The interaction between a tidal estuary and a shallow unconfined aquifer : a saltwater intrusion and environmental impacts in the riparian zone

Werner, Adrian Deane (2004). The interaction between a tidal estuary and a shallow unconfined aquifer : a saltwater intrusion and environmental impacts in the riparian zone PhD Thesis, School of Engineering, The University of Queensland.

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Author Werner, Adrian Deane
Thesis Title The interaction between a tidal estuary and a shallow unconfined aquifer : a saltwater intrusion and environmental impacts in the riparian zone
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2004
Thesis type PhD Thesis
Supervisor Lockington, D.
Volker, R.
Total pages 1v.
Collection year 2004
Language eng
Subjects L
290801 Structural Engineering
680203 Civil
Formatted abstract

In this thesis, the contribution to groundwater salinization from an estuary in a system similar to the Sandy Creek - alluvium system in the Pioneer Valley, northern Queensland is explored. Initially, collation and interpolation of the large data set of field observations is undertaken. Field observations of electrical conductivity, water table elevations and groundwater geochemistry, along with hydrogeologic interpretations present evidence that such groundwater salinization processes as estuarine saltwater intrusion, oceanic saltwater intrusion and relic seawater up-coning are occurring. The analyses of field observations also provide a framework for a conceptual model of the interaction between Sandy Creek and the adjacent alluvial aquifer.

A conceptual model of estuary-aquifer interaction is used as the basis for a two-dimensional numerical model of estuarine saltwater intrusion in a system similar to the Sandy Creek - alluvium system. The model is unable to reproduce observed salinity distributions with realistic aquifer hydraulic properties and groundwater hydraulic gradients. Thus, it would appear that estuarine saltwater intrusion is not the sole source of salinity in the aquifer adjacent to Sandy Creek, although further field monitoring is required, possibly in the form of trace element monitoring to better define the contribution of estuaries to regional groundwater salinization.

A more generic study of estuary-aquifer interaction at the irrigation scale (2km) is then conducted. Here, the focus is on estimation of both rate of salinization and salt plume extents, and water table salinization in a range of aquifer materials. Irrigation scale modelling is used to ascertain the horizontal propagation distances of estuarine induced groundwater waves. Wave propagation distances and salt plume extents are used to define the horizontal dimension of a more detailed riparian scale (180m) study.

The riparian scale study adopted a higher level of spatial discretization so that more advanced system characteristics such as estuarine tidal fluctuations, recharge, and the effect on water table salinization and salt plume extent could be explored. Tidal fluctuations appear to have a considerable impact on water table salinities in the riparian zone, on the salt distribution in the underlying aquifer, and on the groundwater flow patterns, in particular the location of discharging groundwater, in the adjacent aquifer. Increasing the tidal amplitude induced larger impacts in the adjacent aquifer. Surprisingly, tidal fluctuations had only minor influences on the prediction of broader-scale saltwater intrusion, as defined by relative salinities less than 0.50 (relative to seawater). Thus it would appear that neglecting tidal fluctuations may not necessarily introduce errors in the prediction of saltwater intrusion at >60m from the estuary bank in the simulation of similar systems. However, any study aimed at predicting the near-estuary environmental impacts such as water table and vadose zone salinization caused by land use changes, especially groundwater pumping, needs to include the water table fluctuations induced by estuarine tide.

The scale of the riparian scale study and limitations in the modelling software (SUTRA) restricted the estimation of vadose zone salinization resulting from increasing the water table salinity. Therefore, the potential for vadose zone and ground surface salinization was examined in one-dimensional numerical experiments using a more specialized variably saturated transport model, HYDRUS-ID. Evaporative stresses at the ground surface were shown to exacerbate vadose zone salinity when the water table was sufficiently shallow. Where the time-averaged water table depth was 1 m below the ground surface, evaporative stresses initiated an accumulation of ground surface salts. This salt accumulation occurred more rapidly in a sandy clay loam, than in a sand or sandy clay, and it was shown that solute longitudinal dispersion and water table wave magnitude are important considerations. The remediation of a saline soil profile through rainfall fiushing during a single wet season of three months was shown to be difficult, particularly for salts at the soil surface. The limitations of the one-dimensional modelling were apparent by comparison to simulated water table salinity changes from two-dimensional simulations, which predicted a limited reduction in water table salinity from freshwater infiltration, particularly in the short-term. Thus, two-dimensional effects are considered important in prediction of vadose zone salinization. Neglecting hysteresis in simulation was shown to produce errors in the prediction of evaporative salt build-up, although in rainfall infiltration simulations, hysteresis effects were minimal.

Close examination of the prediction from HYDRUS-ID identified an artificial pumping error caused by the adopted method of moisture retention hysteresis (i.e. the method of Kool and Parker, 1987). The magnitude of this error inherent in the hysteresis approach is systematically quantified using fluctuating capillary pressure scenarios and a range of hypothetical parameters. Artificial pumping error was found to be largest in coarse materials and when simulation began on the main drainage moisture retention curve. The error was also dependent on the characteristics of the capillary pressure fluctuations combined with the shape of the moisture retention envelope (i.e. the area bounded by the main drainage and imbibitions curves).

A correction to the hysteresis algorithm in HYDRUS-ID was proposed based on the Parker and Lenhard (1987) model, which was modified to improve its computer memory requirements. The modified hysteresis model was implemented in HYDRUS-ID, along with improvements in the management of moisture reversal points (eg. from wetting to drying). Changes to HYDRUS-ID eliminated artificial pumping errors and generally improved the accuracy of prediction.

The magnitude of hysteresis is explored using the modified HYDRUS-ID program by comparison of hysteretic prediction to the results of non-hysteretic simulations that adopt either the main drainage curve or an average moisture retention curve, defined by taking the arithmetic average of the van Genuchten (1980) parameters used to describe moisture retention hysteresis. Quantifying the magnitude of hysteresis was attempted using water table wave characteristics defined by Nielsen and Perrochet (2000), namely the magnitude and argument of a complex water table response function. Hysteresis is shown to account for discrepancies between non-hysteretic Richards model prediction conducted by Nielsen and Perrochet, and their laboratory experiment results. The use of a single-valued mean retention curve, as advocated by some authors, fails to provide a match between the simulated and observed behaviour of the Nielsen and Perrochet parameters, but is shown to be adequate for predicting time-averaged soil moisture profiles. Relationships between the magnitude of hysteresis and such simulation parameters as the porous medium hydraulic properties or the frequency of the forcing pressure wave were illustrated, although the relationships appeared to be extremely complex and simple assertions of the relationships were difficult. In fact, no correlation was evident between the hydraulic conductivity or the porosity and the magnitude of hysteresis. There is evidence that the magnitude of hysteresis slightly increases with reducing water table depth (below ground surface).

Keyword Estuaries
Saltwater encrouchment

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
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Created: Fri, 24 Aug 2007, 18:35:15 EST