There’s something in the water: alum dosing plants on lake tributaries, Rotorua, NZ

Hi there,

It’s been a while since I last posted. Times have been busy what with teaching and getting research students up and running. July-October is my busiest time of year as I am full-on with teaching 2nd year Environmental Chemistry and Geochemistry. I’m also fortunate enough to be working with some very talented undergraduates (Ben Shirley and Alistair Lockley), supervising their undergraduate research projects.

Left to right: Alistair Lockley, Ben Shirley and UoW technician Warrick Powrie.
Left to right: Alistair Lockley, Ben Shirley and UoW technician Warrick Powrie.

Ben and Alistair are investigating the chemical effects of alum dosing in two small streams feeding two lakes near Rotorua, Bay of Plenty. Alum is a potassium aluminium sulfate salt which is added to these streams in order to render dissolved phosphorus (P) chemically and biologically inert, thereby reducing the flux of bio-available P into the lakes.

We had two fine days in late July to collect samples of water and sediments from the streams and lakes. Alum dosing has been carried out for several years on the Utuhina Stream, which discharges into Lake Rotorua, and this appears to have been successful, with corresponding reductions in the concentration of P in Rotorua lake water over the same period. Dosing is also occurring at a second site, but the efficacy of the dosing here is less evident. The second site is located on a small geothermal stream which passes through a dense wetland before discharging into lake Rotoehu (see map below).

Lakes of the Rotorua area
Lakes of the Rotorua area

The two steams are really quite geochemically distinct having very different temperatures, ionic contents and pH values. Our working hypothesis is that the difference in pH (about 2 units) between the streams causes differing amounts of charge to be developed on aluminium particles that are precipitated following alum addition. When alum is added to the stream water. aluminium ions (Al) bind with hydroxide ions (OH) forming a white precipitate known as Gibbsite (Al(OH)3). These particles can develop positive or negative charge depending on the water pH value. When phosphorus ‘sees’ the particles the determining factor in whether it will interact is the charge difference between P and AlOH3. P will be negatively charged, forming mainly H2PO4- and HPO42- ions. So it all depends on what the charge state of the gibbsite is since like charges repel. The alternative is that Al3+ binds directly with PO43- forming AlPO4, or that the alum dosing causes P bound to natural particle surfaces in the stream to be destabilised. Only time will tell, but it should be quite interesting and pretty useful seeing as whatever we discover, this new information will help the regional council understand mechanics of what’s going on at a chemical level.

Here’s some more pics of the fieldwork.

Cheers,

Adam

Sunset over Lake Rotorua
Sunset over Lake Rotorua
Soda Springs, Lake Rotoehu
Soda Springs, Lake Rotoehu
White gibbsite precipitate visible downsteam of the alum dosing plant at the Soda Spings, Lake Rotoehu
White gibbsite precipitate visible downsteam of the alum dosing plant at the Soda Spings, Lake Rotoehu

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