Wet season hydrochemistry of Bribin Cave
Wet season hydrochemistry of Bribin Cave
in Gunung Sewu Karst, Indonesia
Tjahyo Nugroho Adji
Environmental Earth Sciences
ISSN 1866-6280
Environ Earth Sci
DOI 10.1007/s12665-012-1599-x
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Environ Earth Sci
DOI 10.1007/s12665-012-1599-x
ORIGINAL ARTICLE
Wet season hydrochemistry of Bribin Cave in Gunung Sewu
Karst, Indonesia
Tjahyo Nugroho Adji
Received: 17 March 2011 / Accepted: 2 February 2012
Ó Springer-Verlag 2012
Abstract This research was conducted on the Bribin
River, the most important underground river in the Gunung
Sewu Karst, Gunung Kidul, Java, Indonesia. The main
purpose of this study was to define the wet-season hydrochemistry of this river. This research also focuses on
identifying the relationship between hydrochemical
parameters to provide better aquifer characterization.
Water-level monitoring and discharge measurements were
conducted over a 1-year period to define the discharge
hydrograph. Furthermore, baseflow-separation analysis is
conducted to determine the diffuse-flow percentage
throughout the year. Water sampling for hydrogeochemical
analysis is taken every month in the wet season and every
2 hours for two selected flood events. To describe the
hydrogeochemical processes, a bivariate plot analysis of
certain hydrochemical parameters is conducted. The results
show that the diffuse-flow percentage significantly controls
the river hydrochemistry. The domination of diffuse flow
occurs during non-flooding and flood recession periods,
which are typified by a high value of calcium and bicarbonate and low CO2 gas content in water. Conversely, the
hydrochemistry of flood events is characterized by the
domination of conduit flow and CO2 gas with low calcium
and bicarbonate content. According to the wet-season
hydrochemistry, it seems that the small- and medium-sized
fractures in the Bribin aquifer still provide storage for the
diffuse and fissure flows, although the conduit fracture is
already developed.
T. N. Adji (&)
Department of Environmental Geography,
Gadjah Mada University, Jogjakarta 55281, Indonesia
e-mail: adji@geo.ugm.ac.id
Keywords Hydrochemistry Bribin Cave
Gunung Sewu Karst Indonesia
Introduction
The Gunung Sewu Karst area was initially described by
Danes (1910) and Lehmann (1936). The area is located
within the central part of the island of Java, Indonesia. In
Javanese traditional language, ‘‘Gunung’’ means mount or
hill, while ‘‘Sewu’’ means thousand; thus, Gunung Sewu
means the area with a thousand hills. This karst area is
characterized by the development of positive formations of
blunt conical hills (kegelkarst). Kegelkarst, according to
Sweeting (1972), are categorized as tropical karst formations. Several other geomorphologists affirm Lehmann’s
opinion (Flathe and Pfeiffer 1965; Balazs 1968, 1971;
Verstappen 1969; Waltham et al. 1983). Balazs (1968)
verifies that the number of hills in Gunung Sewu amounted
to approximately 40,000, with a density of approximately
30 hills/km2. Recent studies related to this karst region
have been published by Haryono and Day (2004), Ahmad
et al. (2005), and Urushibara-Yoshino and Yoshino (1997).
The major underground river system in this region is the
Bribin-Baron system, which has an outlet discharge of up
to 8,000 l/s (MacDonalds and Partners 1984). This system
is the most important river in the Gunung Sewu Karst
(Fig. 1). In terms of water resources, the number of people
who depend on this water supply system is over 200,000.
The Bribin River catchment area was first defined by
Fakultas Kehutanan (1993) as being bounded on the
upstream (north) side by a massive old-volcanic mountain
and by the ancient Ponjong-Polje in the west (Srijono and
Aldilla 2006), while the southern and eastern boundaries
have not yet been defined. This paper discusses the results
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Fig. 1 Bribin River catchment
and Bribin-Baron system
(MacDonalds and Partners
1984)
of a quantitative evaluation of wet-season hydrogeochemistry in Bribin Cave, with the primary focus on a better
understanding of karst aquifer characteristics within this
area.
Climate and hydrogeological situation
The most recent research related to rainfall condition in
Gunung Sewu was conducted by Brunsch et al. (2011),
who note that there are spatial and temporal difference in
rainfall condition. The spatial variation depends on the
proximity to the sea and the elevation, whereas the temporal variation is regulated by climate circumstances.
Long-term rainfall declines slightly, whereas there is a
clear decrease in precipitation from 2002 to 2009. The
rainfall intensity rises from December to February and then
decreases in most of the other months. Even propensities
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toward an extended dry predict period could be perceived.
In addition, the El Nin˜o and La Nin˜a events have influences
on the rainfall distinction in the dry period with years of
either extremely low or high rainfall rates. Some studies on
global climate conditions in Gunung Sewu were also
conducted by Verstappen (1997) and Urushibara-Yoshino
and Yoshino (1997). Studies related to climate in the
Quaternary period in the vicinity of the study (Java Island)
were performed by Urushibara-Yoshino (1995), Verstappen (1975, 1994), Dam (1994), Morley (1982), and Budel
(1975). Meanwhile, information on climate conditions in
the Bribin River catchment can be found in publications by
Adji and Nurjani (1999), Suryanta (2001), MacDonalds
and Partners (1984), BMG (2000, in Sutikno and Tanudirjo, 2006), and Fakultas Kehutanan (1993). These
studies show that the monthly average temperature in the
Bribin River catchment area ranged from 22 to 28°C.
Meanwhile, the annual rainfall in Gunung Sewu ranges
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Environ Earth Sci
from 2,000 to 2,500 mm/year from the data obtained at 12
rain-gauge stations in the period of 1947–2000. The
research brief by Adji (2010) indicates that the air humidity
in Bribin River catchment ranges between 60 and 90%,
with a temperature varying between 24 and 28°C. Because,
the catchment is located in a tropical region, knowledge of
climatic conditions, especially rainfall and temperature, is
relevant to the water input component associated with the
karst hydrogeochemistry.
Geologically, the study area is dominated by Miocene
limestone of the Wonosari Formation, which consists of
massive coral reef limestone in the south and bedded
chalky limestone in the north (Balazs 1968; van Bemmelen
1970; Waltham et al. 1983; Surono et al. 1992). The results
of an interpretation of a 1:50,000 scale aerial photo and a
1:1,000,000 scale ERS image by Kusumayudha (2005)
show that the moment structure consisting of thick cracks,
faults, and fractures in Gunung Sewu have a general
structure direction from northwest to southeast and from
northeast to southwest. The regional structure is then
divided into blocks bounded by faults that also define the
hydrogeological system since the creation of the low-level
and high-level configuration (Kusumayudha, 2005). The
catchment of Bribin River is included in the WonosariBaron Subsystems, which have a volcanic rock (bedrock)
configuration as a graben with a northeast-southwest
direction (Fig. 2).
Bribin River was first described by MacDonalds and
Partners (1984). This river begins at the surface of Pentung
River and then disappears into the Sawahombo sinkholes.
The river comes out in Luweng Jomblangan, emerges again
in Gilap Cave, Luweng Jomblang Banyu, Luweng
Jurangjero, and at last appears in Bribin Cave before
coming out as a large resurgence at Baron Beach in the
Indian Ocean. In the section between Luweng Jurang Jero
and Bribin Cave, it leaks into the Ngreneng Cave, which
also flows into Baron Beach. Adji (2011) notes that the
minimum discharge is approximately 1,600 l/s, while the
maximum discharge is recorded at approximately 2,500 l/s.
Methods
Water-level data logger and discharge measurements were
conducted for a 1-year period to obtain the discharge
hydrograph throughout the year. The stage discharge rating
curve used is based on research by Suryanta (2001). The
diffuse-flow proportion (baseflow) was defined using the
automated baseflow separation by digital filtering method
(Eckhardt 2005) based on a recession constant value in the
hydrograph, which was then correlated to the value of base
flow indices (BFI) within a karst aquifer. The following
formula was used:
qbðiÞ ¼
ð1 BFImax Þaqbði1Þ þ ð1 aÞBFImax qi
1 aBFImax
where qb(i) is the baseflow at time i, qb(i-1) is the baseflow
at the previous time i-1, qi is the total flow at time i, a is
the recession constant and BFImax is the maximum
baseflow that can be measured or performed. To find the
constant of the recession, we used the formula:
Qt ¼ Q0 eat
where Qt is the flow rate at time t, Q0 is the initial discharge
in the recession segment, and is a constant. Furthermore,
e- may be replaced by k, which is known as a recession
constant, or depletion factor. This constant is often used as
an indicator of the continuity of baseflow (Nathan and
McMahon 1990). Then, the value of k is compared with the
classification of the karst underground river recession by
Worthington (1991, in Gillieson 1996).
Water sampling was conducted once a month for a year
to cover two seasons (wet and dry). In addition, detailed
sampling was performed twice hourly during two flooding
Fig. 2 West–East
hydrogeological conceptual
model of Gunung Sewu
(Kusumayudha 2005)
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events. Chemical analysis of the samples in the laboratory
included the cations Ca2?, Mg2?, Na?, and K? and the
anions Cl-, SO42-, and HCO3-. The volumetric method
was used for the elements Ca2?, Mg2?, CO3- and SO42-,
the spectrophotometric method was used for HCO3-, and
the flame photometry method was used for Na? and K?.
Another analysis was the calculation of PCO2 (partial
pressure of carbon dioxide gas), which is assumed to have
reached equilibrium with the water samples (Stumn and
Morgan 1981; Drever 1988; White 1988). With the assistance of Wateq-4F software (Ball and Nordstrom 1991),
the PCO2 in all samples was determined.
þ
PCO2 ¼ HCO
3 ½H = K1 KCO2
where PCO2 is the partial pressure of carbon dioxide gas in
water; [HCO3-] is the activity of bicarbonate ions, [H?] is
hydrogen ion activity, K1 is the equilibrium constant
reaction solvent at 25°C, and KCO2 is the equilibrium
constant of CO2 in water at 25°C. To describe the hydrogeochemical processes, a scatter-plot analysis with a small
sample size (non-discrete parameters) was conducted that
included the following: (1) discharge-major ion concentration, (2) specific conductivity-calcium and bicarbonate,
(3) discharge-diffuse flow, (4) diffuse flow-log PCO2 , (5)
diffuse flow-calcium and bicarbonate, and (6) sodium
chloride. A chemograph analysis was also performed to
interpret the relative contribution of different waters and
was compared with three models of chemographs described
in the literature (in Perrin 2003).
Result of wet-season hydrochemistry of Bribin Cave
Figure 3 shows the wet season hydrochemograph of the
underground river in Bribin Cave, which is based on
sampling measurements from May 2006 to April 2007.
This hydrochemograph shows that the dissolved constituents at the flood events vary and tend to decrease, as
there are rainfall recharges through the conduit. Comparing
between the beginning and end of the rainy season, the
temperature and conductivity values are nearly identical,
i.e., in the range of 27°C and 500 lS/cm. This result proves
that the diffuse flow component has a dominant role at
times when flooding does not occur. Differences are shown
in calcium, bicarbonate, and pH values, which do not reach
the initial conditions, allowing the dilution of the underground river to still proceed by the fissure flow component.
However, this dilution does not affect the value of temperature and conductivity. Meanwhile, the bivariate plot
between the diffuse flow percentage and discharge variation has a negative relationship, with R2 = 0.91 (Fig. 4). It
also appears that a slight deviation occurs at the beginning
and end of the rainy season. These changes occur during
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the dominance of the flow component due to the time lag of
the fissure and diffusion to recharge the underground river
as a result of different rain intensities throughout the wet
season. Next, a bivariate plot between the diffuse flow and
calcium or bicarbonate and between the diffuse flow and
CO2 partial pressure are presented in Figs. 4 and 5.
According to Fig. 5, there is a grouping of sample positions based on characteristic stages of flood events (flood
peak and recession stages), and it is influenced by the time of
the beginning or the end of the rainy season. This result may
be caused by high variations in diffuse flow during flood
events. In addition, bivariate plotting between diffuse flow
and calcium or bicarbonate indicates that the dissolved
constituent in water at the beginning of the rainy season
consists of higher diffuse flow and calcium content than all
other samples because of the rainwater dilution in the peak of
the wet season. In contrast, the samples at the time of the
flood peak have the lowest content in diffuse flow and dissolved constituents. Another interesting phenomenon is that
the samples in the flood recession of the 22 February flood
event exhibit lower major dissolved constituent (Ca2? and
HCO3-) contents than the 19 March flood event. This result
indicates that the groundwater storage within the karst
aquifer is recharged sufficiently by rainwater, thereby
increasing the percentage of diffuse flow and the relatively
more saturated fissure of carbonate minerals that have
undergone a process of water–rock interaction. Consequently, the diffuse flow in the 19 March flood event
(end of rainy season) increases. Next, Fig. 6 shows the
relationship between CO2 partial pressure and diffuse
flow percentage.
According to Fig. 6, there is a positive correlation
between CO2 partial pressure and the diffuse flow. In
detail, the flood peak in 22 February is characterized by a
lower value of the log PCO2 and low diffuse flow content.
This condition differs greatly from the March 19 flood
peak, which shows a greater log PCO2 and diffuse flow
percentage. These values are still higher when compared
with the samples during the first flood recession of 22
February. In addition, the non-flooding conditions present
higher diffuse flow content and log PCO2 values than at the
time of flooding. It can be summarized that the variation of
log PCO2 is associated with the percentage of diffuse flow,
so that the process of interaction of water–gas–rock is more
dominant than the water–rock interaction only.
Result of Bribin Cave hydrogeochemistry within flood
events
Sampling within flood event conditions in Bribin was
conducted for two flooding incidents (22 February and 19
March 2007) to obtain a detailed description related to
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Fig. 3 Wet-season
hydrochemograph of Bribin
River
hydrogeochemical fluctuations within flood events. Here,
samples are taken every 2 h. Next, the chemograph that
describes the fluctuations of discharge, diffuse flow, dissolved constituent, and the partial pressure of CO2 in the
flood hydrograph is presented in Fig. 7. In the left part of
Fig. 7, it seems that the diffuse-flow percentage decreases
if the discharge increases at the 22 February 2007 flood
event; in other words, the diffuse flow decreases with
increasing discharge to reach its peak discharge. Meanwhile, the response of diffuse flow on the 19 March flood
event exhibits a similar trend (Fig. 7, right). In summary,
from the two flood occasions in Bribin Cave, it can be
explained that the diffuse flow has a tendency to decrease
with increasing discharge to the peak discharge and then
gradually increases shortly after passing the peak
discharge.
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Fig. 4 Bivariate plot between diffuse flow and discharge during wet
season
Fig. 6 Bivariate plot between diffuse flow and log PCO2
The relationship between discharge and diffuse flow at
the two flood events in Bribin Cave is presented in Fig. 8.
It seems clear that there is a negative relationship between
discharge and diffuse flow in both flood events, and even
the value of R2 reaches a significant correlation (above
0.99). This decrease in percentage is a result of the
replacement of diffuse flow from karst aquifers by conduit
flow, which is directly recharged from rainfall through the
conduit passage. In the period of decline in diffuse flow,
the process of dilution by precipitation occurs along with
the increase in the Bribin River discharge. Then, the high
correlation between the discharge increase and the diffuse
flow decrease, or vice versa, indicates that the karst-aquifer
system is controlled by good conduit development.
Figure 8 shows that there are different variation tendencies (PCO2 and discharge values) between flood
events for 22 February and 19 March. In the 22 February
flood event, the PCO2 shows a downward trend at the
time to reach peak discharge and even continues for
some time after the peak discharge. The decreasing PCO2
value indicates that the water is still affected by remnants of diffuse and fissure flow, which are driven by the
rainwater that comes afterward; thus, it has a tendency to
gradually decrease. These results are in accordance with
those described by Vesper and White (2004), Raeisi and
Karami (1997), and Liu et al. (2004). At the beginning
of the recession period, the PCO2 increases significantly,
although it does not reach the initial condition (before
the flood event). This behavior may be caused by the
mixing process between diffuse, fissure, and conduit
flow. In addition, the PCO2 variation within the 19 March
flood event (end of wet season) exhibits a lower correlation with diffuse flow. The variation and correlation
between diffuse flow and the PCO2 value at the two flood
events the possible domination of water–gas–rock interaction within the flood events rather than only water–
rock interaction, as in the dry season. However, the 22
February flood tends to show a stronger water–gas–rock
interaction than the 19 March flood event, which indicates a shifting from the water–gas–rock interaction to
the water–rock interaction. This tendency can be further
confirmed by looking at the variations of Ca2?, HCO3-,
and electrical conductivity values, and comparing the
correlation between diffuse flow and PCO2 .
Fig. 5 Bivariate plot between diffuse flow-calcium (left) and diffuse flow-bicarbonate (right)
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Fig. 7 Bribin cave hydrochemograph within the 22 February 2007 flood event (left) and within 19 March (right)
Fig. 8 Bivariate plot between diffuse flow and discharge within the 22 February (left) and 19 March (right) flood events
The variation of electrical conductivity (EC), calcium,
and bicarbonate in the two flood events (Fig. 7) is similar
to the model described in Perrin (2003). In his second
flood-chemograph model, he explains that the content of
dissolved constituents in water gradually rises when the
discharge rises to its peak (rising limb), is followed by
water influenced by dilution by the precipitation process,
reaches its peak some time after the peak discharge (EC
down), and then slowly falls back to the water chemistry
condition before the flood event. If the fracture type in the
Bribin River aquifer is dominated by a large conduit only,
the rainwater will be rapid and will exhibit the same travel
time with the discharge increasing. However, the Bribin
River aquifer also contains fissure and diffuse storage in
medium and small fractures (high dissolved element contents) and is subsequently pushed by conduit water, which
led to EC increasing first, followed by the domination of
rainwater (EC decreases).
The calcium value in the 22 February flood event
showed a decreasing trend with increasing discharge, while
the bicarbonate shows similar trend as the EC value. Furthermore, the analysis of bivariate plots between diffuse
flow and calcium or PCO2 have correlation (R2) values of
approximately 0.5. This result indicates that both the
water–gas–rock and water–rock interactions during the
floods had a nearly balanced contribution (Fig. 9). This
result is in contrast to the negative correlation in the dry
season (Adji 2010). However, the correlation value within
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Fig. 9 Bivariate plot between diffuse flow and PCO2 in Gua Bribin (left) and between diffuse flow and calcium (right) at the 22 February flood
event
Fig. 10 Bivariate plot between diffuse flow and PCO2 in Gua Bribin (left) and between diffuse flow and bicarbonate (right) at the 19 March flood
event
Fig. 11 Bivariate plot between natrium and chloride in the 22 February (left) and 19 March (right) flood events
approximately 0.5 indicates that the diffuse flow and CO2
gas are not the only factors controlling water dissolved
constituents at flood events. In addition, the 19 March flood
event shows that increasing diffuse flow is associated with
decreasing bicarbonate (R2 = 0.63), as shown in Fig. 10.
This result is different from what happened in the first flood
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event, as this second flood event has already shown a stable
diffuse and fissure flow in the river. In general, conduit
flow may directly dilute river water, so there is no large
time lag between the flood peak and decreasing bicarbonate
or calcium content. Here, the effect of CO2 gas at this flood
event still shows a substantial correlation (R2 = 0.36),
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although its role has been reduced compared with the first
flood event (22 February), which is related to the
decreasing intensity of rainfall (end of wet season).
The variation of pH and temperature within the flood
hydrograph is theoretically more dependent on the karst
flow components that dominate at the time of measurement. Bivariate plot of Na? and Cl- at the two flood events
are presented in Fig. 11. This figure shows that the ratio of
sodium-chloride ions at times of peak flood and shortly
thereafter have a ratio close to the rainwater ratio (1:1),
while the sample position farthest from the line ratio of 1:1
is during the period of recession prior to flood events.
Furthermore, the position of the samples during the period
of recession after the flood peak is located between the
position of the pre-flood and the flood peak.
These results indicate that the increase in discharge or
decrease in diffuse flow causes the ratio of sodium-chloride
to approach the rainwater ratio (line 1:1). This pattern
indicates the start of conduit flow domination through the
process of dilution by precipitation within the underground
river, whereas in periods of recession after peak flood, this
ratio begins to slowly increase and move away from the 1:1
line. An interesting pattern is shown by the ratio of sodiumchloride ions in the 19 March flood, where the ratio at the
peak flood is enough to deviate from the 1:1 line, while the
ratio pre- and post-flood shows no significant difference.
This result can be explained by the second flood event,
which is already entering the end of the wet season; thus,
the diffuse flow is largely dominated by dissolved minerals
compared to the time of the first flood. Consequently, the
ratio of sodium-chloride ions at the peak of the flood is
greater than at the time of the first flood event, or if plotted
in the graph, it is relatively positioned away from the 1:1
line ratio. Meanwhile, the ratio before and after flooding is
not affected more by the flood events.
Conclusions
The wet season Bribin hydrochemistry is strongly influenced by the percentage of diffuse flow in water. During
flood events, river water is dominated by conduit flow,
which causes decreasing values of calcium and bicarbonate
while the CO2 content increases. In this condition, the
correlation between diffuse flow and major dissolved elements (calcium and bicarbonate) is low, which shows the
process of water–gas–rock interaction. During the nonflood and flood recession periods, the domination of conduit flow is replaced by diffuse flow, which causes the
water–rock interaction to dominate. This stage is characterized by increasing calcium and bicarbonate in the water
and decreasing CO2 content, and the hydrochemistry is
nearly identical to that found in the dry season, when the
diffuse flow is highly dominant. Lastly, the hydrochemograph during the flood and non-flood events shows that the
small-medium size fracture in the Bribin aquifer still serves
to store diffuse and fissure flows, although the conduit
fracture is already developed, as indicated by its dominance during the flood events.
Acknowledgments The author wishes to fully express his profound
gratitude to Professor Sudarmadji, Professor Suratman and Dr. Heru
Hendrayana for their support from the beginning until the finalization
of this research. In addition, sincere thanks and appreciation are due
to Bagus Yulianto, Badi Hariadi, Lili Ismangil, Zaenuri Putro Utomo,
Ari Purwanto, and Acintyacunyata Speleological Club (ASC) for their
accompaniment during the equipment installation and fieldwork.
Thanks also to Dr. Eko Haryono, who assisted this project financially,
and reviewer Prof. Julia Ellis Burnet, who provided valuable input for
the improvement of this manuscript.
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in Gunung Sewu Karst, Indonesia
Tjahyo Nugroho Adji
Environmental Earth Sciences
ISSN 1866-6280
Environ Earth Sci
DOI 10.1007/s12665-012-1599-x
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DOI 10.1007/s12665-012-1599-x
ORIGINAL ARTICLE
Wet season hydrochemistry of Bribin Cave in Gunung Sewu
Karst, Indonesia
Tjahyo Nugroho Adji
Received: 17 March 2011 / Accepted: 2 February 2012
Ó Springer-Verlag 2012
Abstract This research was conducted on the Bribin
River, the most important underground river in the Gunung
Sewu Karst, Gunung Kidul, Java, Indonesia. The main
purpose of this study was to define the wet-season hydrochemistry of this river. This research also focuses on
identifying the relationship between hydrochemical
parameters to provide better aquifer characterization.
Water-level monitoring and discharge measurements were
conducted over a 1-year period to define the discharge
hydrograph. Furthermore, baseflow-separation analysis is
conducted to determine the diffuse-flow percentage
throughout the year. Water sampling for hydrogeochemical
analysis is taken every month in the wet season and every
2 hours for two selected flood events. To describe the
hydrogeochemical processes, a bivariate plot analysis of
certain hydrochemical parameters is conducted. The results
show that the diffuse-flow percentage significantly controls
the river hydrochemistry. The domination of diffuse flow
occurs during non-flooding and flood recession periods,
which are typified by a high value of calcium and bicarbonate and low CO2 gas content in water. Conversely, the
hydrochemistry of flood events is characterized by the
domination of conduit flow and CO2 gas with low calcium
and bicarbonate content. According to the wet-season
hydrochemistry, it seems that the small- and medium-sized
fractures in the Bribin aquifer still provide storage for the
diffuse and fissure flows, although the conduit fracture is
already developed.
T. N. Adji (&)
Department of Environmental Geography,
Gadjah Mada University, Jogjakarta 55281, Indonesia
e-mail: adji@geo.ugm.ac.id
Keywords Hydrochemistry Bribin Cave
Gunung Sewu Karst Indonesia
Introduction
The Gunung Sewu Karst area was initially described by
Danes (1910) and Lehmann (1936). The area is located
within the central part of the island of Java, Indonesia. In
Javanese traditional language, ‘‘Gunung’’ means mount or
hill, while ‘‘Sewu’’ means thousand; thus, Gunung Sewu
means the area with a thousand hills. This karst area is
characterized by the development of positive formations of
blunt conical hills (kegelkarst). Kegelkarst, according to
Sweeting (1972), are categorized as tropical karst formations. Several other geomorphologists affirm Lehmann’s
opinion (Flathe and Pfeiffer 1965; Balazs 1968, 1971;
Verstappen 1969; Waltham et al. 1983). Balazs (1968)
verifies that the number of hills in Gunung Sewu amounted
to approximately 40,000, with a density of approximately
30 hills/km2. Recent studies related to this karst region
have been published by Haryono and Day (2004), Ahmad
et al. (2005), and Urushibara-Yoshino and Yoshino (1997).
The major underground river system in this region is the
Bribin-Baron system, which has an outlet discharge of up
to 8,000 l/s (MacDonalds and Partners 1984). This system
is the most important river in the Gunung Sewu Karst
(Fig. 1). In terms of water resources, the number of people
who depend on this water supply system is over 200,000.
The Bribin River catchment area was first defined by
Fakultas Kehutanan (1993) as being bounded on the
upstream (north) side by a massive old-volcanic mountain
and by the ancient Ponjong-Polje in the west (Srijono and
Aldilla 2006), while the southern and eastern boundaries
have not yet been defined. This paper discusses the results
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Environ Earth Sci
Fig. 1 Bribin River catchment
and Bribin-Baron system
(MacDonalds and Partners
1984)
of a quantitative evaluation of wet-season hydrogeochemistry in Bribin Cave, with the primary focus on a better
understanding of karst aquifer characteristics within this
area.
Climate and hydrogeological situation
The most recent research related to rainfall condition in
Gunung Sewu was conducted by Brunsch et al. (2011),
who note that there are spatial and temporal difference in
rainfall condition. The spatial variation depends on the
proximity to the sea and the elevation, whereas the temporal variation is regulated by climate circumstances.
Long-term rainfall declines slightly, whereas there is a
clear decrease in precipitation from 2002 to 2009. The
rainfall intensity rises from December to February and then
decreases in most of the other months. Even propensities
123
toward an extended dry predict period could be perceived.
In addition, the El Nin˜o and La Nin˜a events have influences
on the rainfall distinction in the dry period with years of
either extremely low or high rainfall rates. Some studies on
global climate conditions in Gunung Sewu were also
conducted by Verstappen (1997) and Urushibara-Yoshino
and Yoshino (1997). Studies related to climate in the
Quaternary period in the vicinity of the study (Java Island)
were performed by Urushibara-Yoshino (1995), Verstappen (1975, 1994), Dam (1994), Morley (1982), and Budel
(1975). Meanwhile, information on climate conditions in
the Bribin River catchment can be found in publications by
Adji and Nurjani (1999), Suryanta (2001), MacDonalds
and Partners (1984), BMG (2000, in Sutikno and Tanudirjo, 2006), and Fakultas Kehutanan (1993). These
studies show that the monthly average temperature in the
Bribin River catchment area ranged from 22 to 28°C.
Meanwhile, the annual rainfall in Gunung Sewu ranges
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Environ Earth Sci
from 2,000 to 2,500 mm/year from the data obtained at 12
rain-gauge stations in the period of 1947–2000. The
research brief by Adji (2010) indicates that the air humidity
in Bribin River catchment ranges between 60 and 90%,
with a temperature varying between 24 and 28°C. Because,
the catchment is located in a tropical region, knowledge of
climatic conditions, especially rainfall and temperature, is
relevant to the water input component associated with the
karst hydrogeochemistry.
Geologically, the study area is dominated by Miocene
limestone of the Wonosari Formation, which consists of
massive coral reef limestone in the south and bedded
chalky limestone in the north (Balazs 1968; van Bemmelen
1970; Waltham et al. 1983; Surono et al. 1992). The results
of an interpretation of a 1:50,000 scale aerial photo and a
1:1,000,000 scale ERS image by Kusumayudha (2005)
show that the moment structure consisting of thick cracks,
faults, and fractures in Gunung Sewu have a general
structure direction from northwest to southeast and from
northeast to southwest. The regional structure is then
divided into blocks bounded by faults that also define the
hydrogeological system since the creation of the low-level
and high-level configuration (Kusumayudha, 2005). The
catchment of Bribin River is included in the WonosariBaron Subsystems, which have a volcanic rock (bedrock)
configuration as a graben with a northeast-southwest
direction (Fig. 2).
Bribin River was first described by MacDonalds and
Partners (1984). This river begins at the surface of Pentung
River and then disappears into the Sawahombo sinkholes.
The river comes out in Luweng Jomblangan, emerges again
in Gilap Cave, Luweng Jomblang Banyu, Luweng
Jurangjero, and at last appears in Bribin Cave before
coming out as a large resurgence at Baron Beach in the
Indian Ocean. In the section between Luweng Jurang Jero
and Bribin Cave, it leaks into the Ngreneng Cave, which
also flows into Baron Beach. Adji (2011) notes that the
minimum discharge is approximately 1,600 l/s, while the
maximum discharge is recorded at approximately 2,500 l/s.
Methods
Water-level data logger and discharge measurements were
conducted for a 1-year period to obtain the discharge
hydrograph throughout the year. The stage discharge rating
curve used is based on research by Suryanta (2001). The
diffuse-flow proportion (baseflow) was defined using the
automated baseflow separation by digital filtering method
(Eckhardt 2005) based on a recession constant value in the
hydrograph, which was then correlated to the value of base
flow indices (BFI) within a karst aquifer. The following
formula was used:
qbðiÞ ¼
ð1 BFImax Þaqbði1Þ þ ð1 aÞBFImax qi
1 aBFImax
where qb(i) is the baseflow at time i, qb(i-1) is the baseflow
at the previous time i-1, qi is the total flow at time i, a is
the recession constant and BFImax is the maximum
baseflow that can be measured or performed. To find the
constant of the recession, we used the formula:
Qt ¼ Q0 eat
where Qt is the flow rate at time t, Q0 is the initial discharge
in the recession segment, and is a constant. Furthermore,
e- may be replaced by k, which is known as a recession
constant, or depletion factor. This constant is often used as
an indicator of the continuity of baseflow (Nathan and
McMahon 1990). Then, the value of k is compared with the
classification of the karst underground river recession by
Worthington (1991, in Gillieson 1996).
Water sampling was conducted once a month for a year
to cover two seasons (wet and dry). In addition, detailed
sampling was performed twice hourly during two flooding
Fig. 2 West–East
hydrogeological conceptual
model of Gunung Sewu
(Kusumayudha 2005)
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events. Chemical analysis of the samples in the laboratory
included the cations Ca2?, Mg2?, Na?, and K? and the
anions Cl-, SO42-, and HCO3-. The volumetric method
was used for the elements Ca2?, Mg2?, CO3- and SO42-,
the spectrophotometric method was used for HCO3-, and
the flame photometry method was used for Na? and K?.
Another analysis was the calculation of PCO2 (partial
pressure of carbon dioxide gas), which is assumed to have
reached equilibrium with the water samples (Stumn and
Morgan 1981; Drever 1988; White 1988). With the assistance of Wateq-4F software (Ball and Nordstrom 1991),
the PCO2 in all samples was determined.
þ
PCO2 ¼ HCO
3 ½H = K1 KCO2
where PCO2 is the partial pressure of carbon dioxide gas in
water; [HCO3-] is the activity of bicarbonate ions, [H?] is
hydrogen ion activity, K1 is the equilibrium constant
reaction solvent at 25°C, and KCO2 is the equilibrium
constant of CO2 in water at 25°C. To describe the hydrogeochemical processes, a scatter-plot analysis with a small
sample size (non-discrete parameters) was conducted that
included the following: (1) discharge-major ion concentration, (2) specific conductivity-calcium and bicarbonate,
(3) discharge-diffuse flow, (4) diffuse flow-log PCO2 , (5)
diffuse flow-calcium and bicarbonate, and (6) sodium
chloride. A chemograph analysis was also performed to
interpret the relative contribution of different waters and
was compared with three models of chemographs described
in the literature (in Perrin 2003).
Result of wet-season hydrochemistry of Bribin Cave
Figure 3 shows the wet season hydrochemograph of the
underground river in Bribin Cave, which is based on
sampling measurements from May 2006 to April 2007.
This hydrochemograph shows that the dissolved constituents at the flood events vary and tend to decrease, as
there are rainfall recharges through the conduit. Comparing
between the beginning and end of the rainy season, the
temperature and conductivity values are nearly identical,
i.e., in the range of 27°C and 500 lS/cm. This result proves
that the diffuse flow component has a dominant role at
times when flooding does not occur. Differences are shown
in calcium, bicarbonate, and pH values, which do not reach
the initial conditions, allowing the dilution of the underground river to still proceed by the fissure flow component.
However, this dilution does not affect the value of temperature and conductivity. Meanwhile, the bivariate plot
between the diffuse flow percentage and discharge variation has a negative relationship, with R2 = 0.91 (Fig. 4). It
also appears that a slight deviation occurs at the beginning
and end of the rainy season. These changes occur during
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the dominance of the flow component due to the time lag of
the fissure and diffusion to recharge the underground river
as a result of different rain intensities throughout the wet
season. Next, a bivariate plot between the diffuse flow and
calcium or bicarbonate and between the diffuse flow and
CO2 partial pressure are presented in Figs. 4 and 5.
According to Fig. 5, there is a grouping of sample positions based on characteristic stages of flood events (flood
peak and recession stages), and it is influenced by the time of
the beginning or the end of the rainy season. This result may
be caused by high variations in diffuse flow during flood
events. In addition, bivariate plotting between diffuse flow
and calcium or bicarbonate indicates that the dissolved
constituent in water at the beginning of the rainy season
consists of higher diffuse flow and calcium content than all
other samples because of the rainwater dilution in the peak of
the wet season. In contrast, the samples at the time of the
flood peak have the lowest content in diffuse flow and dissolved constituents. Another interesting phenomenon is that
the samples in the flood recession of the 22 February flood
event exhibit lower major dissolved constituent (Ca2? and
HCO3-) contents than the 19 March flood event. This result
indicates that the groundwater storage within the karst
aquifer is recharged sufficiently by rainwater, thereby
increasing the percentage of diffuse flow and the relatively
more saturated fissure of carbonate minerals that have
undergone a process of water–rock interaction. Consequently, the diffuse flow in the 19 March flood event
(end of rainy season) increases. Next, Fig. 6 shows the
relationship between CO2 partial pressure and diffuse
flow percentage.
According to Fig. 6, there is a positive correlation
between CO2 partial pressure and the diffuse flow. In
detail, the flood peak in 22 February is characterized by a
lower value of the log PCO2 and low diffuse flow content.
This condition differs greatly from the March 19 flood
peak, which shows a greater log PCO2 and diffuse flow
percentage. These values are still higher when compared
with the samples during the first flood recession of 22
February. In addition, the non-flooding conditions present
higher diffuse flow content and log PCO2 values than at the
time of flooding. It can be summarized that the variation of
log PCO2 is associated with the percentage of diffuse flow,
so that the process of interaction of water–gas–rock is more
dominant than the water–rock interaction only.
Result of Bribin Cave hydrogeochemistry within flood
events
Sampling within flood event conditions in Bribin was
conducted for two flooding incidents (22 February and 19
March 2007) to obtain a detailed description related to
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Fig. 3 Wet-season
hydrochemograph of Bribin
River
hydrogeochemical fluctuations within flood events. Here,
samples are taken every 2 h. Next, the chemograph that
describes the fluctuations of discharge, diffuse flow, dissolved constituent, and the partial pressure of CO2 in the
flood hydrograph is presented in Fig. 7. In the left part of
Fig. 7, it seems that the diffuse-flow percentage decreases
if the discharge increases at the 22 February 2007 flood
event; in other words, the diffuse flow decreases with
increasing discharge to reach its peak discharge. Meanwhile, the response of diffuse flow on the 19 March flood
event exhibits a similar trend (Fig. 7, right). In summary,
from the two flood occasions in Bribin Cave, it can be
explained that the diffuse flow has a tendency to decrease
with increasing discharge to the peak discharge and then
gradually increases shortly after passing the peak
discharge.
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Fig. 4 Bivariate plot between diffuse flow and discharge during wet
season
Fig. 6 Bivariate plot between diffuse flow and log PCO2
The relationship between discharge and diffuse flow at
the two flood events in Bribin Cave is presented in Fig. 8.
It seems clear that there is a negative relationship between
discharge and diffuse flow in both flood events, and even
the value of R2 reaches a significant correlation (above
0.99). This decrease in percentage is a result of the
replacement of diffuse flow from karst aquifers by conduit
flow, which is directly recharged from rainfall through the
conduit passage. In the period of decline in diffuse flow,
the process of dilution by precipitation occurs along with
the increase in the Bribin River discharge. Then, the high
correlation between the discharge increase and the diffuse
flow decrease, or vice versa, indicates that the karst-aquifer
system is controlled by good conduit development.
Figure 8 shows that there are different variation tendencies (PCO2 and discharge values) between flood
events for 22 February and 19 March. In the 22 February
flood event, the PCO2 shows a downward trend at the
time to reach peak discharge and even continues for
some time after the peak discharge. The decreasing PCO2
value indicates that the water is still affected by remnants of diffuse and fissure flow, which are driven by the
rainwater that comes afterward; thus, it has a tendency to
gradually decrease. These results are in accordance with
those described by Vesper and White (2004), Raeisi and
Karami (1997), and Liu et al. (2004). At the beginning
of the recession period, the PCO2 increases significantly,
although it does not reach the initial condition (before
the flood event). This behavior may be caused by the
mixing process between diffuse, fissure, and conduit
flow. In addition, the PCO2 variation within the 19 March
flood event (end of wet season) exhibits a lower correlation with diffuse flow. The variation and correlation
between diffuse flow and the PCO2 value at the two flood
events the possible domination of water–gas–rock interaction within the flood events rather than only water–
rock interaction, as in the dry season. However, the 22
February flood tends to show a stronger water–gas–rock
interaction than the 19 March flood event, which indicates a shifting from the water–gas–rock interaction to
the water–rock interaction. This tendency can be further
confirmed by looking at the variations of Ca2?, HCO3-,
and electrical conductivity values, and comparing the
correlation between diffuse flow and PCO2 .
Fig. 5 Bivariate plot between diffuse flow-calcium (left) and diffuse flow-bicarbonate (right)
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Fig. 7 Bribin cave hydrochemograph within the 22 February 2007 flood event (left) and within 19 March (right)
Fig. 8 Bivariate plot between diffuse flow and discharge within the 22 February (left) and 19 March (right) flood events
The variation of electrical conductivity (EC), calcium,
and bicarbonate in the two flood events (Fig. 7) is similar
to the model described in Perrin (2003). In his second
flood-chemograph model, he explains that the content of
dissolved constituents in water gradually rises when the
discharge rises to its peak (rising limb), is followed by
water influenced by dilution by the precipitation process,
reaches its peak some time after the peak discharge (EC
down), and then slowly falls back to the water chemistry
condition before the flood event. If the fracture type in the
Bribin River aquifer is dominated by a large conduit only,
the rainwater will be rapid and will exhibit the same travel
time with the discharge increasing. However, the Bribin
River aquifer also contains fissure and diffuse storage in
medium and small fractures (high dissolved element contents) and is subsequently pushed by conduit water, which
led to EC increasing first, followed by the domination of
rainwater (EC decreases).
The calcium value in the 22 February flood event
showed a decreasing trend with increasing discharge, while
the bicarbonate shows similar trend as the EC value. Furthermore, the analysis of bivariate plots between diffuse
flow and calcium or PCO2 have correlation (R2) values of
approximately 0.5. This result indicates that both the
water–gas–rock and water–rock interactions during the
floods had a nearly balanced contribution (Fig. 9). This
result is in contrast to the negative correlation in the dry
season (Adji 2010). However, the correlation value within
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Fig. 9 Bivariate plot between diffuse flow and PCO2 in Gua Bribin (left) and between diffuse flow and calcium (right) at the 22 February flood
event
Fig. 10 Bivariate plot between diffuse flow and PCO2 in Gua Bribin (left) and between diffuse flow and bicarbonate (right) at the 19 March flood
event
Fig. 11 Bivariate plot between natrium and chloride in the 22 February (left) and 19 March (right) flood events
approximately 0.5 indicates that the diffuse flow and CO2
gas are not the only factors controlling water dissolved
constituents at flood events. In addition, the 19 March flood
event shows that increasing diffuse flow is associated with
decreasing bicarbonate (R2 = 0.63), as shown in Fig. 10.
This result is different from what happened in the first flood
123
event, as this second flood event has already shown a stable
diffuse and fissure flow in the river. In general, conduit
flow may directly dilute river water, so there is no large
time lag between the flood peak and decreasing bicarbonate
or calcium content. Here, the effect of CO2 gas at this flood
event still shows a substantial correlation (R2 = 0.36),
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although its role has been reduced compared with the first
flood event (22 February), which is related to the
decreasing intensity of rainfall (end of wet season).
The variation of pH and temperature within the flood
hydrograph is theoretically more dependent on the karst
flow components that dominate at the time of measurement. Bivariate plot of Na? and Cl- at the two flood events
are presented in Fig. 11. This figure shows that the ratio of
sodium-chloride ions at times of peak flood and shortly
thereafter have a ratio close to the rainwater ratio (1:1),
while the sample position farthest from the line ratio of 1:1
is during the period of recession prior to flood events.
Furthermore, the position of the samples during the period
of recession after the flood peak is located between the
position of the pre-flood and the flood peak.
These results indicate that the increase in discharge or
decrease in diffuse flow causes the ratio of sodium-chloride
to approach the rainwater ratio (line 1:1). This pattern
indicates the start of conduit flow domination through the
process of dilution by precipitation within the underground
river, whereas in periods of recession after peak flood, this
ratio begins to slowly increase and move away from the 1:1
line. An interesting pattern is shown by the ratio of sodiumchloride ions in the 19 March flood, where the ratio at the
peak flood is enough to deviate from the 1:1 line, while the
ratio pre- and post-flood shows no significant difference.
This result can be explained by the second flood event,
which is already entering the end of the wet season; thus,
the diffuse flow is largely dominated by dissolved minerals
compared to the time of the first flood. Consequently, the
ratio of sodium-chloride ions at the peak of the flood is
greater than at the time of the first flood event, or if plotted
in the graph, it is relatively positioned away from the 1:1
line ratio. Meanwhile, the ratio before and after flooding is
not affected more by the flood events.
Conclusions
The wet season Bribin hydrochemistry is strongly influenced by the percentage of diffuse flow in water. During
flood events, river water is dominated by conduit flow,
which causes decreasing values of calcium and bicarbonate
while the CO2 content increases. In this condition, the
correlation between diffuse flow and major dissolved elements (calcium and bicarbonate) is low, which shows the
process of water–gas–rock interaction. During the nonflood and flood recession periods, the domination of conduit flow is replaced by diffuse flow, which causes the
water–rock interaction to dominate. This stage is characterized by increasing calcium and bicarbonate in the water
and decreasing CO2 content, and the hydrochemistry is
nearly identical to that found in the dry season, when the
diffuse flow is highly dominant. Lastly, the hydrochemograph during the flood and non-flood events shows that the
small-medium size fracture in the Bribin aquifer still serves
to store diffuse and fissure flows, although the conduit
fracture is already developed, as indicated by its dominance during the flood events.
Acknowledgments The author wishes to fully express his profound
gratitude to Professor Sudarmadji, Professor Suratman and Dr. Heru
Hendrayana for their support from the beginning until the finalization
of this research. In addition, sincere thanks and appreciation are due
to Bagus Yulianto, Badi Hariadi, Lili Ismangil, Zaenuri Putro Utomo,
Ari Purwanto, and Acintyacunyata Speleological Club (ASC) for their
accompaniment during the equipment installation and fieldwork.
Thanks also to Dr. Eko Haryono, who assisted this project financially,
and reviewer Prof. Julia Ellis Burnet, who provided valuable input for
the improvement of this manuscript.
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