R. Ian Acworth

Field Measurements of Beachface Salinity Structure using Cross-Borehole Resistivity Imaging

Abstract The dynamics of groundwater within beaches has implications for sediment transport in the swash zone and mass transport across the land-ocean boundary. Groundwater forcing by nearshore processes (e.g., wave runup, setup and ocean tides), and regional and local aquifer flows, result in the complex exchange of fresh and saline waters across the beachface boundary. The new application of a cross-borehole resistivity imaging technique was used to observe the salinity structure along a 20 m shore-normal transect within the upper 3 m of an open-coast beachface. Several intriguing (and unanticipated) observations were made. These include the horizontal layering of saline and fresh pore-waters, the presence of salty water overlying fresh water, and evidence of significant advective/dispersive mixing, within a groundwater environment where the prevailing hydraulic gradients did not suggest this to be the case. An advantage of the resistivity imaging technique over direct point sampling with spear points to extract pore-water, is that a dense and fully 2-D profile of the subsurface salinity structure can be mapped. The cross-borehole technique is intrusive in the sense that some disturbance is caused during installation, but this occurs only in the immediate vicinity of the vertical electrode strings, and post-processing is effective to remove the influence of these data. A high correlation achieved between measured bulk resistivity and fluid conductivity confirmed that the results of resistivity imaging can be used to calculate the pore-water EC (salinity), and hence the relative contributions from the adjacent freshwater aquifer and ocean sources. Though not attempted at the deployment described here, the burial of all electrodes also makes the technique suitable for examining the variation through time of saline-fresh pore-waters within the beachface.

River‐aquifer interactions in a semiarid environment investigated using point and reach measurements

A critical hydrological process is the interaction between rivers and aquifers. However, accurately determining this interaction from one method alone is difficult. At a point, the water exchange in the riverbed can be determined using temperature variations over depth. Over the river reach, differential gauging can be used to determine averaged losses or gains. This study combines these two methods and applies them to a 34 km reach of a semiarid river in eastern Australia under highly transient conditions. It is found that high and low river flows translate into high and low riverbed Darcy fluxes, and that these are strongly losing during high flows, and only slightly losing or gaining for low flows. The spatial variability in riverbed Darcy fluxes may be explained by riverbed heterogeneity, with higher variability at greater spatial scales. Although the river-aquifer gradient is the main driver of riverbed Darcy flux at high flows, considerable uncertainty in both the flux magnitude and direction estimates were found during low flows. The reach-scale results demonstrate that high-flow events account for 64% of the reach loss (or 43% if overbank events are excluded) despite occurring only 11% of the time. By examining the relationship between total flow volume, river stage and duration for in-channel flows, we find the loss ratio (flow loss/total flow) can be greater for smaller flows than larger flows with similar duration. Implications of the study for the modeling and management of connected water resources are also discussed. Key Points Losing riverbed fluxes under high flows and approximately neutral under low flows Event driven riverbed fluxes dominate reach losses Smaller events can have higher loss ratio than larger events

Detection of subsurface soil cracks by vertical anisotropy profiles of apparent electrical resistivity

To date, an understanding of crack dynamics has been fundamentally hampered by the lack of available techniques to observe or monitor crack dynamics below the soil surface. A new technique relates the growth of soil cracks to a progressive increase in the electrical anisotropy of the soil. Although a single measurement of anisotropy is possible using a surface array of electrodes, the use of four strings of electrodes installed vertically at the corners of a square provides a valuable picture of the crack pattern at depth. In addition, time-lapse electrical surveys allow the growth of cracks to be clearly monitored. The electrical anisotropy is defined as the ratio of the α -to- β apparent resistivity for the square array and is determined for each coplanar set of four electrodesusing one electrode from each of the four vertical strings. In a laboratory, we measured the electrical anisotropy in a sand-filled lysimeter with a plastic sheet, introduced to represent an electrically insulating crack. Measurem...

A New Method for Estimating Recharge to Unconfined Aquifers Using Differential River Gauging

In semiarid and arid environments, leakage from rivers is a major source of recharge to underlying unconfined aquifers. Differential river gauging is widely used to estimate the recharge. However, the methods commonly applied are limited in that the temporal resolution is event-scale or longer. In this paper, a novel method is presented for quantifying both the total recharge volume for an event, and variation in recharge rate during an event from hydrographs recorded at the upstream and downstream ends of a river reach. The proposed method is applied to river hydrographs to illustrate the method steps and investigate recharge processes occurring in a sub-catchment of the Murray Darling Basin (Australia). Interestingly, although it is the large flood events which are commonly assumed to be the main source of recharge to an aquifer, our analysis revealed that the smaller flow events were more important in providing recharge.

An objective frequency domain method for quantifying confined aquifer compressible storage using Earth and atmospheric tides

The groundwater hydraulic head response to the worldwide and ubiquitous atmospheric tide at 2 cycles per day (cpd) is a direct function of confined aquifer compressible storage. The ratio of the responses of hydraulic head to the atmospheric pressure change is a measure of aquifer barometric efficiency, from which formation compressibility and aquifer specific storage can be determined in situ rather than resorting to laboratory or aquifer pumping tests. The Earth tide also impacts the hydraulic head response at the same frequency, and a method is developed here to quantify and remove this interference. As a result, the barometric efficiency can be routinely calculated from 6-hourly hydraulic head, atmospheric pressure, and modeled Earth tide records where available for a minimum of 15 days duration. This new approach will be of critical importance in assessing worldwide problems of land subsidence or groundwater resource evaluation that both occur due to groundwater abstraction

Vertical groundwater storage properties and changes in confinement determined using hydraulic head response to atmospheric tides

Accurate determination of groundwater state of confinement and compressible storage properties at vertical resolution over depth is notoriously difficult. We use the hydraulic head response to atmospheric tides at 2 cpd frequency as a tracer to quantify barometric efficiency (BE) and specific storage (Ss) over depth. Records of synthesized Earth tides, atmospheric pressure, and hydraulic heads measured in nine piezometers completed at depths between 5 and 55 m into unconsolidated smectitic clay and silt, sand and gravel were examined in the frequency domain. The barometric efficiency increased over depth from ∼0.05 in silty clay to ∼0.15 in sands and gravels. BE for silty clay was confirmed by calculating the loading efficiency as 0.95 using rainfall at the surface. Specific storage was calculated using effective rather than total moisture. The differences in phase between atmospheric pressure and hydraulic heads at 2 cpd were ∼180° below 10 m indicating confined conditions despite the low BE. Heads in the sediment above a fine sand and silt layer at 12 m exhibited a time variable phase difference between 0° and 180° indicating varying confinement. Our results illustrate that the atmospheric tide at 2 cpd is a powerful natural tracer for quantifying groundwater state of confinement and compressible storage properties in layered formations from hydraulic heads and atmospheric pressure records without the need for externally induced hydraulic stress. This approach could significantly improve the development of conceptual hydrogeological model used for groundwater resource development and management.

The influence of syndepositional macropores on the hydraulic integrity of thick alluvial clay aquitards

Clay‐rich deposits are commonly assumed to be aquitards which act as natural hydraulic barriers due to their low hydraulic connectivity. Postdepositional weathering processes are known to increase the permeability of aquitards in the near surface but not impact on deeper parts of relatively thick formations. However, syndepositional processes affecting the hydraulic properties of aquitards have previously received little attention in the literature. Here, we analyze a 31 m deep sediment core recovered from an inland clay‐rich sedimentary sequence using a combination of techniques including particle size distribution and microscopy, centrifuge dye tracer testing and micro X‐ray CT imaging. Subaerial deposition of soils within these fine grained alluvial deposits has led to the preservation of considerable macropores (root channels or animal burrows). Connected pores and macropores thus account for vertical hydraulic conductivity (K) of urn:x-wiley:00431397:media:wrcr23221:wrcr23221-math-0001 m/s (geometric mean of 13 samples) throughout the thick aquitard, compared to a matrix K that is likely < urn:x-wiley:00431397:media:wrcr23221:wrcr23221-math-0002 m/s, the minimum K value that was measured. Our testing demonstrates that such syndepositional features may compromise the hydraulic integrity of what otherwise appears to have the characteristics of a much lower permeability aquitard. Heterogeneity within a clay‐rich matrix could also enhance vertical connectivity, as indicated by digital analysis of pore morphology in CT images. We highlight that the paleo‐environment under which the sediment was deposited must be considered when aquitards are investigated as potential natural hydraulic barriers and illustrate the value of combining multiple investigation techniques for characterizing clay‐rich deposits.

Quantifying compressible groundwater storage by combining cross-hole seismic surveys and head response to atmospheric tides

Groundwater specific storage varies by orders of magnitude, is difficult to quantify, and prone to significant uncertainty. Estimating specific storage using aquifer testing is hampered by the nonuniqueness in the inversion of head data and the assumptions of the underlying conceptual model. We revisit confined poroelastic theory and reveal that the uniaxial specific storage can be calculated mainly from undrained poroelastic properties, namely, uniaxial bulk modulus, loading efficiency, and the Biot‐Willis coefficient. In addition, literature estimates of the solid grain compressibility enables quantification of subsurface poroelastic parameters using field techniques such as cross‐hole seismic surveys and loading efficiency from the groundwater responses to atmospheric tides. We quantify and compare specific storage depth profiles for two field sites, one with deep aeolian sands and another with smectitic clays. Our new results require bulk density and agree well when compared to previous approaches that rely on porosity estimates. While water in clays responds to stress, detailed sediment characterization from a core illustrates that the majority of water is adsorbed onto minerals leaving only a small fraction free to drain. This, in conjunction with a thorough analysis using our new method, demonstrates that specific storage has a physical upper limit of urn:x-wiley:jgrf:media:jgrf20879:jgrf20879-math-0001 m−1. Consequently, if larger values are derived using aquifer hydraulic testing, then the conceptual model that has been used needs reappraisal. Our method can be used to improve confined groundwater storage estimates and refine the conceptual models used to interpret hydraulic aquifer tests.