Google Earth image showing approximate locations of mining exploration

Thomson Dam catchment showing in (yellow) the sites of past alluvial gold workings and (red) sites of old mining batteries where mercury was used - including one battery which would be submerged when the dam is full! Most of the alluvial workings were located in Red Jacket Creek, the Jordon River and Thomson River (where the existing dam is located). Note clearfells on western side of dam.

Mining

p71 "The Thomson and Aberfeldy Valleys were intensively mined for gold during the latter half of the 19th and early 20th centuries. At least five auriferous reefs were located in the Thomson Valley just to the south of Aberfeldy township. Another thirty were located in the Aberfeldy Valley (including Donnelly and Fulton Creeks). As well as these mines there is still evidence of extensive alluvial mining along the Jordan River, the Aberfeldy River and its tributaries and the Thomson River south of its confluence with the Jordan. This undoubtedly caused large scale devestation of the vegetation in these areas. This has since regenerated, but it is unlikely that these areas have truly reverted to their original state. The effect of this era on the quality of the river may still persist, but if so, insufficient data exists to quantify this. In view of the quality today these effects must be small at this time.

The existence of old mines and particularly batteries suggest that large quantities of mercury were imported into the area during this period, and large quantities may still be present. Mercury is commonly used to seperate gold from crushed ore. In the past, before the toxicity of mercury was fully appreciated, this process was extremely wasteful. A significant proportion of the mercury used was discarded with the processed ore. Consequently, even after many years, metallic mercury can be found in mullock heaps associated with old mining batteries.

The Board has long been aware of this problem. It has also been raised by members of the public following the public displays of the details of the proposed Thomson development. Seven known mining batteries have been located in the study area. Two of these are in the Thomson Valley and the remainder in the Aberfeldy Valley. Presumably some or all of those utilized mercury for extracting gold from crushed ore...

The environmental chemistry of mercury has been intensively studied in recent years but not fully understood. It is known that certain mercury compounds are considered more toxic than others, particularly organomercury compounds which may be synthesised biologically from metallic mercury. The inorganic salts exhibit this characteristic. Furthermore these organomercury compounds by virtue of their chemical properties accumulate in the tissues of aquatic species such as mussels and fish and may even reach levels considered dangerous for human consumption."

Mercury Analysis of Samples Obtained From The Thomson Valley

Sample Site	Concentration of Mercury mg/1000 g (dry wt. basis)	Comments
Hg1	0.006	River Sediment
Hg2	0.027	River Sediment
Hg3	0.020	Sediment from mouth of Simpson Gully Creek
Hg4	<0.003	Sand deposit above waterline
Hg5	40	Crushed Rock Deposit
Hg6	90	Crushed Rock deposit at battery
Hg7	0.128	River at bank below battery
Hg8	0.042	River Sediment
Hg9	0.012	River Sediment
Hg10	0.012	River Sediment
Hg11	0.032	River Sediment
Hg12	<0.003	River Sediment
Hg13	<0.003	River Sediment
Hg14	0.006	River Sediment

p72 "A survey of the Thomson Valley recently completed by the Board has shown that while quite significant concentrations of mercury can be found in the soil in the immediate vicinity of old batteries, the areas where mercury can be detected are small, and very little appears to be finding its way into the river. Sediment and silt samples were taken from the river bed upstream and downstream of a battery east of the river below Cast Iron Point. Additional soil samples were obtained from the vicinity of the battery on the river. No samples were obtained from the other battery as this was inaccessible at the time.

While the presence of mercury in any form appears undesirable in a water catchment area it is inevitable that mercury should be present owing to its natural distribution in rocks and soil. The mercury content of soils averages about 0.1 mg/1000 grams and varies within relatively narrow limits. Sedimentary rocks resulting from weathering and deposited by physical, chemical and biological processes also generally average less than 0.01mg/1000 grams and seldom exceed 0.02mg/1000grams.

The mining of gold with its associated use of mercury in Victorian water catchment areas is not uncommon. For example, gold mining, has occurred in the catchments of the Hume, Eildon, Eppalock, Cairn Curran, Moondarra, Tarago, Rosslynne and Lal Lal Reservoirs. Of these, Moondarra, Tarago and Rosslynne supply water for human consumption while other reservoirs supply water for agricultural purposes which would include stock watering. In the case of Eildon, in addition to mercury used in gold mining operations, mercury as the mineral cinnabar, was mined in the catchment without causing problem levels to occur in the water.

At present no large scale gold mining operation is carried out in the area. Consequently the importation and use of mercury within the catchment has ceased. However, as the area is considered to be still rich in gold the possibility remains that mercury could be again used, especially if the world price of gold continues to rise. Any consequent discharge of mercury to the environment would need to be tightly controlled under the Environment Protection Act, 1970, and the Mines Act." Source: Thomson River Water Supply Development - Report on Environmental Study into Thomson Dam and Associated Works. Vol 1 MMBW July 1975.

Southern end of Thomson Dam (August 2007) 23% full.

Thomson Dam (August 2007) near the site of an old gold mine battery which when the dam is full is likely to be underwater.

Thomson Dam northern end - looking into the Jordan River.

Thomson Yarra Tunnel pipeline near Swingler Intake. Water from Melbourne is piped from the northern part of the dam.

Swingler Intake

Swingler Intake

Dam at Swingler Intake

August 2007: Logging off Marshall Spur Road. Most logs end up here. Young trees will take up much of the water that should be flowing into the depeleted reservoir. Bell Clear Creek sub-catchment on left and Whitelaw Creek sub-catchment on right.

August 2007: More thirsty clearfells in Bell Clear Creek.

Dead Myrtle Beech (cool temperate rainforest species) near Bell Clear Creek in a Site of Rainforest Significance.

Logging in close proximity to rainforest species - Bell Clear Creek Catchment

Ditto