Spatial and temporal dynamics of stream chemistry in a forested watershed impacted by atmospheric deposition

Abstract

We analyzed spatial and temporal dynamics of solute chemistry in a forest watershed impacted by atmospheric deposition in the Adirondack Mountains of New York State, USA. Spatial dynamics of solute chemistry and natural abundance isotopes of nitrate (15N and 18O) were examined in 6 locations and the watershed outlet in 2001 and 2002. Temporal dynamics were examined during 5 discharge periods: winter, snowmelt, spring, summer, and fall, which were based on discharge levels at the outlet. Solute concentrations were statistically significantly different (p≤0.05) among stream sampling locations and discharge periods, with no interaction effects. Groundwater sources located in upper watershed controlled stream chemistry at higher elevations with highest pH, Ca2+, sum of base cations, Si, NO3-, total N, and SO42- and lowest Al concentrations. Two low elevation wetlands had a substantial influence over stream chemistry at those locations contributing lowest NO3-, total N, and highest DOC and DON. Snowmelt exhibited among the lowest pH, sum of base cations, and SO42-, and highest NO3-, total N, DON, and total Al; snowmelt appeared to dilute groundwater, and flush stored soil-derived solutes. Summer discharge, composed mainly of groundwater, exhibited the lowest flow, among the highest Mg2+, Ca2+, and lowest DON, DOC, and total Al concentrations. Isotopic analysis together with patterns of NH4+ versus NO3- dynamics indicated that NO3- was microbial, generated in fall and accumulated in winter in upper watershed soils, and flushed to stream during high discharge events. Highest discharge in snowmelt 2001, a summer drought in 2002, and fall storms following the drought were further evaluated for their specific effects on stream chemistry. Snowmelt 2001 had the lowest pH and highest NO3-, base flow during summer drought had the lowest total Al, and storms in fall 2002 had highest SO42- of all periods, but all other solute concentrations were comparable to other discharge periods in this study. Depending on objectives, watershed outlet alone may sufficiently represent solute dynamics in the watershed, and high-discharge events may sufficiently describe solute fluxes for the watershed.