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State of Delaware
DELAWARE GEOLOGICAL SURVEY
John H. Talley, State Geologist
REPORT OF INVESTIGATIONS NO. 66
GROUND-WATER RECHARGE POTENTIAL MAPPINGIN KENT AND SUSSEX COUNTIES, DELAWARE
by
A. Scott Andres
University of Delaware
Newark, Delaware
2004
0 5 10 15 202.5
RECHARGE POTENTIAL
Excellent
Good
Fair
Poor
Water/marsh/swamp area
Pit/fill
Miles
Su
ssex C
ou
nty
Ke
nt C
ou
nty
RESEARCH
DELAWARE
GEOLOGICALSURVEY
EXPL
ORA
TIO
N
SERVICE
State of Delaware
DELAWARE GEOLOGICAL SURVEY
John H. Talley, State Geologist
REPORT OF INVESTIGATIONS NO. 66
GROUND-WATER RECHARGE POTENTIAL MAPPINGIN KENT AND SUSSEX COUNTIES, DELAWARE
by
A. Scott Andres
University of Delaware
Newark, Delaware
2004
RESEARCH
DELAWARE
GEOLOGICALSURVEY
EXPL
ORA
TIO
N
SERVICE
Use of trade, product, or firm names in this report is fordescriptive purposes only and does not imply endorsementby the Delaware Geological Survey.
PageABSTRACT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Previous Regional Recharge Evaluations . . . . . . . . . . . . . . . . 2Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
METHODS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Mapping Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Aquifer Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Data Management and Mapping. . . . . . . . . . . . . . . . . . . . . . . . . 4Performance Testing by Modeling. . . . . . . . . . . . . . . . . . . . . . . 5
Page RESULTS AND DISCUSSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Relationships Between Lithology, Recharge Potential,
and Hydraulic Conductivity . . . . . . . . . . . . . . . . . . . . . . . . 8Comparison with Johnston (1976) Water-Budget Study. . 9Results of Ground-Water Flow Modeling . . . . . . . . . . . . . . 10
CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
REFERENCES CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
CONTENTS
ILLUSTRATIONS
Page
Table 1. State government programs using recharge potential maps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Lithologic rating and thickness categories. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Recharge potential rating categories. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Input data for ground-water flow models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Loading data for solute transport models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Proportions of recharge potential categories in Kent and Sussex counties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Statistics on recharge ratings of well, boring, and outcrop record data by category and county. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Areally weighted recharge potential and water budget in four Delaware watersheds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
TABLES
Page
Figure 1. Location of study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. Recharge potential rating example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Illustration of flow and contaminant transport simulation strategy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Recharge potential map of Kent and Sussex counties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Box plot showing relationships between material categories and hydraulic conductivity (K). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Example hydraulic conductivity (K) calculations for different recharge potential and lithologic categories. . . . . . . . . . . . . . . . . . 8
7. Locations of watersheds studied by Johnston (1976). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Comparison of areally weighted recharge potential (AWRP) and water budget in four Delaware watersheds. . . . . . . . . . . . . . . . 9
9. Simulated water-table profiles for different recharge potential categories. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0
10. Example results of simulated contaminant transport under different recharge potential scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
INTRODUCTIONThe term “ground-water recharge” refers to both a
process and a quantity of water. As a process, recharge is themovement of water downward from land surface throughthe unsaturated zone to the top of the saturated zone or watertable. Recharge as a quantity is the volume of water thatreaches the top of the saturated zone or water table. Quantityis commonly expressed as an average rate per unit area,which typically is 12 to 14 inches per year in Delaware, orabout 570,000 to 670,000 gallons per day per square mile(Johnston, 1973, 1976). Recharge typically occurs duringNovember through April when evaporation and transpirationrates are low (Johnston, 1973, 1976; Talley, 1988); however,ground-water level monitoring has documented the occur-rence of recharge during summer months (Andres andHoward, 2002; unpublished data)
Ground-water recharge potential maps of Kent andSussex counties, Delaware (location map shown in Fig. 1)are the result of a 1:24,000-scale evaluation of the water-transmitting properties of the interval between land surfaceand 20 ft below land surface (bls). Andres (1991a) devel-oped the mapping methodology for the geologic characteris-tics of the Atlantic Coastal Plain portion of Delaware.Methodology development started in 1990, and the finalmaps were completed in 2002 and 2003 in digital format(Andres et al., 2002), and in printed versions of Kent(Andres, 2003a) and Sussex counties (Andres, 2003b).
The ground-water recharge potential mapping(GWRM) methodology was developed after evaluation ofmethods of several other recharge evaluation programs inDelaware (Petty, et al., 1983, 1985), New Jersey (Charles etal., 1993), Florida (Boniol et al., 1990), Illinois (Kempton,1981; Berg et al., 1984; Kempton and Cartwright, 1984),Kansas (Sophocleous and McAllister, 1990), and the U. S.Environmental Protection Agency (Aller et al., 1985). Themethod used in Delaware is most similar to the stack-unit
method developed by the Illinois Geological Survey(Kempton, 1981; Berg et al., 1984; Kempton andCartwright, 1984). General methods of water-budget analy-sis, soil infiltration potential, and flow-net analysis werealso considered.
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GROUND-WATER RECHARGE POTENTIAL MAPPINGIN KENT AND SUSSEX COUNTIES, DELAWARE
A. Scott Andres
ABSTRACTGround-water recharge potential maps support decision-making and policy development in land use, water-resources
management, wastewater disposal systems development, and environmental permitting in state, county, and local govern-ments. Recently enacted state law requires that counties and towns with more than 2,000 residents provide protection toareas with excellent recharge potential in comprehensive land use plans. Approximately 14 percent of Kent County and 8percent of Sussex County have areas with excellent recharge potential.
Ground-water recharge potential maps show land areas characterized by the water-transmitting capabilities of the first20 feet below land surface. Ground-water recharge potential mapping in Kent and Sussex counties was done using geologicmapping techniques and over 6,000 subsurface observations in test borings, wells, borrow pits, natural exposures, and ditch-es. Hydraulic testing of more than 200 wells shows that the four recharge potential categories (excellent, good, fair, poor) canbe used as predictors of the relative amounts and rates at which recharge will occur. Numerical modeling shows that rechargerates in areas with excellent recharge potential can be two to three times greater than rates in fair and poor recharge areas.
Because of the association of recharge potential map categories with hydraulic properties, map categories are indica-tors of how fast contaminants will move and how much water may become contaminated. Numerical modeling of contami-nant transport under different recharge potential conditions predicts that greater masses of contaminants move more quicklyand affect greater volumes of water under higher recharge potential conditions than under lower recharge potential condi-tions. This information can be used to help prioritize and classify sites for appropriate remedial action.
AtlanticOcean
Ma
ryla
nd
N
0
New Jersey
20Miles
KentCounty
NewCastleCounty
SussexCounty
Figure 1. Location of study area.
Purpose and ScopeGWRM was done in recognition of the importance of
ground water to environmental quality and to the economyof Delaware. Ground-water discharge constitutes a majorityof total fresh-water stream flow. Ground water providesnearly all fresh water for potable, irrigation, agriculture, andindustrial uses in areas south of the Chesapeake andDelaware Canal. It has long been recognized that ground-water recharge strongly influences the availability and quali-ty of ground-water resources, and that recharge is dependenton the infiltration of local precipitation and water transmit-ting properties of subsurface geologic materials. The toolsand results of GWRM provide objective, technical means tocharacterize the recharge of ground water.
The maps assist state, county, and local governmentswhen making decisions involving water resources. They areused in the Delaware Department of Natural Resources andEnvironmental Control (DNREC) water resource, solidwaste, and hazardous substance management, permitting, andpollution prevention programs (DNREC, 1993; Table 1). Asa result of State legislation enacted in 2001 (7 DelawareCode, Chapter 60, Subchapter VI), Delaware counties andmunicipalities with more than 2,000 residents are directed toinclude the most sensitive ground-water recharge areas intheir comprehensive land use plans.
The GWRM program was conducted in Kent andSussex counties; however, the methods developed in thisproject (Andres, 1991a) were used to complete a similarrecharge potential mapping project for New Castle County(Butoryak and Talley, 1993), in which only the excellentrecharge areas were mapped. These areas are identified asground-water recharge resource protection areas and areafforded additional land use restrictions by county landdevelopment codes (New Castle County UnifiedDevelopment Code, Chapter 40), by the City of Newark’sWellhead Protection Ordinance (Wellhead ProtectionOrdinance, Article VII), and by the Town of Townsend’sEnvironmental Protection Regulations (TownsendEnvironmental Protection Regulations, Article XI). KentCounty recently adopted GWRM maps in wastewater plan-
ning and construction decisions (Kent County Code,Chapter 180) and has initiated the process to incorporate themaps into the planning and zoning process (Kent CountyCode, Chapter 187). Several towns preparing comprehen-sive land use plans have also recently included rechargepotential maps and discussed recharge protection in theirplans.
Andres (1991a) developed procedures to challenge therecharge potential maps in anticipation of the use of themaps for regulatory purposes. These procedures are reprint-ed in the appendix.
GWRM has supported other regulatory and non-regu-latory programs in Delaware. The Delaware Department ofAgriculture (DDA) Pesticide Compliance Section usesground-water recharge potential maps in support of compli-ance monitoring efforts. Data derived from GWRM havesupported projects in the Delaware Geological Survey(DGS) hydrologic (Andres, 1994, 2001; Andres andHoward, 2002) and geologic (Andres and Ramsey, 1995,1996; Andres and Howard, 2000; Ramsey, 2001; Ramseyand McKenna, 2002) programs. Ground-water rechargepotential maps are used in projects at the DGS, includingwellhead protection area (WHPA) delineation and evalua-tion of surface-water and ground-water quality.
Previous Regional Recharge EvaluationsBecause of the porous nature of the sedimentary units
of the Coastal Plain, low land-surface elevations, and lowtopographic relief, streams are usually in direct contact withground water in the underlying aquifer. In this setting, previ-ous investigators of ground-water recharge on the DelmarvaPeninsula concluded that long-term average or median ratesof fair-weather stream flow, or base flow, are in equilibriumwith ground-water recharge rates. On the basis of analysesof hydrograph separations from six watersheds, Johnston(1973) found an average recharge rate of approximately 14inches per year. Johnston’s (1977) digital ground-water flowmodel produced similar results. In a detailed study of fourwatersheds that included analyses of climatic water budgetsand stream hydrograph separations, Johnston (1976) identi-fied recharge rates on the order of 12 to 16 inches per year.Cushing et al. (1972) concluded that in many streams, medi-an unit stream flow (median flow per watershed area) is agood estimator of average base flow and report rechargerates ranging from about 6 to 16 inches per year (Cushing etal., 1972, fig. 24) for watersheds in Kent and Sussex coun-ties. Comparison of these recharge estimates to the water-shed’s recharge potential provides an independent dataset toanalyze interactions of surface water and ground water.
AcknowledgmentsCurrent and former DNREC staff members John T.
Barndt, Nancy Goggin, Jennifer McDermott, and Philip J.Cherry deserve special recognition for their efforts to finan-cially support this program. The following federal, state, andlocal government entities are thanked for providing logisti-cal support and access to facilities: U. S. Natural ResourcesConservation Service; U. S. Fish and Wildlife Service; U. S.Geological Survey; DDA; DNREC divisions of Parks andRecreation; Fish and Wildlife; Soil and Water Conservation;
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Table 1. State government programs using recharge potentialmaps. Modified from DNREC (1993).
Department of Natural Resources and Environmental Control
Underground Storage TankSolid WasteSite Investigation and RemediationHazardous WasteEnvironmental ResponseWastewater Spray IrrigationUnderground Injection ControlParks and Recreation (land acquisition)Source Water ProtectionNon-Point Source PollutionBiosolids Application
Department of Agriculture
Pesticide Compliance
and Water Resources; Delaware Department ofTransportation; and Delaware Solid Waste Authority. Iwould also like to thank Kent County, Sussex County,Bridgeville, Delmar, Greenwood, Harrington, Lewes,Millsboro, Milton, Georgetown, Seaford, and Selbyville. Iappreciate the cooperation of a number of land owners inproviding access to property and facilities, AmericanOriginal Foods Inc., Chesapeake Forest Products Inc.,DelAgra Inc., Delaware Wildlands, Inc., Draper-King ColeInc., Glatfelter Pulp and Paper, Co., Inc., and The NatureConservancy.
DGS staff members played significant roles in the pro-gram. Roland E. Bounds operated the truck-mounted drillrig. Johan J. Groot provided palynological and paleoenvi-ronmental analyses and interpretations. C. Scott Howard andTodd A. Keyser conducted day-to-day operations and co-authored maps and reports during the last four years of theprogram. William S. Schenck guided the computer aideddrafting efforts during the first few years of the program.Lillian T. Wang provided GIS support in the final phases ofmap compilation. More than 30 students participated in theprogram; Jennifer E. Athey, Bruce W. Brough, Dawn A.Denham, Lisa J. Donohoe, Andrew Grundstein, and JamesA. Maio are recognized for their expertise and multipleyears of service.
John T. Barndt, Scott C. Blaier, Martin W. Wollaston,and Jeffery P. Raffensperger are thanked for criticallyreviewing the manuscript.
The program was partially funded by DNREC withfederal grant funds from sections 106 and 319 of the CleanWater Act and the Safe Drinking Water Act. The DGS pro-vided support in the forms of direct funding and in-kind ser-vices of staff time and logistics.
METHODS
Mapping ProceduresRecharge potential maps are the result of 1:24,000-
scale evaluation of water-transmitting properties of the earthmaterials between land surface and 20 ft bls, and are used toidentify those areas where materials permit the most waterto move into the shallow aquifer the most rapidly. As statedin Andres (1991a, p. 5) “This layer and thickness were cho-sen because:1. Almost all water that recharges the ground water moves
through this layer.2. This interval almost always contains the water table, and
therefore, ground-water recharge occurs in this layer.3. This thickness is great enough to filter some local-scale
heterogeneity and allow for correlation, but not so largeas to filter out sub-regional heterogeneity or localtrends.”
Recharge potential mapping criteria include threematerial thickness categories, three lithologic categories,and four recharge categories. Tables 2 and 3, and Figure 2depict the mapping criteria and a recharge-rating example,respectively. In general, the more coarse-grained and uni-form the material, the higher the recharge rating.
The recharge potential rating and mapping proceduresrely primarily on descriptive logs of drill holes and bore-holes from selected water well drillers, test boring operators,
geologists, and engineers, as well as descriptions of materi-als exposed in outcrops, borrow pits, drainage ditches, andhand auger borings. Grain size distribution tests were run onselected samples to calibrate and check the accuracy of visu-al descriptions by DGS staff. Test borings were completed inareas where there were questions about the accuracy ofdrillers’ logs.
Two mapping categories have no recharge potentialrating; water/swamp/marsh areas and pit/fill areas. A smallnumber of recharge potential data points are located in theseareas, but recharge potential polygons are not mappedbecause little data are available, or because geologic map-ping techniques are not applicable. “Water area” indicatesareas of open water, marsh, and swamp, areas which typical-ly have limited available data. “Pit/fill” identifies areas ofborrow pits and dredge spoil disposal, and sites where exca-vation and filling render geologic mapping techniquesinvalid.
The process of constructing recharge potential maps issimilar to weighted estimation where the weighting criteriaare surficial and subsurface geologic mapping methods andgeologic models. Geologic mapping methods included inter-preting cross sections, identifying, characterizing, and corre-lating lithostratigraphic units and lithofacies, interpretingpaleontologic data, evaluating geophysical logs, and analyz-ing geomorphic features. These methods resulted in geolog-ic models formulated to map distributions of materials withsimilar hydrologic characteristics and recharge potentials,and the locations of boundaries that separate materials thatare different. The topology of recharge potential map units issuch that a unit may touch only units from adjacent rechargepotential categories. For example, an area rated good mayshare a boundary with units rated excellent or fair, but notpoor.
In the original methods report, Andres (1991a) alsorecommended that the maps be used in conjunction withmaps of recharge and discharge areas determined by flow-net analysis. Flow-net analysis was discontinued after theinitial mapping project because of time and budgetary con-straints. Additional evaluations of the mapping criteria aredescribed in following sections.
Aquifer TestingTo test the relationships between lithology and
hydraulic conductivity (K), single-well aquifer tests or slugtests were run in more than 200 monitoring wells. Digitaldata logger/silicon-strain gage type transducer systems wereused to collect the test data. Transducer accuracy was peri-odically tested in the laboratory, and wells were retested andinaccurate transducers were replaced as needed. A 2-secondrecording interval and a 10 pounds per square inch transduc-er were used in all tests. Water level displacement methodsincluded addition of water and insertion and removal ofsolid slugs.
The method described by Bouwer (1989) proved tohave the most utility in analyzing test data, and calculatingK. In order to minimize the possibility of using erroneous ornoisy test data, the results of two or more rising and/orfalling head tests were analyzed in greater than 90 percent ofthe wells. The K data from these tests were compared to thedescriptions or grain-size distribution measurements of the
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materials adjacent to the well screen.
Data Management and MappingWell, boring, and outcrop data, and the associated
lithologic and recharge ratings and aquifer test data are
stored and maintained in the DGS’s primary relationaldatabase management system. The system was used forentry, quality assurance, and retrieval of data, as well as foridentification of additional wells and boreholes that couldpotentially be used.
All lithologic and recharge ratings were evaluated bytwo separate project staff members, with the second evalua-tion always done by professional staff. Following data entry,the recharge ratings were checked against the lithologic rat-ings to ensure that the proper recharge ratings wereassigned.
Locational accuracy of data points was verified by oneor more methods. A majority of these data were verified inthe field and plotted on 1:24,000-scale USGS topographicmaps. Aerial photography, global positioning systems, engi-neering drawings, and tax parcel maps were sometimes usedto determine location. Using these methods, approximately25 percent of the well records originally identified as poten-tial data points were not used because the location could notbe verified.
Following verification, data point locations weretransferred from the field maps to a master set of projectdata point maps, and position coordinates were determined.Latitude and longitude coordinates were determined to thenearest second. Coordinates and estimates of positionalaccuracy were recorded in the computer database with thelocation data. Except for points located by precision survey-ing and GPS, resolution of coordinates is one second(approximately 100 ft), and accuracy is estimated to be aradius of about 200 ft.
Draft recharge potential maps were drawn and editedon copies of the master project data point maps, transferredto new paper or stable-base material maps, and digitized ona digitizing tablet. Prior to digitizing, paper quadrangle
4
Table 3. Recharge potential rating categories. From Andres(2003a, b).
MORE SAND MORE MUD
EXCELLENT = E20S20S (l)
GOOD = G20S 10L (�m)20S (m�l)10S 10L (�m)20L 10S (�m)20L (�s)
FAIR = F20L (m�s)20S 10M (�l)10S 10M (�l)10M 10S 10M20L 10M (�s)
POOR = P20M 10S (�l)10M 10L (�s)20M 10L (�s)20M (�s,l)
Table 2. Lithologic rating and thickness categories. FromAndres (2003a, b).
Lithologic Category Symbols
S = Sand with trace (0 to 12%) of silt or clay and coarser,including: Gravel, sandy, silty; Sand, gravelly, silty*;and Gravel, silty*
L = Silty sand - sand with 12% to 35% silt
M = Mud - all finer than silty sand
* When stratified into sorted layers then S; when unstrati-fied then L.
Thickness Category Symbols
0 to < 5 ft = (lower case)*5 to 10 ft = 10
10.1 to 20 ft = 20
* Lithologies with a total thickness of 1 ft or less maybe combined with another lithologic category.
Figure 2. Recharge potential rating example. Lithologic rating andthickness symbols are described in tables 2 and 3.
maps were registered to coordinate frames plotted by com-puter on a stable base material. Plots of the digitized mapswere then visually compared to the originals. Final digitalcompilation was done using ARCMap v8.1 (ESRI, 2000)after all quadrangle mapping was completed. Problems withmismatched recharge polygon boundaries across quadrangleboundaries and quadrangle boundary lines were removedduring the process.
Because mapping was done in different areas at differ-ent times, the mapping process was iterative. Rechargepotential maps completed during earlier years of the pro-gram were evaluated and modified when additional data col-lected later indicated that changes were necessary. Thespecific software and hardware used changed several timesduring the course of the program. Readers interested in thecomputer hardware and software used over the project peri-od may contact the author.
The processes for determining data point coordinatesand creating, drafting, and digitizing data point and rechargepotential maps cause some data points to be placed withininappropriate recharge polygons. For example, points ratedfair may occur within polygons rated good. These occur-rences are acceptable where the data point falls within thelocational error tolerance (60 m), and in areas where pointrecharge ratings were averaged because of clustering of datapoints.
Performance Testing by ModelingThe expected performance of the recharge categories
and maps was tested with simple ground-water flow andcontaminant transport simulations. The intent of using sim-ple models is to compare how materials with differentrecharge potentials will behave under similar hydrologicconditions, a scenario that would be expensive and difficultto measure in a field experiment. Using this guideline, thestarting conditions of each simulation are the same across allrecharge potential categories, and K and recharge ( R ) with-in each simulation are assumed to be homogeneous.
Water-table profiles and gradients generated bysteady-state ground-water flow simulations constructed inMODFLOW (MacDonald and Harbaugh, 1988) and VisualMODFLOW (Waterloo Hydrogeologic, Inc., 2002) werecompared to data from monitoring well networks (Andres,1991b, 1992; Denver, 1989, 1993) and USGS HydrologicAtlas Series maps (Adams and Boggess, 1964; Adams et al.,1964a, 1964b; Boggess and Adams, 1964, 1965; Boggess etal., 1964). Model grid spacings for MODFLOW and VisualMODFLOW simulations were uniformly 20 ft. A constanthead boundary was set at one end of the grid to provide areference base level. Model lengths were set from 2,000 to3,000 ft long, sizes typical of the distances between first andsecond order streams and the drainage divides of theirwatersheds. Effects of model thickness were tested by vary-ing thickness from 20 to 40 ft. The ratio of horizontal K(Kh) to vertical K (Kv) was set to 10.
A test of the recharge potential mapping methods wasdone by comparing long-term ground water (Qgw) andoverland runoff (Qo), and evapotranspiration (ET) valuesreported by Johnston (1976) to proportions of land coveredby each recharge potential category and wetlands. Johnston
(1976) equated Qgw to R for long-term conditions. The pro-portion calculations were performed with ARCMap (ESRI,2000) by assigning a ranked numerical attribute to each cat-egory and computing the areally weighted recharge potential(AWRP) for the watershed:
AWRP =((Area of excellent x 4) + (Area of good x 3) +
(Area of fair x 2) + (Area of poor x 1))/Total Area
In addition, the percentage of wetlands present in 1997 landcover data (Delaware Office of State Planning Coordination,1999) was computed with ARCMap (ESRI, 2000) and com-pared to Johnston’s (1976) estimated ET rates.
Contaminant transport modeling was done by Andresand Brough (1994) and Andrew Grundstein (written com-munication, 1995) using a combination of simulation tools(Fig. 3). MODFLOW and Visual MODFLOW were used tosimulate flow fields as described in preceding paragraphs. Asimple finite-difference model (Campbell, 1985) was usedto simulate steady-state gravity drainage and contaminanttransport with dispersion. The latter model, which we namedSOLTRAN, simulates the timing of contaminant input froma source located at land surface to the water table as a func-tion of time. AT123D (Yeh, 1992) is an analytic-equationbased model and was used to simulate two-dimensional con-taminant transport with diffusion and dispersion. Hundredsof different combinations of K, R, and contaminant inputswere simulated with these models to calculate the ranges ofpossible results associated with different hydraulic conduc-tivities, porosities, dispersivities, and water saturation per-centages (Tables 4 and 5). AT123D simulation periodsincluded runs of 180 and 360 days.
RESULTS AND DISCUSSIONThe sequence of mapping areas was determined by a
survey of program priorities within DNREC (DNREC,1993) and other state agencies that use the maps. High prior-ity was given to areas experiencing more rapid develop-ment, areas containing larger numbers of sites ofground-water pollution, and areas where permitting of sub-surface wastewater discharge facilities were an issue.Individual reports containing descriptions of geologic units
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Table 4. Input data for ground-water flow models. Vertical K is10 percent of horizontal K.
Recharge Potential LongitudinalCategory K (ft/d) Porosity Dispersivity (ft)
Excellent 50 - 300 0.34 20-40Good 30 - 150 0.34 30-50Fair 15 - 70 0.27 40-60Poor 0.5 - 15 0.24 60-100
Table 5. Loading data for solute transport models. Loadingrates from Andres and Brough (1994) and Grundstein(1995, written communication).
Recharge Potential Loading LoadingCategory Rate (kg/ha) Period (days)
Excellent 26 - 34 5 - 25Good 18 - 26 10 - 51Fair 11 - 26 20 - 103Poor 11 - 26 37 - 184
6
encountered in the area, hydrologic functions of the units,and cross sections were produced for each project area, and1:24,000-scale maps of recharge potential and data pointswere submitted to DNREC for each quadrangle.Compilation of county maps was done following comple-tion of the 1:24,000 mapping.
General DescriptionThe recharge potential of most of the land area in
Kent and Sussex counties is in the good and fair categories(Table 6). This is a result of the larger number of lithologic
assemblages in the good and fair categories (Table 3).Although slightly larger, the percentage of land area ratedexcellent (10 percent) is similar to the eight percent ratedexcellent in the pilot study area located in southeasternSussex County (Andres, 1991a). Kent County has a signifi-cantly higher percentage of area rated excellent than SussexCounty. Sussex County has a significantly higher percent-age of area rated poor than Kent County. Percentages ofareas rated fair and good are virtually the same between thecounties.
The overall map pattern (Fig. 4) reflects the complexspatial associations of geologic units present within 20 ft ofland surface. The large contiguous areas of excellent andgood recharge potential in central and southern Kent Countyare associated with relatively coarse-grained deposits of theColumbia and Beaverdam formations. The large contiguousareas of poor and fair recharge potential in southeastern andsouth-central Sussex County reflect the underlying relative-ly fine-grained deposits of the Cypress Swamp and Omarformations.
Table 7 shows statistics on recharge ratings of well,boring, and outcrop record data by category and county. Intotal, nearly 6,400 well and boring records were evaluated.
Figure 3. Illustration of flow and contaminant transport simulation strategy. MODFLOW and Visual MODFLOW are used to simulatewater-table gradient. SOLTRAN is used to simulate temporal characteristics of contaminant loading to the water table. Resultsfrom MODFLOW and SOLTRAN are used as input to AT123D. AT123D is used to simulate contaminant transport through thesaturated zone. Amount of contaminated water determined from AT123D output. Modified from Andres and Brough (1994).
Table 6. Proportions of recharge potential categories in Kentand Sussex counties. From Andres (2003a, b).
Area Excellent Good Fair PoorKentCounty 14 41 40 5
SussexCounty 8 40.5 42.5 9
Allareas 10.5 40.5 41.5 7.5
7
0 5 10 15 202.5
RECHARGE POTENTIAL
Excellent
Good
Fair
Poor
Water/marsh/swamp area
Pit/fill
Miles
Su
ssex C
ou
nty
Ke
nt C
ou
nty
Figure 4. Recharge potential map of Kent and Sussex counties. Map compiled from data of Andres et al. (2002).
This included more than 600 test borings completed by DGSstaff, and more than 200 outcrops and hand-auger boringsdescribed by DGS staff. The density of observations in thenon-water covered areas of Kent and Sussex counties isabout 4.8 observations per square mile, which on the1:24,000 scale of the mapping base, translates into 4 to 5observations within a 2.5 by 2.5-in block.
Ranges of recharge ratings were assigned to approxi-mately 600 locations where (1) the boring did not penetrate20 ft, (2) nearby data indicated a possible inaccuratedescription, or (3) the descriptive log was not detailedenough to assign a single value. Locations that wereassigned ranges of recharge ratings were used to assistrecognition of overall trends and to determine boundariesbetween map units.
Relationships Between Lithology, Recharge Potential, and Hydraulic Conductivity
A common theme of questions from users of therecharge potential maps is “What is the K value of an excel-
lent recharge potential area?” The answers to this and simi-lar questions are derived from evaluation of the results of Ktesting and corresponding lithologic data. Single-wellaquifer, or slug, tests provide data representative of the K oflithologies or materials within a few feet of the well(Bouwer, 1989). There are more geologic material cate-gories than recharge lithologic categories in this evaluationin order to account for the more detailed observations in adescriptive log compared to the three general lithologic rat-ing classes used in determining recharge potential, andbecause there can be multiple lithologies present in the zonearound the well. In general, the results confirm the expectedtrends; more uniformly coarse-grained geologic materialshave higher K values than more uniformly fine-grained geo-logic materials (Fig. 5). Examples of calculated K values forselected lithologic and recharge potential ratings are present-ed in Figure 6 .
Overlapping K ranges (Fig. 5) between adjacent mate-rial categories are due to natural factors and errors in thedata. Natural factors affecting K include bulk grain size dis-tribution, bedding, grain shape, and packing (Freeze andCherry, 1979; Bouwer, 1989). For example, poorly-sortedsands will have a lower K than well-sorted sands (Sheppard,1989). As a result, K of poorly-sorted sand may have a
8
Figure 6. Example hydraulic conductivity (K) calculations for dif-ferent recharge potential and lithologic categories.Except for K value shown for the thin bed of sand, silt,and clay, the K values used for calculation are log aver-ages of different material categories.
Figure 5. Box plot showing relationships between material cate-gories and hydraulic conductivity (K). There is overlapamong the material types in ranges of K but differencesbetween the average K values. Freeze and Cherry (1979)report minimum K values for mud 2 to 3 orders of mag-nitude less than shown in this illustration. K valuesreported in ft/d. L.A. = log average.
Table 7. Statistics on recharge ratings of well, boring, and out-crop record data by category and county. FromAndres (2003a, b).
DensityArea Excellent Good Fair Poor (pts/sq mi)KentCounty 748 759 733 385 5.5
SussexCounty 731 1025 971 600 4.3
Kent andSussex 1479 1784 1704 985 4.8
lower K than a well-sorted sand with scattered laminae ofsilty sand or sandy silt. There also are indeterminate errorsin descriptive logs and errors in processing and interpretingslug test data that will tend to expand the ranges of K associ-ated with different geologic materials. Because these errorsarise from conditions that cannot be directly observed, thereis no way to state which data are in error and which are not.
Comparison with Johnston (1976) Water-Budget StudyIn all ground-water flow simulations, comparison to
an independently measured parameter such as flow is animportant step in calibrating and evaluating ground-watermodel results. These comparisons are needed because thesame model observations can result from multiple combina-tions of geometry, K, and R. Johnston’s (1976) estimates ofQgw, which is equivalent to R, Qo, and ET rates forBeaverdam Branch, Stockley Branch, Nanticoke River, andSowbridge Branch watersheds (Fig. 7), provide a means to
compare simulation results with flow data. Johnston’s valueswere generated by independent evaluations of stream hydro-graphs (Table 8 and Figs. 8A-8B).
There is a clear positive trend (Fig. 8A) betweenrecharge potential and percentage of Qgw in the water bud-
get for the three smaller watersheds (Beaverdam Branch,Stockley Branch, and Sowbridge Branch), while the data forthe larger Nanticoke River watershed show a higher dis-charge rate than would be predicted by the recharge poten-tial trend of the smaller watersheds. There is indication thatthe lower percentage of Qgw for the Nanticoke is related toa higher proportion of ET in the water budget (Fig. 8B). Thehigher ET proportions, however, are not correlated with thepercentage of wetlands (Table 8).
It is also possible that the Nanticoke River and water-shed may interact with ground-water flow paths in ways thatthe recharge potential map units cannot represent. Fine-grained beds below the recharge potential mapping horizonare common in the upper Beaverdam Formation (Andres,
9
Table 8. Areally weighted recharge potential and water budget in four Delaware watersheds. ET= evapotranspiration, Qgw = ground-water runoff, ppt = precipitation. Quantities are shown as percentages.
100 x Qgw/ 100 x ET/ percentWatershed total precipitation total precipitation AWRP wetlandsSowbridge Branch 34 56 3.1 3.3Stockley Branch 31 62 2.2 0.7Beaverdam Branch 33 58 2.7 1.4Nanticoke River 29 61 2.8 1.0
Figure 8. Comparison of areally weighted recharge potential(AWRP) and water budget in four Delaware watersheds.The formula used to compute AWRP is discussed in themethods section. 8A. AWRP and ground-water runoff(Qgw) as a percentage of total precipitation (ppt). 8B.Evapotranspiration (ET) as a percentage of precipitation(ppt) by watershed.
52
56
60
64
SowbridgeBranch
StockleyBranch
BeaverdamBranch
NanticokeRiver
ET
pe
rce
nta
ge
B.
0
1
2
3
4
27 29 31 33 35
100 x Qgw/total precipitation
AW
RP
Nanticoke River
Stockley Branch
Sowbridge Branch
Beaverdam Branch
A.
0 5Miles
CountyKent
CountySussex
Nanticoke River
Sowbridge Branch
Beaverdam Branch
Stockley Branch
10 20
Figure 7. Locations of watersheds studied by Johnston (1976).
1994; Andres and Ramsey, 1995, 1996). These low perme-ability geologic features would tend to deflect deeperground-water flow paths up and discharge into the riverabove the stream gaging station. Data needed to test thishypothesis are not available. The Nanticoke River and itstributaries were intensively ditched in the 1950s with themain channel deepened by 6 to 7 ft (Johnston, 1976).Johnston (1976) noted a significant drop in the long-term ele-vation of the water-table in an observation well and a changein the relationship between depth to ground water and riverdischarge following the ditching project. The deepening ofthe river could also be causing the river to intercept flowpaths from below the recharge potential mapping horizon.
Results of Ground-Water Flow ModelingAnother test of recharge potential and lithologic cate-
gories was done by evaluating water-table profiles simulatedby digital ground-water flow models that use K valuesderived from slug tests (Figs. 9A-9B). Water-table gradientsobserved within small watersheds in the Coastal Plain ofDelaware almost always fall in the range of 0.0001 to 0.003ft/ft. Model-generated gradients above this range or water-
table profiles that rise above the top of the model grid indi-cate that K is too small or R is too large, or both. Model-generated gradients below this range indicate that K is toolarge or R is too small, or both. As expected, model resultspredict that overlapping ranges of R values associated withadjacent recharge potential categories (i.e., excellent andgood) produce reasonable gradients.
R values associated with excellent recharge potentialand those associated with poor recharge potential differ by afactor of three. This is an independent indication that givensimilar amounts of precipitation, areas of greater (i.e., excel-lent and good) recharge potential transmit more water intothe Columbia aquifer than areas of lesser recharge potential(i.e., fair and poor). It is a logical extension, therefore, thatrestricting the amount of impervious surfaces in areas ofexcellent and good recharge potential areas will help main-tain the adequacy of future ground-water supplies.
Contaminant transport simulations show trends similarto those predicted by the water-table profile simulations;that is, overlapping ranges of results between adjacentrecharge potential categories. There are substantial differ-ences between the mean and median results of rechargepotential categories. Contaminants reach the water tablemore quickly and travel faster through the aquifer (Fig.10A) affecting larger quantities of ground water (Fig. 10B)as K values increase. This supports the hypothesis that moredamage to ground-water quality occurs more quickly asrecharge potential increases.
Although the contaminant-transport simulations are rel-atively simplified compared to field conditions, the resultsprovide information useful for developing resource protectionpolicies and interpreting ground-water monitoring data.Unacceptable concentrations of ground-water contaminantscan occur under any recharge potential condition if sufficientcontaminants are released into the ground. Because clean-upof contaminants is almost always easier and less expensive ifdone before they reach the water table, it is much more criti-cal to begin clean-up in an area of excellent recharge potentialas soon as possible after contaminant release. In evaluatingthe effectiveness of clean-up efforts with monitoring of waterquality, recharge potential can help estimate the lag betweencontaminant release and when contaminants reach a monitor-ing point in the underlying aquifer.
CONCLUSIONSGround-water recharge potential maps show land
areas characterized by their abilities to transmit waterthrough the first 20 feet below land surface. The mappingwas done using geologic mapping techniques and over6,000 subsurface observations in test borings, wells, borrowpits, natural exposures, and ditches. On the basis ofhydraulic testing, the four recharge potential categories(excellent, good, fair, poor) are a predictor of the relativeamounts and rates at which recharge will occur. Numericalmodeling shows that recharge rates in areas with excellentrecharge potential can be two to three times greater thanrates in fair and poor recharge areas.
Ground-water recharge potential maps provide decision-making support and guide policy development for such topicsas land use, water-resources development and management,
10
0
20
40
60
80
100
0 200 400 600 800 1000 1200
Wat
er-t
able
hei
gh
t (f
t)
K=5 ft/d
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K=50 ft/d
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5
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15
0 200 400 600 800 1000 1200
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hei
gh
t (f
t)
K=5 ft/d, R=5 in/yr
K=75 ft/d, R=20 in/yr
K=75 ft/d, R=15 in/yr
Distance (ft)
A.
B.
Figure 9. Simulated water-table profiles for different rechargepotential categories. A. Profiles are for steady-state con-ditions with a recharge rate of 15 inches per year andK=hydraulic conductivity in ft/d. B. Profiles are forsteady-state conditions with variable recharge rates (R).
wastewater disposal systems development, and environmentalpermitting in state, county, and local governments. Recentlyenacted state law requires that counties and towns with morethan 2,000 residents provide protection to areas rated withexcellent recharge potential in comprehensive land use plans.This is only one type of use for the maps. The hydraulic con-ductivity data associated with the good, fair, or poor rechargepotential map units can be applied in modeling tools used forguiding water resource management, environmental and landuse permitting, and pollution prevention and control efforts.
Because recharge potential map categories are associat-ed with physical properties of earth materials, they are indica-tors of how fast contaminants will move and how much watercan be contaminated. Numerical modeling of contaminant
transport under different recharge potential conditions predictsthat greater masses of contaminants move more quickly andaffect greater volumes of water under higher recharge potentialconditions than under lower recharge potential conditions.
REFERENCES CITED
Adams, J. K., and Boggess, D. K., 1964, Water-table, surface-drainage, and engineering soils map of the Harbeson quadran-gle, Delaware: U.S. Geological Survey HydrologicInvestigations Atlas HA-108, scale 1:24,000.
Adams, J. K., Boggess, D. K., and Coskery, O. J., 1964a, Water-table, surface-drainage, and engineering soils map of theFrankford area, Delaware: U.S. Geological Survey HydrologicInvestigations Atlas HA-119, scale 1:24,000.
Adams, J. K., Boggess, D. K., and Davis, C. F., 1964b, Water-table, surface-drainage, and engineering soils map of theLewes area, Delaware: U.S. Geological Survey HydrologicInvestigations Atlas HA-103, scale 1:24,000.
Aller, L., Bennett, T., Lehr, J. H., and Petty, R. J., 1985, DRASTIC:A standardized system for evaluating ground water pollutionpotential using hydrogeologic settings: U.S. EnvironmentalProtection Agency, EPA/600/2-85/018, 163 p.
Andres, A. S., 1991a, Methodology for mapping ground-waterrecharge areas in Delaware’s Coastal Plain: DelawareGeological Survey Open File Report No. 34, 18 p.
___1991b, Results of the coastal Sussex County, Delaware ground-water quality survey: Delaware Geological Survey Report ofInvestigations No. 49, 28 p.
___1992, Estimate of nitrate flux to Rehoboth and Indian Riverbays, Delaware, through direct discharge of ground water:Delaware Geological Survey Open File Report No. 35, 36 p.
___1994, Geohydrology of the Seaford area, Delaware: DelawareGeological Survey Hydrologic Map No. 9, scale 1:24,000.
___ 2001, Geohydrology of the Smyrna-Clayton area, Delaware:Delaware Geological Survey Hydrologic Map No. 10, scale1:24,000.
___ 2003a, Ground-water recharge potential of Kent County,Delaware: Delaware Geological Survey Hydrologic Map No.11, scale approximately 1:100,000.
___ 2003b, Ground-water recharge potential of Sussex County,Delaware: Delaware Geological Survey Hydrologic Map No.12, scale approximately 1:110,000.
Andres, A. S., and Brough, B. W., 1994, Simulation of aquifer con-tamination under varying recharge potential conditions: unpub-lished report to the Delaware Department of Natural Resourcesand Environmental Control 23 p.
Andres, A. S., and Howard, C. S., 2000, The Cypress SwampFormation, Delaware: Delaware Geological Survey Report ofInvestigations No. 62, 14 p.
___ 2002, Results of hydrogeologic studies of the Cypress SwampFormation, Delaware: Delaware Geological Survey Report ofInvestigations No. 64, 21 p.
Andres, A. S., Howard, C. S., Keyser, T. A., and Wang, L. T.,2002, Ground-water recharge potential maps of Kent andSussex Counties, Delaware: Delaware Geological SurveyDigital Data Product DDP02-02, version 1.5, ESRI E00format.
Andres, A. S., and Ramsey, K. W., 1995, Geology of the Seafordarea, Delaware: Delaware Geological Survey Geologic MapNo. 9, 1:24,000.
___1996, Geology of the Seaford area, Delaware: DelawareGeological Survey Report of Investigations No. 53, 23 p.
11
Figure 10. Example results of simulated contaminant transportunder different recharge potential scenarios. A. Massesof contaminants pass a point 150 ft downflow of therelease area more quickly and at a higher loading rateat higher K values even when similar masses of con-taminants (e.g., constant load) are released. Rechargevolume weighted loadings (e.g., variable load) increasethe differences in contaminant masses betweenrecharge potential categories. B. Areas underlain bycontaminated water are greater with increasingrecharge potential, even when similar masses (e.g.,constant loading) of contaminant are added.
Berg, R. C., Kempton, J. P., and Cartwright, K., 1984, Potential forcontamination of shallow aquifers in Illinois: IllinoisGeological Survey Circular 532, 30 p. with maps.
Boggess, D. K., and Adams, J. K., 1964, Water-table, surface-drainage, and engineering soils map of the Bethany Beacharea, Delaware: U.S. Geological Survey HydrologicInvestigations Atlas HA-122, scale 1:24,000.
___1965, Water-table, surface-drainage, and engineering soils mapof the Millsboro area, Delaware: U.S. Geological SurveyHydrologic Investigations Atlas HA-122, scale 1:24,000.
Boggess, D. K., Adams, J. K., and Davis, C. F., 1964, Water-table,surface-drainage, and engineering soils map of the RehobothBeach area, Delaware: U.S. Geological Survey HydrologicInvestigations Atlas HA-109, scale 1:24,000.
Boniol, D., Munch, D. A., and Williams, W., 1990, Recharge areasof the Floridian aquifer in the Crescent City Ridge of southeastPutnam County, Florida — a pilot study: St. Johns River WaterManagement District, Technical Publication SJ 90-9, 72 p.
Bouwer, Herman, 1989, The Bouwer and Rice slug test – anupdate: Ground Water, vol. 27, no. 3, pp. 304-309.
Butoryak, K. R., and Talley, J. H., 1993, Delineation of ground-water recharge resource protection areas in the Coastal Plain ofNew Castle County, Delaware: unpublished report to WaterResources Agency for New Castle County, 26 p.
Campbell, G. S., 1985, Soil physics with BASIC: New York,Elsevier Science Publishing Co., Inc., 150 p.
Charles, E. G., Behroozi, C., Schooley, J., and Hoffman, J. L.,1993, A method for evaluating ground-water recharge areas inNew Jersey: New Jersey Geological Survey Report GSR-32,95 p.
City of Newark Wellhead Protection Ordinance, Article VII,Section 30-52, Ord. No. 91-16, Amend. No. 1, [4/22/1991].
Code of Kent County, Delaware, v. 8, 2003, Part II GeneralLegislation, Chapter 180, Sanitary Standards [08/15/2003].
___ 2003, Part II General Legislation, Chapter 187, Subdivisionand Land Development [6/24/2003].
Cushing, E. M., Kantrowitz, I. H., and Taylor, K. R., 1972, Waterresources of the Delmarva Peninsula: U.S. Geological SurveyProfessional Paper 822, 58 p.
Delaware Code, Title 7, Chapter 60, Subchapter VI, 2001,[06/27/2001].
Delaware Department of Natural Resources and EnvironmentalControl, 1993, Report on the use of the ground water recharge-potential maps in the State of Delaware: DNREC report to theU.S. Environmental Protection Agency, 11 p.
Delaware Office of State Planning Coordination, 1999, 1997 landuse / land cover data – Sussex County: Delaware Office ofState Planning Coordination, ESRI coverage format.
Denver, J. M., 1989, Effects of agricultural practices and septicsystem effluent on the quality of water in the unconfinedaquifer in parts of eastern Sussex County, Delaware: DelawareGeological Survey Report of Investigations No. 45, 66 p.
___1993, Herbicides in shallow ground water at two agriculturalsites in Delaware: Delaware Geological Survey Report ofInvestigations No. 51, 28 p.
ESRI, Inc., 2000, ArcMap software v. 8.1: Redlands, CA.
Freeze, R. A. and Cherry, J. A., 1979, Groundwater: EnglewoodCliffs, NJ, Prentice-Hall, Inc., 604 p.
Johnston, R. H., 1973, Hydrology of the Columbia (Pleistocene)deposits of Delaware: an appraisal of a regional water-tableaquifer: Delaware Geological Survey Bulletin 14, 78 p.
Johnston, R. H., 1976, Relation of ground water to surface water infour small basins of the Delaware Coastal Plain: DelawareGeological Survey Report of Investigations No. 26, 56 p.
Johnston, R. H., 1977, Digital model of the unconfined aquifer incentral and southeastern Delaware: Delaware GeologicalSurvey Bulletin 15, 47 p.
Kempton, J. P., 1981, Three-dimensional geologic mapping forenvironmental studies in Illinois: Illinois Geological SurveyEnvironmental Geology Note 100, 43 p.
Kempton, J. P. and Cartwright, K., 1984, Three-dimensional geo-logic mapping: a basis for hydrogeologic and land-use evalua-tions: Association of Engineering Geologists Bulletin vol. 21,no. 3, pp. 317-335.
MacDonald, M. G., and Harbaugh, A. W., 1988, A modular three-dimensional finite-difference ground-water flow model: U.S.Geological Survey Techniques of Water-ResourcesInvestigtations of the U.S. Geological Survey Book 6, ChapterA1.
Petty, S., Miller, W. D., and Lanan, B. A., Potential for ground-water recharge in the Coastal Plain of New Castle County,Delaware, Sheet 1, northern New Castle County (1983); Sheet2 Chesapeake and Delaware Canal Area (1985), Woodruff, K.D., ed.: Delaware Geological Survey Open-File Report No. 28,maps with discussion, scale 1:24,000.
Ramsey K. W., 2001, Geologic map of the Milton and Ellendalequadrangles: Delaware Geological Survey Geologic Map No.11, scale 1:24,000.
Ramsey, K. W., and McKenna, K. K., 2002, An evaluation of sandresources, Atlantic offshore, Delaware: Delaware GeologicalSurvey Report of Investigations No. 63, 37 p.
Sheppard, R. G., 1989, Correlations of permeability and grain size:Ground Water, vol. 27, no. 5 p. 633-638.
Sophocleous, M. and McAllister, J., 1990, Hydrologic-balancemodeling of the Rattlesnake Creek watershed, Kansas: KansasGeological Survey, Ground Water Series 11, 72 p.
Talley, J. H., 1988, Ground-water levels in Delaware, January 1978- December 1987:Delaware Geological Survey Report ofInvestigations No. 44, 58 p.
Town of Townsend Environmental Protection Regulations, ArticleXI, Section 1100.
Unified Development Code of New Castle County Delaware, 2003,Chapter 40, Article 10, Section 110, [12/14/1999].
Waterloo Hydrogeologic Inc., 2002, Visual MODFLOW v.3.0:Waterloo, Ontario, Canada, Waterloo Hydrogeologic Inc.
Yeh, G. T., 1992, AT123D v1.21 – Analytic transient one-, two-,and three-dimensional simulation of waste transport in theaquifer system: International Ground-Water Modeling CenterFOS 38.
12
APPENDIX
PROCEDURES FOR CHALLENGINGRECHARGE POTENTIAL MAPS
The following American Society for Testing andMaterials (ASTM) methods are listed as being acceptablefor ground-water recharge potential mapping. Alternativemethods may be acceptable but are not listed below. If alter-native methods are to be employed, they should be approvedbefore commencement of the project. These procedureshave been adapted from Andres (1991).
Test Boring and Logging MethodsTest borings should be completed according to ASTM
D1452 to a depth of at least 25 feet. This allows for the soilhorizon, down to the top of the “C” horizon, to be removedfrom the determination of the recharge potential rating.Removal of the soil layer from the recharge potential ratingis a reasonable procedure because soil permeability is notonly controlled by grain size distribution but is also greatlyaffected by soil structures (Bouwer, 1991). Split barrel(ASTM D1586) or thin walled tube (ASTM D1587) samplesshould be collected at five foot intervals. The test boringreport should include the information collected duringdrilling and the descriptions and classifications of all materi-als penetrated in each boring.
All materials occurring in the depth interval of inter-est, even those not sampled by split barrel or thin-walledtube methods, are to be fully described, including the infor-mation recorded during drilling, according to method ASTMD2488, parts 1, 2, 3.11, 3.13 through 3.15, and 3.17 through13, or an equivalent. Acceptable alternate classisficationmethods include ASTM D2487 parts 1 through 4 and 5.13through 15 (Unified System), American Society forEngineering Education (Burmeister) system, or other visualor laboratory methods that accurately describe the grain sizedistribution, grain shapes, composition, compactness, con-sistency, homogeneity, and degree of cementation of thesample.
Seive analyses may be employed (ASTM D422) toassist the description of individual core samples. It shouldbe noted that the sample preparation procedures used forsieve analysis can homogenize a sample and therefore maynot accurately describe the materials as they occur in theground or in the core.
The dimensions of sand are 0.074 mm (or U. S. SieveNo. 200) to 2.0 mm (or U. S. Sieve No. 60). The minimumdimension of gravel is 2.0 mm (or U. S. Sieve No. 60)(Hunt, 1984, p. 342, 355 [c.f. Burmeister, 1948, 1949,1951]). The dimensions for gravel and sand given in ASTMD2487-5.11 and -5.12, and D2488-3.12 and -3.16 (UnifiedSystem) are not acceptable.
The system for describing sand and gravel sizes givenabove is a hybrid of the Udden-Wentworth (Ehlers andBlatt, 1982) and Unified System, the two grain size scalesmost commonly used by geologists and engineers in theUnited States. These sand and gravel sizes are used in orderto accomodate the considerable volume of research on thehydraulic properties of materials done by engineers, and torecognize the importance of gravel in geologic interpreta-tion. In addition, it is recognized that most driller’s logs are
made using the Unified System and that driller’s logs consti-tute the bulk of subsurface information used in rechargemapping. In terms of evaluating logs for the recharge map-ping process, materials described as very coarse sand by theUnified System would be classified as granules or fine grav-el by the Udden-Wentworth scale.
Boring Locations and Statistical FilterFor sites up to 20 acres a minimum of two test borings
per site should be required. If borings used to construct theexisting recharge map are located on the site, then those bor-ings should be used in the evaluation and can be substitutedfor one of the required additional borings. In addition, ifborings used to construct the existing recharge map arelocated within 500 feet of the site boundaries, then thoseborings should be included in the evaluation. For sitesbetween 20 and 200 acres an additional boring per 20 acresshould be required for the total acreage between 20 and 200acres. The maximum number of borings for sites 200 acresand smaller is 10. For sites larger than 200 acres an addi-tional boring per 40 acres should be required for the totalacreage exceeding 200 acres. In all cases the borings shouldbe arranged, as much as possible, to be evenly distributedover the site and conform to a rectangular grid.
Averaged lithologic ratings are determined (usingtable 2 from the text) for each pair of adjacent borings byadding together the thicknesses of each lithology (e.g., sand,silty sand, or mud) and dividing the result by two. This pro-cedure averages the results of borings and smooths localscale heterogeneity. Averaged recharge potential ratings arethen determined (using table 3 from the text) from the aver-aged lithologic rating and plotted on the map at the mid-point between the adjacent borings (see Fig. A1). If theresultant recharge rating(s) at the mid-point rating positiondiffers by less than two ranks (i.e., excellent to good) fromthe original map, then the existing map rating for that loca-tion should be considered valid. If new data (e.g., the aver-age recharge ratings) show that a recharge rating or ratingsat the mid-point rating position(s) differ by two or more
13Delaware Geological Survey Report of Investigations No. 66
ranks (i.e., excellent to fair) from the existing map, then themap should be revised by the agency responsible for man-aging recharge areas. The revision should consider only thedata points from the existing recharge map and the averagedrecharge potential ratings. The revision should also be guid-ed by cross-sections, isopachous maps, or other tools com-monly used in geologic mapping.
References Cited
American Society for Testing and Materials (ASTM), 1990,Annual Book of Standards Volume 4.08, soil and rock; build-ing stones; geotextiles: Philadelphia, PA, ASTM.
Andres, A. S., 1991, Methodology for mapping ground-waterrecharge areas in Delaware's Coastal Plain: DelawareGeological Survey Open File Report No. 34, 18 p.
Bouwer, H., 1991, Simple derivation of the retardation equationand application to preferential flow and macrodispersion:Ground Water, vol. 29, no. 1, pp. 41-46.
Burmeister, D. M., 1948, The importance and practical uses of rel-ative density in soil mechanics: ASTM Vol. 48, Philadelphia,PA, ASTM.
Ehlers, E. G., and Blatt, H., 1982, Petrology - igneous, metamor-phic, and sedimentary: San Francisco, W. H. Freeman and Co.,732 p.
Hunt, R. E., 1984, Geotechnical engineering investigation manual:New York, McGraw-Hill Book Co., 983 p.
Delaware Geological Survey Report of Investigations No. 66
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RESEARCH
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GEOLOGICALSURVEY
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Delaware Geological SurveyUniversity of Delaware
Newark, Delaware 19716