A hydropedological approach to quantifying groundwater recharge in various glacial settings of the mid‐continental USA

Data from six monitoring stations were combined with a soil‐water dynamics model (HYDRUS 1D) to achieve physically‐based estimates of shallow water‐table recharge in representative hydropedological settings of the glaciated midwestern U.S.A. Calibration involved inverse modeling that yielded optimized hydraulic parameters. Root mean square errors for modeled versus measured soil moisture contents were generally within 3% for all soil layers at the six study sites. The optimized models also accurately simulated recharge values that corresponded to observed water‐table fluctuations. Optimized parameter values were consistent with estimates from a pedotransfer function, lab analyses, and field experiments. Forward modeling indicated that shallow water‐table recharge in mid‐continent glacial settings is approximately 35% of precipitation, but interannual and monthly variability is significant. Soil parent materials and horizon characteristics influence recharge primarily through their control on K_s with clay‐rich till parent materials producing values as low as 16% and coarse‐grained outwash parent materials producing values as high as 58% of precipitation. During the three‐year study period, distinct seasonality of recharge was observed with most recharge occurring in the winter (seasonal mean of all sites was 66% of precipitation) and lesser but interannually stable amounts in the spring (44%), summer (13%), and autumn (16%). This research underscores the importance of incorporating pedological information into models of soil‐water dynamics and groundwater recharge. © 2015 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.     

Hydrogeologic Controls on Induced Seismicity in Crystalline Basement Rocks Due to Fluid Injection into Basal Reservoirs

A series of M_b 3.8–5.5 induced seismic events in the midcontinent region, United States, resulted from injection of fluid either into a basal sedimentary reservoir with no underlying confining unit or directly into the underlying crystalline basement complex. The earthquakes probably occurred along faults that were likely critically stressed within the crystalline basement. These faults were located at a considerable distance (up to 10 km) from the injection wells and head increases at the hypocenters were likely relatively small (～70–150 m). We present a suite of simulations that use a simple hydrogeologic‐geomechanical model to assess what hydrogeologic conditions promote or deter induced seismic events within the crystalline basement across the midcontinent. The presence of a confining unit beneath the injection reservoir horizon had the single largest effect in preventing induced seismicity within the underlying crystalline basement. For a crystalline basement having a permeability of 2 × 10^?17 m² and specific storage coefficient of 10^?7/m, injection at a rate of 5455 m³/d into the basal aquifer with no underlying basal seal over 10 years resulted in probable brittle failure to depths of about 0.6 km below the injection reservoir. Including a permeable (k_z = 10^?13 m²) Precambrian normal fault, located 20 m from the injection well, increased the depth of the failure region below the reservoir to 3 km. For a large permeability contrast between a Precambrian thrust fault (10^?12 m²) and the surrounding crystalline basement (10^?18 m²), the failure region can extend laterally 10 km away from the injection well.     

Partitioned by process: Measuring post-fire debris-flow and rill erosion with Structure from Motion photogrammetry

After wildfire, hillslope and channel erosion produce large amounts of sediment and can contribute significantly to long-term erosion rates. However, pre-erosion high-resolution topographic data (e.g. lidar) is often not available and determining specific contributions from post-fire hillslope and channel erosion is challenging. The impact of post-fire erosion on landscape evolution is demonstrated with Structure from Motion (SfM) Multi-View Stereo (MVS) photogrammetry in a 1 km² Idaho Batholith catchment burned in the 2016 Pioneer Fire. We use SfM-MVS to quantify post-fire erosion without detailed pre-erosion topography and hillslope transects to improve estimates of rill erosion at adequate spatial scales. Widespread rilling and channel erosion produced a runoff-generated debris-flow following modest precipitation in October 2016. We implemented unmanned aerial vehicle (UAV)-based SfM-MVS to derive a 5 cm resolution digital elevation model (DEM) of the channel scoured by debris-flow. In the absence of cm-resolution pre-erosion topography, a synthetic surface was defined by the debris-flow scour's geomorphic signature and we used a DEM of Difference (DoD) to map and quantify channel erosion. We found 3467 ± 422 m³ was eroded by debris-flow scour. Rill dimensions along hillslope transects and Monte Carlo simulation show rilling eroded ~1100 m³ of sediment and define a volume uncertainty of 29%. The total eroded volume (4600 ± 740 m³) we measured in our study catchment is partitioned into 75% channel erosion and 25% rill erosion, reinforcing the importance of catchment size on erosion process-dominance. The deposit volume from the 2016 event was 5700 ± 1140 m³, indicating ~60% contribution from post-fire channel erosion. Dating of charcoal fragments preserved in stratigraphy at the catchment outlet, and reconstructions of prior deposit volumes provide a record of Holocene fire-related debris-flows at this site; results suggest that episodic wildfire-driven erosion (~6 mm/year) dominate millennial-scale erosion (~5 mm/Ka) at this site. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Gas-partitioning tracer test to quantify trapped gas during recharge

Dissolved helium and bromide tracers were used to evaluate trapped gas during an infiltration pond experiment. Dissolved helium preferentially partitioned into trapped gas bubbles, or other pore air, because of its low solubility in water. This produced observed helium retardation factors of as much as 12 relative to bromide. Numerical simulations of helium breakthrough with both equilibrium and kinetically limited advection/dispersion/retardation did not match observed helium concentrations. However, better fits were obtained by including a decay term representing the diffusive loss of helium through interconnected, gas-filled pores. Calculations indicate that 7% to more than 26% of the porosity beneath the pond was filled with gas. Measurements of laboratory hydraulic properties indicate that a 10% decrease in saturation would reduce the hydraulic conductivity by at least one order of magnitude in the well-sorted sandstone, but less in the overlying soils. This is consistent with in situ measurements during the experiment, which show steeper hydraulic gradients in sandstone than in soil. Intrinsic permeability of the soil doubled during the first six months of the experiment, likely caused by a combination of dissolution and thermal contraction of trapped gas. Managers of artificial recharge basins may consider minimizing the amount of trapped gas by using wet, rather than dry, tilling to optimize infiltration rates, particularly in well-sorted porous media in which reintroduced trapped gas may cause substantial reductions in permeability. Trapped gas may also inhibit the amount of focused infiltration that occurs naturally during ephemeral flood events along washes and playas.