<p> Hydrogeologically, Rapa Nui (Easter Island, Chile) is one of the least understood islands in Polynesia. There are no surface streams, the soils are poor in productivity and highly permeable, and the water table sits far below the surface of the island. One of the many mysteries of Rapa Nui is how the ancient inhabitants survived with so few sources of freshwater. </p><p> Fieldwork was conducted to identify terrestrial sources of freshwater and to evaluate the occurrence of submarine groundwater discharge (SGD). We documented observable surface water features located in the interior and coast of the island and made field measurements of water temperature and salinity to identify areas of SGD along the coast. The limited number of interior surface water features, periods of drought, permeable aquifers, and existence of <i> puna</i> (dug wells) along the coast of Rapa Nui lead us to conclude that coastal seeps were an important prehistoric source of freshwater.</p>

<p> Managed Aquifer Recharge (MAR) systems have become an increasingly important approach to the management of groundwater in Southern California in recent years. This thesis describes an experimental investigation of the vadose zone dynamics beneath a recharge basin used by the Orange County Water District. Soil moisture probes, pressure transducers, and fiber optic distributed temperature sensing (FODTS) at multiple depths beneath the basin were used to monitor infiltration. The purpose was to measure the diurnal temperature flux using heat as a tracer of infiltrating water to gain insight on the influence of basin stage (i.e., water level) on infiltration rate. To increase the temperature resolution from the standard 1 m, we installed a wrapped fiber optic cable at two locations using direct push technology. The wrapped FODTS cable was spliced to a trenched cable that ran laterally across the basin at depths of 30 cm and 1 m, installed during a previous experiment. The wrapped cable was then installed vertically at two locations to observe both the spatial distribution and vertical dynamics of fluid flow at 10 cm intervals. Propagation of the diurnal heat flux at the surface was related to water velocity. The infiltration behavior was affected by subtle changes in stratigraphy below the basin. The heat tracer suggests strong components of horizontal flow due to the presence of thin fine-grained hydrostratigraphic units. Water movement during initial saturation was particularly complex and suggested that simple one-dimensional vertical flow models will not accurately predict infiltration rates. The FODTS system provided high-resolution dynamic imaging of percolation that is not possible using a multi-level transducer system.</p><p>

<p> The purpose of this study was to investigate multiple methods used to locate and identify sources of submarine groundwater discharge (SGD) along the coastline of Santa Catalina Island (Catalina) in Los Angeles, California. The driving hypothesis is that SGD may be identified by higher temperature/lower salinity plumes. To test this hypothesis, temperature and conductivity measurements were made along the shoreline of Catalina and compared with previously collected airborne thermal infrared images. In some locations where potential zones of SGD were identified, samples were collected from the surface water within the plume and analyzed for the presence of radon. Radon in ocean water can be used as a tracer to identify the presence of terrestrial groundwater. The analysis was complicated by dynamic variables including tides, weather, and sample collection. In spite of this uncertainty, one potential zone of SGD was identified using sea surface temperature, electrical conductivity, and radon at Toyon Bay. This zone of SGD is consistent with the location of warm water shown in airborne infrared imagery. These results suggest that high-resolution thermal imagery may be useful for identifying diffuse SGD in southern California, but further field tests will be necessary to confirm this conclusion.</p><p>

<p> Altered waters impacted by serpentinization of Coast Range Ophiolite (CRO) ultramafic units have been reacting with trapped Cretaceous seawaters, meteoric waters, and other surface derived waters since tectonic emplacement of this ophiolite. In 2011, groundwater monitoring wells of various depths were established near Lower Lake, CA, USA in the McLaughlin Natural Reserve, administered by the University of California-Davis, in order to understand ongoing low temperature alterations and biogeochemical interactions taking place. Wells were installed at two sites in the Reserve. There are three Quarry Valley area wells (QV1-1 [23m depth], QV1-2 [14.9m], QV1-3 [34.6m]) and five Core Shed area wells (CSW1-1 [19.5m], CSW1-2 [19.2m], CSW1-3 [23.2m], CSW1-4 [8.8m], CSW1-5 [27.4m]). Water samples were collected from all installed wells, as well as from an older well drilled near the historic core shed (Old Core Shed Well, or OCSW [82m]), and an upper (TC1) and lower (TC2) site sampling a nearby groundwater-fed alkaline seep, at Temptation Creek. Key environmental parameters (temperature, pH, conductivity, oxidation-reduction potential, and dissolved oxygen) were collected in the field using YSI-556 multiprobe meter, and total concentrations for major cations (Ca⁺², Na⁺, Mg⁺², K⁺) were analyzed using Thermo Scientific iCAP 7400 Inductively Coupled Plasma-Atomic Emission Spectrometry, and anions (F^&ndash;, Cl^&ndash;, SO₄^&ndash;2, NO₃^&ndash;) on Dionex Modular DX 500 Ion Chromatography. </p><p> Principal component analysis was conducted to determine key factors and processes controlling water chemistries at CRO. Geochemist&rsquo;s Workbench software was used to model the low temperature alteration of a serpentinization-influenced model water volume passing through serpentinite over a period of 100 million years. Modeling provided insight into the changing pH, Eh, evolving water chemistries, stepwise mineral assemblages, appearance of marker minerals at geochemical transitions in the system, and supported evidence of pervasive impacts of low temperature, oxidative weathering of serpentinites. This work supports the case of incremental dilution and transformation of a deeply sourced Ca²⁺-OH^&ndash; Type II water in this environment, and constrains reaction status of present day CRO waters and those of similar sites, in terms of the progress of serpentinite weathering reactions. Further, the study informs our understanding of serpentinization-related geological environments present on other celestial bodies (<i>e.g.</i>, Mars, Europa, Enceladus) in our Solar System and beyond.</p><p>

<p>A better understanding of groundwater flow through bedrock fractures is critical to applications involving heat and solute transport. Pumping tests performed to characterize these systems are often ill-suited because the radius of penetration quickly expands beyond the inter-well distance, gaining information beyond the well pair of interest. Periodic hydraulic tests allow the radius of penetration to be controlled by the frequency of oscillation, and testing at multiple frequencies gives parameter estimates for a range of spatial scales. Periodic pumping tests were performed at the Mirror Lake experimental fractured rock hydrology field site in New Hampshire. Results suggest a more complex, 3D network of connectivity than previously indicated by constant rate pumping tests. The relative degree of connectivity, given by diffusivity, corresponds to early-time response seen in the constant rate test. This confirms that the periodic tests investigated at a smaller penetration radius than the steady response from constant pumping.

<p> Percolation rates in Managed Aquifer Recharge (MAR) facilities, such as recharge basins and stream channels, can vary widely through both time and space. Natural variations in sediment hydraulic conductivity can create &ldquo;dead zones&rdquo; in which percolation rates are negligible. Clogging is a constant problem, leading to decays in facility percolation rates. Measuring percolation rate variations is important for management, maintenance, and remediation of surface MAR facilities. </p><p> We have used Fiber Optic Distributed Temperature Sensing (FODTS) to monitor percolation in a long narrow river channel separated from an active river by a levee. The alluvial sediment in the river channel varies widely in texture and water balance is difficult to monitor independently. The off-river channel was monitored by installing a fiber optic cable in the subsurface and measuring the propagation rate of the diurnal temperature oscillations carried downward with infiltrating water. In this way, heat was used as a tracer of percolation rates along the section defined by the 1800 meters of buried cable. We were able to confirm the FODTS measurements of percolation in the Off River Channel and demonstrate its wide applicability. Results from the measurements have been used to understand both the hydraulic behavior of percolation in the facilities and to make management decisions regarding facility operations and the potential need for additional surface sediment remediation.</p><p>

<p> The Niles Cone Basin (NCB) within Alameda County, California, contains portions of the basin under perennial and ephemeral artesian groundwater conditions. This study used 349 wells installed throughout the basin&rsquo;s four-aquifer system to delineate the spatial distribution of the 86 wells that have gone artesian between 1995 and 2015. Artesian wells within all four aquifers occur at elevations below 5.2 meters above sea level (MASL) but predominantly below 3.0 MASL. Even at lower elevations, artesian conditions do not occur in regions of major pumping owing to significant drawdown. Within topographically-low regions, wells may not be artesian where well-heads are located at higher elevations, such as on a levee or other elevated landforms. This can be observed throughout the Newark and Centerville aquifers where artesian wells are located near non-artesian wells during the same monitoring event. Precipitation influences artesian conditions with artesian events correlated with increases in precipitation generally during, but not limited to, the early spring months. The water levels of the shallow Newark Aquifer were found to respond independently from the three deeper aquifers. Those deeper aquifers were observed to be in hydraulic connection with one another, displaying synchronous water level changes with time across the basin.</p><p>

Aquifers hosted in fractured crystalline rocks are generally characterized by low porosity and strongly heterogeneous and anisotropic flow paths, with flow and transport dominated by discrete fracture sets. In general, zones of high hydraulic conductivity correlate with zones of high fracture intensity and fracture connectivity. Fractured rock hydraulic conductivity, however, is not only a function of spatial fracture distributions, but also displays dynamic variability due to changes in fracture aperture with changes in effective stress, such as those due to groundwater pumping and seasonal variations in water level. Some studies suggest hydromechanical coupling plays a minimal role in hydraulic conductivity alteration at shallow depths, whereas other studies attribute hydraulic conductivity alteration directly to hydromechanical coupling, thus raising a fundamental science question: what is the role of hydromechanical coupling in shallow fractured rock aquifers? This study investigates the role of hydromechanical coupling in shallow fractured rock aquifers from 3 perspectives. First, a sensitivity study presents the results of analytical and numerical modeling to determine what key hydromechanical parameters are important in the shallow crust under realistic stress states. Results suggest that hydraulic conductivity alteration is dominated by fracture normal closure, with shear dilation playing a minimal role, and that shallow dipping fractures are likely to have the highest hydraulic conductivity relative to steeply dipping fractures due to compressional deviatoric stress states. These results suggest that depth-dependent hydraulic conductivity trends observed in nature may be due in part to hydromechanical phenomena, and thus fractured rock characterization should include hydromechanical characterization. Study two presents the results of an aquifer-scale fractured rock characterization at Gates Pond, Berlin, MA in which hydromechanical variables are constrained in the field and coupled with structural and hydraulic characterization, including stress-tests, long-term water level monitoring, isotope analysis and earth tide analysis, to provide insight into the mechanical properties of the aquifer. Results of this study reveal an interesting field setting in which the mechanical properties of the aquifer are homogeneous throughout the study area, but the aquifer is compartmentalized by foliation parallel fractures that restrict hydraulic connection between wells that are placed perpendicular to foliation. Gates Pond also displays a hydraulic conductivity trend in which Foliation Parallel Fractures (FPF) have a decreasing transmissivity with increasing depth, and tectonic fractures display a decrease in transmissivity with increasing dip. Such observations suggest a conceptual model in which FPFs dominate flow in the shallow subsurface, with transmissivity decreasing with depth, where Tectonic fracture become the dominate flowing set. These results are consistent with the results of analytical and numerical modeling predictions from Chapter 1. Lastly, the third study presents results of a regional scale correlation of critically stressed fractures and fracture transmissivity in the Nashoba terrane, eastern Massachusetts. Whereas Chapter 1 suggests that fracture transmissivity is strongly modified by fracture normal closure, which is supported by field observations in Chapter 2, many workers suggest that flowing fractures are those that are critically stressed, and are thus strongly modified by shear dilation. This study addresses the role of shear dilation by identifying critically stressed fractures at a regional scale and correlating resolved stresses on transmissive fractures to fracture transmissivity. Fracture characteristics, transmissivity and borehole breakouts are characterized for 17 wells from throughout the Nashoba terrane. Critically stressed fractures are identified using inferred stress states, and correlation of critically stressed fractures to fracture transmissivity is investigated. Results suggest that transmissivity is weakly correlated to the ratio of shear to normal stress, and that ratio is strongly correlated to fracture dip. A conceptual model is proposed in which shallow dipping fractures are more likely to be critically stressed, such as FPFs in the shallow subsurface; however, high transmissivity fractures need not be critically stressed. Thus, it is concluded from observations in this dissertation that fractures in the shallow crust are most sensitive to fracture normal closure, although shear dilation may enhance transmissivity. The complex interaction between normal closure and shear dilation results in shallow dipping fractures being the most transmissive in the shallow subsurface, with tectonic fractures becoming more important with increasing depth. Each of the 3 studies presents a unique contribution to the study of hydromechanical coupling in fractured rock aquifers, with each study supporting the hypothesis that hydromechanical coupling may alter hydraulic conductivity of fractures in the shallow subsurface, contributing to observed depth-dependent hydraulic conductivity trends, variable hydraulic conductivity as a function of fracture dip, and dynamic permeability. Results of these studies show that hydromechanical coupling affects hydraulic conductivity of fractures in the shallow crust, and should therefore be incorporated into fractured rock aquifer characterization in conjunction with standard structural and hydrogeologic characterization.

This dissertation is a composite of several studies. First, three of the most common fracture sampling techniques are tested against each other to evaluate the effectiveness of each method to adequately capture the properties of natural discrete fracture networks (DFNs). Numerical simulation is used to evaluate the single scanline, selection and multiple scanline methods in layered rocks. Using statistics from each of the techniques, DFNs are stochastically generated and compared to another network that represents the natural DFN. This model was built with the exact locations, sizes and orientations of fractures as the natural network. Porosity and permeability results reveal that the most effective method to use is the selection method because this method is consistent and performs as well as the other methods but with less expenditure of time and energy. Second, the influence of lithology and rock fabric on fracture attribute distribution in crystalline rock is assessed. Trace lengths, spacings and orientations of joints and foliation-parallel fractures (FPFs) are used to determine the potential influence of fracture type and distribution on the groundwater flow system. Results show that although both joints and FPFs are common, major orientations and spacings are different for both fracture types. Because FPFs possess identical trace lengths but narrower spacing than joints, numerical modeling experiments indicate that they play an important role in controlling the groundwater flow regime by enhancing the transmissive properties of rocks. Third, conceptual models of DFNs that have unique hydraulic character that are based on fracture configurations and properties are developed with the aid of numerical simulations. Sets of persistently parallel fractures are stochastically generated to assess the effects of fracture size and distribution, intensity, number of sets and intersection angle on the hydraulic properties of DFNs. Arbitrarily chosen class intervals of the ratio of DFN permeability to that of a single fracture are used to delineate DFNs with similar hydraulic character. Numerous graphs are created for use in the field to determine and delineate DFNs with distinct hydraulic character.

Hydrological and geochemical data was collected from a small area within the 7.32-km$\sp2$ Cadwell Creek drainage basin in west-central Massachusetts for a period of one year. The hydrologic monitoring network included a tipping-bucket rain gauge, soil-moisture and soil-temperature probes, and continuous water-level recorders to measure changing groundwater levels and stream stage. The geochemical sampling network consisted of precipitation and throughfall collectors, soil water collectors, shallow and deep groundwater monitoring wells, and stream sampling locations. Water samples were collected on a weekly or bi-weekly basis for the entire study period. Both the hydrologic and geochemical information collected during the study period indicated that the majority of water reaching the stream, both during periods of high water level and storm flow and during baseflow periods in the summer months, was derived primarily from the top meter or two of the shallow water-table aquifer. Deeper groundwater exhibited a substantially different chemistry from that in the top 1-2 meters of the aquifer. The geochemical evidence clearly indicated that deep groundwater never entered the stream, even during baseflow periods. Instead, the stream in the vicinity of the study site became dry in the summer. Comparison of groundwater and stream chemistry during periods of high water level clearly indicated that the water in the stream was derived almost solely from the shallow groundwater, with little or no contribution from more dilute precipitation and soil water. Silica concentration, as well as alkalinity and pH values, proved to be an important indicator of the origin of water entering the stream. Specific factors affecting the degree to which acidic precipitation is neutralized before entering surface water within this watershed are primarily hydrologic in nature. The amount of time that precipitation water is in contact with the geologic materials prior to entering the stream appears to determine the degree to which it is neutralized. In this case, the depth to the top of the water table is a controlling factor. Silicate weathering dominated cation exchange as a neutralization mechanism in this watershed, at least in the vicinity of the instrumented site.