Modeling spatial and temporal variations of surface moisture content and groundwater table fluctuations on a fine-grained beach, Padre Island, Texas

The basic goals of this study are to document, represent and model beach surface moisture dynamics. Achieving these goals requires that the dynamics be understood within the context of the key associated processes including evaporation and groundwater table fluctuations. Atmospheric parameters including wind speed, air temperature and relative humidity, evaporation, beach surface moisture content, groundwater table fluctuations and tidal oscillations were directly monitored in an eight-day field experiment. Field measurements demonstrated that beach surface moisture content has a relatively high degree of variability in the cross-shore direction and a relatively low variability in the alongshore direction. The highest levels of variability were found in the middle beach, where daily fluctuations of up to 30% (volume) were common. Long-term variations in surface moisture content are controlled by water table fluctuations, while short-term variations are dominated by either evaporation or groundwater table fluctuations depending on the local water table depth. Two traditional methods to estimate potential evaporation rates were tested, the mass-transfer method and the combined (energy-budget and mass-transfer together) approach. Results showed that the mass-transfer method produces consistent large errors in simulations, even with recalibration of the constants. Simulations utilizing the Penman equation provide much better agreement with field data. It was found that groundwater table fluctuations at the studied beach are mainly forced by tidal oscillations. The numerical solution of the linealized Boussinesq equation provides an accurate approach to simulate tide-forced beach groundwater table fluctuations. These simulations were found to be significantly improved by modeling the system as a sloping beach rather than using the traditional vertical beach approach. Spatial and temporal variations in beach surface moisture were modeled using the numerically solved Richard’s equation and the Force-Restore method. The Force-Restore method underestimates surface moisture contents when the water table is relatively shallow owing to an inherent defect in the model itself. The simulations employing the numerically solved Richard’s equation agree closely surface moisture content from the field. This study represent perhaps the first, certainly the most comprehensive, attempt that has been made to date, to explain intermediate-scale variability in beach surface moisture content in light of the microscale process that drive these dynamics. The findings of this study should be applicable to longer time periods, and larger spatial areas with similar environmental settings. However, more investigations regarding hydraulic properties of local sediment are needed to enhance the model applicability

Physicochemical changes to soil and sediment in managed realignment sites following tidal inundation

PhDThe recognition of the value of salt marshes and concerns over salt marsh loss has led to the adoption of managed realignment in coastal areas. Managed realignment involves the landward relocation of the seawall, allowing an area of agricultural land to be tidally inundated. It is believed that managed realignment sites can act as a sink for contaminants. However, these sites may also act as a contaminant source and pose a risk to estuarine biota. In this thesis, the potential for metal and herbicide release from agricultural soil and dredged sediment in managed realignment sites was investigated by laboratory microcosm experiments. The agricultural soil and dredged sediment were subjected to two different salinities and drying-rewetting treatments. Results indicate the release of metals (Cu, Ni and Zn) and herbicides (simazine, atrazine and diuron) was dependent on their strength of binding to the soil and sediment, and complexation and competition reactions between seawater anions, cations and the sorbed metals. The release patterns indicated that metal and herbicide release into overlying water may continue for extended periods of time after an initial rapid release. The total metal and herbicide loads released into the overlying water followed the order: Cu < Zn < Ni and diuron < atrazine < simazine with a greater release from the soil than sediment. The increase in CO2 release, mineralisation rates, total metal and herbicide loads after drying and rewetting the soil suggested an increase in the mineralisation of organic matter and the release of organic matter associated metals and herbicides. Results of linear regression analyses provided evidence that the release of the metals and herbicides as DOC-complexes was important for soil but not for sediment. These findings indicate that there is a lower potential for contaminant release from managed realignment sites where dredged sediments are beneficially re-used

Numerical modelling of organic contaminant reaction and transport in bed-sediments.

Reactive transport modelling of contaminants in the environment is being increasingly relied upon for a wide range of tasks associated with risk-based decision making, such as interpretation of historical contamination data, optimisation of attenuation and remediation methods, and monitoring of changes resulting from an implemented remediation scheme. However, in the area of contaminant fate and behaviour in bed-sediments, reactive transport modelling has until now stopped short of integration of various mechanistic models to a single modelling environment that would allow a cohesive understanding and prediction of contaminant profiles. This study has developed CoReTranS, a predictive modelling environment that simulates one-dimensional organic contaminant reaction and transport in bed- sediments, using an object-oriented modelling approach. The CoReTranS model has been verified and benchmarked by comparing numerical results of simplified problems with their analytical solutions. The following simulations were undertaken to validate the CoReTranS model: 1. Simulation of the dataset from a diffusion-controlled laboratory experiment for the transport and distribution of selected trace level organic contaminants in a riverine environment gave new numerical results to improve on predicted modelling approach. 2. Simulation of the dataset from a study of marsh sediments contaminated with petroleum-derived hydrocarbons from Wild Harbour, West Falmouth, MA and Kitimat Arm, Douglas Channel, British Columbia resulted in an excellent agreement between the numerical results of the transport model in CoReTranS and the numerical results and data of the original study. The CoReTranS model was also used to interpret results from the following field studies in order to explain key processes that controlled the fate and transport of PAHs and PCBs in bed-sediments: 1. Simulation of the dataset from Kitimat fjord system near Kitimat, British Columbia, wherein PAHs in sediments were purported to be derived from atmospheric particle emissions, wastewater discharges and accidental spillages from a nearby duminium smelter provided a better understanding of the post-depositional reactive transport of PAHs in the fjord system. 2. Simulation of the dataset from a study on the natural recovery of PCB-contaminated sediments at the Sangamo-Weston/Twelvemile Creek/Lake Hartwell Superfund Site in the US showed that it would take nearly 30 years to achieve the 1 mg/kg clean-up goal for total PCB in the chosen transect sites, and 20 years more than the predicted time in the original study. The CoReTrans model was also used to predict the effect of capping contaminated sediments as a remedial strategy. Results from the various simulation scenarios using the CoReTranS model showed that sediment capping as a remedial strategy in managing contaminated sediments can effectively reduce contaminant flux to the overlying water through interaction with the sediment cap matrix and by increasing the dissolved contaminants' transport lengths (i.e., cap thickness). Comparing the results obtained from laboratory experiments or field monitoring studies of bed-sediment systems with different accumulation, degradation and release mechanisms, with the results from the CoReTranS model was critical in identifying the key processes that drive the fate and transport of organic contaminants in bed-sediments. The information derived from the use of the CoReTranS model highlighted recommendations to guide future experiments, field monitoring and model extension which include other relevant transport mechanisms such as colloid- enhanced transport, rate-limited reaction processes and the effect of sediment consolidation to contaminant fate and transport. This information will further enable practical application of such information by engineers to site-specific risk assessment and remediation, as well as continued research and technology development