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Open access publications by faculty, postdocs, and graduate students in the Department of Earth Sciences.

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    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, Shangjia
    This 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.
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    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.
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    Stormwater drives seasonal geochemical processes beneath an infiltration basin
    (Journal of Environmental Quality, 2022-10-11) McQuiggan, Rachel; Andres, Scott A.; Roros, Andreanna; Sturchio, Neil C.
    Deicing salt is an important component of road safety during winter storms. Stormwater infiltration best management practices aim to prevent the salt from polluting streams and waterways, but this may shift pollutants to groundwater resources. In response to limited field studies investigating groundwater quality impacts caused by input of salt from stormwater infiltration best management practices, we monitored water levels and quality of groundwater at various depths in an unconfined aquifer around a stormwater infiltration basin using in situ sensors coupled with grab sampling. Our observations revealed differences in groundwater chemistry with depth in the aquifer and processes that were driven by the seasonal changes in the chemistry of stormwater (salt-impacted in winter and fresh in non-winter) recharging the aquifer. Water–matrix interactions in the vadose zone beneath the basin affected the transport of sodium (Na) into groundwater following non-winter recharge. Sodium movement through the aquifer was delayed relative to chloride (Cl), indicating a longer residence time of Na in the vadose zone. Radium (Ra) concentrations were correlated with Cl concentrations, suggesting salt-impacted recharge caused desorption of Ra into groundwater because of increased salinity. Stormwater-influenced groundwater followed a preferential flow path due to heterogeneity of the aquifer materials, and water chemistry varied with time and location along the flow path. These results highlight the importance of well screen length, placement and depth, and frequency of observations when designing a monitoring network.
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    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.
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    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.
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