Physical and hydraulic characterisation of aquifer and vadose zone media

Abstract

Physical and hydraulic properties of aquifer and vadose zone materials are generally poorly understood due to a variety of reasons including inaccessibility, spatial variability, inadequate techniques, and the lack of resources generally available to assess the subsurface characteristics. This thesis focuses on the use of four case studies to characterise and estimate physical and hydraulic properties of both saturated and unsaturated materials in two New Zealand subsurface environments. Investigations into the properties of the alluvial gravel vadose zone led to the following major conclusions regarding the structure and dynamics of the system; 1) There are 3 major textures within the profile (sandy gravel, pure sand lenses and open framework gravels [OFG'sJ), of which sandy gravel material comprises ~ 80-90% of the profile with the remainder comprising pure sand lenses and OFG's. Estimates of saturated and unsaturated hydraulic properties were determined for each unit; 2) Water content varied spatially with textural changes and temporally with surface flux dynamics reflecting a non-steady state system within the vadose zone; 3) Fine silt/clay layers were observed at the lower and sometimes upper edges of the OFG's, more research is needed to investigate the effect of this material on flow and saturation dynamics and potential denitrification micro-sites. Leaching experiments indicated nonequi1ibrium type flow in the alluvial gravel vadose zone, with significant solute fluxes to depth occurring soon after the application of bromide, followed by a long tail. Effective model parameters were derived from two dual permeability models by inverse methods to represent average transport processes through the heterogeneous profiles. The 'management type' Mixing Cell model (MC) results were comparable to the more complex HYDRUS model (H1DDP), providing similar overall fits to the observed data (similar RMSE ~0.05-0.1) and similar representation of cumulative fluxes. Modelling indicated that macropore flux was significant in transporting solute through the profile (MC range 5-59%; HIDDP range 15-54% of total fluxes). The Mixing Cell model results indicated macropore transporting water contents of ~0.003 (v/v) and matrix domain transporting water contents of around 0.1-0.15 (vIv) for the two field sites. A comparison of pumping tests and tracer tests in the alluvial system indicated pumping and tracer test derived hydraulic conductivity values can be reconciled in an alluvial iii aquifer when the structural heterogeneity of the aquifer is incorporated into the analysis. There was a 2-3 orders of magnitude difference in saturated hydraulic conductivity between the OFG and the sandy gravels and most flow and contaminant flux goes through the OFG even though they comprised only a small proportion (~ 1%) of the profile. In addition greater drawdown was observed along preferential flow paths (OFG's) indicating water extracted from the pumping wells was initially drawn preferentially from these flow paths. Macro and micro X-ray Computed Tomography (XCT) methods were successfully used to derive conceptual understanding, physical and hydraulic properties of a layered coastal sand aquifer on the Coromandel Peninsula. XCT analysis of the core material enabled a greater appreciation of the 3-D internal structure of the aquifer than could be obtained from visual core logging. Major conclusions drawn from the XCT characterisation include 1) XCT analysis enabled 3-D visualisation of heterogeneities at the core and micro/grain scale, and this contributed to greater conceptual understanding of the aquifer structure; 2) Structural and mineralogical properties including layer boundaries, organic matter (mangrove roots) distribution, and mineral (Ilmenite) assemblage distribution were clearly observable using the large and micro scale XCT and, 3) Physical and hydraulic parameters including permeability, porosity, grain shape, pore/throat/grain size distributions and sorting were successfully measured and calculated for the aquifer material using the micro-XCT interpretation tools.