HYDROLOGY AND CHEMISTRY OF A RIPARIAN WETLAND IN THE ADIRONDACKS OF NEW YORK STATE

A riparian wetland in the Sunday Lake watershed was instrumented as part of a study of mercury in Adirondack wetlands, lakes and terrestrial systems. Multiple arrays of shallow groundwater piezometers were installed adjacent to the inlet stream and along the shore of Sunday Lake. Dataloggers equipped with pressure transducers continuously monitor groundwater stage at 4 of the piezometers and surface water stage at 3 gage stations. Water samples are taken monthly and analyzed for major ion chemistry as well as ¹⁸O and ²H.

Head measurements show that the riparian wetlands discharge water to the streams from surrounding areas of stratified drift throughout most of the year. During baseflow periods in the growing season, water levels in piezometers display a daily oscillation in stage of as much as 3 cm due to evapotranspiration. During hydrologic events the groundwater flow system in the riparian wetland can be reversed with water from the stream flowing into the groundwater. Groundwater hydrographs show a rapid rise in stage associated with rapid infiltration from rainfall or snowmelt events. This is immediately followed by a rapid decline and then a slow rise. The slow rise appears to be due to infiltration of stream water into the groundwater during peak stream flow. The peak in the stream hydrograph occurs 10-12 hours after the initial rise in groundwater stage. The reversal of hydraulic gradient extends completely across the riparian wetland.

Riparian groundwater is chemically dilute (SC<40 µS). Base cation concentrations tend to be slightly higher in the groundwater while acid anions such as sulfate tend to have lower concentrations than the adjacent surface waters. Measurements of low dissolved oxygen during the summer show that sulfate reduction is an important process. Stable isotope measurements show significant differences between riparian wetland waters and surface waters for all sample periods.

Piezometers in the riparian wetland immediately adjacent to the inlet stream have the highest methyl Hg concentrations (6 ng/L). This appears to be due to methylation of Hg by sulfate reducing bacteria that flourish in the riparian zone where upwelling groundwater under the wetland provides an abundant supply of sulfate. Riparian wetlands appear to be the principal source of methyl mercury to surface waters.