissolved metals in these samples are listed in Table 1 along with relevant ancillary data. Temporal concentration changes range from -30% of the mean for nickel to over a factor of 5 for chromium. As shown in Table 2, similar con~ntmtions for some of these metals have been reported previously for the Mississippi (TREFRY et al., 1986; samples collected using Go-flo samplers and filtered through 0.4 pm Nuclepore filters). However, our data are generally much lower than those reported by the United States Geological Survey (USGS) for the National Stream Quality Accounting Network (NASQAN) station at St. Francisville, Louisiana (samples collected using depth-integrating sampler and filtered through 0.45 ~trn membrane filters). (The NASQAN data are available from the USGS Water Resources Division, Reston, VA 22092, and are published annually in USGS Water-

Table

1.

Date

Data Reports for individual states.) In the extreme case, the zinc data reported here are over 50 times lower than the mean of the NASQAN data for the same period and, in fact, are about an order of magnitude lower than the NASQAN detection limit. Three factors should be kept in mind when considering these trace metal data. (a) 80% of the water of the lower Mississippi comes from three sources-the Ohio, Missouri. and Upper Mi~~ippi Rivers (BRIGGS and FICKE. 1978: and various USGS Water-Data Reports). The Ohio River is typically about 0.8 pH unit more acid than the other two sources and has a lower suspended load. These differences may lead to some desorption of cations and adsorption of anions when the sediment-rich waters of the Upper Mississippi and Missouri mix with the more acidic waters of the Ohio below Cairo, Illinois. Therefore, dissolved trace metals are not necessarily conservative upon mixing, since a large percentage of the readily reactive forms of some of these elements are adsorbed (e.g., 70-90% of the reactive zinc is adsorbed: SHKLER and BOYLE, 1985). (b) The relative prounion of water coming from the Ohio vs. other sources varies with total discharge (Fig. 2). Therefore, variations of river chemistry with discharge may reflect variations of the source mixture. (c) Major element concentrations in the Mississippi River also vary temporally, with generally decreasing concentrations as discharge increases (illustrated by specific conductivity in Fig. 2). This trend is due in part to dilution by direct runoff and in part to the increased proportion of waters from the less solute-laden Ohio River at high discharge. Dilution could be crudely normalized by linear adjustment to a constant major element concentration. However. the compositiondischarge ~lationship is only approximate and can vary appreciably between rising and falling discharge stages (EVERETT, 197 1; WELLS, 1980). An examination of Figs. 2 and 3 indicates that there is no simple relation between variations in the dissolved concentrations of the metals and the physical parameters. Some metals show a high degree of variability from low to intermediate discharge but relatively constant concentrations from intermediate to high discharge (Fe, V, MO). Other metals show peak concentrations at intermediate discharge with significant de-

Mississippi River Seasonal Data

PH

Suspended (kl$)

Oischarge (102 ,3/s,,)

Sp. Cond. (who)

Cu

Ni

Cd

Fe

Ln

V

Cr

MO

XOhio

______________~~-~~~~(~~l/~g)-___~~-~~___~

22 Apr a2

7.60

159

201

344

24.9

26.2

0.155

33

4.2

22

1.90

a.8

6 Nov a2

7.93

18

67

462

30.9

24.6

0.2M)

14

3.8

36

1.60

24

48 29

14 Mar 83

7.86

121

204

354

21.2

22.4

0.130

31

3.6

23

2.55

10.5

SE

31 Nay 83

7.8

120

416

299

23.8

20.5

0.075

33

1.9

24

0.44

9.9

6%

26 sep a3

a.23

35

57

492

20.0

21.3

0.138

11

1.7

45

1.24

27

38

26 Jan 84

7.98

85

115

389

18.3

25.1

0.189

47

4.1

IS

2.84

12.2

57

la Apr 84

7.72

103

297

349

20.1

23.5

0.122

26

2.8

20

0.86

9.3

58

3215

Trace metals in the Mississippi Table 2.

Comparison of reported dissolved metal cOnCent.rarfons in the Mississippi River (tvnol/kg).

This Work a

;: Cd :", V Cr MO

Trefry et al. b

:: 0.12 30 3.0 23 1.4 11

USGS c

140 60 f

29 24 0.12 45 *_

:E

-;.4 __

1:
a)

Discharge weighted means

b)

Mean of four samples collected in May 1962, September 1982, April 1963 and Noveeber 1983.

c)

USGS maan data calculated assumin that values below detection limit = 0 (except MO. for ti 1ch all values were at or below detection limit). Nua&er of times detected/ntier of samples: CU-27/27; Ni-25126; Cd-13/26; Fe-23/24; Zn-25125; V-11/26; Cr-WlO; MO-l/a. Several high values were deleted from the mean for Fe and Zn. (Data courtesy of USGS Water Reswnes Dlvis~on).

relationships in Fig. 3 are still empirically usell even when the underlying mechanisms are unknown. These data are relevant to questions concerning anthropogenic perturbations of natural trace element chemistry in the Mississippi River. Recently, for example, high dissolved zinc concentrations of probable anthropogenic origin have been reported in a number of tributaries of the Ohio River (SHILLERand BOYLE, 1985), although the cont~bution of these tubule to the total discharge of the Mississippi River is quite low. Table 3 compares discharge-weighted average metaI concentrations for the Mississippi River with metal concentrations of samples from three less dis-

creases at both high and low discharge (Cr, Zn, Cd?).

Two metals show relatively little variation in concentration (Cu, Ni). Dissolved flux-discharge plots (Fig. 3) provide another means of examining metal variability. Low variability in concentration leads to approximately linear flux-discharge relationships; some of the more variable elements reach a peak or limiting flux at intermediate to high discharge (Cr, Zn, Cd). These relationships with flow appear to be systematic and thus may be used as the best available means of estimating concentration. Unfo~unately, with a limited data base one cannot be completely ~nfident that the rne~di~h~e relationships apply under all circumstances. It is still more difficult to ascertain mechanisms since it is possible for these relationships to arise in various ways. The primary mechanistic dilemma is to understand the extent to which chemistry or source determines metal concentrations/fluxes. For example, the relatively constant concentration of an element such as copper may be indicative of control by chemical equilibrium reactions or may be due to flushing of the primary source proportionally to flow. For an element such as chromium, the maximum concentration at intermediate discharge may be indicative of non-conservative mixing of the major tributaries (discharge being a rough estimation of mixing; cf: Fig. 2). Alternatively, the peak flux of chromium suggests a source of limited potential (e.g., flushing of groundwater) which becomes diluted at high discharge (i.e. high surface runoff). Similarly, the curved molybdenum concentration-discharge plot may also indicate non-conservative mixing (adsorption as opposed to desorption for chromium), while the constant flux at low discharge suggests a constant baseline source (e.g., constant ground water or sewage source). Evidently, a more detailed time series, me~u~ments of abuses, and mixing studies will be required to resolve the possible controlling mechanisms. Nonetheless, the discharge

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’ ’ 2’ ’ 5’ ’ 1 Discharge ( 1 04m3/sec)

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F3ci. 2. Ancillary physical and chemical parameters associated with the trace element samples verslls discharge of the Misskippi Rivers a) percentage ~n~bu~on of the Ohio River to the total flow, b) specific conductivity, c) suspended load in near surface river water, d) PH.

3276

A. M. Shiller and E. A. Boyle

347 e

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2

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+-T-T-+

Discharge ( 1 dm’/sec)

FIG. 3. Dissolved concentrations and fluxes of eight trace elements versus discharge of the Mississippi River.

turbed major rivers: the Amazon, Orinoco, and Yang&e (Changjiang) Rivers. A strict comparison of these rivers is inappropriate since p&T,suspended load, and source rocks differ substantiahy. The Yang&e is most similar to the Mississippi in major element composition and pH but has significantly higher suspended loads. Based on this comparison, there is little evidence of major anthropogenic perturbation of dissolved metal concentrations in the Mississippi, with the possible ex-

Table 3.

DissolvedMetals in Major Rivers (mnol/kg).

Missfssfppi CU

23

Ni Cd Fa Zn V Cr

23 0.12

MO

398 1’: 11

Yangtze 18/21 ,~. 213 co.01 O&2

Fdl%?Oi?

24 -5 0.06 180f1300 0.313.8 12/l&5 0.7 1.5

Orfnoco

19 _. 0% 210 044 i.2

Misslsslppidata are dischargeweightedmeans; Yangtze data are from samplingsIn June 1980 and Novclnber 19Gl; Amazon data are from sanrpllngs in June 1976 and December1982; Orfnoco data are from one samplingin June 1982. Two values listed indicatethe concentrationdifferencebetweenhigh and low discharge. Cu. Nf. Cd for Yangtze.&eazon. and Orfnoco,and Fe at high flow in Ainazonfrom Refs. Boyle, ifu*sW and Grant (1982) and Edmond et al. (1985). Al? other data are from thfs work, Shtllerand Boyle (1985).or A.M. Shfllerand E. Boyle, onpubltsheddata.

ception of cadmium and nickel; although even these differences might be due to natural causes. Of course, it is also possible that anthropogenic additions ofparticuiates and organic matter might alter the natural di~~v~/ad~r~ metal ~st~butions and thus coinciden~ly offset ~thro~genic inputs. The most appropriate comparison would involve fluxes of adsorbed and dissolved metah; such data are not presently available for other river systems, however. In conclusion, the data reported here indicate that concentrations of dissolved metals in the Mississippi River are lower than often reported; particularly the trace element data of NASQAN. Evaluation of longterm trends in metal concentrations due to anthropogenic perturbations (e.g., SMITH et al., 1987) will require more detailed time series from carefully collected and analyzed sampIes, and comparison with major river systems which are less disturbed. The variability of dissolved metal inundations in the Mississippi River indicates that single samples may be representative only of the magnitude of fluvial trace element concentrations. There are systematic relations between dissolved metals and discharge which at present are the best estimators of variations in metal concentrations. Anthropogenic contributions are uncertain but have not increased most of these metals beyond concentrations observed in less disturbed systems. Ac~owZ~~~~s-We

thank L.-H. Chan (Louisiana State University) and members of the Louisiana ~~rncnt of

Trace metals in the Mississippi Natuml Resources for aid in collecting samples during 1983 and 1984; M. R. Scott (Texas A & M) and the captain and crew of the R/V Gyre for assistance during the 1982estuarine transects; and J. M. Edmond and A. Spivack (MIT) for the Orinoco and Yangtze samples. Support for this work was provided by the NSF through a Postdoctoral Fellowship (AMS) and research grant QCE 81-17929 (EAB). Editorial haloing:

D. M. Shaw REFERENCES

BOYLE E. A., HUESTED S. S. and

JONESS. P. (1981) On the distribution of copper, nickel, and cadmium in the surface waters of the North Atlantic and North Pacific Ocean. L Geophys. Res. 86,8048-8066.

A., HUESTEDS. S. and GRANT B. (1982) The chemical mass balance of the Amazon Plume-II. Copper, nickel, and cadmium. Deep-Sea Res. 29, 13% 1364. BRIMS J. C. and FICKEJ. F. (1978) Quality of rivers of the United States, 1975 water year. USGS Open File Report 78-200,436 p. BRULANDIL W. (1983) Trace elements in sea-water. In BOYLE E.

3271

Chemical Oceanography Vol. 8 (ds. J. P. RILEYand R. CHESTER). _,DD. . . 157-220. Academic Press. EDMOND J. M., SPIVACK A., GRANTB. C., Hu M.-H., CHEN Z., CHEN S. and ZENG X. (1985) Chemical dynamics of the estuary of the Changjiang River. Cont. Shelf: Res. 4, 17-36. EVERETT D. E. (197 1) Hydrologic and quality characteristics of the lower Mississippi River. Louisiana Dept. public Works Tech. Rep. 5, 48~. SHILLER A. M. and BOYLEE. A. (1983) Trace metals in the dume of the Misaissinoi River. Eos 64. 1021. SHELLER A. M. and BOOZEE. (1985) Dissolved zinc in rivers. Nature 317,49-52.

SMITHR. A,, ALEXANDER R. B. and WOLMAN M. G. (1987) Waterquality trends in the nation’s rivers. Science 235, 1607-1615. TREFRYJ. H., NELSON T. A., TROCINER. P., METZS. and VETTERT. (1986) Trace metal fluxes through the Mississippi River delta system. In Contaminant Fluxes Through the Coastal Zone, (ed. G. KULLENBERG), pp. 277-288. Rapp. P.-v. Reun. Const. Int. Explor. Mer, 186. WELLSF. C. (1980) Hydrology and water quality of the lower MississippiRiver. Louisiana Dept. Fbbiic Works Tech. Rep. 21, 83~.