GEOCHEMISTRY OF A STRATIFIED POOL IN A FLOWING STREAM, MOUNT MORRIS, NEW YORK

A small pool approximately 10 m in diameter and 0.7 m deep in Buck Run, Livingston County, New York, becomes anoxic during periods of low flow. Water samples were collected for the purpose of characterizing the general chemistry of the water and verifying the suspected stratification. Measurements of pH, Eh, temperature, and dissolved oxygen were taken in October, 2000, seven days after a 1.3 inch rain event. The temperature of both waters was 18 °C. The anoxic water had a pH of 6.2 and Eh of -230 mV. The oxygenated water had a pH of 7.3, and Eh of 110 mV. Dominant ions in the anoxic water (Na+ > Ca2+, SO42- > Cl-) were measured using ion chromatography. Dominant ions in the oxygenated water were similar (Ca2+ > Na+), but at lower concentrations. The geochemistry indicates the water in the pool is stratified, with the boundary between oxygenated water above and anoxic water below occurring at approximately 40 cm beneath the surface. The chemistry of the anoxic water resembles that of groundwater in shale, which underlies and surrounds the stream. Subsequent samples obtained during the summer of 2001 verify the existence of the stratification and that the pool remains stratified until rain events occur that induce mixing and flush the pool, after which the stratification is re-established. Development of an anoxic environment in a shallow, clear flowing stream conducive to mixing is unusual. The anoxia appears to be due to the discharge of groundwater that has percolated through pyritic black and gray shales.