Environ. Sei. Technol. 1987, 21, 21 1-216
Zinc Cycling in an Acidic Adirondack Lake Jeffrey R. White"
School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana 47405 Charles 1. Drlscoll
Department of Civil Engineering, Syracuse University, Syracuse, New York 13210 Acidification of surface waters by acidic deposition has reportedly caused increased concentrations of Zn. Elevated concentrations of Zn are of interest due to potential toxicity to aquatic organisms. Researchers have also examined the chronology of Zn deposition to lake sediments as an indicator of changes in atmospheric deposition. However, there have been few studies that focus on Zn chemistry and transport in acidic lakes. Elevated concentrations of Zn (0.15-1.2 pmol L-l; mean = 0.39 pmol L-l) were observed in acidic Darts Lake. Although peak concentrations of 1.2 pmol of Zn L-l were observed during snowmelt, spatial and temporal variations in Zn concentration were minor and limited to melt water in streams and at the lake surface. On the basis of mass balance calculations and sediment trap observations, Zn did not appear to be significantly retained in Darts Lake. Long- and short-term variations in in-lake retention of Zn, caused by surface water acidification, may complicate quantitative interpretation of Zn deposition in sediments. Introduction Acidification of dilute surface waters results in elevated concentrations of trace metals (1-5). Trace metal buffering capacity (complexation of aquo metal) is low in these waters due to low concentrations of aqueous and particulate complexing ligands. Generally, the aquo form of trace metals is most toxic to aquatic organisms (6-8). In particular, laboratory and field studies have demonstrated the importance of Zn as a potential toxicant at submicromolar concentrations in aquatic ecosystems (9-11). Therefore an evaluation of Zn chemistry and cycling in dilute acidic waters is significant to our understanding of the potential impact of lake acidification. Although Zn is a potentially toxic trace metal, there have been very few studies of Zn chemistry and transport in acidic surface waters. Surface water concentrations of Zn in southern Norway (median = 0.23 pmol L-l, n = 136 lakes) were noted to increase closer to industrial centers ( 4 ) . The concentration of Zn in lakes of northern Sweden ranged from 0.15 to 0.46 pmol L-l (1). Using results of a synoptic survey, Baker (12) reported that Zn concentrations increased exponentially with decreases in the pH of Adirondack surface waters (0.02-1.0 pmol of Zn L-l from pH 7.0 to pH 4.2). Troutman and Peters (13) evaluated the Zn transport of three lake-watershed systems, with acidic [mean pH 4.7; mean acid neutralizing capacity (ANC) -30 pequiv L-l], intermediate (mean pH 5.6; mean ANC 30 pequiv L-l), and circumneutral (mean pH 6.2; mean ANC 147 pequiv L-l) drainage waters. The authors observed that bulk deposition of Zn to the watershed containing the circumneutral pH lake substantially exceeded efflux of Zn from the watershed within drainage water; the other two more acidic lakewatersheds exhibited a net loss of Zn. Schindler et al. (14) demonstrated significant release of Zn (1.4 pmol of Zn L-l for control enclosure at pH 6.7; 5.4 pmol of Zn L-l for enclosure at pH 5.0) from littoral sediments following experimental acidification of lake enclosures. Santschi et al. (15) attributed 0013-936X/87/0921-0211$01.50/0
the high steady-state concentrations of Zn in lake enclosures to a lower KD (distribution coefficient) at lower pH. Acidic lake systems undergo seasonal changes in acidbase chemistry (16). Sping snowmelt waters in acidic Adirondack New York lakes are typically elevated in H', Al, and NO3-. During summer stratification, biologically generated acid-neutralizing capacity results in neutralization of H+ and A1 acidity (17). Increases in pH may result in a decrease in A1 solubility and conversion of aqueous Al to a particulate form. Metal oxide solid phases (Al, Fe, Mn, Si) are known to regulate trace metal chemistry through surface adsorption reactions (18-21). Therefore, in-lake formation of particulate A1 may modify the transport of trace metals through sorption reactions (e.g., adsorption, coprecipitation). We recently demonstrated (22) that, for an acidic Adirondack lake, seasonal variations in pH and soluble A1 concentrations affected the transport of Pb. During periods of low flow and thermal stratification, the deposition of P b to sediments was found to be strongly dependent on A1 deposition rates and represented a major component of Pb flux in the lake (22).
The fate of Zn in dilute surface waters may also be controlled by interactions with suspended particulates and sediment materials. The nature and extent of this interaction may be influenced in part by pH changes occurring in the system (14, 15, 23). Schindler et al. (14),working in dilute soft-water lakes of the Canadian Shield, experimentally acidified enclosures in a dilute lake, to which were added radioisotopes of several metals. In neutral-pH (6.7) control enclosures 65Znwas extensively (62%) sorbed to particulate matter. Acidification of these waters to pH 5.1 resulted in a reduction in Zn partitioning to particles in suspension; only 14% of added Zn was removed from solution through sorption to particles. Other work involving the same lake systems demonstrated that Zn was strongly sorbed to particulate organic matter present in the lake (24). In a more recent study using radiotracers in lake enclosures, Santschi et al. (15) illustrated that Zn transport within lakes may be controlled by both physical transport of particles and pH-related chemical processes. They reported a significant decrease in the distribution coefficient for Zn (as determined by sediment trap particles) as pH was reduced from 6.5 (log KD = 4.8) to 4.8 (log KD = 3.7). Batch sediment titrations revealed a similar trend wlth pH; a pH change of 6.5 to 4.3 resulted in a decrease in log KD from 4.9to 3.3 (15). However, Zn is not a strongly hydrolyzing metal, and laboratory experiments suggest that adsorption to oxide surfaces may not be significant below pH 5.5 (21, 25-27). In addition to potential biological effects, there is considerable interest in using the sediment record of Zn deposition as a tracer of anthropogenic inputs to lake systems. Several investigators have reported that increased deposition of Zn to sediments started in the late 1800s, peaked, and then declined in more recent years (5, 28). Carignan and Tessier (29) investigated sediment porewater chemistry and suggested that Zn profiles were af-
0 1987 American Chemical Society
Environ. Sci. Technol., Vol. 21, No. 2, 1987
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fected by sediment diagenesis, unrelated to acidification history. In order to better understand the Zn cycle, the objective of this investigation was to evaluate spatial and temporal variations in the concentration and transport of Zn within an acidic lake. Moreover, we assessed Zn transport to and from the lake system.
Experimental Section Site Description and Analyses. A water column and sediment trap [following Bloesch and Burns (31)]monitoring program was used to investigate mechanisms regulating the chemistry and transport of trace metals (Zn, Pb, Mn) in Darts Lake. A detailed description of the sampling program is presented in a recent volume of this journal (22). Sediment cores were collected in triplicate from the profunda1 sediments near the pelagic sampling station, with a 5-cm gravity coring device. Sediment cores were sectioned on site into l-cm intervals to be used for the determination of surficial sediment pools of Zn and other metals. Core sections of sediment were stored in the dark at 4 “C for no more than a few weeks before chemical extractions were completed. Field measurements included stream flow, pH, dissolved oxygen (30),water column temperature, and daily on-site climatological data. The stream and water column chemical parameters monitored included dissolved organic carbon (33),monomeric A1 forms (34, 35), and Zn. To determine “dissolvedZn”, centrifugation (5720g for 30 min) was used to separate particulates from solution (36),and the supernatant was acidified (0.5% v/v Ultrex “0,). Sediment trap samples were analyzed for metals (Al, Fe, Mn, Zn, Pb), particulate carbon (33),and suspended solids (30). To determine the acid-labile metal content of sedimenting material, dissolved concentrations of each element were subtracted from acid-leachable concentrations [a detailed description of the method is available elsewhere (22)].The flux of acid-labile metals (gross deposition rate, Dg) was of interest since this operationally defined extraction probably reflects recently sorbed material rather than native metals within the particulate matrix (37). Similar methods of acid digestion (including hydrogen peroxide treatment) were used to quantify the acid-labile pool of Zn in sediments. The total Zn content of sediments was determined by summing fractions in a sequence of extractions (38). In addition to the lake sampling program, laboratory batch adsorption experiments were conducted. Aliquots of fresh wet surficial sediment from the pelagic sampling station of Darts Lake (dry weight = 0.1 g) were suspended in polyethylene bottles containing l-L solutions of 1.0 pmol L-l Zn. The suspensions were adjusted with HN03 and NaOH to yield a range of pH levels from 3.0 to 6.0. Following equilibration for 24 h, suspensions were separated by centrifugation (5720g for 30 min) and analyzed for pH and “dissolved Zn”. Data from lake enclosure experiments suggest that 24 h is sufficient time for the adsorption of Zn to natural lake particles to reach equilibrium (14,15). Separate aliquots of sediment were also analyzed for dry weight to determine the mass of adsorbed Zn per unit mass of sediment. Analysis of metals in sediment collections and water samples was conducted by atomic absorption spectrophotometry with a graphite furnace. The total error (sampling and_ analytical error from triplicate analyses presented as X , SD, 95% CI) associated with soluble Zn, Mn, Fe, and total monomeric A1 analyses was 0.52 f 0.03 pmol L-l (95% CI = 0.44-0.59 pmol L-l), 1.38 f 0.02 pmol L-l (95% CI = 1.32-1.43 pmol L-l), 1.18 f 0.04 pmol L-’ 212
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(95% CI = 1.08-1.28 pmol L-I), and 19.5 f 0.2 pmol L-l (95% CI = 19.2-19.8 pmol L-l), respectively. The fractional uncertainties for sediment trap collection and analysis of Mn, Al, Fe, and organic C are available elsewhere (22). The fractional uncertainty for Zn, expressed as mean coefficient of variation, was 11.6 f 7.1% (95% CI = 8.9-14.3%). Computative Methods. Gross deposition rates of substances into sediment traps (D,, for suspended solids, Al, Mn, Fe, organic matter, and Zn) were calculated from the mass of particulate-associated substances accumulated during the collection period, expressed on an areal basis: Dg = CrnV/(Att) (1) where D, is equal to the gross deposition rate (mmol m-2 d-l), C, is the sediment trap concentration (mmol ~ m - ~ ) of particulate-associated acid-labile metal, V is the sediment trap volume (cm3),A, is the area of the sediment trap opening (m2),and t is the collection period (days). In addition to gross deposition, specific deposition rates were determined by normalizing the D, value for the mass of soluble metal present above the sediment collector during the collection period: D, = D g / M (2) where D, (day-l) is the specific deposition rate in moles of particulate-associated metal per mole of soluble metal per day, D, is gross deposition rate (eq l),and M (mmol m-2) is the quantity of soluble metal per square meter above the trap. High specific deposition rates indicate efficient in-lake retention, while low values of D, suggest conservative behavior. The distribution coefficient (KD) is a parameter that is indicative of the intrinsic affinity of a metal for particulate matter in an aqueous system. It is a measure of the equilibrium mass distribution of an adsorbate (trace metal) between solution and solid-phase adsorbent (particulates).. By use of sediment trap data of particulate concentrations and concentrations of acid-labile trace metals, water column values of KD can be determined as KD = Crn/(CpMi) (3)
KD(cm3g-l) is a function of the concentration of particulate-bound trace metal (Crn),particulate concentration [C,, total solids in g cm-3 (30)],and the volume-weighted mean concentration of soluble trace metal in the water column above the sediment trap (Mi, mmol ~ m - ~ The ). distribution coefficient was also calculated for the laboratory batch adsorption studies by using the mass of particulate-bound metal associated with a known mass of adsorbent at equilibrium. Particulate-bound metal is determined by the difference between the soluble metal concentration added to suspension and the soluble metal concentration at equilibrium. Results and Discussion Water Chemistry. A detailed description of the water chemistry of Darts Lake is beyond the scope of this paper. However, information on water chemistry for the study year 1981-1982 is available elsewhere. Previous reports provide details on the chemistry and cycling of A1 (17,32, 40), P b (22), amd Mn (41) for Darts Lake during this period. The rates of Zn influx-efflux through stream water followed patterns previously reported for Pb (22)and Mn (41);flow was most important in influencing Zn flux to and from Darts Lake. Approximately 38% of the annual Zn influx-efflux occurred during the March, April, and May snowmelt period of high stream discharge. An increase in soluble Zn occurred in midwinter in the inlet and was
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COVER
Soluble Zinc Isopleth(micromois.i-' )
2-
Flgure 1. Temporal and spatial variations in soluble Zn. Triangles represent sample collection dates. Isopleth lines were determined by linear interpolation between observations in data.
associated with a minor melt event where dissolved organic carbon (DOC) concentrations were also elevated (38). Overall, influx rates of Zn were nearly equivalent to efflux rates throughout the study year. It should also be emphasized that particulate Zn comprised typically less than 6% of the total concentration of Zn in these waters, and concentrations of total suspended solids in the inlet stream were less than 1 mg L-' (39). Water Column Chemistry. Darts Lake is dimictic; complete turnover (isothermal conditions) occurred in November and April of the study year. Orthograde distributions in soluble Zn (Figure 1) and other chemical parameters (17, 22, 32) were noted during turnover. During winter stratification, elevated concentrations of Zn (0.7-1.0 pmol L-I) were observed in the epilimnion and corresponded with elevated concentrations of DOC (Figure 2a) and decreases in pH (Figure 2b). The low pH and relatively high concentrations of soluble Zn and DOC at the lake surface during snowmelt may be attributed to inputs of low-density acidic melt water. On the basis of on-site climatological data, minor snowmelt events were likely during these periods. During summer stratification no significant temporal or spatial variations in soluble Zn were observed. However, a clinograde DOC distribution was noted, with concentrations ranging from 0.24 mmol of C L-l in the epilimnion to 0.34 mmol of C L-l in the lower waters. The source of hypolimnetic DOC may have been the decomposition of sediment organic matter and diffusion into the water column. Elevated concentrations of DOC, Fe, Mn, and A1 in the vernal hypolimnion suggest some disruption of the sediment matrix serving as the source of these solutes. Although anoxia was not observed in the hypolimnion, DO concentrations were low (minimum summer DO = 0.06 mmol L-l), and reduced rates of Fe and Mn oxidation within hypolimnetic waters may account for their increase in concentration. The chemical disruption of the sediment matrix, apparently through breakdown of Fe and Mn oxides and organic matter, appeared to cause remobilization of P b to the water column in Darts Lake (22) but did not result in the remobilization of Zn to the water column. Therefore, despite extensive temporal and spatial variations in pH and DOC (Figure 2), Zn concentrations remained relatively constant throughout much of the study period. Concentrations of soluble Zn observed in Darts Lake (0.15-1.2 pmol L-l; X f SD = 0.30 f 0.15 pmol of Zn L-l, n = 160) were higher than total metal concentrations measured in lakes of central and northern Norway (4) and northern Sweden (5), which are lakes considered to represent background concentrations since the lakes do not receive highly polluted precipitation (4). Darts Lake concentrations of soluble Zn
O
N
D
J
F
M
A M J TIME (months)
J
A
S
O
N
Flgure 2. Temporal and spatial variations in (a) dissolved organic carbon and (b) pH. Triangles represent sample collection dates. Isopleth lines were determlned by linear interpolation between observations in data.
Table I. Multiple Linear Regression of Concentrations of Acid-Labile Zn in Darts Lake Sediment Traps as a Function of Particulate Constituents constituent
p
