SPATIAL AND SEASONAL PATTERNS IN SURFACE AND SUBSURFACE WATER CHEMISTRY: IMPLICATIONS FOR HYDROLOGIC FLOWPATHS

Recently there has been considerable interest in investigating the mechanisms responsible for the export of dissolved organic carbon (DOC) and nitrate-N. We explored the mechanisms of solute export and their variation with topography and season for a forested catchment located in the Adirondack Park, NY. The 133ha Archer Creek catchment was comprised of an assemblage of subcatchments representing a range in topography, soils, drainage, and vegetative features. Headwater subcatchments had steep hillslopes, bench-step topography, surface springs, and older deciduous vegetation. In contrast, subcatchments located lower in the watershed were poorly drained with a disproportionately greater share of wetlands and coniferous vegetation. We hypothesized that the potential for export of solutes such as DOC and nitrate from these contrasting subcatchment regimes would differ and would be regulated by topography, drainage, and seasonal conditions. We tested this hypothesis by conducting a continuous synoptic survey of stream and groundwater chemistry for subcatchments located in the headwaters and the lower portions of the catchment. The subcatchment located in the lower part of the catchment had highest yearly mean DOC (816.67 umol/l) and lowest yearly mean nitrate-N concentration (11.719 ueq/l). In contrast, subcatchments located in the headwaters had lowest yearly mean DOC (137.24 umol/l) and highest yearly mean nitrate-N (56.05 ueq/l), suggesting the controls of deep flow paths on solute export. Export of nitrate was influenced by season whereas export of DOC showed little seasonal variation. Nitrate-N concentrations reached their peak during spring for both the lower and headwater subcatchments (51.98 ueq/l and 70.11 ueq/l respectively) whereas the lowest values were recorded during the summer and fall (undetectable ueq/l and 38.18 ueq/l respectively). This seasonal pattern in nitrate concentration suggests a flushing response in the spring and an alternate export mechanism during summer/fall. This study suggests that export of solutes from this catchment occur via different processes, which vary with topography and season. Future research will explore these seasonal differences of nitrate and DOC export in greater detail.