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Core Ideas Two new subcatchments are used to test the importance of lithology and land use. Differences in lithology and land use result in differences in soils and waters. Despite differences, all catchments have a shallow and a deep water table. The relative importance of flow paths controls distinct chemistry response to discharge. Cross‐site comparison will ultimately enable upscaling from the catchment to large scale. The footprint of the Susquehanna Shale Hills Critical Zone Observatory was expanded in 2013 from the forested Shale Hills subcatchment (0.08 km2) to most of Shavers Creek watershed (163 km2) in an effort to understand the interactions among water, energy, gas, solute, and sediment. The main stem of Shavers Creek is now monitored, and instrumentation has been installed in two new subcatchments: Garner Run and Cole Farm. Garner Run is a pristine forested site underlain by sandstone, whereas Cole Farm is a cultivated site on calcareous shale. We describe preliminary data and insights about how the critical zone has evolved on sites of different lithology, vegetation, and land use. A notable conceptual model that has emerged is the “two water table” concept. Despite differences in critical zone architecture, we found evidence in each catchment of a shallow and a deep water table, with the former defined by shallow interflow and the latter defined by deeper groundwater flow through weathered and fractured bedrock. We show that the shallow and deep waters have distinct chemical signatures. The proportion of contribution from each water type to stream discharge plays a key role in determining how concentrations, including nutrients, vary as a function of stream discharge. This illustrates the benefits of the critical zone observatory approach: having common sites to grapple with cross‐disciplinary research questions, to integrate diverse datasets, and to support model development that ultimately enables the development of powerful conceptual and numerical frameworks for large‐scale hindcasting and forecasting capabilities.

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Subsurface water flow in South Florida is largely controlled by the heterogeneous nature of the karst limestone in the Biscayne aquifer and its upper formation, the Miami Limestone. These heterogeneities are amplified by dissolution structures that induce changes in the aquifer's material and physical properties (i.e., porosity and dielectric permittivity) and create preferential flow paths. Understanding such patterns are critical for the development of realistic groundwater flow models, particularly in the Everglades, where restoration of hydrological conditions is intended. In this work, we used noninvasive ground penetrating radar (GPR) to estimate the spatial variability in porosity and the dielectric permittivity of the solid phase of the limestone at centimeter-scale resolution to evaluate the potential for field-based GPR studies. A laboratory setup that included high-frequency GPR measurements under completely unsaturated and saturated conditions was used to estimate changes in electromagnetic wave velocity through Miami Limestone samples. The Complex Refractive Index Model was used to derive estimates of porosity and dielectric permittivity of the solid phase of the limestone. Porosity estimates of the samples ranged between 45.2 and 66.0% and showed good correspondence with estimates of porosity using analytical and digital image techniques. Solid dielectric permittivity values ranged between 7.0 and 13.0. This study shows the ability of GPR to image the spatial variability of porosity and dielectric permittivity in the Miami Limestone and shows potential for expanding these results to larger scales and other karst aquifers.