Water quality in streams settles after eruption

Ruapehu Bulletin, Volume 13, Issue 618, 28 December 1995, Page 8

Water quality in streams settles after eruption

The potential contamination of water supplies from ash fallout was a major concern when Mount Ruapehu first erupted in September. Ten weeks later the issue of water quality has diminished from that of a threat of civil emergency status in the entire region, to the spoiling of fishing on the Mangaturuturu Stream. However, it will be five or six years before the rainbow trout, native fish and blue ducks that inhabited the Mangaturuturu will feel it' s safe to go back. The ash sitting on the upper slopes of Mt Ruapehu will take at least half a decade to wash into streams and the fishing will be no good until all the ash has been washed away, says Harry Brown of the Taranaki Fish and Game Council. When the mountain first erupted the acidity in the Mangaturuturu shot to a pH of 3.5, and slipped to a toxic 5 on 27 October. Fish can only live in a water body where the pH is above 5.5. and by comparison the acidity was a neutral pH 7 before the eruption. But although the high acidity in the Mangaturuturu is unpleasant, it is the ash that kills, as ash gets into the gills of fish and causes them to drown. Initially a lot of fish were killed at the confluence of the Mangaturuturu and the Manganui a Te Ao streams and those that managed to escape the lethal Mangaturuturu migrated to the Manganui a Te Ao. Blue duck chicks could have been adversely affected at the time of the eruption as the chicks would be more vulnerable to acidity and ash than their parents. However Mr Brown says no-one can be certain about the effect on the blue ducks until the population is next monitored. Mr Brown says that fish in the popular Tongariro River took a hammering when there was a big ash fallout over the Desert Road. However, the water quality is okay now, because the fish are okay. So don't worry if you fall out of your raft, your wet suit is not going to dissolve!