Frederick D. Day-Lewis

Time-lapse imaging of saline-tracer transport in fractured rock using difference-attenuation radar tomography

[1] Accurate characterization of fractured-rock aquifer heterogeneity remains one of the most challenging and important problems in groundwater hydrology. We demonstrate a promising strategy to identify preferential flow paths in fractured rock using a combination of geophysical monitoring and conventional hydrogeologic tests. Cross-well differenceattenuation ground-penetrating radar was used to monitor saline-tracer migration in an experiment at the U.S. Geological Survey Fractured Rock Hydrology Research Site in Grafton County, New Hampshire. Radar data sets were collected every 10 min in three adjoining planes for 5 hours during each of 12 tracer tests. An innovative inversion method accounts for data acquisition times and temporal changes in attenuation during data collection. The inverse algorithm minimizes a combination of two functions. The first is the sum of weighted squared data residuals. Second is a measure of solution complexity based on an a priori space-time covariance function, subject to constraints that limit radarattenuation changes to regions of the tomograms traversed by high difference-attenuation ray paths. The time series of tomograms indicate relative tracer concentrations and tracer arrival times in the image planes; from these we infer the presence and location of a preferential flow path within a previously identified zone of transmissive fractures. These results provide new insights into solute channeling and the nature of aquifer heterogeneity at the site. INDEX TERMS: 0910 Exploration Geophysics: Data processing; 0915 Exploration Geophysics: Downhole methods; 1829 Hydrology: Groundwater hydrology; 1832 Hydrology: Groundwater transport; 1894 Hydrology: Instruments and techniques; KEYWORDS: radar tomography, fractured rock, ground-penetrating radar, geophysics, hydrogeophysics

Improved hydrogeophysical characterization and monitoring through parallel modeling and inversion of time-domain resistivity andinduced-polarization data

Electrical geophysical methods have found wide use in the growing discipline of hydrogeophysics for characterizing the electrical properties of the subsurface and for monitoring subsurface processes in terms of the spatiotemporal changes in subsurface conductivity, chargeability, and source currents they govern. Presently, multichannel and multielectrode data collections systems can collect large data sets in relatively short periods of time. Practitioners, however, often are unable to fully utilize these large data sets and the information they contain because of standard desktop-computer processing limitations. These limitations can be addressed by utilizing the storage and processing capabilities of parallel computing environments. We have developed a parallel distributed-memory forward and inverse modeling algorithm for analyzing resistivity and time-domain induced polar-ization (IP) data. The primary components of the parallel computations include distributed computation of the pole solutions in forw...

Time-lapse inversion of crosswell radar data

The combination of differential radar tomography with conventional tracer and/or hydraulic tests facilitates high-resolution characterization of subsurface heterogeneity and enables the identification of preferential flow paths. In dynamic imaging, each tomogram is typically inverted independently, under the assumption that data sets are collected quickly relative to changes in the imaged property (e.g., attenuation or velocity); however, such “snapshot” tomograms may contain large errors if the imaged property changes significantly during data collection. Acquisition of less data over a shorter time interval could ameliorate the problem, but the resulting decrease in ray density and angular coverage could degrade model resolution. To address these problems, we propose a new sequential approach for time-lapse tomographic inversion. The method uses space-time parameterization and regularization to combine data collected at multiple times and to account for temporal variation. The inverse algorithm minimizes the sum of weighted squared residuals and a measure of solution complexity based on an a priori space-time covariance function and a spatiotemporally variable mean. We demonstrate our approach using a synthetic 2-D time-lapse (x; z; t) data set based loosely on a field experiment in which difference-attenuation radar tomography was used to monitor the migration of a saline tracer in fractured rock. We quantitatively show the benefits of space-time inversion by comparing results for snapshot and time-lapse inversion schemes. Inversion over both space and time results in superior estimation error, model resolution, and data reproduction compared to conventional snapshot inversion. Finally, we suggest strategies to improve time-lapse cross-hole inversions using ray-based inversion constraints and a modified survey design in which different sets of rays are collected in alternating time steps.

Identifying fracture‐zone geometry using simulated annealing and hydraulic‐connection data

A new approach is presented to condition geostatistical simulation of high-permeability zones in fractured rock to hydraulic-connection data. A simulated-annealing algorithm generates three-dimensional (3-D) realizations conditioned to borehole data, inferred hydraulic connections between packer-isolated borehole intervals, and an indicator (fracture zone or background-K bedrock) variogram model of spatial variability. We apply the method to data from the U.S. Geological Survey Mirror Lake Site in New Hampshire, where connected high-permeability fracture zones exert a strong control on fluid flow at the hundred-meter scale. Single-well hydraulic-packer tests indicate where permeable fracture zones intersect boreholes, and multiple-well pumping tests indicate the degree of hydraulic connection between boreholes. Borehole intervals connected by a fracture zone exhibit similar hydraulic responses, whereas intervals not connected by a fracture zone exhibit different responses. Our approach yields valuable insights into the 3-D geometry of fracture zones at Mirror Lake. Statistical analysis of the realizations yields maps of the probabilities of intersecting specific fracture zones with additional wells. Inverse flow modeling based on the assumption of equivalent porous media is used to estimate hydraulic conductivity and specific storage and to identify those fracture-zone geometries that are consistent with hydraulic test data.

A physical explanation for the development of redox microzones in hyporheic flow

Recent observations reveal a paradox of anaerobic respiration occurring in seemingly oxic-saturated sediments. Here we demonstrate a residence time-based explanation for this paradox. Specifically, we show how microzones favorable to anaerobic respiration processes (e.g., denitrification, metal reduction, and methanogenesis) can develop in the embedded less mobile porosity of bulk-oxic hyporheic zones. Anoxic microzones develop when transport time from the streambed to the pore center exceeds a characteristic uptake time of oxygen. A two-dimensional pore-network model was used to quantify how anoxic microzones develop across a range of hyporheic flow and oxygen uptake conditions. Two types of microzones develop: flow invariant and flow dependent. The former is stable across variable hydrologic conditions, whereas the formation and extent of the latter are sensitive to flow rate and orientation. Therefore, pore-scale residence time heterogeneity, which can now be evaluated in situ, offers a simple explanation for anaerobic signals occurring in oxic pore waters.

Continuous resistivity profiling to delineate submarine groundwater discharge—examples and limitations

Aquifer-ocean interaction, saline intrusion, and submarine groundwater discharge (SGD) are emerging topics in hydrology and oceanography with important implications for water-resource management and estuarine ecology. Although the threat of saltwater intrusion has long been recognized in coastal areas, SGD has, until recently, received much less attention. It is clear that SGD constitutes a major nutrient flux to coastal waters, with implications for estuarine ecology, eutrophication, and loss of coral reefs; however, fundamental questions regarding SGD remain unanswered: What are the spatial and temporal distributions of SGD offshore? How do seasonal and storm-related variations in aquifer recharge affect SGD flux and nutrient loading? What controls do aquifer structure and heterogeneity impose? How are SGD and saline recirculation related? Geophysical methods can provide insights to help answer these questions and improve the understanding of this intriguing and environmentally relevant hydrologic pheno...

Investigation of aquifer‐estuary interaction using wavelet analysis of fiber‐optic temperature data

Fiber-optic distributed temperature sensing (FODTS) provides sub-minute temporal and meter-scale spatial resolution over kilometer-long cables. Compared to conventional thermistor or thermocouple-based technologies, which measure temperature at discrete (and commonly sparse) locations, FODTS offers nearly continuous spatial coverage, thus providing hydrologic information at spatiotemporal scales previously impossible. Large and information-rich FODTS datasets, however, pose challenges for data exploration and analysis. To date, FODTS analyses have focused on time-series variance as the means to discriminate between hydrologic phenomena. Here, we demonstrate the continuous wavelet transform (CWT) and cross-wavelet transform (XWT) to analyze FODTS in the context of related hydrologic time series. We apply the CWT and XWT to data from Waquoit Bay, Massachusetts to identify the location and timing of tidal pumping of submarine groundwater.

Origin and Extent of Fresh Paleowaters on the Atlantic Continental Shelf, USA

While the existence of relatively fresh groundwater sequestered within permeable, porous sediments beneath the Atlantic continental shelf of North and South America has been known for some time, these waters have never been assessed as a potential resource. This fresh water was likely emplaced during Pleistocene sea-level low stands when the shelf was exposed to meteoric recharge and by elevated recharge in areas overrun by the Laurentide ice sheet at high latitudes. To test this hypothesis, we present results from a high-resolution paleohydrologic model of groundwater flow, heat and solute transport, ice sheet loading, and sea level fluctuations for the continental shelf from New Jersey to Maine over the last 2 million years. Our analysis suggests that the presence of fresh to brackish water within shallow Miocene sands more than 100 km offshore of New Jersey was facilitated by discharge of submarine springs along Baltimore and Hudson Canyons where these shallow aquifers crop out. Recharge rates four times modern levels were computed for portions of New Englands continental shelf that were overrun by the Laurentide ice sheet during the last glacial maximum. We estimate the volume of emplaced Pleistocene continental shelf fresh water (less than 1 ppt) to be 1300 km(3) in New England. We also present estimates of continental shelf fresh water resources for the U.S. Atlantic eastern seaboard (10(4) km(3)) and passive margins globally (3 x 10(5) km(3)). The simulation results support the hypothesis that offshore fresh water is a potentially valuable, albeit nonrenewable resource for coastal megacities faced with growing water shortages.

Geoelectrical evidence of bicontinuum transport in groundwater

Bicontinuum models and rate-limited mass transfer (RLMT) explain complex transport behavior (e.g., long tailing and rebound) in heterogeneous geologic media, but experimental verification is problematic because geochemical samples represent the mobile component of the pore space. Here, we present geophysical evidence of RLMT at the field scale during an aquifer-storage and recovery experiment in a fractured limestone aquifer in Charleston, South Carolina. We observe a hysteretic relation between measurements of pore-fluid conductivity and bulk electrical conductivity; this hysteresis contradicts advective-dispersive transport and the standard petrophysical model relating porefluid and bulk conductivity, but can be explained by considering bicontinuum transport models that include first-order RLMT. Using a simple numerical model, we demonstrate that geoelectrical measurements are sensitive to bicontinuum transport and RLMT parameters, which are otherwise difficult to infer from direct, hydrologic measurements.

Understanding Water Column and Streambed Thermal Refugia for Endangered Mussels in the Delaware River

Groundwater discharge locations along the upper Delaware River, both discrete bank seeps and diffuse streambed upwelling, may create thermal niche environments that benefit the endangered dwarf wedgemussel (Alasmidonta heterodon). We seek to identify whether discrete or diffuse groundwater inflow is the dominant control on refugia. Numerous springs and seeps were identified at all locations where dwarf wedgemussels still can be found. Infrared imagery and custom high spatial resolution fiber-optic distributed temperature sensors reveal complex thermal dynamics at one of the seeps with a relatively stable, cold groundwater plume extending along the streambed/water-column interface during midsummer. This plume, primarily fed by a discrete bank seep, was shown through analytical and numerical heat-transport modeling to dominate temperature dynamics in the region of potential habitation by the adult dwarf wedgemussel.