Natural neutralization of an acidic mine discharge by a small wetland

In the set of pictures below, I am measuring the pH of acidic water emerging from a waste rock dump, as it is neutralized by a small swampy area.

Water flowing out of a waste rock dump, pH 1.1. In the photograph on the left, the water pH measures 1.1. Note the pronounced orange staining caused by iron precipitation.
Water flowing 3-4 feet behind, through mossy area, pH 3.3. This photograph was taken about 3-4 feet back from the first (i.e., the first photograph is in the foreground). The pH of the water is 3.3! The water flowed (slowly) through a mossy area, and it was no visibly diluted by water from another source.
Water flowing through a small wetland, 6-7 feet from the first, pH 6.6. This photograph is taken a further 3 feet back, where the pH measured 6.6, nearly completely neutralized! The water flowed through the detritus layer of grasses seen in the background of the middle photograph. Again, no other source of water could be seen that might explain the increase in water pH.

Clearly, the water pH increased from 1.1 to 6.6 because it flowed through the vegetated areas. Somehow, the water acidity was neutralized by alkalinity injected into the water column. The most likely explanation is that this alkalinity results from microbial activity in the vegetated areas.

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Last modified Monday, April 21, 1997