CONVERSION OF DRASTIC MAPS FROM FASTCAD DRAWINGS TO ArcInfo COVERAGES

BY DAVID ADAMS, JOSEPH NORTH AND GARY MADDOX - FLORIDA DEPARTMENT OF ENVIRONMENTAL PROTECTION (FDEP), DIVISION OF WATER FACILITIES, AMBIENT MONITORING SECTION; SUVRAT KHER - FDEP, DIVISION OF WATER FACILITIES , TECHNICAL SUPPORT SECTION

 

ABSTRACT

DRASTIC is a system developed jointly by the U.S. Environmental Protection Agency (USEPA) and the National Water Well Association (NWWA) to map potential aquifer vulnerability. The methodology utilizes existing hydrogeological information to produce color-coded maps which display areas that are more or less likely to be affected by pollution introduced on the ground’s surface. These maps depict areas which are more or less sensitive to land use changes which may affect ground water quality.

Drastic maps are composites made up of eight individually mapped hydrogeological parameters. Seven of these parameters are used to derive the DRASTIC summary index score. The eighth, hydrogeologic setting, is useful for general map interpretation but, it is not included in the summary index

A separate polygon coverage was built for each parameter and then the eight coverages were composited into a single coverage with all information from the originals for the FASTCAD drawings. These drawings were then converted to ArcInfo coverages which were edited by county. The county coverages were then edgematched and mapjoined into a statewide coverage. The statewide coverages, for each aquifer, was then dissolved for the individual DRASTIC parameters for further error checking of the composite coverages. Once all errors were corrected, the composite attributes were recalculated for the final statewide DRASTIC coverages.

 


INTRODUCTION

 

DRASTIC is a system developed jointly by the U. S. Environmental Protection Agency (USEPA) and the National Water Well Association (NWWA) to map potential aquifer vulnerability. The methodology utilizes existing hydrogeological information to produce color-coded maps which display areas that are more or less likely to be affected by pollution introduced on the ground’s surface. These maps depict areas which are more or less sensitive to land use changes which may affect ground water quality.

DRASTIC maps are composites made up of eight individually mapped hydrogeological parameters. Seven of these parameters are used to derive the DRASTIC summary index score. The eighth, hydrogeologic setting, is useful for general map interpretation, but it is not included in the DRASTIC score. A separate FASTCAD polygon drawing was digitized for each parameter, then the eight drawings were composited into a single coverage for each county, with all the information from the originals. The values for the DRASTIC parameters were summed to produce a DRASTIC score for the composite polygons. Composite polygons smaller than 100 acres were deleted, since the DRASTIC method is not detailed enough to map below that threshold. The source data, in Florida, is for the most part very generalized. The source maps were regional or statewide in scale and scope of coverage, except for soils and topography.

DRASTIC maps are most useful as a generalized tool for assessing regional relative potential aquifer vulnerability.

The eight mapped DRASTIC parameters and their associated weights are:

DRASTIC parameter value

X weight

= weighted score

D - depth to water

5

dd

R - net recharge

4

rr

A - aquifer media

3

aa

S - soil media

2

ss

T - topography

1

tt

I - impact of vadose zone

5

ii

C - hydraulic conductivity

3

cc

hydrogeologic setting

(not scored)

 

DRASTIC Index = (dd + rr + aa + ss + tt + ii + cc)

The eighth layer, hydrogeologic setting, reflects general geomorphology and varies nationally by region. Florida lies within region 11, the Southeast Coastal Plain, of the Ground water region of the United States as described by Heath in USGS Water Supply Paper 2242. Four Settings predominate in region 11 and they are:

11A - Karst limestone areas ( limestone near the surface)

11B - Coastal Deposits (thick clastic marine sediments)

11C - Swamps and Wetlands

11D - Beaches and Bars (sandy coastal areas and relict beach ridges)

The Ground Water Quality Monitoring Program (GWQMP) of the FDEP, in 1986, contracted with the five Florida water management districts and Alachua County to produce DRASTIC maps for their respective areas. These maps, when they were completed, were turned over to the GWQMP which converted them to a standard electronic format i.e., a Computer Aided Drafting (CAD) polygon map and an associated DBASE data file. These coverages were divided by county and aquifer.

 

Three aquifer systems were mapped for Florida: the Floridan, the Intermediate and the Surficial. None of these aquifer systems is continuous throughout the whole state and not all areas where they exist were mapped. Mapping was restricted to where the aquifer system was used as a drinking water supply. The Surficial was the most extensive followed by the Floridan and the Intermediate was a very limited coverage. Each aquifer system was digitized on county base maps.

 

 

THE CONVERSION PROCESS

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The FASTCAD county map drawings were then converted to ArcInfo county coverages via the ARC dxfarc function. After conversion, the coverages had to be built as point coverages as they lacked topology. The next step involved cleaning as line coverages in ARC with specific danglelengths and fuzzy tolerances. The cleaned covers were checked for dangles and overshoots, which resulted from the original digitization, to ensure that all polygons were closed and that each polygon contained the appropriate label point. Once the above editing was completed all the county covers were built as polygon coverages and checked for labelerrors.

Two very useful AMLs written by S Kher to help speed up the editing process. Copies of these AMLs are available from the author.

 

Procedures for Joining of DBF Files and Polygon Attribute Tables

When the above procedures were completed, the DBASE DBF files, that contained the DRASTIC values, were joined with the polygon attribute tables (PAT) generated by ArcInfo during the cleaning process. This was a seven step procedure.

Step One - In ARCVIEW add a field named polygon to the PAT. Export the DBF file as a delimited text file to an ArcInfo workspace.

Step Two - In ArcInfo use the ARC pullitem command on the coverages’ xcode files to create a new info file containing two items e.g., cover-id and dxf-text.

Step Three - In ArcInfo use the ARC joinitem command to join the PAT file with the file created in Step Two.

Step Four - In ARCVIEW open the coverage as a theme. Copy the dxf-text values to the polygon field. Delete dxf-text and save the table.

Step Five - In ArcInfo at the INFO prompt create a temporary .DAT file and create fields duplicating all the fields from the original PAT file. Add the delimited text file, created in Step One, to the blank DAT file.

Step Six - In ArcInfo use the ARC joinitem command to join the cover PAT file to the DAT file utilizing polygon as the relate item.

Step Seven - In ARCVIEW check that the tables were joined correctly.

 

 

EDGEMATCHING, MAPJOINING AND DISSOLVING OF COUNTY COVERAGES

Edgematching of the county coverages was done in ARCEDIT using the edgematch function. Edgematching along county boundaries was done by linking nodes that lie along the boundary. Identity links were also added at the ends of the common boundary for the purpose of nailing down these positions during rubbersheeting.

The ARC command mapjoin was used for creating new coverages containing the all the attributes and features of the edgematched coverages. Once the coverages are joined, further editing of polygon slivers and mismatched polygons along the boundary was required.

The mapjoined coverage is then dissolved on the individual DRASTIC parameters using the ARC dissolve command. This results in eight coverages in which the county boundary has been dissolved between polygons with like DRASTIC parameter values. The places where the county boundary remained possibly indicated errors for that DRASTIC value and required further investigation.

Examples of the dissolved coverages for the Surficial aquifer system are in the graphic links below.

Depth to Water Topography
Net Recharge Impact to Vadose Zone
Aquifer Media Hydraulic Conductivity
Soil Media Hydrogeologic Setting

 

When all problems were resolved, the eight individual dissolved were brought back together with the ARC intersect command. The overall Drastic coverages then had to be projected from UTM 17-27 to Albers-HPGN to be consistent with FDEP library layers already in existence. When the coverages are dissolved all fields except for area, perimeter, cover-id, cover# and the parameter dissolved on are eliminated. This necessitates recreating the weighted DRASTIC values and the overall index value. This was accomplished in ARCVIEW via the table editing capabilities.

Examples of Drastic maps for the three aquifer systems are in graphic links below.

Surficial Aquifer System Intermediate Aquifer System Floridan Aquifer System

 

The final step in the project was the incorporation of the above coverages into the FDEP GIS library along with the appropriate metadata.

ACKNOWLEDGEMENTS

We are grateful for the direction and support of Mark Dietrich, Data Processing Manager of the Technical Support Section, Division of Water Facilities, FDEP, whose help was instrumental in the completion of this project.

REFERENCES

Aller, L., Bennett, T., Lehr, J. H., and Petty, R.J. 1985. DRASTIC: A standardized system for evaluating ground water pollution potential using hydrogeologic settings. NWWA/EPA Series EPA/600/2-85/018

Aller, L., Bennett, T., Lehr, J. H., Petty, R.J. and Hackett, G. 1987. DRASTIC: A standardized system for evaluating ground water pollution potential using hydrogeologic settings. NWWA/EPA Series EPA-600/2-87-035

Hatchitt, J. and Maddox, G. L. 1993. Using DRASTIC methods to monitor the quality of Florida’s groundwater. Geo Info Systems Vol 3 #1, pp 42-5.

Herr, J. 1990. Application of DRASTIC ground pollution mapping methodology to the SFWMD. South Florida Water Management District (SFWMD) Technical Publication 90-02.

 


AUTHOR INFORMATION

David Adams
Environmental Specialist II
FDEP2600 Blair Stone Road
Tallahassee, FL 32399-2400
850/414-8550 Fax 850/921-5655
adams_d@dep.state.fl.us
Suvrat Kher
Environmental Specialist II
FDEP
2600 Blair Stone Road
Tallahassee, FL 32399-2400
850/921-9492
kher_s@dep.state.fl.us
Joseph North
Environmental Specialist II
FDEP
2600 Blair Stone Road
Tallahassee, FL 32399-2400
850/921-9869 Fax 850/921-5655
north_j@dep.state.fl.us
Gary Maddox
Professional Geologist II
FDEP
2600 Blair Stone Road
Tallahassee, FL 32399-2400
850/921-9425 Fax 850/921-5655
maddox_g@dep.state.fl.us