Longitudinal and temporal trends in the water chemistry of the North Branch of the Moose River

Abstract

Surface water acidification is potentially a problem in regions with low ionic strength drainage waters. Atmospheric deposition of sulfuric acid has generally been implicated as the causative agent of this problem, although other sources of acidity may contribute. The Adirondack region of New York State is an area with “acid-sensitive” surface waters and an abundance of acidic lakes. The intent of this study was to evaluate the processes regulating the acid/base chemistry of a series of lakes draining a large heterogeneous watershed in the Adirondack region of New York. The study site, the North Branch of the Moose River, is heterogeneous in its soil and geological characteristics. This variability was reflected through differences in water chemistry that occurred within the basin. The northern headwaters generally drain subcatchments with shallow, acidic soils. The resulting water chemistry was acidic (equivalence of acidic anions exceeded equivalence of basic cations) with high concentrations of Al and dissolved organic carbon (DOC). As this water migrated through a large lake (Big Moose Lake) with a moderate hydrologic retention time (∼0.5 yr), considerable loss of DOC was evident. As acidic water was transported through the drainage area, it mixed with waters that were enriched in concentrations of basic cations from the eastern subbasins. As a result, there was a successive increase in the acid neutralizing capacity (ANC) and a decrease in Al concentrations as water migrated from the northern reaches to the outlet of the watershed. In addition to these general trends, short-term changes in water chemistry were evident. During low flow summer periods concentrations of basic cations were elevated, while concentrations of SO 4 2− and NO 3 − were relatively low. These conditions resulted in less acidic waters (higher ANC) with relatively low concentrations of Al. During high flow winter/spring conditions, elevated concentrations of SO 4 2− and NO 3 − were evident, while concentrations of basic cations were reduced resulting in low pH (low ANC) waters with high concentrations of Al. Variability in the processes regulating the pH buffering of waters was apparent through these short-term changes in water chemistry. In the northern subbasin short-term fluctuations in ANC were minimal because of the buffering of Al under low pH conditions. Seasonal changes in the ANC were more pronounced in the eastern subbasin because of the predominance of inorganic carbon buffering in the circumneutral pH waters. Lakes in the west-central Adirondacks have characteristically short hydraulic residence times and elevated nitric acid inputs. As a result these waters may be more susceptible to surface water acidification than other “acid-sensitive” lake districts in eastern North America. Given the apparent interregional differences, extrapolation of chemical trends in the Adirondacks to other areas may be tenuous.