VARIABILITY IN GROUNDWATER FLOW DIRECTION WITHIN SOILS OF A MOUNTAIN HEADWATER CATCHMENT

Saturated groundwater flow direction within soils of headwater mountain catchments is often assumed to mimic land surface topographic gradients. However, groundwater hydraulic gradients are also influenced by subsurface permeability contrasts and time-variable water levels producing variability in flow direction and magnitude that can deviate from the land surface. In this study, we investigate the relationship between shallow groundwater flow direction, surface topography, and the topography of low permeability units in soils of a glaciated headwater catchment at the Hubbard Brook Experimental Forest (HBEF) in North Woodstock, NH. Groundwater levels were continuously monitored throughout several seasons (Mar. 2019 to Jan. 2020) in a network of wells installed in the upper hillslopes of Watershed 3 of the HBEF. Five clusters of three wells per cluster were screened from 0.18 – 1.1 m depth at the base of the solum. 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 47 slug tests across the well network to determine the saturated hydraulic conductivity (K­_sat­) of the materials surrounding each well.

Results show that the mean shallow groundwater flow direction deviates from surface topography by 2-10 degrees. During lower water table regimes, flow direction can deviate as much as 56 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 K­_sat­ values 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 (1.6×10^-5 to 5.1×10^-5 m/s). Thus, a marked permeability contrast exists between the solum and C horizon.

Overall, results of this study 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 soils.