Using Hydraulic Head Chloride And Electrical Conductivity Data To

using hydraulic head, chloride and electrical conductivity data to distinguish between mountain-front and mountain-block recharge to basin aq

Using hydraulic head, chloride and electrical conductivity data to
distinguish between mountain-front and mountain-block recharge to
basin aquifers
Etienne Bresciani1,2, Roger. H. Cranswick1,3, Eddie W. Banks1, Jordi
Batlle-Aguilar1,4, Peter G. Cook1, Okke Batelaan1
1National Centre for Groundwater Research and Training, School of the
Environment, Flinders University, Adelaide, SA 5001, Australia
2Korea Institute of Science and Technology, Seoul, 02792, Republic of
Korea
3Department of Environment, Water and Natural Resources, Government of
South Australia, Adelaide, SA 5000, Australia
4Kansas Geological Survey, University of Kansas, Lawrence, KS 66047,
USA
Correspondence to: Etienne Bresciani
([email protected])
Abstract. Numerous basin aquifers in arid and semi-arid regions of the
world derive a significant portion of their recharge from adjacent
mountains. Such recharge can effectively occur through either stream
infiltration in the mountain front zone (mountain-front recharge, MFR)
or subsurface flow from the mountain (mountain-block recharge, MBR).
While a thorough understanding of recharge mechanisms is critical for
conceptualizing and managing groundwater systems, distinguishing
between MFR and MBR is difficult. We present an approach that uses
hydraulic head, chloride and electrical conductivity (EC) data to
distinguish between MFR and MBR. These variables are inexpensive to
measure, and may be readily available from hydrogeological databases
in many cases. Hydraulic heads can inform on groundwater flow
directions and stream-aquifer interactions, while chloride
concentrations and EC values can be used to distinguish between
different water sources if these have a distinct signature. Such
information can provide evidence for the occurrence or absence of MFR
and MBR. This approach is tested through application to the Adelaide
Plains basin, South Australia. The recharge mechanisms of this basin
have long been debated, in part due to difficulties in understanding
the hydraulic role of faults. Both hydraulic head and chloride
(equivalently, EC) data consistently suggest that streams are gaining
in the adjacent Mount Lofty Ranges and losing when entering the basin.
Moreover, the data indicate that not only the Quaternary aquifers but
also the deeper Tertiary aquifers are recharged through MFR and not
MBR. It is expected that this finding will have a significant impact
on water resources management in the region. This study demonstrates
the relevance of using hydraulic head, chloride and EC data to
distinguish between MFR and MBR.
1 Introduction
==============
Numerous basin aquifers in arid and semi-arid regions receive a
significant portion of their recharge from adjacent mountains, largely
because the latter typically benefit from higher precipitation and
lower evapotranspiration (Winograd et al., 1998; Wilson and Guan,
2004; Earman et al., 2006). Two recharge mechanisms can be recognized
(Wahi et al., 2008): mountain-front recharge (MFR), which
predominantly consists of stream infiltration in the mountain front
zone, and mountain-block recharge (MBR), which consists of subsurface
flow from the mountain towards the basin. Here the mountain front zone
is defined after Wilson and Guan (2004) as the upper zone of the
basin, between the basin floor and the mountain block (Figure 1a). The
term MFR has traditionally been used to encompass the two recharge
mechanisms described above, but it may be more appropriate to use it
for the first one only. Following Wahi et al. (2008), the collective
process of MFR and MBR is referred to as mountain system recharge
(MSR).
The distinction between MFR and MBR is important. The
conceptualization of a basin groundwater system critically depends on
whether recharge occurs through MFR or MBR, as each of these
mechanisms implies different groundwater flow paths, groundwater age
and geochemical characteristics. MFR and MBR can also imply different
responses to land and water resource management practices (both in the
basin and the mountain) as well as to climate change. A good
understanding of these mechanisms is thus essential for an effective
coordinated management approach of water resources in basins and
adjacent mountains (Manning and Solomon, 2003; Wilson and Guan,
2004).
While various methods exist to estimate MSR as a bulk, characterizing
the individual contributions of MFR and MBR is difficult. For
instance, Darcy’s law calculations and inverse groundwater flow
modelling typically provide bulk MSR estimates (e.g., Hely et al.,
1971; Anderson, 1972; Maurer and Berger, 1997; Siade et al., 2015).
It is possible to consider MFR and MBR independently in a groundwater
flow model, but the solution to the inverse problem is more likely to
be non-unique (e.g., Bresciani et al., 2015b). The water balance and
chloride mass balance methods also provide bulk MSR estimates when the
analysis is performed at the base of the mountain front zone or
further downstream in the basin (e.g., Maxey and Eakin, 1949;
Dettinger, 1989). Environmental tracers such as noble gases (e.g.,
Manning and Solomon, 2003), stable isotopes (e.g., Liu and Yamanaka,
2012) and radioactive isotopes (e.g., Plummer et al., 2004) can
help to determine which of MFR or MBR is the dominant mechanism, but
their analysis remains expensive and their interpretation can be
difficult. The most robust approach for characterizing MFR and MBR
might be the integrated analysis of all available hydraulic,
temperature and concentration data through the coupled modelling of
groundwater flow, heat and solute transport in the combined
basin-mountain system (e.g., Manning and Solomon, 2005) – but it is
also arguably the most complex approach.
In this study, we explore alternatives to expensive and complex
methods to investigate whether MSR to basin aquifers is dominated by
MFR (Figure 1b) or MBR (Figure 1c), or if both recharge mechanisms are
significant (Figure 1d). We focus on the use of hydraulic head,
chloride (Cl-) and electrical conductivity (EC), which are inexpensive
to measure and may be readily available from hydrogeological databases
in many cases. The general utility of hydraulic head and Cl- data to
infer groundwater dynamics is well established (e.g., Domenico and
Schwartz, 1997; Herczeg and Edmunds, 2000). Furthermore, EC values
can be converted to Cl- concentrations (as demonstrated later), and
hence can be used in a similar manner to Cl-. However, studies
demonstrating the specific use of these data for the characterization
of MSR mechanisms appear to be rare (Feth et al., 1966). This may
reflect a traditionally low data density along mountain fronts, which
are not generally the prime locations for drilling groundwater wells
due to an expected lower yield than elsewhere in the basin (aquifer
thickness may typically be smaller, and prevailing recharge conditions
are not favourable to well yield).
After presenting a general rationale for the use of hydraulic head, Cl-
and EC data to distinguish between MFR and MBR, the Adelaide Plains
basin in South Australia (Figure 2) is used as a case study to test
the relevance of the approach. This semi-arid region features a
sedimentary basin bounded by a mountain range – the Mount Lofty
Ranges, from which most of the recharge is believed to be derived
(Miles, 1952; Shepherd, 1975; Gerges, 1999; Bresciani et al., 2015a).
Groundwater in this basin has been used for over a century for
industry, water supply and agriculture. Nonetheless, and despite
several recent studies, the relative contributions of MFR and MBR is
still subject to debate. In particular, the hydraulic role of faults
(i.e., acting as barrier or conduit to flow) that run along the
mountain front remains unclear (Green et al., 2010; Bresciani et al.,
2015a; Batlle-Aguilar et al., 2017). As a result of the common use of
groundwater, a relatively high density of wells exists in the region,
including in the mountain front zone. Therefore, this case study
provides a good example of the potential of the proposed approach.
2 Rationale
===========
In this section, a generic rationale is presented for the use of
hydraulic head and Cl- (or EC-derived Cl-) data to distinguish between
MFR and MBR to basin aquifers. Hydraulic head and Cl- data can be used
independently, but as they are of different nature, it is expected
that their simultaneous use will result in a more complete and
reliable characterization of the recharge mechanisms.
2.1 Using hydraulic head
------------------------
Hydraulic heads directly relate to groundwater dynamics. Consequently,
hydraulic head patterns could theoretically enable the identification
of groundwater flow paths, both in mountains and basins. Specifically,
four types of analysis are suggested that could inform the likely
occurrence or absence of MFR and MBR:
1.
Assessment of the correlation between hydraulic head and
topography. In the mountain block, a good correlation would
suggest that groundwater flow is dominated by local flow systems
as opposed to regional flow systems (Tóth, 1963). This would
imply that only a small portion of the recharge occurring over the
mountain would make its way towards the basin. In fact, in this
case MBR would be mostly limited to the recharge occurring over
triangular facets in between stream catchments at the base of the
mountain block (Figure 3). Here, the recharge is less likely to be
routed towards mountain streams, and instead it may be routed
towards the basin (Welch and Allen, 2012). In the mountain front
zone, a good correlation between hydraulic head and topography
would suggest that groundwater discharges to streams, so that MFR
from stream leakage would be limited or non-existent.
2.
Analysis of the shape of head contours adjacent to surface water
features to identify losing and gaining stream conditions. It is
well known that head contours show a curvature pointing in the
downstream direction where the contour lines cross a losing stream
(due to the mounding induced by groundwater recharge) (Figure 4a),
whereas they show a curvature pointing in the upstream direction
where the contour lines cross a gaining stream (due to the
depression induced by groundwater discharge) (Figure 4b) (Winter
et al., 1998). Performing such analysis in the mountain block
should indicate whether mountain groundwater appears mostly routed
towards local streams, which would make it less likely for MBR to
be significant. Additionally, performing such analysis in the
mountain front zone should allow for testing the occurrence or
absence of MFR (at least in the form of stream infiltration, which
is the predominant form of MFR (Wilson and Guan, 2004)).
3.
Comparison of stream levels with nearby groundwater levels. A
stream level higher than nearby groundwater levels would indicate
a potential for stream infiltration, while the opposite would
indicate a potential for groundwater discharge to stream. If the
data density is low, this analysis may be preferable over the
previous one (#2) as it does not require head contours to be
accurately determined. However, it can only inform on a potential
interaction: groundwater discharge or recharge would be
significant only if the hydraulic conductivity of the streambed is
high enough. In contrast, the previous analysis (#2) could give a
more definite answer, because the curvature of head contours at
some distance from the stream should only be visible if the
groundwater-surface water interaction is significant relative to
other flow components (i.e., horizontal flow).
4.
Evaluation of the vertical head gradient in the mountain front
zone. Recharge areas are associated with a decrease of hydraulic
head with depth, while discharge areas are associated with an
increase of hydraulic head with depth (e.g., Wang et al., 2015).
Hence, in the mountain front zone, a head decrease with depth
would suggest that MFR occurs (at a rate that depends on the
vertical hydraulic conductivity of the aquifer). In contrast, an
absence of head decrease (or a head increase) with depth would
suggest that MFR does not occur.
In cases where faults run between the basin and the mountain, it may
be tempting to study the difference in hydraulic head between the two
sides of the fault zones. Intuitively, a large head difference would
indicate that a fault zone constitutes a barrier to flow in the
direction perpendicular to it (e.g., Bense et al., 2013), and
consequently that MBR would be low. However, a large head difference
across a fault zone may not always imply that the fault zone
constitutes a hydraulic barrier. Let us consider the hypothetical case
of a sedimentary layer overlying a basement of relatively low
hydraulic conductivity and that features a sharp transition in
elevation as a consequence of faulting (Figure 5a). The hydraulic
conductivity of the fault zone itself is not different from that of
the embedding materials (i.e., the fault only implies a difference in
basement elevation). In this simple configuration, if the groundwater
level right below the fault is lower than the basement elevation above
of the fault – as a result of downstream hydraulic controls, the
groundwater level above the fault is essentially ‘disconnected’ from
the lower part (Figure 5b). This is because in all cases, the
groundwater level above the fault has to satisfy a minimum height
(i.e., transmissivity) for groundwater to flow there. Hence, this
example demonstrates that a large difference in head can exist across
the fault zone despite the fact that the fault zone itself has no
specific (low) hydraulic conductivity. It should also be noted that
regardless of the cause, the implications of a large difference in
head in terms of the amount of flow eventually crossing the fault zone
is far from obvious, as it depends on the hydraulic conductivity of
the fault or the basement (which is in either case difficult to
determine). A more relevant analysis may be to investigate whether or
not the hydraulic head above of the fault will be so high (relative to
topography) as to imply local groundwater discharge to mountain
streams instead of lateral flow across the fault towards the basin. In
other words, what matters is the partitioning of the mountain
groundwater between these two pathways. This is precisely what the
first three types of analysis presented above should contribute to
determine.
2.2 Using chloride
------------------
Chloride (Cl-) is a naturally occurring ion in groundwater that is
generally considered as conservative in geochemical studies (Clark
and Fritz, 1997). If there is no removal or addition of Cl- in the
aquifer, and if the effects of dispersion (i.e., mixing of water from
different flow paths) can be neglected, the Cl- concentration will be
constant along each groundwater flow path (Bresciani et al., 2014).
Under such conditions, if the potential MFR and MBR sources have a
different Cl- concentration, Cl- could be an excellent tracer to
distinguish between these two recharge mechanisms.
In many cases, Cl- in groundwater primarily originates from
atmospheric deposition (Allison et al., 1994). The rate of
atmospheric deposition depends on a number of factors including
distance to the source (oceanic or terrestrial), elevation, terrain
aspect, slope, vegetation cover and climatic conditions (Hutton and
Leslie, 1958; Guan et al., 2010b; Bresciani et al., 2014). Other
potential sources of Cl‑ in groundwater include anthropogenic inputs
(e.g., salting of roads, irrigation, application of fertilizer,
leakage from septic/sewage systems) and dissolution of Cl-bearing
minerals. Cl- removal from solution is unlikely as Cl does not easily
adsorb onto clays or precipitate as mineral (Clark and Fritz, 1997).
The Cl- concentration in recharge water also depends on
evapotranspiration, which leaves Cl- in solution, implying its
enrichment (Eriksson and Khunakasem, 1969; Allison et al., 1994) (a
growing vegetation could in theory counter this effect since Cl is a
nutrient for plants (White and Broadley, 2001), but in practice the
uptake of Cl from soil by most plant species is insignificant
(Allison et al., 1994)). Thus, depending on how variable the above
controlling factors are, the Cl- concentration in mountain groundwater
– i.e., the potential MBR source – may show significant spatial and
temporal variability. On the other hand, the Cl- concentration in
stream water entering the basin – i.e., the potential MFR source –
strongly depends on the streamflow generation mechanisms. If the
mountain streams are supported by large proportions of overland flow
or interflow, the Cl- concentration in stream water entering the basin
will tend to be lower than that in mountain groundwater, because these
streamflow generation mechanisms imply relatively little
evapotranspiration. In contrast, if the mountain streams are mostly
supported by groundwater discharge, the Cl- concentration in stream
water entering the basin will tend to have an integrated value of the
mountain groundwater Cl- concentration. In conclusion, while no
general statement can be made, chances are that the potential MFR and
MBR sources have a distinct Cl- signature.
In this study, the proposed strategy consists of analysing three types
of water for Cl-: groundwater in the basin, groundwater in the
mountain, and stream water in the mountain front zone. Assuming
steady-state concentrations and conservative Cl-, groundwater in the
basin should have the same concentration as stream water in the
mountain front zone in the case of MFR (further assuming that
transpiration from plants after stream infiltration and potential
mixing with diffuse recharge are negligible), while it should have the
same concentration as groundwater in the mountain in the case of MBR.
As much as possible, the analysis should focus on comparing points
that are not too far apart from another and along presumed flow paths,
such as to reduce risks of misinterpretation caused by spatiotemporal
variability in Cl- concentration and dispersion. In particular, MBR
should be most reliably assessed by comparing the groundwater Cl-
concentration in the uppermost part of the basin to that in the
lowermost part of the mountain, along lines running perpendicular to
the mountain front.
Electrical conductivity (EC) values are known to be strongly
correlated to Cl- concentrations (Guan et al., 2010a). Therefore, EC
data can be converted to Cl- data if a relationship between the two
can be assumed. Typically, EC is more routinely measured than Cl-, and
thus this should significantly increase the dataset. Ideally, an
empirical relationship between EC and Cl- should be developed based on
available pair measurements in the study area.
3 Case study
============
3.1 Study area and background
-----------------------------
The Adelaide Plains (AP) basin is a coastal sedimentary embayment of
1,700 km2 in South Australia (Figure 2Error: Reference source not
found). The area is bounded by the Mount Lofty Ranges to the east and
south, by the Light River to the north, and by the Gulf Saint Vincent
to the west. It can be split into two sub-basins: the Central Adelaide
Plains (CAP) sub-basin south of Dry Creek, and the Northern Adelaide
Plains (NAP) sub-basin north of Dry Creek. The topographic gradient is
more pronounced in the CAP and adjacent mountains (regional slopes of
about 0.8 % and 7 %, respectively) than in the NAP and adjacent
mountains (regional slopes of about 0.3 % and 2.5 %, respectively).
Torrens River and Gawler River are the largest rivers in the CAP and
in the NAP, respectively. A number of streams run down from the Mount
Lofty Ranges, either feeding these rivers or flowing directly into the
ocean.
Precipitation is relatively low and potential evapotranspiration is
high in this semi-arid area. The average rainfall is 445 mm yr-1 (no
snowfall) and the annual average maximum daily temperature of 21.6 °C
at Adelaide Airport station located near the coast (station number
23034, 1970–2013; Australian Government, Bureau of Meteorology).
Direct recharge from rainfall in the basin is thus expected to be
relatively low. Instead, most of the recharge is believed to be
derived from the adjacent Mount Lofty Ranges. The latter receive an
average rainfall of 983 mm yr-1 and negligible snowfall (i.e., only
exceptionally and in insignificant quantities) at Mount Lofty Cleland
Conservation Park (station number 23810, 1970–2013; Australian
Government, Bureau of Meteorology), i.e., more than twice than that of
the basin. It also experiences cooler temperatures with an annual
average maximum daily temperature of 15.2 °C at Mount Lofty (station
number 23842, 1993–2007; Australian Government, Bureau of
Meteorology). The majority (77 %) of the rainfall in the Mount Lofty
Ranges occurs during autumn and winter (May–September) (station number
23810, 1970–2013), suggesting a strong seasonality of the recharge.
The basin comprises complex, spatially-dependent sequences of
Quaternary and Tertiary sedimentary deposits (Gerges, 1999). The
Quaternary sediments are dominated by fluvio-lacustrine clay
interbedded with sand and gravel. The Tertiary sediments are dominated
by sand, sandstone, limestone, chert, marl and shell remains
interbedded with clay. A number of faults dissect the basin. Among
these, the Eden-Burnside Fault and the Para Fault are of primary
interest in this study since they run along the foothill, almost at
the margin of the CAP and the NAP sub-basins, respectively (Figure
2Error: Reference source not found). The total thickness of the
sedimentary units increases sharply on the downthrown side of the
major faults (up to 400 m in places). The thickness of the Quaternary
sediments ranges from 0 to about 140 m across the basin (Figure 6a),
while that of the Tertiary sediments ranges from 0 to about 500 m
(Figure 6b). The Tertiary sediments are directly outcropping in the
northeast part of the CAP. The basement of the basin and the Mount
Lofty Ranges are mostly comprised of Proterozoic fractured rocks of
various lithologies including slate, phyllite, quartzite, limestone
and dolomite. Superficial sedimentary deposits (typically less than 20
m in thickness) also exist locally in the Mount Lofty Ranges.
Up to six semi-confined aquifers (named Q1 to Q6) are recognized
within the Quaternary sediments from the central to western side of
the basin (Gerges, 1999) (i.e., downstream of the mountain front
zone). These aquifers contain water of variable salinity with a median
value of around 1,300 mg L-1. The underlying Tertiary sediments are
generally subdivided into four aquifers (named T1 to T4) over a large
part of the basin. However, there is no clear hydrogeological
distinction between the various Tertiary sediments along most of the
mountain front in both sub-basins, and thus in this area they are
considered to form a single undifferentiated Tertiary aquifer
(Gerges, 1999; Zulfic et al., 2008; Baird, 2010). Simplified
cross-sections of the aquifers in the NAP and CAP sub-basins are shown
in Figure 7a and Figure 7b, respectively. Salinity is relatively low
in the upper aquifer (T1) with a median value of around 600 mg L-1,
slightly higher in the T2 aquifer with median values of around 1,000
mg L-1, and significantly higher in deeper aquifers with median values
of 8,400 mg L-1 and 40,000 mg L-1 in T3 and T4, respectively (but note
that very few data are available from the latter two aquifers).
Because they present large areas of good salinity and yield, the T1
and T2 aquifers have been used since 1914 for occasional water supply,
irrigation and industrial activities, and are currently the main
targets of groundwater extraction in the AP (Gerges, 1999; Zulfic et
al., 2008). Long-term, large cones of depression in both of these
aquifers and forecasted increases in groundwater demand raise concerns
about the sustainability of extraction in the coming years (Bresciani
et al., 2015a). Risks are related to both potential depletion of the
resource and rise in salinity from the migration of higher-salinity
groundwater, which could make groundwater unusable. To better assess
these risks, a thorough understanding of the recharge mechanisms to
these aquifers is necessary.
Early investigations suggested that the natural (i.e.,
pre-development) recharge to the Tertiary aquifers of the basin was
dominated by stream infiltration along the mountain front (i.e., MFR)
(Miles, 1952; Shepherd, 1975). In contrast, subsequent
investigations suggested that the natural recharge of the Tertiary
aquifers was dominated by subsurface flow from the Mount Lofty Ranges
(i.e., MBR) (Gerges, 1999, 2006). The latter conceptual model has
formed the basis of most investigations of the Tertiary aquifers since
its presentation and underpinned the development of a number of
groundwater flow and transport models of the basin aquifers (Jeuken,
2006a, b; Zulfic et al., 2008; Baird, 2010; Georgiou et al., 2011;
Bresciani et al., 2015b). However, studies from Green et al. (2010)
and Bresciani et al. (2015a) produced results supporting the
hypothesis that both MFR and MBR could be significant. To further
investigate this question, the present study provides a re-appraisal
of available hydraulic head, Cl- and EC data through application of
the above rationale.
3.2 Datasets
------------
3.2.1 Hydraulic head dataset
Hydraulic head data in the AP catchment (i.e., the area including both
the basin and contributing mountain areas based on surface topography)
were retrieved from the WaterConnect database
(www.waterconnect.sa.gov.au, Government of South Australia) on
04/11/2016. The collection dates span more than a century, the
earliest measurements being from 1906 and the latest from 2016. The
data were filtered out for unsuitable measurements such as
measurements taken during pumping, aquifer testing or drilling. After
filtering, 111,538 hydraulic head measurements from 9,561 wells were
obtained.
The data were subsequently split according to three aquifer groups:
the AP Quaternary aquifers, the AP Tertiary aquifers (‘AP’ in these
expressions will be omitted in the remaining text) and the Mount Lofty
Ranges aquifers. This grouping is relevant in view of the
hydrogeological characteristics of the system and the objective of the
study. In particular, we did not distinguish between the T1 and T2
aquifers (i.e., the two main aquifers of the AP basin) because, as
mentioned earlier, they are undifferentiated along most of the
mountain front. In the Mount Lofty Ranges, the presence of complex
fracture networks and high relief can induce the blurring of otherwise
depth-dependent hydraulic signals, and so splitting the data according
to depth in this area may not be very meaningful, while it would
reduce data density. The name of the aquifer into which the wells were
screened was informed in the database for about two thirds of the
wells (6,209). This allowed for assignment of these wells to one of
the above aquifer groups. For the remaining one third of wells, the
aquifer group for the wells located in the basin was determined by
comparing the well mid-screen elevation to the bottom elevation of the
Quaternary sediments and to the top elevation of the basement. The
largest number of wells was from the Quaternary aquifers (3,964),
followed by the Mount Lofty Ranges aquifers (3,589) and the Tertiary
aquifers (1,768). Wells screened into the basement of the basin were
disregarded (240 wells).
Groundwater level fluctuations can be an issue for data
interpretation. In particular, as this study focuses on natural
recharge mechanisms, the impact of pumping constitutes a potentially
important bias. It should be noted that the density of hydraulic head
data is higher in areas of lower groundwater salinity, which coincides
with areas that have experienced greater changes due to pumping. The
measurements made before the main development period (i.e., before
1950) may have been less affected by pumping than more recent
measurements, but limiting the analysis only to these measurements
would dramatically reduce the data density. In addition, even the
earliest measurements may not be free of pumping influence, since it
is likely that these were precisely taken to monitor the impact of
pumping. Hence, instead of subjectively fixing an arbitrary date
beyond which the data would be excluded, all data were retained
regardless of the measurement date. For each of the wells that had
multiple measurements, the temporal mean hydraulic head was calculated
in an effort to smooth out the measurement errors and temporal
fluctuations. The analysis focuses on these mean values.
3.2.2 Chloride dataset
Groundwater Cl- data in the AP catchment were also retrieved from the
WaterConnect database on 04/11/2016. This dataset was extended using
the more commonly available EC data from the database. A strong
relationship between EC and Cl- was found from 1,559 pair measurements
(R2 = 0.9996, Figure 8). In wells where only EC data were available,
EC values were converted into Cl- concentrations using this
relationship. In total, 34,145 Cl- or EC-converted Cl- data (simply
referred to as Cl- data) from 12,660 wells were obtained (i.e.,
slightly more than for hydraulic heads, partly due to a
less-restrictive filtering, i.e., keeping measurements taken during
pumping or aquifer testing). The collection dates span the same period
as for the hydraulic head data.
The Cl- data were subsequently split according to the same three
aquifer groups as indicate above for the hydraulic head data. The same
procedure was also applied to determine the aquifer group to which the
wells belong. The largest number of wells was from the Quaternary
aquifers (4,963), followed by the Mount Lofty Ranges aquifers (4,395)
and the Tertiary aquifers (2,963).
Pumping may also have impacted Cl- concentrations by inducing
migration of the original groundwater, whose concentration is
spatially variable (although this effect may be seen later than on
hydraulic heads since solutes travel times are typically longer than
pressure travel times). Also, as for hydraulic head data, the density
of Cl- data is higher in areas that have experienced pumping.
Additionally, in the uppermost aquifers, irrigation may have locally
influenced the Cl- concentration. Nonetheless, following the same
reasoning as for hydraulic heads, all available Cl- data were retained
regardless of the measurement date. For each of the wells that had
multiple measurements, the temporal mean Cl- concentration was
calculated. The analysis focuses on these values.
Flow rate and EC data from streams running down from the Mount Lofty
Ranges into the AP basin were also retrieved from the WaterConnect
database. Six gauging stations were located close enough to the
mountain front zone to be relevant to the current study. Details on
this dataset are given in Table 1. The reported EC values of surface
water were converted into Cl- concentrations using the same
relationship as developed for groundwater. This approach is deemed
appropriate given the common origin of these waters, even though
potentially different chemical reactions might slightly affect the
relationship.
3.3 Data analysis
-----------------
Given the relatively large area investigated, the analysis presented
below concentrates on two “focus areas” that cover the transition
between the Mount Lofty Ranges and the AP basin: one at the margin of
the NAP sub-basin and one at the margin of the CAP sub-basin
(locations indicated in Figure 2). Figures for the entire study area
are also available in Supplementary Material (Figures S1-S12). These
do not call for a different interpretation.
3.3.1 Hydraulic heads
Hydraulic head maps were constructed for the three aquifer groups
(Quaternary aquifers, Tertiary aquifers and Mount Lofty Ranges
aquifers) (Figure 9 and Figure 10). In constructing these maps, the
choice of the interpolation method and associated parameters revealed
to be critical. The classical Inverse Distance Weighting method would
produce the well-known ‘bull’s eye’ effect around individual data
points. This could severely compromise the interpretation of head
contours. Instead, the Diffusion Kernel interpolation method from the
Geostatistical Analyst extension of ArcGIS 10.4.1 was used. This
method allows for a more realistic interpolation when the underlying
phenomenon governing the data is diffusive, as is the case for
hydraulic heads. The most important parameter in this method is the
bandwidth, which is used to specify the maximum distance within which
data points are used for prediction. Taking this parameter too small
would undermine the prediction capability as many areas would remain
uncovered by the interpolation, while taking it too large would
produce overly-smoothed results. A good compromise was found by
setting this parameter to 1,200 m for the NAP focus area and to 800 m
for the CAP focus area – reflecting a higher density of streams and
data in the latter case. Topographic contours were also constructed.
To facilitate the comparison with head contours, these were created
after application of a circular moving-average window to the
topography using a radius that matches the bandwidth used in the
interpolation method for hydraulic head (i.e., 1,200 m in the NAP
focus area and 800 m in the CAP focus area).
Figure 9a displays hydraulic head and topographic contours in the NAP
focus area, showing the Quaternary aquifers on the basin side and the
Mount Lofty Ranges aquifers on the mountain side. The results are
quite contrasted between the mountain and the basin. In the mountain,
head contours follow the topographic contours relatively closely, and
their shape is most often indicative of gaining stream conditions.
Note that one should not expect to see sharp “V” shapes where head
contours cross streams (i.e., as in Figure 4) due to the limited data
density. Instead, head contours are smoothly curved. In the basin,
head contours do not closely follow the topographic contours, and
their shape is generally indicative of losing stream conditions
(especially close to the basin margin). Figure 9b also displays
hydraulic head and topographic contours in the NAP focus area, but
showing the Tertiary aquifers on the basin side instead of the
Quaternary aquifers. Head contours in the Tertiary aquifers are
generally indicative of focused recharge along streams, but at a
somewhat larger-scale, i.e., showing wider curvatures than in the
Quaternary aquifers (mostly around Gawler River and Little Para
River). Figure 9c and Figure 9d display analogous results for the CAP
focus area. Similarly to above, in the mountain, head contours are
relatively well correlated with topographic contours and their shape
is generally indicative of gaining conditions. In the basin, head
contours in the Quaternary aquifers are not very distinct from
topographic contours, but nevertheless tend to indicate losing rather
than gaining stream conditions close to the basin margin (Figure 9c).
In the Tertiary aquifers, head contours are quite distinct from
topographic contours and are quite clearly indicative of focused
recharge along a majority of streams (Figure 9d). Near Glen Osmond
Creek and Brownhill Creek, groundwater flow predominantly appears
oriented towards the southwest, which may result from the bedrock
sloping in this direction (the Tertiary sediments thickness can be
seen to increase in Figure 6b).
Figure 10a-d display the difference, in every point, between river
head (approximated by the topographic elevation of the nearest river)
and groundwater head. Figure 10a shows the NAP focus area, with the
Quaternary aquifers on the basin side and the Mount Lofty Ranges
aquifers on the mountain side. In the mountain, the results generally
reveal a potential for gaining stream conditions along large portions
of the main rivers (i.e., North Para River, South Para River and
Little Para River). Potential losing stream conditions are indicated
around the upper reaches of streams, suggesting that these are not
supported by groundwater discharge, but are rather initiated by
overland flow or interflow. This observation is consistent with the
fact that most of the stream headwaters in the Mount Lofty Ranges are
ephemeral. Potential losing stream conditions are also observed
locally around a few streams in the lowest part of the Mount Lofty
Ranges (e.g., South Para River and Smith Creek). This observation is
not in line with the interpretation of head contours made from Figure
9a. This inconsistency might be an artefact of the temporal averaging
of hydraulic heads, i.e., the hydraulic heads might be on average
lower than the river head but the stream might still be gaining due to
important groundwater discharge in some periods (but this explanation
remains a hypothesis). In the basin, the Quaternary aquifers are
revealed as potentially receiving water from streams everywhere, and
especially close to the basin margin where the head difference is the
largest. Figure 10b shows the head difference between the Quaternary
aquifers and the Tertiary aquifers on the basin side, such as to
investigate the vertical connection between these aquifers (on the
mountain side, this figure is identical to Figure 10a). The hydraulic
head appears larger in the Quaternary aquifers than in the Tertiary
aquifers over most of the area. This indicates a potential for
downward groundwater leakage from the Quaternary aquifers to the
Tertiary aquifers. The rate at which this leakage occurs is
nonetheless difficult to estimate, since it is also function of the
effective vertical hydraulic conductivity and vertical distance
between these units, which are largely unknown. Similar observations
and interpretations can be made of the CAP focus area (Figure 10c and
Figure 10d).
Most observations from Figure 9 and Figure 10 suggest that groundwater
flow is dominated by local flow systems in the Mount Lofty Ranges.
This indicates that only a small proportion of the recharge occurring
over the mountain may make its way towards the basin. Hence, if MBR
occurs, it would be probably limited to the routing of the recharge
occurring over triangular facets in between stream catchments at the
base of the mountain (see section 2.1). By contrast, the results
suggest that MFR is an important recharge mechanism for both the
Quaternary aquifers and the Tertiary aquifers of the AP basin.
3.3.2 Chloride concentrations
Cl- concentration maps were also constructed for the three aquifer
groups (Quaternary aquifers, Tertiary aquifers and Mount Lofty Ranges
aquifers) (Figure 11). Here the Inverse Distance Weighting
interpolation method from the Geostatistical Analyst extension of
ArcGIS 10.4.1 was used. This method is appropriate for the Cl-
concentrations because their analysis does not especially make use of
the shape of concentration contours, and hence the ‘bull’s eye’ effect
is not really an issue. Furthermore, Cl- cannot be assumed to result
from a diffusive process since advection typically dominates at the
scale of this study, and so the Diffusive Kernel method would be
inappropriate. The Inverse Distance Weighting interpolation method
also has the advantage of being exact at the data points. The power
parameter was set to 2, and a standard neighbourhood was used with 15
maximum neighbours and 10 minimum neighbours. The same parameters were
used for both NAP and CAP focus areas.
Figure 11a shows the Cl- concentrations in the Quaternary aquifers and
the Mount Lofty Ranges aquifers in the NAP focus area. This figure
reveals a strong correlation between low Cl- concentration zones and
the location of the main rivers on the basin side (Gawler River and
Little Para River). It seems unlikely that such a correlation would be
observed if MBR was the main recharge mechanism. By contrast, no
obvious correlation can be found in the mountain. Here, the Cl-
concentration mostly appears correlated with elevation, with lower
values occurring at higher elevations. This trend is expected, since
the rate of evapotranspiration – which largely controls Cl-
concentration – is expected to decrease with elevation as a result of
higher rainfall and lower temperature. In line with these
observations, there is a clear discontinuity in Cl- concentration at
the transition between the mountain and the basin almost everywhere
along the mountain front. This suggests that little or no hydraulic
connection occurs between the mountain and the basin through the
subsurface (i.e., MBR is probably insignificant). Similar observations
hold in Figure 11b, where the Tertiary aquifers are shown in the basin
instead the Quaternary aquifers. The zones of low Cl- concentration
around Gawler River and Little Para River are wider in these aquifers
than in the Quaternary aquifers, which is consistent with the above
observation that the head contours display a wider curvature around
these rivers. Figure 11c and Figure 11d show analogous results for the
CAP focus area. The Cl- concentration is generally lower than in the
NAP focus area, especially in the mountain, most likely a result of
lower evapotranspiration associated with the higher elevation of this
area. A strong correlation between zones of low Cl- concentration and
stream locations can be seen in the basin. These zones appear wider
and somewhat less distinct in the Tertiary aquifers than in the
Quaternary aquifers, but it should be noted that the data density is
lower in these aquifers. In both cases, a sharp change in Cl-
concentration can be seen at the transition between the mountain and
the basin, therefore suggesting that MBR is insignificant.
The Cl- concentration in streams running down from the Mount Lofty
Ranges into the AP basin was analysed to investigate if stream leakage
can explain the groundwater Cl- concentrations measured in the basin.
A summary of available flow rate, electrical conductivity and derived
Cl- concentration data for six monitoring stations located near the
transition between the mountain and the basin is presented in Table 1.
The location of the stream gauges is indicated in Figure 2Error:
Reference source not found. The relationship between streamflow rate
and Cl- concentration is shown from a scatter plot in Figure 12. The
stream Cl- concentration displays significant variations, with a
decreasing trend as flow increases. The relationship between flow rate
and Cl- concentration nevertheless varies between different streams.
This probably reflects different catchment characteristics that are
likely to influence the streamflow generation mechanisms (i.e.,
topography, climate, geology, landuse). The variations of streamflow
rate and Cl- concentration show a strong seasonality, as illustrated
through selected time series for Gawler River and Brownhill Creek in
Figure 13a and Figure 13b, respectively. These time series confirm
that low Cl- concentrations occur during periods of high flow, which
coincide with the wet season (May–September). During this season,
lower Cl- concentrations in stream water can be explained by a
relatively large contribution of overland flow or interflow to
streamflow generation, as these processes should experience little
evapotranspiration relative to subsurface flow contribution. The
significance of overland flow or interflow is also supported by other
observations mentioned above. During high flow periods, the
infiltration potential in the mountain front zone should be enhanced
due to higher stream water levels and wider wetted areas. We therefore
propose to estimate a representative value of the MFR Cl-
concentration by calculating the flow-weighted average Cl-
concentration in stream water. A more rigorous approach would require
knowledge of the timing and rate of stream leakage, which are not
available. The flow-weighted average Cl- concentration is 221, 115,
146, 67, 107 and 91 mg L-1 in the North Para River, South Para River,
Gawler River, Dry Creek, First Creek and Brownhill Creek, respectively
(Table 1). These data show that streams are a plausible source for the
low Cl- concentrations observed in the basin aquifers (Figure 11).
4 Discussion
============
4.1 Strengths and limitations of using hydraulic head and chloride
data
------------------------------------------------------------------
One of the main strengths of hydraulic head data is that they can
indicate the contemporary flow direction (if hydraulic conductivity
can be considered to be isotropic). In this study, the analysis of
head contours gave indications that groundwater flows for a large part
in local systems feeding streams in the Mount Lofty Ranges. It also
allowed for the identification of losing stream conditions in the
mountain front zone, where leakage from streams appears to recharge
not only the Quaternary aquifers but also the deeper Tertiary
aquifers. Studying the head variation with depth in the basin further
gave evidence that groundwater flows from the Quaternary aquifers to
the underlying Tertiary aquifers, even though the rate of this flow is
unknown.
The main limitation of hydraulic head data is probably that they are
quite sensitive to pumping. This is problematic when the objective is
to study the natural (i.e., pre-development) recharge mechanisms.
Pumping in the AP basin mostly affects groundwater levels in the
western part of basin, where large cones of depression exist in the
Tertiary aquifers due to extensive historical and ongoing pumping
(Bresciani et al., 2015a). Therefore, for the purpose of this study
which focuses on the eastern part of the basin (where the mountain
front zone is located), the issue may not be as critical. However,
smaller-scale pumping wells surely also exist in the mountain front
zone and in the mountain, and may affect the results to an unknown
degree. This represents a non-negligible source of uncertainty.
Cl- is potentially an effective tool to distinguish between MFR and
MBR. But for this, the stream water Cl- concentration in the mountain
front zone (i.e., potential MFR source) needs to be significantly
different from the groundwater Cl- concentration at the base of the
mountain (i.e., potential MBR source). Different processes involved in
the generation of MFR and MBR imply that these two potential sources
of water may indeed have different Cl- concentrations (see section
2.2). This is certainly the case in the present case study, where the
Cl- concentration at the base of the mountain is seen to be
significantly different from that of the basin. In contrast, the Cl-
concentration in streams appears to be similar to that of the low Cl-
concentration zones of the basin, which are aligned with surface water
features. These observations leave little doubt regarding the recharge
mechanisms in the AP basin, i.e., MFR appears to be the dominant
recharge mechanism.
As for hydraulic heads, pumping can potentially distort the Cl-
concentrations from those of the natural system. In addition,
irrigation may have influenced the Cl- concentration in shallow wells
located in agricultural areas. However, changes in solute
concentrations are expected to be observed later than hydraulic head
changes, and a dramatic shift in Cl- concentrations is unlikely to be
seen in Cl- concentrations away from the main areas of pumping.
Furthermore, if recharge from streams in the basin was only a result
of recent pumping, groundwater should have a very modern (i.e.,
post-development) recharge signature. Groundwater dating shows that
this is not the case (Batlle-Aguilar et al., 2017). It can also be
noted that the correlation between the zones of low groundwater
salinity and stream locations was already observed in the 1950s, i.e.,
using measurements anterior to the main development period (Miles,
1952).
The interpretation of Cl- data also relies on the assumptions of
constant Cl- inputs. This assumption is widely accepted in the
literature for hydrogeological studies and in applications of the
chloride mass balance method to estimate recharge (Wood, 1999;
Scanlon et al., 2006; Crosbie et al., 2010; Healy and Scanlon, 2010).
This assumption may not be strictly satisfied over the AP basin
because groundwater in the Tertiary aquifers can be quite old, as
revealed by numerous samples showing paleo-meteoric origin (> 12,000
y) according to carbon-14 dating and noble gas measurements
(Batlle-Aguilar et al., 2017). This indicates that different
climatic conditions may have prevailed at the time of recharge,
implying possible variations in Cl- inputs. However, such old
groundwater is mostly observed in the western part of the basin.
Groundwater in the mountain front zone is younger (in most cases <
10,000 y according to carbon-14 dating), making it less likely for
these to reflect drastically different paleo-climatic conditions.
Furthermore, even if the Cl- inputs did vary over time, such temporal
variations would not in itself explain the correlation of groundwater
Cl- concentration with stream locations in the basin.
Finally, the assumption of conservative Cl- is deemed reasonable in
view of the geology of the study area, which is not known to bear
evaporite deposits such as halite. This assumption was also tested by
analysing the chloride/bromide (Cl-/Br-) ratio from 161 well samples
distributed over the study area. The average Cl-/Br- ratio from these
samples is 739 +/- 173 (molar) (Figure S13). This shows that
groundwater has a similar Cl-/Br- ratio to seawater (~650 molar),
hence indicating that there is little water-rock interactions or
dissolution of evaporite deposits (Drever, 1997; Davis et al., 1998).
The low variance of the Cl-/Br- ratio also suggests that the dominant
process for the increasing salinity in the groundwater is
evapotranspiration (Cartwright et al., 2006). Furthermore, even if
Cl- was not strictly conservative, focused recharge from streams in
the basin would again appear necessary to explain the fact that the
zones of low groundwater Cl- concentration are found along streams.
Compared to other methods that use noble gas, radioactive or isotopic
tracers, the above approach appears simpler, more cost effective, and
more reliable due to the much higher data density generally achievable
(i.e., given current technologies and budget constraints).
Nevertheless, in contrast with some other methods (e.g., noble gases),
it should be noted that this approach is only qualitative. I.e., it
does not allow for the quantification of the relative proportion of
MFR and MBR and their absolute rate. One way to extend the ideas
presented in this study to gain more quantitative insight would be to
use the data as calibration targets in a groundwater flow and Cl-
transport model. This is also the subject of ongoing efforts
(Bresciani et al., 2015b).
4.2 MFR versus MBR in the AP basin
----------------------------------
In an early study, Miles (1952) noted that the pre-development
groundwater levels along the mountain front of the AP basin were
reflective of unconfined conditions, and that the subsurface materials
in this zone were favourable to stream infiltration. In addition,
Miles (1952) already analysed the groundwater salinity distribution.
He observed that salinity contours were forming fan-shaped zones of
low salinity “mushrooming” outwards from streams, with such patterns
being visible up to more than 100 m below the ground surface. He
concluded that stream infiltration along the mountain front zone was a
major recharge mechanism for the basin aquifers. Later, in a study of
the NAP aquifers, Shepherd (1975) arrived to the same conclusion,
partly using similar arguments and further noting that: (i)
groundwater hydrographs in the Quaternary aquifers were each year
showing a rapid rise in water level shortly after Gawler River and
Little Para River started to flow; (ii) the vertical head gradient and
vertical hydraulic conductivity were indicative of significant
downward flow from the Quaternary to the Tertiary aquifers.
Additionally, a number of studies directly measured groundwater gains
and losses using differential flow-gauging along streams entering the
AP basin (Hutton, 1977; Green et al., 2010; Cranswick and Cook,
2015). All found that several streams were losing a significant
amount of water in the mountain front zone. Finally, Zulfic et al.
(2010) found that bore yield based on air-lift testing conducted at
the time of drilling (e.g., Williams et al., 2004) in the Mount
Lofty Ranges did not increase beyond 100 m depth for most geology
types. This finding can be interpreted as hydraulic conductivity being
relatively low beyond that depth. This would promote local groundwater
flow systems in the mountain, in line with the current analysis.
In contrast, Gerges (1999) and Batlle-Aguilar et al. (2017)
proposed that MBR is the dominant recharge mechanism for the Tertiary
aquifers of the AP basin. A major argument in these studies was based
on the observation that salinity is generally higher in the Quaternary
aquifers than in the T1 and T2 aquifers. From this observation, the
authors suggested that the relatively fresh water found in the T1 and
T2 aquifers could not be the result of downward leakage. Instead, they
proposed that this water should come from the Mount Lofty Ranges
(where the salinity is lower) through subsurface flow. However, along
streams, the Cl- concentration in the Quaternary aquifers is in fact
very similar to that of the underlying Tertiary aquifers (Figure 11).
Furthermore, the possibility that this relatively fresh water
originates from the higher elevation areas of the Mount Lofty Ranges
through deep groundwater flow paths is unlikely since: (i) the
hydraulic head data suggest a predominance of local flow systems in
the Mount Lofty Ranges (section 3.2.1); (ii) there is an important
mismatch between groundwater Cl- concentrations at the base of the
mountain block and those in the upper part of the basin (section
3.2.2); and (iii) groundwater in the deep layers of the basin (i.e.,
T3 and T4 aquifers) generally shows higher salinity (Gerges, 1999),
implying that deep groundwater flow paths cannot explain the observed
fresh water. Another argument in Batlle-Aguilar et al. (2017) was
based on the observation that relatively old groundwater was measured
near the top of Tertiary aquifers (from carbon-14 dating). From this
observation, the authors suggested that groundwater could not be
recharged in the basin, but rather further away, in the Mount Lofty
Ranges. However, most of the old groundwater samples analysed were
from wells found quite some distance away from the mountain front
zone, where the Tertiary aquifers become confined – logically implying
an increase of age with distance from the recharge zone. Furthermore,
relatively young groundwater was found at significant depth near major
faults of the basin, precisely suggesting the occurrence of focused
recharge (Batlle-Aguilar et al., 2017). Finally, neither Gerges
(1999) nor Batlle-Aguilar et al. (2017) proposed a mechanism to
explain how the groundwater could be more saline outside of the
low-salinity corridors under MBR-prevailing conditions, as
consistently observed in both the Quaternary aquifers and the Tertiary
aquifers (Figure 11). These zones of higher salinity directly
contradict the MBR hypothesis, including MBR that would be derived
from the recharge over triangular facets at the base of the mountain.
It seems more likely that the groundwater of higher salinity in the
basin originates from diffuse recharge, which would naturally imply a
much higher salinity as a result of evapotranspiration than focused
recharge from streams.
Hence, on the basis of robust consistent evidence given in this work
(including consistent findings from hydraulic head and Cl- analyses)
and through a critical review of earlier investigations, we propose
that MFR is the most plausible and predominant recharge mechanism for
the relatively fresh water found in the AP basin (i.e., as in Figure
1b). This finding is expected to have important consequences for
future investigations and for the management of water resources in the
Adelaide region. A conceptual model depicting the suggested recharge
mechanisms, and how these can explain the observed Cl- (or salinity)
patterns – at least in the eastern part of the basin – is shown in .
5 Conclusion
============
We presented and demonstrated through a regional-scale example the use
hydraulic head, Cl- and EC data to distinguish between MFR and MBR to
basin aquifers. Hydraulic heads can inform on groundwater flow
directions and stream-aquifer interactions, while chloride
concentrations and EC values can be used to distinguish between
different water sources if these have a distinct signature. This
information can provide evidence for the occurrence or absence of MFR
and MBR.
In the above case study, both hydraulic head and Cl- (equivalently,
EC) analyses gave informative and consistent results (i.e., both
suggested a predominance of MFR), which gives confidence in the
interpretation. The Cl- analysis was particularly straightforward and
authoritative, and it further allowed for the identification of
diffuse recharge in the basin.
Difficulties in the interpretation of hydraulic head and Cl- data may
arise for particular conditions such as when pumping effects are
significant. However, the study of the AP basin demonstrates that even
for a basin that has been subject to long-term groundwater extraction
(for about a century in this case), the data can allow for the
identification of natural recharge mechanisms. Cl- in particular is
expected to be more robust than hydraulic head to pumping effects.
The relevance of the presented approach lies in that the variables
used (hydraulic heads, Cl- concentrations and EC values) have been
routinely measured for decades in many parts of the world; if not,
their measurement is inexpensive provided wells already exist. While
these data cannot tell us everything (e.g., they cannot directly
inform on the recharge rates), we expect that in many cases a
significant dataset would be readily available, bearing an as-yet
unexploited potential to inform on the recharge mechanisms. Such
information is critical for conceptualizing and managing groundwater
systems.
The AP basin serves as an example of a region where the recharge
mechanisms have long been debated in the context of groundwater
resources management. Application of the proposed rationale revealed
to be effective in resolving this debate. It is expected that the
findings of this study will have significant impacts on the management
of water resources in the Adelaide region.
Data availability
=================
Groundwater hydraulic head, Cl- and EC data used in this study are
available on the WaterConnect database
(https://www.waterconnect.sa.gov.au).
Acknowledgements
================
This study was supported by the Goyder Institute for Water Research
through the project I.1.6 “Assessment of Adelaide Plains Groundwater
Resources” (2013–2015), and by the Korea Research Fellowship Program
through the National Research Foundation of Korea (NRF) funded by the
Ministry of Science, ICT and Future Planning (project number
2016H1D3A1908042). The authors thank two anonymous reviewers for
constructive comments.
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North Para River (A5050502)
South Para River (A5051009)
Gawler River (A5050505)
Dry Creek (A5041051)
First Creek (A5040578)
Brownhill Creek (A5040901)
Mean EC (μS cm-1)
3424
1414
3121
1579
1276
808
Flow-weighted mean EC (μS cm-1)
1095
656
758
411
611
545
10th percentile Cl- (mg L-1)
214
99
183
37
76
87
90th percentile Cl- (mg L-1)
852
304
1615
184
259
191
Median Cl- (mg L-1)
1441
457
600
990
432
144
Mean Cl- (mg L-1)
846
288
769
353
257
145
Mean flow rate (ML d-1)
28.05
13.63
82.32
17.20
6.24
5.26
Flow-weighted mean Cl- (mg L-1)
221
115
146
67
107
91
# EC record days
6064
1221
131
944
937
29
Records period
1994–2016
2003–2010
1970–1995
2013–2016
2013–2016
2012–2016
Table 1. Surface water flow rate, EC values and their conversion into
Cl- concentrations for the six gauging stations located near the
mountain front zone (gauging station number in parenthesis; see Figure
2Error: Reference source not found for site locations). Note that the
North Para River and South Para River join about 1 km downstream of
the Para Fault to form the Gawler River.

Figure 1. Conceptual models of the transition between mountain and
basin: (a) physical configuration and (b-c) possible
conceptualizations of MSR where (b) MFR dominates, (c) MBR dominates,
and (d) both MFR and MBR are significant.

Figure 2. Situation map showing elevation (in mAHD, i.e., Australian
Height Datum) and relevant features of this study.

Figure 3. Schematic diagram showing triangular facets at the base of
the mountain block (after Welch and Allen (2012)).

Figure 4. Schematic diagram showing hydraulic head contours and
groundwater flow directions in the horizontal plane for (a) losing and
(b) gaining stream conditions (after Winter et al. (1998)).

Figure 5. Impact of a difference in basement elevation induced as a
result of faulting on hydraulic head in a hypothetical setting. (a)
Hydraulic conductivity field in a vertical cross-section representing
a sedimentary layer overlying a basement having significantly lower
hydraulic conductivity, with the fault zone itself having no different
hydraulic conductivity (i.e., the fault only implies the difference in
basement elevation). (b) Results from an unconfined groundwater flow
simulation in which a constant head (80 m) was specified on the left
boundary and inflow was specified on the right boundary (at a rate
proportional to the hydraulic conductivity). A sharp difference in
hydraulic head is observed across the fault zone. The simulation was
performed using MODFLOW-NWT (Niswonger et al., 2011), with 200 cells
in horizontal direction and 150 cells in the vertical direction.

Figure 6. Total thickness of the Quaternary (Q) sediments (a) and
Tertiary (T) sediments (b) in the AP basin. The colour schemes in (a)
and (b) are different.

Figure 7. Schematic hydrogeological cross-sections in (a) the NAP and
(b) the CAP sub-basins (see Figure 2) (after Shepherd (1975); Gerges
(1999); Zulfic et al. (2008); Baird (2010)). Q: Quaternary aquifers
(lumped together; in reality there are up to six different aquifers
depending on location, separated by clay layers); T1, T2, T3 and T4:
Tertiary aquifers; UTS: Undifferentiated Tertiary Sand aquifers; MPC:
Munno Para Clay aquitard; BPF: Blanche Point Formation aquitard; FR:
Fractured-Rock aquifers; RF: Redbank Fault; AF: Alma Fault; PF: Para
Fault; HVF: Hope Valley Fault; EBF: Eden-Burnside Fault.

Figure 8. Cl- versus EC data in the AP catchment. The fitted function
used to describe the relationship is  , where [Cl-] and [EC]
are in units of mg L-1 and μS cm-1, respectively (coefficient of
determination: R2 = 0.9996).

Figure 9. Head and topographic contours in the NAP (a-b) and CAP (c-d)
focus areas. On the basin side, (a) and (c) show the head in the
Quaternary aquifers, while (b) and (d) show the head in the Tertiary
aquifers. On the mountain side, all four sub-figures show the head in
the Mount Lofty Ranges aquifers. In (a-b), the contour interval is 10
m in the basin and 40 m in the mountain (both for head and
topography), while in (c-d), it is 20 m in the basin and 80 m in the
mountain. Selected groundwater flow directions highlight apparent
gaining stream conditions in the lower part of the mountain and
loosing stream conditions in the mountain front zone.

Figure 10. Head differences in the NAP (a-b) and CAP (c-d) focus
areas. On the basin side, (a) and (c) show the nearest river head
(approximated by topography) minus the head in the Quaternary
aquifers, while (b) and (d) show the head in the Quaternary aquifers
minus the head in the Tertiary aquifers. On the mountain side, all
four sub-figures show the nearest river head minus the head in the
Mount Lofty Ranges aquifers. The same colour scheme is applied
everywhere and is indicated in (a). Reddish colours are for positive
values, indicating a potential for downward flow (i.e., groundwater
recharge). Blueish colours are for negative values, indicating a
potential for upward flow (i.e., groundwater discharge).

Figure 11. Cl‑ concentration in the NAP (a-b) and CAP (c-d) focus
areas. On the basin side, (a) and (c) show the Cl- concentration in
the Quaternary aquifers, while (b) and (d) show the Cl- concentration
in the Tertiary aquifers. On the mountain side, all four sub-figures
show the Cl- concentration in the Mount Lofty Ranges aquifers. The
same colour scheme is applied everywhere and is indicated in (a). The
latter is chosen favourable to the study of relatively low salinity
zones. I.e., all Cl- concentrations larger than 1,400 mg L-1 are
included in the same class (red colour), but in reality much higher
values exist.

Figure 12. Scatter plot of chloride concentration versus streamflow
rate for six streams running down from the Mount Lofty Ranges into the
AP basin.

Figure 13. Stream water chloride concentration and flow rate over
selected periods for (a) Gawler River and (b) Brownhill Creek. The
axes in (a) and (b) have a different scale.

Figure 14. Conceptual model of the recharge mechanisms for the
aquifers of the AP basin, as seen in a cross-section perpendicular to
a stream in the mountain front zone, and how these can explain the
observed groundwater salinity. Blue to red colours indicate low to
high salinity.
39