# Groundwater Vulnerability from Sea Water Intrusion in Coastal Area Cilacap, Indonesia

## Summary (2 min read)

### Introduction

- Because of this location, coastal areas are vulnerable to various problems such as SWI and tidal flooding.
- Partly citiesin Java, Indonesia are located incoastal area, such as:Jakarta, Cirebon, Pekalongan, Semarang, and Surabaya are some of cities located on the northern coast of Java, whereas Cilacap is located on the southern one of Java Island.
- With the GALDIT, it can be seen the environmental condition of a place related to its vulnerability including the distance from the shore line, whereas with the Dupuit Ghyben-Herzberg, the depth of the interface can be known at a certain distance on the shore line.

### Determination of Groundwater Vulnerability

- It was conducted a research using GALDIT method to get the information about groundwater vulnerability fromSWI.
- GALDIT stands for parameters that can cause sea water intrusion.
- The basic principle of this method was determination of vulnerability based on numerical system in weight and rating.
- The weight was determined based on the significance of parameter influence to SWI, whereas rating was specified based on the significancy of variable effect of each parameter to the SWI.

### Determination of Distance and Interface Depth

- For the aquifer type the weight was 1, the aquifer hydraulic conductivity was 3, height of groundwater level was 4, distance from the shore was 4, the ratio of Cl-/[HCO3-+ CO32-] was 1, and for the aquifer thickness was 2.
- It was estimated that the distance from the shoreline was dominant parameter in determining the groundwater vulnerability in the research area because its value was quite varied, and it was similar for the ratings.
- Based on the rating of GALDIT index, the value range was entered in one category and was valued by 2.5.

### Height Calculation of Groundwater Level from Mean Sea Level

- The height of groundwater level from mean sea level is determined by the parameters i.e. specific discharge of groundwater, distance from shoreline, the density of freshwater and saline water, and the aquifer hydraulic conductivity.
- The groundwater specific discharge of each observed wells is based on the calculating results in Table 5, freshwater density that is appropriate with the Law Ghyben-Herzberg is determined by 1.000 g/ cm3, whereas the density of saline water is determined by 1.025 g/cm3 (Todd & Mays, 2005).
- The groundwater level with the height of 0.59 m was founded southern Cilacap village, whereas the groundwater level with the height of 3.05 m is existed in Gumilir village.
- It could be said that the farther distance from the shoreline, the higher the groundwater level.
- This statement is proved by graphic that showed in Figure 3.,.

### Calculation of Freshwater Depth from Sea Level

- The freshwater depth from saline water level is also determined by groundwater specific discharge, distance from the shoreline, density of freshwater and saline water, and aquifer hydraulic conductivity.
- The calculating results showed that the region of South Cilacap located 300 m from the shoreline, had the most shallow groundwater depth, 24.59 m, whereas the region of Gumilir located 2000 m from the shoreline had the largest groundwater depth, 125.93 m.
- This showed the tendency that the farther distance from the shoreline, the greater the depth of interface would be, and otherwise the closer distance from the shoreline, the swallower the depth of interface .
- The completely results related to the calculation of groundwater depth from sea water level could be regarded in Table 7.

### Vulnerability Relationship to SWI with Interface Depth

- The results showed that in the research area had been detected interface with varied depths, ranging from 26.68 m in South Cilacap to 129.74 m in Gumilir.
- This could be understood because of the community well had the depths that was not greater than 25 m.
- There was a quite interesting phenomenon in the observed wells located in TegalKamulyan which had the distance of 500 m from the shoreline.
- Compared with other areas of research, the groundwater specific discharge in the area was low, that was 0.40 m3/day.
- Cases of deep well drilling that took place in Takome Village were a real example that the hydrological conditions of the northern part of Ternate Island were very vulnerable to SWI.

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##### References

187 citations

### "Groundwater Vulnerability from Sea ..." refers methods in this paper

...The specific discharge of groundwater could be calculated by using the method of Darcy (Rushton, 2003; Davie, 2008): q=K.A.dh/dl ……………......................................(3) by dh/dl is hydraulic gradient....

[...]

...Such as explained in research methods, the equations used in the calculation was the general equation of groundwater flow from Darcy....

[...]

...The specific discharge of groundwater could be calculated by using the method of Darcy (Rushton, 2003; Davie, 2008): q=K....

[...]

187 citations

### "Groundwater Vulnerability from Sea ..." refers methods in this paper

...The specific discharge of groundwater could be calculated by using the method of Darcy (Rushton, 2003; Davie, 2008): q=K.A.dh/dl ……………......................................(3) by dh/dl is hydraulic gradient....

[...]

...Such as explained in research methods, the equations used in the calculation was the general equation of groundwater flow from Darcy....

[...]

...The specific discharge of groundwater could be calculated by using the method of Darcy (Rushton, 2003; Davie, 2008): q=K....

[...]

136 citations

### "Groundwater Vulnerability from Sea ..." refers background in this paper

...It would affect to the greater number of groundwater extraction, whereas in another side it would be decreased the groundwater inflow because of the larger open land used for recharge area was converted into residential area (Tillman & Leake, 2010; Fenta & Kifle, 2014; Waikar & Nilawar, 2014)....

[...]

71 citations

58 citations

### "Groundwater Vulnerability from Sea ..." refers background in this paper

...GALDIT vulnerability classes Index Range of GALDIT Vulnerability Classes >7.5 High vulnerability 5-7.5 Moderate vulnerability 5 Low vulnerability Source : (Chachadi & Lobo-Ferreira, 2005) Table 4....

[...]

...G is defined as groundwater occurrence, A is defined as aquifer hydraulic conductivity, L is defined as level of groundwater above mean sea level, D is distance from the shore, I as impact of existing status of SWI and T is thickness of aquifer being mapped (Chachadi & Lobo-Ferreira, 2005)....

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