The geochemical controls on arsenic concentrations in southeastern United States rivers

Chemical Geology, 24 (1979) 315--325 315 © Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

THE GEOCHEMICAL CONTROLS ON ARSENIC CONCENTRATIONS IN SOUTHEASTERN UNITED STATES RIVERS

DENNIS G. WASLENCHUK*

Skidaway Institute of Oceanography, Savannah, GA 31406 (U.S.A.) (Received November 23, 1977; revised and accepted May 16, 1978)

ABSTRACT Waslenchuk, D.G., 1979. The geochemical controls on arsenic concentrations in southeastern United States rivers. Chem. Geol., 24: 315--325. Dissolved As concentrations in rivers in the southeast o f the U.S., among the lowest values reported for world rivers, are controlled by the availability of As by rainwater dilutions, by the extent of complexation by dissolved organic matter, and perhaps by metabolic uptake by aquatic plants. Availability in turn is apparently affected by: (1) the drainagebasin soil type; (2) weathering and erosion rates; and (3) rainfall and soil flushing rates. As complexation by organics prevents adsorptive interactions between dissolved As and active solid-phase organic and inorganic materials. Despite high arsenate solubility, arsenate concentration is limited to levels below saturation, due to reactions which remove the free arsenate ion from solution. The particulate AS load may he as important as the dissolved load with respect to material transport in the rivers. However, only the dissolved load is delivered to the ocean. Those biologically mediated reactions which result in AS species disequilibria in other water bodies have an insignificant effect on As speciation in the rivers.

INTRODUCTION

It has only been possible to make precise concentration measurements of a few trace elements in natural waters since a few years. As a result of recent advances in analytical chemistry, it is n o w possible to approach some of the problems in the aqueous geochemistry o f trace elements. The characterization o f geochemical processes which affect riverine chemistry promises to be a complex problem. Climatic factors are fundamentally important in that they govern soil and weathering types, erosion rates and vegetation, which in turn must affect riverine chemistry. The petrography of the drainage basin will likewise affect riverine chemistry. If we are to understand its intricacies, we must make observations on geochemicals *Present address: Marine Sciences Institute, University o f Connecticut, Avery Point, Groton, CT 06340, U.S.A.

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which participate in the physicochemical and biochemical reactions occurring in the riverine environment. With the advent of a powerful analytical method, As has proved to be an exceedingly good tracer (indicator) for geochemical and biochemical processes, and for rare-element transportational pathways (Waslenchuk, 1978). As, a metalloid, can exist in several valency states at naturally occurring redox potentials, hence several chemical species are possible (Braman, 1975). It is evident that some As species are assimilated and/or transformed by biota, such that As(III) and methylated As forms, in addition to thermodynamically stable AS(V), occur in nature (Ferguson and Gavis, 1972; Waslenchuk, 1977; Andreae, 1978). The geochemical associations of the hydrated As anions are unknown, hence the reactivity and biological availability of arsenicals are also unknown. For instance, ionic As forms might be highly reactive with ubiquitous iron hydroxides and organic ligands and substrates in freshwaters. Furthermore, the atmosphere may be an important reservoir for As in light of the volatility of many arsenic compounds. Importantly, then, the characterization of the riverine chemistry of the metalloid As might provide information on transport mechanisms and processes which affect a number of diverse freshwater components. LOCATION The study area (Fig. 1) contains ten major rivers, the St. Johns, Satilla, Altamaha, Ogeechee, Savannah, Cooper, Santee, Black, Pee Dee and Cape Fear. These comprise over 80~ of the total freshwater input to the continental shelf adjacent to the southeastern Atlantic states. The major factors of the organic and inorganic geochemistry of some representative southeastern rivers have been described by Beck et al. (1974). The rivers contain relatively high amounts of dissolved organic matter and Fe, some are pristine and others support industry, and adjacent cultivated lands are common. River gradients are generally low, and therefore suspended-matter concentrations are also quite low. The annual rainfall in the area is approximately 136 cm.

METHODS

Arsenic speciation analyses The measurements of the various As species concentrations were made by plasma emission spectrophotometry (Braman et al., 1975, 1977). Replicate analyses indicate a + 7% relative variation about the means. Accuracy, as established by analyses of NBS Orchard Leaves is + 5 % of the usual 1--5 ng working level. The detection limit for the various species is about 10 ng/l (ppt).

317

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Dissolved organic carbon (DOC) DOC measurements were made by the m e t h o d o f Menzel and Vaccaro (1964) on 0.45-~m membrane filtered samples. Precision, based on replicate analyses is + 0.1 rag/1 in a range of 0.1--20 rag/1.

Sample preparation Dissolved constituents are operationally defined as those which pass a 0 . 4 5 ~ m membrane filter. The particulate materials retained by the 0.45-~m membrane were homogenized in a few millilitres of concentrated HC1 or HNO3, and aliquots were subsequently analysed in the same manner as aqueous samples. Only that As which resides in crystal lattice positions of resistant minerals is unrecoverable by the method. This fraction is of no importance to the present study. RESULTS AND DISCUSSION

Dissolved arsenic concentrations Water samples were taken every m o n t h from the same location at each o f

0.52 0.52 0.31 0.41 0.47 0.41 0.04 0.53 0.19 0.24

0.40 0.36 0.23 0.45 0.43 0.47 0.08 0.31 0.13 0.20

10/75

0.26 0.25 0.04 0.08 0.13 0.12 0.08 0.20 0.07 0.16

11/75 0.28 0.28 0.16 0.19 0.17 0.15 0.17 0.25 0.25

12/75

(a) = Coastal Plain rivers; (b) = Piedmont rivers.

St. Johns (a) Satilla (a) Altamaha (b) Ogeechee (b) Savannah (b) Cooper (b) Santee (b) Black (a) PeeDee (b) Cape Fear (b)

9/75 0.31 0.33 0.21 0.22 0.33 0.18 0.16 0.15 0.11 0.32

1/76 0.63 0.51 0.47 0.20 0.46 0.19 0.31 0.40 0.32 0.36

2/76 0.55 0.32 0.22 0.18 0.36 0.35 0.18 0.36 0.24 0.49

3/76

0.37 0.24 0.25 0.36 0.27 0.17 0.50 0.25 0.56

4/76

Dissolved arsenate concentration (in ug/1, ppb, as As) in major southeastern rivers

TABLE I

0.42 0.30 0.14 0.28 0.21 0.22 0.17 0.40 0.20 0.46

5/76 0.42 0.44 0.15 0.28 0.19 0.26 0.16 0.47 0.32 0.39

6/76

0.56 0.36 0.14 0.22 0.22 0.24 0.15 0.35 0.20 0.47

7/76

0.65 0.55 0.22 0.30 0.20 0.42 0.18 0.74 0.23

8/76

0.45 0.38 0.21 0.25 0,29 0.27 0.15 0.39 0.21 0.37

Average

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319

the ten major rivers, over the period September 1975--September 1976. The concentrations of arsenate-As, the only dissolved species detected, are listed in Table I. In general, the levels reported are similar to the lowest values found in the literature for other world rivers (Ferguson and Gavis, 1972}. The metabolically produced arsenite (As 3÷) and dimethyl-arsenic (DMA) forms were not detected in the southeastern rivers, presumably reflecting the relative inactivity of biologically mediated arsenic redox and alkylation reactions. An algae-rich swamp in the Black River drainage basin did contain about 0.1 ppb of both As3÷ and DMA, however, but these species apparently do not persist in the river. The average annual concentrations reveal that rivers with drainage basins primarily in the crystalline igneous and metamorphic Piedmont terrain have lower As concentrations (average 0.23 ~g/1) than those having drainage basins restricted to the swampy Coastal Plain sedimentary terrain (average 0.41 ug/1}. This dissimilarity probably reflects the different availability of As in the two drainage-basin types, where moist Piedmont softs do not release As to the extent of wet Coastal Plain soils. It is interesting that dissolved silica behaves in the opposite sense. Beck et al. (1974} found higher SiO2 concentrations in Piedmont rivers than in Coastal Plain rivers. Presumably weathering of the unstable igneous-metamorphic rocks produces an excess of dissolved silica in soil interstitial-water, whereas weathering of the more stable sedimentary rocks of the Coastal Plain does not. However, the predominantly mechanical weathering of the Piedmont rocks is apparently less conducive to the dissolution of rock-bound arsenic than is the chemical weathering typical of the Coastal Plain rocks. The Cape Fear River alone does not fit into this generalization. Although a Piedmont river, the As level is high (0.37 ug/1). The uniquely high (for southeastern rivers) suspended sediment loads of the Cape Fear River, however, reflect higher weathering rates and perhaps, therefore, greater availability of As than do those of the other Piedmont rivers. Supportive to this conclusion are the data of Andreae (1978), which show very high dissolved As concentrations (average 4.3 pg/1) in some silty, western U.S. rivers which flow over quickly weathering volcanic-sedimentary terrain. The As concentration in each southeastern river varies by up to 0.2 gg/l around the annual average. The most notable feature of the annual As concentration variation over the year (Fig. 2) is the low concentration in most rivers during the months from November through January. A similar pattern of variation has been observed for several other trace elements in the Susquehanna River (northeastern states) and in the southeastern rivers (Troup and Bricker, 1975; Windom, 1975), suggesting that trace-element concentrations are at least partially regulated by seasonal climatic variables. Beck et al. (1974) point out that metals are flushed out at high concentrations from swamps, following rains. Hence, river discharge, which is directly related to rainfall where the river stage is not artificially controlled, should be related to trace-element concentration. In fact, a fair relationship between these two variables exists in the pristine Satilla River (Fig. 3).

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Organic complexation In river and estuarine waters, arsenate is complexed by low-molecularweight dissolved organic matter (Waslenchuk and Windom, 1978). Complexation apparently effects conservative behavior of As in the estuary by preventing interactions with active, solid-phase, organic and inorganic materials. Complexation may play a similar role in freshwater; through complexation, dissolved organic matter might partially control the concentration of dissolved As. Amassed DOC data (Table II) correlate strongly with the corresponding arsenic data in Table I, with the exception of a few seasonally high DOC values (marked by *). The relationship, which has a correlation coefficient o f 0.39, is described by the expression (As}= 0.09 (DOC}+ 0.54, and is significant at the 0.001 level. These results are consistent with the possibility that arsenate concentration is an imposed quality of rivers, despite the high solubility of arsenate, due to the existence of reactions that would remove free arsenate ion from solution at levels below saturation (i.e., coprecipitation, adsorption, ion~exchange). The high summer DOC concentrations (marked by * in Table II), which correspond to relatively low arsenate concentrations, probably represent seasonally excessive organic matter, where maximum As concentrations are instead limited by the availability of As.

Downstream variation Within a single river system (the St. Johns River) D O C and arsenate con-

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10/75

*See t e x t for c o m m e n t (p. 321).

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9/75 15.0 16.0 4.4 8.5 5.4 2.4 3.7 13.0 6.5 13.0

11/75

DOC c o n c e n t r a t i o n s (mg/l) in river w a t e r s

T A B L E II

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12/75 10.0 ----4.0 8.0 2.1 2.1 3.5

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3/76 7.0 11.0 5.2 6.1 8.0 5.4 7.5 8.0 9.8 11.0

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5/76

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6/76

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centrations covary strongly (Fig. 4). Seven stations were sampled from near the river's mouth (station 1 ) to the head waters (station 7). In general, the swampy, uppermost reaches of the river (stations 5 -7 on Fig. 4) have the highest dissolved organic C concentrations. Points 5 and 6 also have the highest dissolved As concentrations. Samples from reaches downstream, including points 1--4, have lower As and organic-C levels. The covarience is strong, even though the data point corresponding to station 7 is aberrant. The relatively high organic-C concentration at station 7 is probably due to the uniquely luxuriant vegetation present in the river at that station, and correspondingly, the As level is limited by availability, or perhaps by metabolic uptake by the flora. 2

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The regression excluding station 7 probably represents simple conservative dilution of high-organic-C--high-As waters (derived from the swampy soils) by more pure rainwater and surface runoff. Rainwater collected at nearby Savannah, Georgia, contains an average of 0.2 pg As/1 (Waslenchuk, 1978), and rainwater is reported to contain between 1.0 and 13 rag/1 of dissolved organic-C (Menzel, 1974; Duce and Duursma, 1977), with an average value of about 3.0 mg/1. Hence, rainwater falls at a position on Fig. 4 which is consistent with that of the endmember required to dilute the high-As--highorganic-C water of stations 5 and 6 in the conservative fashion implied by the statistical regression. It may be said, then, that riverine As concentrations are controlled, in part, by rainwater dilutions. In contrast, Beck et al. (1974) found that the relative abundances of major inorganic constituents (Na, Mg, Ca, K, C1, SO4 ) in the southeastern rivers indicate that rain input pre-

324

dominates over rock weathering as the source of these materials; only for SiO2 was there a detectable rock-weathering contribution.

Suspended particulate arsenic Suspended particulate material collected from the Ogeechee River in July 1976, February 1977, and July 1977 contained 9, 5.4 and 10.8 ng/mg As 5+, respectively. At a suspended sediment load o f 5--15 rag/l, this amounts to a 0.03--0.16 pg/1 particulate As load, which is similar to the dissolved As load of 0.08--0.45/~g/1. Hence the association of As with particulates can result in an important As transport mechanism in rivers. However, the As associated with particulates is apparently non-labile. Waslenchuk and Windom (1978) have shown that no significant exchange occurs between dissolved and particulate As fractions in estuaries. Furthermore, the particulate load of southeastern rivers is deposited entirely in estuaries and the coastal zone (Meade, 1969; Windom et al., 1971), hence only the dissolved fraction of As is delivered to the ocean. ACKNOWLEDGEMENTS

I thank Dr. H.L. Windom for many contributions to this research, and Ralph Smith for supervision in some analyses. I thank Dr. R.S. Braman for his assistance and hospitality at the University of South Florida. This research was supported by the United States Energy Research and Development Administration, Contract No. E(38-1)-890, and the Skidaway Institute of Oceanography. REFERENCES Andreae, M.O., 1978. Distribution and speciation of arsenic in natural waters and some marine algae. Deep-Sea Res., 25: 391--402. Beck, K.C., Reuter, J.H. and Perdue, E.M., 1974. Organic and inorganic geochemistry of some Coastal Plain rivers o f the southeastern United States. Geochim. Cosmochim. Acta, 38 : 341--364. Braman, R.S., 1975. Arsenic in the environment. In: E.A. Woolson (Editor), Arsenical Pesticides. Am. Chem. Soc., Syrup. Ser., 7; pp. 108--123. Braman, R.S., Johnson, D.L., Foreback, C.C., Ammons, J. and Bricker, J., 1977. Separation and detection of sub-nannogram quantities of inorganic arsenic and methylarsenic compounds. Anal. Chem., 49: 621--625. Duce, R.A. and Duursma, E.K., 1977. Inputs of organic matter to the ocean. Mar. Chem., 5: 319--339. Ferguson, J.F. and Gavis, J., 1972. A review of the arsenic cycle in natural waters. Water Res., 6: 1259--1274. Meade, R.H., 1969. Landward transport of b o t t o m sediments in estuaries of the Atlantic Coastal Plain. J. Sediment. Petrol., 39: 222--234. Menzet, D.W., 1974. Primary productivity and particulate organic matter, and the sites of oxidation of organic matter. In: E.D. Goldberg (Editor), The Sea, 5. Wiley-Interscience, New York, N.Y:, pp. 659---678.

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Menzel, D.W. and Vaccaro, R.F., 1964. The measurement of dissolved organic and particulate carbon in seawater. Limnol. Oceanogr., 9(1): 138--142. Troup, B.N. and Bricker, O.P., 1975. Processes affecting the transport of materials from continents to oceans. In: T.M. Church (Editor), Marine Chemistry in the Coastal Environment. American Chemical Society, Washington, D.C., pp. 133--151. Waslenchuk, D.G., 1977. The geochemistry of arsenic in the continental-shelf environment. Ph.D. Thesis, Georgia Institute of Technology, Atlanta, Ga., 61 pp. Waslenchuk, D.G., 1978. The budget and geochemistry o f arsenic in a continental-shelf environment. Mar. Chem., 7: 39--52. Waslenchuk, D.G. and Windom, H.L., 1978. Factors controlling the estuarine chemistry of arsenic. Estuar. Coast. Mar. Sci., 6. Windom, H.L., 1975. Heavy-metal fluxes through salt marsh estuaries. In: L.E. Cronin (Editor), Estuarine Research, 1. Academic Press, New York, N.Y., pp. 137--152. Windom, H.L., Neal, W.J. and Beck, K.C., 1971. Mineralogy o f sediments in three Georgia estuaries. J. Sediment. Petrol., 41 : 497--504.