Term Project Outline
Sub-document 1

CE 397 GIS in Water Resources
The University of Texas at Austin

Evaluating migration potential of contaminants through unsaturated subsurface in Texas
Vulnerability Map

by Yosuke Kimura
at http://www.ce.utexas.edu/kimuray/gis/vulnerability.html
Last updated on March 17, 1997

This text summarizes information about vulnerability map. The source of information is Vrba and Zaporozec (1994), and almost all part of this text is from the reference. In this text, "the reference" refers to Vrba and Zaporozec, and other reference are explicitly stated. Following topics are covered.

  1. Concept of Groundwater vulnerability
  2. Contaminants and their subsurface fate (review)
  3. Assessment of groundwater vulnerability
  4. Presentation with GIS
  5. Uses and Limitation of vulnerability maps

  6. and
  7. Comments by yosuke
Example of vulnerability map (DRASTIC method) is found in TNRCC's site.

1. Concept of Groundwater vulnerability

The fundamental concept of groundwater vulnerability is that some land areas are more vulnerable to groundwater contamination than others. The goal of the vulnerability mapping is to divide an area into several units having different potential for groundwater contamination. The results of this assessment is shown typically as polygons on a map.

A generally recognized and accepted definition of vulnerability has, however, not been developed yet. Attempts are listed later in this text. The general definition follows: "Vulnerability is an intrinsic property of a groundwater system that depends on the sensitivity of that system to human and/or natural impacts."

2. Contaminants and their subsurface fate (review)

Before discussing vulnerability, problem of soil contamination is reviewed in this section. Two major factors in groundwater contamination problem are contaminants and their recipient, which is the ground.

  1. Contaminants
  2. Recipient (the ground)
Both of structure and processes in the ground would be considered. Structure is important since the nature of the ground vary dramatically as we go across subsurface vertically. Many processes with different nature take place in subsurface environment, and each of them has different meaning to the fate of contaminants.


Contaminants are likely to fall in one of following categories.

Subsurface structure

Subsurface is stratified, and they are grouped into following four components.

Subsurface processes

Processes which affects fate of subsurface contaminants could be categorized into following four processes.

Comment by yosuke

So far so good.

3. Assessment of groundwater vulnerability

As stated earlier, there is no general method for assessing groundwater vulnerability. This section exemplifies existing methods for assessing the ground water vulnerability.

Principal attributes

  1. Recharge
  2. Net annual recharge is almost always a fundamental part in assessing groundwater vulnerability.

  3. Soil properties
  4. The properties of soils which play major part in assessing vulnerability include follows.

    It should be noted that soil itself is very vulnerable. Therefore it is important to consider if the soil in a region is under environmental stress, such as agricultural activity and acid deposition.

  5. Characteristics of unsaturated/saturated zones
  6. Especially important in the area where soil is not well developed such as hilly area. Following properties are primary factors.

    Among following properties of aquifer material, hydraulic conductivity is an especially important parameter

Comment by yosuke

Well, here goes my concerns. The reference does not provide why a particular parameter is important. I can guess, but I am afraid that I might misunderstand/overlook the significance of one. For example, the depth of aquifer (distance from surface to water table) may be important because long traveling time in vedose zone provide time for chemicals to be consumed somehow before it hit the flow of water where they just move fast. Is this the only reason we account for the depth? Another concern is that the properties seems to be redundant. For example, why do we still need effective porosity of aquifer when we know hydraulic conductivity?

Assessment Methods

The reference is a little chaotic in categorizing different assessment method. To fully understand what has been done so far in this area, I have to either (1) find a better reference or (2) go through original papers referred in the reference. I wouldn't do either of them, and I just presented what I understood from the reference.

There were at least two type of assessment method for vulnerability with a quantitative index as an output.

  1. Coverage method

  2. This method first selects limited number of parameters, and define a rating system for each parameters so that each parameter has several classes. Then create a coverage map for each parameters, then create a map of intersection of all coverage maps. This creates a single coverage map with each polygon having a unique array of ratings of parameters.

    A multidimensional matrix in prepared for evaluating vulnerability of each coverage based on the array attached to it. The dimension of matrix is the number of parameters, and number of rows (or columns) in each index is the number of class in the rating of the corresponding parameter. The elements of this matrix are words describing the degree of vulnerability corresponding to the combination of rating of the parameters assigned to the position of the element in the matrix (for example, extreme, high, moderate, etc.).

    Provided arrays of parameter rating in every polygon in the map and the matrix which convert the array into vulnerability, the vulnerability of all polygon are evaluated. The result can be presented as a coverage map with attributes of vulnerability.

  3. Grid method

  4. In this method, more parameters are used than in the matrix method described above, since the map is handled as a grid rather than a coverage. An array of ratings of each parameter is prepared for each grid point on the map. The elements of the array are then used to calculate the vulnerability at the grid point by predefined arithmetic calculations. A parameter would be assigned with a larger weighting than others, based on its importance in determining vulnerability. Carrying out the calculation for each grid point, we have a grid map having a vulnerability index assigned to each grid point.

    A scale is needed to allow interpretation of the value of index into vulnerability. The scale has several classes with description of vulnerability at different degree (low to high). This scale assign a description of vulnerability to each grid point provided with the index value (result of the arithmetic calculation) at the point. A coverage map can be created based on this grid.

Examples of vulnerability assessment methods include follows.

Comment by yosuke

The reference does not have enough space to go through each method to allow comparison of better method. If I will compare the DRASTIC method and another method, I need to find another reference which describe the method in depth, or read the original. (I am not saying that I will do this!)

4. Presentation with GIS

Comment by yosuke

Let me take care of this section later. (3/17/97)

5. Uses and Limitation of vulnerability maps

Comment by yosuke

I want to finish this part before I go any further. (3/17/97)


Aller et al. DRASTIC: A standardized system for evaluating ground water pollution potential using hydrogeologic settings. EPA/600/2-87/035 USEPA, Ada, 1987

Civita et al. 1990

Foster 1987

Vrba and Zaporozec (eds.). Guidebook on Mapping Groundwater Vulnerability. Intl. Assoc. Hydrol., Hannover, 1994.

Comment by yosuke, a.k.a., where to go?

It seems that I am behind, and I have to figure out what I do using GIS real quick! All I did using ArcInfo, ArcView is that download a vulnerability map of Texas, which was ArcInfo export file of coverage polygons. I was expecting that the file contains the original data to come up with the index values (eg., hydraulic conductivity, weighting parameters etc.), but I was disappointed that it was only the final result.

The project I have in my mind right now are...

  1. Calculate the components of DRASTIC index which can be calculated from STATSGO (They include at least indices for soil texture are topography), and compare that with some more sophisticated model for contaminant transfer in soil.
  2. Compare DRASTIC method with another method
  3. Add specific objects which may cause concern in groundwater quality (e.g., gas station and landfill), and come up with composite vulnerability map (intrinsic and specific vulnerability)
The first topic is the closest to the original idea of this project (see the outline). Problem is that it can be done without having any work on GIS? Well, I can at least show the comparison on map... I am not sure. The second one is probably too much work for me as I am novice with the idea of groundwater vulnerability. The nature of third topic fit well with GIS. I am tending to do the third one if it wasn't the first one.

Things I may want now is...

  1. Database for aquifer (STATSGO covers only surface properties, not subsurface)
  2. Programs which calculate vulnerability index, e.g. DRASTIC index, provided necessary datafile.
  3. Database of specific objects which may cause trouble in groundwater quality

Last updated on March 17, 1997
Comment to: yosuke kimura
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