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The ability to identify, at potential managed aquifer recharge sites, the presence of connected pathways of hydraulically conductive sediments from the ground surface to the water table could help minimize costs and risks associated with recharge operations. A spatially dense dataset had previously been acquired in an almond [Prunus dulcis (Mill.) D.A. Webb] grove in Tulare, CA, using tTEM, a towed transient electromagnetic (tTEM) geophysical method. In order to interpret reliable information about sediment type from the tTEM data, a transform from the tTEM-derived property, electrical resistivity, to sediment type is required. The uncertainty associated with derived models of sediment type can be significantly reduced if a site-and datasetspecific transform is used. Cone penetrometer testing (CPT) was conducted at five locations, strategically selected based on a review of the tTEM data. Co-located measurements of sediment type, derived from the CPT, and electrical resistivity, derived from the tTEM data, were used to create a resistivity-to-sediment-type transform, with sediment type classified as either coarse-grain-dominated (sand and gravel) or fine-grain-dominated (silt and clay) material. The transform captured the uncertainty associated with variable water salinity and content, the resolution of the tTEM data, and other components of the tTEM measurement workflow. Using this transform, models of sediment type were generated for the unsaturated zone at the site. Within these models are features, which we interpret as potential recharge pathways, corresponding to high fractions of coarse-grain-dominated material amongst regions of fine-grain-dominated material. The workflow developed at this site can provide a framework for using tTEM and CPT for recharge site assessment.

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Mapping and monitoring of saltwater intrusion are critical to the sustainable management of groundwater in coastal aquifers around the world. Increasingly, geophysical methods, such as electrical resistivity tomography (ERT), have been used to address these needs. We identified methods for the inversion of ERT data that would most accurately map the location and geometry of an intrusion wedge. This was accomplished using both laboratory and synthetic experiments, with the classic representation of an intrusion wedge perpendicular to the coast. The laboratory experiments allowed us to collect ERT data on a saltwater intrusion wedge in an environment where we had supporting data that provided (1) the distribution of salinity within the tank with which to verify our inversion results, (2) the resistivity, porosity and permeability of the porous medium, and (3) the transform between resistivity and salinity. The synthetic experiments allowed to explore issues of specific interest related to the presence of lithologic heterogeneity at a field site: the role of lithologic heterogeneity in introducing complexity both the resistivity-salinity relationship and the geometry of the saltwater intrusion wedge. We found that using a reference model with a good approximation of the wedge to inform the inversion greatly improved the ability of the resulting resistivity profile to map the wedge. Where there was no, or limited lithologic heterogeneity, a parametric approach, which constrained the range of possible solutions by solving for a sharp interface between the saltwater and freshwater regions, was very effective at capturing the wedge location and geometry. Where there was lithologic heterogeneity, a hybrid between the parametric and informed inversion approaches was most effective, resolving the wedge with a high level of accuracy with little a priori information.

With climate change and population growth, there is increasing concern about the depletion of groundwater resources. Evidence of this is a petition signed by over 1,300 scientists from 100 countries calling for action to ensure that groundwater benefits society now and into the future (Gleeson et al., 2019). A groundwater model is the foundation on which to build sustainable groundwater management. Required to inform the development of the groundwater model is information about the subsurface that captures spatial heterogeneity at the level needed as the input for flow modeling.All groundwater models include some representation of the large-scale structure of the groundwater system-the hydrogeologic units and other major features relevant for modeling flow. We define "large" to be the scale at which we represent the architecture of the subsurface in terms of the boundaries between areas with significant differences in hydrogeologic properties, for example, the boundary between an aquitard and aquifer, the top of the bedrock. Information derived from driller's logs, recorded when wells are drilled, is typically used to build the model. The logs potentially provide valuable information at point locations, but the quality of the driller's logs