Biscayne Aquifer Research Articles

Beach nourishment for coastal aquifersimpacted by climate change and population growth using machine learning approaches

Groundwater in coastal regions is threatened by saltwater intrusion (SWI). Beach nourishment is used in this study to manage SWI in the Biscayne aquifer, Florida, USA, using a 3D SEAWAT model nourishment considering the future sea level rise and freshwater over-pumping. The present study focused on the development and comparative evaluation of seven machine learning (ML) models, i.e., additive regression (AR), support vector machine (SVM), reduced error pruning tree (REPTree), Bagging, random subspace (RSS), random forest (RF), artificial neural network (ANN) to predict the SWI using beach nourishment. The performance of ML models was assessed using statistical indicators such as coefficient of determination (R2), Nash–Sutcliffe efficiency (NSE), means absolute error (MAE), root mean square error (RMSE), and root relative squared error (RRSE) along with the graphical inspection (i.e., Radar and Taylor diagram). The findings indicate that applying SVM, Bagging, RSS, and RF models has great potential in predicting the SWI values with limited data in the study area. The RF model emerged as the best fit and closely matched observed values; it obtained R2 (0.999), NSE (0.999), MAE (0.324), RRSE (0.209), and RMSE (0.416) during the testing process. The present study concludes that the RF model could be a valuable tool for accurate predictions of SWI and effective water management in coastal areas.

Effectiveness of different mixed physical barriers in controlling seawater intrusion in homogeneous and layered coastal aquifers

The intrusion of salt water into coastal regions threatens water resources, especially in arid and semi-arid regions. It damages large quantities of fresh water in these regions, and the productivity of the freshwater abstraction wells declines. Management of seawater intrusion (SWI) is therefore needed to improve fresh groundwater in these regions. This study investigated 12 different configurations of mixed physical subsurface barriers (MPBs) to control SWI in homogeneous and heterogeneous layered aquifers. The effectiveness of different MPB locations and configurations was tested, including (i) a barrier wall on the landward side and the subsurface dams on the seaward side, (ii) a barrier wall on the seaward side and a subsurface dam on the landward side, and (iii) the barrier wall was placed above the subsurface dam, both with different permeabilities. All simulations were based on the SEAWAT code. The numerical model was validated against experimental data. The results showed that a permeable cut-off wall above an impermeable subterranean dam (case MPB-3) with different permeabilities resulted in a reduction of the seawater wedge of 91% and 92% for homogeneous and heterogeneous layered aquifers, respectively. When the barrier wall was placed on the land side and the dam on the seaside (case MPB-1), the reduction of the seawater wedge reached 83% and 85% for homogeneous and heterogeneous layered aquifers, respectively. In contrast, when the dam was placed on the land side and the wall on the seaside (case MPB-2), the saltwater wedge was reduced by 73% for both homogeneous and heterogeneous layered aquifers. In addition, a case study was conducted on the Biscayne aquifer, southeast Florida, USA, with homogeneous conditions. Seawater intrusion was reduced by 36% and 44% in case MPB-1, 41% and 38% in case MPB-2, and 43% and 46% in case MPB-3. These seawater intrusion control methods offer numerous benefits, including improving freshwater storage, effectively controlling salinity during droughts, and potentially improving contaminant management.

Using Geochemical Tracers to Determine Seasonal Inputs of Freshwater to a Coastal Estuary: Biscayne Bay, FL

Biscayne Bay is a coastal estuary that historically relied on rainfall and groundwater inputs from the karst Biscayne aquifer. The construction of major canals along the coastline has released point-source freshwater inputs into the bay, detrimentally affecting the Bay’s ecosystem balance. This project investigates the proportional inputs of freshwater between the wet and dry seasons in Deering Estate, adjacent to Biscayne Bay. The objective of this project was accomplished by analyzing the water chemistry of the bay using naturally occurring geochemical tracers. Water sampling occurred from May to August (wet season 2022) and January to March (dry season 2023); at an inland freshwater spring and on Biscayne Bay. Water samples were analyzed for δ18O and δ2H values, and Sr2+/Ca2+ ratios as geochemical tracers. The highest and lowest salinity values observed in the wet and dry seasons, at both the freshwater spring and Biscayne Bay sites, were before and after a major rain event, respectively. The chemical analysis supports that rain is the dominant source of freshwater input into the bay at our sampling location, and the freshwater spring is dominated by groundwater and canal water during the wet season. During the dry season, groundwater and canal water are the dominant source for the sampling location in Biscayne Bay and the dominant source of freshwater input for the freshwater spring. However, all three endmembers contribute seasonally. Understanding freshwater inputs to this crucial estuary will provide important information for current restoration efforts of Biscayne Bay, specifically around the Deering Estate area.

Groundwater salinization risk in coastal regions triggered by earthquake-induced saltwater intrusion

Anthropogenic factors such as over-pumping and natural events such as earthquakes impact coastal aquifers by reducing freshwater recharge, aquifer water budgets, and increasing saltwater intrusion (SWI). This study investigates the impact of hydrodynamic forces induced by earthquakes on SWI in one hypothetical case, namely, the Henry problem, and a real case of the Biscayne aquifer located in Florida, USA. The analysis was carried out using the analytical solution of estimating the earthquake’s induced hydrodynamic pressure and applying the SEAWAT code to investigate the SWI for the base case and three scenarios, namely for the horizontal acceleration (αh) by 0.10 g, 0.20 g, and 0.30 g. The results show that earthquakes might considerably increase the SWI in coastal aquifers. Moreover, the rise in salinity across expansive land areas significantly threatens agricultural productivity and jeopardizes food security. Namely, in the case of Biscayne aquifer, salinity was increased by 12.10%, 21.90%, and 45.70% for the horizontal seismic acceleration of 0.1 g, 0.20 g, and 0.30 g, respectively. Hence, the conclusions drawn from this study underscore the need for carefull consideration of earthquake impacts in future planning and water management strategies for coastal regions. This proactive approach is crucial to preemptively address and mitigate the groundwater salinization hazard associated with SWI fluctuations due to earthquakes.

Shoreline subsurface dams to protect coastal aquifers from sea level rise and saltwater intrusion

Fresh groundwater in arid and highly populated regions is limited. In coastal aquifers, the deterioration of fresh groundwater is accelerated by saltwater intrusion, primarily occurring through lateral encroachment and vertical movements in the proximity of discharging wells. Coastal regions have high salinity due to saline intrusion, where many abstraction wells are turned off by this high salinity, which leads to increased freshwater supply costs. This study investigates the performance of new approach using the shoreline subsurface dams (SSDs) for mitigating the saline water wedge in coastal aquifers, where the dams are installed at the shoreline (distance from shoreline = 0). Specifically, the current study's novelty is testing the effectiveness of SSDs by different relative heights ranging from 0.05 to 0.50 in the test case (Henry problem) and from 0.09 to 0.53 relative to the aquifer thickness in the field scale aquifer (Biscayne aquifer, Florida, USA). The results showed an exponential increase in salt repulsion for increasing SSDs height, reaching a maximum of + 0.70%, + 1.80%, + 3.25%, + 5.80%, + 10.45%, and + 18.40% for the dam height to aquifer thickness ratios of 0.09, 0.18, 0.26, 0.35, 0.44 and 0.53, respectively, in the field scale case. The SSDs increase the freshwater storage at the coastal zones where the low salinity occurs and reduces the freshwater supply cost. Despite the positive impact of height on repulsion, important factors such as economics, construction aspects, geographical suitability, and environmental impacts must be considered for real applications. This is crucial to develop feasible solutions applicable globally under the growing pressure of sea level rise.

Freshwater cooling injection to mitigate saltwater intrusion and support sustainable groundwater management

This study examined the influence of thermal forcing on saltwater intrusion into nearshore aquifers (SWI) in the context of sea level rise due to climate change and continued groundwater withdrawal due to increasing demand. The study used the SEAWAT code to test two case studies: the Henry problem as a benchmark and the Biscayne Aquifer, Florida, USA, as a real case. Simulations were performed for the base case, sea level rise (SLR), and inland groundwater recharge reductions. A new management method was proposed for mitigating SWI in nearshore aquifers using the Abstraction of saline water, Desalination, Cooling the desalinated water, and Injection into the aquifer (ADCI) strategy. The method was tested with different freshwater temperatures of 25, 20, 15, 10, and 5 °C. The results showed that freshwater recharge in coastal regions is sensitive to the thermal regime, resulting in the attenuation of SWI. The results of this study are important for the sustainable management of groundwater resources in coastal areas to meet increasing water demands. The Biscayne salt repulsion reached +9.80 %, +10 %, +10.20 %, +10.30 %, and + 10.50 % for changing the injection recharge by 25, 20, 15, 10, and 5 °C. Future planning, design, and development of SWI mitigation measures should consider thermal regime effects. In addition, a cost–benefit analysis for this method using 3D models is needed to determine the feasibility and economic aspect of cooling water supply compared to current techniques.

Sustainable management of two‐directional lateral and upconing saltwater intrusion in coastline aquifers to alleviate water scarcity

AbstractLateral saltwater intrusion (LSWI) and vertical saltwater intrusion (VSWI) are the major problems in coastal aquifers. Coastal aquifers are subjected to natural and artificial changes, including sea‐level rise, decreasing inland recharge, and overexploitation of production wells' water resources. Therefore, this study investigates the influence of changing the coastal aquifers boundary conditions considering climate changes' effect. Parameters describing the upward saline well parameters based on the abstraction conditions, also, the top layer properties were studied based on the erosion and sediments, irrigation, and drainage conditions. For this purpose, the SEAWAT model was employed to simulate lateral SWI based on Henry's problem principle. The sensitivity analysis of the mentioned parameters was accomplished considering as a case study the Biscayne aquifer in the Cutler Ridge area near Deering Estate, Florida, USA, where the LSWI and VSWI were also investigated. The results indicate that the boundary conditions, abstraction well parameters, and top layer cost significantly impact aquifer LSWI and water upconing. The simulation results of the Biscayne aquifer for a saline water head of 84.86 cm for 2060 indicate a decrease in freshwater recharge by 30% and increasing the saltwater concentration by 14.30%. At the same time, the LSWI increased by 3.30%, 22.80%, and 8.70%, while the relative upconing heights reached 0.63, 0.60, and 0.47 compared with 0.42 for the current situation. Increasing the upward saline abstraction well discharge rates by 50% increases the LSWI by 9.40%. In contrast, increasing the depth by 10%; decreases the distance by 30%, the screen length by 30%, the LSWI decreased by 1.80%, 25.10%, and 1.30%, respectively. At the same time, the relative VSWI height reached 0.60, 0.39, 0.48, and 0.40, respectively. Moreover, decreasing the top layer hydraulic conductivity by 25% and increasing the aquitards layer depths by 200% decreased the LSWI by 4.90% and 7.80%, respectively. At the same time, the relative VSWI height reached 0.34 and 0.30, respectively. In addition, the saline water heads, a freshwater recharge should be controlled to manage SWI; the upward well should be utilized at a low abstraction rate, far from the seaside, and with minimum screen length. The agriculture activity for decreasing the soil permeability for the top layer must be extended for better management of the SWI. The solution‐oriented findings resulting from this study might be useful for decision‐makers and engineers to develop effective management measures to tackle the SWI issue in coastal aquifers.

Cost-effective management measures for coastal aquifers affected by saltwater intrusion and climate change.

Sustainable management of natural water resources and food security in the face of changing climate conditions is critical to the livelihood of coastal communities. Increasing inundation and saltwater intrusion (SWI) will likely adversely affect agricultural production and the associated beach access for tourism. This study uses an integrated surface-ground water model to introduce a new approach for retardation of SWI that consists of placing aquifer fill materials along the existing shoreline using Coastal Land Reclamation (CLR). The modeling results suggest that the artificial aquifer materials could be designed to decrease SWI by increasing the infiltration area of coastal precipitation, collecting runoffs from the catchment area, and applying treated wastewater or desalinated brackish water-using coastal wave energy to reduce water treatment costs. The SEAWAT model was applied to verify that it correctly addressed Henry's problem and then applied to the Biscayne aquifer, Florida, USA. In this study, to better inform Coastal Aquifer Management (CAM), we developed four modeling scenarios, namely, Physical Surface Barriers (PSB), including the artificial aquifer widths, permeability, and side slopes and recharge. In the base case scenario without artificial aquifer placement, results show that seawater levels would increase aquifer salinity and displace large amounts of presently available fresh groundwater. More specifically, for the Biscayne aquifer, approximately 0.50% of available fresh groundwater will be lost (that is, 41,192 m3) per km of the width of the aquifer considering the increasing seawater level. Furthermore, the results suggest that placing the PSB aquifer with a smaller permeability of <100 m per day at a width of approximately 615 m increases the available fresh groundwater by approximately 45.20 and 43.90% per km of shoreline, respectively. Similarly, decreasing the slope on the aquifer-ocean side and increasing the aquifer recharge will increase freshwater availability by about 43.90 and 44.50% per km of the aquifer. Finally, placing an aquifer fill along the shallow shoreline increases net revenues to the coastal community through increased agricultural production and possibly tourism that offset fill placement and water treatment costs. This study is useful for integrated management of coastal zones by delaying aquifer salinity, protecting fresh groundwater bodies, increasing agricultural lands, supporting surface water supplies by harvesting rainfall and flash flooding, and desalinating saline water using wave energy. Also, the feasibility of freshwater storage and costs for CAM is achieved in this study.

Experimental and Numerical Study to Investigate the Impact of Changing the Boundary Water Levels on Saltwater Intrusion in Coastal Aquifers

Experimental and numerical models can be used to investigate saltwater intrusion (SWI) in coastal aquifers. Sea level rise (SLR) and decline of freshwater heads due to climate change are the two key variables that may affect saltwater intrusion. This study aims to give a better understanding of the impact of increasing seawater levels and decreasing freshwater heads due to climate change and increasing abstraction rates due to overpopulation using experimental and numerical models on SWI. The experimental model was conducted using a flow tank and the SEAWAT code was used for the numerical simulation. Different scenarios were examined to assess the effect of seawater rise and landside groundwater level decline. The experimental and numerical studies were conducted on three scenarios: increasing seawater head by 25%, 50% and 75% from the difference between seawater and freshwater heads, decreasing freshwater head by 75%, 50% and 25% from the difference between seawater and freshwater heads, and a combination of these two scenarios. Good agreement was attained between experimental and numerical results. The results showed that increasing the seawater level and decreasing freshwater head increased saltwater intrusion, but the combination of these two scenarios had a severe effect on saltwater intrusion. The numerical model was then applied to a real case study, the Biscayne aquifer, Florida, USA. The results indicated that the Biscayne aquifer is highly vulnerable to SWI under the possible consequences of climate change. A 25 cm seawater rise and 28% reduction in the freshwater flux would cause a loss of 0.833 million m3 of freshwater storage per each kilometer width of the Biscayne aquifer. This study provides a better understanding and a quantitative assessment for the impacts of changing water levels’ boundaries on intrusion of seawater in coastal aquifers.

Populations using public-supply groundwater in the conterminous U.S. 2010; Identifying the wells, hydrogeologic regions, and hydrogeologic mapping units

Most Americans receive their drinking water from publicly supplied sources, a large portion of it from groundwater. Mapping these populations consistently and at a high resolution is important for understanding where the resource is used and needs to be protected. The results show that 269 million people are supplied by public supply, 107 million are supplied by groundwater and 162 million are supplied by surface water. The population using public supply drinking water was mapped in two ways: the census enhanced method (CEM) evenly distributes the population across populated census blocks, and the urban land-use enhanced method (ULUEM) distributes the population only to certain urban land use designations. In addition, a two-dimensional polygon dataset was created for the conterminous U.S. that identifies 177 unique Hydrogeologic Mapping Units (HMUs) with similar hydrogeologic characteristics. The HMUs do not overlap, but they can delineate areas where stacked hydrogeologic regions (HRs) contribute drinking water from below the surface. HRs are waterbearing geologic regions identified as either a principal aquifers (PA) or secondary hydrogeologic regions (SHR). Within each HMU, the wells were used to determine the proportion of each HR that is providing groundwater to the HMU. In 63% of the HMUs, a single HR is providing water to the public supply wells located within it, while the rest of the HMUs show that the wells are tapping up to a maximum of four stacked HRs. In total, groundwater from 108 HRs provide drinking water for public supply, six of which provide more than 50% of the groundwater used for public supply drinking water. The aquifer serving the largest number of equivalent people (>17 million) is the glacial aquifer. The HR providing the greatest number of people per km2 is the Biscayne aquifer in Florida at nearly 453 people per km2.

Sustainable saltwater intrusion management in coastal aquifers under climatic changes for humid and hyper-arid regions

Saltwater Intrusion (SWI) in many coastal areas is accelerated by freshwater boundary changes due to over-pumping and decreasing aquifer recharge and sea-level rise. This study aims to control SWI due to the rise in sea levels in three climate regions: (i) humid and wet regions using storage dams at different heads and recharge ponds, (ii) hyper-arid and arid regions using physical barriers and wastewater treatment aquifer, brackish water abstraction, and desalination (TRAD) method, and (iii) semi-arid and semi-humid regions using a combination of flooding water recharge well-field and cut-off wall. The study was developed using the world benchmark problem of Henry's problem and Biscayne aquifer in the Cutler Ridge area near Deering Estate, Florida, USA. The finite-difference code SEAWAT was used in numerical simulations. The numerical results indicated that increasing the artificial recharge lake capacity minimizes the SWI in humid and wet regions with a high level of flooding. Moreover, the intrusion is mitigated using physical barriers and the TRAD method for hyper-arid and arid regions. Combining physical barriers during dry seasons and recharge wells for wet seasons resulted in suitable measures to managed the SWI in semi-arid and semi-humid regions. Finally, the best strategy to mitigate the SWI and minimize the desalination costs depends on the precipitation rates. Therefore, the methodology applied to this study represents a valuable tool in order to select the best method based on the climate conditions and particularly the precipitation intensity to increase the water budget in freshwater aquifers.

Lithium in groundwater used for drinking-water supply in the United States

Lithium concentrations in untreated groundwater from 1464 public-supply wells and 1676 domestic-supply wells distributed across 33 principal aquifers in the United States were evaluated for spatial variations and possible explanatory factors. Concentrations nationwide ranged from <1 to 396 μg/L (median of 8.1) for public supply wells and <1 to 1700 μg/L (median of 6 μg/L) for domestic supply wells. For context, lithium concentrations were compared to a Health Based Screening Level (HBSL, 10 μg/L) and a drinking-water only threshold (60 μg/L). These thresholds were exceeded in 45% and 9% of samples from public-supply wells and in 37% and 6% from domestic-supply wells, respectively. However, exceedances and median concentrations ranged broadly across geographic regions and principal aquifers. Concentrations were highest in arid regions and older groundwater, particularly in unconsolidated clastic aquifers and sandstones, and lowest in carbonate-rock aquifers, consistent with differences in lithium abundance among major lithologies and rock weathering extent. The median concentration for public-supply wells in the unconsolidated clastic High Plains aquifer (central United States) was 24.6 μg/L; 24% of the wells exceeded the drinking-water only threshold and 86% exceeded the HBSL. Other unconsolidated clastic aquifers in the arid West had exceedance rates comparable to the High Plains aquifer, whereas no public supply wells in the Biscayne aquifer (southern Florida) exceeded either threshold, and the highest concentration in that aquifer was 2.6 μg/L. Multiple lines of evidence indicate natural sources for the lithium concentrations; however, anthropogenic sources may be important in the future because of the rapid increase of lithium battery use and subsequent disposal. Geochemical models demonstrate that extensive evaporation, mineral dissolution, cation exchange, and mixing with geothermal waters or brines may account for the observed lithium and associated constituent concentrations, with the latter two processes as major contributing factors.

Mitigation of seawater intrusion in coastal aquifers using coastal earth fill considering future sea level rise

Saltwater intrusion (SWI) is a physical problem that threatens many coastal aquifers all over the world. Saltwater intrusion is increasing with abstraction and rise in sea level. Coastal aquifer protection is essential to protect groundwater resources in these areas. A number of methods have been developed to protect coastal aquifers from SWI. This paper presents the impact of sea level rise on SWI in coastal aquifers and application of coastal earth fill as a new technique to control SWI. Different future sea level rise scenarios were studied and different coastal earth fill with an appropriate soil to extend the coastline towards the sea in order to control SWI was studied using SEAWAT model. The proposed control measure is numerically assessed by Henry's problem and then applied to a real case study of Biscayne aquifer, Florida, USA. For each aquifer, the corresponding relation was developed between the intrusion length of saltwater wedge and the width of fill. The results showed that increasing the fill width resulted in decreasing the intrusion length. In the case of Biscayne aquifer, increasing the fill width by 10, 20, 30, and 40% of the aquifer length resulted in retarding the intrusion to 329, 192, 42, and - 48m respectively. Using 150- and 300-m fill widths retards the intrusion length by 32.3% and 60.5%. In addition, increasing the fill width to 465m can retard SWI by 91.3%. This approach is capable to control the future risks of SWI and sea level rise.

Impact of facies and diagenetic variability on permeability and fluid flow in an oolitic grainstone—Pleistocene Miami Oolite

AbstractThe Miami Oolite of South Florida is representative of a grainstone‐rich carbonate unit that has been surficially karsted, and therefore may be considered as an analogue for subsurface reservoirs/aquifers with ‘high’ permeability extremes. The deposit can potentially serve to improve a conceptual understanding of heterogeneity as imparted by shallow‐marine facies changes and early meteoric diagenetic modification. Reviewed here are recent studies of the Miami Oolite with the intent to emphasize those key aspects of the facies and early diagenesis that most impact permeability and fluid flow. The Miami Oolite displays the preserved morphology of a fossilized ooid sandbody, even though it has been subaerially exposed in a tropical climate since its deposition approximately 120 kyr bp during the last interglacial highstand. The depositional motif is one of a dip‐oriented, tidal bar belt of shoals and shallow channels fronted by a strike‐oriented barrier bar. The barrier bar comprises cross‐stratified grainstones and locally burrowed grain/packstones, while the tidal shoals and channels are more commonly burrowed pack/grainstones. Surficial karst features (dolines and stratiform caves) have been added during the ca 120 kyr of subaerial exposure, but of more significance is the associated solution‐enhancement of the widespread burrowed facies. Since the Miami Oolite is the uppermost portion of the Biscayne Aquifer, there is also an understanding of fluid flow through the deposit that sheds valuable insight on the larger scale, shallow subsurface plumbing. The pore system comprises matrix pores (interparticle and separate vugs) and touching‐vug macropores that are commonly associated with burrowed [Ophiomorpha] intervals. Ground‐penetrating radar, well and flow test data indicate that matrix porosity provides most of the groundwater storage, whereas the touching vug macropores account for the majority of flow. The dolines and shallow caves seem to be sufficiently spaced as to generally not be in direct connection, with the result that they are less important in terms of regional flow than the prevailing pore system.

Numerical analysis of physical barriers systems efficiency in controlling saltwater intrusion in coastal aquifers.

Saltwater intrusion (SWI) increases salinity of aquifers and depletion of groundwater resources in coastal aquifers. Different methods have been used to control SWI in the coastal aquifers in order to protect groundwater. In this paper, applicability of physical subsurface barriers (PSB) methods to control SWI in the Biscayne aquifer in Florida, USA, is studied. Numerical models have been developed to study and compare performance of two types of the PSB namely cutoff wall and subsurface dam for SWI control. The developed numerical models have been verified through simulation of benchmark examples and then have been used to simulate a semi-hypothetical case study relying on hydrogeological data measured in the Biscayne aquifer. Different scenarios of barriers depths, locations, and permeability have been analyzed. The results indicated that the PSB can effectively control the intrusion of saline into coastal aquifers. However, cutoff wall gave higher retardation than sub-surface dams.

Assessing Risk of Injection of Reclaimed Water into the Biscayne Aquifer for Aquifer Recharge Purposes

The recharge of non-potable water into a drinking water aquifer is one means to overcome decreasing groundwater supplies and maintain availability of these resources for current and future generations. However, health concerns exist regarding the use of waters of “impaired quality” such as reclaimed wastewater for aquifer recharge. The objective of this study is to evaluate the potential risk to drinking water from the use of reclaimed water for recharge purposes using computational modeling with MODFLOW and MT3D groundwater transport simulation based on an actual situation using rotavirus as a surrogate. The results from the simulation showed that after seven months, the risk of contamination based on concentration contours from the injection well to the production wells was stabilized below 10-6.

Rebuttal to \u201cThe case of the Biscayne Bay and aquifer near Miami, Florida: density-driven flow of seawater or gravitationally driven discharge of deep saline groundwater?\u201d by Weyer (Environ Earth Sci 2018, 77:1\u201316)

A recent paper by Weyer (Environ Earth Sci 2018, 77:1–16) challenges the widely accepted interpretation of groundwater heads and salinities in the coastal Biscayne aquifer near Miami, Florida, USA. Weyer (2018) suggests that the body of saltwater that underlies fresh groundwater just inland of the coast is not a recirculating wedge of seawater, but results instead from upward migration of deep saline groundwater driven by regional flow. Perhaps more significantly, Weyer (2018) also asserts that established hydrologic theory is fundamentally incorrect with respect to buoyancy. Instead of acting along the direction of gravity (that is, vertically), Weyer (2018) claims, buoyancy acts instead along the direction of the pressure gradient. As a result, Weyer (2018) considers currently available density-dependent groundwater flow and transport modeling codes, and the analyses based on them, to be in error. In this rebuttal, we clarify the inaccuracies in the main points of Weyer’s (2018) paper. First, we explain that Weyer (2018) has misinterpreted observed equivalent freshwater heads in the Biscayne aquifer and that his alternative hypothesis concerning the source of the saltwater does not explain the observed salinities. Then, we review the established theory of buoyancy to identify the problem with Weyer’s (2018) alternative theory. Finally, we present theory and cite successful benchmark simulations to affirm the suitability of currently available codes for modeling density-dependent groundwater flow and transport.

The case of the Biscayne Bay and aquifer near Miami, Florida: density-driven flow of seawater or gravitationally driven discharge of deep saline groundwater?

Coastal groundwater flow investigations at the Cutler site of the Biscayne Bay south of Miami, Florida, gave rise to the dominating concept of density-driven flow of seawater into coastal aquifers indicated as a saltwater wedge. Within that wedge, convection-type return flow of seawater and a dispersion zone were concluded to be the cause of the Biscayne aquifer ‘seawater wedge.’ This conclusion was merely based on the chloride distribution within the aquifer and on an analytical model concept assuming convection flow within a confined aquifer without taking non-chemical field data into consideration. This concept was later labeled the ‘Henry problem,’ which any numerical variable-density flow program has to be able to simulate to be considered acceptable. Revisiting the summarizing publication with its record of piezometric field data (heads) showed that the so-called seawater wedge was actually caused by discharging deep saline groundwater driven by regional gravitational groundwater flow systems. Density-driven flow of seawater into the aquifer was not found reflected in the head measurements for low and high tide which had been taken contemporaneously with the chloride measurements. These head measurements had not been included in the assumption of a seawater wedge and associated dispersion zone and convection cell. The Biscayne situation emphasizes the need for any chemical interpretation of flow pattern to be backed up by head data as energy indicators of flow fields. At the Biscayne site density-driven flow of seawater did not and does not exist. This conclusion was confirmed by five independent methods. The hydrostatic use of vertical buoyancy forces needs, under hydrodynamic boundary conditions, to be replaced with buoyancy forces along the direction of the pressure potential forces [(grad p)/density] which, in the subsurface, can be pointed in any direction in space.

Prediction of Groundwater Quality Trends Resulting from Anthropogenic Changes in Southeast Florida.

The effects of surface water flow system changes caused by constructing water-conservation areas and canals in southeast Florida on groundwater quality under the Atlantic Coastal Ridge was investigated with numerical modeling. Water quality data were used to delineate a zone of groundwater with low total dissolved solids (TDS) within the Biscayne aquifer under the ridge. The delineated zone has the following characteristics. Its location generally coincides with an area where the Biscayne aquifer has high transmissivities, corresponds to a high recharge area of the ridge, and underlies a part of the groundwater mound formed under the ridge prior to completion of the canals. This low TDS groundwater appears to be the result of pre-development conditions rather than seepage from the canals constructed after the 1950s. Numerical simulation results indicate that the time for low TDS groundwater under the ridge to reach equilibrium with high TDS surface water in the water-conservation areas and Everglades National Park are approximately 70 and 60 years, respectively. The high TDS groundwater would be restricted to the water-conservation areas and the park due to its slow eastward movement caused by small hydraulic gradients in Rocky Glades and its mixing with the low TDS groundwater under the high-recharge area of the ridge. The flow or physical boundary conditions such as high recharge rates or low hydraulic conductivity layers may affect how the spatial distribution of groundwater quality in an aquifer will change when a groundwater flow system reaches equilibrium with an associated surface water flow system.

Rapid and Intense Phosphate Desorption Kinetics When Saltwater Intrudes into Carbonate Rock

It is important to understand how phosphate sorption dynamics of coastal carbonate aquifers are affected by seawater intrusion, because many coastal aquifers are composed of carbonate rocks and subject to an increase in saltwater intrusion during relative sea-level rise. Twelve carbonate rock and unconsolidated sediment specimens were acquired from a test corehole spanning the full thickness of the Biscayne aquifer in southeastern Florida. All 12 samples exhibit low phosphorus content but variable contents of iron. Column leaching experiments were conducted with two carbonate aquifer samples, alternating between freshwater and saltwater flow. With the first influx of saltwater, phosphate concentration in leachate increased rapidly from a freshwater value of approximately 0.2 μM to peaks of between 0.8 and 1.6 μM. The phosphate concentration began to diminish as saltwater continued to flow, but sustained desorption continued for over 2 h. Overall, seawater drove sorption behavior much more than chemical composition for the aquifer rocks and sediment from the seven rock samples for which we did isotherm sorption experiments. Our results indicate that an immediate and intense pulse of phosphate desorption from carbonate rock and sediment with low phosphorus content occurs in response to an influx of seawater and that the duration of desorption will vary by layer within a single aquifer.