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1. Geospatial Modeling Approach to Determine Potential Sinkholes Risk Probability

   Robertson, J. (January 2020, 2020 ATE Conference)

   null (Ed.)

   Sinkholes are common and naturally occurring in certain areas such as Florida and Southern Georgia. The region’s aquifer is often covered by limestone or dolomite carbonate rock, which are made up of minerals that can dissolve in water under the right conditions. Anthropogenic changes are leading to an increased risk of sinkholes in susceptible areas. The formation of these geologic features is hastened by the improper management of ground water, the increase in watershed pollution and runoff, and the mismanagement of underground fresh and wastewater pipes and structures. The goal of this study is to develop an automated geospatial model to determine areas within the study having a potential high risk for sinkholes. Eleven types of geospatial data were collected, processed, and analyzed in ArcGIS Pro Model Builder to calculate sinkhole vulnerability layers in the study area. The eleven data types were geology, soil, land use, aquifer, ground water measurements, road, fault line, elevation precipitation, and evapotranspiration. From this data, ten sinkhole vulnerability layers were produced: 1) subsidence or surface change, 2) average aquifer well depth, 3) ground water vulnerability (DRASTIC), 4) road density, 5) groundwater travel time, 6) aquifer media (Suwannee Limestone) , 7) geology type, 8) slope, 9) land use, and 10) distance from fault lines. Each layer was reclassified and reassigned a value from 1 to 10 according to its sinkhole vulnerability. The weighted layers were analyzed interpretively using ArcGIS Pro’s weighted sum tool producing a Sinkhole Risk Probability Raster. The sampling tool was used for accuracy assessment by comparing the obtained result with historical sinkhole data. This method showed 77% accuracy between known sinkholes and those shown on the sinkholes probability raster. This study is useful to environmental planners/managers and other stakeholders for decision support. 

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2. Distribution of a Sulfolane-Metabolizing Rhodoferax sp. Throughout a Contaminated Subarctic Aquifer and Two Groundwater Treatment Systems

   https://doi.org/10.3389/fmicb.2021.714769  

   Kasanke, Christopher P.; Willis, Michael D.; Leigh, Mary Beth (August 2021, Frontiers in Microbiology)

   An extensive plume of the emerging contaminant sulfolane has been found emanating from a refinery in Interior Alaska, raising questions about the microbial potential for natural attenuation and bioremediation in this subarctic aquifer. Previously, an aerobic sulfolane-assimilating Rhodoferax sp. was identified from the aquifer using stable isotope probing. Here, we assessed the distribution of known sulfolane-assimilating bacteria throughout the contaminated subarctic aquifer using 16S-rRNA-amplicon analyses of ~100 samples collected from groundwater monitoring wells and two groundwater treatment systems. One treatment system was an in situ air sparging system where air was injected directly into the aquifer. The other was an ex situ granular activated carbon (GAC) filtration system for the treatment of private well water. We found that the sulfolane-assimilating Rhodoferax sp. was present throughout the aquifer but was significantly more abundant in groundwater associated with the air sparge system. The reduction of sulfolane concentrations combined with the apparent enrichment of sulfolane degraders in the air sparging zone suggests that the addition of oxygen facilitated sulfolane biodegradation. To investigate other environmental controls on Rhodoferax populations, we also examined correlations between groundwater geochemical parameters and the relative abundance of the Rhodoferax sp. and found only manganese to be significantly positively correlated. The sulfolane-assimilating Rhodoferax sp. was not a major component of the GAC filtration system, suggesting that biodegradation is not an important contributor to sulfolane removal in these systems. We conclude that air sparging is a promising approach for enhancing the abundance and activity of aerobic sulfolane-degraders like Rhodoferax to locally stimulate sulfolane biodegradation in situ . 

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3. Hydro‐economic modeling of managed aquifer recharge in the lower Mississippi

   https://doi.org/10.1111/1752-1688.13141  

   Ali, Ahmed A.; Tran, Dat Q.; Kovacs, Kent F.; Dahlke, Helen E. (June 2023, JAWRA Journal of the American Water Resources Association)

   Abstract The Mississippi Embayment aquifer is one of the largest alluvial groundwater aquifers in the United States. It is being excessively used, located along the lower Mississippi River covering approximately 202,019 km²(78,000 square miles). Annual average groundwater depletion in the aquifer has been estimated at 5.18 billion cubic meters (Gm³) (4.2 million acre‐feet) in 1981–2000. However, since 2000, annual groundwater depletion has increased abruptly to 8 Gm³(2001–2008). In recent years, multi‐state efforts have been initiated to improve the Mississippi Embayment aquifer sustainability. One management strategy of interest for preserving groundwater resources is managed aquifer recharge (MAR). In this study, we evaluate the impact of different MAR scenarios on land and water use decisions and the overall groundwater system using an economic model able to assess profitability of crop and land use decisions coupled to the Mississippi Embayment Regional Aquifer Study (MERAS) hydrogeologic model. We run the coupled model for 60 years by considering the hydrologic conditions from the MERAS model for the years 2002–2007 and repeating them 10 times. We find MAR is not economically attractive when the water cost is greater than $0.05/m³. Groundwater storage is unlikely to improve when relying solely on MAR as groundwater management strategy but rather should be implemented jointly with other groundwater conservation policies. 

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4. Using δ¹⁸O and δ²H to Detect Hydraulic Connection Between a Sinkhole Lake and a First‐Magnitude Spring

   https://doi.org/10.1111/gwat.13105  

   Ahmed, Nur; Ye, Ming; Wang, Yang; Greenhalgh, Tom; Fowler, Karlee (April 2021, Groundwater)

   Abstract Oxygen and hydrogen isotopes were used in this study to detect a hydraulic connection between a sinkhole lake and a karst spring. In karst areas, surface water that flows to a lake can drain through sinkholes in the lakebed to the underlying aquifer, and then flows in karst conduits and through aquifer matrix. At the study site located in northwest Florida, USA, Lake Miccosukee immediately drains into two sinkholes. Results from a dye tracing experiment indicate that lake water discharges at Natural Bridge Spring, a first‐magnitude spring 32 km downgradient from the lake. By collecting weekly water samples from the lake, the spring, and a groundwater well 10 m away from the lake during the dry period between October 2019 and January 2020, it was found that, when rainfall effects on isotopic signature in spring water are removed, increased isotope ratios of spring water can be explained by mixing of heavy‐isotope‐enriched lake water into groundwater, indicating hydraulic connection between the lake and the spring. Such a detection of hydraulic connection at the scale of tens of kilometers and for a first‐magnitude spring has not been previously reported in the literature. Based on the isotope ratio data, it was estimated that, during the study period, about 8.5% the spring discharge was the lake water that drained into the lake sinkholes. 

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5. Groundwater‐surface water interaction, dissolved organic carbon oxidation and dissolution in carbonate aquifers

   https://doi.org/10.1002/esp.5830  

   Oberhelman, Andrew; Martin, Jonathan_B; Flint, Madison_K (April 2024, Earth Surface Processes and Landforms)

   Abstract The high primary porosity and permeability of eogenetic karst aquifers permit water recharged through secondary dissolution features to be temporarily stored in aquifer matrix porosity. The recharged water contains elevated dissolved organic carbon (DOC) concentrations that, when oxidized, enhance limestone dissolution and impact carbon cycling. We evaluate the relationship between DOC oxidation and limestone dissolution using observations at a stream sink‐rise system and reversing spring in the Floridan aquifer, north‐central Florida, USA, where subsurface residence times of recharged water are days and months, respectively. We estimate water chemical compositions during surface water‐groundwater interactions at these two systems with mixing models of surface water and groundwater compositions and compare them with measured DOC, dissolved inorganic carbon (DIC), Ca²⁺and dissolved organic nitrogen (DON) concentrations. Differences between measured and modelled concentrations represent net changes that can be attributed to calcite dissolution and redox reactions, including DOC oxidation. DOC losses and Ca²⁺gains exhibit significant (p < 0.01) inverse linear correlations at both the reversing spring (slope = −0.9, r² = 0.99) and the sink‐rise system (slope = −0.4, r² = 0.72). DOC oxidation in both systems was associated with decreases in the molar C:N ratio (DOC:DON). Significant (p < 0.01) positive linear correlations between increases in Ca²⁺and DIC concentrations after correcting for DIC derived from calcite dissolution occurred at both the reversing spring (slope = 1.3, r² = 0.99) and the sink‐rise system (slope = 1.61, r² = 0.75). Greater deviations from the expected slope of −1 or +1 at the sink‐rise system than at the reversing spring indicate DOC oxidation contributes less dissolution at the sink‐rise system than at the reversing spring, likely from shorter storage in the subsurface. A portion of the deviation from expected slope values can be explained by the dissolution of Mg‐rich carbonate or dolomite rather than pure calcite dissolution. Despite this, slope values reflect kinetic effects controlling incomplete consumption of carbonic acid during dissolution reactions. 

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