Karst dissolution structures present a global challenge, particularly in urban settings, impacting water resource management and infrastructure stability. Damage prevention and monitoring of these structures often involve identifying them before they manifest at the ground surface. Geophysical methods offer a cost-effective means to non-invasively identify and monitor these structures. This study integrates multiple geophysical techniques, including electromagnetic (EM) and seismic methods, with direct geotechnical coring to identify karst dissolution features in an urban wellfield in South Florida. Our geophysical and geotechnical results show good agreement and suggest the presence of a water-filled dissolution feature or potential sinkhole near the location of a previously detected slurry pond leak. The study underscores the importance of non-invasive exploration methods in karst assessment for sustainable urban development and water resource management strategies.

The Rio Icacos watershed in the Luquillo Mountains (Puerto Rico) is unique due to its extremely rapid weathering rates. The watershed is incised into a quartz diorite that has developed a large knickzone defining the river profile. Regolith thickness within the watershed generally decreases from 20 to 30 m at the ridges to several meters in the quartz diorite-dominated valley to tens of centimeters near the major river knickpoint, as determined from previous studies. Above the knickzone, we observe spheroidal corestones, but below this weathering is much less apparent. Measured erosion rates from previous studies are also high in the knickzone compared with upper elevations within the river profile. A suite of near-surface geophysical methods (i.e. ground penetrating radar and terrain conductivity) capable of fast data acquisition in rugged landscapes, was deployed at kilometer scales to characterize critical zone structure. Concentrations of chaotic ground penetrating radar (GPR) reflections and diffraction hyperbolas with low electrical conductivity were observed in vertical zones that outcrop at the land surface as areas of intense fracturing and spheroidally weathered corestones. The width of these fractured and weathered zones showed an increase with proximity to the knickpoint, and was attributed to dilation of these sub-vertical fractures near the knickpoint, as postulated theoretically by a stress model calculated for the topographic variability across the knickzone in the Rio Icacos, and that shows a release of compressive stress near the knickpoint. We hypothesize that erosion rates increase in the knickzone because of this inferred dilation of fractures. Specifically, opened fractures could enhance access of water and in turn promote spalling, erosion, and spheroidal weathering. This study shows that ground-based hydrogeophysical methods used at the landscape-scale (traditionally applied at smaller scales) can be used to explore critical zone architecture at the scales needed to explain the extreme variability in erosion rates across river profiles. (c) 2018 John Wiley & Sons, Ltd.

The role of subtropical peatlands as a source for methane gas is not well understood, partly due to uncertainties surrounding environmental controls on gas ebullition patterns. Past studies have pointed to an array of environmental factors controlling ebullition, although we have found that ebullition patterns can be replicated by a model considering only physical parameters of the peat matrix. Here we tested a computer model for gas ebullition and storage against a natural system for the first time, using a suite of field measurements in the Florida Everglades. Modeled ebullition showed patterns similar to those observed in the field in terms of frequency distribution and magnitude, specifically from areas of higher density peat fabric. These results suggest that the internal structure of the peat soil is an important control on spatial and temporal patterns of ebullition in the Everglades and should be considered when investigating environmental controls on ebullition patterns.

The spatial and temporal variability in production and release of greenhouse gases (such as methane) in peat soils remains uncertain, particularly for low‐latitude peatlands like the Everglades. Ground penetrating radar (GPR) is a hydrogeophysical tool that has been successfully used in the last decade to noninvasively investigate carbon dynamics in peat soils; however, application in subtropical systems is almost non‐existent. This study is based on four field sites in the Florida Everglades, where changes in gas content within the soil are monitored using time‐lapse GPR measurements and gas releases are monitored using gas traps. A weekly methane gas production rate is estimated using a mass balance approach, considering gas content estimated from GPR, gas release from gas traps and incorporating rates of diffusion, and methanotrophic consumption from previous studies. Resulting production rates range between 0.02 and 0.47 g CH4 m−2 d−1, falling within the range reported in literature. This study shows the potential of combining GPR with gas traps to monitor gas dynamics in peat soils of the Everglades and estimate methane gas production. We also show the enhanced ability of certain peat soils to store gas when compared to others, suggesting that physical properties control biogenic gas storage in the Everglades peat soils. Better understanding biogenic methane gas dynamics in peat soils has implications regarding the role of wetlands in the global carbon cycle, particularly under a climate change scenario.

The Atlantic Coastal Ridge in Dade County is an ooid shoal formed during the late Pleistocene. The ridge is composed of the eogenetic karst Miami Limestone, characterized by a heterogeneous distribution of porosity that can manifest as large touching-vug macroporous features with sizes in the meters and tens of meters. Direct evidence for the presence of such large dissolution features is only visible when exposed to the surface which may be problematic in urbanized areas. For that reason it is critical to detect any potential precursors for such dissolution features at the subsurface level. The purpose of this study was to investigate the ability of ground penetrating radar to detect areas of macroporosity by quantifying changes in electromagnetic wave velocity and relate them to changes in porosity after application of the complex refractive index model (CRIM). In order to constrain volumetric water content (VWC) in the CRIM, an experiment using Miami Limestone samples was designed to understand: (1) the range of typical VWCs; and (2) the effect of capillary fringe on water table elevation. The results show several areas where increases in EM wave travel time associated with contrasts in porosity exceeding 40% cannot be explained by changes in VWCs typically shown in the Miami Limestone. This study may help understand porosity variability in the unsaturated part of the Miami Limestone and if expanded to larger scales may aid groundwater flow models by better capturing distributions of macroporous areas that may contribute to direct recharge of the Biscayne aquifer. (C) 2015 Elsevier B.V. All rights reserved.

PreviousNext No AccessNear-Surface Asia Pacific Conference, Waikoloa, Hawaii, 7-10 July 2015Investigating carbon stocks and fluxes in subtropical peatlands using ground penetrating radar (GPR)Authors: Xavier Comas*William WrightMathew McClellanXavier Comas*Department of Geosciences, Florida Atlantic UniversitySearch for more papers by this author, William WrightDepartment of Geosciences, Florida Atlantic UniversitySearch for more papers by this author, and Mathew McClellanDepartment of Geosciences, Florida Atlantic UniversitySearch for more papers by this authorhttps://doi.org/10.1190/nsapc2015-084 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Subtropical peatlands are important carbon (C) reservoirs and sources of greenhouse gases such as methane (CH4) and carbon dioxide (CO2), however the spatial and temporal distribution of these gases is very uncertain, particularly when compared to boreal systems. In this work we present an array of surveys at several locations and scales of measurement using hydrogeophysical methods (and mainly ground penetrating radar, GPR), to better understand biogenic gas dynamics in subtropical peat soils. Keywords: GPR, environmental, carbonPermalink: https://doi.org/10.1190/nsapc2015-084FiguresReferencesRelatedDetails Near-Surface Asia Pacific Conference, Waikoloa, Hawaii, 7-10 July 2015ISSN (online):2159-6832Copyright: 2015 Pages: 501 publication data© 2015 Published in electronic format with permission by the Society of Exploration Geophysicists, Australian Society of Exploration Geophysicists, Chinese Geophysical Society, Korean Society of Earth and Exploration Geophysicists, and Society of Exploration Geophysicists of JapanPublisher:Society of Exploration Geophysicists HistoryPublished Online: 10 Jul 2015 CITATION INFORMATION Xavier Comas*, William Wright, and Mathew McClellan, (2015), "Investigating carbon stocks and fluxes in subtropical peatlands using ground penetrating radar (GPR)," SEG Global Meeting Abstracts : 320-323. https://doi.org/10.1190/nsapc2015-084 Plain-Language Summary KeywordsGPRenvironmentalcarbonPDF DownloadLoading ...

The spatial and temporal variability in accumulation and release of greenhouse gases (mainly methane and carbon dioxide) to the atmosphere from peat soils remains very uncertain. The use of near-surface geophysical methods such as ground penetrating radar (GPR) has proven useful during the last decade to expand scales of measurement as related to in situ gas distribution and dynamics beyond traditional methods (i.e., gas chambers). However, this approach has focused exclusively on boreal peatlands, while no studies in subtropical systems like the Everglades using these techniques exist. In this paper GPR is combined with gas traps, time-lapse cameras, gas chromatography, and surface deformation measurements to explore biogenic gas dynamics (mainly gas buildup and release) in two locations in the Everglades. Similar to previous studies in northern peatlands, our data in the Everglades show a statistically significant correlation between the following: (1) GPR-estimated gas content and gas fluxes, (2) GPR-estimated gas content and surface deformation, and (3) atmospheric pressure and both GPR-estimated gas content and gas flux. From these results several gas-releasing events ranging between 33.8 and 718.8mg CH4 m(-2)d(-1) were detected as identified by the following: (1) decreases in GPR-estimated gas content within the peat matrix, (2) increases in gas fluxes captured by gas traps and time-lapse cameras, and (3) decreases in surface deformation. Furthermore, gas-releasing events corresponded to periods of high atmospheric pressure. Changes in gas accumulation and release were attributed to differences in seasonality and peat soil type between sites. These results suggest that biogenic gas releases in the Everglades are spatially and temporarily variable. For example, flux events measured at hourly scales were up to threefold larger when compared to daily fluxes, therefore suggesting that flux measurements decline when averaged over longer time spans. This research therefore questions what the appropriate spatial and temporal scale of measurement is necessary to properly capture the dynamics of biogenic gas release in subtropical peat soils.

We tested a set of biogenic gas traps combined with time‐lapse cameras to investigate the heterogeneous nature of biogenic gas ebullition events in subtropical peat soils at both the laboratory and field scale. The main findings are: (1) ebullition events in peat soils are highly heterogeneous; (2) estimates of flux rate are directly influenced by temporal scale of measurement with rapid (i.e., hourly) releasing events exceeding daily averages by one order of magnitude; and (3) increases in atmospheric pressure result in gas release from shallow peat soils into the atmosphere (i.e., ebullition), as indicated by a positive linear relation between changes in biogenic gas content and changes in atmospheric pressure. These results suggest that biogenic gas releases from shallow subtropical peat soils are not constant with larger than average daily fluxes being potentially released within hours during periods of increased atmospheric pressure. Furthermore, this study also shows the potential of time‐lapse cameras for autonomously assessing the temporal variation in biogenic gas flux to the atmosphere from peatlands, and questions what temporal scale of measurement should be appropriate to infer dynamics of biogenic gas release in peat soils.