GIS Techniques for Groundwater contamination Risk Mapping

ABSTRACT:
While water supply is a crucial issue, there is also evidence to suggest that the quality of groundwater supplies is also under threat in recent years. This is a result of salinisation and an increase in the use of agrochemicals. This paper attempts to produce groundwater vulnerability and risk maps using DRASTIC system. These maps are designed to show areas of greatest potential for groundwater contamination on the basis of hydro geological conditions and human impact. Geographical Information System (GIS) is used to create a groundwater vulnerability map by overlaying the available hydro geological data from thematic maps. The resulting map is then integrated with a landuse map to assess the potential risk of groundwater to pollution in the area. The final map shows interesting results and stresses the need for the GIS to test and improve on the groundwater contamination risk assessment methods. This is a very important contribution of GIS towards environmental management.

GROUNDWATER VULNERABILITY:
Groundwater is generally a safe source of drinking water. Certain problems have beset the use of groundwater to human users and to the environment due to its over-use or overdraft. Groundwater vulnerability refers to the sensitivity of groundwater to contamination. The seriousness of the impact on water use depends on the extent and magnitude of the pollution and the value of the groundwater resource. Groundwater vulnerability mapping is used as a guide in determining which areas are more susceptible to groundwater contamination within the mapped area. This paper aims at mapping groundwater contamination using GIS techniques

DRASTIC SYSTEM:
DRASTIC is an empirical model developed by U.S. Environmental Protection Agency. It is widely used for evaluating relative groundwater pollution susceptibility by using hydro geological factors. DRASTIC is an acronym for the most important hydro geologic features which control groundwater pollution.

These features are:
• D - Depth to water table
• R - (Net) Recharge
• A - Aquifer media
• S - Soil media
• T - Topography (slope)
• I - Impact of Vadose Zone Media
• C - Conductivity (Hydraulic) of Aquifer.

For further accurate analysis, lineament density, analyzed from lineament map is overlaid to DRASTIC system.

METHODOLOGY:
Each DRASTIC factor is assigned a weight based on its relative significance in affecting pollution potential. Each factor is further assigned a rating for different ranges of the values or significant media types based on its impact on pollution potential. The typical ratings range from 1 to 10 and the weights from 1 to 5. DRASTIC index is calculated for each basin. The ArcView (ESRI, 1997) Geographic Information System (GIS) is used to compile the geospatial data, to compute the DRASTIC indices, and to generate the final vulnerability map. The higher the DRASTIC index, the greater the relative pollution potential.

DRASTIC INDEX:

The equation for determining the DRASTIC index is:
DrDw + RrRw + ArAw + SrSw + TrTw + IrIw + CrCw = DRASTIC Index
where the letters D, R, A, S, T, I, C represent the seven hydro geologic factors, r designates the rating, and w the weight. The smallest possible DRASTIC index rating is 23, and the largest is 226.



DEPTH TO WATER:
Depth to water constitutes the thickness of ground travelled by a contaminant before reaching the groundwater table. Shallow water tables are more susceptible to contamination than deep water tables. Depth of water is computed from land surface topography (as DEM) and water table surface topography. Water table contours are digitized, geo-referenced and rasterized .Water table elevations are subtracted from land surface elevation on a pixel by pixel basis to compute depth of water using well data.

Factors	Range (feet)	Rating
Depth to water	0 - 5 5-15 15-30 30-50 50-75 75-100 100 +	10 9 7 5 3 2 1

NET RECHARGE:
Net Recharge represents the amount of water that penetrates the ground surface and reaches the water table. Precipitation data is extracted from meteorological station report. Run off and Evapotranspiration are calculated using estimated value by land use. Net recharge = precipitation – evaporation – run off. The more the recharge, greater the contamination of ground water.

Factors	Range(inches/year)	Rating
Net Recharge	0 - 2 2 - 4 4 - 7 7 - 10 10+	1 3 6 8 9

AQUIFER MEDIA:
The material of the aquifer determines the mobility of the contaminant through it. An increase in the time of travel of the pollutant through the aquifer results in more attenuation of the contaminant. All information is obtained from geological database.

Factors	Range	Rating	Typical rating
Aquifer media	Massive shale Metamorphic/igneous Weathered metamorphic/igneous Glacial Till Bedded sandstone, limestone, shale Massive sandstone ,massive limestone Sand and gravel Basalt Karst limestone	1 - 3 2 - 5 3 - 5 4 - 6 5 - 9 4 - 9 4 - 9 2 - 10 9 - 10	2 3 4 5 6 6 8 9 10

SOIL MEDIA
Soils of different types have differing water holding capacity and influence the travel time of the contaminant. Soil database provides the required information.

Factors	Range	Rating
Soil media	Thin or Absent ,Gravel Sand Peat Shrinking and/or aggregated clay Sandy loam Loam Silty loam Clay loam Muck Nonshrinking and nonaggregated clay	10 9 8 7 6 5 4 3 2 1

TOPOGRAPHY:
Topography is considered as the slope and slope variability of the land surface. It helps to control pollutant run off or retention on the surface. High degrees of slope increases run off and erosion which is composed of the pollutant. Slope percentages are calculated using the Digital Elevation Model (DEM) data in a Topography database.

Factors	Range(percent slope)	Rating
Topography (%).	0 - 2 2 - 6 6 - 12 12 - 18 18 +	10 9 5 3 1

IMPACT OF VADOSE ZONE:
The vadose zone is defined as the ground portion found between the aquifer and the soil cover in which pores or joints are unsaturated or only discontinuously saturated. The texture of the vadose zone determines the time of travel of contaminant through it.

Factors	Range	Rating	Typical rating
Impact of the vadose zone media	Confining layer Silt/clay Shale Limestone Sandstone, Bedded limestone, sandstone, shale, Sand and gravel Metamorphic/igneous Sand and gravel Basalt Karst limestone	1 2 – 6 2 – 5 2 – 7 4 – 8 2 - 8 6 – 9 2 - 10 8 - 10	1 3 3 6 6 4 8 9 10

HYDRAULIC CONDUCTIVITY:
Hydraulic conductivity of the soil media determines the amount of water percolating to the ground water through the aquifer. Hydraulic conductivity maps are obtained from the maps of Transmissivity and saturated Thickness.

Factors	Range	Rating
Hydraulic Conductivity (GPD/Ft2)	1 - 100 100 - 300 300 - 700 700 - 1000 1000 - 2000 2000 +	1 2 4 6 8 10

MODIFIED DRASTIC SYSTEM:
Modified DRASTIC system is sum of DRASTIC system and lineament density. Lineaments are geological structures such as fractures and joints. The lineament is closely related to groundwater flow and contaminants migration. Lineament density is obtained by using photolineament factor value from aerial photographs. Higher lineament density values may represent more potential to groundwater contamination.

Modified DRASTIC index = DRASTIC index + (Distribution density rating x 5)

SPATIAL DATABASE DESIGN AND CONSTRUCTION USING GIS:

Topographic database design and construction
Topographic database is constructed using 1:50,000 scale topographic map (contour interval: 20m).

Drainage database design and construction
Drainage database is constructed using 1:50,000 scale topographic map. The drainage database has line and point attributes. The line attribute represents river and stream and the polygon attribute represents reservoir, lake and large river.

Well database design and construction:
Well database includes information about well and groundwater. The data from the inventory are well location, owner, address, phone number, installation year, use, depth, diameter, static water table, survey date, depth to water level, temperate, pH, EC and TDS and they are constructed to spatial database using GIS.

Geologic database design and construction
Geologic database is constructed using 1:50,000 scale geological map. Geologic database has polygon attribute.

Soil database design and construction
Soil database is constructed using 1: 25,000 scale detailed soil map. The database has polygon attribute.

Land use database design and construction
Land use database has polygon attribute. Land use database is constructed by converting from the result of image processing of LANDSAT TM image.

Lineament database design and construction
Lineament database has line attribute. The lineament database is obtained from the aerial photo interpretation and used for calculating lineament density.



ASSESSMENT OF GROUNDWATER POLLUTION SUSCEPTIBILITY:

The steps of assessment include

• data collection about study area
• database construction using the collected data
• extraction of hydro geologic factors from the database
• Overlay analysis of the factors.

OVERLAY ANALYSIS:
Groundwater Contamination Risk Mapping is carried out by overlay of layers representing the DRASTIC parameters. Theoretically an overlay is necessary for each parameter; however DRASTIC parameters are frequently closely associated. In some areas the vadose zone and aquifer media are the same. In other areas, soil and topography are intimately related. In such cases it is not necessary to create seven separate overlays.

It is easier to choose the aquifer media as the starting parameter because the values chosen for other parameters may depend on the choice of aquifer for mapping. Drastic system is best suited for areas that are 40 hectares or larger in size. The data used to generate the DRASTIC map is produced at a variety of scales. Values for hydraulic conductivity are frequently extrapolated from only a few points of reference or simply estimated from aquifer media. When creating the map it is therefore important to attempt to "justify" the scales by either making generalizations or finding the most detailed information available.

For each parameter a raster map is made through interpolation in the Arc View GIS software using the well data. Map of soils is scanned and then processed from the Soil Map. The slope map is obtained from the digital terrain model (DTM). Each parameter is classified on certain vulnerability classes with values from the digital terrain model. Record the corresponding weight and rating for the area and multiply the two numbers. All seven DRASTIC parameters are mapped with a different color. Finally, through a function specific to the GIS software – the overlay function, the seven maps are combined through the Map Calculator function from the Spatial Analyst extension resulting in the Vulnerability Map of groundwater.

We have to evaluate the hydrologic settings which are present on the map. The areas on the final map are labeled with the appropriate hydro geologic setting. The DRASTIC index for the region is calculated.



Overlay of DRASTIC layers to produce Groundwater vulnerability Map

DRASTIC INDEX:
  DRASTIC index helps to identify areas that are more likely to be susceptible to ground water contamination .The higher the DRASTIC index, the greater the vulnerability to contamination. The index generated provides only a relative evaluation tool and is not designed to produce absolute answers or to represent units of vulnerability.

The maps are color-coded using ranges depicted on the map legend. The color codes used are part of a national color-coding scheme .The color codes are chosen to represent the colors of the spectrum, with warm colors (red, orange, and yellow) for areas of higher vulnerability (higher pollution potential indexes) and cool colors (greens, blues, and violet) for areas of lower vulnerability to contamination.



GROUNDWATER VULNERABILITY CLASSIFICATION:
Five classes of vulnerability ranking are chosen to describe the relative assessment of the probability of a groundwater resource to contamination. Low, moderately low, moderate, moderately high and high shown as distinct colors on the vulnerability map.

HIGH
High vulnerability ranked groundwater resources are found predominantly with shallow water tables, high-moderate recharge potential and permeable soils. Other areas include the fractured rock areas.

MODERATELY HIGH
Moderately high vulnerability ranked groundwater resources are found in terrains where high recharge potential, depth to water table, geology and Vadose Zone play a very important role. Areas that are locally recharged through direct infiltration from the surface and have seasonally variable water tables have moderately high ground water vulnerability.

MODERATE
Moderate vulnerability ranked groundwater resources are areas that include much of the unconsolidated sediments. Low slope (2-10 %), relatively deep water tables and soil permeabilities (moderate to slow), fractured rock terrain are the dominant factors affecting this vulnerability class.

LOW MODERATE
Low to moderate vulnerability ranked groundwater resources are characterized predominantly by moderate to steep slopes, deep water table and fractured geology of meta-sedimentary and granite terrains. Clay rich mineralogy and alluvial sediments affects the permeability of the sediments in this class.

LOW
This class is predominantly characterized by very steep slopes, deep water tables, low recharge potential and fractured metasedimentary and/or granite geology. A site with a low DRASTIC index is not free from groundwater contamination, but it is less susceptible to contamination compared with the sites with high DRASTIC indices.

Maps that are overlayed to form the groundwater vulnerability map of Younggwang County in Korea


RESULT:



Groundwater assessment for developments that require consent

Vulnerability classification	Groundwater assessment requirements
Low	A desk study is required to identify the concerns and potential risk to groundwater or the environment and the need for any further action to be presented in the development application.
Low-moderate	A potential risk is indicated by the vulnerability map requiring site investigation and groundwater monitoring. The work involves limited amount of site investigation, soil and water sampling and testing, definition of flow systems and reporting.
Moderate	For moderate vulnerability areas, a detailed groundwater site investigation is required. The work includes an ongoing monitoring program, details on the protection design factors (natural attenuation, physical barriers, etc)
Moderately high	The risk to groundwater in this area cannot be tolerated. The work includes detailed site investigation and implementation of an on-going monitoring program, protection design system incorporating natural attenuation, hydraulic barriers and physical barriers. The proposal should include a feasibility plan for a clean-up, detailed monitoring and ongoing assessment program.
Vulnerability classification	Groundwater assessment requirements
High	Identifies the area having a potential risk that needs remedial action plan. The work includes site investigations, ongoing monitoring, demonstrated remedial action plan for clean-up, which analyses the effectiveness of the remediation approach in achieving designated water quality. The financial capacity should also be evaluated. The risk to groundwater is unacceptable and activity may be banned by the responsible authority.

APPLICATIONS OF GROUNDWATER VULNERABILITY MAPS:
The groundwater contamination risk mapping program offers a wide variety of applications. These maps are prepared to assist planners, managers, and state and local officials in evaluating the relative vulnerability of areas to ground water contamination. An important application of the pollution potential maps for many areas will be assisting in land use planning and resource expenditures related to solid waste disposal. More site-specific information can be collected and combined with other local factors to determine site suitability.

A groundwater vulnerability map can assist in developing ground water protection strategies. By identifying areas more vulnerable to contamination, officials can direct resources to areas where special attention or protection efforts might be warranted. This information can be utilized effectively as an educational tool to promote public awareness of ground water resources. Areas that are identified as being vulnerable to contamination may benefit from increased ground water monitoring for pollutants or from additional efforts to clean up an aquifer. Developers proposing projects within ground water sensitive areas may be required to show how ground water will be protected.

MAP LIMITATIONS:
Groundwater vulnerability can be assessed only for areas greater than 40 hectares.
The hydro geologic basins include both major and minor aquifers, but are not always the same as an aquifer.

The vulnerability map shows the relative vulnerability of the hydro geologic basins, and is based on average values for each entire basin. The map is acceptable for evaluating relative vulnerability of the basins, but it should not be used in place of site-specific assessments.

The map does not show areas that will be contaminated or areas that cannot be contaminated. Whether a specific site will ever have groundwater contamination depends on the likelihood of contaminant release, the type and quantity of contaminant released, and the hydro geologic characteristics at that location.

The vulnerability assessment was based on available data. The DRASTIC indices and vulnerability map can be updated as additional or new information becomes available.

CONCLUSION:
The use of GIS techniques for groundwater contamination risk mapping is primarily due to the automatization of certain operations thereby working time and the working personnel are reduced. The database that is “behind” each layer can anytime be updated. In addition the use of GIS facilitates the rapid visualization of some elements in the map through selecting them from the attribute table. Vulnerability digital maps, together with the land use maps, data on contamination sources and groundwater quality can be used in view of a rapid and correct evaluation of pollution risk. By using this technology we are assured that the information will be used in a consistent, efficient and flexible manner.

REFERENCES:

• Aller, L., T. Bennett, J.H. Lehr, R.J. Petty, and G. Hackett, 1987. "DRASTIC: A Standardized System for Evaluating Ground Water Pollution Potential Using Hydro geologic Settings, " Environmental Protection Agency Report



• “Regional Groundwater Pollution Susceptibility Analysis Using DRASTIC System and Lineament Density” by Saro Lee , Dae-Ha Lee , Soon Hak Choi , Won Young Kim and Seung-Gu Lee



• “Estimating groundwater vulnerability to nonpoint source pollution from nitrates and pesticides on a regional scale” Bernard Engel, Kumar Navulur, Brian Cooper Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907-1146, USA



• Groundwater vulnerability map explanatory notes-Lachlan Catchment, Published by Centre for Natural Resources