GIS-BASED ASSESSMENT OF GROUNDWATER VULNERABILITY …

Proceedings, 6th International Conference on Cartography and GIS, 13-17 June 2016, Albena, Bulgaria

ISSN: 1314-0604, Eds: Bandrova T., Konecny M.

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GIS-BASED ASSESSMENT OF GROUNDWATER

VULNERABILITY TO ARSENIC CONTAMINATION IN THE

FLOODPLAIN OF THE OGOSTA RIVER, NW BULGARIA

Velimira Stoyanova, Tsvetan Kotsev

Dr. Velimira Stoyanova

National Institute of Geophysics, Geodesy and Geography

Bulgarian Academy of Sciences

Acad. Georgi Bonchev Str., Bl. 3, 1113 Sofia, Bulgaria

Tel.: +3592 979 3396; GSM 0889885934

E-mail: [email protected]

Assoc. Prof. Dr.Tsvetan Kotsev

National Institute of Geophysics, Geodesy and Geography

Bulgarian Academy of Sciences

Acad. Georgi Bonchev Str., Bl. 3, 1113 Sofia, Bulgaria

GSM 0894413096

E-mail: [email protected]

Abstract

The aim of this study is to evaluate the specific groundwater vulnerability of arsenic contamination in the specific

environmental conditions of river floodplains. For this purpose DRESPI modification of DRASTIC index is elaborated

and tested in the arsenic polluted Ogosta Valley, NW Bulgaria. The assessment takes into account the following six

parameters: depth to groundwater table, net recharge, soil texture, impact of the thickness of soil, redox potential and

pH of soil. The respective layers are generated in ArcGIS (ESRI, 2015) and the DRESPI index is computed to generate

the final vulnerability map. The values of the vulnerability index for the floodplain of the Ogosta River are in the range

of 55 - 182 points. The most vulnerable areas are associated with the lowest sections of the floodplain close to the river,

while the less threatened are the lands of the high floodplain in the valley.

Keywords: DRASTIC index, DRESPI index, GIS, Map Algebra

INTRODUCTION

Economic activities, e.g. ore dressing and smelting, tend to concentrate in the river valleys due to their favorable

conditions for development of settlements and industrial infrastructure and this often results in chemical contamination

of soil, especially in the river floodplains. The problem of arsenic contamination of aquifers in the valleys and river

lowlands affects millions of people across the world: Asia – Bangladesh, China, India, Taiwan (Fazal et. аl 2001;

Тuinhof et. al 2006; Smedley, 2008); North America – Mexico; South America – Argentina, Chile (Berlin et al., 2001;

Rodriguez et al., 2002); Europa – Poland, Hungary and Spain (Jain et al., 2000) (Figure 1). Preconditions for

enrichment of groundwater with As exist also in the Ogosta Valley, NW Bulgaria, as a consequence of the historic

mining in the local area. The global significance of the ‘arsenic problem’ mobilizes huge resources and efforts by

international organizations, state governments, local authorities and scientists for research and remediation activities.

An accurate assessment of groundwater vulnerability of As pollution in the river floodplains would help the delineation

of the most threatened areas and prevent valuable water resources from degradation. The index method of DRASTIC

(Aller et al, 1987) and its variations GLA (Hölting et al., 2005), PI (Goldscheider et al., 2000), SINTACS (Civita,

1994), EPIK (Doerfliger and Zwahlen, 1998), REKS (Malik et al., 1999), GOD (Foster, 1987), AVI (van Stemproot et

al., 1993) are often used for evaluation of groundwater intrinsic vulnerability, but we were not able to find in the

literature an application specifically developed for arsenic under conditions of riparian environment.



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Figure 1. Countries with established contamination of aquifers with arsenic (Source:Rahman et al., 2006; Mukherjee et

al., 2006)

1. Poland; 2. Brazil; 3. NewZealand; 4. Spain; 5. Hungary; 6. USA; 7. Argentina; 8. Mexico; 9. Taiwan; 10. Chile; 11.

Sri Lanka; 12. Canada; 13. China; 14. Bangladesh; 15. India; 16. Vietnam; 17. Afganistan; 18. Pakistan; 19. Egypt;

20. Ghana; 21. Cambodia; 22. Sweden; 23. Finland; 24. United Kingdom; 25. Germany; 26. Romania; 27. Bulgaria;

28. Greece; 29. Austria; 30. Myanmar; 31. Iran; 32. Japan; 33. Laos; 34. Nepal; 35. Switzerland; 36. Thailand

The latter is not taken into account by Guo et al. (2007) and Ckakraborty et al. (2007) though both research teams

studied aquifer vulnerability to As contamination with DRASTIC models.The PI method was applied for the basin of

the Ogosta River by Orehova et al. (2009) and the aquifer intrinsic suscptibility in the Ogosta Valley was rated as

medium. The results of this study does not show any differentiation of the estimates within the valley floor and the

specific vulnerability of groundwater to As pollution has not been considered also.

Given the foregoing, the aim of the current paper is to propose a modification of the DRASTIC index method for

assessment of groundwater vulnerability to arsenic pollution in river floodplain and to present the results of its

application for a stretch of the Ogosta Valley.

MATERIAL AND METHODS

Study Area

The study area is situated in the upper stretch of the Ogosta Valley, NW Bulgaria (Figure 2) and extends between the

parallels of latitude N43˚22 and N43˚25 and meridian of longitude E22˚48 and E23˚70. It covers an area of around 14,5

sq. km with mean elevation of 491 m and average inclination of slopes about 3˚. The valley extends over part of the

Western Balkan mountain range and Western Fore-Balkan. It includes parts of three municipalites: Chiprovtsi, Georgi

Damyanovo and Montana. Extraction and dressing of iron-ore and lead-ore took place near the town of Chiprovtsi in

the upper reach of the Ogosta River from 1951 to 1999. Due to the tailings dam failure in 1964 and to the mine waste

discharge into the Ogosta River in the period 1964-1979, the floodplain soil in the Ogosta Valley received significant

amounts of arsenic and heavy metals (Spectroteh, 1994; Kotsev, 2003; Mandaliev et al., 2013; Kotsev et al., 2015).



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Figure 2. Study area

A - “Ogosta” Reservoir; B - Ogosta River; C, D – monitoring wells in the Ogosta Valley

Elevated As levels in the groundwater are registered for some sites in the valley (Kotsev et al., 2006), but the problem is

not well studied yet.

DRESPI index method

DRESPI is rather an modification of the DRASTIC model than a complete new index method. It is aimed at evaluation

of specific vulnerability to As pollution of groundwater in river floodplains. The acronym is created from the first letters

of the following six parameters integrated in the final assessment: Depth to groundwater table (D); net Recharge (R);

Eh - redox state of soil (E); Soil texture (S); pH - active soil reaction (P); Impact of the soil thickness (I). The indices of

DRESPI can be divided into two groups: (1) for evaluation of hydrogeological settings – D, R, S, I; (2) for assessment

of As mobility in the vadose zone – E, P. Compared with DRASTIC, the new application of the model does not include

the indices related to lithology and conductivity of the aquifer as well as the topography (inclination of slopes) because

of their poor spatial differentiation within the river floodplain, at least in the study area (table 1). Redox state and active

reaction (pH) are the major factors to control the solubility and water migration of chemical elements and they were

integrated in the DRESPI model to make the assessment more specific with regard to As. It is well known that under

reducing conditions arsenic tends to form AsIII

species which can be easily transported within the vadose zone (Nriagu,

1994). Under oxic conditions predominant are AsV species, which are strongly absorbed by iron oxide-hydroxides and

are mostly fixed in situ in soil (Nriagu, 1994; Wang et al., 2006; Pendias, 2011). Given the fact that information on

absolute values of Eh of soil solutes is rather scarce, this index is reduced to registration of lack or presence of

indications for periodic or permanent reduction of iron and manganese species in soil. Because of the lack of

information the two intervals for periodic and predominant reducing conditions are merged into one for this case study.

Soil pH intervals are borrowed from the USDA classification of soil active reaction (USDA, 1998). The rating of the

intervals is determined considering the decreasing adsorption of AsV species onto iron oxide-hydroxides with increasing

of pH (Raven et al., 1998; Amita et al., 1999; Dixit et al., 2003). DRESPI keeps the same weight ranking as DRASTIC

for the indices related with hydrological settings. Although the content of the I-index is altered in DRESPI, its weight

factor remains equal to 5 because of the significant protective role of soil cover against As penetration to the

groundwater. The maximum weight of the Eh parameter reflects the significant role of the redox state of soil for As



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mobility and transport in the vadose zone. Arsenic mobility is less dependent on the soil reaction and this is the reason

pH to be considered less important than Eh.

The DRESPI index is calculated as the sum of product of ratings and weights assigned to each of the parameters.

Di = DR*DW + RR*RW + ER*EW +SR*SW + PR*PW + IR*IW (1)

Where: Di – DRESPI index, R – rating, W – weight.

The minimum DRESPI index value is 23 and the maximum is 219 and it is very close to the range of DRASTIC 26-

228. This allows DRESPI to use vulnerability classes identical to those of DRASTIC, which makes the perception of

results easier. The only difference is the merger of middle classes 4 and 5 of DRASTIC into one class in DRESPI. It

resulted in dividing the whole range into the following seven classes: 23-79 (no risk area), 80-99 (very low

vulnerability), 100-119 (low vulnerability), 120-159 (moderate vunerability), 160-179 (high vulnerability), 180-199

(very high vulnerability) and 200-219 (extremely high vulnerability). However, the estimates of the two models are not

comparable due to differences between content and intervals of the used parameters.

Table 1. DRASTIC and DRESPI Rating and Weighting Values for the Various Hydrogeological Parameter Settings

(Aller et al, 1987; Stoyanova, 2015)

DRASTIC (Aller et al., 1987) DRESPI (Stoyanova, 2015)

Parameters Range Rating R DRASTIC

Weight

W

Total

Score

Parameters Range Rating

R

DRESPI

Weight

W

Total

Score

D - Depth

to water

(m)

30,5>

22,9-30,5

15,2-22,9

9,1-15,2

4,6-9,1

1,5-4,6

0-1,5

1

2

3

5

7

9

10

5 5

10

15

25

35

45

50

D - Depth to

groundwater

tabel

(m)

5>

4-5

3,5-4

3-3,5

2,5-3

2-2,5

1,5-2

1-1,5

0,5-1,0

0-0,5

1

2

3

4

5

6

7

8

9

10

5 5

10

15

20

25

30

35

40

45

50

R - Net

recharge

(mm)

0-50,8

50,8-101,6

101,6-177,8

177,8-254

254>

1

3

6

8

9

4 4

12

24

32

36

R –

net

Recharge

(mm/y)

0-50

51-100

101-150

151-200

201-250

251-300

301-350

351-400

401-450

451>

1

2

3

4

5

6

7

8

9

10

4 4

8

12

16

20

24

28

32

36

40

A -

Aquifer

media

Massive Shale

Metamorphic

Weathered

Metamorphic

Glacial Till

Bedded Sandstone,

Limestone and

Shale Sequences

Massive Sandstone

Masive Limestone

Sand and Gravel

Basalt

Karst Limestone

2

3

4

5

6

6

6

8

9

10

3 6

9

12

15

18

18

18

24

27

30

Not

applicable

Not

applicable

E - Redox

state of the

soil – Eh

Predominant

oxic

conditions

Alternation

of oxic and

reducing

1

5

4 4

20



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conditions

Predominant

reducing

conditions

10

40

S - Soil

media

Nonshrinking

Muck

Clay Loam

Silty Loam

Loam

Sandy Loam

Shrinking

Peat

Sand

Gravel

Thin ar absent

1

2

3

4

5

6

7

8

9

10

10

2 2

4

6

8

10

12

14

16

18

20

20

S – Soil

texture

Share of

particle size

fraction

<0,01mm

(%)

85>

75-85

60-75

45-60

30-45

20-30

10-20

5-10

<5

1

2

3

4

5

6

7

8

9

2 2

4

6

8

10

12

14

16

18

Not

applicable

P - Active

soil reaction

– pH

<5,5

5,6-6,0

6,1-6,5

6,6-7,3

7,4-7,8

7,9-8,4

8,4>

1

2

3

4

5

6

7

3 3

6

9

12

15

18

21

T -

Topograph

y (Slope)

(%)

18>

12-18

6-12

2-6

0-2

1

3

5

9

10

1 1

3

5

9

10

Not

applicable

I - Impact

of the

vadose

zone

media

Confining Layer

Silt/Clay

Shale

Limestone

Sandstone

Bedded

Limestone,

Sandstone, Shale

Sand and Gravel

wit Silt and Clay

Metamrphic/

Igneous

Sand and Gravel

Basalt

Karst Limestone

1

3

3

6

6

6

6

4

8

9

10

5 5

15

15

30

30

30

30

20

40

45

50

I - Impact of

the

thickness of

soil (m)

1,80>

1,61-1,80

1,41-1,60

1,21-1,40

1,01-1,20

0,81-1,00

0,61-0,80

0,41-0,60

0,21-0,40

0-0,20

1

2

3

4

5

6

7

8

9

10

5 5

10

15

20

25

30

35

40

45

50

C -

Conductivi

ty of the

aquifer

(GPD/FT2)

1-100

100-300

300-700

700-1000

1000-2000

2000>

1

2

4

6

8

10

3 3

6

12

18

24

30

Not

applicable

Spataial data processing in GIS

Assessment of groundwater vulnerability with DRESPI, as well as with DRASTIC, produces not only a numeric score,

but also maps for each index and an integrated vulnerability map. The whole process takes four major steps using ESRI

ArcGIS, ArcMap 10.3 (Figure 3). The first step produces files into raster format for each six parameters of DRESPI.

Depth to groundwater table is obtained by subtracting the values of altitude in the available hydrodynamic map from the

altitude of the detailed DTM 1x1 m. The limited number of sites with information on the soil texture and soil thickness

did not allow us to interpolate these data and average values were calculated for the main morphographic zones

delineated by using the morphographic map of the valley (Tcherkezova, 2015). The map of the zones is then

reclassified with the relevant mean values for each zone. Areas of low redox potential in soil are outlined to the sections

of shallow groundwater table with depth less than 50 cm from the surface. Locations of studied soil profiles with

described gleyic colour patterns fitted very well to these areas. The second step of data processing includes

reclassification of the resulted maps of the six indices considering the rating of the predefined intervals for each

parameter. The values of the rated maps are then multiplied with the relevant weight factors to produce the final maps

of each index of DRESPI (Figure 4). The third step uses the Spatial Analyst Tools - Map Algebra - Raster Calculator of



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ArcMap to combine all the index maps into one preliminary vulnerability map. The latter is reclassified in the last step

of data processing taking into account the predefined vulnerability classes.

Figure 3. Flowchart of groundwater vulnerability analysis using DRESPI index in GIS

Figure 4. Evaluation maps of DRESPI indices



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Each class is characterized by a different color: no risk area – dark green (RGB – 38/118/0); very low vulnerability –

green (RGB – 10/204/0); low vulnerability- light green (RGB – 170/255/0); moderate vunerability – yellow (RGB –

255/255/0); high vulnerability – orange (RGB – 255/209/15); very high vulnerability – red (RGB - 255/0/0); extremely

high vulnerability – violet (RGB – 194/0/187).

RESULTS AND DISCUSSION

The calculated values of DRESPI index for the Ogosta Valley are within the range 55-182 and fall into six classes of

vulnerability with different coverage of the study area (% of total area) : no risk area – 2.48; very low vulnerability -

11.05; low vulnerability – 33.76; moderate vulnerability – 29.54; high vulnerability – 23.13; very high vulnerability

0.03 (Figure 5).



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Figure 5. Groundwater Vulnerability map

Results show that moderate and higher levels of vulnerability are considered for more than 50% of the Ogosta Valley

lands between the village of Beli Mel and the “Ogosta”reservoir. Groundwater in these areas is prone to significant

threat of arsenic pollution. Depth to groundwater table, net recharge, redox potential and soil thickness are the factors

with the greatest contribution to the vulnerability estimates. Sections with highest scores are associated with the lowest

parts of the floodplain along the Ogosta River channel due to the shallow groundwater table (0-1.5m), the higher

probability for reducing conditions in soil, its less thickness (0.4-0.8 m) and coarser texture. The aquifer is much better

protected from arsenic pollution in the higher floodplain where most of the indices show low scores. The DRESPI

groundwater vulnerability map clearly shows three areas of signifficant concern with regard to possible As pollution

where high vulnerability level (shades of orange) are registered. The biggest area is located close to the reservoir where

the low floodplain is wide spread. The other two areas are located in the lands of the villages of Beli Mel and Gorna

Kovatchitsa where low sections of the floodplain are also well developed.

CONCLUSIONS

The proposed DRESPI modification of the DRASTIC model integrates new indices to provide more accurate evaluation

of groundwater vulnerability to As contamination in river floodplains. The elaborated map shows significant spatial

differentiation of vulnerability within the study area of the Ogosta Valley, which is mostly due to the detailed DTM

used for preparation of some of the input data layers. The levels of vulnerability are closely associated with the

morphology of the river floodplain as a result of the fluvial genesis of the study area. The aquifer is most susceptive to

As contamination in the lowest sections of the floodplain where the highest levels of soil pollution in the valley can be

expected. The DRESPI estimates for the Ogosta Valley are dependant mostly on the indices Depth to groundwater, net

Recharge, Redox state of soil and the Impact of soil thickness. The evaluation elaborated with DRESPI can be

incorporated as a first step in the risk assessment of groundwater pollution with arsenic in riparian zones. The range and

intervals of the index of net Recharge take into account the hydro-climatic conditions of Bulgaria and for this reason the

current setting of DRESPI makes it most appropriate for the river valleys on its territory. Nevertheless, the model can be

easily adapted to the features of other regions.

ACKNOWLEDGEMENTS

This study has been accomplished in the frame of the research project ASCOR “Arsenic contamination of Ogosta River:

Linking biogeochemical processes in floodplain soils with river system dynamics”, № IZEBZO-142978, Bulgarian-

Swiss Research Programme (2011-2016).



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BIOGRAPHY

Dr. Velimira Stoyanova works at the National Institute of Geophysics, Geodesy and Geography –

Bulgarian Academy of Sciences, Department of Geography, section Physical Geography. She is a

member of Bulgarian Cartographic Association and Bulgarian Geographical Society. She received

a Doctoral degree in Physical Geography and Landscape Studies in 2015. Her PhD research is

related to vulnerability of arsenic contamination of groundwater in the floodplain of Ogosta River

between Martinovo Village and “Ogosta” Reservoir. She is Master's degree of Physical Geography

and Landscape Studies and Regional Development and Management at Sofia University “St.

Kliment Ohridski”, Faculty of Geology and Geography, 2011. Current research interests of Dr.

Stoyanova are in Environmental Geochemistry, Landscape Ecology, Hydrogeology, Geographic

Information Systems.

Assoc. Prof. Dr.Tsvetan Kotsev works at the National Institute of Geophysics, Geodesy and

Geography - Bulgarian Academy of Sciences, Department of Geography, section Physical

Geography. Dr. Kotsev received his Master Degree in Geography from Sofia University“St.

Kliment Ohridski” in 1998 and his Doctoral Degree in Physical Geography in 2003. Current

research interests of Dr. Kotsev are in the fields of Environmental Geochemistry, Environmental

Quality Assessment and Landscape Ecology.

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_052208.pdf

GIS-BASED ASSESSMENT OF GROUNDWATER VULNERABILITY …

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