A pinboard by
Sarah McKnight

PhD Student, University of Massachusetts


I predict and model saline-to-freshwater interfaces under different climatic and geologic conditions

A lot of research has focused on the intrusion of saltwater, also known as brine, in coastal regions because of drinking water concerns. However, saltwater also develops beneath inland deserts, where high rates of evaporation creates groundwater with high salinity. For example, the aquifer underlying the southeastern extent of an arid salt flat known as Salar de Atacama (SdA) in northern Chile contains an 8 km-long interface between fresh and saline fluids that exhibits a slope of approximately 5 degrees. Homogenous, isotropic models fail to capture this geometry, predicting slope angles for an interface at approximately 42 degrees. I investigate the conditions that impact brine-to-freshwater interface dynamics. Those conditions include both hydrologic factors, such as increasing rates of evaporation and decreasing rates of recharge, and geologic factors, such as faulting and low-permeability layers of rock. I use the USGS-made SEAWAT program to model how the interface responds to those different hydrologic and geologic factors, and I then compare those results to the field conditions that I observe. Metrics for evaluating interface response include slope angle, migration rate, and time to reach dynamic equilibrium. From this modeling, I expect saltwater creep to occur when recharge is less than average, which will likely happen in many deserts as a result of climate change. I also expect that geologic layers with low permeability will encourage the horizontal migration of the salt-to-freshwater interface by acting as a sort of platform that supports the denser brine’s expansion beneath the fresh groundwater. For now, I am focusing on SdA, but I look forward to extending this work to other desert regions as well. Results from my work has implications for the stratigraphic impact on brine migration, arid hydrogeology under changing climate conditions, and groundwater development in brine-rich, arid regions.


From soil to sea: the role of groundwater in coastal critical zone processes

Abstract: Near coasts, surface water–groundwater interactions control many biogeochemical processes associated with the critical zone, which extends from shallow aquifer to vegetative canopy. For example, submarine groundwater discharge delivers a significant fraction of weathering products such as silica and calcium to the world's oceans. Owing to changing fertilizer and land use practices, submarine groundwater discharge is also responsible for high nitrogen loads that drive eutrophication in marine waters. Submarine groundwater discharge is generally unmonitored due to its heterogeneous and diffuse spatial patterns and complex temporal dynamics. Here, we review the physical processes that drive submarine groundwater discharge at various spatial and temporal scales and highlight examples of interdependent critical zone processes. Like the inland critical zone, the coastal critical zone is undergoing rapid change in the Anthropocene. Disturbances include warming air and sea temperatures, sea‐level rise, increasing storm severity, increasing nutrient and contaminant inputs, and ocean acidification. In a changing world, it is more important than ever to understand complex feedbacks between dynamic surface water‐groundwater interaction, rocks, and life through long‐term monitoring efforts that extend beyond inland rivers to coastal groundwater.For further resources related to this article, please visit the WIREs website.

Pub.: 17 May '16, Pinned: 14 Aug '17

Impacts of physical and chemical aquifer heterogeneity on basin-scale solute transport: vulnerability of deep groundwater to arsenic contamination in Bangladesh

Abstract: Aquifer heterogeneity presents a primary challenge in predicting the movement of solutes in groundwater systems. The problem is particularly difficult on very large scales, across which permeability, chemical properties, and pumping rates may vary by many orders of magnitude and data is often sparse. An example is the fluvio-deltaic aquifer system of Bangladesh, where naturally-occurring arsenic (As) exists over tens of thousands of square kilometers in shallow groundwater. Millions of people in As-affected regions on deep (≥150 m) groundwater as a safe source of drinking water. The sustainability of this resource has been evaluated with models using effective properties appropriate for a basin-scale contamination problem, but the extent to which preferential flow affects the timescale of downward migration of As-contaminated shallow groundwater is unknown. Here we imbed detailed, heterogeneous representations of hydraulic conductivity (K), pumping rates, and sorptive properties (Kd) within a basin-scale numerical groundwater flow and solute transport model to evaluate their effects on vulnerability and deviations from simulations with homogeneous representations. Advective particle tracking shows that heterogeneity in K does not affect average travel times from shallow zones to 150 m depth, but the travel times of the fastest 10% of particles decreases by a factor of ∼2. Pumping distributions do not strongly affect travel times if irrigation remains shallow, but increases in the deep pumping rate substantially reduce travel times. Simulation of advective-dispersive transport with sorption shows that deep groundwater is protected from contamination over a sustainable timeframe (>1000 y) if the spatial distribution of Kd is uniform. However, if only low-K sediments sorb As, 30% of the aquifer is not protected. Results indicate that sustainable management strategies in the Bengal Basin should consider impacts of both physical and chemical heterogeneity, as well as their correlation. These insights from Bangladesh show that preferential flow exerts a strong control on breakthrough of both conservative and reactive solutes even at large spatial scales, with implications for predicting water supply vulnerability in contaminated heterogeneous aquifers worldwide.

Pub.: 13 Oct '16, Pinned: 14 Aug '17

A method of estimating bulk potential permeability in fractured-rock aquifers using field-derived fracture data and type curves

Abstract: A method is devised for estimating the potential permeability of fracture networks from attributes of fractures observed in outcrop. The technique, which is intended as a complement to traditional approaches, is based on type curves that represent various combinations of fracture lengths, fracture orientations and proportions (i.e., intensities) of fractures that participate in flow. Numerical models are used to derive the type curves. To account for variations in fracture aperture, a permeability ratio (R) defined as the permeability of a fracture network in a domain divided by the permeability of a single fracture with identical fracture apertures, is used as a dependent variable to derive the type curves. The technique works by determining the point on the type curve that represents the fracture characteristics collected in the field. To test the performance of the technique, permeabilities that were derived from fractured-rock aquifers of eastern Massachusetts (USA) are compared to permeabilities predicted by the technique. Results indicate that permeabilities estimated from type curves are within an order of magnitude of permeabilities derived from field tests. First-order estimates of fracture-network permeability can, therefore, be easily and quickly acquired with this technique before more robust and expensive methods are utilized in the field.

Pub.: 10 Nov '12, Pinned: 14 Aug '17

Investigating the role of hydromechanical coupling on flow and transport in shallow fractured-rock aquifers

Abstract: Fractured-rock aquifers display spatially and temporally variable hydraulic conductivity generally attributed to variable fracture intensity and connectivity. Empirical evidence suggests fracture aperture and hydraulic conductivity are sensitive to in situ stress. This study investigates the sensitivity of fractured-rock hydraulic conductivity, groundwater flow paths, and advection-dominated transport to variable shear and normal fracture stiffness magnitudes for a range of deviatoric stress states. Fracture aperture and hydraulic conductivity are solved for analytically using empirical hydromechanical coupling equations; groundwater flow paths and ages are then solved for numerically using groundwater flow and advection-dispersion equations in a traditional Toth basin. Results suggest hydraulic conductivity alteration is dominated by fracture normal closure, resulting in decreasing hydraulic conductivity and increasing groundwater age with depth, and decreased depth of long flow paths with decreasing normal stiffness. Shear dilation has minimal effect on hydraulic conductivity alteration for stress states investigated here. Results are interpreted to suggest that fracture normal stiffness influences hydraulic conductivity of hydraulically active fractures and, thus, affects flow and transport in shallow (<1 km) fractured-rock aquifers. It is suggested that observed depth-dependent hydraulic conductivity trends in fractured-rock aquifers throughout the world may be partly a manifestation of hydromechanical phenomena.

Pub.: 18 Jun '14, Pinned: 14 Aug '17

Rapid recharge of fresh water to the halite-hosted brine aquifer of Salar de Atacama, Chile

Abstract: We document, analyse, and interpret direct and rapid infiltration of precipitation to the southern margin of the Salar de Atacama halite-hosted brine aquifer during two intense precipitation events in 2012–2013. We present physical, geochemical, and stable and radioactive isotope data to detail this influx of water. The two events differ distinctly in the mechanisms of recharge. The 2012 event did not produce direct precipitation onto the salar surface, while the 2013 event did. Both events are recorded by abrupt changes in head in observation wells along the halite aquifer margin. Spatially distributed water levels rose by over 30 cm during the larger 2013 event consistent with remotely sensed observations of surface water extent. The lithium concentration and stable isotopic composition of water indicate dilution of brine and dissolution of salt with fresh water. Tritium measurements of precipitation, surface water, and groundwater all indicate modern influx of water to the halite aquifer along the southern margin. We extend these observations by examining the response of the halite aquifer as a whole to precipitation events during the period of 2000–2010. This study suggests that local recharge to the aquifer during sporadic precipitation onto the halite nucleus is an important component of the modern water budget in this hyper-arid environment. The rapid dissolution and salinization along the southern margin of the salar halite nucleus are aided by such precipitation events contributing a modern fresh water component to the water budget of the economically valuable lithium-rich brine. Copyright © 2016 John Wiley & Sons, Ltd.

Pub.: 24 Oct '16, Pinned: 14 Aug '17

Assessing hydrogeologic controls on dynamic groundwater storage using long-term instrumental records of water table levels

Abstract: This study analyzes a long-term regional compilation of water table response to climate variability based on 124 long-term groundwater wells distributed across New England, USA, screened in a variety of geologic materials. The New England region of the USA is located in a humid-temperature climate underlain by low-storage-fractured metamorphic and crystalline bedrock dissected by north–south trending valleys filled with glacial and post-glacial valley fill sediments. Uplands are covered by thin glacial till that comprises more than 60% of the total area. Annual and multi-annual responses of the water table to climate variability are assessed to understand how local hydraulic properties and hydrogeologic setting (located in recharge/discharge region) of the aquifer influence the hydrologic sensitivity of the aquifer system to climate variability. This study documents that upland aquifer systems dominated by thin deposits of surface till comprise ~70% of the active and dynamic storage of the region. Total aquifer storage changes of +5 to −7 km3 occur over the region during the study interval. The storage response is dominated by thin and low permeability surficial till aquifer that fills and drains on a multi-annual basis and serves as the main mechanism to deliver water to valley fill aquifers and underlying bedrock aquifers. Whereas the till aquifer system is traditionally neglected as an important storage reservoir, this study highlights the importance of a process-based understanding of how different landscape hydrogeologic units contribute to the overall hydrologic response of a region.

Pub.: 21 Feb '17, Pinned: 14 Aug '17