Environ. Sci. Technol. 1994, 28, 1211-1218
Changes in Inorganic Carbon Chemistry and Deposition of Onondaga Lake, New Yorkt Charles 1. Drlscoll'
Department of Civil and Environmental Engineering, 220 Hinds Hall, Syracuse University, Syracuse, New York
13244-1 190
Steven W. Effler and Susan M. Doerr
Upstate Freshwater Institute, P.O. Box 506, Syracuse, New York
13214
Changes in Ca2+,alkalinity, dissolved inorganic carbon (DIC) and pH have occurred in the epilimnion of Onondaga Lake over a 10-yr period, in response to a major reduction in external Ca2+loading that resulted from the closure of an adjoining soda ash manufacturer. These changes included reduced Ca2+concentrations, diminished depletion of DIC and alkalinity in the upper waters in the summer, and a shift to higher pH values. Results of analyses from sediment trap collections before and after closure of the facility indicate approximately 3-fold reductions in particulate inorganic carbon (PIC), and suspended solids deposition have occurred as a result of the closure. Despite these changes, the water column remains oversaturated with respect to the solubility of CaC03. Moreover, there was no apparent relationship between the degree of CaC03 saturation and PIC deposition. Changes'in the extent of CaC03 precipitation have altered the acid-base chemistry of the lake which, in turn, affects prevailing pollution problems associated with inputs of domestic waste, such as NH3 toxicity.
adjoining soda ash (Na2C03) manufacturer. The loading of this ionic waste has decreased dramatically in the interim, due to the closure of the facility. This closure has presented an opportunity to examine the impact of the waste discharge on the in-lake concentrations of Ca2+ and related species and on the precipitation and deposition of CaC03. In addition, through this change it is possible to gain insights into factors regulating DIC and Ca2+ chemistry and CaC03 precipitation in hard-water lakes. In this paper, we document the changes in epilimnetic values of Ca2+,alkalinity, DIC, and pH in Onondaga Lake that resulted from decreases in the loading of ionic waste associated with the closure of an adjoining Na2C03 facility. Water chemistry conditions with respect to CaC03 solubility were evaluated for periods before and after closure with a chemical equilibrium model; the model was also used to examine the mechanism(s) responsible for an observed increase in pH. Finally, changes in the deposition rates of particulate inorganic carbon (PIC) since closure of the industry are documented and corroborated by alkalinity and DIC losses from the water column.
Introduction Inorganic carbon constitutes the major pH buffering system in most fresh waters. Many hard-water lakes become oversaturated with respect to the solubility of CaC03 during the productive summer period, for both physicochemical (I, 2) and biochemical (e.g., photosynthesis; refs 3-6) processes can stimulate CaC03 precipitation. The precipitation and deposition of CaC03 (usually as calcite) is a widely observed phenomenon in hard-water lakes (7-9). This process results in the removal of aqueous Ca2+ and dissolved inorganic carbon (DIC) and often results in the accumulation of CaC03 in underlying sediments (9). The phenomenon has broader importance, as it may influence the cycling of other constituents such as P (10, 111, dissolved organic carbon (DOC) (121, and particles (e.g., clays, organic particles) that serve as nuclei for precipitation (13). Further, water clarity (e.g., as measured with a Secchi disc) may decrease greatly during periods of CaC03 precipitation due to increases in light scattering (14-16). Temporal and vertical variations in the aqueous chemistry of dissolved inorganic carbon (DIC) and Ca2+ and precipitation of CaC03 in the epilimnion of Onondaga Lake have been documented for the summer-fall interval of 1980 (4). Extremely high concentrations of Ca2+(mean value 15.6 mmol L-I) prevailed in the lake during that study as a result of the input of ionic waste from an f
Contribution No. 126 of the Upstate Freshwater Institute.
* Author to whom inquiries should be addressed;e-mail address:
[email protected]. 0013-936X/94/0928-1211$04.50/0
0 1994 American Chemical Society
Methods Study Site. Onondaga Lake is a polluted (13,hypereutrophic (181,dimictic lake located within metropolitan Syracuse, NY. The lake has a surface area of 11.7 km, a volume of 131 X 106 m3, a mean depth of 12.0 m, and a maximum depth of 20.5 m. The lake flushes rapidly, four times per year on average (19). Soda ash production began on the western shore of the lake in 1884,using the Solvay process. The abundance of limestone (CaC03) and NaCl brines and deposits in the Syracuse area and the proximity of the lake for disposal of wastes and as a source of cooling water made the shores of the lake an ideal location for the production of Na~C03. The generation of ionic waste was a nearly stoichiometric relationship with Na2C03 production; for each mole of Na2C03 produced, approximately 1.0,2.8, and 4.7 mol of Ca2+,Na+, and C1-, respectively, were released as waste. Prior to about 1945,this waste was released directly to the lake; in recent years, it reached the lake via a fluvial input or with a domestic waste discharge (17). The Na2C03 production facility closed in February 1986. The reductions in the annual loading rates of C1-, Na+ and Ca2+associated with the closure of the NazC03 facility are depicted in Table 1, by comparing averages for 3 yr intervals before (1983-1985) and after (1987-1989) the closure. Inputs for the earlier interval are generally indicative of conditions that prevailed for at least 15 yr before the closure. Loads are based on a biweekly monitoring program of the major tributaries (20). Techniques used to develop the annual loads for these materials have been described in detail elsewhere (21). Reductions of about 75,60, and 70% have occurred for C1-, Na+, and Environ. Sci. Technol., Voi. 28,
No. 7, 1994 1211
Table 1. Average Annual Loads for C1; Na+, and Ca2+ for Two Intervals, 1983-1985 and 1987-1989
constituent
c1-
Na+ Ca2+
loading ( X 108 mol yr-1) 1983-1985 1987-1989 18.9 7.7 5.9
av concn % reduction
76 59
4.6 3.1 1.8
-
CaCO,
+ H+ f C1-
(1)
The decrease in CaC03 precipitation following closure would be accompanied by reduced H+ production and thereby a tendency for increases in pH. Field and Laboratory Methods. Data reported here are from two different programs, a long-term program (conducted since 1969; used for information on cation concentrations only) in which lake-water samples are collected biweekly at 3-m depth intervals for the early spring-late fall interval, and recent studies that have included more intensive monitoring and investigations of in-lake processes (e.g., refs 4 and 22). Lake monitoring is conducted at a single deep water location in the southern basin of the lake, found to be representative of overall lake conditions (23). Temperature profiles were made with a Montedoro-Whitney thermistor. Water chemistry samples were collected in the late morning. Opaque collection bottles were filled completely and tightly closed to minimize gaseous loss of COz. Measurements of pH were made in the laboratory, usually within 2 h of sample collection. Although water chemistry data are presented for 1980-1990, we have focused our analysis on three study periods: 198011981, a period of very high Ca2+ loading and water column concentrations; 1985,a year of somewhat lower water column Ca2+ concentrations but prior to closure; and 1989,a year of much lower Ca2+concentrations due to reduced industrial discharge. Sediment traps ( 1 1 ) were deployed in triplicate at the routine monitoring site at a depth of 10 m for intervals of 1 wk for the June-October period of 1985 and for the April-October period of 1989. The epilimnion was contained within the upper 10 m until mid-August 1985 and until mid-September 1989. The traps were simple cylinders with aspect ratios of 6:l (length:diameter), consistent with the recommendations of various investigators (24,251. Average deposition rates are presented here. 1212 Envlron. Scl. Technol., Vol. 28, No. 7, I994
1m depth, 1989
analyte Ca2+
70
ea2+,respectively, as a result of closure of this industry (Table 1). Approximately 50,40, and 35 % of the prevailing loads of these materials are estimated to be associated with the continuing input of ionic waste from adjoining waste beds [ (21);as leachate and contaminated groundwater]. The estimated average hydrologic loading for the two 3-yr periods of Table 1were similar, 14.9 and 13.8 m3 s-l, respectively. The external loading of P to the lake has also been reduced by a factor of 2 since 1980, through improved removal of P at the adjoining municipal wastewater treatment plant that discharges to the lake (22). Despite this change, phytoplankton growth in Onondaga Lake is not limited by the availability of P (22). Changes in the loading of ionic waste have implications for the acid-base status of Onondaga Lake. Inputs of Ca2+ as a neutral salt (e.g., C1-) followed by CaC03 precipitation results in the production of H+, e.g. Ca2++ C1- f HCO;
Table 2. Laboratory Analysis Used in This Study and Average Values of Water Chemistry at 1 m Depth for 1989
alkalinity (HC03-) PH
method
ref
Water Chemistry Methods atomic absorption 26 spectrophotometry titration, HzS04 (pH of 4.3) 26
laboratory 26 titration, Hg (N03)2 26 turbidimetric, Bas04 26 Na+ atomic absorption 26 spectrophotometry Mg2+ atomic absorption 26 spectrophotometry temperature thermister Sediment Trap Methods ss gravimetric 26 PIC gas chromatography 27 POC gas chromatography 27
c1sod2-
(mmol L-1) 3.9 3.2 meq L-1 8.00unit.s 12.3 1.7 8.5 1.0 17.1 "C
The principle water column analytes for this study were pH, alkalinity and Ca2+. Other constituents measured that were utilized to support equilibrium model calculations included C1-, Na+, Mg2+,and S042-. These analyses were performed on unfiltered samples. Sample handling and analysis were conducted according to Standard Methods (26). Particulate inorganic (PIC) and organic carbon (POC) and total suspended solids (SS) analyses were conducted on aliquots of sediment trap collections. Analytical methods are summarized in Table 2. Calculations. Chemical equilibrium calculations were conducted with the model MINEQLf (28). Thermodynamic data used in this analysis were obtained from the chemical equilibrium model WATEQ (29). Two types of calculations were conducted with the model. Chemical speciation was calculated for fixed (ambient)pH conditions in which chemical precipitation was not allowed to occur. Dissolved inorganic carbon (DIC) concentrations and values of the partial pressure of C02 (PcoJwere calculated from measured values of alkalinity and pH with correction for temperature and ionic strength. Saturation index values (SI) were determined from these calculations, according to SI = log Qp/Kp where SI is the saturation index with respect to calcite solubility, Qp is the ion activity product ((Ca2+)(C03"))of the solution, and Kp is the thermodynamic solubility constant for calcite [ ~ K s = o 8.475 (2913. Positive, negative, and zero SI values suggest that a solution is oversaturated, undersaturated, or in equilibrium, respectively, with respect to calcite solubility. The second type of chemical equilibrium calculation included a chemical "titration" to examine the process responsible for the observed shift in pH in the upper waters. In this calculation, Ca2+added as a C1- salt was varied from 1.25 to 22.4 mmol L-l to assess changes in the water column chemistry that occurred due to changes in the Ca2+ loading to the system. The upper range of the Ca2+ concentration is indicative of concentrations when the Na2C03 manufacturing facility was in operation. The lower values are more representative of conditions after closure. Calculations were made using water chemistry components
(other than Ca2+ and C1-) that are average values for 1989 (Table 2) and assuming the system was in equilibrium with atmospheric C02 (Pco, = 10-3.5 atm). The downward fluxes of suspended solids (dry weight), PIC, and POC (LR,; mg or mol of x ; x m2 day1) were determined for the individual deployment periods from the mass of the constituent ( x ) collected in the trap (W,), the sediment trap deployment period (T; days), and the area of the trap opening ( A ; m2), according to the relationship LR, = W,/(AT)
(3)
20n
.-
5
;
E
W
.-5 0
0
12. 8. 4
0
0. n
r
1
4
=
3
J
Results and Discussion Water Column Measurements. Surface water concentrations of Ca2+in Onondaga Lake were very high in the early 1980s (11-17 mmol L-I) and declined somewhat through the mid-1980s (Figure la). The major feature of the time series is the large decrease in Ca2+ concentration that immediately followed the closure of the Na2C03 facility. The rapid response is a manifestation of the high flushing rate of the lake (19). The average Ca2+concentrations over the 1983-1985 and 1987-1989 periods were 11.3 and 4.3 mmol L-1, respectively. The reduction in lake Ca2+ concentrations (62%) was similar to that reported for loading over the same interval (Table 1). A large disparity in the seasonality of Ca2+concentrations before and after closure also was evident (Figures l a and 2a). Calcium [as well as C1- and Na+ (30)l concentrations increased greatly in the epilimnion during the stratification period before closure of the facility. At first consideration, this pattern may seem inconsistent with the precipitation and deposition of large quantities of CaC03 ( 4 ) . Indeed, epilimnetic depletion of Ca2+and DIC is widely observed during the summer (2, 5, 31) in unpolluted hard-water lakes. The atypical seasonal pattern of Ca2+concentration reflects the hydrodynamics of the ionic waste entering Onondaga Lake. Inputs of the high-density saline waste plunged to the lower waters during periods of (vertical) isodensity, but low ambient turbulence (30). Thereafter, the waste entered the upper layers, resulting in the progressive enrichment of Ca2+ (30)and thereby masking the loss of Ca2+associated with precipitation and deposition of CaCOs. The absence of this strong seasonality following closure of the facility is a manifestation of the greatly diminished hydrodynamic impact of the reduced loads of ionic waste (21). The continuing load of ionic waste (Table 1)is apparently still enough to obscure clear manifestations of decalcification in the epilimnetic time series of Ca2+ (Figures l a and ea). In sharp contrast, decalcification of the epilimnion of Onondaga Lake has been clearly manifested annually by the marked depletion of alkalinity and DIC during summer stratification (shown as surface alkalinity in Figures l b and 2b, as surface DIC in Figure Id, and as selected water column alkalinity profiles for June, July, and August in Figure 3a-c). This pattern coincideswith avery high Ca2+: DIC molar ratio in the upper waters prior to closure (Figure le). The increases in alkalinity and DIC in the fall reflect continued loading, entrainment of enriched lower waters (21) with the approach to fall turnover, and probably reductions in precipitation in the CaC03. The extent of alkalinity and DIC depletion has decreased since closure of the Na2C03 facility in 1986, indicating a reduction in CaC03 precipitation in the lake. This reduction in
16.
W
.3
.--
2
C
SU ’ 0 8.5
f(C>
*
.
.
.
.
.
t
8 .o
Ip 7.5
7.0
6.5 h
-
4
I
-I
-0‘
3
E
2
0 a
1
E
W
0
12
9n
..
9
(5:s 3 0
80
.
.
82
84
.
.
86
.
.
88
.
.
90
Year Figure 1. Time series of Ca2+ (a), alkalinity (b), pH (c), dissolved inorganiccarbon (d), and Ca:DIC molar ratio (e) for the surface waters of Onondaga Lake.
decalcification was also clearly evident in individual water column profiles (Figure 3a-c). Prior to closure, marked depletion of alkalinity and DIC in the epilimnion occurred early during the summer (June, July; Figure 2b). Following reductions in Ca2+loading, loss of alkalinity and DIC in the water column was not evident until later during stratification (August; Figure 2b). A strong relationship was evident between depletion of alkalinity (AALK) and mean Ca2+in the surface waters of Onondaga Lake for the summer period of the years of study [AALK (mequiv L-1) Environ. Sol. Technol., Vol. 28, No. 7, 1994
1213
Alkalinity
(a) epiiirninion 0-0
15
0-0
”-
1985 1989
0
0
2
4
0
(meq . L-’)
2
4
0
2
4
6
4 n
-E 8
5
2 12
n
16
0 -’
20
(b) epilimnion
pCo2 (10-3 0.1
5
10
15 0.1
5
10
a
otm)
15 0.1
5
10
15
0
:3
4 n
vE
8
5
’ 1 12
0
0
Apr
May
Jun
JuI
Aug
Sep
Oct
Figure 2. Temporal patterns in the concentrations of Ca2+ (a) and alkalinity (b) in the surface waters of Onondaga Lake for 1985 and 1989.
= 0.13 Ca2+(mmol L-l) -t0.89; r2 = 0.72, n = 81. Concentrations of alkalinity and DIC have increased throughout the study period, particularly since 1986 (Figure 1). This pattern coincides with the decline in Ca2+concentrations and is consistent with decreased deposition of PIC from the water column (discussed below). Calculations of PcoZ indicate near equilibrium with atmospheric COz in the upper waters and increase with depth (Figure 3d-f). The water column of Onondaga Lake was oversaturated with respect to atmospheric C02 for virtually all dates sampled. The accumulation of Pco, in the lower waters has declined markedly from 1980to 1989, presumably due to declines in the deposition of POC (discussed below). Deposition. Average deposition rates of suspended solids, PIC, and POC for the May-September interval are presented in Table 3. Levels of precision of sediment collection by the traps compared favorably with values reported in the literature (24,25). For example,the average coefficient of variation for PIC deposition was 10.0% in 1985 and 12.4% in 1989. The deposition rate of PIC was much lower in 1989 than in 1985 (by about a factor of 3; Table 3). The reduction in PIC (i.e., CaC03) deposition following closure of the Na2C03 facility documented here is consistent with the decrease in depletion of alkalinity and DIC in the water column (Figures 1-3) over the same period. Calcium carbonate deposition played a dominant role in overall deposition in the lake in both years: Assuming PIC deposition occurred as CaC03, approximately 60% of the total dry weight deposition was associated with CaC03 for both study periods (Table 3). The downward fluxes of other materials (i.e.,P and organic C) may also be coupled to CaC03 deposition (4, IO). The downward flux of POC is considered an indicator of the level of lake primary productivity (32). Values for Onondaga Lake (Table 3) are consistent with the literature for other eutrophic lakes (21,32). About a 40% reduction 1214
Environ. Scl. Technol., Vol. 28, No. 7, 1994
16
20
Calcite Saturation Index 0.0
0.5
1.0
1.5
[j! -Figure 3. Selected profiles of alkalinlty (a-c), partial pressure of C02 (Pco,; d-f), and CaC03 saturation index (SI;g-i) for June-August in 1980 (0)and 1989 (0).
in POC deposition is reported for the later interval. However,much of this reduction may reflect the increased losses of phytoplankton organic C to zooplankton grazing (22) and the decreases in coprecipitation of organic particles with CaC03 (13). The reduction in PIC (Le., CaC03) deposition from 1985 to 1989 (64%; based on total study periods in Table 3) matches the relative decreases in external loading (70% ) and epilimnetic Ca2+concentrations (62 % ) observed since closure of the Na2C03 facility. The reduction in Ca2+ loading that accompanied the closure is likely the primary cause for the observed decrease in CaC03 (i.e., suspended solids) deposition in Onondaga Lake, although the possibility that a modest coincident reduction in primary production may have contributed to the decrease in the downward flux of CaC03 cannot be discounted. CaC03 Solubility Calculations. Chemical equilibrium calculations suggest that the water column of Onondaga Lake was oversaturated with respect to the solubility of CaC03 prior to and following closure; results for the upper waters (depth of 1m) are presented in Figure 4. Average and median values of SI were similar despite large changes in Ca2+ loading. These consistent values may reflect the compensation for decreased Ca2+ con-
Table 3. Summary of Average and Standard Deviation of Deposition Values from Sediment Traps for 1985 and 1989
suspended solids (mgm-2 d-l) SD av 39 400 14 030 13 230 7 730
war 1985 1989
particulate inorganic carbon (mmol of C m-2 d-9 av SD n 215 95 20 78 59 21
n
20 21
particulate organic carbon (mmol of C m-2 d-1)
SD 65 33
av 101 54
n 18 21
40 A
avg = 0.83 std = 0.27 n = 46
(a) 1980 & 1981
30
300 20
1
A
e
1985 1989
A A A
A
1 10
0.0
0 (b)
ovg = 0.61 std = 0.18 n = 21
1985
n
30
20
10
0 avg = 0.98 std = 0.22 n = 23
( c ) 1989
30
20
1I
10
0 (
Calcite Saturation Index ( l m )
Figure 4. Distribution of CaC03 saturation index for surface waters for the May-September Interval of 1980/1981 (a), 1985 (b), and 1989 (C).
centration by the increase in pH in recent years. The dynamics of CaC03 SI in the upper waters generally track those of pH (41,as observed in other productive systems (3-5,31). The CaC03SI valuesfor Onondaga Lake (Figure 4) fall within the range reported for other productive hardwater lakes (4). Example profiles of CaC03 SI for 1980 and 1989 (Figure 3g-i) demonstrate that conditions of oversaturation prevail throughout the water column for the early spring to late fall interval (and probably year-round). Moreover, water column profiles of Ca2+do not show any increase in lower water concentrations during summer stratification. Thus, deposited CaC03 does not appear to be remobilized through dissolution reactions in the hypolimnion. This pattern is atypical. In hard-water lakes, dissolution of at least a portion of the deposited CaC03 generally occurs by
0.5 1 .o Calcite Saturation Index
1.5
0
(lm ovg)
Figure 5. Deposition of partlculate inorganlc carbon from the upper waters as a function of CaC03 saturation index in the surface waters for 1985 and 1989.
late summer due to the production of C02 and the decreases in pH in the lower waters. This process is usually manifested as positive CaC03 SI values in the epilimnion with decreases to negative values in the hypolimnion and increases in Ca2+concentration in the hypolimnion (5, 31). Note that errors in CaC03 SI calculations for the lower waters may occur due to changes in pH associated with CO2 loss after sample collection. While this error is difficult to quantify, calculations with MINEQL+ suggest that Onondaga Lake waters remain oversaturated with respect to CaC03 solubility over a wide range of Pco, values. It does not appear that errors associated with C02 degassing in water samples alter our assessment that the entire water column of Onondaga Lake is oversaturated with respect to CaC03 solubility. Water quality models that depict the deposition of CaCO3 generally utilize rate expressions that are a function of the degree of oversaturation with respect to CaC03 solubility (i.e., second-order rate dependence; ref 33). However, for Onondaga Lake no relationship was evident between paired observations of CaC03 SI and PIC deposition in sediment traps (Figure 51, despite the major reduction in PIC deposition since closure of the NazCOa facility (Table 3). This lack of a relationship demonstrates that CaC03 SI is not a good indicator of the magnitude of CaC03 precipitation and deposition, and a functional relationship between the extent of CaC03 saturation and CaC03deposition probablyshouldnot be invoked in waterquality modeling efforts. We expectthis poor relationship to extend to other hard-water lakes that experience CaC03 precipitation. For example, Stabel (34) reported that seasonal CaC03 SI maxima did not coincide with the maximum downward fluxes of CaCO3 in Lake Constance. Previously, we demonstrated that seasonal variations in Ca2+deposition and CaC03SI were only weakly correlated in Onondaga Lake (4). Impacts and Implications for Future Remediation Efforts. The ionic waste discharge from the Na2C03 facility has clearly promoted precipitation and deposition of CaC03in OnondagaLake. The magnitude of the impact is demonstrated by comparison with deposition rates Envlron. Scl. Technol., Vol. 28, No. 7, 1004
1215
4c
Table 4. Comparison of Literature Deposition Rates of Suspended Solids (Sediment Traps) to Values Reported Here for Onondaga Lake (Modified from Ref 35)
( a ) 1980 & 1981
ovq = 7.62 std = 0.27
30
location Blelham Tarn Canadargo Lake Lawrence Lake Windermere Ennerdale Water Wastewater Lake Suwa Char Lake Konigsee’ Oberlinger See4 Lake of Lucerne4 Rotsee4 Bay of Quinte, 1971 Limno Corrals in Bay of Quinte Lake Constance Wintergreen Lakes Onondaga Lake 1985 1989 a
trophic state eutrophic oligotrophic mesotrophic oligotrophic oligotrophic oligotrophic oligotrophic meso-/eutrophic mesotrophic eutrophic eutrophic eutrophic hypereutrophic range hypertrophic
n = 46
deposition rate (mg m-2 d-1) 9000 1000-5000 500 600 500 9000 lo00 650 1250-3100b 3150 2700 5000-18000 2600-20400
ref 35 36 37 38 38 38 35 35 39 39 39 39 40 35
450-1140 34 3700-6 100b 41 100-30000 25 39 400 13 230
this study this study
20
r 10
0 (b) 1985
0
5
20
8
w
n = 21
1
10
0 (c) 1989 30
1216 Environ. Scl. Technol., Vol. 28, No. 7, 1994
r
C
Average for 1 yr, b Range of different years.
reported for other systems. Both the 1985 and 1989 downward fluxes of PIC (CaC03) for Onondaga Lake (Table 3) greatly exceed the limited number of values reported in the literature (21, 32, 34). The associated deposition rates of suspended solids are also exceedingly high by comparison to the more substantial list of fluxes availablefor this parameter (Table 4). The 1985suspended solids dry weight flux for Onondaga Lake exceeds allvalues encountered in our review of the literature; the 1989 rate exceeds all but one (Table 4). The sediments of the lake are enriched in CaC03. Auer et al, (42) found that the mean CaC03 content of the surficial sediments was 60% (on a dry weight basis); higher than any concentration encountered in the literature. The very high downward fluxeshave resulted in an unusually high net sedimentation rate (0.6-0.7 cm yr-l) in the pelagic zone of the lake (43). Additional decreases in the downward flux of CaC03 (i.e., dry weight) are expected with further reductions in the continuing loading of Ca2+-enrichedionic waste. Based on the documented near-linear response to loading reductions to date, elimination of the continuing ionic waste loading would be expected to result in a similar reduction in CaC03 deposition. This would be accompanied by further reductions in the summer depletion of DIC and a shift to yet higher pH values (discussed below). One of the most intriguing effects of changes in Ca2+ loading and CaC03 deposition in Onondaga Lake is the acid-base chemistry. Plant metabolism is a principal regulator of the dynamics of pH in epilimnia (3, 5, 311, particularly in productive systems such as Onondaga Lake (4). Thus, the temporal patterns of pH in productive epilimnia are recurring only in so far as the temporal structure of plant photosynthesis and respiration continue. For example,OnondagaLake annually experiencesan early spring phytoplankton bloom (18) during which some of the highest pH values occur (4). However, other seasonal (or shorter term) features are not recurring, largely because of the stochastic nature of important regulating environmental forcing conditions (e.g., light, nutrient loading). Two notable changes in the distribution of epilimnitic pH
avg = 7.61 std = 0.18
30
avg = 8.17 std = 0.17 n = 23
20
10
9
-t--s
6.6 PH
Flgure6. Distributionof pHfor surface waters over the May-September interval for 1980/1981 (a), 1985 (b), and 1989 (c).
have occurred over the period of this study (Figure 6). First, during the early 1980s, pH values exhibited a wide range (Figures IC and 6a). Second, pH values have increased following closure (Figure 6c). The wider range of pH values occurring during conditions of very high Ca2+ concentrations prior to closure were a manifestation of the lower buffering capacity of the water column that prevailed as a result of the lower alkalinity and DIC caused by the higher deposition of CaC03. The median pH value for the 1980 and 1981 observations was 7.82, the median value for 1989 and 1990 was 8.22. We attribute this shift largely to the change in the magnitude of CaC03 precipitation (eq 1). This mechanism is demonstrated with the theoretical CaClz “titration” for Onondaga Lake made using MINEQL+ (Figure 7). Added CaCl2 resulted in increases in Ca2+concentration and the precipitation of CaC03, as the solution was saturated with respect to the solubility of CaC03 throughout the titration. Increases in added CaClz coincided with decreases in pH due to the production of HC1, a byproduct of CaC03 precipitation (eq 1). The CaC03 precipitation phenomenon interacts with existing domestic waste-related problems of the lake. The lake receives very high loads of P and N (particularly as ammonia), mostly from the adjoining sewage treatment plant (19,22).Precipitation of CaC03 may have prevented full photosynthetic utilization of the P load by coprecipitation of a fraction of the input. Based on the
25
t
O2 0.0
t 4
1.50
1.75
2.00
2.25
2.50
2.75
I
3.00
Added pCaCl2
Figure 7. Concentratlonsof Ca2+(a)andpH (b) from theoretlcaltitration of eplimnitlc Onondaga Lake water (Table 2) wlth additions of CaCI2. The system was assumed to be in equilibrium with atmospheric CO?
(Pco, =
10-3.5 atm).
application of a selective extraction procedure, Wodka et al. (11)speculated that as much as 30 % of the downward flux of P in the spring-fall interval of 1980 was coprecipitated with CaC03. Toxicity criteria for free ammonia ("3) to protect fish are routinely contravened throughout the lake as a result of inputs from the sewage treatment plant (22). Conditions are most severe in the epilimnion, as higher pH values prevail there (22),causing more of the total ammonia to exist as NH3 (23). Thus, shifts to higher pH values, which would be expected to accompany continued reductions in CaC03 precipitation (by reducing Ca2+ loads), would exacerbate the NH3 toxicity problem in the lake.
Conclusions An approximately 70% reduction in Ca2+ loading to polluted, hypereutrophic Onondaga Lake, associated with the closure of an adjoining Na2C03 facility, resulted in nearly equivalent decreases in the epilimnetic concentration of Ca2+and in CaC03 deposition from the epilimnion. The reduction in CaC03 precipitation has resulted in less depletion of alkalinity and DIC, a narrower distribution of pH, and a shift to higher pH values in the epilimnion. However, the entire lake remains oversaturated with respect to the solubility of Ca2+,as observed before closure of the facility. The magnitude of SI (>O) in the epilimnion has not been a good indicator of the downward flux of CaC03 in Onondaga Lake, as the values of SI before and after closure of the facility were similar. Further changes of the form documented here are expected in response to additional reductions in loading of the Ca2+-enrichedionic waste. The high rates of CaC03 precipitation and deposition caused by the industrial waste have implications with respect to the character, extent, and manageability of certain domestic waste problems in Onondaga Lake.
Literature Cited (1) Strong, A.; Eadie, B. J. Limnol. Oceanogr. 1978,23, 877887. (2)Brunskill, G. J. Limnol. Oceanogr. 1969, 14, 839-847. (3)Effler, S.W. J. Great Lakes Res. 1984, 10, 3-14. (4) Effler, S.W.; Driscoll, C. T. Environ. Sci. Technol. 1985, 19, 716-720. (5) Effler, S. W.; Field, S. D.; Quirk, M. Freshwater Biol. 1982, 12, 139-147. (6) Otsuki, A,; Wetzel, R. G. Arch. Hydrobiol. 1974,73,14-30. (7) Effler, S. W. J. Am. Water Works Assoc. 1987,79, 80-82. (8) Kelts, K.; Heu, K. J. In Lakes, Chemistry, Geology and Physics; Lerman, A., Ed.; Springer-Verlag: New York, 1978; pp 295-324. (9) Jones, B. F.;Bowser, C. J. In Lake, Chemistry, Geology and Physics; Lerman, A., Ed.; Springer-Verlag: New York, 1978; pp 179-227. (10) Murphy, T..; Hall, K. J.; Tesaki, I. Limnol. Oceanogr. 1983, 28, 58-69. (11)Wodka,M. C.;Effler,S. W.;Driscoll,C.T.Limnol. Oceanogr. 1985,30, 833-843. (12) Otsuki, A.; Wetzel, R. G. Limnol. Oceanogr. 1973,18,490493. (13) Johnson, D. L.;Jiao, J.; Dos Santos, S. G.; Effler, S. W. Environ. Sci. Technol. 1991,25, 736-743. (14)Effler,S. W.;Perkins,M.G.;Freer,H.;Johnson,D.L. Water Resour. Bull. 1987,23, 189-196. (15) Effler, S.W.; Perkins, M. G.; Wagner, B. A. J. Great Lakes Res. 1991,17, 109-119. (16)Weidemann, A. D.; Bannister, T. T.; Effler, S. W.; Johnson, D. L.Limnol. Oceanogr. 1985,30, 1078-1083. (17) Effler, S. W. Water, Air, Soil Pollut. 1987, 33, 85-115. (18)Auer, M. T.; Storey, M. L.; Effler, S. W.; Auer, N. A.; Sze, P. Hydrobiologia 1990,200/201, 603-617. (19)Devan, S. P.; Effler, S. W. J. Environ. Eng. Diu. ASCE 1984,100, 93-109. (20) Onondaga County. Onondaga Lake Monitoring Report(s), 1984-1990 (annual); Onondaga County: Syracuse, NY, 1984-1990. (21)Effler, S. W.; Brooks, C. M.; Addess, J. M.; Doerr, S. M.; Storey, M. L.; Wagner, B.A. Water, Air, Soil Pollut. 1991, 55, 427-444. (22)Effler, S.W., Ed. State of Onondaga Lake; Onondaga Lake Management Conference: Syracuse, NY, 1994. (23) Effler, S.W.; Brooks, C. M.; Auer, M. T.; Doerr, S. M. Res. J. Water Pollut. Control Fed. 1991, 63, 280-282. (24)Bloesch, J.; Burns, N. M. Schweiz. 2.Hydrol. 1990,42,1555. (25)Rosa, F.; Bloesch,J.;Rathke, M. In Handbook of Techniques forAquatic Sediments Sampling; Mudrock, A.; MacKnight, S. C., Eds.; CRC Press: Ann Arbor, MI, 1991. (26)American Public Health Association. Standard Methods for Analysis of Water and Wastewater, 16th ed.; American Public Health Association: Washington, DC, 1.975. (27) Stainton, M. P. J. Fish. Res. Board Can. 1973, 30, 14411445. (28) Schecher, W. D.; McAvoy, D. C. Cornput. Environ. Urban Syst. 1992,16, 65-76. (29)Ball, J. W.; Nordstrom, D. K.; Jenne, E. A. Additional and Revised Thermochemical Data for WATEQ2: A Computerized Model for Trace and Major Element Speciation and Mineral Equilibria of Natural Waters; U.S.Geological Survey Water Resources Investigations: Menlo Park, CA, 1980;Report 78-116. (30)Owens, E. M.; Effler, S. W. Water Resour. Bull. 1989,25, 587-597. (31) Effler, S.W.; Field, S. D.; Wilcox, D. J. J.Freshwater Ecol. 1981,1, 141-153. (32) Kelly, C. A.; Chynoweth, D. P. Limnol. Oceanogr. 1981,26, 891-897. (33) Morel, F. M. M. Principles of Aquatic Chemistry; Wiley: New York, 1983. (34) Stabel, H. H. Limnol. Oceanogr. 1986,31, 1081-1093. Environ. Scl. Technol., Vol. 28, No. 7, 1994 1217
Charlton, M. N. Verh. Int. Verein.Limnol. 1975,11,267272. Fuhs, G.W. Limnol. Oceanogr. 1973,18,989-993. White, W. S.;Wetzel, R. G. Verh.Int. Verein.Limnol. 1975, 19, 330-339. Pennington, W. J. Ecol. 1973, 62,215-251. Stabel, H.H.Chemical Processes in Lakes; Stumm,. W.,. Ed.; John Wiley & Sons: New York, 1985. Johnson, M. G.;Brinkhurst, R.0.J.Fish. Res. Board Can. 1971,28,1715-1725. Molongoski, J. J.; Klug, M. J. Freshwater Biol. 1980, 10, 497-506.
1218 Environ. Sci. Technol., Vol. 28. No. 7. 1994
(42) Auer, M. T.; Johnson, N.; Penn, M.; Effler, S.W. In State of Onondaga Lake; Effler, S . W., Ed.; Onondaga Lake Management Conference: Syracuse, NY, 1992. (43) Rowell, C. H. Ph.D. Dissertation, State University of New York, College of Environmental Science and Forestry, Syracuse, NY, 1993. Received for review June 29, 1993.Revised manuscript received March 18, 1994. Accepted March 21, 1994." @
Abstract published in Advance ACS Abstracts, May 1, 1994.
