ACID BUFFERING CAPACITIES OF ST. LAWRENCE COUNTY RIVERS: INFLUENCE OF BEDROCK GEOLOGY

The western Adirondacks are heavily impacted by acid deposition. For 8 weeks during the summer of 2008 we monitored the Oswegatchie, Grasse, Raquette, and St. Regis rivers to determine the downriver buffering potential of each river. Distinct bedrock regions from headwaters to the St. Lawrence River included the Adirondack Highlands (highly metamorphosed acidic crystalline rocks), the Adirondack Lowlands (metamorphosed gneisses of variable composition and marbles), and the St. Lawrence Lowlands (Paleozoic sedimentary rocks including substantial carbonate-bearing units). Monitoring sites were selected along each of the four rivers to represent these three regions. In addition, 13 samples from tributaries of the Oswegatchie River were sampled to investigate smaller drainage basins. Water samples were collected for ICP-MS analysis (72 elements) and used to calculate acid neutralizing capacity (ANC). The results were entered into an ARC-GIS database. The results of the ANC testing showed progressive downstream increases in ANC from waters that were extremely sensitive (0-40 μeq/l) to highly insensitive (>200 μeq/l). At the three sampling sites pH varied downstream from 5.66-7.27-6.76 (Oswegatchie), 6.45-7.48-7.38 (Grasse), 6.46-6.74-7.45 (Raquette), and 6.76-7.24-7.36 (St. Regis). Sampling conducted at the beginning and end of the monitoring period found that river water chemistry in the Adirondack Highlands was particularly sensitive to precipitation with lower pH values during heavy precipitation events. This was reflected in water chemistry. For example, the Oswegatchie at Rt. 3 site, which had 222 ppb Al in the 2nd week of the study had 373 ppb Al during the 7th. Total dissolved solids increased from 5 to 96 ppm downstream in the rivers. The two western rivers (Oswegatchie and Grasse), which traverse extensive areas of marble in the Adirondack Lowlands and show rapid increases in pH have significantly more Ca (up to 50% of TDS). Eastern rivers (Raquette and St. Regis) traverse less marble and show delayed recovery. Potassium/rubidium ratios increase in the St. Lawrence Lowlands likely due to the use of fertilizer in agricultural areas. This work suggests that the buffering of rivers draining the western Adirondacks occurs rapidly in conjunction with bedrock chemistry.