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The Loy Yang Ash Pond was commissioned in 1982. During construction of the dam wall for the LYAP, additional material was excavated from the pond near its centre to use as fill material for the dam wall (Pedler & Raisbeck, 1988; Daniels et al., 1993). This over-excavation exposed the HHF sands near the unconformity, providing a direct conduit for seepage flow into HHF aquifers beneath the LYAP. Within a relatively short period, significant ash pond seepage was observed under the dam embankment. The seepage was creating a small artesian head on the base of the dam wall, and there was concern about the geotechnical stability of the dam wall. The seepage was thought to be occurring through the old borrow pit (Reinsch et al., 1982). A water balance and tracer study by Wood et al. (1982) estimated a seepage loss of 1,000 m3/day.

In 1986 seepage was found to be discharging to the southern batters of Loy Yang B Station (see Figure 1.2). Investigative drilling was carried out on the ridge but failed to intercept seepage; ongoing monitoring of these bores show that they remain dry (Daniels et al., 1993). A cut-off drain was installed to capture seepage flows and lower the water level below the ground surface (Pedler & Raisbeck, 1988), although this was not completely effective at capturing all seepage (Daniels et al., 1993). The existing network of groundwater monitoring bores was further developed to monitor groundwater chemistry. As part of environmental management and EPA licence requirements additional bores were also added to the network, namely 3135, 3281 and 3282 (Mulder & Pedler, 1990; Daniels et al., 1993).

A seepage modelling study by Pedler & Raisbeck (1988) estimated a total seepage loss of 890 m3/day. The seepage flow at the B Station measured in 1993 was 104 m3/day, although not all seepage was known to be collected and monitored in flow readings (Daniels et al., 1993).

Mulder & Pedler (1990) undertook a solute transport modelling study of the seepage from the LYAP. The HHF aquifer was assumed to be a uniform 5 m thick sand, with the groundwater velocity (taken as a constant) calculated from the outflow of the Relief Well Pit (RWP) system. Both one- and two-dimensional (1-D / 2-D) solute transport models were used, with the calibration of the 1-D model MYGRT (EPRI, 1986) used to refine model parameters for the 2-D model MOC (Knoikow & Bredehoeft, 1978). The aquifer properties were based on measured values from the Loy Yang mine and previous experience in the Latrobe Valley. The values were considered typical for the sands of the HHF. The influence of groundwater and aquifer chemistry on solute migration was modelled through the use of retardation coefficients for Na and SO4, while for Cl conservative transport with no reactions were assumed. The retardation coefficients of 1.1 were adopted through the use of the 1-D model and visual calibration to the 8 years of limited monitoring data available at that time. The results predicted that seepage related impacts on the HHF aquifer should remain within the boundaries of the Loy Yang site. The complex geology was recognised as a major uncertainty in this study. A historical timeline of ash seepage is compiled below in Table 3.2. It is summarised from Reinsch et al. (1982), Wood et al. (1982), Pedler & Raisbeck (1988), Mantyvirta & Clancy, (1989), Mulder & Pedler (1990), Daniels et al. (1993), Daniels (1994a), McKinley (1992; 1993; 1994a & 1994b). The EPAV has permitted existing ponds to continue to operate providing their seepage does not affect the beneficial uses of the aquifers at the project boundaries. The State Environment Protection Policy (Groundwaters of Victoria) (the "SEPP"), issued by the EPAV in December 1997, specifically allows for an attenuation zone for an ash pond operated at a coal-fired power station (EPAV, 1997). The beneficial uses of groundwater within this zone are not required to be met, providing that such uses are met at the boundary of the zone.

Table 3.2 - History of Ash Pond Seepage at Loy Yang 1979-

1981

• Loy Yang Ash Pond is constructed in two seasons - the Haunted Hill sands are exposed in an internal burrow pit used for building the dam wall;

1982 • Seepage is observed emanating from the western toe of the ash pond; a water balance study estimates the seepage is approximately 1,000 m3/day;

1983 • investigation reveals that migration of leachate from the ash pond is occurring and a series of relief wells are installed to capture seepage flows and reduce hydrostatic pressures; sampling shows localised contamination; 1986 • seepage discharge found on the Loy Yang B Bench;

1987 • regular groundwater monitoring program initiated;

1988 • detailed seepage analyses are undertaken, including field investigation and computer modelling in a north-south section (through to Loy Yang B) and in an east-west section (through to the dam toe and relief well pit); estimates of seepage flows were 890 m3/day;

1989 • a review of ash production rates shows that there is insufficient capacity in the Ash Pond, which would fill by 1995 if no remedial options were undertaken; they recommended that pond capacity be increased;

1990 • a solute transport study is undertaken including both field work and computer modelling of SO4 and Cl movement through the Haunted Hills aquifer;

• the study concludes that pollution is currently localised to the shallow aquifers only (Haunted Hill), and that the pollution will remain within Loy Yang boundaries for the duration of the ash pond's design life;

1992 • a review of groundwater quality monitoring data shows that seepage is now reaching the intermediate aquifers, particularly the M2C aquifer;

1993 • a more detailed investigation of the seepage at Loy Yang B revealed that it did contain ash pond seepage; this report showed :

− the possible connection of intermediate and deeper aquifers through the presence of inferred faults in the ash pond and Loy Yang B region,

− seepage at the Loy Yang B bench was a combination of natural water, fire service reservoir water and ash pond seepage;

• annual review of groundwater quality monitoring shows :

− TDS, SO4 and Cl concentrations have stabilised within the HHF, except bore 3135U, which had rising Cl concentrations and low SO4,

− Hydrogen sulfide can be detected in most groundwater bores,

− North-east of the ash pond, high Cl concentrations indicate a higher background salinity, although high SO4 concentrations are also found, indicating ash pond seepage,

− the salinity of the Loy Yang B bench seepage increased markedly between 1992 and 1993; analysis of the field data shows that the seepage pathway is indirect, emanating from the eastern end of the pond and not in a north-south manner as might be expected;