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STATE OF DELAWARE

DELAWARE GEOLOGICAL SURVEY

REPORT OF INVESTIGATIONS NO. 13

THE OCCURRENCE OF SALINE GROUND WATER IN DELAWARE AQUIFERS

By

K. D. Woodruff

Newark, Delaware

August, 1969

THE OCCURRENCE OF SALINE GROUND WATER IN DELAWARE AQUIFERS

By

K. D. WOODRUFF

Hydrologist, Delaware Geological Survey

August, 1969

CONTENTS

Page

ABSTRACT ••• • • • • • • • • • • • • • • • • • • • • • • • • • • • 1

• • • •• • • •• • • • • • • •

122

• •• •

• • • • •• • • • •

• • • •• • • •• • •

• • •INTRODUCTION •••••••••••••Previous Work ••••••••••Acknowledgements • • • •

SALINITY DETERMINATIONS, RANGES, AND COLLECTIONOF DATA ••••••••••• "•••• • • • • • • • • • • • • • • 3

RELATIONSHIP OF GROUND-WATER LEVELS TO FRESH­SALT- WATER INTERFACES •••••••••••••• • • • • 4

OCCURRENCE OF SALT WATER IN AQUIFERS •••••••••• 6Holocene Aquifer s •••••••••••••••••••••• 6Columbia Aquifer s • • • • • • • • • • • • • • • • • • • • •• 6Chesapeake Group Aquifers ••••••••••••••••• 11Paleocene-Eocene Aquifers ••••••••••••••••• 12Marine Cretaceous Aquifers • • • • • • • • • • • • • • • •• 16Magothy Aquifer s • • • • • • • • • • • • • • • • • • • • • •• 16Potomac Aquifer s •••••••••••••••••• •• 18

PUMPING LIMITA TIONS • • • • • • • • • • • • • • • • • • • • • • • 19

RECOMMENDATIONS FOR FUR THER STUDY · . . . . . . . . . . 20

SUMMARY . . '. . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

REFERENCES • • •• • • • • C) • • • • • • • • • • • • • • • • • • • 44

ILLUSTRATIONS

Figure 1. Generalized geologic map of Delaware (after Spoljaricand Jordan, 1966) • • • • • • • • • • • • • • • • • • • • 9

2. Geologic cross section of Delaware (Modified afterJordan, 1962) •••••••••••••••••••• • • 10

3. Map of fresh- salt-water interface positions accordingto Upson, 1966 and Back, 1966 showing key welllocations • • • • . • • • • • • •• • • . • • • • • • • •. 17

Figure 4.

5.

6.

ILLUSTRATIONS (Cont.)

Well locations for which chloride analyses areavailable in New Castle County, Delaware ••

Well locations for which chloride analyses areavailable in Kent County, Delaware •• 0 •••

Well locations for which chloride analyses areavailable in Sussex County, Delaware ••••

TABLES

· . . .

• • • •

• • • •

Page

22

23

24

Table 1.

2.

Chloride analyses of water from Holocene sands ••

Theoretical water levels required in selected wellsto maintain a fresh-water aquifer at given depths••

• •

. .

8

15

3. Chloride analyses of water from Delaware wells ••• 26

THE OCCURRENCE OF SALINE GROUNDWATERIN DELAWARE AQUIFERS

ABSTRACT

The location of the fresh- salt-water- boundary in the deeper aquifersof Delaware is related mainly to head values. Near coastal areas, dynamicconditions may prevail that affect the interface position within shallow aqui­fers open to the sea.

Holocene and Columbia sands which form Delaware's shallow water­table aquifers contain brackish water in scattered coastal areas while brack­ish water in the artesian aquifers is found at various depths. Water fromChesapeake Group sediments (Miocene) is fresh in Kent County but is saltyin poorly defined areas of Sussex County. The interface in the Piney PointFormation (Eocene) lies just north of Milford and extends in a northeast­southwesterly direction across the State. Brackish water exists in theMagothy and Potomac formations of Cretaceous age a few miles south ofMiddletown.

Heavy pumping near sources of brackish water should be avoidedfor the present. Proper location of monitoring wells is necessary fordetection of future chloride movement.

INTRODUCTION

Ground water is one of Delaware's most important natural resources.All of Kent and Sussex Counties (excepting some irrigation from streams)and about 55 per cent of the population of New Castle County depend onground water to supply their daily water needs. The State has many water­bearing formations but in some areas the use of these aquifers is limitedby the nearness of two major types of salt-water sources: (1) large sur­face bodies of saline water including the Delaware and Chesapeake baysand the Atlantic Ocean; and (2) saline water presently existing within cer­tain aquifer s. The exact reason for the occurrence of high chloride waterwithin aquifers some distance from the ocean is not always known. Informations deposited under marine conditions sea water may have beentrapped within the sediments at the time of deposition or, in some cases,salt water may have entered due to higher sea-level stands during thegeologic past. It is improbable that there is a direct hydraulic connectionwith the ocean to artesian aquifers under inland areas even though suchaquifers may now contain saline water. Lithologic changes within asingle formation and the presence of overlying silts or clays usuallyimpede or modify ground-water flow.

1

Increasing population and water use have caused some concern aboutthe possibility of salt-water encroachment. Such fears are not entirely un­founded because improper management of the State's ground-water resourcescould indeed lead to encroachment under certain conditions ..

This study (1) evaluates existing data and chemical analyses pertinentto the salt-water problem, (2) points out those areas where further data areneeded, and (3) recommends or reaffirms those areas where ground-waterdevelopment should be limited because of potential salt-water problems.

Previous Work

In 1958 Barksdale and others discussed the theoretical position andpossible movement of water in Cretaceous aquifers under and adjacent tothe lower Delaware River. Their postulated fresh-salt water interfacepositions appear today to be essentially correct, at least for Delaware.Rasmussen and others (1958) studied the chloride concentrations in aquifersalong the Chesapeake and Delaware Canal. Three monitoring wells wereinstalled in an attempt to determine the effect of deepening and wideningthe Canal. It was tentatively concluded that such construction would in­crease the opportunity for recharge from the Canal to then unaffectedaquifer s by a small, but unknown, amount.

More recently, Back (1966) and Upson (1966) have separately con­sidered the problem for the northern Atlantic Coastal Plain of the· UnitedStates. Recent drilling has indicated that in Delaware the location oftheir postulated fresh- salt water interface for Cretaceous aquifers isbasically valid.

Acknowledgements

Analyses included in this report were made by a number ofagencies. Most analyses before about 1962 were made by or underthe direction of the U. S. Geological Survey either independently orin cooperation with the Delaware Geological Survey. After that dateanalyses have been made by private consulting laboratories, variousindustries, the Delaware Water and Air Resources Commission, andthe Delaware Geological Survey.

Dr. John C. Kraft of the Department of Geology, University ofDelaware, provided correlations on parts of the cross section of Dela­ware (Figure 2). Dr. Robert R. Jordan, Mr. Nenad Spoljaric, andDr. Thomas Pickett of the Delaware Geological Survey were especially

2

helpful in discussions of stratigraphic problems. Dr. J. J. Groot,Mr. Ray Sundstrom and Dr. Robert D. Varrin of the University ofDelaware reviewed the manuscript and offered many helpful sugges­tions.

SALINITY DETERMINATIONS, RANGES,AND COLLECTION OF DATA

Salinity of water is usually expressed in terms of the chlorideion content, the chloride being initially in the form of sodium chloride.Titration, or reacting, with a measured amount of silver nitrate is thestandard laboratory procedure for determining the exact amount ofchloride in a water sample. Other analytical methods do exist andselection of technique depends on the materials available, personalpreference, and accuracy desired. Chloride concentration is usuallyexpressed as parts per million (ppm), i. e., parts by weight of chloridein one million parts of water. Lately, efforts have been made to stan­dardize analytical results and to express all concentrations in milligramsper liter, another measure of weight. For most practical purposesmilligrams per liter is equal to parts per million.

Sea water has a chloride content of about 18,000 ppm. Actualchloride ion concentrations in Delaware aquifers that have undergonesalt-water intrusion or have residual chlorides do not appear to be ashigh as that of sea water except in one or two cases. The highest con­centration found in any aquifer to date is about 17,000 ppm of chloride.However, as will be discussed below, this is not a typical occurrence,and more commonly two or three thousand ppm of chloride is maximum.

Fresh water in Delaware and most of the eastern United Statesis usually considered to be water with a chloride content under 250 ppm.This is the highest limit of concentration recommended by the U. S.Public Health Service. Few people can taste a concentration of 250ppm as the taste threshold for most individuals seems to be around500 to 600 ppm. Indeed, in some western portions of the country waterwith 500 ppm chloride is locally considered to be fresh.

Water for industrial and irrigation use often requires lowchlorides. Certain industrial processes cannot tolerate even a fewppm and Rima et al (1964) state that any concentration over 100 ppmis toxic to most plants.

Chlorides in Delaware's tidal surface waters vary considerablyand consequently affect the water use. In Delaware Bay, for example,chlorides may range from a few hundred parts per million at Delaware

3

Memorial Bridge to essentially sea water concentrations in the centralportion of the Bay. Tidal fluctuations exist as far north as Trenton,New Jersey and during periods of low fresh-water inflow the 50 ppmchloride line may extend up river into the Philadelphia area. Thesalinity in smaller ungated, tidal streams also varies greatly, depend­ing on tide and fresh-water run-off. During high run-off periodswater in these streams is fresh while at other times the chloridesat the same points may reach as high as 6500 parts per million. Thispoints out the need for careful management of the shallow ground-wateraquifers which contribute the bulk of fresh-water inflow to Delaware'ssurface streams in the Coastal Plain.

Chemical analyses and ground-water levels used in this reportwere obtained mainly from Delaware Geological Survey files and fromexisting publications. Some additional field data were recently collectedby the Delaware Survey and the Delaware Water and Air ResourcesCommission. A few chemical analyses were not used because ofquestionable field procedure or disagreement with duplicate samples.Except for those analyses made by the Delaware Geological Survey,the sampling procedure was not always known. Factor s such as thepumping time before sampling and the depth of the water sample mayhave effected some results. Because of the variable nature of localwater supply problems, available data are not uniformly distributedwithin the State. In those areas where a great deal of informationexists it was not considered necessary to list all available analyses,especially where chlorides are known to be due to waste discharges.

Well locations in this report are numbered according to theDelaware Geological Survey's well numbering system. In this system';;the State is divided into 5-minute blocks of latitude and longitude witheach block being designated by a combination of one upper case andone lower case letter. These two letters constitute the first part ofthe well number.

Each 5-minute block is further subdivided into 25 one-minuteblocks, each one-minute block bearing a two digit number accordingto its location in the 5-minute block. This number follows the twoletters in the first part of the code. The last part of the code is asequential number which indicates the order in which the well in­formation was "scheduled" or entered into the well files. The loca-tion of each well is plotted on a series of master grid maps and retainedpermanently.

RELATIONSHIP OF GROUND-WATER LEVELS TOFRESH-WATER INTERFACE

The boundary between fre sh and salt water in an aquifer, orwater- bearing stratum, can be sharp or can be a mixed or diffused

4

wedge- shaped zone of varying height and width. The exact shape ofthe interface depends on a number of factors including permeabilityand aquifer thickness. In monitoring any movement of the interfaceit is important to locate the diffused zone or the edge of the interfacerather precisely to be sure of detecting any actual movement. Ingeneral, under static conditions in the deeper aquifer s the interfacewill occur where the head or weight of fresh water balances or equalsthe weight of underlying salt water. A larger volume of fresh waterthan salt water is required for this balance because of the greaterdensity of salt water. Also, some loss of fresh-water head may occurin a long flow path. More precisely, salt water would theoretically befound about 41 feet below sea level per foot of fresh water above sealevel according to the Ghyben-Herzberg Principle. Mathematicallythis can be stated as:

h= (d-l)Hwhere:h=height of fresh water above sea leveld=density of salt waterH=total depth of fresh water below sea level

(1)

Taking the density of sea water as about 1.024 and the freshwater head as 1 foot then equation (1) becomes:

hH=- =

d-l

H=4l feet

1

1. 024-1(2)

Water levels in artesian aquifers can, therefore, be a generalguide to the depth at which the interface can be expected. The chlorideconcentration at the interface would not usually be the same as that ofthe adjacent salt or brackish water body, but would be of some lowervalue due to mixing occuring at the interface. Such mixing apparentlyoccurs to some degree even under essentially static conditions.

In shallow aquifers near coastal areas open to the sea, otherfactors may modify the interface position. Experimental work byCahill (1967) indicates that a cyclic flow exists on the salt-water sideof an interface depending on tidal amplitude and the amount of freshwater inflow. Salt water may thus be continually moving landwardonly to be swept seaward again under the influence of a lateral freshwater head. This had been suggested earlier by a number of workersincluding Cooper and others (1946).

5

OCCURRENCE OF SALT WATER IN AQUIFERSHolocene Aquifer s

Along coastal areas the size and pattern of land forms is con­stantly changing.. The process may be due to the gradual work of wind,currents, and waves, or may be extremely sudden such as occurredin Delaware during the devastating storm of 1962. In any case, newlyformed shoreline features may have salt water trapped within theirsediments. Also, the nearness of such f~atures to salt water providesopportunity for hydraulic connection with the ocean. In Sussex County,Delaware, there are many instances of high chlorides in what are main­ly Holocene deposits. At Indian River Inlet, a chloride content of17,150 ppm was recorded at about 11 feet in test hole Pj42-11 (seeFigure 6 for location). At 40 feet chlorides were also high, about14,600 ppm. Approximately two and one-half miles north of IndianRiver Inlet, in hole Pj2l-3, the chloride content of water squeezedfrom core samples varied from 650 ppm to a high of about 6900 ppmdepending on depth. Chlorides in test holes Pj22-l and Pj22- 2, bothdue west of Pj2l-3, also show similar variations in salinity with depth.

Table 1 is based on an unpublished report by Peter B. Smoor,hydrologist, formerly with the Delaware Geological Survey, and showsthe results of chloride analyses made on water samples squeezed fromcores, including data from the test holes mentioned above. The coreswere taken in a series of test holes located from Indian River Inletnorth to Cape Henlopen. The possibility exists that some of thesetest holes may penetrate both Holocene and older, Columbia, deposits.South of Dewey Beach,· Columbia, rather than Holocene sediments, areknown to outcrop both inland and along beach areas (J. C. Kraft, per­sonal communication). However, in this case, the difference is notsignificant from a hydrologic standpoint as both units are water-tableaquifers. Probabty no large scale pumping would be feasible from anyshallow wells in this area and in most instances there is no guaranteethat even small yield domestic wells will tap fresh water. However,there are scattered coastal areas where small amounts of fresh waterare skimmed off the top of underlying salt water by wells as shallow asten feet.

Inland, known sediments of Holocene age are not extensive orthick enough to be seriously considered as water- supply sources.

Columbia· Aquifers

Throughout most of Delaware sediments that were either deriveddirectly from glacial melt-water or were otherwise deposited during

6

the Pleistocene (glacial) Period mantle the surface and overlie oldersediments. These rocks are referred to as the Columbia Formationin the northern part of the State and the Columbia Group in SussexCounty where more than one formation of Pleistocene age exists(Jordan, 1962). In New Castle County, the Columbia Formation isgenerally thin, having an average thickness of about 30 feet exceptin paleochannels where the maximum known thickness is about 105feet. The Columbia thickens to the south and in Sussex County themaximum thickness exceeds 150 feet. Much vertical recharge todeeper aquifers probably takes place through the Columbia Forma­tion. Hundreds of shallow domestic wells and many irrigation wellstap Columbia sands and in northern Delaware some high-yieldingindustrial wells are located in Columbia paleochannels. In Kent andSussex Counties many municipal wells also are screened in theColumbia.

The data reveal a few incidents of abnormally high chloridesin known Columbia aquifers. Three such cases - New Castle, Lewes,and Rehoboth Beach have been known for some time and have beendiscussed by Marine and Rasmussen (1955). Lewes has alleviatedits problem by drilling new wells farther inland with resultant lower­ing of chlorides in the older wells nearer Delaware Bay, indicatingthat a reversal of the intrusion took place. Getty Oil Company re­ported another instance of brackish water from well Ec13-ll (seeFigure 4). Chlorides in this well were as high as 360 ppm duringAugust, 1966. This is probably indicative of general lateral migra­tion of salt water in the Delaware City area. Pumpage from thiswell, which taps a Columbia channel, has been curtailed. In allcases contamination was induced from nearby surface bodies ofsaline or brackish Water.

Recently, high chlorides occurred in well Ec23-6 (see Figure 4)located at the Gunning- Bedford School just north of the Chesapeake andDelaware Canal. In August, 1967, a high iron content and unusualtastes were reported in water from this well. However, no analyseswere made until January, 1968 when over 800 ppm chloride werefound. The well penetrates at least 80 feet of Columbia material andanalysis of electric logs made in recent test holes shows the highchloride water to exist from about 70-100 feet below land surface.The apparent immediate source of chloride appears to be the dredg-ing spoil area on the north bank of the Chesapeake and Delaware Canal,1000 to 1500 feet south of the well. Very probably the high chlorideswill eventually be flushed from the area but there is no indication asto how long this might take.

7

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EXPLANATION

r.-:.v:::<l~

AMPHIBOLITE

r rWISSAHleKO. SCHiST

FaDENS! MICACEOUS SCHIIT,.NEII8AND MIIMATITE

LOW EftPALEOZOte G~:~~" COCKEYSVILLE MARBLE

(?) ~

D!NIE, WHITE CRYSTALLINELIMESTONE AND DOLOMITE

I II II I

f:;:··: ~ISERPENTINE

COWMBIA'M.

DJIIJIlYELLOW AND REODtlH BROWNQUARTZ SAND WITH GRAVELAND LITTLE CLAY

[~'Orn~~ARDEN GRANITE

~~

8AIIPIO

1:.:0.1]IANDEO INEIII

SHORE-LINE DEPOSITS

I!m!m

c:::!QPEGMATITE

WENONAH I"M.

Q!JGRAY AND RUST-IROWN FINE TOMEDIUM, MICACEOUS, SPARINGLYGLAUCONITIC OUAftTZ SAND

REDBANK 'M.~

ftEDDISH· BftOWN. SLlIHTLYMICACEOUS AND" ILAUCONITIC,FINE TO MEDIUIII lAND

liT. LAUREL -NAVESINK Fill.

I Kmlns IDARIt' IRI!ENISH-IftOWN AND DARIt'IRAY, HI8HLY GLAUCONITfC IANDYSILT AND IILTY SAND

POTOMAC FM.

~VAlUEUTe:O liLTS AND CLAYSWITH 1101 0' QUARTZ lAND

YELLOW AND WHITE, WELL IORTED.MEDIUM QUARTZ lAND OflBEACHI!IAND DUN!I

ESTUARINE DEPOSITS

mml1

MERCHANTVILLE FM.

~DARK GftAY TO BLUIIH-GRAYSLIGHTLY 'LAUCONITIC, MICACEOUSSILTY, VERY FINE lAND

I: ·,rd. :1POftT DEPOIIT GftANOOIORITE

MOTTLED, GRAY AND BROWN, SILTAND CLAYEY QUARTZ SAND

OTHER NEARSHORE DEPOSITS

~YELLOW AND GRAY QUARTZ SANDWITH SOME INTERBEDDED gRAYliLTS; CHIEFLY NERITIC.O,,.HOREBAR, AND LAgOONAL 'ACIES

MAGOTHY FM.

~WHITE AND lUII'll' QUAftTZ ....NOWITH IEDI OP' GRAY AND ILACKCLAYEY liLT~

MONMOUTHBftOUP

MATAWANGROUP

WILMINGTONCOM~LEX

AGE UNDETERMINED

LOWER ~CRETACEOUS

II

BRYN MAWR FY.

h·~~.':·:;JftED AND IROWN QUARTZ lAND WITHSILT. CLAY AND FINE .RAVEL

C~

MIOCENE ~BWISH .ftAY SILT WITH QUARTZSAND AND SOME IHELL IEDS

C-" UNITC

~GftAYIIH- GftEEN, CLAYEY

EOCENE GLAUCONITIC SILT AND lAND

RANCOCAS FII.

~

.Al.EOCENELGRAYISH-GREEN AND GftEEN,IILTY,GLAUCONInC lAND

UNIT B

QQGREENISH-GRAY SANDY AND CLAYEYGLAUCON I11C II LT

UPPERCRETACEOUS

T!RTIARY

QUATPNARY ~LEIITOC!N

10ICAL! 0' MILES

•

MARYLAND

Figure 1. Generalized geologic map of Delaware (after Spoljaricand Jordan, 1966).

9

Most of the natural flow of water within the Columbia is towardexisting surface streams and water bodies. As long as local pumpingdoes not reverse the gradient, lowered stream flows and salt-waterintrusion will not be a problem. The exact amount of pumpage necessaryto do this depends on the individual situation and few areas in the Statehave been evaluated in this regard. Trained personnel, basic data, andtime have so far been insufficient to permit much detailed study. Incoastal areas some fresh water can usually be found in the upper partof the Columbia, especially north of Lewes, although there are someexceptions. Baker et al. (1966) reported salt-water contamination inthe shallow Columbia sands at Slaughter Beach. Also, as mentionedabove, the high chloride content of water from test holes south of DeweyBeach could be at least partly from the Columbia. Salt-water canusually be expected in the deeper Columbia in southern Delawarecoastal areas. Although hydraulic connection with the ocean is probablythe main reason for high chlorides, upward vertical leakage from theunderlying Chesapeake Group could account for higher than normalchlorides in some Columbia wells near the coast.

Chesapeake Group

Miocene age sediments crop out in southern New Castle Countyand thicken towards the southeast as shown in Figures 1 and 2. Through­out Maryland and Delaware they are known collectively as the ChesapeakeGroup. Generally, south of about Dover, at least two distinct sandswithin the Chesapeake Group can be recognized, and are known informallyas the Frederica (upper) and Cheswold (lower) aquifer s. These aquifer sare an extremely important source of water for central and southernKent County. In areas of present pumping, natural chlorides are lowand no evidence can be found of induced abnormally high chlorides. Wateranalyses from a recently drilled well at Milford (Me15-29) show thechloride content of water from the Cheswold (?) aquifer to be 64 ppm.There is no reason to believe that there would be any danger of salt-waterintrusion considering the locations of present and potential heavy pumpingareas in central Delaware.

In Sus sex County the situation is somewhat different. Inland, theFrederica and Cheswold aquifers are thought to be fresh in the northernand western part of the county although there are few data available.Data are especially meager for the southern inland portion of SussexCounty due to the lack of wells drilled to the Chesapeake Group. AtSlaughter Beach, Baker et ale (1966) reported fresh water in wells tap­ping the Frederica and Cheswold aquifers at depths of 240 to 300 feet.At Prime Hook Beach local drillers report fresh water at about 300 feetbut salt water at shallower depths. This is believed to be substantiatedby electric logs from the area. However, the Cheswold and Frederica

11

aquifers apparently contain brackish water in most central and southernSussex County coastal areas. Water from well Pjll-l (see Figure 6for location) near Dewey Beach reportedly contained 1200 ppm chloridesat 300 feet and also at 530 feet. Water from well Nh13-l, drilled in 1940at Broadkill Beach, reportedly was high in chlorides at 380 feet althoughthe evidence is poor. However, the theoretical interface or 300 ppmisochlor line at this depth according to Back (1966), apparently basedon water levels is several miles south of Broadkill Beach. In general,the fresh-salt-water interface appears to cut across the southeast cornelof Delaware somewhere near Rehoboth Beach and extends in a south­westerly direction. At present, the exact position of this line is notknown with certainty.

In Sussex County two additional sands, younger and thus higherin the stratigraphic section than the Frederica and Che swold, havebeen recognized and are called the Pocomoke and Manokin aquifers.It is not everywhere possible to distinguish these individual sand layersbut this classification has proven quite helpful in dealing with strati­graphic problems of a general nature. Usually water in the Pocomokeand Manokin aquifers is low in chlorides. Data for well Oj22-l, whichtaps the Manokin at Sussex Shores, shows brackish water at 80 feet butfresh water at about 178 feet. Approximately 50 feet of clay or sandyclay is present between the two strata, the upper of which is probablyof Pleistocene age. The electric log of a well at Bethany Beach,Oj32-l0, also seems to indicate fresh water from about 185 feet to215 feet, but brackish water occurs in sands shallower than 185 feet.Two analyses, first reported by Rasmussen and others (1960), areevidence of local, apparently natural, intrusion in the Manokin aquifernear Lewes due to the nearness of Delaware Bay and the Lewes-RehobothCanal.

Paleocene-Eocene Aquifers

The major Paleocene-Eocene aquifers in Delaware are theRancocas and the Piney Point formations, the Piney Point being slightlyyounger than the Rancocas Formation. As can be seen on Figure 2,the Rancocas outcrops in south-central New Castle County and dipsin a southeasterly direction beneath younger sediments. South ofSmyrna the Rancocas appears to grade into a silt called Unit C (Jordan,1962) and interfinger s laterally into another silt named Unit A. Theelectric log of a deep well (Id31- 26) drilled north of Dover and south ofCheswold shows about 10 feet of Rancocas sand and only a short distanceto the south of this well the Rancocas disappears entirely.

Very few reliable chemical analyses exist for water from thePaleocene-Eocene sediments, particularly the Rancocas Formation.

12

Data from one well, Hd44-1, drilled in 1957 about four miles southeastof Smyrna, shows the water to have a chloride content of over 900 ppm.Correlation by Kraft et ale (1966) on the basis of electric logs indicatesthat the bottom of the well is in Unit C, a short distance above the topof the Rancocas, although Unit C is not usually water-bearing. Theanalyses on this well are thought to be entirely reliable as two commerciallaboratories are in reasonable agreement on the chloride content. Further­more, the static water level within the aquifer was only two to three feetabove sea level at time of d~illing and according to the Ghyben-Herzbergprinciple at least five feet of fresh-water head would be needed to main­tain a fresh-water aquifer at this location and depth (see Table 2). It isquite possible that the high chlorides are residual, due to incompleteflushing of ancient sea water. However, pumping from the Rancocas inthe Clayton area, recorded in the period from 1943 to 1966, could alsolower the levels near Smyrna as shown by calculations which used atransmissibility of 16,800 gallons per day per foot and a storage co­efficient of .00019 (aquifer coefficients from Sundstrom et ale 1968).

Data from well He12-1, drilled to the Paleocene (Rancocas?) in1946 at Woodland Beach indicates the water is brackish and not suitablefor drinking. Unfortunately, no actual analyses were made available.As shown in Table 2, the static level was four feet above sea level and,theor~tically, should be slightly greater than four feet to prevent salt­water encroachment.

Based on available information, it appear s then that salt-waterexists in at least part of the Rancocas Formation and possibly in Unit Calso. A brackish-water line apparently extends from Woodland Beachon Delaware Bay in a southeasterly direction, passing about two to threemiles south of Smyrna. Farther to the north, the level in well Fb33-3,drilled in 1930 to the Rancocas Formation at Middletown was over 44feet above sea level, more than enough to inhibit salt-water intrusionat this location. Little or no control exists other than the wells mentionedabove and it is presently not possible to extend the line farther southwest withconfidence. Moreover, it is possible that part of the chloride content re­ported in these cases could be residual. Also, other unpublished work byKraft indicates rapid lateral facies changes within the Rancocas. Thusit becomes extremely difficult in northern Kent County to determine thecause or the possibility of salt water within the Rancocas and the Eoceneage sediments in general.

The Piney Point Formation, the other major aquifer in the Paleoceneand Eocene series does not crop out in Delaware but underlies the Chesa­peake Group in the Dover area. To the north it grades into Unit A ofJordan (1962) and is absent, as an aquifer at least, a mile or two southof Cheswold. However, there is electric log evidence that the PineyPoint as a geologic unit is found north of Dover but has become finergrained. The thickest section of Piney Point found in Delaware so far

13

is at Dover Air Force Base. To the south and westward into Maryland thePiney Point thins and loses its identity in southern Sussex County (see Figure2). The Piney Point Formation is one of the more important water producersin central Delaware, yielding as high as 1100 to 1200 gallons per minute atthe Dover Air Force Base and about 600 gallons per minute in central Dover.Farther to the south as the formation becomes finer grained the yield dropsoff considerably. At Milford, Delaware a maximum yield of only 65 gpmwas obtained in a partially developed test well (Me15-29) drilled in January,1968 and pump test results showed a very low transmissibility.

Piney Point water in areas of present pumping is low iin:chlorides.One of the earlier reports on Piney Point water quality was from wellsHe45-1 and He45-2 drilled at Bombay Hook in 1938 and 1940 respectively.These wells were reported to flow above ground surface at the time ofdrilling and the water was reported to be fresh. A driller's report alsoexists for another Piney Point (?) well, Hd52-2 drilled in 1953 a littleless than two miles southeast of well Hd44-1 (see Figure 5). The waterquality was reported to be good although no actual analyses were made.The static level in Hd52-2 was measured as nine feet below sea level atthe time of drilling. Normally, high chlorides would be expected undersuch conditions in a Delaware aquifer.

The static level at the time of drilling in well Je32-4 (Dover AirForce Base), which is screened both at the top and bottom of the PineyPoint and produced fresh water, was also lower than might theoreticallybe expected (see Table 2). The stratigraphy of the Piney Point and theoverlying and adjacent formations probably playa role in keeping theaquifer fresh in Kent County.

The Piney Point undoubtedly contains brackish or salty water tothe south. Prior to the drilling of a Piney Point well at Milford (Me15-29)there was some question as to the exact location of the interface in thatarea. Two screens were installed in the Milford well in an attempt tolocate more exactly the depth of the interface. One screen was placedfrom 640' to 700' (land surface datum), near the top of the aquifer, andone near the bottom of the Piney Point from 780' to 785'. A series ofwater quality samples were taken and samples for both zones containedabout 537 ppm chloride. The static level was one foot above sea levelin February, 1968 while according to the Ghyben-Herzberg principle afresh-water head of at least 18 feet above sea level should be needed tomaintain a fresh-water aquifer at this location. On this basis, the inter­face is probably at least a mile or two north of Milford. On Figure 3,the postulated interface extends in a northeast- southwest direction acrossDelaware and consequently passes just north of Milford and thencebeneath the southwest corner of Delaware. Some questionable furthersupport for the existence of salt-water in the Piney Point is providedby data from an 1080 foot well drilled in 1898 at Lewes. Correlationby Rasmussen and others (1960) indicated that the well penetratedEocene sediments and water from the interval 1064 to 1080 'PineyPoint?) reportedly was saline. The stratigraphic correlation howeveris in some doubt (Jordan, personal communication) and no trace of thewell can be found today.

14

Tab

le2

-T

heo

reti

cal

wate

r-le

vels

req

uir

ed

inse

lecte

dw

ell

sto

main

tain

afr

esh

-wate

raq

uif

er

at

giv

en

dep

ths.

Wel

ln

o.

Fo

rmati

on

or

aq

uif

er

scre

en

ed

an

dsc

reen

sett

ing

(feet

bel

ow

msl

)

Dep

thto

bo

tto

mo

faq

uif

er

(feet

be­

low

msl

)

Sta

tic

lev

el

at

tim

eo

fd

rill

ing

an

dd

ate

(feet

abo

ve

msl

)

Th

eo

reti

cal

lev

el

req

uir

ed

tom

ain

­ta

infr

esh

-wate

raq

uif

er

(feet

abo

ve

msl

)

Ch

lori

des

pp

m

bd

33

-4(a

)PM

ag

oth

y.

---·

--6

58

55

/61

15

.6

(scre

en

sett

ing

60

7-6

39

)

27

0

Gd

33

-4(b

)M

on

mo

uth

(37

6-4

09

)4

58

3.4

5/6

11

0.9

8

Hd

44

-l

Hel2

-l

Id3

1-2

6

Je3

2,,

:4

Eo

cen

e(1

90

-2

50

)

Ran

co

cas

(op

enat

21

9)

Mag

oth

y

Pin

ey

Po

int

(33

7-3

67

,5

31

-54

1)

25

03

5/5

76

.09

22

(bo

tto

mo

fsc

reen

)*

21

94

19

46

5.1

rep

ort

ed

(bo

tto

mo

fb

rack

ish

well

)*1

03

41

6/6

62

4.8

94

6

56

95

.74

/57

13

.16

Me1

5-2

9P

iney

Po

int

(63

3..

79

3,

77

3-7

78

)

79

81

1/6

81

8.1

53

7

*D

epth

tob

ott

om

of

aq

uif

er

no

tk

no

wn

.

15

Marine Cretaceous Aquifers

The marine Cretaceous sediments in Delaware include the Monmouthand Matawan groups. The younger Monmouth Group has been divided in theoutcrop area into the Redbank. Formation and the underlying Mt. Laurel­Navesink. Formation; the Matawan Group includes the Wenonah and Mer­chantville formations. However, downdip all four of these formationslose their identity and the terms Monmouth and Matawan are lowered toformation status.

In general, the marine Cretaceous sediments are silts and veryfine sands and therefore are not high-yielding aquifers. Some water­bearing sands do occur, but, they are generally very thin. Chloridesare low from the Monmouth sediments in central New Castle Countyand, in view of the light with,drawals, there is no immediate reasonto expect any occurrence of high chlorides in areas of present pumping.However, head values for the Monmouth Formation in well Gd33-4(a)on Thoroughfare Neck, in southeastern New Castle County indicate thatthis well location is probably close to an interface. Table 2 shows thecalculated head value necessary to theoretically limit salt water intru­sion at this point. In the vicinity of the Chesapeake and Delaware Canalground-water discharge appears to be towards the Canal, thus limitingintrusion from this source (Rasmussen et al., 1958).

Magothy Formation

The Magothy is recognized as a transitional unit between youngermarine units and the older non-marine Potomac Formation. TheMagothy Formation crops out in the Chesapeake and Delaware Canalnear Chesapeake City, and generally dips to the southeast. Magothysands form an important aquifer in northern Delaware and, except forsome local thinning, generally persist beneath the entire State southof the outcrop area. Several analyses confirm the presence of brack­ish water in the Magothy south of a northeast- southwest trending linepassing about five miles south of Middletown. Water from wellGd33-4(b) near Deakyneville, in southern New Castle County containsabout 300 ppm chloride and farther south, water from the Magothy wellId3l-26, located between Dover and Cheswold, contains over 1000 ppmchloride. These analyses are in good agreement with the theoreticalinterfaces proposed by Upson (1966) and by Back (1966).

The presently fresh-water bearing portions of the aquifer lieadjacent to surface salt or brackish-water sources in New CastleCounty and thus careful management of the aquifer is necessary.These sources are (1) the Chesapeake and Delaware Canal, (2)Delaware Bay to the east and (3) possibly, the Chesapeake Bay to thewest. Atpresent, where the Magothy underlies the Canal, the freshwater gradient within the Magothy appears to be towards the Canal

16

.ld31-26I

\II

\,•

\ ~"-29, / Milford

~------/T,I

\ srid,n,,:: ?

• I

is..... I:1\ I, I

'lL!I--------

tN

I

0 5 10 mil••i

• well locatioll

'.n.n".n.. approximate localion of interface

I. Cretaceoul 350 ppm chloride line.Back (1966).

2.Cratanoul 250 ppm chloride line.Uplon (1966).

3. Piney Poinl Formation 250 ppmchlorl d. lin., modified ofter Uplon (1966).

Figure 3. Map of fresh-salt-water interface positions according toUpson, 1966 and Back, 1966 showing key well locations.

17

(Rasmussen et at., 1958) and salt-water intrusion would not occurunless future pumping reversed the gradient. Beneath Delaware Baythe situation is somewhat more complicated with regard to both thestratigraphy and ground-water movement. Apparently fine-grainedHolocene sediments overlie the Bay floor from at least DelawareMemorial Bridge south to about Port Mahon (Jordan etal., 1962).In addition, the Merchantville Formation, younger than the Magothy,may overlap the Magothy in a northerly direction (Spoljaric, personalcommunication) and help prevent direct connection with the Bay. Thisis not known with any certainty, however, and much work remains tobe done on the stratigraphy beneath Delaware Bay. Grqund-waterflow directions along and under Delaware Bay south of New Castleare probably towards the Bay. Farther to the northeast in thePhiladelphia- Camden area, local pumping has rever sed thisgradient toward the centers of pumping (Barksdale and others, 1958).No actual cases of intrusion into the Magothy due to pumping havebeen reported in southeastern New Castle County and in general,pumping from the Magothy in the Delaware Bay area is light.

The possibility of intrusion from Chesapeake Bay appears tobe less of a problem than the other two sources mentioned above althoughan interface probably exists in the Chesapeake Bay area (Back, 1966).Sundstrom et al. (1967) believe that pumpage from the older PotomacFormation in the western end of the Chesapeake and Delaware Canalwould have little effect on this interface and the same should alsogenerally hold true for the Magothy.

Potomac Formation

The Potomac either outcrops or lies directly beneath a cover ofColumbia sediments over about 95-100 square miles in New CastleCounty. It is the chief water-producing formation of northern Delawarewith the possible exception of Columbia paleochannels. The strati­graphy and basic hydrology have been rather extensively covered inother reports, especially by Sundstrom et al." (1967), and thus willnot be treated here in detail. Sands within the Potomac Formation,like those of the Magothy, are geographically close to large brackishsurface water bodies. In the Chesapeake and Delaware Canal area,flow does not appear to be toward the Canal, as in shallower aquifer$,but rather is toward centers of pumping. In this case, the center ofthe main cone of depression is in the Delaware City area at the easternend of the Canal. Thick intervening clays apparently exclude at leastlocal recharge to the Potomac from surface water sources and saltwater intrusion is not generally thought to be an immediate threat.Sundstrom et al., (1967) has given considerable attention to this problemand reference should be made to this paper for detailed explanations.

18

Over-all, the possibility of declining water levels are in themselvesthought to be more of a problem than salt-water intrusion.

Water from the Potomac at some point downdip undoubtedlycontains high chlorides •. The theoretical fresh- salt-water interfaceappears to occur at about the same position as that for the Magothy(see Figure 3). Geophysical logs from well Gd33-4, which penetratedPotomac sediments, indicate the presence of high chlorides and tendto confirm the 250 ppm chloride line postulated by Upson (1966) andthe 350 ppm line of Back (1966). In addition, Rima and others (1964)place the fresh- salt-water interface (chloride value not specified) innearly the same position as did Upson (1966) but apparently somewhatsouth of the 350 ppm line proposed by Back (1966). The electric logof the deep well at the Dover Air Force Base (Je32-4) likewise mayindicate the presence of chlorides in the upper part of the Potomac.Only a single point resistivity log is quite tenuous. The interfaceapparently bends north quite sharply on the New Jersey side ofDelaware Estuary, reflecting pumping in that area. It should benoted that the interface position in Delaware as shown on Map I-514-Bof the map series "Engineering Geology of the Northeast Corridor,Washington, D. C. to Boston, Massachusetts: Coastal Plain andSurficial Deposits (1967)" is inconsistent with present data.

PUMPING LIMITATIONS

Two general situations exist where large ground-water withdrawalsshould be controlled to prevent salt-water intrusion. These are (1) at orjust on the fresh-water side of the interface in artesian aquifers, and(2) in the water-table or shallow aquifers immediately adjacent to saltor brackish water bodies. In Delaware's artesian aquifers however,the fresh-salt water boundary does not always appear to be well-definedbut rather seems to be a broad, diffused zone that may extend for severalmiles. Chlorides in such a zone are generally only a few hundred partsper million and seem to change very little along the dip of any particularformation. Data from both the Magothy and Piney Point formations inparticular indicate the existence of such zones. Pos sibly, thoseDelaware aquifers formed under marine conditions at least are stillin the process of being£lushed by fresh water and the chlorides nowpresent are remnants of ancient sea water. Moreover, dynamic con­ditions may prevail in shallow water-table aquifers of Pleistocene orHolocene age opening to the sea and cautious use of the Ghyben-Herzbergis necessary in all cases.

Specifically, additional large amounts of water should not bepumped from the Potomac and Magothy formations in a zone across theState bounded by Townsend on the north and Smyrna on the south, atleast until the movement of water within the formations call be monitoredand the exact nature of the interface defined. The same should holdtrue for the Piney Point Formation in an area bounded by a northeast-

19

southwest line two to three miles north of Milford and by a line passingthrough Milford on the south. However, some limited planned withdrawalscould probably be made relatively close to these arbitrary boundarieswith proper monitoring.

In most Atlantic Shore areas north of Cape Henlopen fresh watercould probably continue to be skimmed off the top by small withdrawalswithout undesireable results, especially if the withdrawals were seasonal.

However, any specific coastal area should be studied as anentity in itself. The ground-water flow patterns in the water tableand shallow aquifers are known to some degree but the detailedpicture is rather complex and any fresh- salt-water interface inthese shallow aquifers could probably be affected in a number ofways. Detailed stratigraphic knowledge of shallow sediments maybecome important when considering dredging projects, especiallyin Sussex County. As long as fresh-water heads are high the dangerof encroachment is minimal. Unrestricted dredging in salt-waterareas might initiate intrusion in certain cases. Thus in consideringany large scale ground-water projects, or even a number of small ones,it becomes important to know fresh-water heads and to predict theexact flow patterns.

Increased pumping from the shallow Columbia Formation nearNew Castle should be done cautiously and with adaquate monitoringdue to the nearness of Delaware Bay and the possibility of resultantintrusion. However, this would not apply to areas just west of thecity and east of U.S. Route 13 where a Columbia paleochannel offersexcellent yield possibilities with no apparent salt-water problems.

Finally, the possibilities of pumping from the Rancocas twoto three miles south of Middletown should be viewed with caution.Theoretically, an interface should occur somewhere in this area anda limited amount of field data does indicate the presence of brackishwater although the exact source of this water is not known.

RECOMMENDATIONS FOR FURTHER STUDY

Four specific lines of investigations are recommended uponreview of the existing data:

(1) Determine the nature and position of the fresh- salt-waterinterface in the Piney Point Formation in Delaware. The yield of theformation is generally high and further development is quite certain.In northern Sussex County and southern Kent County the exact positionof the interface is critical from a water- supply planning viewpoint.At least one deep well to the Piney Point one to three miles north ofMilford would be required as a minimum to locate and subsequently

20

monitor the interface. If increased development of the area took place,one or more wells could be added on a northeast-southwest line acrossthe State. Chlorides should be monitored at least once a year and waterlevels several times a year. Considering the high cost of well installa­tions, one well, if correctly located, would probably be adequate formany years, if no additional heavy pumping of the Piney Point occurred.Actually, the interface would move extremely slowly, if at all, andprobably is not moving any perceptible amount at present.

(2) Refine the generally known data on chloride distributionin the Magothy and Potomac formations. Such refinement is importantfrom the standpoint of development for southern New Castle andnorthern Kent counties. The Magothy and Potomac formations, withthe exception of Columbia channels, are generally the only reliableproducers of large amounts of water in the area. The Rancocas willyield, in places, as high as 500 gpm to properly developed wells,but more often the maximum yield is about 250-300 gpm. Again,one well to each formation just north of Townsend would provide much­needed control in this area. Water-level measurements and qualitydata should be taken on the same basis as outlined for the Piney Pointwell. Because of the several sand zones or hydrogeologic units withinthe Potomac there may be more than one interface depending on headrelationships between these sandy zones. This is illustrated by tworecently drilled wells (Ec32-3 and Ec32-7) near St. George's sClleenedin separate. sands within the Potomac and having a head differentialbetween the two sandy zones of about 30 feet. Figure 2 partiallydifferentiates two of these Potomac zones.

(3) Investigate the reported high chlorides in the RancQcasFormation in northern Kent County as suggested by analyses fromwell Hd44-1 and well He44-1. To confirm the initial analyses wouldbe a relatively easy matter but, to determine the position of anysalt-water interface by conventional drilling would be more expensive.Nevertheless, the presence of chlorides should be actually confirmedby at least one hole. Water-level measurements should also be madein wells tapping the Rancocas in order to define flow directions. Thestratigraphic situation in the Rancocas is complicated somewhat by afacies change downdip where the Rancocas sands apparently grade orinterfinger laterally into Unit A, an aquitard. No reliable water leveldata or chloride analyses exist for Unit A because of its non-wateryielding nature. Although this general area southeast of Smyrna is onlylightly pumped at present, the water supply possibilities should beaccurately defined as part of future planning.

(4) Fresh-salt-water boundary positions in the Columbia sandsalong coastal areas need to be determined, specifically for pumpingareas near New Castle, Delaware City, Lewes, Rehoboth Beach,Bethany Beach and Fenwick Island. Each locality in itself is probably

21

lIlCdI5-1: well location ond number

KENT CO.

N

1oI

2 milesI

Figure 4. Well locations for which chloride analyses are available inNew Castle County, Delaware.

22

2 milesI

oI

N

t

Harringtonr-', ,~JLdSI-1

SUSSEX CO

Milford"­

Houston I "[J / "-

(-..)-

~ lBHeI2-I:welllocation and number

/....._---'~- -_..-/

NEW CASTLE------

Figure 5. Well locations for which chloride analyses are available inKent County, Delaware.

23

.0014"1: nil M'-rLr7 I j',../ 1/1,;_

/

DIlENT:..0:.. /

-­,-I ......

II

.1~,~,

II

\I

..~"'."'.-",

•.,..... i o I 2 ..11..LL..J

Figure 6. Well locations for which chloride analyses are available inSussex County, Delaware.

24

a considerable project and may involve rather complex head relation­ships. There does not appear to be any immediate danger of drasticallychanging existing boundaries. However, continued growth and develop­ment of these areas dictate that we under stand what is happening or whatmight happen within the shallow aquifers.

SUMMARY

Brackmh water is present in nearly all of Delaware's aquifers.The exact depth and location of the salt-water interface varies witheach aquifer and is dependent mainly on fresh-water heads, centersof pumping, and lithology. The interface positions for the artesianPotomac and Magothy Formations are fairly well known but the inter­face positions for younger formations are not adequately defined atpresent.

Salt-water intrusion in water-table aquifers underlying coastalareas is spotty. The fresh-water heads of the Columbia sands whichmake up the water-table aquifer s are lowest near the ocean, and intru­sion would be expected to occur naturally in some areas. Furtherdevelopment of the Columbia Formation throughout the State shouldbe done cautiously as these sands provide most of the recharge todeeper aquifers and sustain base flow to tidal surface streams.

Monitoring of salt-water movement is generally inadequatebut not critical at present from the standpoint of water use. Furthermonitoring will be needed in the future as ground-water developmentincreases. Most heavy centers of ground-water pumping are not inan immediate danger of salt-water intrusion but proper managementand control of pumping is essential.

25

Table 3 - Chloride Analyses of water from Delaware Wells Page 26na - not a vailable

Total Depth GeologicWell No. Owner Depth Screened Unit Date of ppm

In Feet (land surface Screened Sample Chloridesdatum)

Ca55-1 City of Newark 76 na Potomac 4/26/51 12

Ca55-3 City of Newark 64 na Potomac 4/26/51 12

Ca55-5 City of Newark 63 na Potomac 4/26/51 16

" " " " " " " 12/18/51 "Ca55-7 Phillips Packing 79 na Potomac 9/22/53 .9. O'

Cb51-2 City of Newark 62 na Potomac 4/26/51 12

Cc34-8 Town of Newport 71 na Potomac 9/20/55 13

CompositeCr.54-1,

New Castle Co. 197, 159, 221 Potomac 1/11/51 3.4Cc45-2, na

" " " " " " " " 1/21/54 5.0Cc55-1

Cc45-1 New Castle Co. 197 187-197 Potomac 9/3/57 5.3

Cc55-1 New Castle Co. 221 211-221 Potomac 4/3/57 2.5Cc55-1 " " " " na " 9/3/57 5.0

Cd15-1 Ludlow Mfg. 98 na Potomac 6/ /53 15.7Cd15-1 " " " " " 6/28/55 35

Cd33-2 Cork Insulation Co. 120 na Potomac 1/18/54 '515

Cd33-2 " " " " II " 6/28/55 218Cd33-2 " II II " " " 4/18/57 174

Cd42-l Collins ParkWater Co. 72 60-72 Potomac 1/18/54 25

Pag

e2

7

To

tal

Dep

thG

eolo

gic

Wel

lN

o.

Ow

ner

Dep

thS

cre

en

ed

Un

itD

ate

of

pp

mIn

Feet

(lan

dsu

rface

Scre

en

ed

Sam

ple

Ch

lori

des

dat

um

)C

d4

2-5

Co

llin

sP

ark

Wate

rC

o.

11

31

08

-11

3P

oto

mac

1/1

8/5

47

Cd

43

-lA

tlas

Ch

em

ical

52

36

-51

Co

lum

bia

6/2

8/5

51

4

Cd

43

-3A

tlas

Ch

em

ical

11

79

2-1

12

Po

tom

ac

1/4

/50

7.1

""

""

"8

/60

15

8

Cd

43

-4A

tlas

Ch

em

ical

11

09

4-1

04

Po

tom

ac

6/2

9/4

91

4

Cd

43

-5A

tlas

Ch

em

ical

94

70

-90

Po

tom

ac

1/4

/50

28

Cd

43

-11

Atl

as

Ch

em

ical

88

75

-85

Co

lum

bia

8/1

0/5

11

0.7

Cd

43

-11

""

""

"6

/28

/55

38

Cd

43

-11

""

"If

"4

/18

/57

30

Cd

43

-l2

Dela

ware

Sta

teH

igh

way

Dep

t.5

24

4-4

9P

oto

mac

7/2

2/5

21

5

Cd

5l-

8T

ow

no

fN

ewC

ast

le1

50

65

-82

Po

tom

ac

5/5

/67

9

Cd

52

-lT

ow

no

fN

ewC

ast

le2

4o

pen

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.8G

rou

p

Hd

44

-lW

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orn

eli

us

271

21

0-2

70

Eo

cen

e5

/15

/57

92

2

Hd

44

-2W

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.C

orn

eli

us

50

na

Ch

esa

peak

e7

/2/5

73

0G

rou

p

Pag

e36

To

tal

Dep

thG

eolo

gic

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lN

o.O

wne

rD

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nit

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eo

fp

pm

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eet

(lan

dsu

rfac

eS

cree

ned

Sam

ple

Ch

lori

des

dat

um

)

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1-2

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tern

atio

nal

1200

11

32

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47

Mag

othy

6/2

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694

6L

atex

6/2

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610

986

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1050

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3-4

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yo

fD

ov

er53

03

01

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int

10

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7.0

Jd3

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Do

ver

A.F

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31

90

-21

0C

hes

wo

ld1

/1/4

32

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Je3

1-1

Do

ver

A.F

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.27

01

85

-19

5?

Ch

esw

old

3/4

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2.8

Je3

1-1

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""

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3.7

Je3

1-1

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3.6

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1-2

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ver

A.F

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3n

aC

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12

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ver

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14

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18

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ver

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2.8

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4/2

9/6

22

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ver

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223

61

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1,

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eyP

oin

t4

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6.2

55

5-5

66

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51

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of

240

op

enF

red

eri

ca

10

/27

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2.9

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rin

gto

n

Pag

e3

7

To

tal

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thG

eolo

gic

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lN

o.

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ner

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thS

cre

en

ed

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of

pm

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(lan

dsu

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ple

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lori

des

dat

um

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yo

fM

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42

22

0-2

36

Fre

deri

ca

4/1

3/3

12

.5M

e15

-3"

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./2

0/5

61.

5"

Me1

5-1

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ity

of

Mil

ford

22

4o

pen

fro

m2

00

?F

red

eri

ca

4/1

3/3

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Me1

5-1

3C

ity

of

Mil

ford

24

22

20

-23

6F

red

eri

ca

12

/18

/51

3.-4

Me1

5-1

3"

""

""

"6

/18

/54

2.4

Me1

5-2

9C

ity

of

Mil

ford

86

76

40

-70

0,

Pin

ey

Po

int

2/1

6/6

85

37

78

0-7

85

2/2

0/6

854

1

Nc2

5-2

614

1-6

1C

olu

mb

ia2

/16

/56

9.5

Ng

42

-2T

ow

no

fM

ilto

n68

na

Co

lum

bia

2/1

7/5

66

.5

Nh

35

-16

2n

aM

ano

kin

11

/4/5

36

30

0

Ni2

5-1

See

Tab

leI,

Pag

e8

.

Ni3

5-3

See

Tab

leI,

Pag

e8

.

Ni4

1-1

To

wn

of

Lew

es

11

080

Co

lum

bia

12

/28

/43

19N

i41

-1"

""

""

"2

/13

/44

18

.0N

i41

-1"

""

""

"3

/44

20

.2N

i41

-1"

""

""

"8

/26

/47

13

.0

Ni4

2-2

To

wn

of

Lew

es

60

-65

na

Co

lum

bia

6/1

6/4

41

8N

i42

-2"

""

""

"9

/22

/44

18

Ni4

2-2

""

""

""

10

/5/4

517

Pag

e38

To

tal

Dep

thG

eolo

gic

Wel

lN

o.

Ow

ner

Dep

thS

cre

en

ed

Un

itD

ate

of

pp

mIn

Feet

(lan

dsu

rface

Scre

en

ed

Sam

ple

Ch

lori

des

dat

um

)

Ni4

2-8

To

wn

of

Lew

es6

4n

aC

olu

mb

ia3

/10

/44

42

2N

i42

-8"

""

""

"4

/8/4

44

52

Ni4

2-8

""

""

""

6/1

6/4

46

60

Ni4

2-8

""

""

"u

9/4

41

19

0N

i42

-8"

""

""

"1

Z/5

/44

10

40

Ni4

Z-8

""

""

""

1/4

51

28

0N

i4Z

-8"

""

""

"Z

/Z3

/45

73

8N

i4Z

-8"

""

""

"1

0/5

/45

605

Ni4

Z-8

""

""

""

11

/3/5

3

Ni4

Z-9

To

wn

of

Lew

es6

0-6

5n

aC

olu

mb

ia6

/16

/44

Z5N

i4Z

-9"

""

""

"9

/ZZ

/44

7ZN

i4Z

-9"

""

""

"1

Z/5

/44

147

Ni4

Z-9

""

""

""

1/2

2/4

513

9N

i42

-9"

11

""

""

10

/5/4

51

84

Ni4

Z.1

0T

ow

no

fL

ewes

64

na

Co

lum

bia

4/8

/44

Z9N

i42

-10

""

""

""

6/1

6/4

4Z

8N

i4Z

-10

""

""

""

Z/2

3/4

5Z6

Ni4

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0"

""

""

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0/5

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31

Ni4

Z-1

1T

ow

no

fL

ewe

ssh

allo

w,

na

Co

lum

bia

5/4

485

5e~act

dep

thu

nk

no

wn

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Z-l

l"

""

""

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482

5N

i42

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9/Z

Z/4

41

40

0N

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lZ/5

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14

20

Ni4

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l"

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/23

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11

70

Ni4

Z-1

1"

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0/5

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10

60

Ni5

l-3

To

wn

of

Lew

es8

46

4-8

4C

olu

mb

ia1

94

416

Pag

e39

To

tal

Dep

thG

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gic

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lN

o.

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ner

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ed

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of

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(lan

dsu

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en

ed

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ple

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des

dat

um

)

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l-l5

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ph

Mart

in3

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riv

eP

oin

tC

olu

mb

ia5

/16

/44

31

Ni5

l-l6

To

wn

of

Lew

es97

78

-96

Co

lum

bia

2/Z

3/5

615

Ni5

l-1

7T

ow

no

fL

ewes

16

21

30

-16

2C

olu

mb

ia1

0/4

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10N

i51

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II"

""

""

Z/Z

3/5

69

Ni5

Z-Z

Hen

lop

enP

ou

ltry

80

na

Co

lum

bia

5/1

5/4

413

Ni5

Z-3

v.L

.D

enn

isS

hal

low

,ex

act

Dri

ve

Po

int

Co

lum

bia

5/1

6/4

41

4d

epth

un

kn

ow

n

Ni5

2-4

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C.

Mar

shall

65D

riv

eP

oin

tC

olu

mb

ia5

/16

/44

11

Ni5

Z-5

E.

H.

Mau

ll6

5D

riv

eP

oin

tC

olu

mb

ia5

/16

/44

15

Ni5

2-6

L.

Mit

chel

l8

0D

riv

eP

oin

tC

olu

mb

ia5

/16

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1Z

Ni5

2-8

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rin

gto

n7

2D

riv

eP

oin

tC

olu

mb

ia5

/15

/44

12

Ni5

3-3

U.S

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ov

ern

men

t1

47

(?)

na

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lum

bia

3/1

/44

15

Ni5

3...3

""

""

"9

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/44

13N

i53

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""

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10

/24

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13N

i53

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""

""

lO/Z

5/4

413

Ni5

3-3

""

""

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0/3

1/4

4lZ

Ni5

3...

3"

""

""

81

z6/4

719

Ni5

3-3

""

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12

Ni5

3-4

U.S

.G

ov

ern

men

t10

3n

aC

olu

mb

ia9

/22

/44

lZ

Nj5

1-Z

See

Tab

leI,

Pag

e8

.

Pag

e40

To

tal

Dep

thG

eolo

gic

Wel

lN

o.

Ow

ner

Dep

thS

cre

en

ed

Un

itD

ate

of

pp

mIn

Feet

(lan

dsu

rface

Scre

en

ed

Sam

ple

Ch

lori

des

dat

um

)

Oc1

4-8

To

wn

of

Bri

dg

ev

ille

119

10

0-1

19

Co

lum

bia

2/1

6/5

611

Of4

2-1

Wate

r&

Su

pp

lyC

o.

128(

1)1

20

-12

8(1

)C

olu

mb

ia2

/13

/56

38o

fG

eo

rgeto

wn

0i1

1-1

C.

Bak

er

80D

riv

eP

oin

tC

olu

mb

ia5

/16

/44

18

0i1

1-3

W.

Mu

rray

Sh

allo

w,

ex

-D

riv

eP

oin

tC

olu

mb

ia5

/16

/44

11act

dep

thu

nk

no

wn

OiU

-5W

.C

arp

en

ter

90D

riv

eP

oin

tC

olu

mb

ia5

/15

/44

11

0i3

4-1

To

wn

of

Reh

ob

oth

131

69

-13

8C

olu

mb

ia2

/13

/56

14

(4sc

reen

s)

0i3

4-3

To

wn

of

Reh

ob

oth

116

10

2-1

12

Co

lum

bia

19

56

115

0i3

4-4

J.B

oy

d35

Dri

ve

Po

int

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lum

bia

5/1

2/4

415

0i3

4-7

To

wn

of

Reh

ob

oth

125

(1)

na

Co

lum

bia

1964

20

0i3

5-1

To

wn

of

Reh

ob

oth

110

(1)

na

Co

lum

bia

12

/19

/44

13

0i3

5-2

110

na

Co

lum

bia

12

/15

/44

13

0i3

5-1

5A

tlan

tic

Ice

Mfg

.C

o.

136

na

Co

lum

bia

9/2

2/4

416

0i3

5-2

1S

tate

of

Dela

ware

128

na

Co

lum

bia

2/2

0/4

628

0i3

5-2

5R

.H

all

28D

riv

eP

oin

tC

olu

mb

ia19

52?

17

.1

Pag

e41

To

tal

Dep

thG

eolo

gic

Wel

lN

o.

Ow

ner

Dep

thS

cre

en

ed

Un

itD

ate

of

pp

mIn

Feet

(lan

dsu

rface

Scre

en

ed

Sam

ple

Ch

lori

des

dat

um

)

Oj2

1-3

R.

Do

dg

e9

3D

riv

eP

oin

tC

olu

mb

ia7

/1/4

47

20

Oj2

1-3

""

"."

"9

/1/4

48

60

Oj2

1-3

""

""

"8

/1/4

59

10

Oj2

1-3

""

""

"4

/51

70

Oj3

1-1

To

wn

of

Reh

ob

oth

11

0n

aC

olu

mb

ia1

93

11

8O

j31

-1"

""

""

"1

93

83

6O

j31

-1"

""

""

"1

94

486

Oj3

1-1

""

""

""

19

53

95

Oj3

1-3

Blu

eH

enT

heate

r1

36

·n

aC

olu

mb

ia9

/1/4

49

10

Oj3

1-3

""

""

""

6/2

0/4

56

55

Oj3

1.3

""

""

""

8/1

/45

91

0

Oj3

1-5

L.

Mil

ler

11

31

06

-11

2C

olu

mb

ia1

95

1?

15

Oj3

1-6

L.

Mil

ler

38

32

-38

Co

lum

bia

19

51

?2

2

Oj3

1-7

R.

Lin

gen

felt

er

27

Dri

ve

Po

int

Co

lum

bia

19

50

23

.9

Oj3

1-1

0J.

Hem

meri

ck

25

Dri

ve

Po

int

Co

lum

bia

5/5

23

9.3

Oj4

1-1

63

/10

/44

21

Oj5

1-1

See

Tab

leI

Pag

e8

.

Oj5

1..

2S

eeT

ab

leI

Pag

e8

.

Oj5

1-3

See

Tab

leI

Pag

e8

.

Pag

e4

2

To

tal

Dep

thG

eolo

gic

Wel

lN

o.

Ow

ner

Dep

thS

cre

en

ed

Un

itD

ate

of

pp

mIn

Feet

(lan

dsu

rface

Scre

en

ed

Sam

ple

Ch

lori

des

__

__

__

__

__

__

__

__

__

__

__

_d_a

_t...;,u

;...m.....:

)_

To

wn

of

Seafo

rd

To

wn

of

Seafo

rd

To

wn

of

Seafo

rd

To

wn

of

Seafo

rd87

53

-86

Co

lum

bia

10

/9/5

311

95

65

-95

Co

lum

bia

2/2

7/5

31

2.0

""

"1

0/9

/53

6

""

u2

/21

/56

4

11

57

0-9

8C

olu

mb

ia1

/60

31

80

na

Co

lum

bia

10

/9/5

31

0

90

48

-68

Co

lum

bia

10

/9/5

31

4

876

7-8

7(1

)C

olu

mb

ia3

/7/4

57

.5

76

56

-76

(1)

Co

lum

bia

3/7

/45

8.4

11

49

7-1

08

Man

ok

in7

/51

7

84

72

-85

Co

lum

bia

2/1

4/5

69

53

65

00

+M

ioce

ne

7/5

91

20

0

26

71

03

-10

7M

ioce

ne

7/2

8/5

92

97

2

" """

" "

Vil

lag

eo

fM

ills

bo

ro

Du

Po

nt

Co

mp

any

To

wn

of

Seafo

rd

Du

Po

nt

Co

mp

any

Heck

man

Pro

du

cts

Sta

teo

fD

ela

ware

Sta

teo

fD

ela

ware

Pc2

3-1

2

Pc5

5-1

Pc3

3-6

Pg

53

-9

Pjl

l-1

Pc'

24

-8

Pc2

3-3

Pc2

3-3

Pc2

3-3

Pc3

3-4

Pc2

3-1

Pc2

4-1

Pj1

1-2

Pj2

1-2

See

Tab

leI,

Pag

e8

.

Pj2

1-3

See

Tab

leI,

Pag

e8

.

Pj2

2-1

See

Tab

leI,

Pag

e8

.

Pj2

2-2

See

Tab

leI,

Pag

e8

.---~-

...-

Pag

e4

3

To

tal

Dep

th.G

eolo

gic

Wel

lN

o.

Ow

n'er

Dep

thS

cre

en

ed

Un

itD

ate

of

pp

mIn

Feet

(lan

dsu

rface

Scre

en

ed

Sam

ple

Ch

lori

des

dat

um

)

Pj4

2-1

1'

See

Tab

leI.

Pag

e8

.

Qd

21

-3T

ow

no

fL

au

rel

91M

ult

iple

Co

lum

bia

2/2

1/5

66

.5

Oh

3l-

2,

6A

tlan

tic

Ice

Co

.1

6,

16

Dri

ve

Po

int

Co

lum

bia

2/1

4/5

64

0

Oh

5l.

.7V

illa

ge

of

Fra

nk

ford

10

2n

aC

olu

mb

ia2

/14

/56

14

Qi5

5..

lD

ela

ware

Po

lice

11

7n

aC

olu

mb

ia8

/23

/53

16

Ch

ief'

sA

sso

cia

tio

n

Oj3

2-7

W.

D.

Sh

ort

7161

..6

9C

olu

mb

ia9

/14

/54

58

Rd

31

-8T

ow

no

fD

elm

ar

12

61

15

..1

25

Co

lum

bia

2/1

4/5

61

2

Rh

32

-lT

ow

no

fS

elb

yv

ille

95

85

-95

Co

lum

bia

11/2

4/5

21

0.6

Rh

32

-7T

ow

no

fS

elb

yv

ille

49

5n

aC

olu

mb

ia4

/5/6

52

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SELECTED BIBLIOGRAPHY

Back, W., 1966, Hydrochemical Facies and Ground-Water Flow Patterns inthe Atlantic Coastal Plain: U. S. Geol. Survey Prof. Paper498-A, 42 p.

Barksdale, H. C., Greenman, D. W., Lang, S. M., Hilton, G. S., and Outlaw,D. E., 1958, Ground-Water Resources in the Tri-State RegionAdjacent to the Lower Delaware River: Special Report 13,State of New Jersey Dept. of Conservation and EconomicDevelopment, 190 p.

Cahill, J. M., 1967, Hydraulic Sand-Model Study of the Cyclic Flow of SaltWater in a Coastal Aquifer: U. S. Geol. Survey Prof. Paper575-B, p. B240-B244.

Cooper, H. H., Kohout, F. A., Henry, H. R. and Glover, R. E., 1964, SeaWater in Coastal Aquifers: U. S. Geol. Survey Water SupplyPaper 1613-C, 84 p.

1967, Engineering Geology of the Northeast Corridor, Washington,D. C. to Boston, Massachusetts: Coastal Plain and Sur­ficial Deposits; U. S. Geol. Survey Miscellaneous GeologicalInvestigations Map I-5l4-B.

Gill, H. E., 1962, Ground-Water Resources of Cape May County, N. J., SaltWater Invasion of Principal Aquifers: State of New Jersey,Dept. of Conservation and Economic Development, Div. ofWater Policy and Supply, Special Report 18, 171 p.

Jordan, R. R., 1962, Stratigraphy of the Sedimentary Rocks of Delaware:Delaware Geol. Survey Bull. 9, 51 p.

Jordan, R. R., and Groot, J. J., 1962, Some observations on the Sedimentsof the Delaware River South of Wilmington: Delaware Geol.Survey Report of Investigations No.6, 12 p.

Kraft, J. C., and Maisano, M. D., 1968, Geologic Cross-Section of DelawareShowing Stratigraphic Correlations, Aquifer Distribution andGeologic Setting Within the Atlantic Coastal Plain ­Continental Shelf Geosyncline: A Contribution of theUniversity of Delaware Water Resources Center.

Marine, I. W., Rasmussen, W. C., 1955, Preliminary Report on the Geologyand Ground- Water Resources of Delaware: Delaware Geol.Survey Bull. 4, 336 p.

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Rasmussen, W. C., Groot, J. J., and Beamer, N. H., 1958, Well for theObservation of Chloride and Water Levels in Aquifers thatCross the Chesapeake and Delaware Canal; Delaware Geol.Survey Report of Investigations No.3, 22 p.

Rasmussen, W. C., Groot, J. J., and Depman, A. J., 1958, High-Capacity TestWell Developed at the Air Force Base, Dover, Delaware:Delaware Geol. Survey Report of Investigations No.2, 36 p.

Rasmussen, W. C., Wilkens, R. S., and Beall, R. M., 1960, Water Resourcesof Sussex County, Delaware: Delaware Geol. SurveyBull. 8, 228 p.

Rima, D. R., Coskery, O. J., Anderson, P. W., 1964, Ground-WaterResources of Southern New Castle County, Delaware:Delaware Geol. Survey Bull. No. 11, 54 p.

Spoljaric, N., and Jordan, R. R., 1966, Generalized Geologic Map ofDelaware.

Sundstrom, R. W. and Others, 1967, The Availability of Ground- Water fromthe Potomac Formation in the Chesapeake and DelawareCanal Area, Delaware: University of Delaware, WaterResources Center Publication, 95 p.

Sundstrom, R. W., and Pickett, T. E., 1968, The Availability of Ground Waterin Kent County, Delaware with Special Reference to DoverArea: University of Delaware Water Resources CenterPublication, 123 p.

Upson, J. E., 1966, Relationships of Fresh and Salty Ground Water in theNorthern Atlantic Coastal Plain of the United States:U. S. Geol. Survey Prof. Paper 550-C, p. C235-C243.

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