Department of Earth Sciences

Permanent URI for this community

https://udspace.udel.edu/handle/19716/10389

The Earth Sciences Department is part of the College of Earth, Ocean, & Environment. We have an excellent faculty with research strengths in coastal and marine geology and geophysics, surface processes and geomorphology, hydrogeology, geomicrobiology, Quaternary geology, paleobiology and micropaleontology, and environmental geophysics.

Visit the Department's website for more information.

Browse

Now showing 1 - 20 of 75

Alegria Canyon Terrace, Santa Barbara County, California
(2012-05-15) Wehmiller, John F.
Alegria Canyon Terrace, west of Santa Barbara, California. Modern terrace platform at sea level; Pleistocene (MIS 5) terrace at level of railroad tracks.
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.
Amino acid racemization age estimates for Pleistocene marine deposits in the Eukeka-Fields Landing Area, Homboldt County, California
(U.S. Department of the Interior Geological Survey, 1977) Wehmiller, John F.; Kennedy, George L.; Lajoie, Kenneth R.
Amino acid enantiomeric (D/L) ratios in fossil Saxidomus samples from four localities in the Eureka - Fields Landing area of the Humboldt Bay region, California yield age estimates of 180,000 to 280,000 years for exposed and slightly deformed bay and estuarine deposits.
Amino acid racemization dating of some coastal plain sites, southeastern Virginia and northeastern North Carolina
(University of Delaware, Newark, DE, 1985) Mirecki, June
Bivalve mollusks (g. Mercenaria and g. Anadara) have been analyzed from 4 southeastern coastal plain sites using the amino acid racemization (AAR) technique for correlation and age estimate purposes. As seen previously (Wehmiller and Belknap, 1982), a conflict exists between age estimates obtained by AAR and V-series methods at the Norris Bridge, VA locality. Here, V-series dating of coral yields a 187 ±20 KA age, while AAR methods suggest a 400 to 500 KA age. The following methods are employed to understand this discrepancy at the Norris Bridge site: 1). Analysis of standard samples to ensure uniform sample preparation and system operation; 2). Quantification of variation in Mercenari'a and Anadara samples for each field site; 3). Qualitative comparison of AAR behavior between Mercenaria and Anadara to assess suitability of Anadara for dating purposes; 4). Aminostratigraphic correlation of field sites using both genera, and 5). Estimation of AAR ages using the the non-linear model of Wehmiller and Belknap (1982). Standard samples analyzed for this study show relatively good preCISIon. Coefficients of variation (CVs) of D/L leucine values are 5.7% (ILC-A) and 9.1% (ILC-B). Overall, both ILC-A and -B show accuracy within 6% when compared to other University of Delaware analyses. Several valves of each genus were analyzed from all sites to determine field site precision. CVs for multiple Mercenaria samples from each site are as follows: Gomez Pit, VA (5.2%); New Light Pit, VA (7.0%); Moyock Pit, N.C. (8.8%) and Norris Bridge, VA (4.4%). CVs for Anadara samples are slightly higher, but comparable for each field site. No Anadara were analyzed from the New Light Pit locality. On the basis of an identical order of racemization rates (i.e. ASP2:ALA> LEU>VAL) and similar ASP/LEU values in both genera, Mercenaria and Anadara qualitatively show similar racemization behavior. 3 clusters of DjL leucine values have been discerned from these field site analyses. These D/L leucine values are used to correlate fossiliferous strata in the southeastern Virginia and northeastern North Carolina Using the non-linear kinetic model, these Coastal Plain. D/L values suggest the following ages: 0.21 to 0.25 (75 KA) and 0.530 (approximately 400 KA). D/L values ranging between 0.33 and 0.38 are tentatively estimated to be 100 to 200 KA in age.
Aminostratigraphy, geochronology and geochemistry of fossils from Late Cenozoic marine units in southeastern Virginia
(University of Delaware, Newark, 1990) Mirecki, June
Paired Amino Acid Racemization (AAR) and Electron Spin Resonance (ESR) data have been obtained from individual mollusc valves (g. Mercenaria) collected from stratified nearshore deposits at three sites in southeastern Virginia (Gomez Pit, Norris Bridge and Yadkin Pit). AAR and ESR data are used here to define aminozones in the region, and to infer age relationships among these superposed aminozones. These data are compared to U-Th analyses of corals from both sites (Szabo, 1985). Age differences among aminozones are estimated using models of racemization kinetics. Previous workers have generally concluded that the lithologic section at Gomez Pit represents a single transgression followed by minor regression. However, AAR and ESR data indicate at least one unconformity in this transgressive sequence, representing approximately 100 ka. The uppermost aminozone lla is defined by a mean ALLO/ISO values of 0.141+/-0.026 (n = 55). Aminozone lla directly overlies aminozone llc (ALLO/ISO = 0.335+/-0.028; n = 19). Molluscs representing a third, lowest aminozone (lld; ALLO/ISO = 0.471+/-0.038; n = 9) are recognized, but taphonomic and ESR data suggest that lld molluscs in Gomez Pit may be reworked. At Norris Bridge, only aminozone lld (ALLO/ISO = 0.473+/-0.020; n = 5) is recognized. ESR age estimates for aminozone lla range from 93-125 ka (n = 4), in general agreement with a Stage 5 estimate of 75+/-5 ka from U-Th analyses of corals from the same unit. ESR age estimates of 220 and 262 ka were calculated from 2 shells showing ALLO/ISO = 0.33; these data support a Stage 7 or Stage 9 interpretation for aminozone llc. AAR kinetic models suggest an age $>$400 ka for aminozone lld shells, inconsistent with a 187+/-20 ka age from a solitary coral at Norris Bridge. Attempts to obtain age estimates using ESR have been unsuccessful. Shells representing excellent, good, fair and poor preservation conditions from Gomez Pit aminozone lla were compared to determine the effects of condition on precision of amino acid data. These data were used to test the hypothesis that leaching by groundwater selectively removes the most extensively epimerized (free) amino acids from the mollusc shell, thus lowering the ALLO/ISO value. No statistically significant difference in amino acid concentrations, amino acid fractions or ALLO/ISO values was found among these Gomez Pit shells representing different preservation characteristics.
Caloosa Shell Pit, Florida
(2012-05-15) Wehmiller, John F.
Cape Blanco, Oregon
(2012-05-15) Wehmiller, John F.
Cape Hatteras and Core Banks, North Carolina beach shells
(2012-05-15) Wehmiller, John F.
Cedar Neck, Sussex County, Delaware
(2012-05-15) Wehmiller, John F.
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.
Comparison of approaches to dating Atlantic coastal plain sediments, Virginia Beach, Virginia
(Washington, DC : U.S. Nuclear Regulatory Commission, 1998-03) Lamothe, Michel; Wehmiller, John F.; Noller, Jay S.
Compilation of Florida Amino Acid Racemization Data
(2014-07-24) Wehmiller, John F.
Correlation and chronology of Pacific Coast marine terrace deposits of continental United States by fossil amino acid stereochemistry technique, evaluation, relative ages, kinetic model ages, and geologic implications
(U.S. Department of the Interior Geological Survey, 1977) Wehmiller, John F.; Lajoie, Kenneth R.; Kvenvolden, Keith A.; Peterson, Etta; Belknap, Daniel F.; Kennedy, George L.; Addicott, Warren O.; Vedde, John G.; Wright, Robert W.
Enantiomeric (D/L) ratios of six or more amino acids have been determined in Pleistocene mollusks from fifty-one terrace localities on the Pacific coast of the United States from Puget Sound, Washington, to San Diego, California. Samples have been selected for the purpose of evaluation of various aspects of the amino acid dating technique as well as for the application of the technique to unresolved chronologic problems. Samples with known stratigraphic relationships (upper and lower Pleistocene, Pliocene) have been examined to document the relationships between geologic age and extent of racemization. Most genera investigated exhibit the expected trend of increasing extent of racemization with increasing age, but some genera do not document this trend, probably because of contamination during diagenesis. Of all genera examined in this manner, the bivalve mollusk Saxidomus appears to be the most reliable.
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.
Davenport, California
(2012-05-15) Wehmiller, John F.
Descriptive text for Gomez Pit stratigraphy and geochronology
(2013) Wehmiller, John F.
Development of an amino acid racemization database for coastal plain sites in North Carolina
(University of Delaware, 2004) Pellerito, Vincent
An extensive collection of unpublished and published AAR, radiometric and taphonomic characteristic data for mollusk samples from coastal North and South Carolina have been arranged into a relational database. Organizing over two decades of regional AAR data is particularly important for ongoing chronostratigraphic studies of coastal North Carolina, where active study of an extensive Quaternary sequence is underway as part of the North Carolina Coastal Geology cooperative. A relational database design allows for integrated querying of multiple parameter datasets and ensures the database remains adaptable by removing any dependency on software. We also make use of current data sharing standards for the Microsoft® Windows® platform, employing data analysis software and GIS. Examination of this integrated dataset using advanced visualization techniques should improve understanding of the North Carolina coastal plain stratigraphy and help refine current chronostratigraphic estimates for the region. Furthermore, it builds on efforts to hone the accuracy and applicability of the AAR method as a chronological tool by incorporating numerous analyses over a thoroughly studied region such as the North Carolina coastal plain. Future endeavors such as web accessibility of the database and possible incorporation into a larger data repository is assisted with proper design early on. In addition, a user-friendly database interface has been developed for continued chromatographic data collection for an active AAR laboratory.
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.
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.
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.

Browse

Browsing Department of Earth Sciences by Title

Results Per Page

Sort Options