Analysis of Geochemical Processes

In Sections 3.5 to 3.8 the transport of SO4 in HHF groundwaters was analysed and investigasted in detail. On the basis of observed data, it was argued that the major geochemical control on SO4 transport is through dissimilatory SO4 reduction. This attenuation process is evident from the following factors :

• an abundance of organic matter within aquifer sediments;

• hydrogen sulfide can be detected (by odour) and at low concentrations in some bores;

• reduced form of N and P species;

• presence of siderite (FeCO3) and pyrite (FeS2) in aquifer sediments, which are formed by reaction with the alkalinity and sulfides released by SO4 reduction;

• indicative presence of SO4 -reducing bacteria (eg. Desulfotomaculum sp.).

Before the construction of the Loy Yang Ash Pond, the abundance of organic matter and the limited supply of SO4 to groundwater would have constrained the degree of SO4 reduction and subsequent sulfide production, as described by equation 3-11 and Section 3.6.2. With the commencement of the LYAP and the influx of high SO4 seepage waters into HHF groundwaters, SO4 was no longer limiting and higher rates of SO4 reduction could thus develop. The RWP or western embankment zone of the LYAP could therefore be expected to be accumulating the precipitated sulfides. The main uncertainty remaining in assessing the geochemistry of SO4 in the HHF is the acid buffering of the groundwater against the influx of 15 years of alkaline seepage from the LYAP. The process of SO4 reduction generally leads to the consumption of acidity and the release of alkalinity, as per equation 3-11, compared to HHF groundwaters which have maintained or slightly increased in acidity since the detection of seepage in 1982. There are two processes that may act to release or control acidity - carbon dioxide buffering (carbonic acid) and the formation of metal sulfides. These will now be analysed further.

The carbonic acid system is based on the dissolution of gaseous CO2 (pCO2) into solution (Stumm & Morgan, 1996; Langmuir, 1997) :

at 15 0C CO2 (g) ⇔ CO2 (aq) where Keq =

] pCO [ ] CO [ 2 (aq) 2 = 10-1.34 3-14 where Keq - equilibrium constant

Using 3-14 it is possible to convert from the dissolved concentration measured in the field to partial CO2 gas pressures (pCO2). The dissolved CO2 can then form carbonic acid or H2CO3, a diprotic acid. This can release acidity (H+) according to the following reactions (Appelo & Postma, 1994) :

CO2 (aq) + H2O ⇔ H2CO3 3-15a

H2CO3 ⇔ H+ + HCO3- Keq = 10-6.3 3-15b

HCO3- ⇔ H+ + CO32- Keq = 10-10.3 3-15c

The second possible source of acidity is through the sulfide released from SO4 reduction. The sulfide is thought to precipitate as iron sulfide minerals, and in doing so can release the hydrogen ions (H+) into solution according to (Dvorak et al., 1992; Stumm & Morgan, 1996; Langmuir, 1997) :

M2+ + H2S (aq) ⇔ MS + 2H+ 3-16

On the basis of the high SO4 seepage over the operational life of the LYAP, it may be possible that the precipitation of sulfides is releasing sufficient acidity to control the pH of HHF groundwaters, due to protons from the hydrogen sulfide or bisulfide.

One approach to ascertain the effect of dissolved CO2 and sulfides on groundwater chemistry is through the use of geochemical modelling of the aqueous chemistry. A geochemical study of the mineral saturation states of HHF groundwaters will be undertaken using the geochemical model PHREEQC (Parkhurst, 1995). All data used in Table 3.13 is from April 1998, as this represents a reliable and extensive data set.

In this analysis, the following bores and monitoring points were used :

• 3138U Background groundwater chemistry before the influence of ash seepage;

• LYAP Ambient chemistry of the ash pond;

• RWP Seepage collection pit at the toe of the ash pond embankment (200 m west from the LYAP);

• 2124U Intermediate monitoring bore (550 m west from the LYAP);

• 3135U Downgradient monitoring bore (1,350 m west from the LYAP).

Table 3.13 - Groundwater Chemistry Data Used for PHREEQC Analysis

Site pH Eh Na K Ca Mg Fe SO4 HS- Cl HCO3 CO2 (aq) 3138U 6.5 45 120 3.1 19 7.2 27 23 - 210 - -

LYAP 9.0 - 1,800 74 360 46 0.27 4,300 - 590 321 - RWP 4.9 125 1,200 32 64 75 23 2,800 0.5 550 12 238 2124U 4.3 136 330 0.5 8 110 18 810 1.0 340 <2 343 3135U 4.6 38 200 1.6 15 50 130 <5 <0.1 380 310 1,936

Notes - 3138U data is from April 1997, due to no sample from April 1998; LYAP data is from Feb. 1998;

pH, Eh (mV) and CO2 are measured in the field; 1 - CO32- concentration is 24 mg/L.

A temperature of 15 0C was used, according to measured data from groundwater monitoring. The alkalinity was expressed as analysed from monitoring data (as total equivalent CaCO3 for LYAP, and HCO3 for bores). The concentration of CO2 (aq) was converted to equivalent partial pressure (in atm) using equation 3-14. The chemistry for each monitoring point was then equilibrated to its respective pCO2 (measured as dissolved CO2 for the RWP, 2124U and 3135U, and assumed as atmospheric pCO2 for the LYAP). The value used for atmospheric pCO2 was 3.16x10-4 atm (or 10-3.5 atm) (Langmuir, 1997). The sulfate-sulfide redox couple was used to calculate a redox potential, as a comparison to field measured data. The sulfide content of the LYAP and bore 3135U was set equal to the detection limit of 0.1 mg/L. The results from PHREEQC are given in Table 3.14.

Table 3.14 - Geochemical Analysis of HHF Groundwaters : Saturation States

LYAP RWP

Mineral Formula 3138U

Prior pCO2 Prior pCO2

Aragonite CaCO3 - 0.59 0.58 -4.44 -4.63

Calcite CaCO3 - 0.74 0.73 -4.29 -4.48

Carbon Dioxide CO2 (aq) - -4.77 -4.72 -0.91 -0.63

Siderite FeCO3 - -5.73 -0.14 -2.44 -2.64 Dolomite CaMg(CO3)2 - 0.81 0.78 -8.29 -8.68 Anhydrite CaSO4 -2.99 -0.31 -0.31 -1.10 -1.10 Gypsum CaSO42H2O -2.74 -0.06 -0.06 -0.85 -0.85 Hydrogen Sulfide H2S - -6.93 -7.11 -3.92 -3.92 Mackinawite FeS - -3.89 1.46 -1.45 -1.92

FeS (am) FeS -4.62 0.73 -2.18 -2.66

Pyrite FeS2 - 24.29 8.07 15.54 9.21

Redox (calc.) (mV) - -299 -28

2124U 3135U

Mineral Formula

Prior pCO2 Prior pCO2 Aragonite CaCO3 -6.15 -6.16 -3.65 -2.90

Calcite CaCO3 -6.00 -6.00 -3.49 -2.75

Carbon Dioxide CO2 (aq) -0.73 -0.47 0.75 0.00

Siderite FeCO3 -3.37 -3.38 -0.40 0.35 Dolomite CaMg(CO3)2 -10.65 -10.67 -6.29 -4.80 Anhydrite CaSO4 -2.21 -2.21 -4.64 -4.64 Gypsum CaSO42H2O -1.96 -1.96 -4.39 -4.39 Hydrogen Sulfide H2S -3.63 -3.63 -4.95 -4.96 Mackinawite FeS -2.27 -2.54 -2.08 -0.59

FeS (am) FeS -3.00 -3.27 -2.81 -1.33

Pyrite FeS2 14.44 9.02 9.74 8.73

Redox (calc.) (mV) 11 -20

Note - "Prior" is before equilibration by PHREEQC to the given pCO2.

The redox potentials calculated by PHREEQC are generally mildly reducing, correlating with the interpreted conditions in HHF aquifers. The redox potentials measured in the field, which are mildly oxidising, suggest they are influenced by the introduction of oxygen at the ground surface and by the use of bailers in sampling. At the ground surface, the presence of oxygen would lead to strongly oxidising conditions in near surface waters, although at the base of the LYAP where seepage emanates into the HHF (up to 30 m in depth) it is expected that the calculated redox potential would be more realistic due to the lack of oxygen. It is recommended that future sampling investigate the use of down-hole (or in-situ) probes to obtain more accurate field redox values.

The PHREEQC results in Table 3.14 show that the ambient waters in the ash pond are over-saturated with respect to carbonate minerals, and that calcite (CaCO3), aragonite (CaCO3) or dolomite (CaMg(CO3)2) may be expected to precipitate in the ash pond. This is expected, as calcite mineralisation has been identified in ash sediments previously in the Latrobe Valley (Black, 1990a). The geochemistry of ash pond waters and ash leachates is analysed and discussed further in Chapter 7.

The degassing of carbon dioxide observed during groundwater monitoring and chemical analysis can be expected from equation 3-14. Drever (1997) states that for mildly acidic waters the CO2 (aq) will dissociate to a gas rather than form HCO3. For bore 3135U, it is important to consider the source of such high concentrations of dissolved CO2. The reduction of SO4 produces 2 moles of CO2 (aq) for every mole of SO4 (cf. Eq. 3-11). The CO2 (aq) measured in the field was consistently around 2,000 mg/L or higher, giving about 45 mmol/L. From Section 3.5.6, if the maximum groundwater velocity is assumed for HHF aquifers, the SO4 concentration would be approximately 1,000 mg/L or about 10 mmol/L. This represents less than one quarter of the SO4 required to produce the measured CO2 (aq) at bore 3135U, giving a production of CO2 (aq) greater than that attributable to SO4 reduction. The high CO2 content of bore 3135U is therefore anomalous, assuming it is produced strictly from SO4 reduction.

As discussed earlier in Section 3.6.2, SO4 reduction is part of broader biogeochemical processes through which bacteria or microorganisms thrive and break down organic matter (eg. Berner, 1980; Jørgensen, 1983). The SRB can only metabolise short chain labile (reactive) organics, whereas other bacteria are able to effectively degrade the higher molecular mass and less reactive organics, converting them into the short chain organics that SRB can utilise (Jørgensen, 1983). The degradation of the more refractory organic content (generally higher molecular mass organics) of the sediments may be partly responsible for the higher CO2 content. This hypothesis is preliminary, however, and more detailed microbiological studies and investigations of both dissolved phase and sediment-bound organic compounds would be required to ascertain the source of the high dissolved CO2 in bore 3135U.

The effect of pCO2 on siderite (FeCO3) is especially evident. For ambient ash pond water, the difference in the saturation index of siderite before and after equilibration with atmospheric CO2 is over five orders of magnitude. For bore 3135U, the saturation index of siderite shifts from an undersaturated to oversaturated state after equilibration. This bore also has a consistantly higher Fe concentration than other bores in the vicinity of the LYAP. This change in saturation state for siderite explains its formation within HHF groundwaters in the Loy Yang region, as observed by Bolger (1984).

All water chemistries show significant undersaturation with respect to hydrogen sulfide. The lack of high concentrations of sulfide in most groundwater samples suggests that the sulfide is reacting with aquifer sediments to form insoluble sulfides. The saturation index data for sulfides in Table 3-13 display a wide range, where the formation of the intermediate sulfide minerals, mackinawite (FeS) and greigite (Fe3S4), appear to become oversaturated in ash pond waters after equilibration to atmospheric carbon dioxide. The formation of mackinawite and greigite are thermodynamically unstable, and are transition products in the formation of pyrite during bacterial SO4 reduction (Berner, 1981; Goldhaber & Kaplan, 1974).

The geochemical modelling of dissolved CO2 and sulfide behaviour has demonstrated the complex controls on groundwater geochemistry. The high CO2 (aq) concentrations appear to influence groundwater chemistry and also appear to be responsible for the precipitation of siderite in the Loy Yang area, as identified by Bolger (1984). Due to the high quantities of SO4 in seepage from the LYAP, the formation and subsequent precipitation of sulfides could release acidity. The combination of these two processes could act as acidic buffer in HHF groundwaters. The extended period of time over which this process has been occurring at the LYAP could therefore be expected to lead to the formation of a sulfide zone around the ash pond embankment area. The concentration of sulfides within aquifer sediments has not been tested, and consequently no comparison can be made to concentrations before the onset of ash pond seepage. Bolger (1984) identified the presence of sulfidic minerals in HHF sediments in the Loy Yang area, corresponding well with the above geochemical model.