Hydrogeophysical Characterization of the Unique Groundwater Discharge Features Termed "Soap Holes" on the Great Plains

Soap holes are localized areas with little to no soil strength that manifest themselves on the surface in a variety of morphologies and dimensions (diameters ≤ 8 m, depths > 2 m, and heights ≤ 2 m). Anecdotally, they are known to change height seasonally. They pose a hazard for livestock and farm equipment, and can render land non-arable locally. Although they have been interpreted as groundwater discharge features for the past 50 years, no comprehensive study has evaluated their subsurface structure, hydrogeology, groundwater source(s), and seasonal variations. We use time-series porewater pressure and temperature data, electrical resistivity tomography (ERT), and grain-size analyses, along with the geochemical and isotopic composition from two soap holes in Alberta (Canada) to provide new insights into these unique groundwater phenomena.

The soap holes developed in smectite-rich, dominantly colloidal sized (< 1 µm) clay that was saturated at ground surface. Estimated hydraulic conductivity values (K) beneath the soap holes were three orders of magnitude greater than the surrounding sediments of similar depth (i.e. 10^-7 m/s vs. 10^-10 m/s). Consistently upward hydraulic gradients (between 0.2 and 1.15) increased with depth, and were close to critical hydraulic gradient exceedance (i.e. soil effective stress decreases to zero) below the soap hole base. The soap holes are thus interpreted as near-surface manifestations of preferential flow zones through a thin confining layer, resulting in locally higher upward groundwater flux. Upward Darcy flux estimates below the soap hole exceeded 0.7 m/day, and substantially exceeded estimated Darcy fluxes within the soap holes ( 2 x 10^-6 m/day). The lack of observed overland flow was consistent with Darcy flux estimates within the soap hole that were lower than average annual evaporation rates (2.2 x 10^-3 m/year).

ERT surveys imaged higher resistivity zones interpreted as preferential flow through the confining layer (e.g. fractures and/or interconnected higher K lenses), suggesting an underlying confined aquifer water source, and also lateral spreading occurring in the shallow saturated zone. Water chemistry and isotopes were consistent with recharge of dominantly winter precipitation, and were similar to nearby water wells completed in the confined aquifer.