DESCRIPTION AND APPLICATION OF CAPTURE ZONE …

DESCRIPTION AND APPLICATION OF CAPTURE ZONE DELINEATION FOR A WELLFIELD AT HILTON HEAD ISLAND, SOUTH CAROLINA

By James E. Landmeyer U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 94-4012

Prepared in cooperation with the SOUTH CAROLINA DEPARTMENT OF HEALTH AND ENVIRONMENTAL CONTROL

Columbia, South Carolina 1994

U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director

For additional information write to:

District Chief U.S. Geological Survey Stephenson Center- Suite 129 720 Gracern Road Columbia, SC 29210-7651

Copies of this report can be purchased from: U.S. Geological Survey Earth Science Information Center Open-File Reports Section Box 25286, Mail Stop 517 Denver Federal Center Denver, CO 80225

CONTENTS

Page

Abstract .......................................................................... 1 Introduction....................................................................... 1

Purpose and scope........................................................... 2 Previous studies............................................................. 2 Description of study area............................................................ 3 Hydrogeologic framework.................................................... 3 Ground-water hydrology..................................................... 6 Description of the capture zone delineation process..................................... 9 Hydrologic characteristics ................................................... 11 Criteria and criteria threshold values.......................................... 11 Analytical models .......................................................... 12

Arbitrary fixed radius model.......................................... 12 Calculated fixed radius model......................................... 13 Theis model......................................................... 14 Numerical (semi-analytical) models........................................... 15 RESSQC model...................................................... 15 MWCAP model...................................................... 16 Application of capture zone delineation for a wellfield at Hilton Head Island, S.C.......... 16 Model input parameters..................................................... 17 Analytical models .......................................................... 18 Arbitrary fixed radius model.......................................... 18 Calculated fixed radius model......................................... 19 Theis model......................................................... 21 Numerical (semi-analytical) models........................................... 25 RESSQC model...................................................... 25 MWCAP model...................................................... 25 Comparison of analytical and numerical simulation results ...................... 28 Summary and conclusions ......................................................... 29 References cited................................................................... 31

ILLUSTRATIONS

Figure 1. Map showing study area in the southern part of Hilton Head Island, Beaufort County, and location of wells used for hydrogeologic fence diagram............................................................4

2. Schematic showing lithology, aquifer names, geologic formations, water-bearing properties, and depths to aquifers in the vicinity of Hilton Head Island, Beaufort County........................................5

3. Fence diagram for wells open to the Upper Floridan aquifer, southern Hilton Head Island ...............................................7

4. Map showing the predevelopment potentiometric surface and ground-water flow direction in the Upper Floridan aquifer, 1885................8

5. Map showing the postdevelopment potentiometric surface and ground-water flow direction in the Upper Floridan aquifer, 1985............... 10

ILLUSTRATIONS-Continued

Page

6-8. Diagrams showing: 6. Typical capture zone delineation using the arbitrary fixed radius model ...............................................................12 7. Typical capture zone delineation using the calculated fixed radius model based on the volumetric-flow equation............................. 13 8. Typical capture zone delineation using the calculated fixed radius model and the volumetric-flow equation ................................. 14

9-12. Maps showing: 9. Capture zones delineated by using the calculated fixed radius model with the volumetric-flow equation and a time-of-travel criterion for selected wells on Hilton Head Island, S.C.................................22 10. Capture zones delineated by using the calculated fixed radius model with the Theis analytical model and a drawdown criterion for selected wells on Hilton Head Island, S.C..............................................23 11. Capture zones delineated by using the RESSQC model, for selected wells on Hilton Head Island, S.C.........................................26 12. Capture zones delineated by using the MWCAP model, for selected wells on Hilton Head Island, S.C.........................................27

TABLES

Table 1. Selected hydrologic characteristics of the Upper Floridan aquifer used in the capture zone models...................................................... 17

2. Travel times calculated for capture zone radii of 100,500, and 1,000 feet, using the arbitrary fixed radius model....................................... 19

3. Radii of capture zones for wells in the study area as determined by the volumetric-flow equation, with a time-of-travel criterion of 0.5,1,2,5, and 10 years .................................................20

4. Radii of capture zones for wells in the study area as determined by the Theis analytical model ..............................................24

IV

DESCRIPTION AND APPLICATION OF CAPTURE ZONE DELINEATION FOR A WELLFIELD AT HILTON HEAD ISLAND, SOUTH CAROLINA

By James E. Landmeyer

ABSTRACT Ground-water capture zone boundaries for individual pumped wells in a confined aquifer were delineated by using ground-water models. Both analytical and numerical (semi-analytical) models that more accurately represent the ground-water-flow system were used. All models delineated 2-dimensional boundaries (capture zones) that represent the areal extent of groundwater contribution to a pumped well. The resultant capture zones were evaluated on the basis of the ability of each model to realistically represent the part of the ground-water-flow system that contributed water to the pumped wells. Analytical models used were based on a fixed radius approach, and included; an arbitrary radius model, a calculated fixed radius model based on the volumetric-flow equation with a time-of-travel criterion, and a calculated fixed radius model derived from modification of the Theis model with a drawdown criterion. Numerical models used included the 2-dimensional, finite-difference models RESSQC and MWCAP. The arbitrary radius and Theis analytical models delineated capture zone boundaries that compared least favorably with capture zones delineated using the volumetric-flow analytical model and both numerical models. The numerical models produced more hydrologically reasonable capture zones (that were oriented parallel to the regional flow direction) than the volumetric-flow equation. The RESSQC numerical model computed more hydrologically realistic capture zones than the MWCAP numerical model by accounting for changes in the shape of capture zones caused by multiplewell interference.

The capture zone boundaries generated by using both analytical and numerical models indicated that the currently used 100-foot radius of protection around a wellhead in South Carolina is an underestimate of the extent of ground-water capture for pumped wells in this particular wellfield in the Upper Floridan aquifer. The arbitrary fixed radius of 100 feet was shown to underestimate the upgradient contribution of ground-water flow to a pumped well.

INTRODUCTION Nationwide, hundreds of potential sources of ground-water contamination have been identified in both local studies and regional surveys. Improved analytical detection of potentially harmful substances introduced to the ground-water environment will most likely identify new sources in the future. Past ground-water contamination management practices have been primarily retroactive, involving remediation attempts after the occurrence of contamination. Adopting a proactive approach, however, could increase the possibility of ground-water protection while it remains uncontaminated. As such, Amendments to the Safe Drinking Water Act in June 1986 included a proactive provision, called Wellhead Protection.

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