Using Geochemical Indicators to Assess the Vulnerability of Springs to Mining
Creators: name: Campbell, Angus
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 401101
subject: Air pollution modelling and control
classificationCode: 370701
subject: Contaminant hydrology

description: Springs and groundwater-fed streams sustain some of the most significant groundwater dependent
ecosystems (GDEs) worldwide. Currently, many of these ecosystems are in decline, or are at risk of extinction,
due to groundwater extraction for agriculture and mining. To accurately assess the vulnerability of a GDE to
drawdown and manage groundwater usage in mining sustainably, a clear conceptual understanding of the
ecosystems’ hydrology and its associated hydrogeological system is critical.
 
In north-eastern Australia, contention has surrounded the development of Australia’s largest coal mine, the
Carmichael Coal Mine, and its potential environmental impacts. A key issue is whether mine dewatering will
impact groundwater flows to the ecologically and culturally significant Doongmabulla Springs Complex and
the Carmichael River. Uncertainty regarding: 1. the source(s) of water that sustain the Doongmabulla springs,
2. exchanges between the Carmichael River and groundwater and 3. the extent of inter-aquifer connectivity,
have prevented clear assessments of the GDEs’ vulnerability to damage or extinction as a result of mining. This
PhD study aimed to resolve these three key conceptual uncertainties and re-assess the vulnerability of the
Doongmabulla springs and Carmichael River to mining impact. This research was carried out using a suite of
environmental isotope tracers (2H, 18O, 3H, 14C, 36Cl, and 87Sr/86Sr) and a diverse range of geochemical,
geophysical and hydrogeological datasets.

Isotope tracers indicate that an array of different groundwater sources with different residence times sustain
the various springs within the Doongmabulla complex. Springs in the west of the complex are predominantly
sustained by very old groundwater from the Early-Mid Pleistocene (residence time &gt;500ka), while
groundwater recharged during the Holocene (residence time &lt;4ka) and Late Pleistocene (residence time 50k
– 150ka) is the main water source of the eastern springs. These data suggest the Clematis Sandstone is the
predominant source aquifer of the springs. However, Holocene groundwater critical to the eastern springs
also transitions through the (younger) Cenozoic sediments and (older) Dunda Beds, in areas near the mine
which are likely to experience significant drawdown. Deeper units (e.g. the Dunda Beds) also cannot be ruled
out as the source of very old groundwater in the western springs. This revised conceptualisation contradicts
the conceptual model of the springs that was used to predict the impacts of the Carmichael mine, that
assumed that all springs are sustained solely by the Clematis Sandstone. It has major implications for the
ecosystems’ vulnerability to drawdown.

Isotope tracers and major ion concentrations in the Carmichael River indicate that spring discharge and influx
of very old (residence time &gt;500ka) and intermediate (residence time &gt;50ka) groundwater sustains baseflow to the river near to the Doongmabulla springs. Further upstream, surface runoff and shallow groundwater are
the main water sources, while no regional groundwater influx to the river appears to occur downstream of
the springs within the mine lease. During major flood events, groundwater levels and chloride concentrations
adjacent to the stream indicate river losses to groundwater occur near the Doongmabulla springs. However,
downstream near the Carmichael mine, river leakage after heavy rainfall is much less significant. These
conclusions indicate that the numerical models used to predict the Carmichael mine’s groundwater impacts,
which assumed perennial streamflow, may have overestimated baseflow in the Carmichael River, and the
potential for river leakage to buffer drawdown in shallow aquifers below and adjacent to the stream channel.

Within the mining lease, aerial electromagnetic surveys and geological logs revealed weathered sandy zones
of substantially increased permeability (relative to un-weathered material) occur within several stratigraphic
layers, and are common in large areas to the south of the current mine pit. These weathered sediments contain
fresh, relatively young groundwater (residence times of hundreds to thousands of years) and are likely to be
preferential pathways for recharge, groundwater flow and inter-aquifer mixing. Since mine dewatering began
in 2019, drawdown has migrated preferentially south of the mine through these high permeability zones, and
into shallow units above the coal-bearing sediments that are not targeted by dewatering. Drawdown is also
migrating gradually into shallow aquifer units west of the mine, towards the Doongmabulla springs and
Carmichael River. These findings indicate that inter-aquifer connectivity and geological heterogeneity is
significantly greater than was assumed in the Carmichael mine’s impact assessment, and shallow aquifers
which sustain the springs, the Carmichael River and pastoral groundwater resources are more vulnerable to
drawdown than has been predicted.

These revised conceptualisations of the Doongmabulla springs, Carmichael River and inter-aquifer
connectivity indicate that dewatering for the Carmichael Coal Mine is likely to cause greater impacts to GDEs
and pastoral groundwater resources than was anticipated in the mine’s approved impact assessment.
Groundwater flow paths near the mine that sustain the eastern springs are likely to be diverted away from
the springs. The current extent of drawdown and evidence of inter-aquifer connectivity also indicates that
drawdown in the aquifers which sustain the Doongmabulla springs, Carmichael River and pastoral
groundwater resources will be greater, occur sooner and migrate further than was originally anticipated. The
relatively sporadic, spatially variable leakage of surface water from the Carmichael River into the underlying
aquifer units is unlikely to buffer drawdown impacts along the river corridor (including beneath the springs),
as was conceptualised in the mine’s impact assessment. These impacts to the hydrogeological system will
likely cause reductions in flow and/or the extinction of these significant GDEs.

This study is one of the first globally to critically re-evaluate the conceptual hydrogeological model
underpinning the environmental impact predictions of a major mining project, using multiple sets of direct
observational data (isotope tracers, geochemistry, geophysics and geology). Its findings have significant management implications for the Carmichael mine and can be used to ensure the protection of the
Doongmabulla springs and Carmichael River. This PhD also demonstrates the value of isotopic and
geochemical datasets for developing accurate conceptual hydrogeological models, which in turn is required
for robust prediction of the impacts of significant groundwater extraction. It presents a widely applicable
methodology, that can be used across industry and the research community to better understand and protect
GDEs vulnerable to groundwater extraction in other contexts.