The application of biotechnology to mine water can reduce the industry's water treatment costs and improve water quality in streams and rivers. Biological treatment of mine waste water is typically conducted in a series of small excavated ponds that resemble, in a superficial way, a small marsh area. The ponds are engineered to first facilitate bacterial oxidation of iron; ideally, the water then flows through a composted organic substrate that supports a population of sulfate-reducing bacteria. The latter process raises the ph. During the past 4 years, over 400 wetland water treatment systems have been built on mined lands as a result of research by the U.S. Bureau of Mines.