Kumar A. Narayan

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.

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.

The impact of flooding on modelling salt transport processes to streams

Abstract The development of many of the worlds arid and semi-arid regions has resulted in the salinisation of land and water resources. In these areas, soils and groundwaters are often naturally saline and any disturbance of the delicate hydrological balance results in mobilisation of the stored salt. The salt transport mechanisms are often highly complex, the understanding of which necessitates the use of computer modelling in combination with field studies. In this paper the transport of salt between groundwater and streams on the Chowilla floodplain in south-eastern Australia was modelled and compared with available field data. The large salinity contrast between the fresh stream and floodwater and the saline groundwater results in density-dependent flow behaviour, and hence necessitated the use of a variable density flow and solute transport model (SUTRA). The model was applied in cross-section over a 6.1-km-long transect across the floodplain. Time varying boundary conditions were employed at the locations of three streams on the transect to simulate the interaction between the rising and falling streams and the adjacent aquifer during and after floods. The model was used to assess the importance of overbank floods in the transport of salt to floodplain streams by carrying out simulations under various recharge scenarios. The simulations showed that the mixing of floodwater and groundwater within the bank storage adjacent to the streams could predict the observed short-term (

Modelling density-dependent flow and solute transport at the Lake Tutchewop saline disposal complex, Victoria

Abstract Intercepted saline groundwaters and drainage effluent from irrigation are commonly stored in both natural and artificial saline disposal basins throughout the Murray-Darling Basin of Australia. Their continued use as wastewater evaporation sites requires an understanding of existing groundwater dynamics. The useful of individual basins, their sustainability and possible environmental impacts remain largely unknown. In this work, the movement of salt to the underlying groundwater system from Lake Tutchewop, a saline disposal complex in north-central Victoria, was modelled in cross-section. Due to the salinity contrast between the hypersaline basin waters and the regional groundwater, it was necessary to simulate density-dependent flow behaviour. Under certain conditions, these density-stratified systems may become unstable leading to the onset of convective behaviour, which greatly increases the movement of salt from the basin to the groundwater system. Modelled concentration profiles in the aquifer system and calculated seepage rates from the basin show that Lake Tutchewop is stable under its present operating regime. The downward movement of salt is mainly controlled by diffusion and dispersion. The calibrated model was used to assess the impact of several management scenarios using time-dependent boundary conditions for lake salinity and water levels. The influence of heterogeneous basin linings on ensuing salt flux rates is examined, and results show that increased solute transport will occur under such conditions. A sensitivity analysis performed on governing variables showed that salt fluxes were most sensitive to lake salinity levels. A solute Rayleigh number defined in terms of basin salinity and hydrogeologic parameters is seen to be an effective tool for predicting the long term behaviour of such saline disposal basins. The models and concepts developed in this work may find application in the design and management of saline disposal complexes.

Simulation of groundwater interception at Lake Ranfurly, Victoria, incorporating variable density flow and solute transport

Abstract The movement of salt from Lake Ranfurly West towards the River Murray and the associated groundwater interception scheme has been modelled in cross-section using the SUTRA model. Recharge from irrigation areas, and saline seepage from the lake have been taken into account. Owing to the salinity contrast between Lake Ranfurly West and groundwater, it was considered appropriate to simulate density-dependent flow behaviour. Concentration profiles in the aquifers and salt loads to the river under various management scenarios were computed under conditions of both density-dependent and non-density-dependent flow. The model simulations have shown that the salt load to the river is (1) dependent on the rate of pumping from interception bores, (2) dependent on the aquifer(s) in which groundwater is intercepted, and (3) marginally greater (11–15%) for density-dependent flow behaviour at less than full interception compared with calculations which neglect density-dependent flow.

Integrated groundwater flow and agronomic modelling for management of dryland salinity of a coastal plain in southern Australia

Abstract An ‘integrated modelling approach’ was used to explore a range of options for land management to control dryland salinity. Three models were developed for this study: a numerical groundwater flow model which predicts groundwater levels for various land uses; agronomic models quantifying crop and pasture yield response to shallow saline watertables and seasonal rainfall; and a financial model of farm revenues, costs and discount rates. When combined, these provided information on groundwater levels, agricultural production, and farm incomes for a 105-km 2 site on a coastal plain in southern Australia. The simulations included current as well as various alternative land management options for a 20-year period. The results indicated that the establishment of deep-rooted perennial pastures can reduce both rates of groundwater recharge and the area of salinised land, thereby enhancing productivity. However, the cost of reclaiming salinised land is high. Compared with current land management practices, the economic benefit of perennial pastures is marginal, and depend strongly on future farm commodity prices and discount rates. The methodology described herein represents a holistic means of dealing with a variety of environmental problems of agricultural management. Its advantages and disadvantages are also discussed.

The effectiveness of management options for dryland salinity control at Wanilla, South Australia

Abstract A calibrated groundwater flow model was used to assess several options for management of dryland salinity at Wanilla on Lower Eyre Peninsula of South Australia. The management options were chosen as potentially able to rectify the hydrological imbalance produced by the clearing of native vegetation which will reverse the salinisation process. The model predicted that current agricultural practices will result in groundwater levels continuing to rise in saline areas over the next 20 years (although tending towards equilibrium) increasing soil salinisation. As groundwater levels rise seepage of groundwater to the local drainage (natural) line will also increase, flowing down-gradient towards the catchment outlet and a seasonally flooded lower lying basin. A 50–90% reduction in catchment recharge, achieved with improved management of existing crops and establishment of perennial pastures, is predicted to reduce groundwater levels across the catchment over a 10–20 year period. This will lead to recovery of saline areas and lower baseflow. However, the model predicts that management of recharge needs to be implemented at least at the catchment scale to be effective. Treatments at the sub-catchment scale (e.g. property) have limited impact. Short term benefits (less than 1 year) can be gained from pumping groundwater from a high transmissivity zone close to discharge area. Groundwater levels are lowered over a small area leading to less evaporative discharge and reduced salt accumulation in the root zone. This option needs to work in conjunction with longer term management such as recharge control.