Water Journal : Water Journal September 2012-1
refereed paper rural pipeline management water SEPTEMBER 2012 77 8 and 9. This pH range is suitable for non-potable domestic supply and stock watering. Once the carbon dioxide has been added, the pH tends not to increase again as the leached calcium and alkalinity from the upstream section of cement-lined pipeline is adequate to partially stabilise the water. Also, ironically, it is suspected that the elevated pH conditions near Lake Bellfield act to supress the establishment of any Bryozoa within this section of the WMP. It has been observed, however, that pH levels prior to the carbon dioxide addition point have reduced in recent months. This is probably due to the drop-off in "easy to leach out lime" in the cement-lined pipes and shorter detention time as demand increases. The elevated pH problem may, therefore, be a relatively short-term problem. Direct connection of previously remote catchments Lake Bellfield is the main supply for the southern area of the WMP (supply systems No 1 through No 4). Storms in the Lake Bellfield catchment in January 2011 caused widespread damage to the landscape. The floods are believed to have resulted in a saturated soil profile in the catchment and caused a geological response of overlying mudstone slipping off the underlying sandstone. This resulted in multiple landslips throughout the catchment and some directly into the reservoir. The water in the storage became highly turbid immediately after the event. However, electron microscope work indicated that there was virtually zero organic matter in this turbidity. The turbidity levels at the surface were up to 100 NTU, while the bottom layers were > 2,000 NTU. The elevated turbidity required GWMWater to switch to supplying from Taylors Lake, a secondary source north of the Grampians that is typically of poorer quality. Taylors Lake had also been affected by the storms, although to a lesser extent. Also, the outlet tower for Lake Bellfield, constructed in 1967, can only draw water from the bottom of the storage. Therefore, to maintain a better quality supply from this source to the WMP, GWMWater instituted emergency measures to pump water from the surface of the storage. The discharge pipe went over the dam wall and into the outlet pipe from the Lake. This operational response resulted in avoidance of initially very high turbidity in the near bottom water (Figure 6). Some towns supplied by the WMP without filtration also required a boil water notice to be issued. Fortunately, at Lake Bellfield "average" turbidity dropped substantially within four months but "average" colour has remained high for over a year (Figure 6). In saying this, however, both colour and turbidity near the surface have usually been substantially higher than near the bottom (Figure 7). Finally, as occurs in most reservoirs, there has been a seasonal (over summer) rise in iron and manganese levels in near bottom zones in the Lake where dissolved oxygen levels fall below roughly 4mg/L to 5mg/L (Figure 8). Improvement of water quality Improvement options assessed for Lake Bellfield included upgrade of the existing aeration-mixing system at the dam and the addition of a multi-level outlet structure. Aeration-mixing aims to prevent low dissolved oxygen conditions near the bottom of the reservoir that trigger release of manganese and iron from sediments in summer. The multi-level outlet would provide the ability to avoid poor quality stratified inflows, especially during winter refilling periods when cold inflows would tend to enter as a thin layer along the reservoir floor. New Opportunities Due to Establishment of the WMP and NMP Systems Various new opportunities have been presented by the establishment of this large rural pipeline network: • Environmental flows Water efficiency improvements by the decommissioning of the channel system means there is more water available to be returned to the streams and rivers across the Wimmera and Mallee regions while providing far greater drought resilience for towns and farms. Figure 8. Manganese/iron vs. DO relationship at Lake Bellfield. Figure 7. True colour and turbidity versus distance from the bottom of the reservoir.
Water Journal November 2012-1
Water Journal August 2012