SUBMARINE GROUNDWATER DISCHARGE (SGD) MEASUREMENT ON THE SANDY UNCONFINED AQUIFER
IN THE CARNAVAL BEACH-ANCOL, JAKARTA BAY
Hendra Bakti1*, Rachmat Fajar Lubis1, Robert M. Delinom1 and Makoto Taniguchi2
1Research Centre for Geotechnology, Indonesian Institute for Science, Bandung, Indonesia
2Research Institute for Humanity and Nature, Kyoto, Japan
*Correspondence author : hendra.bakti@geotek.lipi.go.id
ABSTRACT
Submarine groundwater discharge (SGD) is defined as all direct discharge of subsurface fluids into coastal zone. Components of SGD consist of fresh submarine groundwater discharge and the recirculated saline seawater discharge. SGD could act as a pathway for the transport of anthropogenic contaminants and nutrients to coastal waters. Measurement SGD at Carnaval Beach, Ancol, Jakarta was focussed on unconfined groundwater system. The method of quantified used automatic seepage meter for measured of SGD and installed conductivity temperature depth. The average SGD rate was 0.21 mm/min on March 20-31, 2009 consist of 19.05% fresh water, 80.95% recirculated seawater. April 1-23, 2009, the average SGD rate was 0.81 ml/min which consisted of 16.04% fresh water, 83.96% recirculated seawater.
SGD fluctuation was opposite with the tide. As a result, submarine groundwater discharge at Jakarta coastal area was defined and can be measured to quantify.
Keywords : Submarine groundwater discharge, measurement, unconfined aquifer, Jakarta Bay
INTRODUCTION
Internationally, study of submarine groundwater discharge (SGD) has been increased in the last few years. SGD is a significance pathway of anthropogenic contaminants and nutrients transport to coastal waters which is recognised by Burnett & Dulaiova (2003); Taniguchi et al (2006). Michael et.al.(2003); Martin et.al (2004). SGD is representing all direct discharge of subsurface fluids across the land–ocean interface and it consists of submarine fresh groundwater discharge (SFGD) and recirculated saline groundwater discharge (RSGD). Therefore, the total SGD includes both a terrestrial component of net fresh groundwater and an oceanic component of recirculated seawater (Taniguchi et.al, 2002, 2006).
The SGD study in Indonesia has already conducted by Research Centre for Geotechnology, The Indonesian Institute of Sciences, since year 2006 in collaboration with the Research Institute for Humanity and Nature, Kyoto, Japan, especially in the area of Jakarta. At the field, SGD can be present as submarine springs and seepage
(Burnett et.al, 2001; Taniguchi et al., 2002), and seepage more important volumetrically than discrete springs as it is difficult to detect groundwater seepage through sediment (Taniguchi et al. 2002, 2006).
Jacob et.al (2009) suggested that the magnitude of SGD and its generation mechanisms are indeed poorly understood mainly because of lack of accurate measurement techniques, the sources are invisible, slow moving, spatially and temporary variable. Therefore, the objective this study is to characterize of SGD on unconfined sandy aquifer system by direct measurement. It is expected that the phenomena of SGD in Jakarta Bay can be understood accurately.
STUDY AREA
The Carnaval Beach, Ancol is a part of Jakarta Bay, lied in Jakarta Province (Figure 1). This coastal has average depth of 2 meter until 10 metre, some of coastal areas was reclaimed and the sediment under the sea surface are mud, clay, and fine to coarse sand. The geological setting in surround areas of regional scale consists of quaternary sediment as alluvial system
(Turkandi et.al, 1992). The previous study which was done by Umezawa et.al (2009) has recognized the presence of SGD by using Radon-222 as natural tracer. The result showed that detected radon-222 around Ancol site is very little, approximately 0.8–3.0 dpm/L. They suggested to apply the other method for quantifying SGD continuously. Delinom et.al (2009) reported that the water resistivity on this location at 2,5 meter depth is around 0,5 ohm-m s/d 5 ohm- meter, it has interpreted as fresh groundwater as the anomaly resistivity at 40-45 meter from the coast.
MATERIALS AND METHODS
The method for continuous of quantify SGD measurement in this study is directly measuring by using automatic seepage meter which developed by Taniguchi and Iwakawa (2004). One of the automatic seepage meter was laid around 45 meter from the minimum tide condition (the shore line) on sediment at sea bed.
Automatic seepage meter consists of chamber with 56 cm periphery and 20 cm high. Two conductivity temperature depth (CTD) sensors were installed inside and outside of the chamber. The inside CTD used to monitor SGD and the outside CTD for monitoring coastal sea water. We also
installed barometric probe on 2 meter above surface topography. The data of depth on outside CTD obtained a tide data on the site to be corrected with the pressure on the barometric probe. All of the equipment record the data set with the interval time of 10 minutes. In the terrestrial, the electrical conductivity (EC) of fresh groundwater we measured on the dug well, which have ~100 m (or ~50 meter) from the shore line and distance from the chamber with the depth
~1 meter.
Methodology to separate component of SGD which consist of SFGD and RSGD that flowing into the seepage meter was based on Michael et.al (2003) formula. It can calculate the proportion of SFGD and RSGD from the salinity and volume of discharge. The equation of mass balance and salt balance also described detail on Hays and Ullman (2007) and Garisson et.al (2003) as:
qfw = qt {(Ssw – St)/(Ssw – Sfw)}... (1) Where St is the salinity (or any other conservative property of water) and Ssw is the salinity of the near-shore estuarine water, and Sfw is the salinity of the fresh groundwater. qfw is the fresh water and qt is the volume of SGD from the chamber. We modified St, Ssw and Sfw in this study changed by the EC value.
RESULTS
Resume of the result of the SGD studied on the location, shown on the table 1, where in the characterize of hydro- physicochemical are the average EC SGD (inside CTD) ranged between 52,21 mS/cm to 55,68 mS/cm and the average EC coastal sea water (outside CTD) varied from 16,71 mS/cm to 48,94 mS/cm, respectively. It was recorded data in period of five months, from 20th March until 9th July 2009. The data that determined from 25 April to 31 October 2010, the average EC coastal sea water is ranged between 25,34 mS/cm to 41,28 mS/cm. Furthermore, the average EC temporary of fresh groundwater in the dug well on the site was approximately 1,0
mS/cm. Magnitude of the EC SGD between maximum and minimum only some point.
The total SGD discharge on the location observed large variability at over time. SGD data on the site pointed out that always were not flow if we set automatic seepage meter at interval measurement every 10 minute. The average of SGD rate varied from 0,21 ml/min to 0,81 ml/min (20 March – 23 April 2009) and 0,11 ml/min to 253 ml/min (25 April – 5 August 2010).
Temporal variability of SGD and tide can be seen on figure 2. The average tide on the site about less than 1 meter and have semidiurnal tide.
Estimate of the SGD component on the location calculated at 20 March – 23
April 2009. At the other times as long as measured the either EC SGD or SGD (total SGD) were not always available, because the equipment has easy to broken, to be corrosive by saline sea water. But from
short recorded we can saw on the figure 3, that ilustrated SGD on the location. The average of fresh component SGD are approximately 19,05% (20 to 31 March 2009) and 16,04% (01 to 23 April 2009).
DISCUSSIONS
EC of SGD in this point measurement, showed relatively stagnant or un-fluctuate than the EC coastal sea water.
It stands to reason that coastal sea water got directly recharge from the rain also had been supply the fresh water from the rivers around location (ex. Ciliwung river and Sunter river), which distance about 1 km and 3 km respectively from SGD chamber.
The SGD were indicated as saline water, but coastal sea water sometimes became fresh water on this point. Compare with the other SGD study, example in the Waquoit bay USA, there was resulted SGD was largely saline (Michael et.al, 2003). So that the relatively high salinity indicates that mixing and recirculation of sediment pore water must be effective process over flow paths at least several meters long (Bokuniewicz et.al, 2008).
There is showed that SGD controlled by tide, when sea level rise, the SGD decrease and it will inverse when sea level tide drop. It were confirm with the SGD studied by Garisson et.al (2003), Taniguchi et.al (2006), and Povinec et.al (2006), but suggested by Bokuniewicz et.al (2008) that the interaction was non-linear and, the correlation was weak at tidal ranges less than 1 m. For while at generally, we founded that the pattern of SGD in location study tends to decrease from April 2010 to August 2010, and can be seen in table 1, the greatest of maximum SGD were found on April 2010. That condition, perhaps due to precipitation effect on the site wherein rainy season much more at firstly month in every year. And furthermore, compared with the resulted SGD research by Taniguchi (1995) that precipitation were the main factor of input into the basin at Biwa Lake, Japan. So its reasonable wherefore there were the SGD relatively decrease in successively month at
above as long as in measurement in location of study.
Component of SGD on this point more dominated by recirculated saline water than fresh water. The conditions same with experienced by Taniguchi and Iwakawa (2004), which conducts research in Tannowa, Osaka Bay, Japan. Wherein the fresh water component ranged at 4% to 29%. Likewise, studied at Cape Henlopen, Delaware bay USA, is 32% ± 19% of the total groundwater discharge (Hays and Ullman, 2007).
Dominated of saline groundwater discharge near in the coast is commonly in the many area but its importance for nutrient concentrated during recirculation (Burnett et.al.2003 in Taniguchi et.al 2007).
The our result was presented above, it is the first study in Jakarta Bay and only recorded temporary SGD so the new challenge of the future will observe detail of the spatially SGD perpendicular to the shore line.
CONCLUSIONS
SGD research on coastal shallow aquifer in Jakarta bay has been done successfully. Measurements using the automatic seepage meters and mini CTD who have succeeded in giving accurate information about the character of SGD in the study area. Groundwater seepage from time to time vary controlled by the tides and rainfall. The results of the study at a single point measurements show that the amount of seepage of groundwater more dominated by circulated saline sea water when compared with fresh groundwater.
However, this number will mean considering a very wide coastline Jakarta.
This has enormous consequences for the environmental management of coastal areas
of Jakarta. Because if there is contamination in the groundwater on the land it will be carried away by the flow of groundwater and out in the ocean that would degrade the health of the marine environment. Therefore, control of pollution of groundwater in the mainland to be important from time to time.
ACKNOWLEDGEMENT
This research is part of the research collaboration between Research Centre for
Geotechnology, Indonesian Institute of Sciences and Research Institute for Humanity and Nature , Japan. Hereby, we are grateful for all support and fruitful discussion given by Prof. Dr. Sudarto Notosiswoyo of Bandung Institute of Technology, Dr. Shinichi Onodera of Hiroshima University, Japan, Dr. Mitsuyo Saito of Ehime University, Japan, also Dr.
Sri Yudawati Cahyarini of Indonesian Institutes of Sciences.
Figure 1. Map of Jakarta Bay and design automatic seepage meter in location study. The chamber of automatic seepage meter conduct to sediment about 18 cm depth.
Table 1. The characterize of hydro-physicochemical of SGD in location study.
Figure 2. Tide variation and SGD discharge with interval 10 minute.
(12.10 WIB, March 20, 2009 to 13.00 WIB, April 23, 2009, above and 9.30 WIB, April 25, 2010 to 16.00 WIB, August 05, 2010, below)
Figure 3. Component SGD consists of fresh ground water and recirculated saline water. SGD on the site dominated by recirculated saline water (12.10 WIB, March 20, 2009 to 13.00 WIB,
April 23, 2009)
Time
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