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We were thrilled to see HydroGeoSphere so well represented at this years MODFLOW and More conference, so we’ve invited some of the users to deliver their presentations to the rest of the Aquanty network. This webinar will be of special interest to users interested in matrix diffusion in discrete fracture networks, saltwater intrusion, and the thermal energy transport capabilities offered by HydroGeoSphere.
Join Aquanty in welcoming HGS users Dr. Anner Paldor, Ryan Fredericks, Holly Michael and Katie Fogg and enjoy the following presentations.
Modeled salinities in coastal aquifers depend on high- and low-frequency fluctuations in sea level, controlled by storage properties.
Anner Paldor [1*], Ryan S. Frederiks [1], Holly A. Michael [1,2]
1 Department of Earth Sciences, University of Delaware, Newark, DE, USA.
2 Department of Civil and Environmental Engineering, University of Delaware, Newark, DE, USA.
* Correspondence: annerpal@udel.edu
Modeling groundwater flow and salt transport is a crucial part in the management of coastal aquifers, which supply freshwater to nearly half of the world population. Models aimed to estimate present-day average salinities typically assume steady sea level, thus neglecting the effect of cyclical forcings on the average salinity distributions. This study uses numerical modeling to assess the validity of this assumption. The modeled processes include multi-scale fluctuations in sea level on timescales from tides to glacial cycles. Results indicate that average salinities under high-frequency fluctuations in sea level differ from the steady state distribution produced by average sea level. Low-frequency forcing generates discrepancies between present-day salinities modeled with and without considering the cyclical forcing. These results imply that the estimated salinities in coastal aquifers may be erroneously simulated when assuming steady-state conditions, since present distributions are likely part of a dynamic steady state that includes forcing on multiple timescales. This has important implications for calibration of hydrogeological models of coastal aquifers and for managing vulnerable coastal groundwater resources.
Vulnerability to Sea-Level Rise and Storm-Surge Salinization Differs for Topography-limited and Recharge-limited Systems: Insights from Groundwater Flow and Transport Modeling at Assateague Island, MD
Ryan S. Frederiks [1*], Anner Paldor [1] , Holly A. Michael [1,2]
1 Dept. of Earth Sciences, University of Delaware, Newark, DE, USA.
2 Dept. of Civil and Environmental Engineering, University of Delaware, Newark, DE, USA.
* Correspondence: rsfreder@udel.edu
Coastal groundwater is susceptible to salinization due to climate change impacts on both sea-level rise and storm surge frequency/intensity. While topography-limited systems, i.e. regions with water tables close to the land surface, are expected to be the most vulnerable to sea-level rise, recharge-limited systems are expected to buffer the effects of climate change as water tables can rise and prevent the freshwater-saltwater interface from intruding inland. However, the susceptibility of these systems to storm-surge overwash has not been investigated. In this study, we collected hydraulic head and specific conductance data and calibrated a 2D groundwater flow and transport model for an overwash scar on Assateague Island, MD. Using the calibrated model, we investigated the long-term vulnerability to sea-level rise and increased storm surge frequency/intensity. We find that all systems are more vulnerable to changes in storm surge frequency than to sea-level rise. High hydraulic conductivity and low elevation topography-limited systems are almost as vulnerable to sea-level rise as to storm surge overwash. On the other hand, recharge-limited systems are substantially more susceptible to storm-surge overwash and show a consistent trend towards salinization over the long term. These results imply that mitigation of saltwater intrusion should consider both sea-level rise and the intensification of storm surges, as well as the interplay between these processes. This has implications for management of coastal groundwater resources and for mitigating the impacts of future changes.
Impact of ocean surge profiles on overwash-driven salinization in coastal aquifers
Rachel Housego [1*], Fengyan Shi [2], Anner Paldor [1], Ryan Fredericks {1], Holly A. Michael [1,2]
1 Dept. of Earth Sciences, University of Delaware, Newark, DE, USA.
2 Dept. of Civil and Environmental Engineering, University of Delaware, Newark, DE, USA.
* Correspondence: rhousego@udel.edu
As a result of rising sea level and intensifying storm conditions, coastal communities around the world face an increasing risk from surge-driven inundation. During storm surges, inland propagation of ocean water drives infiltration of high salinity seawater into the fresh groundwater, jeopardizing coastal water resources. Prior studies of surge-driven salinization have typically treated storm surge as normally distributed in time, but in natural systems these profiles can have significant variability in shape and duration which will impact the infiltration of seawater. To understand the impact of the surge profile on groundwater salinization, we simulated 2DV groundwater flow and salt transport during overwash events using a coupled surface-subsurface numerical model (HydroGeoSphere). In these simulations, we varied the shape of the surge profile (timing of water level rise and retreat, and duration) and simulated the recovery of the aquifer for one year after the storm. For each surge case, we examined the influence of the surge profile on groundwater flow paths, extent of saltwater infiltration and recovery time.
In addition to variations in surge profile shape, there can also be large spatial variations in storm surge magnitude along coasts, resulting from differences in bathymetry, coastline shape, and the path and characteristics of the storm. We used the surge generated for the Delaware Inland Bays during Hurricane Sandy with NearCOM-TVD, a surface water hydrodynamic model, as boundary conditions for 2DV HydroGeoSphere simulations. These simulations were applied to assess how the regional surge variations affected the salinization risk. The goal of both sets of simulations is to identify the surge conditions that present the greatest salinization risk for coastal communities.
Effects of floodplain shading on hyporheic aquifer temperatures: Implications for restoration
Katie Fogg [1*], Geoff Poole [1,2], Scott O’Daniel [3], Byron Amerson [1]
1 Dept. of Land Resources and Environmental Studies, Montana State University, Bozeman, MT, USA.
2 Montana Institute on Ecosystems, Montana State University, Bozeman, MT, USA.
3 Confederated Tribes of the Umatilla Indian Reservation, Pendleton, OR, USA.
* Correspondence: sarah.fogg@montana.edu
In alluvial rivers, the continuous, bidirectional exchange of stream water with underlying sediments (i.e. hyporheic exchange) decreases the amplitude of daily and seasonal stream channel temperature oscillations. However, the hyporheic zones of floodplain streams are often wide and shallow, which facilitates heat exchange between the hyporheic zone and atmosphere via floodplain sediments. Riparian forests have the potential to mediate atmosphere-aquifer heat exchange by shading the floodplain surface, thereby influencing stream channel temperatures indirectly via hyporheic exchange of water and heat. Commonly, restoration activities reduce floodplain shade during channel construction, which could lead to unintentional warming of hyporheic and channel temperatures. We investigated the interactions between riparian forest density, the spatial pattern of floodplain shade and hyporheic flow paths using a case study of the semi-arid Meacham Creek floodplain (Oregon, USA). We used HydroGeoSphere, a coupled groundwater-surface water modeling software, to simulate water and heat transport in a 3-dimensional finite element model of the Meacham Creek floodplain under variable riparian forest densities. Model results showed floodplain shade mediated the warming of hyporheic and channel water. Therefore, in alluvial rivers where hyporheic zones extend beyond the channel margins, consideration of floodplain shade management, both during and after restoration actions, may improve outcomes for restoration of stream channel temperatures.