This study combines electrical resistivity tomography (ERT), geological information from boreholes and outcrops, and hydrogeologic data to investigate field-scale fault-zone cementation of the Loma Blanca Fault in the Rio Grande Rift. We collected electrical resistivity data from 16 transects and geological samples from 29 boreholes (completed as groundwater wells to 30 m depth) across and around the fault. 2D ERT profiles, whose interpretations are constrained by geological data, indicate: (1) a high resistivity zone in cemented portions of the fault below the water table, and (2) in the unsaturated zone, a low resistivity feature along the cemented portions of the fault. The high resistivity zone below the water table is consistent with a 10% reduction in porosity due to the fault zone cementation. With the same porosity in the unsaturated zone, the low resistivity feature in the cemented fault zone is consistent with saturation >0.7, in contrast to saturation 0.2-0.7 for sediment outside of the cemented fault zone. In addition, subsurface samples and ERT profiles delineate a buttress unconformity (i.e., steeply dipping erosional contact) corresponding to a paleo-valley margin. This unconformity truncates the cemented fault zone and separates Pliocene axial-fluvial sand (deposited by an ancestral Rio Grande) from late Quaternary sand and gravel (deposited by the Rio Salado, a Rio Grande tributary). The cemented fault zone in the southern portion of the study area is a hydrogeologic barrier; north of the buttress unconformity, where the cemented fault zone has been removed by erosion, the fault is not a hydrogeologic barrier. The integration of geological, geophysical, and hydrogeological observations was key to developing our understanding of this complex system, and allowed us to demonstrate the utility of ERT in detecting subsurface fault-zone cementation.

Fault zones are an important control on fluid flow, affecting groundwater supply, contaminant migration, and carbon storage. However, most models of fault seal do not consider fault zone cementation, despite the recognition that it is common and can dramatically reduce permeability. In order to study the field-scale hydrogeologic effects of fault zone cementation, we conducted a series of aquifer pumping tests in wells installed within tens of meters of the variably cemented Loma Blanca Fault, a normal fault in the Rio Grande Rift. In the southern half of the study area, the fault zone is cemented by calcite; the cemented zone is 2-8 m wide. In the center of the study area, the cemented fault zone is truncated at a buttress unconformity that laterally separates hydrostratigraphic units with a ∼40X difference in permeability. The fault zone north of the unconformity is not cemented. Constant rate pumping tests indicate that where the fault is cemented, it is a barrier to groundwater flow. This is an important demonstration that a fault with no clay in its core and similar sediment on both sides can be a barrier to groundwater flow by virtue of its cementation; most conceptual models for the hydrogeology of faults would predict that it would not be a barrier to groundwater flow. Additionally, the lateral permeability heterogeneity across the unconformity imposes another important control on the local flow field. This permeability discontinuity acts as either a no-flow boundary or a constant head boundary, depending on the location of pumping.

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We present a series of simple three-dimensional numerical flow models of to examine how different barrier types impact the local subsurface flow regime and head distribution. Pumping is simulated near barriers including a linear boundary (similar to image well superposition), conductive faults structure, and laterally opposing facies (i.e. sharp permeability change). For each of these scenarios, vertical barriers (90°) were compared to dipping barrier orientations (45°). Each simulation was run for a duration of 1000 h with a fully penetrating well pumping at 6.3E-03 m 3 /s (100 USGPM) under confined aquifer conditions. A finite element method based multiphysics software (COMSOL) was utilized for model mesh generation and flow simulation. Transient average well drawdowns were evaluated for each run. The linear barrier models produced the most substantial average well drawdown, which is attributed to the barrier's impermeability. Opposing facies simulations produced more average drawdown at the well than their conductive fault model counterparts. The impacts of barrier dip angle in late test time were only discernible in the conductive fault model runs. Furthermore, we include preliminary examples of a simple field study analogue where both opposing facies and a conductive fault are present. Additional perturbation analyses that vary both barrier dip and well proximity are likely to provide further insights to the flow regime sensitivity.