DETERMINATION OF AQUIFER VULNERABILITY USING DRASTIC MODEL ACROSS CONTERMINOUS UNITED STATES

 

 

Karthik Kumarasamy

Graduate Student

Department of Civil and Environmental Engineering

Email: karthikkumar@cc.usu.edu

 

Objective: The focus of this project is to produce a map of Aquifer vulnerability across conterminous United States using the DRASTIC model.

Introduction: Groundwater is an important natural resource for numerous human activities, accounting for more than 50% of the total water used in the United States. It is vulnerable to contamination by numerous organic and inorganic pollutants such as nitrates, heavy metals, and pesticides. The remediation of polluted aquifer resources is always expensive and protracted, and is often abandoned, leading to loss of valuable resources at a considerable economic cost. This is the motivating factor for protecting zones, which are more vulnerable with respect to others. Assessment of groundwater vulnerability aids in the management of limited groundwater resources. The breadth of nitrate concentration data in groundwater allows for a reliable comparison of the performance of the DRASTIC model.

DRASTIC model:

Background: This is an empirical model developed by National Water Well Association in conjunction with the EPA to determine the aquifer vulnerability on a regional basis. Although DRASTIC is physically based the final DRASTIC index is just a numerical index. This method was created to evaluate the aquifer vulnerability of any area in the United States. This model can only be used for area more than 100 acres or larger. Due to wide variability of the pollutants a generic pollutant was selected. It is assumed that the pollutant has the mobility of water. This model does not readily assess the condition of leaky aquifers or confined aquifers (Aller et al, 1987).   

Description of Factors: The system encompasses two portions, namely, the hydro geologic settings and the relative ranking of the hydro geologic parameters. In the hydro geologic setting are the physical characteristics, which affect the pollution potential of the groundwater. The parameters that are considered in the DRASTIC model are depth to water table, recharge, aquifer media, soil media, topography (slope), impact of vadose zone media, and (aquifer hydraulic) conductivity. These factors give rise to the acronym DRASTIC. The numerical ranking system consists of three significant parts: weights, ranges, and ratings. Each parameter in DRASTIC has been assigned a weight based on its relative importance with respect to other parameters.

Assumptions: The DRASTIC model is developed based on four major assumptions, namely,

  1. Introduction of the contaminant is at the ground surface.
  2. Precipitation enables the contaminant to be flushed into the groundwater.
  3. The mobility of the contaminant is similar to water.
  4. DRASTIC model is used to evaluate areas equal to or larger than 100 acres.

 

Data required and Sources: The data used in this project are as follows,

 

Depth to Water Table: The data for depth to the water table was obtained from STATSGO database developed by United States Department of Agriculture.  The field in the STASGO attribute table is wtdeph, which is the maximum value for the range in depth to the seasonally high water table during the month specified. This field is found in the comp table.

Mean Annual Precipitation: The average annual precipitation is needed to estimate the aquifer net recharge. This map layer shows polygons of average annual precipitation in the contiguous United States, for the climatological period 1961-1990. The data for depth to the water table was obtained from STATSGO database developed by National Atlas.gov. 

Soil data: The soil data was obtained from STATSGO soil map developed by United States Department of Agriculture. This database consists of Soil hydrologic group, which was used as a surrogate for the actual soil texture classification data.

Hydrogeologic features: This data is available from The Ground Water Atlas of the United States available at National Atlas.gov.

Hydraulic Conductivity of the aquifer: This information was estimated for the aquifer types from the values given in Freeze and Cherry (1979).

Nitrate concentration values were obtained from the USGS water quality database. 

Methodology

1.      Determination of RATING for each of the factors considered in the evaluation of DRASTIC.

a.      Depth to water

·        The shape file for a particular state and the corresponding comp table are added in ArcMap.

·        The attribute table of the state map is linked to the component table with the common MUID field in both the tables. The field wtdeph is used to get the required value. This procedure is done for all the states.

·        The shape files for all states are appended to get one shape file for conterminous United States. In order to accomplish this append tool is used. Note: The append tool did not work when all shape files were appended simultaneously. This tool allowed appending of one shape file at a time.

·        The final shape file for US is converted to a raster. This raster file is rated for different ranges using the raster calculator. The final depth to water rating raster is obtained by adding all the rated files. The Fig 1 shown below is the final raster.

Fig 1 Depth to water ratings

b.      Net Recharge: In order to determine net recharge, precipitation map and soil hydrologic group is needed. The soil hydrologic group is used to estimate the recharge based on Williams and Kissel’s equation. The equations are given below.

·        The field hydgrp is used to get the soil hydrologic group. The shape file prepared in the above case is used to prepare a raster file for soil hydrologic groups.

·        The raster calculator is used to determine the net recharge using the precipitation raster and the soil hydrologic group raster.

·        This raster is then used to find the ratings. The Fig 2 shown below is the final raster.

PI = (P - 10.28)2/(P + 15.43) for hydrologic soil group A

PI = (P - 15.05)2/(P + 22.57) for hydrologic soil group B

PI = (P - 19.53)2/(P + 29.29) for hydrologic soil group C

PI = (P - 22.67)2/(P + 34.00) for hydrologic soil group D

PI for A/D, B/D, and C/D is the average PI for each of the corresponding groups

where,

                                    PI = Percolation index

P = Annual average precipitation

 

Fig 2 Ratings of Net recharge

c.       Aquifer media:

·        The shape file for the shallow aquifers for conterminous United States is rated and the rated shape file is converted to a raster. The final rated file is shown in Fig 3 shown below.

·        The ratings used are given below

Other Rocks: 3

Carbonate-rock aquifers: 8

Igneous and metamorphic rock aquifers: 3

Sand stone and carbonate rock aquifers: 6

Sandstone aquifers: 6

Semiconsolidated sand aquifers: 4

Unconsolidated sand and gravel aquifers: 8

 

 

 

Fig 3 Rating for the Aquifer media

d.      Soil Media:

·        The field hydgrp is again used to get the required ratings.

·        The shape file for conterminous United States prepared as in Depth to water is again used to determine the raster to find the ratings. The Fig 4 shown below is the final raster.

·        The ratings used to produce the map of soil media is given below

A:  8, B:  5, C:  4, D:  3, A/D:  6, B/D:  4, C/D:  4

 

 

Fig 4. Ratings for the soil media

e.      Topography:

·        The two fields found in the STATSGO attribute file for United States, namely, Slopel (lower limit of surface slopes for this component), Slopeh (upper limit of surface slopes for this component) is used to calculate the final slope value.

·        The average of the above two fields is used to determine the final slope value which is used to determine the ratings for this factor. This map and the ratings used are shown in Fig 5 below.             

 

Fig 5 Ratings for the topography

f.        Impact of the Vadose zone media

·        The same shape file used for the calculation of aquifer media is used with the assumption that the geology present just above the water table will be the similar to the geology below the water table. With this assumption the ratings map is prepared. The ratings are shown below.

Other Rocks: 1

Carbonate-rock aquifers: 6

Igneous and metamorphic rock aquifers: 4

Sand stone and carbonate rock aquifers: 6

Sandstone aquifers: 6

Semiconsolidated sand aquifers: 5

Unconsolidated sand and gravel aquifers: 8

 

Fig 6 Ratings for the Impact of the vadose zone media

g.      Hydraulic conductivity (Aquifer)

·        The shape file for the shallow aquifers for conterminous United States was used along with the hydraulic conductivity values for the corresponding aquifers.

·        This is rated and the rated shape file is converted to a raster. The final rated file is shown in Fig 7 shown below.

The ranges for hydraulic conductivity are given below;

Other Rocks: little or no permeability

Carbonate-rock aquifers: 10-2 – 101

Igneous and metamorphic rock aquifers: 10-7 – 103

Sand stone and carbonate rock aquifers: 10-3 – 101

Sandstone aquifers: 10-3 – 101

Semiconsolidated sand aquifers: 1-103

Unconsolidated sand and gravel aquifers: 102 – 106

 

 

Corresponding to the ranges shown above, the hydraulic conductivity is rated as shown below;

 

 

Other Rocks: 1

Carbonate-rock aquifers: 1

Igneous and metamorphic rock aquifers: 1

Sand stone and carbonate rock aquifers: 1

Sandstone aquifers: 1

Semiconsolidated sand aquifers: 8

Unconsolidated sand and gravel aquifers: 10

 

 

 

Fig 7 Ratings for the hydraulic conductivity of the aquifer

2.      Computation of DRASTIC index: Final DRASTIC Index = 5D + 4R + 3A + 2S + 1T + 5I + 3C

The DRASTIC score is computed using the above equation. The raster calculator is used to calculate the final values of DRASTIC.

 

Fig 8 Final DRASTIC index

 

Nitrate Distribution: The dataset used to produce the Nitrate concentration map is from NAWQA program’s NO3- plus NO2- concentration values, which is used for the comparison of DRASTIC model. As the concentration of nitrite in groundwater is insignificant in comparison to nitrate concentration (Hem 1989), and also because this combination provides more wells to compare the result of the model, the data used is nitrite plus nitrate in mg/l of nitrogen. The NAWQA Program began in 1991 to describe the quality of the Nation’s water resources, using nationally consistent methods. Hence, this data consisted of data only from the year 1991. The value used for comparison is the median of the concentration data for each well. The procedure used to produce the map is as follows;

1.      The nutrients data is downloaded from the USGS water quality dataset

2.      This is then filtered to obtain the Nitrate plus Nitrite concentration values

3.      A code was written to compute the median nitrate concentration values from the raw data.

4.      The station numbers were used to obtain the coordinates of the wells

5.      Then using GIS tools such as Make XY Event Layer the final concentration distribution map is produced.

The areas that showed pronounced problems from this dataset were: 1. northeastern USA, 2. intensely farmed area of the central USA grain belt, 3. irrigated agricultural regions of California and Idaho.

Fig 9. Map showing the distribution of the wells where nitrate concentration was measured in the United States from the NAWQA program

 

RESULTS AND DISCUSSIONS: It can be observed from the map that results given by DRASTIC model agree very well with the nitrate concentration data shown in the Fig 9. According to the DRASTIC model Nebraska shows very high vulnerability, this observation is confirmed by the nitrate concentration data. Similar trends are observed parts of California, Idaho and in Utah. The predictions by the model are very generic in the sense of the contaminant. Nitrate is a contaminant with characteristics similar to the generic contaminant assumed in the development of the model. The model predicts that the some parts in the state of Kansas and most of Florida has very high vulnerability zones, this trend is not shown by the nitrate concentration map. The discrepancy in the prediction could possibly be because of lower inputs of nitrate or other factors such as inaccurate prediction by the model. 35 % of area in the United States is in the high and very high vulnerability zone. Raster calculator is used to compute this result.

 

REFERENCES

 

Aller et al, 1987, DRASTIC: A standardized system for evaluating groundwater pollution potential using hydro geologic settings, EPA, and National Water Well Association.

 

Hem J.D., 1989, Study and Interpretation of the Chemical Characteristics of Natural Water; U.S. Geological Survey, Water-Supply Paper 2254.