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w h e re th e t e mp e rature difference between 10 m and 30 m was greater than 1°C . M easu rements were generally taken in the morning, however, the tem.perature diffe rence between 10 m and 30 m w as selected as the best indicato r of persistent stratification as this avoids confounding effects due to diurnal surface heati ng, and time of sa mpling. There was a temperature difference of greater tha n 1 °C for 224 days in the year 1985/86 wh ic h gave rise to maximu m iron and manganese concentrations of 2.55 mg L- 1, 1.8 m g L- 1, respe ctively (T able 1). In the year 1995/96 there was on ly 28 days where therm al strati ficatio n between 1 Om and 30111 occurred and consequen tly the maximum iron and mangan ese concentrations were low, 0.386 m g L- 1 and 0087 mg L- 1, respectively. The yearly maximum concentration of iron at 30111 was significa ntl y co rrelated with the duration of strati fication (Spearman no n-parametric correlatio n, r,=O. 73 28, P=0.0044). The max imum manganese co ncen tration was also significa ntl y correlated with th e duration of stratifi cation (S p ear man non-parame tri c correlation r,=0.8303, P=0.0004). The yearly maximum manga nese co ncentrati on corre lat e d w ith the yea rl y m ax i mum ir on co nc e ntrati o n s (Spearman non-param etric correlation r,=0 .9133, P <0.0001) indicating that manganese resolubilisati on coincided with iron resolubilisation.
1996). Th e bubble plume aerator at Myponga ha s b een s u ccessfu l at m ainta ining manganese conce ntratio ns below this level. M anganese wou ld not be considered a health threat unless the concen tration exceeded 0.5 mgL- 1, a level which has not been realised since aerator operatio n .
References Aust ralian D rin king Water Gu idelines ( 1992) . N ational H ea lt h and M edical Research Council. Raman, R.. K. and Arbuckle, B. R. (1989) . Long-term Destratification in an Illinois Lake. Joumal of tl,e A111erica11 Water Works Associatio11. 81: 66-7 1 Sherman , B. S., Whittington , J. and Oliver, rt. L. (2000). T h e impact of destracification on water quality in Chaffey Dam. , Proc . Kinneret Symposium on Limnology and Lake Management 2000+. A rcl,iv fiir H ydrobiologie (in press) Stumm, W. and Morgan, J. J. (1996). Aquatic C/1e111istry: cl,e111ical equilibria mid rates i11 11atural waters. Th ird edition . John Wiley and Sons, New York. Visser, P. M ., lbeli ngs, 13., van der Veer, B., Koedoods, J . and Mur, L. ( I 996). Artificial mixing prevents nuisance bl ooms of the cyanobacterium Microcystis in Lake Nie uwe Meer, the Net herlands. Fres/111mter Biology. 36: 435-450.
Wetzel (1983). L/111110/ogy. Second Edition. C BS Coll ege Publish ing, Philadelphia.
Authors Dr Justin Brookes is a R esearch Scientist in volved in a large interdisciplinary study at the CRC for Water Quality an d Treatment, examining the use of mec han ical mixers and aerato rs to dest racify reservo irs and co n trol cyanobacteria. CRC for Water Quality and T reatmen t, PMB 3, Sali sbury, SA, 51 OS.P h . (08) 8 259 0222 e- m ai l justin.brookes@sawater.sa .gov .au Michael Burch is Projec t Leader for t h e CRC for Water Quality and T reatment proj ect " Artificial M ix ing fo r D estratifi cation and Co n tro l of Cya nobacteri al Growth in Reservoirs". H e is also the ARMCANZ National Algal Manager. Ph. (08) 8259 0352. email mi ke. burch@sawater. sa.gov.a u Peter Tarrant is M anager W ater Q uali ty, Bulk Water Division of the South Austra lian Water Corporation. Peter has 20 years ex peri ence in water supply e ngin eering and water qu ali ty m anageme nt with SA W ater. Ph.: (08) 8207 134 l , e- m ail: p et e r. tarran t@ saw ater. sa .gov .au.
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The bubble plume aerator deployed in M y p o n ga R ese r vo ir ha s bee n successfu l at reduc ing the conce ntratio n of iro n and m an ga nese ions which in crease treatment costs and contribute to "dirty water' problems du ri ng distribu tion . According to th e Australian Drinking Water Gu idelines (1996) the conc entration of iron in d rinkin g water should not exceed 0.3 mgL- 1• T hi s guideline is based upon the concentration at wh ic h iron precipitates fro m solution and the caste th reshold. Prio r to th e installatio n of a destratifier at Myponga R ese rvo ir the solubl e iron concentration frequen tl y exceeded the gu ideli n e . H owever, since ae ra t o r ope ration the iron concentration is ma intain ed closer to the gu ide line concentration . Based o n aesthetic conside ratio ns the concen tration o f m anga nese in drinking water should n ot exceed 0.1 mgL- 1 (Australian Drinking Water Guideli nes,
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