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Item Saltwater Intrusion Into a Confined Island Aquifer Driven by Erosion, Changing Recharge, Sea-Level Rise, and Coastal Flooding(Water Resources Research, 2024-01-08) Stanic, S.; LeRoux, N. K.; Paldor, A.; Mohammed, A. A.; Michael, H. A.; Kurylyk, B. L.Aquifers on small islands are at risk of salinization due to low elevations and limited adaptive capacity, and present risks will be exacerbated by climate change. Most studies addressing small-island saltwater intrusion (SWI) have focused on homogeneous sandy islands and one or two hydraulic disturbances. We herein investigate SWI dynamics in a layered, confined island aquifer in response to multiple environmental perturbations related to climate change, with two considered in tandem. Our field and modeling work is based on an island aquifer that provides the drinking water supply for an Indigenous community in Atlantic Canada. Observation well data and electrical resistivity profiles were used to calibrate a numerical model (HydroGeoSphere) of coupled groundwater flow and salt transport. The calibrated model was used to simulate the impacts of climate change including sea-level rise (SLR), storm surge overtopping, changing aquifer recharge, and erosion. Simulated aquifer conditions were resilient to surges because the confining layer prevented deeper saltwater leaching. However, reduced recharge and erosion resulted in saltwater wedge migration of 170 and 110 m, respectively when considered individually, and up to 295 m (i.e., into the wellfield) when considered together. Despite the confining conditions, SLR resulted in wedge migration up to 55 m as the confining pressures were not sufficient to resist wedge movement. This is the first study to harness an integrated, surface-subsurface hydrologic model to assess effects of coastal erosion and other hydroclimatic stressors on island aquifers, highlighting that climate change can drive extensive salinization of critical groundwater resources. Key Points - A surface-subsurface numerical model is used to investigate climate change impacts on island groundwater resources used for water supply - The confined aquifer is resilient to storm surges which only salinize the unpumped surficial aquifer before being flushed - Coastal erosion and recharge reductions result in the most saltwater intrusion and can work in tandem to threaten future water supply Plain Language Summary Due to their limited resources and adaptive capacity, small islands are highly vulnerable to climate change impacts, including saltwater intrusion. Freshwater needs on small islands are often sourced from small aquifers that are in delicate balance between conditions in the ocean, atmosphere, and land. In this study, we investigate the movement of saltwater into the freshwater aquifer of a small island that provides drinking water resources for an Indigenous First Nation. We consider climatic changes in the ocean (sea-level rise (SLR), storm surges, and related coastal erosion) and atmosphere (changes to net precipitation) and associated impacts to the island's fresh groundwater resources. We use field data paired with a mathematical model and demonstrate that the pressurized conditions of the layered island aquifer make it more resilient to SLR than unconfined aquifers in sandy islands are. However, the aquifer's freshwater volume is susceptible to coastal erosion and reduced precipitation, particularly when these happen at the same time. Results point to coastal erosion as a potential widespread driver of freshwater loss along eroding portions of the global coastline.Item Trace elements in abyssal peridotite olivine record melting, thermal evolution, and melt refertilization in the oceanic upper mantle(Contributions to Mineralogy and Petrology, 2023-09-06) Lin, Kuan-Yu; Warren, Jessica M.; Davis, Fred A.Trace element concentrations in abyssal peridotite olivine provide insights into the formation and evolution of the oceanic lithosphere. We present olivine trace element compositions (Al, Ca, Ti, V, Cr, Mn, Co, Ni, Zn, Y, Yb) from abyssal peridotites to investigate partial melting, melt–rock interaction, and subsolidus cooling at mid-ocean ridges and intra-oceanic forearcs. We targeted 44 peridotites from fast (Hess Deep, East Pacific Rise) and ultraslow (Gakkel and Southwest Indian Ridges) spreading ridges and the Tonga trench, including 5 peridotites that contain melt veins. We found that the abundances of Ti, Mn, Co, and Zn increase, while Ni decreases in melt-veined samples relative to unveined samples, suggesting that these elements are useful tracers of melt infiltration. The abundances of Al, Ca, Cr, and V in olivine are temperature sensitive. Thermometers utilizing Al and Ca in olivine indicate temperatures of 650–1000 °C, with variations corresponding to the contrasting cooling rates the peridotites experienced in different tectonic environments. Finally, we demonstrate with a two-stage model that olivine Y and Yb abundances reflect both partial melting and subsolidus re-equilibration. Samples that record lower Al- and Ca-in-olivine temperatures experienced higher extents of diffusive Y and Yb loss during cooling. Altogether, we demonstrate that olivine trace elements document both high-temperature melting and melt–rock interaction events, as well as subsolidus cooling related to their exhumation and emplacement onto the seafloor. This makes them useful tools to study processes associated with seafloor spreading and mid-ocean ridge tectonics.Item Pre- and postsettlement depositional processes and environments of the 3rd- to 5th-order White Clay Creek watershed, Piedmont Province, Pennsylvania and Delaware, USA(GSA Bulletin, 2023-08-23) Pizzuto, J.E.; Huffman, M. E.; Symes, E.We extend two hypotheses based on studies of 1st- to 3rd-order Piedmont watersheds of southeastern Pennsylvania, USA, by collecting data in a larger 3rd- to 5th-order watershed nearby. One hypothesis posits that presettlement river corridors were dominated by wetlands, and the other suggests that river valleys were filled by millpond sedimentation following European settlement. Both hypotheses support new river restoration practices, so their generality is important to assess. Ten lithofacies indicate depositional environments, while pedostratigraphic criteria and 14C dating define presettlement and postsettlement stratigraphic units. Basal gravels similar to modern stream bed sediments represent presettlement channels with active bedload transport. Wedge-shaped gravel deposits resembling modern bars further document presettlement bedload transport by channelized flows. Extensive presettlement and postsettlement units of massive, organic-poor, fine-grained sediment formed when overbank flows inundated floodplains. Peat deposits, exposed at a single site (but absent elsewhere), represent a presettlement wetland. Decimeter-thick, discontinuous, massive carbonaceous fine-grained sediments occasionally overlie basal gravels; these may represent localized wetlands adjacent to presettlement channels or hydraulic backwater environments. Laminated sand and mud accumulated behind one 3-m-high mill dam, but these millpond deposits are absent at other sites. Instead of being dominated by wetlands, presettlement river corridors are better described as a complex mosaic of riparian environments including older colluvial landforms, floodplains (some of which may have been seasonally inundated wetlands), primary (and possibly secondary) channels, and depending on geomorphic setting, either localized or valley-spanning wetlands. After European settlement, millponds were important locally, but their deposits represent a minor component of the stratigraphic record.Item Physiochemical Controls on the Horizontal Exchange of Blue Carbon Across the Salt Marsh-Tidal Channel Interface(Journal of Geophysical Research: Biogeosciences, 2023-06-06) Fettrow, Sean; Jeppi, Virginia; Wozniak, Andrew; Vargas, Rodrigo; Michael, Holly; Seyfferth, Angelia L.Tidal channels are biogeochemical hotspots that horizontally exchange carbon (C) with marsh platforms, but the physiochemical drivers controlling these dynamics are poorly understood. We hypothesized that C-bearing iron (Fe) oxides precipitate and immobilize dissolved organic carbon (DOC) during ebb tide as the soils oxygenate, and dissolve into the porewater during flood tide, promoting transport to the channel. The hydraulic gradient physically controls how these solutes are horizontally exchanged across the marsh platform-tidal channel interface; we hypothesized that this gradient alters the concentration and source of C being exchanged. We further hypothesized that trace soil gases (i.e., CO2, CH4, dimethyl sulfide) are pushed out of the channel bank as the groundwater rises. To test these hypotheses, we measured porewater, surface water, and soil trace gases over two 24-hr monitoring campaigns (i.e., summer and spring) in a mesohaline tidal marsh. We found that Fe2+ and DOC were positively related during flood tide but not during ebb tide in spring when soils were more oxidized. This finding shows evidence for the formation and dissolution of C-bearing Fe oxides across a tidal cycle. In addition, the tidal channel contained significantly (p < 0.05) more terrestrial-like DOC when the hydraulic gradient was driving flow toward the channel. In comparison, the channel water was saltier and contained significantly (p < 0.05) more marine-like DOC when the hydraulic gradient reversed direction. Trace gas fluxes increased with rising groundwater levels, particularly dimethyl sulfide. These findings suggest multiple physiochemical mechanisms controlling the horizontal exchange of C at the marsh platform-tidal channel interface. Plain Language Summary Tidal salt marshes store large amounts of carbon belowground in soils, but there is also a significant amount of carbon flowing into and out of these ecosystems via tidal channels. We investigated the carbon flowing between the channel bank and surface water in a salt marsh in Delaware. We found that soil minerals (i.e., iron oxides) control the mobility of carbon as iron oxides retain carbon during ebb tides and release carbon during flood tides as the minerals dissolve. The gradient between the groundwater and surface water elevation (i.e., hydraulic gradient) controls the flow direction for dissolved carbon, altering the concentration and source of carbon found in the tidal channel across tidal cycles. In addition, gases trapped in channel banks are pushed out of the soils as the tide rises. These findings will improve our understanding of carbon cycles in these critical carbon sinks. Key Points - Physiochemical mechanisms control horizontal exchange of carbon across marsh-tidal channel interfaces, affecting lateral carbon flux - Dissolution and reprecipitation of carbon-bearing Fe oxides during flood and ebb tides control the horizontal mobility of carbon - Hydraulic gradients control the carbon character in the tidal channel, and rising tides push greenhouse gases out of the channel bankItem Predicting Subsurface Architecture From Surface Channel Networks in the Bengal Delta(Journal of Geophysical Research: Earth Surface, 2023-03-19) Xu, Zhongyuan; Khan, Mahfuzur R.; Ahmed, Kazi Matin; Zahid, Anwar; Hariharan, Jayaram; Passalacqua, Paola; Steel, Elisabeth; Chadwick, Austin; Paola, Chris; Goodbred, Steven L. Jr.; Paldor, Anner; Michael, Holly A.Groundwater is the primary source of water in the Bengal Delta but contamination threatens this vital resource. In deltaic environments, heterogeneous sedimentary architecture controls groundwater flow; therefore, characterizing subsurface structure is a critical step in predicting groundwater contamination. Here, we show that surface information can improve the characterization of the nature and geometry of subsurface features, thus improving the predictions of groundwater flow. We selected three locations in the Bengal Delta with distinct surface river network characteristics—the lower delta with straighter tidal channels, the mid-delta with meandering and braided channels, and the inactive delta with transitional sinuous channels. We used surface information, including channel widths, depths, and sinuosity, to create models of the subsurface with object-based geostatistical simulations. We collected an extensive set of lithologic data and filled in gaps with newly drilled boreholes. Our results show that densely distributed lithologic data from active lower and mid-delta are consistent with the object-based models generated from surface information. In the inactive delta, metrics from object-based models derived from surface geometries are not consistent with subsurface data. We further simulated groundwater flow and solute transport through the object-based models and compared these with simulated flow through lithologic models based only on variograms. Substantial differences in flow and transport through the different geologic models show that geometric structure derived from surface information strongly influences groundwater flow and solute transport. Land surface features in active deltas are therefore a valuable source of information for improving the evaluation of groundwater vulnerability to contamination. Key Points: - We demonstrate a novel approach that harnesses land surface characteristics to inform groundwater modeling in deltas - The subsurface lithologic data of an active delta is more consistent with surface features than that of an inactive delta - Incorporation of surface information can improve the prediction of contaminant transport in aquifers Plain Language Summary: The structure of groundwater aquifers affects how groundwater and contaminants move through them. In deltas, dynamic river networks are responsible for depositing sediments that ultimately form subsurface aquifers. Therefore, the characteristics of the surface river channel network should provide information about the structure of the subsurface. We tested this idea using a large set of sedimentary data from the Bengal Basin. We created models of the subsurface based on the surface network and showed that the subsurface data reflect the model characteristics in deltas that are actively depositing sediment. Using these subsurface models as input for groundwater flow models, we showed that incorporating this surface information is important for being able to predict how contaminants move in groundwater.Item The role of iron in the formation of Ediacaran ‘death masks’(Geobiology, 2023-02-26) Gibson, Brandt M.; Schiffbauer, James D.; Wallace, Adam F.; Darroch, Simon A. F.The Ediacara biota are an enigmatic group of Neoproterozoic soft-bodied fossils that mark the first major radiation of complex eukaryotic and macroscopic life. These fossils are thought to have been preserved via pyritic “death masks” mediated by seafloor microbial mats, though little about the chemical constraints of this preservational pathway is known, in particular surrounding the role of bioavailable iron in death mask formation and preservational fidelity. In this study, we perform decay experiments on both diploblastic and triploblastic animals under a range of simulated sedimentary iron concentrations, in order to characterize the role of iron in the preservation of Ediacaran organisms. After 28 days of decay, we demonstrate the first convincing “death masks” produced under experimental laboratory conditions composed of iron sulfide and probable oxide veneers. Moreover, our results demonstrate that the abundance of iron in experiments is not the sole control on death mask formation, but also tissue histology and the availability of nucleation sites. This illustrates that Ediacaran preservation via microbial death masks need not be a “perfect storm” of paleoenvironmental porewater and sediment chemistry, but instead can occur under a range of conditions.Item Aqueous Geochemical Controls on the Sestonic Microbial Community in Lakes Michigan and Superior(Microorganisms, 2023-02-17) Rani, Asha; Ranjan, Ravi; Bonina, Solidea M. C.; Izadmehr, Mahsa; Giesy, John P.; Li, An; Sturchio, Neil C.; Rockne, Karl J.Despite being the largest freshwater lake system in the world, relatively little is known about the sestonic microbial community structure in the Laurentian Great Lakes. The goal of this research was to better understand this ecosystem using high-throughput sequencing of microbial communities as a function of water depth at six locations in the westernmost Great Lakes of Superior and Michigan. The water column was characterized by gradients in temperature, dissolved oxygen (DO), pH, and other physicochemical parameters with depth. Mean nitrate concentrations were 32 μmol/L, with only slight variation within and between the lakes, and with depth. Mean available phosphorus was 0.07 μmol/L, resulting in relatively large N:P ratios (97:1) indicative of P limitation. Abundances of the phyla Actinobacteria, Bacteroidetes, Cyanobacteria, Thaumarchaeota, and Verrucomicrobia differed significantly among the Lakes. Candidatus Nitrosopumilus was present in greater abundance in Lake Superior compared to Lake Michigan, suggesting the importance of ammonia-oxidating archaea in water column N cycling in Lake Superior. The Shannon diversity index was negatively correlated with pH, temperature, and salinity, and positively correlated with DO, latitude, and N2 saturation. Results of this study suggest that DO, pH, temperature, and salinity were major drivers shaping the community composition in the Great Lakes.Item Mixotrophy broadens the ecological niche range of the iron oxidizer Sideroxydans sp. CL21 isolated from an iron-rich peatland(FEMS Microbiology Ecology, 2023-02-14) Cooper, Rebecca E.; Finck, Jessica; Chan, Clara; Küsel, KirstenSideroxydans sp. CL21 is a microaerobic, acid-tolerant Fe(II)-oxidizer, isolated from the Schlöppnerbrunnen fen. Since the genome size of Sideroxydans sp. CL21 is 21% larger than that of the neutrophilic Sideroxydans lithotrophicus ES-1, we hypothesized that strain CL21 contains additional metabolic traits to thrive in the fen. The common genomic content of both strains contains homologs of the putative Fe(II) oxidation genes, mtoAB and cyc2. A large part of the accessory genome in strain CL21 contains genes linked to utilization of alternative electron donors, including NiFe uptake hydrogenases, and genes encoding lactate uptake and utilization proteins, motility and biofilm formation, transposable elements, and pH homeostasis mechanisms. Next, we incubated the strain in different combinations of electron donors and characterized the fen microbial communities. Sideroxydans spp. comprised 3.33% and 3.94% of the total relative abundance in the peatland soil and peatland water, respectively. Incubation results indicate Sideroxydans sp. CL21 uses H2 and thiosulfate, while lactate only enhances growth when combined with Fe, H2, or thiosulfate. Rates of H2 utilization were highest in combination with other substrates. Thus, Sideroxydans sp. CL21 is a mixotroph, growing best by simultaneously using substrate combinations, which helps to thrive in dynamic and complex habitats.Item Frequent Storm Surges Affect the Groundwater of Coastal Ecosystems(Geophysical Research Letters, 2023-01-09) Nordio, Giovanna; Frederiks, Ryan; Hingst, Mary; Carr, Joel; Kirwan, Matt; Gedan, Keryn; Michael, Holly; Fagherazzi, SergioRecent studies have focused on the effect of large tropical cyclones (hurricanes) on the shore, neglecting the role of less intense but more frequent events. Here we analyze the effect of the offshore tropical storm Melissa on groundwater data collected along the North America Atlantic coast. Our meta-analysis indicates that both groundwater level and specific conductivity significantly increased during Melissa, respectively reaching maximum values of 1.09 m and 25.2 mS/cm above pre-storm levels. Time to recover to pre-storm levels was 10 times greater for groundwater specific conductivity, with a median value of 20 days, while groundwater level had a median recovery time of 2 days. A frequency-magnitude analysis indicates that the percent of time with salinization is higher for Melissa than for energetic hurricanes. Given the high frequency of these events (return period of 1–2 years), and the long time needed for groundwater conditions to return to normal levels, we conclude that increasingly frequent moderate storms will have a significant impact on the ecology of vegetated shorelines. Key Points: - Salinization - Coastal areas - Groundwater Plain Language Summary: Salinization and flooding events due to sea level rise and storm surges threaten coastal ecosystems, changing groundwater characteristics. Moderate and more frequent storm surges can have a significant impact on coastal ecology, similar to larger tropical cyclones. Salinity and water table elevation need time to recover to normal conditions. The recovery time is compared to the frequency of these moderate storm surge events to determine the effect on the coastal groundwater.Item Coastal topography and hydrogeology control critical groundwater gradients and potential beach surface instability during storm surges(Hydrology and Earth System Sciences, 2022-12-02) Paldor, Anner; Stark, Nina; Florence, Matthew; Raubenheimer, Britt; Elgar, Steve; Housego, Rachel; Frederiks, Ryan S.; Michael, Holly A.Ocean surges pose a global threat for coastal stability. These hazardous events alter flow conditions and pore pressures in flooded beach areas during both inundation and subsequent retreat stages, which can mobilize beach material, potentially enhancing erosion significantly. In this study, the evolution of surge-induced pore-pressure gradients is studied through numerical hydrologic simulations of storm surges. The spatiotemporal variability of critically high gradients is analyzed in three dimensions. The analysis is based on a threshold value obtained for quicksand formation of beach materials under groundwater seepage. Simulations of surge events show that, during the run-up stage, head gradients can rise to the calculated critical level landward of the advancing inundation line. During the receding stage, critical gradients were simulated seaward of the retreating inundation line. These gradients reach maximum magnitudes just as sea level returns to pre-surge levels and are most accentuated beneath the still-water shoreline, where the model surface changes slope. The gradients vary along the shore owing to variable beach morphology, with the largest gradients seaward of intermediate-scale (1–3 m elevation) topographic elements (dunes) in the flood zone. These findings suggest that the common practices in monitoring and mitigating surge-induced failures and erosion, which typically focus on the flattest areas of beaches, might need to be revised to include other topographic features.Item Hydrologic Control on Arsenic Cycling at the Groundwater–Surface Water Interface of a Tidal Channel(Environmental Science and Technology, 2023-01-10) Yu, Xuan; LeMonte, Joshua J.; Li, Junxia; Stuckey, Jason W.; Sparks, Donald L.; Cargill, John G.; Russoniello, Christopher J.; Michael, Holly A.Historical industrial activities have resulted in soil contamination at sites globally. Many of these sites are located along coastlines, making them vulnerable to hydrologic and biogeochemical alterations due to climate change and sea-level rise. However, the impact of hydrologic dynamics on contaminant mobility in tidal environments has not been well studied. Here, we collected data from pressure transducers in wells, multi-level redox sensors, and porewater samplers at an As-contaminated site adjacent to a freshwater tidal channel. Results indicate that sharp redox gradients exist and that redox conditions vary on tidal to seasonal timescales due to sub-daily water level fluctuations in the channel and seasonal groundwater–surface water interactions. The As and Fe2+ concentrations decreased during seasonal periods of net discharge to the channel. The seasonal changes were greater than tidal variations in both Eh and As concentrations, indicating that impacts of the seasonal mechanism are stronger than those of sub-daily water table fluctuations. A conceptual model describing tidal and seasonal hydro-biogeochemical coupling is presented. These findings have broad implications for understanding the impacts of sea-level rise on the mobility of natural and anthropogenic coastal solutes.Item Hydrogen Diffusion in Clinopyroxene at Low Temperatures (195°C–400°C) and Consequences for Subsurface Processes(Geochemistry, Geophysics, Geosystems, 2022-12-12) Bissbort, Thilo; Lynn, Kendra J.; Becker, Hans-Werner; Chakraborty, SumitStudying diffusion of hydrogen in nominally anhydrous minerals, like clinopyroxene, at low temperatures is a challenging task due to experimental and analytical difficulties. In this study, to overcome these problems we have produced H concentration gradients in single crystals of natural diopsidic clinopyroxene by ion implantation and measured the nanoscale profiles before and after diffusion anneals using Nuclear Resonance Reaction Analysis. These steps allowed us to conduct experiments at temperatures between 195°C and 400°C. Obtained diffusion rates show a consistent Arrhenius relation DH= 5.47(±13.98) · 10−8 · exp (−115.64(±11.5) kJ mol−1/RT) m2s−1. Notably, our results lie well within the range of extrapolations from high temperature experiments (≥600°C) of previous studies. This implies that fast diffusion of hydrogen (compared to other elements) extends to low temperatures. We used these results in a non-isothermal diffusion model that simulates the ascent of crystals (0.5, 1.0, and 2.0 mm) along two representative P-T-paths from 600°C to 100°C, to assess potential re-equilibration of H contents in clinopyroxene at low temperatures. Our model highlights the need to carefully consider boundary conditions, which are a function of P-T-fO2, that control the concentration gradient at the crystal's rim. The results from this model help to assess, as a function of crystal size and cooling rate, when re-equilibration must be considered. Key Points - Diffusion rates of hydrogen in clinopyroxene in the low temperature range (195°C–400°C) were quantified for the first time - Diffusion coefficients at low temperatures lie within the range of extrapolations from high temperature experiments - Non-isothermal modeling was applied to evaluate potential re-equilibration of clinopyroxene crystals at low temperaturesItem Critical facility accessibility and road criticality assessment considering flood-induced partial failure(Sustainable and Resilient Infrastructure, 2022-11-25) Gangwal, Utkarsh; Siders, A. R.; Horney, Jennifer; Michael, Holly A.; Dong, ShangjiaThis paper examines communities’ accessibility to critical facilities such as hospitals, emergency medical services, and emergency shelters when facing flooding. We use travel speed reduction to account for flood-induced partial road failure. A modified betweenness centrality metric is also introduced to calculate the criticality of roads for connecting communities to critical facilities. The proposed model and metric are applied to the Delaware road network under 100-year floods. This model highlights the severe critical facility access loss risk due to flood isolation of facilities. The mapped post-flooding accessibility suggests a significant travel time increase to critical facilities and reveals disparities among communities, especially for vulnerable groups such as long-term care facility residents. We also identified critical roads that are vital for post-flooding access to critical facilities. The results of this research can help inform targeted infrastructure investment decisions and hazard mitigation strategies that contribute to equitable community resilience enhancement.Item Surface Water-Groundwater Connections as Pathways for Inland Salinization of Coastal Aquifers(Groundwater, 2022-11-17) Hingst, Mary C.; McQuiggan, Rachel W.; Peters, Chelsea N.; He, Changming; Andres, A. Scott; Michael, Holly A.Coastal agricultural zones are experiencing salinization due to accelerating rates of sea-level rise, causing reduction in crop yields and abandonment of farmland. Understanding mechanisms and drivers of this seawater intrusion (SWI) is key to mitigating its effects and predicting future vulnerability of groundwater resources to salinization. We implemented a monitoring network of pressure and specific conductivity (SC) sensors in wells and surface waters to target marsh-adjacent agricultural areas in greater Dover, Delaware. Recorded water levels and SC over a period of three years show that the mechanisms and timescales of SWI are controlled by local hydrology, geomorphology, and geology. Monitored wells did not indicate widespread salinization of deep groundwater in the surficial aquifer. However, monitored surface water bodies and shallow (<4 m deep) wells did show SC fluctuations due to tides and storm events, in one case leading to salinization of deeper (18 m deep) groundwater. Seasonal peaks in SC occurred during late summer months. Seasonal and interannual variation of SC was also influenced by relative sea level. The data collected in this study data highlight the mechanisms by which surface water-groundwater connections lead to salinization of aquifers inland, before SWI is detected in deeper groundwater nearer the coastline. Sharing of our data with stakeholders has led to the implementation of SWI mitigation efforts, illustrating the importance of strategic monitoring and stakeholder engagement to support coastal resilience.Item Effects of Geologic Setting on Contaminant Transport in Deltaic Aquifers(Water Resources Research, 2022-08-25) Xu, Zhongyuan; Hariharan, Jayaram; Passalacqua, Paola; Steel, Elisabeth; Chadwick, Austin; Paola, Chris; Paldor, Anner; Michael, Holly A.Coastal deltaic aquifers are vulnerable to degradation from seawater intrusion, geogenic and anthropogenic contamination, and groundwater abstraction. The distribution and transport of contaminants are highly dependent on the subsurface sedimentary architecture, such as the presence of channelized features that preferentially conduct flow. Surface deposition changes in response to sea-level rise (SLR) and sediment supply, but it remains unclear how these surface changes affect the distribution and transport of groundwater solutes in aquifers. Here, we explore the influence of SLR and sediment supply on aquifer heterogeneity and resulting effects on contaminant transport. We use realizations of subsurface heterogeneity generated by a process-based numerical model, DeltaRCM, which simulates the evolution of a deltaic aquifer with different input sand fractions and rates of SLR. We simulate groundwater flow and solute transport through these deposits in three contamination scenarios: (a) vertical transport from widespread contamination at the land surface, (b) vertical transport from river water infiltration, and (c) lateral seawater intrusion. The simulations show that the vulnerability of deltaic aquifers to seawater intrusion correlates to sand fraction, while vertical transport of contaminants, such as widespread shallow contamination and river water infiltration, is influenced by channel stacking patterns. This analysis provides new insights into the connection between the depositional system properties and vulnerability to different modes of groundwater contamination. It also illustrates how vulnerability may vary locally within a delta due to depositional differences. Results suggest that groundwater management strategies may be improved by considering surface features, location within the delta, and the external forcings during aquifer deposition. Plain Language Summary: The findings of this study provide insight into the vulnerability of deltaic aquifers to three contamination processes: (a) widespread contaminant transport from the land surface, (b) river water infiltration, and (c) seawater intrusion. We consider how contamination is affected by the location of contaminants and the processes associated with the accumulation of sediments in deltas. Our work shows that vulnerability to contamination depends on how the aquifer is deposited. The results also demonstrate that the distribution of sandy channels preserved in the subsurface, as well as rivers on the surface, controls vertical contaminant transport. We find that these effects vary from upstream to downstream in the delta because of spatial differences in depositional processes. These findings will help to improve predictions of groundwater contamination and manage groundwater development in deltas around the world.Item Dynamic Steady State in Coastal Aquifers Is Driven by Multi-Scale Cyclical Processes, Controlled by Aquifer Storativity(Geophysical Research Letters, 2022-05-24) Paldor, Anner; Frederiks, Ryan S.; Michael, Holly A.Coastal aquifers supply freshwater to nearly half the global population, yet they are threatened by salinization. Salinities are typically estimated assuming steady-state, neglecting the effect of cyclical forcings on average salinity distributions. Here, numerical modeling is used to test this assumption. Multi-scale fluctuations in sea level (SL) are simulated, from tides to glacial cycles. Results show that high-frequency fluctuations alter average salinities compared with the steady-state distribution produced by average SL. Low-frequency forcing generates discrepancies between present-day salinities estimated with and without considering the cyclical forcing due to overshoot effects. This implies that salinities in coastal aquifers may be erroneously estimated when assuming steady-state conditions, since present distributions are likely part of a dynamic steady state that includes forcing on multiple timescales. Further, typically neglected aquifer storage characteristics can strongly control average salinity distributions. This has important implications for managing vulnerable coastal groundwater resources and for calibration of hydrogeological models. Key Points: - Average salinities in coastal aquifers are affected by low-frequency cyclical changes in sea level (SL) - High-frequency cyclical forcings generate episodic discrepancies in salinity when modeled with and without considering these processes - Under these multi-scale fluctuations in SL, dynamic steady states of coastal aquifers are affected by aquifer storage properties Plain Language Summary: Coastal communities rely heavily on groundwater for freshwater supply, and the primary risk for this vital resource is salinization. Multiple processes in the ocean-land interface control the salinity of coastal aquifers, and assessments of salinities typically neglect some of these processes. In this work, we show that some of the typically neglected processes may be responsible for large-scale, systematic discrepancies between actual and estimated salinities. This has important implications for the assessment of risks to coastal groundwater reservoirs and for the long-term management of these resources.Item Spatially averaged stratigraphic data to inform watershed sediment routing: An example from the Mid-Atlantic United States(Geological Society of America Bulletin, 2022-05-05) Pizzuto, James E.; Skalak, K.J.; Benthem, A.; Mahan, S.A.; Sherif, M.; Pearson, A.J.New and previously published stratigraphic data define Holocene to present sediment storage time scales for Mid-Atlantic river corridors. Empirical distributions of deposit ages and thicknesses were randomly sampled to create synthetic age-depth records. Deposits predating European settlement accumulated at a (median) rate of 0.06 cm yr−1, range from ∼18,000 to 225 yr old, and represent 39% (median) of the total accumulation. Sediments deposited from 1750 to 1950 (“legacy sediments”) accumulated at a (median) rate of 0.39 cm yr−1 and comprise 47% (median) of the total, while “modern sediments” (1950−present) represent 11% of the total and accumulated at a (median) rate of 0.25 cm yr−1. Synthetic stratigraphic sequences, recast as age distributions for the presettlement period, in 1900 A.D., and at present, reflect rapid postsettlement alluviation, with enhanced preservation of younger sediments related to postsettlement watershed disturbance. An averaged present age distribution for vertically accreted sediment has modal, median, and mean ages of 190, 230, and 630 yr, reflecting the predominance of stored legacy sediments and the influence of relatively few, much older early Holocene deposits. The present age distribution, if represented by an exponential approximation (mean age ∼300 yr), and naively assumed to represent steady-state conditions, implies median sediment travel times on the order of centuries for travel distances greater than ∼100 km. The percentage of sediment reaching the watershed outlet in 30 yr (a reasonable time horizon to achieve watershed restoration efficacy) is ∼60% for a distance of 50 km, but this decreases to <20% for distances greater than 200 km. Age distributions, evaluated through time, not only encapsulate the history of sediment storage, but they also provide data for calibrating watershed-scale sediment-routing models over geological time scales.Item Floodplain Sediment Storage Timescales of the Laterally Confined Meandering Powder River, USA(Journal of Geophysical Research: Earth Surface, 2022-01-11) Huffman, Max E.; Pizzuto, James E.; Trampush, Sheila M.; Moody, John A.; Schook, Derek M.; Gray, Harrison J.; Mahan, Shannon A.As sediment is transported through river corridors, it typically spends more time in storage than transport, and as a result, sediment delivery timescales are controlled by the duration of storage. Present understanding of storage timescales is largely derived from models or from field studies covering relatively short (≤102 year) time spans. Here we quantify the storage time distribution for a 17 km length of Powder River in Montana, USA by determining the age distribution of eroded sediment. Our approach integrates surveyed cross-sections, analysis of historical aerial imagery, aerial LiDAR, geomorphic mapping, and age control provided by optically stimulated luminescence (OSL) and dendrochronology. Sediment eroded by Powder River from 1998 to 2013 ranges from a few years to ∼5,000 years in age; ages are exponentially distributed (r2 = 0.78; Anderson-Darling p value 0.003). Eroded sediment is derived from Powder River's meander belt (∼900 m wide), which is only 1.25 times its meander wavelength, a value reflecting valley confinement rather than free meandering. The mean storage time, 824 years (95% C.I. 610–1030 years), is similar to the time required to rework deposits of Powder River's meander belt based on an average meander migration rate of ∼1 m/yr, implying that storage time distributions of confined meandering rivers can be quantified from remotely sensed estimates of meander belt width and channel migration rates. Heavy-tailed storage time distributions, frequently cited from physical and numerical modeling studies, may be restricted to unconfined meandering rivers. Plain Language Summary: As sediment moves downstream through a watershed it is intermittently stored in a river's deposits before being eroded and transported farther downstream. Storage times vary from less than a decade to millennia. Storage time greatly exceeds the time sediment is being transported by the river. Consequently, the time required for sediment to reach a point downstream is largely controlled by the time spent in storage. This can influence how the movement of contaminants are monitored and restoration strategies are developed. Sediment particles spend different amounts of time in storage, which can be represented as a probability distribution. Here we date sediment eroded by Powder River in southeastern Montana from 1998 to 2013 and find that the storage time distribution is exponential. Furthermore, the mean storage time of 824 years (which fully characterizes the exponential distribution) can be determined from the meander belt width and the channel migration rate, both of which can be measured using aerial imagery, providing a simple method for assessing storage times in laterally confined rivers.Item Along-Shore Movement of Groundwater and Its Effects on Seawater-Groundwater Interactions in Heterogeneous Coastal Aquifers(Water Resources Research, 2021-12-14) Geng, Xiaolong; Michael, Holly A.Studies of coastal groundwater dynamics often assume two-dimensional (2D) flow and transport along a shore-perpendicular cross-section. We show that along-shore movement of groundwater may also be significant in heterogeneous coastal aquifers. Simulations of groundwater flow and salt transport incorporating different geologic structure show highly three-dimensional (3D) preferential flow paths. The along-shore movement of groundwater on average accounts for 40%–50% of the total flowpath length in both conduit-type (e.g., volcanic) heterogeneous aquifers and statistically equivalent (e.g., deltaic) systems generated with sequential indicator simulation (SIS). Our results identify a critical role of three-dimensionality in systems with connected high-permeability geological features. 3D conduit features connecting land and sea cause more terrestrial groundwater flow through the inland boundary and intensify water exchange along the land-sea interface. Therefore, conduits increase the rate of SGD compared to equivalent homogeneous, SIS and corresponding 2D models. In contrast, in SIS-type systems, less-connected high-permeability features produce mixing zones and SGD nearer to shore, with comparable rates in 3D and 2D models. Onshore, 3D heterogeneous cases have longer flowpaths and travel times from recharge to discharge compared to 2D cases, but offshore travel times are much shorter, particularly for conduit-type models in which flow is highly preferential. Flowpath lengths and travel times are also highly variable in 3D relative to 2D for all heterogeneous simulations. The results have implications for water resources management, biogeochemical reactions within coastal aquifers, and subsequent chemical fluxes to the ocean. Plain Language Summary: The findings of this study provide insight into the complex patterns of groundwater flow under the influence of geologic variability in coastal aquifers. In coastal regions, studies of solute transport processes mainly rely on an assumption of 2D groundwater flow and solute transport in the shore-perpendicular direction. Our results reveal that groundwater does not only flow toward the sea, it also can flow along-shore, especially in aquifers with features that connect the onshore and offshore. This affects exchange and mixing between fresh and saline groundwater, which can strongly impact delivery of contaminants and nutrients to sensitive nearshore marine ecosystems. Results highlight the importance of characterizing the geology of coastal aquifers and representing it in models of groundwater flow and contaminant transport.Item Linking the Surface and Subsurface in River Deltas—Part 2: Relating Subsurface Geometry to Groundwater Flow Behavior(Water Resources Research, 2021-08-02) Xu, Zhongyuan; Hariharan, Jayaram; Passalacqua, Paola; Steel, Elisabeth; Paola, Chris; Michael, Holly A.Understanding subsurface structure and groundwater flow in deltaic aquifers is essential for evaluating the vulnerability of groundwater resources in delta systems. Deltaic aquifers contain coarse-grained paleochannels that preserve a record of former surface river channels as well as fine-grained floodplain deposits. The distribution of these deposits and how they are interconnected control groundwater flow and contaminant transport. In this work, we link depositional environments of deltaic aquifers to stratigraphic (static) and flow and transport (dynamic) connectivity metrics. Numerical models of deltaic stratigraphy were generated using a reduced-complexity numerical model (DeltaRCM) with different input sand fractions (ISF) and rates of sea-level rise (SLR). The groundwater flow and advective transport behavior of these deltas were simulated using MODFLOW and MODPATH. By comparing the static and dynamic metrics calculated from these numerical models, we show that groundwater behavior can be predicted by particular aspects of the subsurface architecture, and that horizontal and vertical connectivity display different characteristics. We also evaluate relationships between connectivity metrics and two environmental controls on delta evolution: ISF and SLR rate. The results show that geologic setting strongly influences both static and dynamic connectivity in different directions. These results provide insights into quantitatively differentiated subsurface hydraulic behavior between deltas formed under different external forcing (ISF and SLR rate) and they are a potential link in using information from delta surface networks and depositional history to predict vulnerability to aquifer contamination. Plain Language Summary: Geologic structure and groundwater flow behaviors influence groundwater resources in delta plains. In deltaic aquifers, channel structures were created by past surface rivers. These channels in the subsurface are “fast-travel” pathways for groundwater and contaminants. We created synthetic delta structures with a numerical model and then simulated groundwater flow through them in order to tie geologic structure to groundwater flow behavior. By using many different models, we investigate how structure and flow relate, and how the subsurface geology and groundwater system are affected by different sediment inputs and sea-level rise rates. The findings will help us better manage delta groundwater resources and provide an opportunity to predict groundwater contamination from surface characteristics.