groundwater vulnerability of pandak and...

J. SE Asian Appl. Geol., May–Aug 2010, Vol. 2(2), pp. 121–128

GROUNDWATER VULNERABILITY OF PANDAKAND BAMBANGLIPURO, YOGYAKARTASPECIAL PROVINCE, INDONESIA

Leakhena Snguon∗1, Doni Prakasa Eka Putra2, and Heru Hendrayana2

1Department of Civil Engineering, Faculty of Engineering, Institute of Technology Cambodia, Cambodia2Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, Indonesia

Abstract

The study of intrinsic vulnerability of groundwa-ter was generated in order to delineate groundwaterprotection zone in Pandak and Bambanglipuro, In-donesia, whose mainly water supply is from ground-water. Two methods of vulnerability mapping arechosen for the evaluation; DRASTIC method andHoelting method. The resulted maps conducted fromthese method are validated using the actual contam-inant concentration through the impact of on-sitesanitation, for instance nitrate as it is proved to bevery stable contaminants in groundwater. Consid-ered in different hydrogeological setting, these twomethods have produced various results at the certainsite. However, its reliability has been drawn uponthe nitrate concentration at the study areas.Keywords: Intrinsic groundwater, vulnerability,DRASTIC, Hoelting methods, nitrate contamina-tion.

1 Introduction

The concept of groundwater vulnerability is de-rived from the assumption that the physical en-vironment may provide some degree of protec-tion of groundwater against natural and humanimpacts (Vrba and Zaporozec, 1994). Techni-cally, there are two types of vulnerability; in-trinsic vulnerability and specific vulnerability.

∗Corresponding author: LEAKHENA SNGUON, De-partment of Civil Engineering, Faculty of Engineering,Institute of Technology Cambodia, Cambodia. E-mail:[email protected]

According to Vrba and Zaporozec (1994), theterm intrinsic vulnerability is used to define thevulnerability of the groundwater to contami-nants generated by human activity as a functionof natural factors such as hydrogeological fac-tors, hydrology, the overlying soil and geologi-cal material. While the term specific vulnerabil-ity is used to define the vulnerability of ground-water to a particular contaminant or grounp ofcontaminants by taking account the contami-nant properties and their relationship with thevarious components of intrinsic vulnerability.

Many intrinsic vulnerabilities methods havebeen developed in the past two decades butbasically there are only three mainly groupsof these techniques referred as hydrogeologicalcomplex and setting methods (HCS), paramet-ric methods (included matrix systems (MS), rat-ing systems (RS), and point count system mod-els (PCSM)), and analogical relation and nu-merical model (AR). The intrinsic methods ap-plied in the case study are both from the pointcount system model. DRASTIC method is amethod developed for the U.S. EnvironmentalProtection Agency (EPA) by Aller et al. (1985,vide Aller et al., 1987). In the other hand, Hoelt-ing method, which takes the unsaturated zoneinto consideration, was developed by the Ge-ological Survey of the individual states of theFederal Republic of Germany in 1995 (Voigt etal., 2004).

Since vulnerability is not an absolute charac-teristic but subjective, relative, non-measurable,dimensionless property (Vrba and Zaporozec,

121

SNGUON et al.

Figure 1: Geology and hydrogeological of thestudy area (adapted from Putra, 2003 and Mac-Donald & Partners, 1984).

1994), the reliability of the each vulnerabilitymaps is still be a topic to be discussed amonghydrogeologist. From many research works,Gugo and Dassargues (2000) has suggested thatthe only way of gaining confidence in vul-nerability is by comparing the results of vari-ous techniques and analyzing their consistencywith the actual contaminant at the study area.

The main objective of this study is, (1) to de-termine the intrinsic vulnerability of ground-water in two study areas using two differentmethods; (2) to evaluate the suitability of themethod for the study areas.

2 Study Area

2.1 Climate

The study areas are located in Pandak and Bam-banglipuro, Bantul Regency, Yogyakarta SpecialPrivince, Indonesia. It lies between 418000 to428200 East and 9118000 to 9128000 North (seeFigure 1). Geomorphologically, the study areais generally flat down to the south. However,there is a small hill at the central part of Pandakwhose elevation varied from 25 to 62.5 m abovesea level.

Situated in the tropical area, its climate char-acterized by a warm monsoon climate withtemperature varies from 25.78 ◦C (in April)to 27.65 ◦C (in March) (PSDA, 2008). Dur-ing 10 years period (2000-2009), the annual

rainfall ranges between 1700 mm/a to almost2000mm/a with the real evapotranspiration es-timated from 1300 to 1450 mm/a.

2.2 Geology and Hydrogeology

According to the regional geological context,the study areas are located in southern part ofYogyakarta basin. It consists of only two for-mation; Yogyakarta Formation which is a resultof Merapi Volcano ejecta in Quaternary succes-sion and composes of sand, gravel, silt and clay;Sentolo Formation, occurred in Tertiary succes-sion and consists of marl, limestone and marlylimestone (see Figures 1, 2) (Putra, 2003, Mac-Donald & Partners, 1984).

The recharge of the groundwater in the studyareas is mainly by infiltration of precipitationwhile the urban or artificial recharge was over-looked due to its groundwater usage cycle. Thiscycle is described from the direct extractionof the water and directly use as main supplythen depose back directly though on-site sani-tation and finally begin with another extraction.Confirmed by Putra (2007) that the ground-water recharge in Yogyakarta can be primar-ily determined by considered only the estima-tion if groundwater recharge due to precipita-tion, with reduction of actual evapotranspira-tion and runoff.

2.3 Application of DRASTIC Method to theStudy Area

DRASTIC is a point count system method de-veloped by Aller et al. (1987) for Environmen-tal Protection Agency (EPA). DRASTIC is anacronym of Depth to water (D), Net Recharge(R), Aquifer media (A), Soil media (S), Topog-raphy (T), Impact of vadose zone (I), HydraulicConductivity of aquifer (C). The degree of vul-nerability is defined according to the DRAS-TIC index value which represents the pollutionpotential of groundwater to be polluted. TheDRASTIC index can be calculated using equa-tion below:

DRASTIC index = 5× DR + 5× RR + 3× AR+2× SR + 1× TR + 1× IR+3× CR

122 c© 2010 Department of Geological Engineering, Gadjah Mada University

GROUNDWATER VULNERABILITY OF PANDAK AND BAMBANGLIPURO

Figure 2: Stratigraphy cross section within the study area.

Figure 3: a) Depth to water and b) Net rechargeranges and corresponding rating according toDRASTIC in the study areas.

2.4 Depth to Water (D)

Depth to groundwater refers either to the depthto the water surface in an unconfined aquiferor to the top of the aquifer where the aquifer isconfined (Aller et al., 1987). From 79 dug-wellsmeasurement at the field within the grid 250m× 250m, the map of depth to groundwater isthen created and transformed into digital ele-vation model on the basis of defined ranges ofDRASTIC (see Figure 3a).

2.5 Net Recharge (R)

According to Aller et al. (1987), net rechargeindicates the amount of water per unit area of

land which penetrates the groundwater surfaceand reached the water table. As mentionedabove, the groundwater recharge in study areasis primarily from the natural infiltration fromprecipitation. Though, runoff is very importantwithin these areas due to its lithology and land-use type. The highest recharge in the area isabout 290 mm/a (see Figure 3b).

2.6 Aquifer Media (A) and Hydraulic Con-ductivity (C)

Based on Aller et al. (1987), aquifer media refersto the consolidated or unconsolidated mediumwhich serves as an aquifer such as sand andgravel or limestone. Meanwhile, hydraulic con-ductivity refers to the ability of the aquifer ma-terials to transmit water, which in turn controlsthe rate at which groundwater will flow undera given hydraulic gradient. Both aquifer mediaand hydraulic conductivity of aquifer are im-portant factor since it controls the time availablefor attenuation and the rate in which a contam-inant will flow away from the source.

Figures 4a and 4b illustrate the aquifer me-dia and hydraulic conductivity of Pandak andBambanglipuro according to the geologicalmap, boreholes data and slug test.

2.7 Soil Media (S)

Soil media refers to that uppermost portion ofthe vadose zone characterized by significant bi-

c© 2010 Department of Geological Engineering, Gadjah Mada University 123

SNGUON et al.

Figure 4: a) Aquifer media and b) Hydraulicconductivity of aquifer ranges and correspond-ing rating according to DRASTIC in the studyarea.

Figure 5: a) Soil media and b) vadose zoneranges and corresponding rating according toDRASTIC in the study area.

ological activity (Aller et al., 1987). Soil is com-monly considered the upper weathered zoneof the earth which averages three feet or less.Soil has a significant impact on the amount ofrecharge which can infiltrate into the groundand hence on the ability of a contaminant tomove vertically into the vadose zone (Figure5a and 5b). Moreover, where the soil zone isfairly thick, the attenuation processes of filtra-tion, biodegradation, sorption, and volatiliza-tion may be quite significant.

2.8 Impact of Vadose Zone (I)

The vadose zone is defined as that zone abovethe water table which is unsaturated. The typeof vadose zone media determines the attenu-ation characteristic of the material below the

Figure 6: Slope variation ranges and corre-sponding rating according to DRASTIC in thestudy area.

typical soil horizon and above the water table(Aller et al., 1987).

2.9 Topography (T)

Topography parameter, in DRASTIC method,refers to the slope and slope variability of theland surface. Basically, topography helps con-trol the likelihood that pollutant will run-off orremain on the surface in one area long enoughto infiltrate, since the greater the chance of infil-tration, the higher the pollution potential asso-ciated with the slope (Figure 6).

2.10 Application of Hoelting Method to theStudy Area

Hoelting Method was developed by the Geo-logical Survey of the individual states of theFederal Republic of Germany in 1995 (Voigtet al., 2004). It is another point-count systemwhich takes the unsaturated zone into consid-eration. The degree of vulnerability is expressesas the protective effectiveness (the ability of thecover above an aquifer to protect the ground-water) of the soil cover down to a depth of1 m (the average rooting depth) and the rockcover (unsaturated zone). The higher the totalnumber of points, the longer the approximateresidence times for water percolating throughthe unsaturated zone and in consequence thegreater the overall protective effectiveness.

Thus the overall protective effectiveness (PT)is calculated using the following formula:

PT = P1 + P2 + Q + HP



Table 1: Value of soil factor according to effec-tive field capacity.

ΣeFC (mm) down to 1m Soil factors (S)

> 250> 200 – 249> 140 – 199> 90 – 139> 50 – 89≤ 50

7505002501255010

Source: Hoelting et al. (1995)

Soil Type eFC (mm/dm)Fine Sand 20.0 – 21.5 Medium – Coarse sand 8.0 – 11.0Sandy loam 12.5 – 21.0Silty sand 13.0 – 26.5Clay 11.0 – 16.0Silty loam 14.5 – 21.0

Source: AGBODEN (1996)

Type of sub-soil: unconsolidated materials

Point of sub-soil factor

Clay 500Loamy Silt, Silt 250Loamy sand, silty sand 90Slightly clayey sand, clayey sand gravel

75

Sand 25Gravelly sand, sandy gravel 10Gravel, Breccia 5Source: Hoelting et al. (1995)

Rock type Factor (O)

Claystone, shale, marlstone, siltstoneSandstone, quartzite, massive igneous rocks, metamorphic rockPorous sandstone, porous effusive volcanic rock (e.g. tuff)Conglomerate, breccias, (tuffaceous) limestone, dolomite, gypsum rock

20

15

10

5Source: Hoelting et al. (1995)

Hydraulic features Factor (F)

Non-jointedSlightly jointedModerately jointed, slightly karsticModerately karsticStrongly jointed, fractured or karsticNot known

254

1.00.50.31.0


U (mm/a) P-ETP (mm/a) Factor (W)

≤ 100> 100 – 199> 200 – 299> 300 – 399≥ 400

– ≤ 100> 100 – 199> 20 – 299> 300 – 399≥ 400

1.751.501.251.000.750.50


Overall protective

effectiveness

Total number of points

Approximate residence time in the

unsaturated zone

Very highHighModerateLowVery low

> 4000> 2000 – 4000 > 1000 – 2000 > 500 – 1000 ≤ 500

>25 years10-25 years3-10 yearsSeveral months to 3 yearsA few days to 1 year; in karstic rocks often less

Source: Hoelting et al. (1995)Table 2: Typical effective field capacity of soil.


> 250> 200 – 249> 140 – 199> 90 – 139> 50 – 89≤ 50

7505002501255010







75




20

15

10




254

1.00.50.31.0



≤ 100> 100 – 199> 200 – 299> 300 – 399≥ 400

– ≤ 100> 100 – 199> 20 – 299> 300 – 399≥ 400

1.751.501.251.000.750.50


Overall protective

effectiveness



unsaturated zone


> 4000> 2000 – 4000 > 1000 – 2000 > 500 – 1000 ≤ 500



Where P1 is the protective effectiveness of thesoil cover, (P1 = S W); S: effective field capac-ity (eFC) of the soil (each eFC class is assigned adifferent rating down to 1 m depth the averagerooting depth), W: percolation factorP2 is the protective effectiveness of the rockcover, P2 = W(R1T1 + R2T2 + ... + RnTn); R:rock type where Rs (Rs = O�F) stands for consol-idated rocks and Ru for unconsolidated rocks,O is a factor for rock type, and F is the degreeof faulting, jointing and karstification), T: thick-ness of the rock cover above the aquifer.Q is bonus points for perched aquifer systems(500 points).HP is bonus points for hydraulic (artesian)pressure condition (1500 points

The values of each parameter are shown onthe Tables 1–6 and the class for overall protec-tive effectiveness is shown on Table 7.

Table 3: Value of sub-soil type factor of uncon-solidated materials.


> 250> 200 – 249> 140 – 199> 90 – 139> 50 – 89≤ 50

7505002501255010







75




20

15

10




254

1.00.50.31.0



≤ 100> 100 – 199> 200 – 299> 300 – 399≥ 400

– ≤ 100> 100 – 199> 20 – 299> 300 – 399≥ 400

1.751.501.251.000.750.50


Overall protective

effectiveness



unsaturated zone


> 4000> 2000 – 4000 > 1000 – 2000 > 500 – 1000 ≤ 500



Table 4: Value of rock type factor of consoli-dated rock (O).


> 250> 200 – 249> 140 – 199> 90 – 139> 50 – 89≤ 50

7505002501255010







75




20

15

10




254

1.00.50.31.0



≤ 100> 100 – 199> 200 – 299> 300 – 399≥ 400

– ≤ 100> 100 – 199> 20 – 299> 300 – 399≥ 400

1.751.501.251.000.750.50


Overall protective

effectiveness



unsaturated zone


> 4000> 2000 – 4000 > 1000 – 2000 > 500 – 1000 ≤ 500



Table 5: Value of factor (F) according to the de-gree of discontinuity.


> 250> 200 – 249> 140 – 199> 90 – 139> 50 – 89≤ 50

7505002501255010







75




20

15

10




254

1.00.50.31.0



≤ 100> 100 – 199> 200 – 299> 300 – 399≥ 400

– ≤ 100> 100 – 199> 20 – 299> 300 – 399≥ 400

1.751.501.251.000.750.50


Overall protective

effectiveness



unsaturated zone


> 4000> 2000 – 4000 > 1000 – 2000 > 500 – 1000 ≤ 500



Table 6: Percolation factor (W) according torecharge rate.


> 250> 200 – 249> 140 – 199> 90 – 139> 50 – 89≤ 50

7505002501255010







75




20

15

10




254

1.00.50.31.0



≤ 100> 100 – 199> 200 – 299> 300 – 399≥ 400

– ≤ 100> 100 – 199> 20 – 299> 300 – 399≥ 400

1.751.501.251.000.750.50


Overall protective

effectiveness



unsaturated zone


> 4000> 2000 – 4000 > 1000 – 2000 > 500 – 1000 ≤ 500




SNGUON et al.

Table 7: Class for overall protective effective-ness.


> 250> 200 – 249> 140 – 199> 90 – 139> 50 – 89≤ 50

7505002501255010







75




20

15

10




254

1.00.50.31.0



≤ 100> 100 – 199> 200 – 299> 300 – 399≥ 400

– ≤ 100> 100 – 199> 20 – 299> 300 – 399≥ 400

1.751.501.251.000.750.50


Overall protective

effectiveness



unsaturated zone


> 4000> 2000 – 4000 > 1000 – 2000 > 500 – 1000 ≤ 500



3 Results

Results of groundwater vulnerability assess-ment from DRASTIC and Hoelting methods areshown in Figure 7. The diagram apparentlyshows that there is no relation or interaction be-tween intrinsic groundwater vulnerability fromDRASTIC model and actual contaminant con-centration in the study area at all. Similar facthas been proved by some researcher that inthe case of DRASTIC model, only little corre-spondence exists between the most vulnerableand the most contaminated . The correlationbetween intrinsic vulnerability from Hoeltingmethod and actual contaminant concentrationrepresent as a curve where average of actual ni-trate concentration in groundwater ranges re-spectively according to the classes; Very High> High > Medium > Low > Very Low.

Since each method of determining vulnera-bility may reflex in different parameters andapplicable at certain hydrogeology system, theonly way of gaining confidence in vulnerabilitymapping is by comparing the results of varioustechniques and analyzing their consistency inpractical case studies where contamination hasalready occurred (Striger et al., 2005; Gogu etal., 2003; Vias et al., 2005; Ferreira and Oliveira,2003).

Believed to be a very conservative ion inthe groundwater, nitrate contaminant has beenchosen as an indicator contaminant for the eval-uation. In order to facilitate the evaluation ofthe vulnerability, a new set of maps is createdby subtracting the assessed vulnerability class

Figure 9: Nitrate vs Chloride concentration ingroundwater of the research area.

from the nitrate contamination class. However,for the evaluation, source of nitrate is also. Thetwo principal anthropogenic sources of nitratein the study area are from chemical fertilizersand domestic wastewater leakage from septictank (or faecal coliform).

From many experiments, ARGOSS (2001) hasconcluded that where the nitrate:chloride ratiois between 1:1 and 8:1, then it is likely that thenitrate is primarily from a faecal source. As infact, the diagram shows that the nitrate:chlorideratio is approximately 5:1, hence the source ofnitrate is originally from on-site sanitation (Fig-ure 9).

4 Discussion and Conclusion

Producing vulnerability map of one area is avery challenge work for hydrogeologist. How-ever, comparing vulnerability map conductedfrom two methods is more difficult since it willexpress carrying distribution pattern, whichhave similarities beneath strong different andeven contradiction. Thus, it is recommendedto all new researchers to choose a method insame system group for the comparison and as-surance of the produced map, as also men-tioned by Gogu et al. (2000), to avoid a wrongjudgement. Because of the limitation of this re-search, some study can not be done. Hence, it isrecommended to others researchers to conductmore research in this area. For instance, thespecific vulnerability map of Nitrate and Chlo-



Figure 7: Map of intrinsic vulnerability of groundwater within the study area using DRASTIC andHoelting method.

Figure 8: Boxplot diagram between groundwater nitrate concentration and a) DRASTIC; b) Hoelt-ing Vulnerability Class.

ride should be done in order to assure the qual-ity of groundwater for the supply. Some fur-ther study should also be done by statistical re-gression to assure the accuracy of the evalua-tion. For the environmental protection sound,it should focus not only the contaminant butalso its source and other affecting factor suchas on-site sanitation improvement, water useplanning, and its land use management.

As mention above, protection should be morehighly concerned, as the remediation is diffi-cult and expensive, especially at the most north-ern part of both areas whose groundwater isvery vulnerable to be polluted. Concerningto the nitrate contamination, on-site sanitationshould be improved to reduce the risks, sinceit is the primary source of nitrate in the studyarea. Otherwise the groundwater will not be us-able as water supply in few months time. Basi-cally, as communities develop, there is increas-ing pressure for the land and thus encroach-ment into areas where sanitation is controlledmay occur over time. Unless the community

and support agencies have plans to counter-actthis. In addition, risk of contamination will in-crease when basic protective measures are notwell-maintained.

References

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Gogu, R.C and Dassargues, A. (2000), Currenttrends and future challenges in groundwater vul-nerability assessment using overlay and indexmethods, Environmental Geology 39 (6)

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