4.4 Spatial Data Analysis.
This stage will process all the input layer from stage 2 and 3 in order to extract a spatial features which are relevant to the groundwater zone. This phase includes various analysis such as table analysis and classification, polygon classification and weight calculation. Polygons in each of the thematic layers were categorised depending on the recharge characteristics and suitable weightages were assigned (Table 1-8). The values of the weightage are based on Krishnamurthy et al. (1996 & 1997).
4.5 Data Integration
The final stage involves combining all thematic layers using the method that is modified from DRASTIC model, which is used to assess ground water pollution vulnerability by Environmental Protection Agency of the United State of America (Aller, 1985). The formula of the groundwater potential model (GP) as shown below:
GP = Rf + Lt + Ld + Lu + Te + Ss + Dd + St
where:
Rf:annual rainfall, Lt:lithology, Ld:lineament density, Lu:land use,
Te:topography elevation, Ss:slope steepness, Dd:drainage density and St:soil type.
The output is then reclassed into five groups such as very high, high, moderate, low and very low using the Quantile classification method (ESRI, 1996). The output that is produced is capable of being used for further investigations and assessments, especially at larger scale.
5.0 Result and Discussion
The ground water potential map of the Langat Basin area is shown in figure 3. In order to produce the map, a GIS model has been used, to integrate thematic maps such as annual rainfall, lithology, lineament density, land use, topography elevation, slope steepness, drainage density and soil type. Each thematic layer consists of a number of polygons, which correspond to different features. The polygons in each of the thematic layers have been categorized, depending on the suitability/relevance to the ground water potential, and suitable weights were assigned. Finally, all the thematic layers were integrated using the ground water potential model to derive the final derived layers. The score values of the area in the final map are shown Table 9.
Table 1 : Landuse
| Landuse
| Weight
|
| Forest
| 20
|
| Agriculture
| 40
|
| Scrub
| 30
|
| Wetland
| 50
|
| Urban
| 10
|
| Cleared Land
| 10
|
| Water Body
| 60
|
Table 2: Lineament Density
| Lineament Density (km/km2)
| Weight
|
| > 0.0075
| 60
|
| 0.0055 - 0.0075
| 50
|
| 0.0035 - 0.0055
| 40
|
| 0.0015 - 0.0035
| 30
|
| < 0.0015
| 20
|
Table 3: Annual Rainfall.
| Annual Rainfall (mm)
| Weight
|
| 2500 - 2750
| 70
|
| 2250 - 2500
| 60
|
| 2000 - 2250
| 50
|
| 1750 - 2000
| 40
|
| 1500 - 1750
| 30
|
Table 4 : Lithology.
| Lithology
| Weight
|
| Alluvium
| 70
|
| Limestone
| 40
|
| Phylite-Schist-Quarzit
| 20
|
| Quartz vein
| 5
|
| Volcanic
| 30
|
| Granite
| 10
|
Table 5 : Topography Elevation
| Elevation (m)
| Elevation Zone
| Weight
|
| < 20
| Almost Flat Topography
| 50
|
| 20 - 100
| Undulating Rolling Hilly
| 40
|
| 100 - 500
| Hilly Steeply Disserted
| 35
|
| 500 - 1000
| Steeply Dissected Mountainous
| 25
|
| > 1000
| Mountainous
| 10
|
Table 6 : Slope Steepness
| % Slope
| Slope Gradient
| Slope Zone
| Weight
|
| 0 - 7
| 0° - 3°
| Almost Flat Topography
| 50
|
| 8 - 20
| 4° - 9°
| Undulating Rolling Hilly
| 40
|
| 21 - 55
| 10° - 24°
| Hilly Steeply Disserted
| 30
|
| 56 - 140
| 25° - 63°
| Steeply Dissected Mountainous
| 20
|
| > 140
| > 63°
| Mountainous
| 10
|
Table 7 : Drainage Density
Drainage Density
(km/km2)
| Weight
|
| > 0.0055
| 10
|
| 0.0040 - 0.0055
| 20
|
| 0.0025 - 0.0040
| 30
|
| 0.0010 - 0.0025
| 40
|
| < 0.0010
| 50
|
Table 8 : Soil Type
| Soil Series
| Soil Type
| Weight
|
| Keranji
| Clay
| 10
|
| Melaka-Durian-Muncung
| Gravel clay-silty
clay-clay
| 20
|
| Muncung-Seremban
| Fine sandy clay
| 20
|
| Prang
| Clay
| 10
|
| Regam-Jerangau
| Coarse sandy clay-clay
| 30
|
| Selangor-Kangkung
| Clay
| 10
|
| Serdang-Bugor-Muncung
| Fine sandy clay
loam-fine
sandy clay-clay
| 30
|
| Serdang-Kedah
| Fine sandy clay loam
| 30
|
| Urban Land
| Sandy clay
| 30
|
| Steep Land
| Coarse sandy clay
| 40
|
| Peat Land
| Clay
| 10
|
| Tanah Lombong
| Sand
| 50
|
Telemung-Akob-Lanar
Tempatan
| Sandy loam-sandy
clay
| 30
|
Table 9: Score values of the area polygons in the final map.
| Score/value
| Class of
groundwater zone
| Estimate of
discharge rate
|
| > 285
| Very High
| > 22 m3/hour/well
|
| 260 - 380
| High
| 18 - 22 m3/hour/well
|
| 245 - 255
| Moderate
| 14 - 18 m3/hour/well
|
| 230 - 240
| Low
| 10 - 14 m3/hour/well
|
| < 225
| Very Low
| < 10 m3/hour/well
|
A summary of the results (Table 10), shows that almost all alluvial plains have high potential of groundwater occurrence. Where as, in steeply mountainous areas underlain by granite with low lineament density, the potential for groundwater is very low. Meanwhile in hard rock areas, the groundwater potential is high in areas with high lineament density and low drainage density.
Borehole data collected by the Minerals and Geoscience Department were used to compare the final results with the actual field data.
