DYNAMICS OF GROUNDWATER FLOW DIRECTION IN THE CRITICAL ZONE OF A FORESTED, GLACIATED CATCHMENT

Shallow groundwater flow direction in hillslopes of headwater mountain catchments is often assumed to mimic the surface topographic gradient. However, groundwater flow direction is also influenced by other variables, such as water levels and subsurface hydraulic conductivity, which can result in temporal variations in both magnitude and direction of flow. In this study, we investigate the temporal variability of groundwater flow direction in the critical zone of a headwater catchment at the Hubbard Brook Experimental Forest in North Woodstock, NH. Groundwater levels were continuously monitored throughout several seasons (March to October 2019) in a network of wells. Five clusters of three wells per cluster were screened from 0.18 – 1.1 m depth at the base of the solum (approximate rooting zone). Water levels were also monitored in five deeper wells, screened from 2.4 - 6.9 m depth within glacial drift of the C horizon. We conducted 50 slug tests across the well network to determine hydraulic properties of the materials surrounding each well.

Results to date show that groundwater flow direction has an arithmetic mean deviating from surface topography by 5-15 degrees. During lower water table regimes, in between recharge events, flow direction can deviate as much as 45 degrees from the ground surface, but under higher water table regimes, in response to recharge events, flow direction mimics surface topography. There is an observable connection between the direction at which the top of the C horizon and the direction groundwater flow deviates from the land surface. Slug test results show the interquartile range of saturated hydraulic conductivity (K­_sat­) within the C horizon (1.5×10^-7 to 9.8×10^-7 m/s) is two orders of magnitude lower than the interquartile range of K­_sat­ values within the solum (2.9×10^-5 to 5.2×10^-5 m/s). Thus, the C horizon is on average a confining unit relative to the solum that may constrict the rate of infiltration of groundwater below the rooting zone.

Our results indicate that 1) shallow groundwater flow direction is dynamic and can deviate from surface topography, and 2) the subsurface topography of the C horizon can influence flow direction. This suggests that temporal dynamics of groundwater flow direction should be considered when calculating hydrologic fluxes in critical zone studies.