The DRASTIC model is the basis for the aquifer sensitivity rating used for these maps. It examines several components that are important in determining the level of aquifer sensitivity, which is the relative ease with which a contaminant (in this case a pesticide) applied on or near a land surface can migrate to the aquifer of interest.
DRASTIC stands for:
D – Depth to water (difference between the well-head elevation and that of the water level in the aquifer)
R – Net Recharge (amount of water that reaches the aquifer)
A – Aquifer Media (primary type of aquifer material)
S – Soil Media (primary type and size of soil particles)
T – Topography (the slope of the land surface)
I – Impact of the Vadose Zone (primary type and size of vadose zone material)
C – Hydraulic Conductivity (the ease at which water is able to move through the aquifer material)
Depth to Water - determines the depth of material which a contaminate must travel to reach the water-table. It uses water-level data provided by the US Geological Survey and the New Mexico Office of the State Engineer. It is important because it helps determine the amount of time the contaminate is in contact with the media.
Net Recharge - represents the amount of water per unit area of land which penetrates the ground surface and reaches the water table; transport contaminant, vertically and horizontally, through the vadose zone to the water table available for dispersion and dissolution.
Aquifer Media - the consolidated or unconsolidated saturated material which yields sufficient quantities of water for use; this information is derived from published geology maps and hydrogeologic studies.
Soil Media - this parameter represents the textural class of the soil; the upper portion of vadose zone, characterized by significant biological activity, significant impact on recharge, and the site of filtration, sorption, and biodegradation. There are missing areas in New Mexico that the SSURGO data has not been developed. This is due to lack of data for many reasons, such as military, forest, and government lands, i.e. White Sands Missile Range. These missing areas are not calculated for the final DRASTIC index; leaving them “missing” from the final aquifer sensitivity of the region.
Topography - this parameter is derived from 1:250,000 scale digital elevation models, which represents the lay-of-the-land, slope and slope variability of the land surface. Helps control likelihood that a pollutant will run off or remain on the surface long enough to infiltrate.
Impact of Vadose Zone - represents the unsaturated portion of the aquifer material; the zone above the water table which is unsaturated.
Hydraulic Conductivity - represents the ease at which water flows through the aquifer material; the ability of the aquifer material to transmit water. Controls rate at which groundwater will flow under a given hydraulic gradient.
Areal Extent - this shows the areal extent to which each level of senstivity occurs. Percentages are based on area covered by all parameters (as shown in Aquifer Sensitivty layers).
Sources of Hydrogeologic Data - Includes DRASTIC Elements, Data Type, Format, and the Source of the data used for each parameter.
References
Combining DRASTIC Parameters
Once the data was compiled and converted to the DRASTIC model ratings, all of the parameters were then combined into a single shapefile; to obtain the final index, all of the parameters where then summed together. This was done by using the “Intersect” tool in ArcMap. ESRI states: “ Intersect creates a new coverage by overlaying the features from the input coverage and intersect polygon coverage. The output coverage contains the input features or portions of the input features that overlap features in the intersect coverage. The output features have the attribute from the original feature from the input coverage and the feature in the intersect coverage, which they intersect.”
The values for the final index’s, i.e. “Low,” “Medium,” etc., were created by first applying the Jenk’s Natural Breaks classification for all regions into five classes, then taking the average for each class and applying the same values to all regions to be consistent across the entire state. The index values are as follows: Very Low = 0-80, Low = 81-102, Medium = 103-122, High = 123-144, Very High = 145-X.
Due to the fact that the shapefiles for the combined parameters were very large datasets (up to one million records) a second shapefile type was created for the regions. A new “dissolved” shapefile allows for a faster display of the aquifer sensitivity, due to it having fewer records to display. This is done by using the “Dissolve” tool in ArcMap that combines adjacent records, which then makes larger polygons of the same type. Also, a third type of aquifer sensitivity display was created. A new raster of the natural sensitivity was produced, which allows for immediate display of the sensitivity. The values from this raster were then classified to match those of the original aquifer sensitivity, thus displaying the same exact colors as the other two shapefiles. These different display methods are viewable at different scales, so that one does not display on top of the other, making the map easier to read.
Each of these layers is weighted based on a pre-determined level of importance in the DRASTIC model. For example, the depths to groundwater and soil composition are the most important in this model because they determine how fast a contaminate will impact the groundwater.
In addition, each layer is classified by distinguishing features. In the topography layer, for instance, five different classes are used to describe the slope percent. Each one of these classes has a certain rating, much like the overall weighting of each layer. Following the equation is a list of all the ratings used for each layer in this particular DRASTIC model.
Both the layer weight and the class rating are incorporated into the model. This can be calculated using an equation, where R is the rating of each class and W is the weighting of the layer.
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Transparent, "see through," layers were created for all of the parameters, including Aquifer Sensitivity at the large scale. These were added so the user can see through the parameter layer to the digital photos layed underneath all of the layers. This creates easy access to view a certain building and the senstivity of the aquifer in that certain part of the area. To view these layers, you must zoom in far enough to see both the parameter and the digital photo, at a scale of 1:24,000.
Depth to Water ( Top )
Depth to water refers to the depth from the land surface to the surface of the saturated zone in an unconfined aquifer or to the top of the confined aquifer. The data for this parameter was derived from well data by the Office of the State Engineer for New Mexico. This data provided the field “DEPTH_WATE” which is the total depth to the water in the well. These points were then used with the Geostatistical Analyst tool and the Kriging method to create a raster coverage of depth to water for the area. ESRI states that, “ Like IDW [Inverse Distance Weighting] interpolation, kriging forms weights from surrounding measured values to predict values at unmeasured locations.” Then, the filled contours were converted to vector polygons in order to be compatible with the other parameters. The ranges of depth were then rated in the field “Dr,” according to the ratings provided by Aller, et al. 1987. The assigned pesticide weight of 5 was then used to calculate the “D” parameter for this project.
Net Recharge ( Top )
To depict this parameter, separation of agricultural areas from nonagricultural areas was necessary. Land use maps were used to accomplish this task. Digital land use maps can be acquired from the USEPA over the internet. Agricultural land that receives irrigation water for successful crop production was assigned a rating of 9, including water bodies, and wetlands. A rating of 1 was assigned to nonagricultural lands, such as urban and built-up areas, rangeland, dry cropland, and barren land. These ratings can be found in the field “Rr.” The assigned pesticide weight of 4 was then used to calculate the “R” parameter for this project.
Aquifer Media ( Top )
Data used for this parameter was derived from geological data provided by New Mexico Tech. The polygons were generalized from their 1:500K geology map of New Mexico, which is based off of Dane and Bachman’s 1965 geology of New Mexico map. A new field, “Ar,” was created for the index. The ratings are based off of the field entitled “GENERALIZE,” where ranges were assigned different ratings based on the type of geology. The field “Aw” was created to display the weight for this layer. The assigned pesticide weight of 4 was then used to calculate the “A” parameter for this project; the two fields multiplied together are displayed in the index field entitled “A.”
Soil Media ( Top )
The data needed for this parameter is soil data from the Soil Data Mart from the Natural Resources Conservation Service (NRCS). The following tables from the SSURGO county database are joined to attain the type of soil texture used to classify the ratings for the type of soil media for the DRASTIC model: component table = this table has the column “mukey” which is used to join to the county .mxd; chorizon table = this table has the column “cokey” which is used to join with the component table; chtexturegrp = this table has the column “chkey” which is used to join with the chorizon table; chtexture = this table has the column “chtgkey” which is used to join with the chtexturegrp table. This table also contains the type of soil texture, which is the field needed for the “S” factor in the DRASTIC model. The ratings for the different soil textures are then calculated for the field, “Sr.” Then, the DRASTIC weight of 5 is assigned to the field, “Sw.” These two fields are then multiplied together to reach the index for the soil, “S.”
Topography ( Top )
A digital elevation model consists of an array of elevations for ground positions that are usually at regularly spaced intervals. Different ratings were assigned to the different ranges of topography; the greater the slope, the lower the rating, due to the importance of runoff. The field “Tr” was created for the ratings, and the field “Tw” was created for the weight for this layer. The assigned pesticide weight of 3 was then used to calculate the topography parameter for this project, which is displayed in the index field “T.”
Impact of Vadose Zone (Top )
As with aquifer media, data used for this parameter was derived from geological data provided by New Mexico Tech. The polygons were generalized from their 1:500K geology map of New Mexico. The ratings for this layer are also based on the field “GENERALIZE.” The typical ratings were used for this parameter, as indicated by Aller, et al., 1987. The field “Ir” was created to show these ratings, and the field “Iw” was created for the assigned pesticide weight of 4. The two fields were then multiplied together in the index field “I.”
Hydraulic Conductivity ( Top )
Data used for this parameter was obtained from geological data provided by New Mexico Tech, entitled, “Generalized Geologic Map of New Mexico,” published in July 2003. The polygons were generalized from their 1:500K geology map of New Mexico, which was modified from Dane and Bachman’s 1965 geologic map. The ranges of values of Hydraulic Conductivity were based off of table 12 (shown below) of Aller, et al.’s 1987 publication. These values were built off the field named “GENERALIZE” in the geologic map shapefile. New ratings for the type of geology were created. This was done because the values in Table 12 are too low for the original ranges for the DRASTIC model. A higher rating is indicative of higher hydraulic conductivity. The new ratings are shown below. The assigned pesticide weight of 2 was then used to calculate the “C” parameter for this project.
Areal Extent ( Top ) - this shows the areal extent to which each level of senstivity occurs within the region. Percentages are based on area covered by all parameters (as shown in Aquifer Sensitivity layers).
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Sources of Hydrogeologic Data ( Top )
References ( Top )
Aller, L., Bennett, T., Lehr, J. H., Petty, R.J., and Hackett G. DRASTIC: A standardized system for evaluating ground water pollution potential using hydrogeologic settings. NWWA/EPA Series, EPA-600/2-87-035, 1987.
Creel, Bobby J., Theodore W. Sammis, John F. Kennedy, Donald O. Sitze, Daniel Asare, Hugh C. Monger and Zohrab A. Samani. Ground-Water Aquifer Sensitivity Assessment. NMWRRI, TR305, May 1998.
Environmental Sciences Research Institute (ESRI). ArcGIS Desktop Help. ArcGIS 9.2.
Natural Resources Conservation Service. Soil Data Mart, SSURGO Series, USDA. <http://soildatamart.nrcs.usda.gov>.
New Mexico Tech. Generalized Geologic Map of New Mexico. July 2003.
Office of the State Engineer. W.A.T.E.R.S. download (wells). Jan 2003. <www.ose.state.nm.us/water-info/gis-maps>.