Tuesday, February 02, 2010

Is acidity a problem in streams of Bukit Timah Nature Reserve (BTNR)?

This is in response to the Straits Times article "Native species in Singapore may be wiped out by acid rain" (14 Sep 09) (I must have somehow missed it last year :-), the reply from Nparks and a recent comment.

BTNR has always held deep memories for me in water quality monitoring as this was where it all started in 2006.

These past few years, the pH of the 3 streams I monitored have indeed been acidic, fluctuating around 3 to 5, with a few instances of 2+ and one instance of 1+ in 2007. Though some websites mentioned that the pH used to be higher in the 1990s, I could find no documentation about this.

The big question is: is this bad for the water inhabitants e.g. crabs, fish? Hard to say. Most literature will insist that such low pH is unthinkable for aquatic lifeforms to thrive or even survive. However, many reports have surfaced about discovering rare and endemic species living in acidic waters with a pH as low as 3. You just need to give living things time to adapt (and evolve?) and they should fit in eventually.

Then the question becomes: has the acidity in BTNR always been like this? We are now back to historical records which are hard to come by.

Nevertheless, here are some possible explanations to the low pH in BTNR (regardless of whether it harms the wildlife there).
  1. Acid rain
    From reports, acid rain generally does not lower the water pH below 4. An accomplice maybe but hardly the main perpetrator.
  2. Decomposing vegetation
    Ah... this sounds like a more likely culprit. Decomposing vegetation can give rise to humic and tannic acids. These dark brown substances produce the well known black waters in the Amazon and the acidic peat swamps in Sumatra and Borneo. Still, they do not generally cause the pH to go below 3.
  3. Acid mine drainageThis can be a real killer. Sulphur or sulphide bearing ore from underground deposits are somehow brought to the surface and oxidised into sulphuric acid. This CAN really depress the pH to deadly levels.
  4. GeologySimilar to acid mine drainage, certain volcanic streams and lakes (probably having the same origins of sulphur or sulphide in the geology) can push the pH down below 1.
  5. PollutionAnd yes, if you dump acids right into the stream, you can get very high acidity.
1 and 2 appear unlikely in view of the low pH in BTNR. I suppose 5 is out too unless some lunatic is dumping acids at the peak of BTNR.

If BTNR has sulphur or sulphide underground, this certainly is interesting. Has anyone done any geological analysis there?

Then I have a new question if it turns out that the pH used to be higher in the last decade: what has happened that causes the sulphur/sulphide to oxidise into sulphuric acid in this decade?

Till the guys at Nparks and NUS finish their investigation, I guess we just wait and see for the results.

Jungle Fall Path stream

Seraya Loop stream

Rock Path stream

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