COMPLEX SURFACE AND GROUNDWATER INTERACTION IN A SMALL WATERSHED, ORR’S ISLAND, MIDCOAST MAINE

Hydrometric and geochemical data were used to study surface and ground water interactions in a small forested headwater catchment on Orr’s Island in midcoast Maine. Fractured metamorphic bedrock is overlain by 0-4 m of fine-grained glacial sediment, into which small stream channels are carved. Sampling during summer and fall 2003 included stream and throughfall stations, several dozen wells in four transects within glacial sediment, and one bedrock well. During both base flow and storm flow, stream samples exhibited lower specific conductance than groundwater samples. Dissolved silica and dissolved organic carbon (DOC) in groundwater were spatially variable and overlapped stream values. The following patterns were observed during a 21 mm rainstorm in November 2003: (1) Stream conductivity initially rose with stage, implying rapid flushing of groundwater, but then showed dilution before peak stage was reached as contributions from surface and/or shallow groundwater increased. (2) Stream DOC concentrations showed a positive correlation with stage, indicating a dramatic increase in DOC flux during the storm event. (3) DOC in groundwater exhibited spatial variation among transects, variably peaking before, during, or after peak stream flow. Collectively, the data indicate that groundwater behavior and runoff generation in this small watershed are complex, and that stream flow cannot be represented simply by models based on mixing of homogeneous precipitation and groundwater end-members.