Carbon Isotope Mass Transfer as Evidence for Contaminant Dilution

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Chapter 14

Carbon Isotope Mass Transfer as Evidence for Contaminant Dilution Laura Toran Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6036

Carbon isotope data provided evidence for dilution of sulfate-contaminated groundwater near an underground mine; this result would not be predicted from thermodynamic reaction path modeling alone. The effect of CO outgassing and carbonate precipitation and dissolution on δ C of dissolved inorganic carbon was modeled with an integrated form of Rayleigh distillation. The observed carbon isotope ratios were 2-4 ‰lighter than those modeled for most samples, indicating an additional source of light carbon. Mixing with uncontaminated water surrounding the mine is hypothesized to explain the discrepancies. Alternative hypotheses include sulfide oxidation and CO outgassing at pH less than or equal to 5 and siderite precipitation which preferentially removes heavy carbon. 2

13

2

Mixing of different groundwaters can be difficult to recognize when dissolved constituents are controlled by similar processes and sources in the mixed waters. Modeling of isotope ratios can provide an additional source of information to indicate mixing. Carbon isotope ratios were measured and calculated for groundwater that has been contaminated with sulfate and undergone C 0 outgassing and carbonate dissolution and precipitation. Geochemical modeling can help explain how sulfate contamination occurred and provide information that will help predict or prevent future contamination. However, additional data on carbon isotopes was needed to try to better understand the observed carbon mass transfer. Carbon isotope modeling suggests additional processes such as dilution that may be important. Although the lack of chemical data from before and, in particular, during mining makes it difficult to obtain a unique reaction path, the modeling in this study points out alternative mechanisms and new data collection needs. Sulfide oxidation in a carbonate environment involves two sets of reactions: oxidation and neutralization. Sulfide minerals exposed to the atmosphere can oxidize to produce sulfate and acidity (Equation 1): 2

FeS, + ]_Oy + Η,Ο - Fe

2+

+

=

+ 2 H + 2SO, .

2

0097-6156/90/0416-0190$06.00/0 c 1990 American Chemical Society

Melchior and Bassett; Chemical Modeling of Aqueous Systems II ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

(1)

14. TORAN

191

Carbon Isotope Mass Transfer

In a carbonate environment, dissolution of minerals such as dolomite neutralizes the acidity and increases the HC0 ", M g , and C a in solution (Equation 2): 2+

2+

3

2H

+

4- 2 S 0

= 4

+ MgCa(C0 ) - 2 S 0 3

2

=

+ Mg

4

2+

+ Ca

2+

+ 2HC0 . 3

(2)

These reactions can cause the system to become saturated with respect to calcite, iron hydroxide, siderite, and some sulfate minerals. Furthermore, if the neutralization takes place in an open system, C 0 outgassing may occur as the dissolved inorganic carbon (DIC) increases the partial pressure of C 0 . 2


2

SITE DESCRIPTION The Shullsburg mines in southwestern Wisconsin present an example of these reactions. The mines are part of the upper Mississippi Valley zinc-lead district and were operated from the 1920s until 1979. While the mines were exposed to air, sulfide minerals in the GalenaPlatteville Formation oxidized to sulfate. When groundwater re-entered the mined formation after mine closure, the high sulfate concentrations spread to the local groundwater system (Figure 1) and forced closure of 11 wells (1,2). However, because the surrounding carbonate rocks neutralized the pH, acidity and trace metals were not an environmental problem. Contaminated water is defined here as having sulfate concentration greater than 1 mmol/L. The contaminated water had higher C a , M g and S 0 than uncontaminated water from the surrounding area (Figure 2). However, contaminated and uncontaminated samples had similar pH and HC0 " values. 2+

2+

=

4

3

R E A C T I O N PATH M O D E L I N G The computer program P H R E E Q E (3) was used for chemical equilibrium and reaction path modeling. P H R E E Q E does speciation and mass transfer calculations to find the distribution of aqueous complexes and the saturation indices of potential mineral phases present. The P H R E E Q E reaction path model of sulfide oxidation in the Shullsburg mines includes five chemical changes. (1) Sulfide oxidation increases S 0 and H according to the stoichiometry of Equation 1. (2) Dolomite plus or minus calcite dissolution buffers pH, increases C a and M g , and initially increases HC0 " in the water. (3) Calcite precipitation occurs when the dissolution of dolomite causes the solution to become saturated with respect to calcite (as it was in all cases modeled). (4) Iron concentration can be reduced by precipitation of an iron mineral such as siderite (FeC0 ), jarosite (NaFe (S0 ) (OH) ), or amorphous iron hydroxide (Fe(OH) ). It was unclear what mineral or phase controls iron concentration, especially since precipitation was sensitive to oxidation potential (pe), and the field redox electrode measurements used in the model are not always relevant for specific redox couples. Since the mine workings flooded before the study was begun, samples of the solid phases could not be collected. (5) To reduce the DIC concentration, C 0 outgassing is fixed by selecting an appropriate log P to match the observed DIC and pH. The amount of carbon observed in the Shullsburg waters was less than the modeled carbon in a closed system. An increase in the amount of sulfide oxidation implies an increase in the amount of carbonate dissolution in a 1:1 stoichiometric ratio (Equation 2). When calcite precipitation, driven by dolomite dissolution, is included in reaction modeling, the relationship between sulfate and carbon becomes nonlinear (Figure 3). Even for this nonlinear model, there is a discrepancy between measured carbon concentrations and modeled closed system carbon concentrations. =

+

4

2+

2+

3

3

3

4

2

6

3a

2

C G 2



Figure 1. Location map, average sulfate concentrations near the Shullsburg mines measured between 1983 and 1985 in the Galena-Platteville formation (IT), and groundwater flow directions measured in 1983 (19).



TORAN


Figure 2. Histogram of major cations and anions for an uncontaminated sample used in modeling (S-27), a contaminated sample (S-18), and results of P H R E E Q E reaction path model.


193


I

5

I

Γ"

7

Ί

•

CJ

Τ~

9

I

13

Alkalinity. meq/L

11

f

Γ

15

-

Ί

17

_

Ί

2

Figure 3. Sulfate versus alkalinity for contaminated Shullsburg samples. Diagonal curves show P H R E E Q E calculations for total carbon production during closed system oxidation and neutralization using high and low initial DIC. Samples to the left of the closed system region have undergone C 0 outgassing at nearly constant pH.

10 -

15

20

25

30

35

_


14. TORAN

195


Loss of carbon could be accounted for by C 0 outgassing in an open or "partially" open system. The predicted amount of carbon lost is not large enough to correspond to a system in equilibrium with a partial pressure of C 0 ( P 2 ) ^ 10 - However, the system would not have been completely closed, since oxidation took place in open mine workings, exposed to air. "Open" and "closed" are end members for what is actually a variable system in open caves (4), and the P in water is difficult to predict (5). The calculated P in contaminated water varies from 10" to 10" atm, which is typical of soil gas P or streams from carbonate caves (6). To model the C 0 outgassing along with the sulfide oxidation and carbonate neutralization, the carbon concentration was matched by selecting a log P , different for each sample modeled. Fixing the amount of C 0 outgassing converts the P H R E E Q E model to a mass balance calculation for carbon since this constraint is not based on equilibration with a known phase. Although the kinetic control on outgassing was not known, alkalinity seemed to have an upper limit of around 10 mmol/L in this carbonatepH buffered system. The P in the models varied from 1 0 ' to 10" atm, the range being close to observed values, but differing slightly because of differences in modeled and measured pH. Matching the carbon rather than the P provides the input data needed for carbon isotope modeling. The modeled and observed cation concentration could be matched closely (Figure 2). The small differences between observed and calculated concentrations can be attributed to differences in starting water, mixing, analytical error, or lack of information about the controlling iron phase. 2

m

2

t n e

T

a l

35 a t m

œ

C Q 2

C Q 2

15

15

C 0 2


2

C02

2

16

2

C Q 2

C G 2

M E T H O D S O F C A R B O N ISOTOPE M O D E L I N G Carbon isotope effects of outgassing and carbonate dissolution and precipitation can be modeled to gain more insight into the reaction mechanism and environment of oxidation. These effects were modeled using an integrated form of Rayleigh distillation derived by Wigley et al. (7). Their equation expresses the isotope consequences of equilibrium fractionation between multiple inputs and outputs. The input parameters for the Rayleigh model are mass transfer coefficients for carbon, fractionation factors for each output, initial carbon isotope ratio, and the carbon isotope ratio of any input carbon. Carbon isotope modeling has been used to distinguish closed and open carbonate systems (8), calculate the s C of C 0 dissolved in water for radiocarbon age determinations (9), investigate different modes of calcite deposition in caves (4), and determine sources of water discharging through an arid canyon (10). For Shullsburg groundwater, the inputs and outputs that may affect carbon isotope ratios included C 0 outgassing, dolomite dissolution, calcite precipitation, siderite precipitation, and dilution during water level recovery. The isotopic composition of carbon in these reactions depend on two factors that can no longer be measured in the Shullsburg mines: the pH during sulfide oxidation and the composition of incoming water that dilutes the contaminated mine water. These factors may be quite variable. The pH in an analogous mine that is still open (measured with pH paper) varied from 5.5 on damp mine walls to 7 in drips from the wall and ceiling (11). During sulfide oxidation, it is possible that acidic microenvironmenls occur, and the outgassing and precipitation might take place in these microenvironments. In any case, the carbon isotope fractionation factors are constant below a pH of about 5. In the carbon isotope modeling, the pH was varied between 5 and 7 to study the

Carbon Isotope Mass Transfer as Evidence for Contaminant Dilution

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