Craig T. Simmons

Impact of Sea-Level Rise on Sea Water Intrusion in Coastal Aquifers

Despite its purported importance, previous studies of the influence of sea-level rise on coastal aquifers have focused on specific sites, and a generalized systematic analysis of the general case of the sea water intrusion response to sea-level rise has not been reported. In this study, a simple conceptual framework is used to provide a first-order assessment of sea water intrusion changes in coastal unconfined aquifers in response to sea-level rise. Two conceptual models are tested: (1) flux-controlled systems, in which ground water discharge to the sea is persistent despite changes in sea level, and (2) head-controlled systems, whereby ground water abstractions or surface features maintain the head condition in the aquifer despite sea-level changes. The conceptualization assumes steady-state conditions, a sharp interface sea water-fresh water transition zone, homogeneous and isotropic aquifer properties, and constant recharge. In the case of constant flux conditions, the upper limit for sea water intrusion due to sea-level rise (up to 1.5 m is tested) is no greater than 50 m for typical values of recharge, hydraulic conductivity, and aquifer depth. This is in striking contrast to the constant head cases, in which the magnitude of salt water toe migration is on the order of hundreds of meters to several kilometers for the same sea-level rise. This study has highlighted the importance of inland boundary conditions on the sea-level rise impact. It identifies combinations of hydrogeologic parameters that control whether large or small salt water toe migration will occur for any given change in a hydrogeologic variable.

On a test case for density‐dependent groundwater flow and solute transport models: The Salt Lake Problem

Like any other computer model, density-dependent mathematical groundwater models must be “verified” to ascertain that they accurately represent the physics implied by a governing set of equations. Very few test cases for density-dependent groundwater numerical models exist. As such, there is still a need for new and more robust tests of these modeling codes. In this paper a numerical model of an idealized evaporating salt lake is produced using the two-dimensional density-dependent saturated-unsaturated transport (SUTRA) model, the results of which are compared with an equivalent laboratory Hele-Shaw cell system developed by Wooding et al. [1997a, b]. Evaporation results in dense brine overlying less dense fluid, which is hydrodynamically unstable and leads to downward convection of salt fingers or plumes. A comparison of experimental and numerical plume growth shows good spatial and temporal agreement. The numerically generated plume pattern is sensitive to changes in random noise level applied just below the evaporation surface that serves as a trigger for the growth of instabilities. Experimental plume patterns were best matched with a noise level corresponding to 1% of the total salinity difference between boundary layer and background fluid concentrations at saturation. In a second comparison the stream-function-based finite difference model described by Wooding et al., [1997a, b] which differs significantly in principle from SUTRA is shown, after revision, to give good spatial and temporal agreement with experimental results. This test for density-dependent groundwater models appears to be the most comprehensive and detailed available to date.

Hydrogeologic controls on disconnection between surface water and groundwater

[1] Understanding how changes in the groundwater table affect surface water resources is of fundamental importance in quantitative hydrology. If the groundwater table below a stream is sufficiently deep, changes in the groundwater table position effectively do not alter the infiltration rate. This is referred to as a disconnected system. Previous authors noted that a low-conductivity layer below the surface water body is a necessary but not sufficient criterion for disconnection to occur. We develop a precise criterion that allows an assessment of whether surface water-groundwater systems can disconnect or not. We further demonstrate that a disconnected system can be conceptualized by a saturated groundwater mound and the development of a capillary zone above this mound. This conceptualization is used to determine the critical water table position at the point where full disconnection is reached. A comparison of this calculated critical water table position with a measurement of the water table depth in a borehole allows the assessment of the disconnection status. A sensitivity analysis of this critical water table showed that for a given aquifer thickness and river width, the depth to groundwater where the system disconnects is approximately proportional to the stream depth and the hydraulic conductivity of the streambed sediments and inversely proportional to the thickness of these sediments and the hydraulic conductivity of the aquifer. The conceptualization also allows the disconnection problem to be analyzed using both variably saturated and fully saturated groundwater models and provides guidance for numerical and analytical approaches.

Mixed convection processes below a saline disposal basin

Abstract Saline groundwater and irrigation drainage are commonly diverted and stored in both natural and artificial depressions throughout the Murray–Darling Basin of Australia. The disposal basin brines that are formed are often denser than ambient groundwater. Under certain conditions these dense brines may become unstable causing them to mix with groundwater over distances several orders of magnitude greater than due to diffusion alone. A model is developed to study the mixed convection processes below a saline disposal basin located between a recharge and discharge zone. Numerical simulations are performed in cross-section using the 2-D density dependent model SUTRA (saturated–unsaturated transport). It is shown that the salt front movement is related to both the ability of the dense saline brines to mix convectively with underlying groundwaters (Rayleigh convection) and the strength of the regional advective velocity. Both homogeneous and heterogeneous aquifer systems are studied and the effects of anisotropy are considered. Our numerical results suggest that the behaviour of a dense brine plume overlying less dense groundwater in a homogeneous porous medium depends on the magnitude of at least two non-dimensional numbers, a Rayleigh number and modified Peclet number, defined in terms of basin scale hydrogeologic parameters including dispersion. It is shown that the onset of gravitational instabilities and the formation of free convective cells begins when the magnitude of a non-dimensional parameter combining the Rayleigh and modified Peclet number exceeds a certain critical value.

Laboratory Investigation of Variable-Density Flow and Solute Transport in Unsaturated–Saturated Porous Media

In many groundwater systems, fluid density and viscosity may vary in space and time as a function of changes in concentration and temperature of the fluid. When dense groundwater plumes interact with less dense ambient groundwater, these density variations can significantly affect flow and transport processes. Under certain conditions, gravitational instabilities in the form of lobe-shaped fingers can occur. This process is significant because it can lead to more rapid and spatially extensive solute transport. This paper presents new experiments carried out in a sand filled glass flow container under both fully saturated and variably-saturated conditions and focuses upon the processes that occur at the capillary fringe and below the water table, as affected by a dense contaminant plumes migration through the unsaturated zone. Source fluids stained with Rhodamine-WT were introduced at the upper boundary of the tank at a range of low and high densities. In addition to the fluid density gradients and porous medium permeability that determine the onset conditions for instabilities in fully saturated experiments, volumetric water content appears critical in the variably-saturated laboratory runs. Plume behaviour at the water table appears dependent upon the density of the fluid that accumulates there. For neutral and low density fluids, plumes accumulate at the water table and then spread laterally above it and the water table forms a barrier to further vertical flow as pore water velocities reduce with increasing water content. For medium and high density fluids, vertical movement continues as instabilities form at the capillary fringe and fingers begin to grow at the water table boundary and move downwards into the saturated zone. In these cases, lateral spreading of the plume is small. Despite these more qualitative observations, the exact nature of the relevant stability criteria for the onset and growth of instabilities in variably-saturated porous media presently remain unclear. All experimental results suggest, however, that the unsaturated zone and position of the water table must be considered in contaminant studies in order to predict the migration pathways, rates and ultimate fate of dense contaminant plumes. It is possible that the results of experiments presented in this paper could form a useful basis for the testing of variable-density (and variably-saturated) groundwater flow and solute transport numerical codes because they offer controlled physical laboratory analogs for comparison. They also provide a strong basis for the development of more rigorous mathematical formulations that are likely to be either developed or tested using numerical flow and solute transport simulators.

Disconnected Surface Water and Groundwater: From Theory to Practice

When describing the hydraulic relationship between rivers and aquifers, the term disconnected is frequently misunderstood or used in an incorrect way. The problem is compounded by the fact that there is no definitive literature on the topic of disconnected surface water and groundwater. We aim at closing this gap and begin the discussion with a short introduction to the historical background of the terminology. Even though a conceptual illustration of a disconnected system was published by Meinzer (1923), it is only within the last few years that the underlying physics of the disconnection process has been described. The importance of disconnected systems, however, is not widely appreciated. Although rarely explicitly stated, many approaches for predicting the impacts of groundwater development on surface water resources assume full connection. Furthermore, management policies often suggest that surface water and groundwater should only be managed jointly if they are connected. However, although lowering the water table beneath a disconnected section of a river will not change the infiltration rate at that point, it can increase the length of stream that is disconnected. Because knowing the state of connection is of fundamental importance for sustainable water management, robust field methods that allow the identification of the state of connection are required. Currently, disconnection is identified by showing that the infiltration rate from a stream to an underlying aquifer is independent of the water table position or by identifying an unsaturated zone under the stream. More field studies are required to develop better methods for the identification of disconnection and to quantify the implications of heterogeneity and clogging processes in the streambed on disconnection.

Modeling Surface Water-Groundwater Interaction with MODFLOW: Some Considerations

The accuracy with which MODFLOW simulates surface water-groundwater interaction is examined for connected and disconnected losing streams. We compare the effect of different vertical and horizontal discretization within MODFLOW and also compare MODFLOW simulations with those produced by HydroGeoSphere. HydroGeoSphere is able to simulate both saturated and unsaturated flow, as well as surface water, groundwater and the full coupling between them in a physical way, and so is used as a reference code to quantify the influence of some of the simplifying assumptions of MODFLOW. In particular, we show that (1) the inability to simulate negative pressures beneath disconnected streams in MODFLOW results in an underestimation of the infiltration flux; (2) a river in MODFLOW is either fully connected or fully disconnected, while in reality transitional stages between the two flow regimes exist; (3) limitations in the horizontal discretization of the river can cause a mismatch between river width and cell width, resulting in an error in the water table position under the river; and (4) because coarse vertical discretization of the aquifer is often used to avoid the drying out of cells, this may result in an error in simulating the height of the groundwater mound. Conditions under which these errors are significant are investigated.

Variation in performance of surfactant loading and resulting nitrate removal among four selected natural zeolites

Surfactant modified zeolites (SMZs) have the capacity to target various types of water contaminants at relatively low cost and thus are being increasingly considered for use in improving water quality. It is important to know the surfactant loading performance of a zeolite before it is put into application. In this work we compare the loading capacity of a surfactant, hexadecyltrimethylammonium bromide (HDTMA-Br), onto four natural zeolites obtained from specific locations in the USA, Croatia, China, and Australia. The surfactant loading is examined using thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. We then compare the resulting SMZs performance in removing nitrate from water. Results show that TGA is useful to determine the HDTMA loading capacity on natural zeolites. It is also useful to distinguish between a HDTMA bi-layer and a HDTMA mono-layer on the SMZ surface, which has not been previously reported in the literature. TGA results infer that HDTMA (bi-layer) loading decreases in the order of US zeolite>Croatian zeolite>Chinese zeolite>Australian zeolite. This order of loading explains variation in performance of nitrate removal between the four SMZs. The SMZs remove 8-18 times more nitrate than the raw zeolites. SMZs prepared from the selected US and Croatian zeolites were more efficient in nitrate removal than the two zeolites commercially obtained from Australia and China.

Process-Based Reactive Transport Model To Quantify Arsenic Mobility during Aquifer Storage and Recovery of Potable Water

Aquifer storage and recovery (ASR) is an aquifer recharge technique in which water is injected in an aquifer during periods of surplus and withdrawn from the same well during periods of deficit. It is a critical component of the long-term water supply plan in various regions, including Florida, USA. Here, the viability of ASR as a safe and cost-effective water resource is currently being tested at a number of sites due to elevated arsenic concentrations detected during groundwater recovery. In this study, we developed a process-based reactive transport model of the coupled physical and geochemical mechanisms controlling the fate of arsenic during ASR. We analyzed multicycle hydrochemical data from a well-documented affected southwest Floridan site and evaluated a conceptual/numerical model in which (i) arsenic is initially released during pyrite oxidation triggered by the injection of oxygenated water (ii) then largely complexes to neo-formed hydrous ferric oxides before (iii) being remobilized during recovery as a result of both dissolution of hydrous ferric oxides and displacement from sorption sites by competing anions.

GRACE satellite observed hydrological controls on interannual and seasonal variability in surface greenness over mainland Australia

Water-limited ecosystems, covering ~50% of the global land, are controlled primarily by hydrologic factors. Because climate change is predicted to markedly alter current hydroclimatic conditions later this century, a better hydrological indicator of ecosystem performance is warranted to improve understanding of hydrological controls on vegetation and to predict changes in the future. Here we show that the observed total water storage anomaly (TWSA) from the Gravity Recovery and Climate Experiment (GRACE) can serve as this indicator. Using the Australian mainland as a case study, where ecosystems are generally water limited, we found that GRACE-observed TWSA can explain changes in surface greenness (as measured by the normalized difference vegetation index, NDVI) both interannually and seasonally. In addition, we found that TWSA shows a significant decreasing trend during the millennium drought from 1997 through 2009 in the region. However, decline in annual mean NDVI during the same period was mainly driven by decline in annual minimum monthly NDVI, whereas annual maximum monthly NDVI remained relatively constant across biomes. This phenomenon reveals an intrinsic sensitivity of ecosystems to water availability that drought-induced reductions in surface greenness are more likely expressed through its influence on vegetation during lower NDVI months, whereas ecosystem activities tend to recover to their maximum level during periods when the combined environmental conditions favor vegetation growth within a year despite the context of the prolonged drought.