1. Assessing Groundwater Vulnerability and Contaminant Pathways at MCAS Beaufort, SC James M. RineProject Leader & P.O.C. John M. ShaferProject Hydrologist, Director ESRI-USC Elzbieta CovingtonG.I.S. Technical Expert Michael WaddellHead Environmental Geophysics William DomorockiGeophysical Expert Sahadeb De Database Development Richard C. BergConsulting Scientist Earth Sciences and Resources Institute Earth Sciences and Resources Institute

2. STUDY SUMMARY Geographic Information System (GIS) mapping of geologic sensitivity and groundwater flow to model aquifer vulnerability are critical aids in establishing protocols for groundwater monitoring, establishing protocols for handling of hazardous substances, siting of future facilities, emergency response, and designing remediation plans for the clean up of toxic spills. GIS technology is utilized to developed maps of the hydrogeology and groundwater flow in and around the Marine Corps Air Station (MCAS) in Beaufort, SC. These maps are used to model the contamination potential or aquifer sensitivity of the upper Floridan Aquifer, a regionally extensive and highly productive groundwater resource. The hydrogeology or geologic sensitivity map is based on mapping of the subsurface geology in three dimensions from land surface to the top of the Floridan Aquifer. Soils data, from U.S.D.A., are also incorporated into the model to better understand how soil drainage and organic matter might enhance or inhibit recharge of groundwater and affect the movement of potential contaminants. The geologic sensitivity map is especially effective in delineating land areas of MCAS, which are most/least sensitive to contaminants that are heavier than water and therefore, tend to move primarily in a downward direction under the influence of gravity. A calibrated 3D groundwater flow model of MCAS and subsequent detailed flow path and travel time analyses are based on the 3D geologic characterization delineated in the geologic sensitivity map. The groundwater flow model can be displayed in two ways. One mode is as an ArcView or ArcGIS project where groundwater pathlines with travel times of 5 to 500 years can be plotted from any 100 ft 2. cell within the boundaries of MCAS Beaufort. Another mode is as a static map, which defines areas of the MCAS where groundwater travel time from the water table to the top of the Floridan Aquifer is < 10 years or < 25 years. This groundwater travel-time analysis can be used to evaluate the impact of particular land uses on the spread of contaminants introduced at the water table and transported to the upper Floridan Aquifer in the dissolved phase. Methodologies developed for this study are applicable to other shallow aquifer settings. GIS tools developed to conduct these analyses and derive the maps have been installed in the MCAS Beaufort GIS along with the database containing the supporting information.

3. STUDY FACTS Three year study begun in 2001 Geologic/hydrogeologic characterization Develop integrated GIS Planning tools for groundwater contamination Facilitate land use planning, emergency response, and compliance monitoring

4. A. Map of Geologic Sensitivity Delineates areas of higher vulnerability to contaminants denser than water. B. Rapid Groundwater Travel Time Vulnerability Areas of rapid transport to Upper Floridan for dissolved phase contaminants. PROJECT SUMMARY MAPS

5. Aerial photo shows locations of all seismic lines (2001 and 2002), wells, boreholes and areas of vertical electrical soundings conducted for this study. 10 - Miles of seismic 47 – VES (vertical Elec. Surveys) 42 – NEW WELLS / BOREHOLES >2000 ft. – CORE 27 – INSTRUMENTED (water level) WELLS

6. Geologic surveys with Geoprobe & Rotosonic Geologic surveys with Geoprobe & Rotosonic. EXAMPLE OF A WELL / BOREHOLE DESCRIPTION:

7. CONSTRUCTION OF INDIVIDUAL HYDROGEOLOGIC UNIT MAPS A geologic sensitivity/aquifer vulnerability analysis is based largely on hydrostratigraphy which is is derived from examination of core samples and geophysical well logs and their correlation with the reflection seismic surveys. Individual unit surfaces were mapped for the study area using both the seismic profiles and the well data. Once the surfaces were derived, individual unit isopachs (thickness maps) were determined by subtraction of surfaces. Unit Surfaces Unit Thickness Maps

8. Unit Thickness Maps Stack-unit map of the 5 units that overlie the Floridan Aquifer. The numbered codes (1 to 153) coincide with a unique combination of stacked units. See the table on the next page for stack-unit combinations, their frequency of occurrence and ranking of their relative sensitivity to contamination. This map, plus the thickness map of the Upper Floridan aquifer portion of the Ocala Formation is the basis of the solids model used for this project’s groundwater model.

9. HOW STACKED-UNITS ARE SCORED An example scoring summation is as follows for the stacked sequence ScoB ’ H. 10000 Co: Olive Clay <12 ’ thick) 2000 (H: Hawthorn 8-20 ’ thick) 20 (S: Surficial Sand 8-16 ’ thick) + 3 (B ’ : Basal Sand 16-24 ’ thick) 12023 (Contamination Potential Index for stacked sequence ScoB ’ H) Ranking and Color Scheme for Stacked- Hydrogeologic Units

10. Generalized geologic map and unit merging scheme. A total of nine combinations of soil hydrologic groups and organic matter percentage groups were selected for the soil portion of the contamination potential analysis. Geologic sensitivity map. See matrix below for key to colors. To create a color scheme for the combined soil and generalized stack-unit map, a 13 x 9 matrix was constructed that integrates the 13 stack units geology groups, shown on the vertical axis, with the nine soil organic matter/hydrologic groups shown on the horizontal axis.

11. Map shows areas of high geologic sensitivity, which are vulnerable to compounds that are heavier than water and tend to sink in the subsurface. Framework for groundwater model

12. 1.Long-term water level monitoring 2.Short term investigative studies (e.g., tidal effects on groundwater) 3.Hydrogeologic properties determination 4.Evaluation and integration of meteorological data Hydrologic Analysis at MCAS Beaufort, SC 27 – INSTRUMENTED (water level) WELLS

13. Well Hydrographs Hourly water level records in Upper Floridan Aquifer (BFT-1952), Basal Sandy unit (BFT-1954) and Lower Sandy unit (BFT-1953) from well cluster along western boundary of MCAS Beaufort for the period July 2001 to October 2003. Local precipitation is shown for period May 2001 through December 2002.

14. MCAS Ground-Water Model Geometry 3-D geologic solids model depicts non-uniform layering of finite-difference groundwater flow model (MODFLOW-2000). Geologic model.

15. Water Table Map Simulated Steady-State Potentiometric Surface of the Upper Floridan Aquifer Simulated steady-state potentiometric surface for model layer 1 (water table). Simulated steady-state potentiometric surface for model layer 6 (Upper Floridan Aquifer).

16. Z = hydrogeologic unit (thickness is variable) X = 100 feet Y = 100 feet ArcView image, in plan view, of pathline after 100 years of travel time. Flow penetrated into Layer 6, the upper Floridan aquifer and after some travel begins to return to the ground surface. Cross section view of pathline after 100 years of travel time. PATHLINE ANALYSIS The MCAS Beaufort groundwater flow model variable spacing, finite-difference grid overlaid on a composite 7.5' topographic map of the modeled area draped over the Digital Elevation Model (DEM) of the groundwater flow model domain.

17. ArcView image of groundwater flow paths for a travel time of 500 years. Colors of pathlines indicate which model layer the pathline is in. Horizontal flow is most pronounced within layer #6 (Upper Floridan Aquifer unit). Radiating pathline pattern is characteristic of an island hydrology. This pathline analysis is available to ArcView/ArcGIS users at MCAS Beaufort for every 100 ft. X 100 ft. cell within the boundaries of MCAS. More detailed explanation of this pathline display technique may be found in the ESRI 2003 User Conference Online Proceedings at http://gis.esri.com/library/userconf/proc03/p1103.pdf. http://gis.esri.com/library/userconf/proc03/p1103.pdf

18. Areas of rapid transport to Upper Floridan (10 and 25 years) The Groundwater Travel-time Vulnerability Map is based on the results of the calibrated groundwater flow model. Map is produced by delineating the start points of flow paths whose travel times from the top of the water table to the Upper Floridan Aquifer are less than or equal to 10 or 25 years. Areas where groundwater transport is the most rapid from the ground surface to the target aquifer (Upper Floridan) coincide with the areas of highest aquifer vulnerability to dissolved contaminants.

19. CONCLUSIONS 1.Accurate mapping of aquifer vulnerability is dependent upon both the proper mapping of geologic sensitivity and the formulation of a realistic groundwater model. The groundwater flow model is the basis for groundwater pathline analyses that predict the major advected trajectory of a dissolved phase contaminant plume. The geologic sensitivity map is the basis for predicting the subsurface paths of free-phase contaminants that are denser than water and sink through holes or thin places in confining units. 2.The more the groundwater systems at installations are understood, the more effectively (and less costly) regulatory compliance can be maintained. 3.By delineating areas of high aquifer vulnerability, an installation is better equipped to prevent accidental or intentional contamination of that aquifer. 4. Placing of results of the MODFLOW groundwater flow path analyses within an ArcView or ArcGIS platform makes the results much more accessible to environmental professionals. Earth Sciences & Resources Institute Columbia, South Carolina