The storage and mobilisation of salts in dual porosity landscapes : a detailed case study on weathered felsic volcanic rocks In Boro, South Eastern NSW

Abstract

In Australia, salinity is considered a major land management challenge that affects the ongoing viability of agricultural production and ecosystem function. Biophysical classification, following the Hydrogeological Landscapes (HGL) framework, has been used by NSW Government agencies to identify and manage areas with associated patterns of salinity expression and recognised salinity hazard. At Boro, near Braidwood in south eastern NSW, Australia, this classification identified a weathered felsic volcanic HGL with moderate land salinity, moderate salt load and high stream salinity (electrical conductivity). Historic landscape characterisation at this site described localised salinity, based on preliminary observations of the rock, weathered rock and soil (regolith) materials. However, it could not explain why the salt expression, particularly during wetter periods, was more extensive than predicted. The transdisciplinary nature of salinity investigation is emphasised in this work. The research is therefore structured in 5 parts that cover: the background and general principles related to salinity; physico-chemical profiling of the regolith; evaluation of solutes and fluid movement in the landscape; evaluation of salt storage potential; and quantification of the model for salt storage. The study incorporates the use of geophysical, mineralogical, geochemical and water chemistry data to understand salinity and land degradation issues. X-Ray micro-computed tomography (XRT) was used with a tracer, to determine the porosity and permeability of fresh and weathered rock samples. This was used to evaluate the role of matrix diffusion in flux models for the storage and mobilisation of salts. Fluid movement and solute transport in the landscape are critical factors in determining where and how salinity is expressed at both the regional and local scale. Investigating these factors in the HGL framework has enabled prediction of the storage and mobilisation of salts in the regolith. In many fields, particularly related to anthropogenic pollution, matrix diffusion is also considered a key process in fluid movement and solute transport. However, prior to this study it has not been considered or applied to understanding landscape salinity. The novel application and investigation of matrix diffusion in this detailed site study has revealed its importance and shown that regolith with higher matrix porosity has a greater capacity to store and release salts. This helps to explain the manifestation of salinity in the weathered felsic volcanic landscape at Boro. Solute profiling identified Na+, Cl- and HCO3 as the dominant solutes present in the regolith. These were present in higher concentrations below the break in slope and in open drainage depressions. Discharge sites were identified below the break in slope and evaporative concentration of salts was common in this part of the landscape. X-ray micro-computed tomography (XRT) enabled characterisation and analysis of the macroporosity and micro-porosity networks in weathered and fresh rock samples. The porosity in these samples was up to 19 times higher than previously understood from similar lithologies, and this had implications for salt storage potential. The tracer solution penetrated the rock matrix where there was no detectable permeability. This was a critical factor in indicating the potential for matrix diffusion in weathered and fresh rock samples. Evaluation of matrix diffusion using Fick’s first and second laws identified faster rates of diffusion in the samples with the highest porosity. Modelling indicated that rapid diffusion (40% change) took place over the shorter term (10 years), while almost complete diffusion (99% change) in the same samples took place over 40 thousand years. Evaluation indicates that short- and long-term climatic conditions and changes to the water balance in the landscape have potential to influence whether salts are in a storage or mobilisation phase. These findings have major implications for the understanding of salt flux in weathered felsic volcanic landscapes, as well as more broadly, in other lithologies with high matrix porosity. The HGL characterisation coupled with detailed biophysical analysis including XRT and modelling has allowed a more detailed understanding of the processes for salt storage, mobilisation and expression in the landscape. Salinity, sodicity and low pH, with potential Al toxicity, were identified as the key land management issues in the LFVN HGL. These issues are commonly present below the break in slope and in open drainage depressions. The presence of salts in the landscape reduces the erosion hazard presented by sodicity, by maintaining a cohesive soil structure. Leaching of salts in this landscape has potential to exacerbate erosion. The complex and interrelated nature of salinity, sodicity and low pH in the landscape means that remediation works require a combination of revegetation with extremely salt tolerant plants, soil engineering works and the use of ameliorants.

Date of Award	2021
Original language	English
Supervisor	Leah MOORE (Supervisor) & Ken Mcqueen (Supervisor)