Science of the Total Environment 662 (2019) 180–191
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Science of the Total Environment
j ourna l homepage: www.e lsev ie r .com/ locate /sc i totenvHydrological and geochemical responses of fire in a shallow cave systemFang Bian a,b,c,⁎, Katie Coleborn b,c, Ingrid Flemons b,c, Andy Baker b,c, Pauline C. Treble b,d, Catherine E. Hughes d,
Andrew Baker e, Martin S. Andersen a,b, Mark G. Tozer f, Wuhui Duan b,g, Christopher J. Fogwill h, Ian J. Fairchild i
a Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Sydney, NSW 2052, Australia
b Connected Waters Initiative Research Centre, University of New South Wales, Sydney NSW 2052, Australia
c School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia
d ANSTO, Lucas Heights, NSW 2234, Australia
e National Parks and Wildlife Service, Bathurst, NSW 2795, Australia
f NSW Office of Environment & Heritage, Hurstville, NSW, Australia
g Key Laboratory of Cenozoic Geology Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
h Geography, Geology and the Environment, Keele University, UK
i School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UKH I G H L I G H T S G R A P H I C A L A B S T R A C T• Severe fire over a shallow cave system
leads to changed soil hydrology.
• Cave drip discharge shows increased
preferential and decreased diffuse
flows.
• Post-fire, stable water isotopes show
that soil water was evaporated.
• 6 months post-fire, drip water isotopes
have returned to the pre-fire mean.
• Nutrient elements were largely
volatilised by the severe fire.⁎ Corresponding author at: Water Research Laboratory
E-mail address: f.bian@unsw.edu.au (F. Bian).
https://doi.org/10.1016/j.scitotenv.2019.01.102
0048-9697/© 2019 Published by Elsevier B.V.a b s t r a c ta r t i c l e i n f oArticle history:
Received 13 July 2018
Received in revised form 3 January 2019
Accepted 10 January 2019
Available online 11 January 2019
Editor: José Virgílio CruzThe influence of wildfire on surface soil and hydrology has been widely investigated, while its impact on the
karst vadose zone is still poorly understood. Amoderate to severe experimental fire was conducted on a plot
(10 m × 10 m) above the shallow Wildman's Cave at Wombeyan Caves, New South Wales, Australia in May
2016. Continuous sampling of water stable isotopes, inorganic geochemistry and drip rates were conducted
from Dec 2014 to May 2017. After the fire, drip discharge patterns were significantly altered, which is
interpreted as the result of increased preferential flows and decreased diffuse flows in the soil. Post-fire
drip water δ18O decreased by 6.3‰ in the first month relative to the average pre-fire isotopic composition.
Post-fire monitoring showed an increase in drip water δ18O in the following six months. Bedrock related
solutes (calcium, magnesium, strontium) decreased rapidly after the fire due to reduced limestone dissolu-
tion time and potentially reduced soil CO2. Soil- and ash-derived solutes (boron, lead, potassium, sodium,
silicon, iodine and iron) all decreased after the fire due to volatilisation at high temperatures, except for
SO2−4 . This is the first study to understand the hydrological impact from severe fires conducted on a karstKeywords:
Karst
Fire
Hydrograph analysis
Groundwater, School of Civil and Environmental Engineering, UNSW Australia, 110 King Street, Manly Vale, NSW 2093, Australia.
F. Bian et al. / Science of the Total Environment 662 (2019) 180–191 181system. It provides new insights on the cave recharge process, with a potential explanation for the de-
creased d18O in speleothem-based fire study, and also utilise the decreased bedrock solutes to assess the
wildfire impacts both in short and long time scales.
© 2019 Published by Elsevier B.V.1. Introduction
The lack of quantification of the impact of fire events on sub-surface
systems, especially in karst environments, limits our understanding of
the hydrological impacts of wildfire and the application of prescribed
burning on karst geochemistry. The few recent studies examining
long-term fire impacts on karst systems focused on changes in soil res-
piration, nutrient uptake and evaporation associated with the transfor-
mation of plant biomass to ash during fire and the subsequent recovery
of plant communities (Coleborn et al., 2016b; Nagra et al., 2016; Treble
et al., 2016). Coleborn et al., (2016b) compared soil CO2 concentrations,
temperatures and moisture between burnt and unburnt soils at an al-
pine karst site in Australia. For the first five years, soil respiration was
depressed in the burnt forested site and less biomass was reported rel-
ative to the unburnt forested site. No significant difference could be seen
in the ten-year post-fire soil groups relative to the unburnt control re-
gions. Treble et al. (2016) reported nine-year data on drip water geo-
chemistry, suggesting that the greatest impact of fire is associated
with long-term decreases in sulfur concentration, due to post-fire accu-
mulation of this ash-derived nutrient in biomass. Aside from ash signals,
higher δ18O and chloride levelswere associatedwith increased evapora-
tion in the soil and shallow vadose zone after a wildfire (Nagra et al.,
2016). Compared with the bi-decadal time period needed for post-fire
habitat and fuel recovery in forest ecosystems (Haslem et al., 2011),
soil CO2 recovery (Coleborn et al., 2016b), and cave drip water isotope,
chlorine and sulfur residuals (Treble et al., 2016) are notable in having a
multi-year temporal response. In contrast, little is known about the im-
pact of fire on flow regimes, which has the potential to induce short-
term fluctuations in response to individual weather events.
Changes in deeper flow regimes may potentially arise as a conse-
quence of widely known impacts of wildfire on soil structures
(Fernandez et al., 1999; Pausas et al., 2009; Scott and Van Wyk, 1990).
Scott and Van Wyk (1990) reported reduced soil wettability following
wildfire. Any influence on soil structures would subsequently affect
preferential flow by the modified macropores properties (Beven and
Germann, 1982; Ghodrati and Jury, 1992). Cave drip discharge is poten-
tially affected by physical characters such as soil capillarity (Fredlund
and Rahardjo, 1993) and preferential flows (Šimůnek et al., 2003). The
fire-inducedmorehydrophobic soil structure can increase post-fire run-
off and erosion in burnt areas (Huffman et al., 2001), leading to signifi-
cant increases in soil loss, and increase in total and quick flow volumes
in streams. In cave systems, it potentially modifies the flow regimes
with changed drip rate records.
Any surface fire signal transmitted to a cave will pass through the va-
dose zone—the unsaturated area between the surface and the water ta-
bles. In karst environments, the vadose zone hosts the flow pathways
andwater storages that feed cave dripwater, and iswhere themajor pro-
cesses of bedrock dissolution, mixing and dilution of stored water with
event water occur (Fairchild and Baker, 2012). Other hydrochemical
and biogeochemical processes along these pathways have also been iden-
tified, such as prior calcite precipitation (Fairchild and Treble, 2009),
evaporation (Cuthbert et al., 2014) andnutrient andwater uptake by veg-
etation (Coleborn et al., 2016a; Treble et al., 2016). The combination of
karst hydrology, water isotope tracers and trace solute mobility are vital
for understanding the full complexity offlowpathways (Kogovšek, 2010).
Precipitation percolates through the vadose zone into caves as either
diffuse or conduit flows (Atkinson, 1977). Diffuse flows occur through
the matrix porosity, while conduit flows occur via larger scale fracturesor conduits (White, 2002). Relatively lower flow rates and more stable
geochemical properties are associated with diffuse flows, while higher
and more variable flow rates and more variable chemistry characterise
conduit flows. Basic hydrological models of unsaturated zone recharge
that include flow rate fluctuations were first developed by Smart and
Friederich, 1987. Baker et al. (1997) identified that antecedent precipi-
tation conditionswere an important control on the individual hydrolog-
ical patterns linked to the state of the vadose zone's water storage
capacity at the time. Automated acoustic drip counting was recently in-
troduced as an alternativemeanof drip recording,which is able to count
falling drips, even during transient events, and record small fluctuations
in drip rate over several years (Collister and Mattey, 2008). Water iso-
topes are related to the properties of precipitation (Jouzel et al., 2000)
and have been used to distinguish between groundwater and surface
water (Sophocleous, 2002), as tracers ofmoisture sources, and tofinger-
print catchment residence times and flow pathways (Tian et al., 2007).
In specific regions, the stable isotopes of water (δ2H and δ18O) can re-
veal links between climatic factors and flow pathways (Soulsby et al.,
2000). The current paper aims to do this for a karst vadose zone.
Solute concentrations in cave drips reflect changes in external forcing
(Tremaine and Froelich, 2013) and changes occurring along flow path-
ways. Multiple lines of evidence are typically crucial in qualitativemodels
of karst vadose zone hydrological behaviour. Theoretically, theMg/Ca and
Sr/Ca ratios are recognized to be important diagnostics in karst hydrology
for the amount of prior calcite precipitation and water-rock interaction
(Ternan, 1972; Fairchild et al., 2000; Tremaine and Froelich, 2013;
Razowska-Jaworek, 2014). Solute abundances vary in response to differ-
ences in climate and cave controls (Wassenburg et al., 2012), the type
of bedrocks (Immenhauser et al., 2007) aswell as the duration of each re-
charge event (Huang and Fairchild, 2001). Hartland et al. (2012) reported
a correlation between natural organic matter-transported metals and cli-
matic signals. Inversely, the quantity of soil organic matter can also be in-
dicated by shifts in the metal ratios of cave drip discharges. Nagra et al.
(2016) compared a burnt cave with a nearby control cave. Biomass-
sourced, ash-derived solutes (SO4 and K), together with dissolved bed-
rock solutes, were both reported as a fire signature. However, it is impor-
tant to note that drip water solute concentrations can vary significantly
even within the same cave chamber (Fairchild and Treble, 2009).
In this research, we aimed to identify the hydrological and geochem-
ical impact of an experimental fire on a shallow karst vadose zone. The
fire was deliberately lit above a cave in order to monitor its impact
under controlled conditions. We analysed the composition of cave
drip water over 2.5 years (Dec 2014–May 2017) in Wombeyan Cave, a
shallow cave system in NSW, Australia. Our monitoring started
1.5 years prior to the fire and continued for one year afterwards. Thus,
unlike the previous studies by Nagra et al. (2016) and Treble et al.
(2016), which contain only post-fire data, this study also includes pre-
fire data to serve as a baseline with which post-fire data may be com-
pared. This greatly assists in our attempts to understand and quantify
the impacts of fire on karst systems. This is the first published research
to directly compare pre- and post-fire hydrogeochemical components
and drip discharges in a shallow karst vadose zone after a severe fire.
Reports of pre- and post-fire discharge patterns and water stable iso-
topes weremade to demonstrate changes to the local vadose zone's hy-
drology, while inorganic geochemical changes were also analysed as
supplementary evidence. It is therefore valuable to directly compare
pre- and post-fire hydrogeochemical components and drip discharge
for commonalities and differences.
182 F. Bian et al. / Science of the Total Environment 662 (2019) 180–1912. Site description
The Wombeyan Caves Karst Conservation Reserve (34° 18″ S, 149°
58″ E) is located in the south-eastern part of New South Wales,
Australia, on the western edge of the Sydney Basin, on a plateau of the
Great Dividing Range, surrounded by agricultural areas (Fig. 1).
Wombeyan Limestone, part of the Bungonia Group, is now a marble
due to the formation of igneous rocks in the surrounded Lower-
Middle Devonian Bindook Porphyry Complex (Brunker and Offenberg,
1970; Osborne, 1984). These Silurian carbonates are highly fractured
marbles with nomatrix porosity remaining (Osborne, 1993). Therefore,
flow is entirely dominated by fracture and conduit flows. Typically, the
land surfaces (b4 cm depth) above the caves contain gravel, marble
fragments, red silty clay and dark humic matter (McDonald and
Drysdale, 2007).
The mean monthly land-surface temperature at Wombeyan Caves
ranges between a maximum of 26.0 °C in January to a minimum of 0.6
°C in July. Annual median long-term precipitation is 684.7 mm
(1942–2017, from Bureau of Meteorology, Australia gauge 063093),
with summer precipitation exceeding winter precipitation by 47%.
However, during December 2014 to May 2017, winter monthly precip-
itation (104 mm) exceeded that of summer by 32% (79 mm).
Wildman's Cave (W456) is a small and shallow cave near the top of a
ridge above Mares Forest Creek Gorge. The cave has a narrow pothole-
type entrance and 42 m of reasonably decorated passage (Wylie and
Wylie, 2004). The single large chamber is approximately 15 m long
and 6 m wide, with b1 m of soil and bedrock overlying the cave.
3. Method
3.1. Experimental fire
An experimental fire covering an area of 10 m × 10 m was con-
ducted above Wildman's Cave on 25th May 2016. Given theFig. 1. Location of Wildman's Cave Conservation Reserve inWombeyan Caves, New SouthWale
angles (b), exposed Wombeyan limestone (c), burnt area pre-fire (d) and post fire (e). Photo (experimental nature of the fire, additional fuel (branches and leaf lit-
ter collected adjacent to the site) was placed within the burn area to
ensure that a moderate to severe intensity burn was achieved.
Within the 10 m × 10 m burn area, shrubs and groundcovers domi-
nated the site. There were no mature trees within the burn site.
3.2. Location of monitoring sites within the cave
Eleven dripping stalactites in Wildman's Cave were included in this
study to monitor the geochemical and hydrological variations associ-
ated with the burn (see Fig. 1b). In the corner of the pothole-type en-
trance, a straw stalactite (Site 01) was utilised (see Fig. 1b). A large
cluster of soda-straws separated the entrance from the inner chamber,
and provided drips for Sites 02 to 05. Sites 06 to 11were located within
the large chamber, while Sites 02 to 05 were a lower elevation than the
others and had a thicker roof.
3.3. In-cave water sampling
In-cave monitoring started in Dec 2014 and ended in May 2017.
Water samples were collected at approximately bi-month intervals
throughout the research program, with sample volumes limited by the
infrequent occurrence of recharge events. Two drip water sampling
methods were employed. Firstly, drip water samples were collected
by leaving bottles in the cave for approximately two months. Wide-
mouth 120 ml HDPE sampling bottles were placed under each dripping
site, with a plastic funnel containing an acoustic data logger placed in-
side. Each of these bottles used for cumulative drip water sampling
contained 2 mm paraffin oil layer to prevent evaporation or exchange
with the atmosphere once drips entered the sampling bottle. Secondly,
opportunistic sampling was conducted during each of the bi-monthly
sampling campaigns, provided recharge was occurring. These drip wa-
ters were sampled directly into 250 ml wide-mouth HDPE sampling
bottles placed directly under the drip sites overnight. The collection ofs, Australia (a), 3D diagram of the study site. Recorded sites are indicated with white rect-
a): Google Earth. Photos (c, d and e): Andy Baker.
F. Bian et al. / Science of the Total Environment 662 (2019) 180–191 183these campaign samples was conducted without funnels or drip loggers
to minimise contamination from the surrounding environment. Refer-
ence groundwater samples were collected from a local borehole in the
Wombeyan Caves campground using 250 ml wide-mouth HDPE con-
tainers. Sixty-two opportunistic samples were collected when the cave
was dripping during the bi-monthly sampling campaigns. There were
fewer samples collected in the dry cold months, especially after fire.
At the day after the fire we succeeded in acquiring ten drip samples
the day, then six more in the first month. After that, water samples
were collected in Dec 2016, and Jan and Mar 2017. Bore hole samples
were collected using 250 ml wide-mouth HDPE containers in
Wombeyan Caves.
3.4. Precipitation
Wombeyan monthly rainfall records were provided via Bureau of
Meteorology climate station (Number. 063093). Precipitation samples
for stable water isotopes analysis were collected from Mount Werong
(34° 04″ S, 149° 55″ E), whichwas the nearest precipitation isotope col-
lection station to Wildman's Cave (~30 km to the northeast). Precipita-
tion samples were collected on an event basis using a sealed HDPE
bottle with a plastic funnel. The design was based on the method of
Gröning et al. (2012) in order to prevent evaporation of the sample or
isotopic exchange with the atmosphere. The rainfall-related collection
here was from August 2014 to May 2017, and is an extension of a
time series previously published by Hughes and Crawford (2013) and
further described by Crawford et al. (2013).
3.5. Drip hydrology monitoring
Hydrology records were comprised of drip rates from stalactites
within the cave, whichwere integrated at 15-min intervals using acous-
tic drip loggers (Stalagmate® Plus Mk 2b, http://www.driptych.com/).
For each drip site, a Stalagmate was placed underneath the stalactite
in a plastic funnel sitting in an HDPE bottle surrounded by a plastic
tube to fix it in position. The minimum distance between the stalactite
and Stalagmatewas 25 cm. The volume of dripswas assumed to be con-
sistent throughout this study (Collister and Mattey, 2008).
3.6. Geochemical analysis
Both the long-term and short-term drip water samples collected
were first filtered through 0.45 μm mixed-cellulose filters into 10 ml
plastic sampling vials. Water samples for stable isotope analysis were
stored in 10 ml vials with zero-headspace. Separate 10 ml vials were
used for solute analyses. Two drops of ACS reagent HNO3 acid (70%)
were added into the cationic samples to prevent precipitation. All the
prepared analytes were refrigerated at b5 °C until analysis.
Dripwater samples were analysed to determine the δ2H and δ18O,
with the results given in per mille (‰) using the conventional delta no-
tation relative to VSMOW (Vienna Standard Mean Ocean Water). A Los
Gatos Research (LGR) Water-Vapour Isotope Analyzer in the UNSW
IceLab was used for the analysis of the δ2H and δ18Ο. The ICELAB was
graded excellent in an international inter-laboratory comparison prior
to the period of sample analysis (Wassenaar et al., 2018). All the isotope
analytes were filtered through 0.45 μmmixed-cellulose filters again be-
fore injection into the isotope analyzer (reported accuracy of ±1.0‰ for
δ2H and±0.2‰ for δ18O). The sampleswere calibrated againstfive stan-
dards, with VSMOW-2 run as a primary standard. Some 109 precipita-
tion samples were analysed at the ANSTO Environmental Isotope
Laboratory using a Picarro L2120-I Water Analyzer (same accuracy as
the LGR Water-Vapour Isotope Analyzer). The ANSTO lab runs a mini-
mum of two in-house standards calibrated against VSMOW/VSMOW2
and SLAP/SLAP2 with samples in each batch. Deuterium excess (D-ex-
cess) is calculated with the equation d = δ 2H – 8 ∗ δ 18O.Only opportunistic over-night collections of water samples were
analysed for cations and anions. Cation (B2+, Ba2+, Ca2+, Fe2+, K+,
Mg2+, Na+, Pb3+, Si4+ and Sr2+) concentrations were determined
using inductively-coupled plasma optical emission spectroscopy (ICP-
OES; Optima™ 7300DV, PerkinElmer, Shelton, USA) and inductively-
coupled plasma mass spectrometry (ICP-MS; NexION 300D,
PerkinElmer, Shelton, USA) at the UNSW Mark Wainwright Analytical
Centre, except for water samples from the final collection (9th May
2017), which were analysed using inductively-coupled plasma-atomic
emission spectroscopy (ICP-AES; ICAP7600, Thermo Fisher) at the
Australian Nuclear Science and Technology Organisation (ANSTO).
Anion (Cl−, I− and SO2−4 ) concentrations were determined using an
ion chromatograph (Dionex DX-600) with a self-regenerating suppres-
sor at the ANSTO facility. Mann–Whitney U tests (Mann and Whitney,
1947) were conducted on cation and anion as a non-parametric test
for the geochemistry data which were not normally distributed over
2 years. U and Z-scores are calculated with the equations:
nðnþ 1Þ U−m
U ¼ R− ; z ¼
2 σ :
where n is the sample size, R is the sum of the ranks, m and σ are the
mean and standard deviation of U. The U-score allows the comparison
of different groups. The Z-score permits the comparison of the standard
normal quantiles to obtain the calculated probability
4. Results
4.1. Experimental fire
The highest recorded temperature was 929 °C in the middle of the
burn area at 12 cm depth (Supplementary Fig. 1) recorded using a ther-
mocouple temperature recorder TCTemp1000 (ThermoworksTM). The
fire intensity was severe (Keeley, 2009), with canopy cover left intact
but the surface litter largely consumed and thick white ash layers gen-
erated at hotspots to a depth of several centimetres. The firewas started
mid-morning, and visible flame lasted approximately 45 min.
4.2. Hydrology results
The overall drip water recharge responses to precipitation are illus-
trated in Fig. 2. Site 03 slowly stopped dripping, while Sites 08–10
weremisaligned postfire. 14major dripwater recharge events occurred
pre-fire, and 21major drip water recharge events occurred post fire. For
periods outside of the recharge events dripping ceased (e.g. the baseline
drip logger signals were constantly 0). Comparing the number and
timing of recharge events to daily precipitation data, a recharge thresh-
old of about 20 mm/day precipitation is inferred. In the 18-month pre-
fire monitoring period, there were 10 days where daily precipitation
exceeded 20 mm, with precipitation exceeding 50 mm/day on one of
these occasions. In the 12-month post-fire monitoring period, there
were 11 dayswhere daily precipitation exceeded 20mm, including pre-
cipitation exceeded 50 mm/day on two occasions. The highest daily
rainfall total over the monitoring period (91.4 mm) occurred one
week after the fire. Despite minor differences in pre- and post-fire
daily rainfall amounts, Fig. 2 shows a notable change in the drip pattern
after the fire.
The descriptive statistics for basic parameters, including duration,
peak rates, average rates, total drips, skewness and kurtosis for all indi-
vidual recharge events are presented in Fig. 3, which uses box plots to
compare discharge patterns pre- and post-fire. Post-fire total drips
were less variable but not significantly different to pre-fire total drips.
Both peak and average recharge rates increased noticeably andwere as-
sociated with decreased duration after the fire. Detailed analyses of the
event hydrographs are presented in Section 5.1. Overall, post-fire dis-
charge events were characterised by higher mean and peak flows that
184 F. Bian et al. / Science of the Total Environment 662 (2019) 180–191
Fig. 2. Precipitation records at Wombeyan Caves (a), compared with the overall discharge into the cave at burnt drip sites (b) and Site 01 (c.). Discharge records at individual sites are
presented using the colour in agreement with subsequent figures. The timing of the fire is indicated by the vertical red line. Records on individual sites could be found in
Supplementary Fig. 2.were of shorter duration than pre-fire discharge events. Increased peak
rechargewas observed at Sites 02, 04, 05, 07, 09 and 11 (Supplementary
Fig. 2) two weeks post-fire. The skewed discharge peaks were shorter
for post-fire recharge events than pre-fire ones. Conversely, there
were no notable changes in dripping pattern for Site 01, which was
not directly under the burnt area (Supplementary Fig. 2).
4.3. Isotope results
Isotopic data for the 184 precipitation events collected at Mt.
Werong from August 2014 to May 2017 (January 2015 was missing
due to insufficient rainfall) show that the values for δ18O (n = 166)
ranged from −17.7‰ to 3.1‰, and from −126.4‰ to 31.6‰ for the
δ2H samples (n=184). Theweightedmean of precipitationwere isoto-
pically higher in the 12 months pre-fire (δ18O = −6.3‰, δ2H =
−31.6‰) than the 12 months post-fire (δ18O = −7.9‰, δ2H =
−48.2‰).Post-fire, in June 2016, the lowest isotope valueswere observed, and
when a prolonged period of rain occurred at both Mt. Werong and
Wombeyan Caves, where 238 mm fell over 22 rain days, more than
three times the long-term mean precipitation for June (69.9 mm). This
was associated with two consecutive eastcoast low-pressure systems
affecting the NSW coast in one month, resulting in record high daily
and monthly rainfall records at many stations. The rain from these
two systems was significantly isotopically lower, with a monthly δ18O
mean of −14.5‰ and a δ2H mean of −98.7‰ in June 2016.
Dripwater δ18O values for 213 samples are presented in Fig. 5which
were controlled by the precipitation conditions and residual time in the
flow paths (Cuthbert et al., 2014; Jouzel et al., 2000). Until May 2016,
δ18O values ranged from −8.5‰ to −3.1‰, and δ2H values ranged
from −50.6‰ to −10.1‰. After the fire, sites behaved differently ac-
cording to their locations (See Fig. 1b). For the shallower sites (Sites
07–11), there was a response the day after the burn, with the mean
δ18O values being 4.5‰ lower than that for all pre-fire samples
F. Bian et al. / Science of the Total Environment 662 (2019) 180–191 185
Fig. 3. Box plots of descriptive statistics for all individual recharge events at the drip sites below the burnt site: duration (a), peak drip rates (b), mean drip rates (c), total drip amount (d),
and skewness and kurtosis (e). Q2 and Q3 are shaded and represent the second and third Quartiles (the inter-quartile range). First and forth quartiles are indicated via the whiskers.(−5.1‰ to −9.6‰). In contrast, there was only a slight difference in
δ18O at Sites 02–06 (−5.5‰ to −6.2‰).
The largest shift occurred during the next sampling campaigns (5
and 21 June 2016). All isotopic values of the ten burnt sites were signif-
icantly lower (−76.5‰ and −84.1‰ in δ2H for Sites 02–06 and 07–11,
respectively, and −11.9‰ and −12.7‰ in δ18O). One month later, the
sites showed a return trend to the pre-fire average until a new peak
was reached in March 2017. The isotopic values of Site 01 were consis-
tently lower than its pre-fire mean values. After December 2016, drips
at the Site 1 were isotopically lower than those at the burnt sites. Over
the post-fire hydrological year, dripwater becamemore isotopically de-
pleted than that pre-fire.
Comparison of drip water and precipitation D-excess identifies a
higher D-excess in the drip waters compared to precipitation pre-fire.
D-excess for dripwaters andprecipitation aremore similar immediately
post-fire. This lower D-excess verified that the fire depleted the stores,
such that old water which would have been affected by evaporation
was removed by the fire and the stores were replenished with new re-
charge (Froehlich et al., 2008).
The pre-fire drip water stable isotope composition was above the
global meteoric water line (GMWL; Fig. 6). The local meteoric waterline (LMWL)was established using precipitation data fromMt.Werong.
Three different groups based on their spatial locations in the cave were
plotted separately in Fig. 6a and b. In Fig. 6a, the slope for Sites 07–11
(slope = 6.5 ± 0.4) was similar to the LMWL (slope = 6.6 ± 0.3),
and Site 01 (slope = 5.8 ± 1.4). Three groundwater samples from the
borehole in Wombeyan camping area were collected, on 23 September
2015, 11 Jan and 23 March 2017, and had an average δ2H of −38.45 ±
5.10‰ and an average δ18O value of −7.31 ± 0.59‰ (Fig. 5, shaded
blue).
4.4. Solute results
The solutes showed great variability between sites. Results of
Mann-Whitney U tests showed there was a significant difference be-
tween different time periods for some drip water solute concentra-
tions (Table 1, and the time series for individual solute in
Supplementary Fig. 4). The concentrations of all bedrock-derived
solutes in drip water (Ca2+, Mg2+ and Sr2+) were significantly
lower after the fire. Barium did not change immediately but de-
creased after six months. The decreases in concentrations of Ca2+,
Mg2+ and Sr2+ were substantial one month post-fire and partially
186 F. Bian et al. / Science of the Total Environment 662 (2019) 180–191
Table 1
Descriptive statistics and Mann-Whitney U test results for solute concentrations. Samples are separated into three groups (pre-fire, 1 month post-fire and 6 months post-fire). Statistical
significance (p b 0.05) is indicated in bold text. Z-scores represent the number of standard deviations between each data point and the mean.
Ion Sampling Time Number Min Quartile 1 Median Quartile 3 Max U Z Exact probN|U|
B2+/μg∙L−1 Pre-fire 7 4.9 11.6 27.4 33.3 62.4
1 mth post-fire 15 2.2 3.5 5.0 10.2 14.4 95.0 3.0 0.0
Ba2+/mg∙L−1 Pre-fire 12 1.4 3.9 4.4 6.4 23.3 –
1 mth post-fire 13 1.6 2.3 3.0 12.8 30.1 105 1.4 0.2
6 mth post-fire 10 2.0 2.0 2.0 4.3 6.0 94 2.2 0.0
Pb3+/μg∙L−1 Pre-fire 14 0.0 0.0 0.1 0.2 0.2
1 mth post-fire 15 0.0 0.0 0.0 0.1 0.1 147.5 1.8 0.1
Ca2+/mg∙L−1 Pre-fire 40 47.0 87.2 107.9 119.1 150.5 –
1 mth post-fire 14 49.4 57.1 65.8 74.4 81.9 525 4.8 0.0
6 mth post-fire 10 67.9 75.4 91.8 99.8 110.0 288 2.1 0.0
Mg2+/mg∙L−1 Pre-fire 29 0.4 0.8 1.3 1.5 2.3 –
1 mth post-fire 14 0.6 0.7 0.7 0.8 1.4 311 2.8 0.0
6 mth post-fire 10 0.7 0.8 1.1 1.2 1.6 179 1.1 0.3
Sr2+/μg∙L−1 Pre-fire 38 1.4 61.4 72.5 84.5 109.4 –
1 mth post-fire 13 0.2 36.2 40.1 71.7 78.9 384 3.0 0.0
6 mth post-fire 10 37.0 46.8 50.5 57.8 64.0 10 3.5 0.0
Na+/mg∙L−1 Pre-fire 20 1.1 1.6 1.8 2.0 2.6 –
1 mth post-fire 14 1.0 1.2 1.4 2.0 2.0 190 1.7 0.1
6 mth post-fire 10 1.1 1.2 1.3 1.9 2.5 142 1.8 0.1
Cl−/mg∙L−1 Pre-fire 27 1.2 2.5 3.1 4.0 11.6 –
1 mth post-fire 14 2.1 2.7 4.5 6.7 13.1 12 −1.5 0.1
6 mth post-fire 10 2.7 2.9 3.7 6.5 14.0 101 −1.2 0.3
K+/mg∙L−1 Pre-fire 21 0.1 0.6 0.8 3.0 25.2 –
1 mth post-fire 14 0.1 0.1 0.4 1.0 1.5 218 2.4 0.0
6 mth post-fire 10 0.1 0.1 0.2 0.3 1.2 190 3.6 0.0
Si4+/mg∙L−1 Pre-fire 39 1.0 1.5 2.0 2.0 2.3 –
1 mth post-fire 14 1.0 1.1 1.2 1.3 1.4 516 4.9 0.0
6 mth post-fire 10 1.2 1.4 1.5 1.6 1.8 303 2.7 0.0
I−/mg∙L−1 Pre-fire 19 0.1 0.1 0.3 1.5 7.3 –
1 mth post-fire 8 0.1 0.5 1.0 1.3 1.5 61 −0.8 0.4
6 mth post-fire 10 0.1 0.1 0.1 0.1 0.1 180 4.0 0.0
SO2−4 /mg∙L−1 Pre-fire 27 0.0 0.1 0.1 0.2 0.4 –
1 mth post-fire 14 0.3 0.4 1.2 6.2 20.6 3 −5.1 0.0
6 mth post-fire 10 0.4 0.6 0.8 1.4 2.6 0 −4.6 0.0
Fe2+/mg∙L−1 Pre-fire 35 0.6 1.5 2.0 2.8 5.5 –
1 mth post-fire 14 0.9 1.1 2.0 3.2 9.7 243 0.0 1.0
6 mth post-fire 6 0.0 0.0 0.0 0.0 0.0 210 3.9 0.0recovered six months post-fire. Chloride remained lower after the
fire. Sulphate increased 10-fold one month post-fire and although
it decreased between one and six months post-fire, it remained
above pre-fire levels. Iodide slightly increased initially, but returned
to below the pre-fire level half a year later.
5. Discussion
5.1. Changes in cave drip hydrology and Karst architecture
By comparing log10-transformed changes in drip rates over time
(see Supplementary Fig. 3), the drainage stages during the recession
stage of individual recharge events can be identified. These stages are
represented conceptually in Fig. 7. Through the changes of the slopes
(the log10-transformed rates of decrease in recharge), we can interpret
the changes to the dominant recharge flow patterns at each site. Three
different stages in the recession stage of recharge for individual precip-
itation events are identified.
a) In the first stage of recession (Fig. 7, green), when both the soil and
karst are saturated with event water, soil moisture is above field ca-
pacity, which permits themaximum possible recharge rate from the
soil to the karst stores. The major pathway for the event water is
through preferential paths in the surface soil directly to the karst
fractures, with bypass or overflow of the karst stores. Flow rates
are mainly restricted by the minimum diameter of the fracture in
the karst, or the internal diameter of the stalactite.
b) In the second stage (Fig. 7, blue), the soil is no longer saturated
enough to support preferential flow in the soil and overflow orbypass flow in the karst. Soil diffuse flow from the soil into the
karst stores becomes the dominant flow pathway. In this way, rela-
tively slower recharge rates from the soil, and longer residence time
in karst stores, generate lower slopes. In some cases, especially in the
inner part of the cave like Site 07, this stage is less frequently ob-
served or even mixed with the first stage. We hypothesise that this
is due to lower soil water storage volumes andweaker capillarity ef-
fects from a thinner soil layer.
c) In the final stage (Fig. 7, yellow), when the surface precipitation has
stopped and soil moisture falls below field capacity, drainage to the
karst by soil diffuse flow also stops. The amount of new event water
is now low. The cave drips are quickly recharged from the residual
karst stores, and the drip rate is again limited by the diameters of
the fractures or stalactites.
There is no sign of any original sedimentary structures in the
Wombeyan Limestone as it is formed of marble and therefore it effec-
tively contains only secondary porosity. Preferential flow in the soil
and overflow or bypass flow in the karst dominates cave recharge in
Wildman's Cave. Theoretically, discharge from soil storage would be
buffered by soil capillarity (Fredlund and Rahardjo, 1993) resulting in
moderate drip rates and soil diffuse flows to the karst and cave below.
When there is no interaction between the soil water store and the bed-
rock, discharge only occurs from the karst water store and drip rate
peaks are likely to decline more rapidly.
Fires can change the physical and chemical properties of soil
(Bonacci et al., 2008). Post-fire, with low soil water content and high
F. Bian et al. / Science of the Total Environment 662 (2019) 180–191 187
Fig. 4.Monthlyweightedmean rainfall atMt.Werong comparing to recorded rainfall amounts. Rainfall D-excess is also displayed. The vertical line indicates the timing of the experimental
fire (25th May 2016 indicated by the red line).air saturations, the relative permeability towater is negligible. Therefore
soil diffuse flow should be severely reduced after the fire, at least ini-
tially (Russo, 1998). Immediately post-fire, in an initially gas saturated
soil, part of the first rechargewill be consumed to fill the capillary reser-
voir, and will therefore not contribute to the flow recorded in the cave
drips. An increase in soil hydrophobicity can affect flow paths within
the soil (Huffman et al., 2001). Stoof et al. (2014) reported that fire-
induced preferential flows are associatedwith a change in soil water re-
pellency. Plaza-Álvarez et al. (2018) also reported the increased soil
water repellency after prescribed burns in forest ecosystems, and a re-
covery trend was monitored. The total storage volume is unchanged
post-fire, with similar total recharge amounts pre- and post-fire (see
Fig. 3c). The post-fire recharge in Wildman's Cave was more intense,
having a higher magnitude, shorter duration and less frequentFig. 5. δ2H (a), δ18O variations and D-excess (b) of drip water from 01/01/2016 to 0101/2017. T
isotope values for local groundwater: δ2H=−38.45‰±5.10‰, δ18O=−7.31‰±0.59‰. The
fire isotope data.hydrographic peaks than the pre-fire recharge. Hereby, a post-fire in-
crease in preferential flow in the soil could explain the observed change
in drip rates in the three stages described in Fig. 7.
Three effects of fire on the recession stages (arrows in Fig. 7) are de-
scribed below:
Effect 1) For the deeper sites, which were covered with a thicker soil
layer (Sites 02–04), A Model predict increases in mean post-
fire drip rates for all precipitation events across all the stages
of recession. The peak flow rates increased slightly. Fire-
induced increases in soil preferentialflowpathways increased
the drip rates in the stage a green. In the second stage, an in-
crease in soil hydrophobicity weakened the soil's capillary ac-
tion, which substantially decreased the duration of the soilhe experimental fire is indicated by the red line. Horizontal blue band represents themean
x-axis scale is expanded for the period 01/03/2016 to 1/09/2016 to better show the post-
188 F. Bian et al. / Science of the Total Environment 662 (2019) 180–191
Fig. 6. δ2H and δ18O data for pre-fire (a) and post-fire samples (b).
Fig. 7. Conceptual models for the fire influence on vadose zone discharge events during indiv
represent the preferential flows dominated stage, b in blue the soil diffuse flows dominated
diffuse flow stage, and B model indicates with extreme low soil preferential flow.diffuse flow controlled stage. In the final recession stage, the
karst stores were unaffected by the surface fire, and there
was no change in the draining of the last water from the frac-
tures.
Effect 2) For the shallower sites (Site 07), B Model describes the ampli-
fied effects of changes in soil hydrophobicity and preferential
flows. At Site 07, soil diffuse flow was the dominant process
pre-fire, and the preferential flows were not obvious. Post-
fire, newly created preferential flows dominated the soil dif-
fuse flows, so that the drip rate for this site was mainly con-
trolled by the first stage.
Effect 3) For sites in themiddle part of the cave, deeper than Site 02 and
with a thinner soil layer than Site 01 (e.g. Site 05), there was a
similar pre-fire drip rate A Model to that at Sites 02–04. In-
creased preferential flows dominated the soil diffuse flows
which originally appeared in the second stage.
Both Sites 06 and 11 were fed by two asynchronously dripping sta-
lactites, making it difficult to identify the specific stages (See Supple-
mentary Fig. 3). However, increased preferential flows possibly still
occurred at both sites. Overall, we propose that the fire-induced in-
crease in preferential flows and the loss of soil capillarity both led to
more intense cave drip recharge.
5.2. Stable isotope behaviour and residence time of drip waters
Fig. 4 showed that Wildman's Cave drip water δ2H and δ18O values
were less variable than the values for Mt. Werong precipitation, indica-
tive of mixing of water in the soil and karst. Figs. 4 and 5 above show
that the variability in drip water isotopic composition broadly follows
that of precipitation, albeit damped and lagged. Post fire, drip water sta-
ble isotope composition has a negative isotopic excursion following a
highmagnitude (164.0mmover three days), isotopically low precipita-
tion event, which occurred one week after the fire.
The post-fire decrease in drip water δ18O, associated with a conver-
gence of drip water and precipitation d-excess, is attributed to the re-
charge from this post-fire precipitation event. No similar δ18O or δ2Hidual precipitation events. Y-axis is displayed in log10 transformation. Stages a in green
stage, c in yellow the residual flows. A model indicates a drip pattern with notable soil
F. Bian et al. / Science of the Total Environment 662 (2019) 180–191 189isotope excursions are observed in the dripwaters in the pre-fire period
(Fig. 4), although no equivalently large and isotopically anomalous pre-
cipitation events occurred over this timeperiod (Supplemental Table 2).
Fromour drip hydrology datawewould infer a loss of soil capillarity and
an increase in preferential flow after the fire, which might be expected
to increase themagnitude of the drip water isotopic response to precip-
itation. Changes in the slopes of δ2H vs δ18O for dripwaters pre and post
fire, in comparison to the local meteoric water line, could help elucidate
changes in soil and vadose zone residence time if drip waters were af-
fected by evaporation, however results are inconclusive as observed
changes in slopes are within their uncertainties (Fig. 6). Comparison
of d-excess of drip water and precipitation shows that drip water d-
excess returns to a higher value than precipitation at the end of the
monitoring period, suggesting a return to pre-fire conditions by this
time.
We therefore summarise that we do not see more positive isotopes
as reported by Nagra et al. (2016), and attributed to increased evapora-
tion and partial enrichment of soil water isotope due to evaporative
fractionation. We hypothesise that for our experimental fire, whose se-
verity was such that soil properties changed, that there was a complete
evaporation of soil and shallow vadose zone water. Drip water isotopic
compositions post-fire therefore represent the isotopic characteristics
of the first recharge, and subsequent recharge events helped replenish
the karst fractures and stores and soil water, whose isotopic composi-
tion gradually returns to an integrated mean of those event after
6 months.Fig. 8. Ratios of Sr/Ca (a), Mg/Ca (b), and ln(Sr/Ca) vs. ln(Mg/Ca) (c). The vertical yellow
line indicates the timing of the experimental fire (25th May 2016).5.3. Drip water solutes signatures
The hydrological interpretation indicates a decrease in water resi-
dence time, which is associated with decreased dissolution of bedrock
and subsequent precipitation of calcite (Fairchild and Treble, 2009).
Bedrock-related solutes including Ca2+, Mg2+ and Sr2+ decreased one
month post-fire (see Table 1). This could reflect a decrease in carbonate
dissolution due to the evaporation of old storage water and a shorter
water residence time. Decreased soil CO2 from the destruction of plant
roots and microbial communities could also enhance this effect.
Comparison of Mg/Ca and Sr/Ca ratios can be used as a geochemical
signature of the amount of bedrockdissolution based on theprior calcite
precipitation effects (Treble et al., 2003; Fairchild and Treble, 2009).
These ratios did not show any notable differences between the pre-
and post-fire groups within the study timescale (see Fig. 8). It suggests
that, for this shallow cave, each precipitation event barely reached satu-
ration for calcite before discharging into the cave.Within-cave evidence
for this interpretation is abundant stalactite formations occurring with-
out associated deposition of stalagmites.
Despite ash being present across the fire site, there was only limited
geochemical evidence of ash-derived solutes in the drip waters. Sulfate
drip water concentration significantly increased ten-fold one month
after fire, whereas concentrations of analysed cations decreased after
thefire experiment. The lack of a post-fire rise in ash-derived cations in-
cluding K, Na and Fe thatwould be expected from ash production, could
be due to the volatilisation of solutes at high temperatures (Bodí et al.,
2014). Thefire temperature exceeded 929 °C in some areas (see Supple-
mentary Fig. 1)which could volatilise organic compounds leavingwhite
ash, which was observed at the experiment site (Fig. 1). Moreover, I−
andCl− concentrations both increased slightly, respectively, and I− con-
centration increased significantly six months after the fire. Concentra-
tions of Cl− also indicated the removal of soil water by the fire. This
agrees with limited saturation index values for calcite calculated from
6 drip activities, from 0.44 ± 0.13 pre-fire to 0.62 ± 0.17 post-fire.
In summary, the combustion of vegetation and soil fauna is
hypothesised to lead to a decrease in limestone dissolution (lower
Mg2+, Ca2+, and Sr2+and a volatilisation in soil- and vegetation-
derived solutes (K+, Na+, Si4+, I− and Fe2+).6. Conclusion
This research has demonstrated the impacts of a high-severity ex-
perimental fire on the karst vadose zone, including 1) short-term com-
plete evaporation of soil water; 2) increased preferential flows and
decreased soil diffuse flows; and 3) increased soil hydrophobicity.
Three different stages of discharge: preferential flows dominated
stage, soil diffuse flows dominated stage and residual flows dominated
stage were defined based on the hydrograph analysis on the cave drip
rates in this study. Based on that, amore intense stage 1 post firewas at-
tributed to the increased preferential flows, and decreased stage 2 to the
decreased importance of soil diffuse flow post fire.
The short-term responses observed in cave drip water isotopic and
hydrochemical composition were large shifts in stable water isotope
composition to lower values and decreased concentrations of bedrock-
derived solutes within the first month post-fire. The lower isotopic
values show that older pre-existing soil water with a higher isotopic
value was removed by evaporation due to the heat of the fire. A shorter
water residence time post fire resulted in relatively lower values with
respect to δ2H and δ18O. A post-fire return to a more enriched isotopic
composition was observed.
The bedrock-related solute concentrations decreased after the fire
experiment because of lower recharge durations and potentially
190 F. Bian et al. / Science of the Total Environment 662 (2019) 180–191decreased partial pressure of carbon dioxide (pCO2). Ash products were
largely volatilised due to the severe intensity of the fire and were not
captured by the cave, instead leaving white-coloured ash above the
cave.
This study has demonstrated a potential explanation for the lower
δ18O values in fire records which is opposite to the higher δ18O from in-
creased albedo observed by Nagra et al. (2016), and provides evidences
to utilise the decreased bedrock solutes as one important factor for
paleo-fire tracing. If another fire experiment was conducted during
dry season, a smaller change in the hydrological regime would be ex-
pected as antecedent soil moisture would already be low. In addition
to this, similar nutrient solutes trends should be observed with similar
fire volatilisation.
Acknowledgements
The researchwas partly funded by Australian Research Council Link-
age LP13010017 and Land & Water Australia grant (ANU52) to Pauline
Treble. The authors appreciate the efforts from team at Wombeyan
Karst Conservation Reserve, especially David Smith, the manager for
his support and assistancewith logistics and accommodation.We thank
Andy Spate and Sophia Meehan for overall project design for
LP13010017. Martina De Marcos and Xiaolin Shan helped with in field
work and Yuyan Yu with documents. Thank you to Bob Cullen for
collecting rainfall samples at Mt. Werong, Barbara Gallagher and
Jennifer van Holst (ANSTO) for analysis of rainfall samples, and the Syd-
ney Catchment Authority for providing rainfall data. And the authors
want to give special thanks to the anonymous reviewers from Science
of the Total Environment, especially on the interpretations of fire
influence.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.scitotenv.2019.01.102.
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