Low-lying coastal plains comprised of unconsolidated infill are internally complex
hydrogeological settings, due to the high level of heterogeneity in the infill material.
In order to resolve the hydrogeological processes active in these complex settings, an
integrated multi-disciplinary, geoscientific approach is required. This research
determines quantitatively, the effects of sedimentary aquifer heterogeneity on
groundwater flowpaths and groundwater processes within a heavily laterised, coastal
plain setting. The study site is the Bells Creek catchment in southeast Queensland,
Australia. The methodology developed in this study provides a new approach to
enable the determination of groundwater flowpaths and groundwater processes at
macroscale resolution within other shallow alluvial and coastal plain aquifers. The
multi-disciplinary approach utilises sedimentological, geophysical, chronological and
hydrogeological techniques (including hydrochemistry and groundwater flow
modelling) to develop a high-resolution aquifer framework, and to determine
accurately, both groundwater flowpaths and relative flow rates.
Sedimentary framework is confirmed to be the principal factor controlling the
distribution of aquifer permeability pathways in any given setting, and is therefore,
the dominant control over groundwater flow and processes. For the Bells Creek
catchment, interpretation of stratigraphic and sedimentary data allowed the
compilation of a detailed sedimentary framework. This interpretation demonstrated
that weathering of the low-lying arkose sandstone bedrock has developed thick
lateritic profiles. Within the weathering profiles, cemented, iron-rich horizons have
resisted erosion and developed raised and elongated ridges in the modern landscape,
while other clay-rich weathered layers have submitted to erosion and downgraded
around those iron-rich ridges. Consequently, alluvial deposition throughout the Late
Quaternary has been restricted to narrow, and relatively deep valleys containing sandrich
channels, and thin floodplains at shallow depth.
From a hydrogeological perspective, there is significant macroscopic aquifer
heterogeneity between fine-grained lateritic mixed clay layers, floodplain clays, ironcemented
ferricrete horizons, and permeable sand-rich alluvial aquifers. This
variability of aquifer material has created a complex subsurface arrangement of
permeability pathways. Application of Ground Penetrating Radar (GPR) in this setting enabled accurate definition of alluvial channel boundaries and the high degree
of connectedness within the channels themselves. Interpretation of a comprehensive
GPR dataset (that covered the entire catchment) allowed refinement of the
sedimentary framework previously established to develop a detailed threedimensional
aquifer framework.
Finite-difference groundwater modelling and particle tracking analysis (using
MODFLOW and MODPATH) has clearly demonstrated that the macroscopic
heterogeneity within the various aquifer materials of the plain has marked impacts on
groundwater pathways, and especially groundwater travel times. The variability
between a maximum residence time of 18 months for groundwater within the
alluvium, compared to hundreds of years for groundwater within the mixed clay
layers of the laterite, clearly demonstrates the importance of accurately defining the
spatial distribution of the various aquifer materials in a groundwater flow
investigation. In this setting, the interconnection of the narrow alluvial channels
feeding into a deeper alluvial delta has provided an effective conduit for shallow
groundwater flow. The role of the alluvial delta in discharging the bulk of fresh
groundwater from the central plain into the coastal and estuarine aquifers to the east,
is certainly critical in preventing saline intrusion from encroaching further west.
Hydrochemical and isotopic indicators have identified the dominant recharge
processes and groundwater flowpaths within the plain, and indicated that the
processes are strongly related to sub-surface permeability distributions determined in
the aquifer framework (and groundwater modelling), as well as seasonal fluctuations
in rainfall. In the northwest of the plain, sandstone hills provide a delayed and
slightly mineralized component of groundwater recharge into adjacent highly
permeable, unconfined alluvial aquifers; these aquifers also recharge directly via
precipitation. Aluminosilicate weathering in the bedrock hills and eastern peripheries
of the laterised bedrock are a source of excess Na, SiO2, and HCO3 to the alluvial
groundwater. As this groundwater flows down-gradient to the east, however, its
chemical composition evolves by sulfate reduction, silica equilibrium and ion
exchange processes into a more mature Na-Cl type.
Within the shallow coastal aquifers proximal to the eastern shoreline, sulfate
enrichment is occurring (associated with increases in Ca, HCO3, Fe and Al) resulting
in major deterioration in groundwater quality. The deterioration is produced by saline
intrusion from the adjacent estuary coupled with oxidation of sulfide materials in
shallow marine and estuarine clays. Reverses in salinity in those coastal aquifers have
been correlated with surges in fresh recharge waters from unconfined coastal dunes
and semi-confined landward alluvium, following significant rainfall events.
The multi-disciplinary methodology developed, provides an effective approach for
accurately defining the three-dimensional distribution of shallow aquifer material of
varying permeability via detailed stratigraphic interpretation and GPR analysis.
Utilising this aquifer framework, finite-difference groundwater modelling aided by
hydrogeological data and hydrochemical analysis, allows accurate determination of
groundwater flowpaths and groundwater processes. This research provides a new
hydrogeological analogue for alluvial channel aquifers within a laterised coastal plain
setting.
Key Words:
groundwater flow, aquifer heterogeneity, numerical modelling, hydrochemistry,
recharge, ground penetrating radar, coastal plain aquifers, weathering, alluvial
channels
