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.
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."
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.
Contaminants are likely to fall in one of following categories.
Subsurface is stratified, and they are grouped into following four components.
Soil has the highest activity of chemical and biological processes. Vulnerability of groundwater is, therefore, highly affected with the nature of soil.
Sorption/desorption, precipitation/dissolution causes delay in chemical transport in unsaturated zone. Chemical and biological processes are less active than soil.
Attenuation of concentration is the significant process in saturated zone. Advection carries the contaminants to a distant area over time. Chemical and biological processes are further less.
Both capacity and transport rate in this zone is very small, and water in this zone is not used directly. Processes in this zone interests us only when we use the zone as a final disposal site of long-life contaminants, such as radioactive wastes.
So far so good.
As stated earlier, there is no general method for assessing groundwater vulnerability. This section exemplifies existing methods for assessing the ground water vulnerability.
Net annual recharge is almost always a fundamental part in assessing groundwater vulnerability.
The properties of soils which play major part in assessing vulnerability include follows.
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
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?
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.
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.
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.
This method is described as an empirical ranking system for the rapid assessment of aquifer vulnerability to contamination (Foster, 1987). The reference mentioned this method as having a simple and pragmatic structure. It accounts only three properties, that are:
This method is developed for the USEPA by Aller et al (1987). Maps are already created for Texas and several counties in Texas. It incorporates following properties in its assessment.
So many variables are factored into the final number (vulnerability index) that critical parameters in the groundwater vulnerability may be subdued any other parameters that have no bearing on vulnerability for a particular setting.
For a better assessment of vulnerability in small scale as required in Italian highly diversified hydrogeology, SINTACS method is developed in Italy (Civita et al, 1990). The six properties used in DRASTIC method is used in SINTACS, too (SINTACS and DRASTIC are acronyms of the same words in Italian and English). The sophistication of SINTACS seems to include follows: (1) Each of six properties are evaluated with a couple of related properties, and (2)the weighting parameters are generated with some algorithm which account for additional local condition (land use etc.?). The detail of the method is not clear from the reference, as this method is more complex.
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!)
Let me take care of this section later. (3/17/97)
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
Vrba and Zaporozec (eds.). Guidebook on Mapping Groundwater Vulnerability. Intl. Assoc. Hydrol., Hannover, 1994.
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.
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