science for a changing world
Prepared in cooperation with the
U.S. Environmental Protection Agency
Distribution of Salinity in Ground
Water from the Interpretation of
Borehole-Geophysical Logs and
Salinity Data, Calf Pasture Point,
Davisville, Rhode Island
Water-Resources Investigations Report 99-4153
U.S. Department of the Interior
U.S. Geological Survey
-------
950R99003
U.S. Department of Interior
U.S. Geological Survey
Distribution of Salinity in Ground
Water from the Interpretation of
Borehole-Geophysical Logs and
Salinity Data, Calf Pasture Point,
Davisville, Rhode Island
By PETER E. CHURCH, U.S. Geological Survey,
and WILLIAM C. BRANDON, U.S. Environmental Protection Agency
Water-Resources Investigations Report 99-4153
Prepared in cooperation with the
U.S. ENVIRONMENTAL PROTECTION AGENCY
Northborough, Massachusetts
1999
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U.S. DEPARTMENT OF THE INTERIOR
BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY
Charles G. Groat, Director
For additional information write to:
Chief, Massachusetts-Rhode Island District
U.S. Geological Survey
Water Resources Division
10 Bearfoot Road
Northborough, MA 01532
Copies of this report can be purchased from:
U.S. Geological Survey
Information Services
Box 25286
Denver Federal Center
Denver, CO 80225
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CONTENTS
Abstract 1
Introduction 1
Description of Study Site 3
Previous Investigations 3
Borehole-Geophysical Logging Methods 5
Interpretation of Borehole-Geophysical Data 6
Natural-Gamma Data 6
Electromagnetic-Induction Data 6
Distribution of Salinity in the Surficial Aquifer 9
Vertical Distribution of Salinity 10
Section A-A1 10
Section B-B1 10
Section C-C 16
Section D-D' 17
Section E-E' 17
Horizontal Distribution of Salinity 18
Summary and Conclusions 20
References Cited 21
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island 25
FIGURES
1. Map of Calf Pasture Point study site, Davisville, Rhode Island 2
2. Mid-tide shallow and deep well head distribution, December 1, 1995 4
3. Relation between salinity and electromagnetic-induction conductivity by lithologic unit 7
4. Geohydrologic section along line A-A' 11
5. Geohydrologic section along line B-B' 12
6. Geohydrologic section along line C-C' 13
7. Geohydrologic section along line D-D1 14
8. Geohydrologic section along line E-E' 15
9. Spatial distribution of major salinity types in ground water by lithologic unit 19
TABLE
1. Wells with borehole-geophysical data including dates of collection, Calf Pasture Point,
Davisville, Rhode Island 9
Contents III
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CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS
CONVERSION FACTORS
Multiply
acre
foot (ft)
foot per day (ft/d)
By
0.4047
0.3408
0.3808
To obtain
hectares (ha)
meter (m)
meter per day (m/d)
VERTICAL DATUM
Sea Level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD
of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of the
United States and Canada, formerly called Sea Level Datum of 1929.
ABBREVIATIONS:
cps count per second
ppt parts per thousand
mS/m millisiemens per meter
IV Contents
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Distribution of Salinity in Ground
Water from the Interpretation of
Borehole-Geophysical Logs and
Salinity Data, Calf Pasture Point,
Davisville, Rhode Island
By Peter E. Church andWilliam C. Brandon
Abstract
The distribution of salinity in ground water
at Calf Pasture Point, a small coastal peninsula
bounded by Narragansett Bay on the east and
Allen Harbor on the west, in Davisville, Rhode
Island, was interpreted from borehole-geophysical
data and previously collected salinity data to help
identify potential flowpaths of contaminated
ground water to surface-water bodies. The surficial
material at this 40-acre site, which ranges in thick-
ness from about 30 to 85 feet, is composed of an
upper sand unit, a silt unit, and a till unit overlying
bedrock. Borehole-geophysical data indicate that
fresh ground water is present in all surficial units
in the northern and northwestern part of the site. In
the central and eastern parts of the site, where
most of the current land surface is composed of
dredged fill placed in a small saltwater embay-
ment, brackish and saline ground water predomi-
nate. Fresh ground water moving into this area
from upgradient and recharge to this extended land
surface from precipitation is diluting the saline
ground water in the upper sand and till units, and
to a lesser extent in the silt unit. In this area, the
freshwater-flow system is slowly expanding
towards Narragansett Bay and the entrance
channel to Allen Harbor.
INTRODUCTION
Chlorinated hydrocarbons were released onto the
land surface and into the shallow unsaturated zone
intermittently between 1960 and 1974 at Calf Pasture
Point, a coastal peninsula bounded by Narragansett
Bay and Allen Harbor at the U.S. Naval Reservation,
Construction Battalion Center, Davisville, Rhode
Island (fig. 1). In the early- to mid-1990's, surface
soils, soils from test and well borings, and water sam-
ples from wells in the southern 40 acres of Calf Pasture
Point were analyzed to characterize the hydrogeology
and distribution of the contaminants in the surficial
materials and underlying bedrock (EA Engineering,
Science, and Technology, 1997). These data revealed a
sequence of glacial sediment, ranging in thickness
from about 30 to 85 ft, overlying quartzite and phyllite
bedrock, and a heterogeneous distribution of fresh,
brackish, and saline waters in the coastal aquifer. These
data also revealed high concentrations of contaminants
in fresh ground water beneath the disposal site, and low
concentrations of contaminants dispersed radially into
fresh and brackish ground water. An understanding of
the distribution of fresh, brackish, and saline ground
water in this coastal setting can be used to determine
ground-water circulation patterns and identify potential
pathways of contaminants to surface-water bodies,
which in turn can be used to develop an effective
ground-water monitoring program.
Introduction 1
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400 FEET
100 METERS
Base map modified from EA Engineering, Science,
Rhode Island stateplane coodinate system
71°45'
71°30'
42°00'
41°45'
41°30'-f
10 KILOMETERS
v V
and Technology, 1997
EXPLANATION
/I'
—I LINE OF GEOHYDROLOGIC CROSS SECTION
WELL SITE LOCATIONS AND TYPE OF LOG
• Electromagnetic-induction and natural-gamma logs
> Natural-gamma log
> Not logged
i- WELL NAME MODIFIERS—Wells numbered (some
/ sites may contain multiple wells) with details
as follows (see appendix for well logs). All well
, Q9D.R names begin with "MW07-"
S = SHALLOW (typically screened in upper sand)
D = DEEP (typically screened in till)
R = BEDROCK (screened in bedrock)
Figure 1. Calf Pasture Point study site, Davisville, Rhode Island.
2 Distribution of Salinity in Ground Water from the Interpretation of Borehole-Geophysical Logs, Calf Pasture Point, Davisville, R.I.
-------
In December 1996 and August 1997, the U.S.
Geological Survey (USGS), in cooperation with the
U.S. Environmental Protection Agency (USEPA), col-
lected borehole-geophysical data to provide a more
detailed description of the distribution of salinity in the
surficial materials at Calf Pasture Point. These bore-
hole-geophysical data were obtained from existing
wells and were interpreted in conjunction with existing
hydrogeologic and salinity data. The purpose of this
report is to describe the horizontal and vertical distribu-
tion of fresh, brackish, and saline water in the surficial
aquifer at this site as interpreted from the existing data
and the borehole geophysical data.
Description of Study Site
Calf Pasture Point, a relatively flat, low-lying
peninsula that juts into Narragansett Bay, is bounded
by Narragansett Bay to the east, the entrance channel to
Allen Harbor to the south, and Allen Harbor to the west
(fig. 1). The present land surface slopes gently from an
altitude of about 20 ft at the northern part of the site to
sea level at the shoreline. According to earlier maps
(EA Engineering, Science, and Technology, 1997), the
current shape and extent of the peninsula differs signif-
icantly from that of the early 1700's when it was a thin
spit jutting to the southwest and much of the current
Calf Pasture Point was a small saltwater embayment in
Allen Harbor. During the next 220 years, the spit elon-
gated in a more westerly direction to form a distinctive
hook. In the early 1940's, the placement of dredged
sediment from Narragansett Bay in the shallow embay-
ment behind the spit created much of the current land
surface (fig. 1). The southern tip of Calf Pasture Point
has formed by coastal sedimentation since 1966.
Previous Investigations
Data collected by EA Engineering, Science, and
Technology at the site include lithologic data from test
holes and boreholes, and ground-water level, hydraulic
conductivity, and salinity data from wells (EA
Engineering, Science, and Technology, 1997). Four
distinct unconsolidated lithologic units overlying bed-
rock were described from land surface to bedrock: an
upper sand unit, a silt unit, a lower sand unit, and a till
unit. The upper sand unit is composed of fine- to
coarse-grained sand with varying amounts of silt,
gravel, and shell fragments, and ranges in thickness
from about 5 to 19 ft. This sand unit contains the
dredged sediment from Narragansett Bay; however, the
dredged sediment is not differentiated from the sand
that surrounded the former saltwater embayment. The
upper sand unit is underlain by a silt unit that ranges in
thickness from 0 to about 47 ft. This silt unit is
described by EA Engineering, Science, and Technol-
ogy (1997) as sandy in the northern part of the site
and as having a component of sand or clay, or both,
in the southeastern part of the study site. In most of
the area, the silt unit is underlain by a basal layer of
till described by EA Engineering, Science, and
Technology (1997) as a silty gravelly sand, with sandy
gravelly silt in some areas, that ranges in thickness
from 0 to about 36 ft. The lower sand unit is generally
composed of very fine to fine sand with some silt and
ranges in thickness from 0 to 28 ft. This unit is present
only in the eastern part of the site and separates the
silt and till units (EA Engineering, Science, and
Technology, 1997). These unconsolidated surficial
deposits overlie a bedrock surface that slopes to the
southeast and south towards Narragansett Bay and the
entrance channel to Allen Harbor. The combined thick-
ness of the silt and till units is very uniform in a wide,
northeast-southwest trending band through the central
part of the site; however, the thickness of the till unit
increases and the thickness of the overlying silt
decreases in a southwest direction within this band.
Introduction 3
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Beneath a small elongated area extending from the
west-central part of the site to the entrance channel to
Allen Harbor (near wells MW07-26S, MW07-19S,D,
and MW07-21 S,D; fig. 1), the silt unit is absent and the
till unit is overlain directly by the upper sand unit.
The water table, which lies within the upper sand
unit, ranges in altitude from about 8 ft above mean sea
level in the northern part of the site to about 2 ft above
mean sea level in wells near the shoreline (fig. 2).
Water-table fluctuations due to tides are about Ito 2 ft
in wells near the shoreline. This tidal effect decreases
landward from the shoreline; the water table is not
affected by normal tides in the northern part of the site.
The ground-water flow direction in this water-table
aquifer is toward the shoreline of Allen Harbor, the
entrance channel to Allen Harbor, and Narragansett
Bay. Ground water in the till and lower sand units flows
to the southeast and south towards Narragansett Bay
and the entrance channel to Allen Harbor (fig. 2). Data
from five wells completed in bedrock indicate a
ground-water-flow direction to the southeast towards
Narragansett Bay. Vertical head gradients in the surfi-
cial materials generally indicate a downward flow, or
recharge, from the upper sand unit to the till unit; how-
ever, upward flow, or discharge, has been measured at
wells near the shoreline. Downward flow of ground
water from the till into bedrock takes place in the
northwestern part of the site, and upward flow from
bedrock into the till takes place in the central and
southeastern part of the site. Results from slug-test
analyses reported by EA Engineering, Science, and
Technology (1997) indicate that hydraulic conductivi-
ties range from 8.9 to 147 ft/d in the upper sand unit,
0.1 to 19 ft/d in the silt unit, 1.2 to 21.8 ft/d in the lower
&.o°°.
sflO1""
»ooof
,0t)0 f
100 METERS
WATER-LEVEL CONTOUR—Shows mid-tide
head distribution from the shallow and deep
wells, in feet above mean sea level,
December 1, 1995. Contour interval
is 1 foot. Datum is mean sea level.
shallow wells
••• deep wells
WELL TYPES
Os Shallow well (typically screened
in upper sand)
Deep well (typically screened
in till or lower sand)
Cluster (shallow and deep wells)
Base map modified from EA Engineering, Science, and Technology. 1997
Rhode Island stateplane coordinate system
Figure 2. Mid-tide shallow and deep well head distribution, December 1, 1995, Calf Pasture Point, Davisville, Rhode Island
(data from EA Engineering, Science, and Technology, 1997).
4 Distribution of Salinity in Ground Water from the Interpretation of Borehole-Geophysical Logs, Calf Pasture Point, Davisville, R.I.
-------
sand unit, and 0.4 to 379 ft/d in the till unit. The highest
hydraulic conductivities in the till are from the shallow
wells MW07-19S (139 ft/d) andMW07-21S (379 ft/d).
These wells are in the upper part of the till near the
entrance channel to Allen Harbor where the silt unit is
absent and the till is directly overlain by the upper sand
unit. Although these hydraulic conductivities appear
anomalously high compared to those in the lower part
of the till, and compared to hydraulic conductivities in
other till deposits in southern New England (Melvin
and others, 1991), high hydraulic conductivities in this
zone may be in part due the lesser amount of silt and
greater amount of sand in the upper part of the till than
in the lower part (EA Engineering, Science, and Tech-
nology, 1997) and the shallow depth where compaction
would be small.
BOREHOLE-GEOPHYSICAL
LOGGING METHODS
The borehole-geophysical methods used at the
site were natural gamma and electromagnetic induc-
tion. These methods are not affected by the chlorinated
hydrocarbon contaminant plume at this site. Borehole
data were obtained at 0.1-ft vertical increments as
probes were raised from the bottom to the top of each
well. Each measurement represents a volume of the
aquifer surrounding the well; its vertical dimension
much greater than the 0.1-ft interval between measure-
ments. As a result of these overlapping volumes with
each subsequent measurement, distinct interfaces
between lithologic units, for example, are recorded as
transitional zones.
Gamma logs are a measure of the natural gamma
radiation emitted from radioactive elements in rock,
primarily from potassium-40 and daughter products of
the uranium- and thorium-decay series (Keys and Mac-
Gary, 1971). In New England, gamma radiation results
largely from potassium-40, a radioisotope in potassium
feldspar (Hansen, 1993). In unconsolidated material,
low levels of natural-gamma radiation generally repre-
sent materials where most of the feldspar has been
removed by transport and depositional processes, such
as in deposits of sand. Higher levels of natural-gamma
radiation are generally emitted from fine-grained
unconsolidated materials, such as silt and clay (Keys
and MacCary, 1971). Silt and clay deposits generally
have higher gamma radiation than sand deposits
because the finer grained material contains more feld-
spar. About 90 percent of the natural-gamma response
recorded by the probe comes from the first foot radially
from the vertical axis of the well (J. H. Williams, U.S.
Geological Survey, oral commun., 1998). Natural-
gamma logs are generally not affected by porosity and
pore fluids. Natural-gamma radiation is expressed in
units of counts per second.
Electromagnetic-induction (EM) logs are a mea-
sure of the electrical conductivity of the aquifer and are
a function of soil or rock type (lithology), porosity,
moisture content, and the concentrations of dissolved
solids in the pore fluid (Biella and others, 1983). The
EM probe measures the response of the aquifer sur-
rounding a well to an induced electromagnetic field
(Hearst and Nelson, 1985). The probe measures the
aquifer response in a zone from about 0.5 ft to 4 ft radi-
ally from the vertical axis of the well. Maximum
response occurs at about 1 ft, and one-half of the
response occurs within about 2 ft from the vertical axis
of the well (McNeill, 1980). EM measurements are
insensitive to organic compounds in borehole and aqui-
fer pore fluids, nonmetallic well casing and screen
material, and sand packs and grout material to borehole
diameters approaching 1 ft. However, EM measure-
ments are very sensitive to metal casing and screens
and to other metallic objects within several feet of the
well casing or screen, and can be affected by grout
materials containing sodium, calcium, and chloride in
boreholes greater than 1 ft in diameter. The presence of
these materials will cause anomalously high or oscillat-
ing EM conductivities. EM conductivity is expressed in
units of millimhos per meter, or millisiemens per meter
as used in this report.
Borehole-geophysical logging at Calf Pasture
Point was conducted in December 1996 with an EM
probe attached to the bottom of a gamma probe (see
table 1 for a list of wells that were logged and dates
when geophysical data were collected). The measuring
points for the EM and gamma probes were 3 ft and 7 ft,
respectively, above the bottom of the combined probes.
Because the probes could not be lowered past the
bottom of the well screens, the aquifer adjacent to the
bottom 3 ft of the well screens was not fully repre-
sented by the EM logs, and the bottom 7 ft was not rep-
resented by the gamma logs. Geophysical data
collection in August 1997 (table 1) was performed with
independent tools for gamma and EM logs. Measuring
points on the gamma and the EM probes were about
0.5 ft and 2 ft, respectively, from the bottoms of the
probes. As a result, geophysical data obtained in
August 1997 were from a thicker portion of the aquifer
adjacent to the well screens than those obtained in
December 1996. Only the aquifer adjacent to the
Borehole-Geophysical Logging Methods 5
-------
bottom 0.5 ft of the well screens was not fully repre-
sented by the gamma logs and the bottom 2 ft by the
EM logs. Gamma and EM conductivity logs obtained
from wells at the Calf Pasture Point study site are
shown in the Appendix 1. Also included are the EM
resistivity logs, which are the inverse of the EM
conductivity logs.
INTERPRETATION OF BOREHOLE-
GEOPHYSICAL DATA
Natural-Gamma Data
Gamma data in the surficial materials at Calf
Pasture Point typically indicate subtle changes with
depth that correspond to changes in lithology, and gen-
erally corroborate the lithologic and stratigraphic inter-
pretations provided by EA Engineering, Science, and
Technology (1997). The gamma radiation measured in
most wells is lower in the upper and lower sand units
than in silt and till units. The gamma radiation data also
indicate apparent changes in grain-size distribution
within lithologic units. At well MW07-19D, the
gamma radiation increases significantly with depth in
the bottom third of the till unit. This increase in radia-
tion is consistent with the composition of the till chang-
ing from silty gravelly sand in the upper two-thirds of
the till unit to gravelly sandy silt in the lower third. A
similar increase in gamma radiation in the lower part of
the till unit takes place at well MW07-21D where the
till is more silty than in the till above. At well MW07-
29D, the gamma radiation in the till unit increases with
depth and corresponds to a change from sand and silt to
gravelly sandy silt (EA Engineering, Science, and
Technology, 1997). Measurements of gamma radiation
in the till unit at wells MW07-23D and MW07-27D
and in the lower sand unit at wells MW07-18D and
MW07-30D indicate measurable variations in grain-
size distribution with depth.
Electromagnetic-Induction Data
EM data collected at Calf Pasture Point indicate
considerable vertical and horizontal variability in
response to changes in pore fluid conductivity, porosity,
and lithology. EM conductivities range from about 20
to 900 mS/m in the upper sand unit, about 20 to
1,000 mS/m in the silt unit, and about 30 to 750 mS/m
in the till unit. At this site, electrical conductivity of the
pore fluid is dominated by the salinity of the water. In
an environment where salinity is the only variable that
changes, there would be a high degree of correlation
between salinity and EM conductivity. The relation
between measured salinities in ground water and EM
conductivities obtained adjacent to the well screens
exhibits a general increase in EM conductivity with
increasing salinity, but also has considerable scatter
(fig. 3). This scatter can be attributed to variations in
lithology, variation in porosity between and within
lithologic units, and possibly other factors, such as well
construction and the difference in time when salinity
measurements and EM measurements were made. The
relative influence of these factors on the EM conductiv-
ity is discussed below.
The effects of lithology, specifically the electri-
cal conductivity of the sediments comprising the litho-
logic units, on EM logs at this site were assessed by
examining data from wells in low salinity areas. EM
conductivities at wells MW07-03D, MW07-10D,
MW07-12D, and MW07-22D (fig. 1), where measured
salinities were low or not detected, are correspondingly
low, change little with depth, and appear to be unaf-
fected by lithology. Similar EM conductivities are
found in the upper sand, silt, and till units. Assuming
the electrical conductivity of the sediments comprising
these deposits varies little within the site, EM conduc-
tivities at these wells indicate that lithology has little
effect on EM conductivities. Although the silt unit in
the southeastern part of the study site, where high EM
conductivities are present (wells MW07-16D, MW07-
18D, and MW07-20D), is described as having a com-
ponent of clay (EA Engineering, Science, and Technol-
ogy, 1997), this clay is not likely to have much effect
on the EM conductivities. Clays in the glacial deposits
of New England are predominately composed of clay-
sized particles of quartz and contribute little to the elec-
trical conductivity of the deposits.
6 Distribution of Salinity in Ground Water from the Interpretation of Borehole-Geophysical Logs, Calf Pasture Point, Davisville, R.I.
-------
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DATA FROM WELL SCREENED
IN LITHOLOGIC UNIT
© Upper Sand
+ Silt
Q Lower Sand
• Till
100 200 300 400 500 600 700
EM CONDUCTIVITY, IN MILLISIEMENS PER METER; LOGS PERFORMED IN DECEMBER 1996 AND AUGUST 1997
800
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Figure 3. Relation between salinity and electromagnetic-induction conductivity by lithologic unit, Calf Pasture Point, Davisville, Rhode
Island.
I
-------
In other wells, particularly MW07-13D and
MW07-29D (fig. 1), an apparent association of EM
conductivity and lithology can be seen: EM conductiv-
ity is about 100 mS/m in the upper sand and till units,
and about 300 mS/m in the silt unit at well MW07-
13D; about 200 to 300 mS/m in the upper sand and till
units, and about 800 to more than 900 mS/m in the silt
unit at well MW07-29D. This apparent association,
however, is more likely due to the different porosities
in these lithologic zones or changes in pore-fluid con-
ductivity, or both, rather than differing conductivities
of the sedimentary materials. Higher porosity in the silt
unit can partially explain the higher EM conductivities
as the conductive pore fluid occupies a larger portion of
the total volume of sediment.
The effect of porosity on EM conductivity in
fully saturated sediment can be roughly estimated by
use of the empirical formula developed by Archie
(1942) where F, the formation resistivity factor, is equal
to RO/RW, the electrical resistivity of the formation
divided by the electrical resistivity of the of the forma-
tion fluid, and the empirical formula developed by
Winsauer (1952), F=l/m where § is porosity and m is
a cementation factor ranging from 1.3 to 2.6 (Telford
and others, 1976). A specific form of this expression
for granular, poorly consolidated rock, called the
Humble formula, is F=0.62())-2-15 (Winsauer, 1952;
Keys and MacCary, 1971; Telford and others, 1976).
By combining these formulae, replacing resistivity
with its inverse, electrical conductivity, and rearrang-
ing, the following equation is derived:
C0=(CW^)2-15)/0.62,
(1)
where
C0 is electrical conductivity of the formation
(EM conductivity); and
Cw is the electrical conductivity of the formation
fluid.
To demonstrate the effect of porosity, equation 1 is
applied to salinity and EM conductivity data from
wells MW07-20S (shallow well) and MW07-20D
(deep well). The shallow well is screened in the upper
sand unit, and the deep well is screened in the till unit.
The salinity in the shallow well (23.7 ppt) is about
0.8 times the salinity in the deep well (29.0 ppt).
However, the EM conductivity in the shallow well
(about 750 mS/m) is nearly 2 times the EM
conductivity in the deep well (about 400 mS/m). By
use of equation 1, the porosities of the upper sand and
till are estimated to be 0.37 and 0.25, respectively. As
these porosities are reasonable values for sand and till
deposits (Fetter, 1980), they can account for the
seemingly high contrast in salinity and EM
conductivity data at these wells. Even though the
salinity is lower in the shallow well, the higher porosity
sand contains a larger volume of saline water than the
till, producing a higher EM conductivity.
EM conductivities from other wells show large
changes within and between lithologic units, such as at
wells MW07-09D, MW07-11D, MW07-16D, MW07-
18D, MW07-24D, MW07-28D, and MW07-30D.
Assuming relatively small changes in porosities within
lithologic zones, these large changes in EM conductiv-
ity are primarily the result of changes in the conductiv-
ity of the pore fluid. This is particularly evident with
the high EM conductivities in the silt zone in wells near
the Narragansett Bay shoreline and the shoreline along
the entrance channel to Allen Harbor. Although few
salinity measurements were taken from the thick and
generally continuous silt zone, the EM logs are consis-
tent with water of high salinity in the silt zone and in
the upper sand in the southeastern and southern area of
Calf Pasture Point.
The scatter in the relation between salinity and
EM conductivity (fig. 3) also may be affected by well
construction, well-screen placement, small-scale varia-
tions within lithologic units, and the different measure-
ments times. Wells were constructed with 2-foot
bentonite seals above the well screens and cement/ben-
tonite grout along the well casings. The EM data do not
appear to be affected by the bentonite seals as no con-
sistent EM response was observed among the wells at
these screen depths. Effects of the casing grout are
assumed to be negligible because the borehole diame-
ters in the surficial aquifer are less than 1 ft, the diame-
ter beyond which the EM logger begins to respond to
the induced current in the aquifer. Well-screen place-
ment, relative to the source of the ground water sam-
pled, could affect the EM logs. About one-third of the
well screens from which the water samples were col-
lected are screened in more than one lithologic unit.
The water sample in which salinity was measured
would have come preferentially from the more porous
or permeable unit adjacent to the well screen. The EM
log, however, represents the electrical conductivity of
the aquifer material and fluid over the full length of the
screen logged, regardless of the permeability of the
material. As variations in grain size, shape, sorting, or
8 Distribution of Salinity in Ground Water from the Interpretation of Borehole-Geophysical Logs, Calf Pasture Point, Davisville, R.I.
-------
packing take place within lithologic units, this prefer-
ential flow can occur even when the well screen is
placed fully within one lithologic unit. Additionally, if
the water sample was collected preferentially from the
lower part of the well screen, the EM conductivity
would be further biased because the EM logger was
unable to fully measure the aquifer adjacent to the low-
ermost 2 ft of the well.
The EM conductivity-salinity relation appears
not to have been affected by temporal variations in col-
lection of EM and salinity data. Salinities were mea-
sured from all wells in December 1995. EM data were
collected in 7 wells in December 1996, 12 months after
the salinity measurements, and 14 wells in August
1997. 20 months after the salinity measurements
(table 1). EM data were collected from two wells,
MW07-19D and MW07-21D, in December 1996 and
August 1997. Comparison of these duplicate data sets
indicate that little change took place during the 8-
month period, suggesting that changes in salinity over
a 20-month period may be small.
Table 1. Wells with borehole-geophysical data including dates of
collection, Calf Pasture Point, Davisville, Rhode Island
Natural-
Well No. aammalog,
December
1996
MW07-03D
MW07-05R
MW07-09D
MW07-10D X
MW07-11D
MW07-12D X
MW07-13D
MW07-16D
MW07-16R
MW07-18D
MW07-19D X
MW07-20D
MW07-21D X
MW07-22D
MW07-23D X
MW07-24D X
MW07-26S X
MW07-27D
MW07-28D
MW07-29D
MW07-30D
MW07-31S
EM conduc- Natural-
tivity log, gamma log,
December August
1996 1997
X
X
X
X
X
X
X
X
X
X
X X
X
X X
X
X
X
X
X
X
X
X
X
EM conduc-
tivity log,
August
1977
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Although a strong correlation between EM con-
ductivity and salinity was unattainable with the existing
data (fig. 3), the EM data collected provide sufficient
data to delineate zones of fresh, brackish, and saline
water in the surficial aquifer at Calf Pasture Point.
Individual anomalies in the EM data at any given
well may not be fully explainable due to the multiple
factors that can affect these data, but, the salinity of
the pore water is interpreted to be the primary factor
affecting the EM data. Although salinities were not
measured from ground-water samples in the thick
silt deposit below the southeastern half of the site, EM
conductivities in most of the silt unit were sufficiently
high to characterize the water as saline. In the remain-
der of this report, references to different EM conductiv-
ities in different lithologic units imply that these
differences are caused by changes in the pore water
conductivity or porosity, or both, and not by changes in
the composition of the sediment comprising the litho-
logic unit.
DISTRIBUTION OF SALINITY IN THE
SURFICIAL AQUIFER
Distribution of fresh, brackish, and saline
water in the surficial aquifer at Calf Pasture Point
was delineated by use of previously collected
salinity data (EA Engineering, Science, and
Technology, 1997) and EM log data. Salinity defi-
nitions of these three categories are—fresh
(<0.5 ppt), brackish (>0.5-10 ppt), and saline
(>10 ppt). Salinity data indicate that the highest
salinities are in the southern and southeastern parts
of the site near the shoreline of Narragansett Bay
and the beginning of the entrance channel to Allen
Harbor. These saline waters were detected prima-
rily in the lower sand and till units. Salinities in
ground water decrease in a north-northwesterly
direction. Brackish water was measured primarily
in the till unit along a wide band that extends from
the northeast to the southwest through the central
part of the site. Freshwater was measured in the
upper sand and till units in the northern part of the
site; however, salinity data were not available in
large areas of the upper sand unit or in most of the
silt unit in this area. Salinities were further defined
in the till unit and extended into the upper sand and
silt units using EM data.
Distribution of Salinity in the Surficial Aquifer 9
-------
Vertical Distribution of Salinity
The vertical distribution of salinity is illustrated
by geohydrologic cross sections, on which interpretive
contours that define zones of fresh, brackish, and saline
water have been drawn (figs. 4-8). Also depicted on
these geohydrologic cross sections are lithology, salin-
ity measurements, and EM logs. Five cross sections
were constructed: four nearly parallel sections trend in
a northwest-southeast direction across Calf Pasture
Point, and one section is about 75 degrees from the
other four and trends approximately north-south, as
shown in figure 1. These sections are based on litho-
logic, salinity, and well construction data from EA
Engineering, Science, and Technology (1997) and EM
logs collected by the USGS. Vertical locations of well
screens at well cluster sites are shown as one well. As
these are straight line sections, some wells were pro-
jected short distances onto the sections. The land-sur-
face profile that existed before the embayment was
filled with dredged sediment from Narragansett Bay
also is shown on the sections.
Section A-A'
This section, shown in figure 4, is in the northern
part of the study site (fig. 1). The upper sand and till
units change little in thickness, as compared to the silt
deposit, which thickens towards the bay. A lower sand
unit separates the silt unit from the till unit at wells
MW07-29D and MW07-30D. Freshwater is present in
the upper sand, silt, and till units in the area of wells
MW07-22D and MW07-03D, consistent with the low
EM conductivity, and at MW07-14D. The slight
increase in EM conductivity in the central part of the
silt unit at wells MW07-22D and MW07-03D could
result from a small increase in porosity or a small
decrease in grain size. The measured salinity in the till
unit at well MW07-28D indicates the presence of
brackish water at this location. Freshwater in the upper
sand unit, brackish water in the upper and lower parts
of the silt unit and in the till unit, and saline water in
the middle of the silt unit are interpreted from the EM
data. Saline water was measured in the till unit at well
MW07-29D and in the lower sand unit at well MW07-
30D. EM data indicate brackish water in the upper sand
unit and saline water in the silt, lower sand, and till
units at both wells.
Well MW07-03D is just west of the area where
the dredged sediment from Narragansett Bay was
placed (fig. 1). The freshwater observed at this well
location is consistent with a freshwater-discharge zone
near the shoreline of the former saltwater embayment.
The presence of freshwater in the upper sand unit and
brackish water in the till unit at well MW07-28D sug-
gests that saline water is being diluted by upgradient
fresh water moving towards the Narragansett Bay since
this area was isolated from the saline embayment water
by the emplacement of the fill. A similar occurrence in
Florida was described by Halford (1998) where filling
salt marshes and other tidally affected areas since 1942
has caused the continual expansion of the freshwater-
flow system. Recharge of freshwater from precipitation
onto this extended area at Calf Pasture Point also is
likely to be a significant factor in reducing the salinity
of ground water in the upper sand unit. The saline
water in the silt unit is also being diluted by freshwater,
but, at a much slower rate than in the upper sand and
till units because of the lower hydraulic conductivity of
the silt. Leaching of the presumed saline water in the
dredged fill material may have contributed to the high
salinity in the silt unit, but the former overlying saltwa-
ter is interpreted as the primary and original source of
the saline water.
Section B-B'
Section B-B' (fig. 5), which is about 400 ft south-
west of section A-A', extends from Allen Harbor to the
northwest to Narragansett Bay to the southeast (fig.l).
The lithology in this section is similar to that observed
in section A-A', except for the large increase in the
thickness of the till unit and a corresponding decrease
in the thickness of the silt unit at well MW07-27D, and
the thin till zones on both ends of this section. Fresh-
water was measured in the till unit and in bedrock at
well MW07-25D near Allen Harbor. Although EM data
were not obtained from this well, it is interpreted that
freshwater also is present in the overlying silt and
upper sand units. This is supported by the EM data and
salinity data collected at well MW07-10D where a
transition from fresh to brackish water is observed.
10 Distribution of Salinity in Ground Water from the Interpretation of Borehole-Geophysical Logs, Calf Pasture Point, Davisville, R.I.
-------
q
I'
or
c
6'
o
i
Sea Level
MW07-22S.D (proiected)
MW07-14D
I
MW07-03S.D
(projected)
\PPROXMATt nil AllliNin- FORMERSAL1
WATER EMBAYMENTAND DREDGED FILL
MW07-29D
(proiected)
MW07-28D
(proiected)
MW07-30D
(projected)
SALINITY-in ppt.
ND = not detected
47
SCREENED,'
INTERVAL X
LINE OF EQUAL SALINITY-m parts
per thousand, interpreted from EM log
\ and salinity data
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
-80 -
-90
100
100 200 300 400
500 600 700 800 900 1,000 1,100 1,200 1,300 1.400 1.500 1,600 1,700
DISTANCE FROM WELL MW07-22D, IN FEET VERTICAL EXAGGERATION: iox
Figure 4. Geohydrologic section along line A-A', Calf Pasture Point, Davisville, Rhode Island.
3,
o'
5'
-------
ro
a
o
3
CO
0)
o
c
3
a
f
O
5
-------
5
£
I
20
10
Sea Level 0
-10
-20
2 -30
III
Q
t -40
-50
-60
-70
-80
-90
MW07-12D
••W'/'A'C.VMM//- LOCATION Ot I-OHM hit Ml. I
WAII K 1-MHMMI:NTJ\N1> IIKI IH,il> IIII
MW07-11D
(projected)
MW07-20S.D
NARKAGANSEJT
HAY
SALINITY-ln ppt
ND = not detected
4.7,
SCREENED /
INTERVAL \
BOTTOM OF_
WELL
LINE OF EQUAL SALINITY-m parts
tj\ per thousand, interpreted from EM log
\ and salinity data
1000
EM CONDUCTIVITY. IN
MILLISIEMENS PER METER
"-300 -200 -100 0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 1,300 1,400 1,500
DISTANCE FROM WELL MW07-12D, IN FEET VERTICAL EXAGGERATION: iox
5'
n>
(a
3,
g°
.'
Figure 6. Geohydrologic section along line C-C', Calf Pasture Point, Davisville, Rhode Island.
-------
i,
D
O
3
o
O
o
c
a
I
o
to
•D
o'
CD
O
I
O
(D
6
(D
O
•
20
10
Sea Level
-10
LJJ
111
U.
~Z -30
UJ
Q
-50
-60
MW07-23S.D
APPROXIMATE LOCATION OF DREDGED
FILL AND FORMER TIDAL ESTUARY
MW07-21S,D,R
(projected)
MW07-24S.D
ALLEN HARBOR
SALINITY-m ppt,
ND = not detected
EM CONDUCTIVITY. IN
MILLISIEMENS PER METER
LINE OF EQUAL SALINITY-in parts
per thousand, interpreted from EM log
and salinity data
-300 -200 -100
100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 1,300 1,400 1,500
DISTANCE FROM WELL MW07-23D, IN FEET VERTICAL EXAGGERATION: iox
in
o'
a)
O
(O
01
o
0)
•u
fi)
CA
•o
o
5'
Figure 7. Geohydrologic section along line D-D1, Calf Pasture Point, Davisville, Rhode Island.
5T
30
-------
E1
20
10
Sea Level
-10
-20
LJJ
LU
LL
? -30
LU
Q
-50
-60
-70
-80
-90
MW07-22S.D
MW07-10S.D
(projected) MW07.26S
(projected)
API'K/IXIHATI: LOCATION OF DREKF.I)
HU-AN1I FORMER TIDAL ESTUARY
ENTRANCE CHANNEL TO ALLF.N HARBOR
SALINE
BRACKISH
WELL AND
NUMBER
SALINITY-in ppt,
ND = not detected
\
4.;
SCREENED/
BOTTOM OF_
WELL
LINE OF EQUAL SALINITY-in parts
per thousand, interpreted from EM log
and salinity data
1000
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
-300 -200 -100 0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 1.300 1,400 1,500
DISTANCE FROM WELL MW07-22D, IN FEET VERTICAL EXAGGERATION: iox
I
B'
Z.
Figure 8. Geohydrologic section along line E-E', Calf Pasture Point, Davisville, Rhode Island.
5T
-------
Brackish water is present in the thick till unit and in the
overlying silt and upper sand units at well MW07-27D.
At well MW07-09D, brackish water is present in the
upper sand unit, upper and lower parts of the silt unit,
and the lower sand and till units. Saline water is present
in the central part of the silt unit and was also measured
in bedrock at this well. At well MW07-18D, which is
near Narragansett Bay, brackish and saline water are
present in the upper sand unit, and saline water is
present in the silt, lower sand, and till units. Although
EM conductivity in the upper part of the lower sand
unit is considerably less than in the lower part, it is
sufficiently high to characterize the water as saline.
As in section A-A', section B-B1 also crosses the
former saltwater embayment (fig. 1). The EM-conduc-
tivity profile at well MW07-09D is similar to that of
well MW07-28D, indicating that fresh ground water
from upgradient and recharge from precipitation has
been diluting the saline water during the nearly 60-year
period since the embayment was filled with dredged
sediment from the Narragansett Bay.
Section C-C'
Section C-C' (fig. 6) is about 300 ft southwest of
section B-B' (fig. 1). The lithology is dominated by the
thick till unit in the northwestern part of the section and
the thick silt unit in the central and southeastern part of
the section. The till unit is directly overlain by the
upper sand unit at well MW07-19D; the silt unit is
locally absent in this area. EM conductivities at well
MW07-12D are low but increase slightly in the till unit;
this is consistent with the measured salinity. Freshwater
is present in the upper sand and silt units at this well.
The EM data from well MW07-19D displays a rela-
tively low EM conductivity in the upper two-thirds of
the till unit and a significant increase in EM conductiv-
ity in the lower one-third of this unit, consistent with
the measured salinities of about 1.1 ppt in well MW07-
19S and 11.6 ppt in well MW07-19D. Relatively low
EM conductivities correspond with low salinity mea-
surements in both the upper sand unit at well MW07-
13S and till unit at well MW07-13D, indicating brack-
ish water. The higher conductivities in the silt unit that
separates the sand unit from the till unit can be inter-
preted as the presence of saline water. However, the
higher conductivity also could be the result of a higher
porosity and is therefore interpreted as a zone of highly
brackish water. The EM-conductivity data from well
MW07-1 ID is consistent with the measured salinity in
the lower sand unit. The EM data from this well also
indicate brackish water in the upper sand unit and in
the upper part of the silt unit, and saline water in the
remaining thick silt deposit. EM conductivities at well
MW07-20D are very high in the upper sand and in the
approximate 47-ft thick layer of silt because of the
well's proximity to the shoreline. Thin zones of sand
and till separate the silt unit from bedrock. The EM
conductivity decreases rapidly from more than
800 mS/m in the silt unit to about 200 mS/m in the till
unit. The salinity measured in the till was 29 ppt, equiv-
alent to the salinity of sea water. The abrupt fall in EM
conductivity from the silt unit to the till unit could be
the result of a significantly lower porosity in the till
unit than in the overlying silt and upper sand units.
Wells MW07-13D and MW07-1 ID are in the
area of the former saltwater embayment. EM data from
these wells show similar characteristics of high salinity
in the silt unit that decreases upward and lower salinity
in the upper sand and till units. This salinity pattern is
the same as observed at wells MW07-09D, MW07-
28D, and MW07-29D, which also are in the filled
embayment. Wells MW07-12D and MW07-19D are
west of the former saltwater embayment. Freshwater in
the upper sand and silt units at well MW07-12D,
slightly brackish water in the upper sand unit and in
most of the till unit, and saline water in the lower part
of the till unit at well MW07-19D are consistent with
fresh and brackish water discharge zones near the
shoreline of Allen Harbor and the former embayment.
16 Distribution of Salinity in Ground Water from the Interpretation of Borehole-Geophysical Logs, Calf Pasture Point, Davisville, R.I.
-------
Because these wells are hydraulically upgradient from
the fill area, the current salinity distribution shown may
also represent the pre-fill conditions.
Section D-D'
Section D-D' (fig. 7) crosses the southern tip of
Calf Pasture Point very close to the entrance channel to
Allen Harbor (fig. 1). It is underlain primarily by the
upper sand unit and a thick till unit over bedrock. Thin
zones of silt are present between the upper sand and till
units at wells MW07-23D and MW07-24D. Salinity
measurements at well MW07-23D indicate the pres-
ence of brackish water in the silt unit and in the lower
part of the till unit. The small increase in EM conduc-
tivity in the middle of the till unit could represent a thin
zone of saline water or an increase in porosity, and is
therefore interpreted as a zone of highly brackish water.
Salinity measurements from wells MW07-21D and
MW07-21R indicate the presence of brackish water in
the upper and lower parts of the till unit and in bedrock.
The EM data from well MW07-21D indicate saline
water in the upper sand unit and in a thin zone just
above the well screen placed at the bottom of the till
unit. At well MW07-24D, saline water was measured
in the lower part of the upper sand unit and in the lower
part of the till unit. The EM data are consistent with the
presence of zones of saline water within these units and
also indicate saline water in the upper part of the silt
unit. Zones of brackish water were observed in the
upper part of the upper sand unit, in the lower part of
the silt unit, and in the upper part of the till unit.
Well MW07-23D is in an area that appears to
have been stable since the early 1700's (EA Engineer-
ing, Science, and Technology, 1997). The brackish
water observed in this backshore setting indicates the
presence of a mixing zone where freshwater discharges
into saline water. Well MW07-21D is in an area that
was the mouth of the former saltwater embayment, and
well MW07-24D is in a former offshore position on the
eastern side of the spit that enclosed the embayment
(fig. 1). The current land area in which wells MW07-
21D and MW07-24D are located was formed by natu-
ral deposition of coastal sediments since 1966 (fig. 1)
(EA Engineering, Science, and Technology, 1997). The
saline zone in the upper sand unit at these wells is con-
sistent with their former shoreline and offshore envi-
ronments. The brackish zones in the till unit at wells
MW07-23D, MW07-21D, and MW07-24D indicate
movement of upgradient freshwater into zones of saline
water. The thin saline zone in the till unit at well
MW07-21D is considerably thicker at well MW07-
24D, consistent with its former offshore position.
Because this area remained offshore until at least 1966,
the brackish water in the upper part of the upper sand
unit at well MW07-24D indicates that saline ground
water was diluted by flow of fresh ground water or
infiltration of fresh surface water, or both, in the past
30 years.
Section E-E'
Section E-E' (fig. 8) is oriented approximately
north-south and is nearly parallel to the eastern shore-
line of Allen Harbor (fig. 1) and perpendicular to the
ground-water-flow direction measured from the deep
wells screened in the till unit (fig. 2). A thick layer of
silt separates the upper sand unit from the till unit in the
northern part of this section but is absent in the south-
ern part. Salinity measurements from well MW07-22D
indicate the presence of freshwater in the upper sand
and till units. EM data indicate freshwater in the silt
unit at this well. At well MW07-10D, salinity measure-
ments indicate the presence of slightly brackish water
in the upper sand, in the lower part of the silt, and in the
till units, which correspond to small increases in EM
conductivity. With the exception of these two thin
zones, the EM data indicate freshwater at this well.
Salinity measurements from well MW07-26S indicate
the presence of brackish water in the upper part of the
till unit. The EM data are consistent with the presence
of brackish water in the upper part of the till unit and
Distribution of Salinity in the Surficial Aquifer 17
-------
also indicate the presence of freshwater in the upper
part of the upper sand unit and brackish water in the
lower part. Salinity measurements and EM data indi-
cate the presence of brackish water in the upper part of
the till unit and saline water in the lower part at well
MW07-19D. Although salinity was not measured, nor
were EM data obtained from the upper sand unit at this
well, the water is considered brackish because of its
proximity to saline water in the upper sand unit as indi-
cated by EM data at well MW07-21D. Salinity mea-
surements and EM data at well MW07-21D also
indicate the presence of a zone of saline water, with
brackish water above and below, within the till unit and
brackish water in bedrock.
Wells MW07-26D, MW07-19D are near the
western shoreline and well MW07-21D is at the mouth
of the former saltwater embayment. Brackish water in
these wells indicates a transition from upgradient fresh-
water to saline water beneath the entrance channel to
Allen Harbor. The saline zone in the lower part of the
till unit at wells MW07-19D and MW07-21D appears
to be continuous with the thicker saline zone in the till
unit at well MW07-24D (section D-D', fig. 6). This
saline zone also appears to extend into bedrock at wells
MW07-19D and MW07-24D but is underlain by
brackish water at well MW07-21D.
Horizontal Distribution of Salinity
Horizontal distribution of fresh, brackish, and
saline water in the upper sand, silt, and till units (lower
sand and till combined) was determined from the inter-
preted salinity contours shown on the geohydrologic-
sections. A composite map was then made to divide the
study site into areas of similar salinity and lithologic
characteristics (fig. 9). Although at some wells, fresh
and brackish water, or brackish and saline water, are
present in the same lithologic unit, the type of water
most represented in the lithologic unit was selected for
this composite map. Ten distinct area types were identi-
fied, ranging from freshwater in all lithologic units to
saline water in all lithologic units. Some of these area
types are represented by only one well whereas others
are represented by several wells.
Freshwater in the upper sand, silt, and till units
was identified in the northern and northwestern parts of
the site which includes wells MW07-03D, MW07-
14D, MW07-22D, and MW07-25D. Two areas were
characterized by brackish water in the upper sand, silt,
and till units. One of these areas is in the central part of
the site and includes wells MW07-13D, MW07-17D
and MW07-27D. The other area is in the southwestern
part of the site near Allen Harbor, represented by well
MW07-23D. The southeastern area along Narragansett
Bay and the beginning of the entrance channel to Allen
Harbor, which includes wells MW07-16D, MW07-
20D, and MW07-24D, contains saline water in all
lithologic zones. Although water is primarily saline in
the till unit at well MW07-24D, a thin zone of brackish
water is present in the upper part of the till unit. This
brackish zone appears to extend to near the entrance
channel to Allen Harbor. Well MW07-09D is in a large
central area where brackish water is present in the
upper sand and till units, and saline water is present in
the intervening silt unit. Wells MW07-1 ID, MW07-
18D, MW07-29D, and MW07-30D are in an area char-
acterized by brackish water in the upper sand unit and
saline water in the underlying silt and till units. In the
area defined by wells MW07-26S, MW07-19D, and
MW07-21D, where the silt unit is absent and the till
unit slopes upward towards the entrance channel to
Allen Harbor, freshwater in the upper sand unit overlies
brackish water in the till unit at MW07-26S, brackish
water in the upper sand unit overlies predominately
brackish water in the till unit at well MW07-19D, and
saline water in the upper sand unit overlies predomi-
nately brackish water in the till unit at well MW07-
21D. A thin zone of saline water in the lower part of the
till unit appears to extend from the entrance channel to
Allen Harbor to the vicinity of well MW07-19D. In the
remaining areas, freshwater is present in the upper sand
and silt units and brackish water in the till unit (wells
MW07-04D, MW07-10D, and MW07-12D). Freshwa-
ter also is present in the upper sand unit with brackish
water in the silt and till units near well MW07-28D.
18 Distribution of Salinity in Ground Water from the Interpretation of Borehole-Geophysical Logs, Calf Pasture Point, Davisville, R.I.
-------
D
I
CT
6'
a
»
,98.°°° ft
22S.D
100 METERS
CODING SYSTEM USED TO
IDENTIFY GEOHYDROLOGY
F = Fresh water in upper sand
B = Brackish water in silt
S = Saline water in lower sand/till
Unit (in this case silt) not present
Southeastern extent of brackish
water in upper part of till
Northern extent of saline
water in lower part of till
WELL LOCATIONS AND WELL
LOGS-Same numbers
and modifiers as in figure 1
• Electromagnetic-induction
and natural-gamma logs
Natural-gamma log
° Not logged
Base map modified from EA Engineering. Science, and Technology, 1997
Rhode Island stateplane coordinate system
.Q
C
Figure 9. Spatial distribution of major salinity types in ground water by lithologic unit, Calf Pasture Point, Davisville, Rhode Island.
-------
SUMMARY AND CONCLUSIONS
Borehole-geophysical logs of wells drilled in the
surficial aquifer at Calf Pasture Point were interpreted
in conjunction with previously collected salinity data to
delineate zones of fresh, brackish, and saline water.
These data may be used to help identify potential path-
ways of contaminants to surface-water bodies in this
coastal aquifer and to formulate an effective ground-
water-monitoring program.
This site is underlain by a sequence of glacial
sediments, marine sands, and fill. The present-day
land surface was formed, in part, when a saltwater
embayment was filled with dredged sediments from
Narragansett Bay in the early 1940's. The distribution
of sediments and overlying dredged material in this
coastal setting has resulted in a complex distribution
of salinity in the surficial aquifer.
Interpretation of the vertical and horizontal dis-
tribution of salinity using borehole electromagnetic-
induction log data indicates that the upper sand unit
contains freshwater (salinities of 0.5 ppt or less) in the
north and northwestern part of the site, brackish water
(salinity greater than 0.5 ppt to 10 ppt) where the
dredged sediments from the Narragansett Bay were
deposited, and saline water (greater than 10 ppt) along
the shore of the Narragansett Bay and the entrance
channel to Allen Harbor. Saline water is present in the
silt in the eastern half of the site, most of which can be
attributed to the residual saline water since the saltwa-
ter embayment was filled with dredged sediment from
the Narragansett Bay. In the western half of the site, the
silt contains fresh or brackish water. Freshwater is
present in till in the western part of the site near Allen
Harbor and in the northwestern part of the site.
Brackish water is present in the till in the central part of
the site, and saline water is present in the till under
about half of the site on the eastern and southeastern
side. Brackish water underlies saline water in areas
along a northeast-southwest band through the middle
of the site. Distinct zones of brackish water are present
within the till in the southwestern part of this band near
the entrance channel to Allen Harbor.
When the former saltwater embayment was filled
with dredged sediment from Narragansett Bay, the
hydrologic-flow regime changed; this created a com-
plex distribution of salinity in the eastern and central
parts of the site. Although saline water from the
dredged material may have been added to the filled
area, filling the former saltwater embayment virtually
eliminated the surface-water source of saline water to
the underlying aquifer. By increasing the land surface
area, more area was made available for infiltration of
fresh water from precipitation. Fresh ground water
from upgradient and local recharge appears to be dilut-
ing the saline water from the former embayment and
gradually expanding the freshwater-flow system as it
travels to Narragansett Bay and the entrance channel to
Allen Harbor. The distribution of salinity in the western
part of the site, which is dominated by freshwater, is
likely to have been less affected by filling of the
embayment. In the eastern and central parts of the site,
however, the distribution of salinity appears to be in a
dynamic state that is responding to the loss of a contin-
uous source of saline water. Additionally, the southern
and southwestern extremeties of the site, along the
Allen Harbor Entrance Channel and its confluence with
Naragansett Bay are also likely to continue to be
20 Distribution of Salinity in Ground Water from the Interpretation of Borehole-Geophysical Logs, Calf Pasture Point, Davisville, R.I.
-------
effected by erosional and/or depositional processes,
which have resulted in the accretion of a substantial
volume of sediment to the shoreline since 1966. The
ground-water flow system can be expected to continue
to evolve in response to these changes.
The present-day ground-water flow regime at
Calf Pasture Point, therefore, represents a complex
system that results from the interaction of a number of
factors and processes operating on different time
scales. In the near-term, interaction of the freshwater
flow system with brackish and saline waters is an
important process relative to contaminant transport.
However, development of an effective long-term moni-
toring program for ground-water quality must also con-
sider those processes which are evolving on other time
scales.
REFERENCES CITED
Archie, G.E., 1942, The electrical resistivity log as an aid in
determining some reservoir characteristics:
Transactions of the Society of Petroleum Engineers of
the American Institute of Mining, Metallurgical, and
Petroleum Engineers, v. 146, p. 54-62.
Biella, Giancarlo, Lozej, Alfredo, and Tobacco, Ignazio,
1983, Experimental study of some hydrogeophysical
properties of unconsolidated porous media: Ground
Water, v. 21, no. 6, p. 741-751.
EA Engineering, Science, and Technology, 1997, Draft
Final, IR Program Site 07, Calf Pasture Point, Phase III
Remedial Investigation, Volume 1, Technical Report,
Naval Construction Battalion Center, Davisville, Rhode
Island: Bedford, Mass., EA Engineering, Science, and
Technology, 282 p.
Fetter, C.W., Ir., 1980, Applied hydrogeology: Columbus,
Ohio, Charles E. Merrill Publishing Company, 488 p.
Halford, K.J., 1998, Ground-water flow in the surficial
aquifer system and potential movement of contaminants
from selected waste-disposal sites at Naval Station
Mayport, Florida: U.S. Geological Survey Water-
Resources Investigations Report 97-4262, 104 p.
Hansen, B.P., 1993, Location of fracture intervals inferred
from borehole logs of eight wells at the Holton Circle
Superfund Site, Londonderry, New Hampshire: U.S.
Geological Survey Open-File Report 92-647, 22 p.
Hearst, J.R. and Nelson, P.H., 1985, Well logging for
physical properties: New York, McGraw Hill, 571 p.
Keys, W.S. and MacCary, L.M., 1971, Application of
borehole geophysics to water-resources investigations:
U.S. Geological Survey Techniques for Water-
Resources Investigation, book. 2, chap. El, 126 p.
McNeill, J.D., 1980, Electrical conductivity of soils and
rocks: Ontario, Canada, Geonics Limited, Technical
Note TN-5, 22 p.
Melvin, R.L., de Lima, Virginia, and Stone, B.D., 1991, The
stratigraphy and hydraulic properties of tills in southern
New England: U.S. Geological Survey Open-File
Report 91-481, 53 p.
Telford,W.M., Geldart, L.P., Sheriff, R.E., and Keys, D.A.,
1976, Applied geophysics: London, Cambridge
University Press, 860 p.
Winsauer, W.O., 1952, Resistivity of brine-saturated sands in
relation to pore geometry: Bulletin of the American
Association of Petroleum Geologists, v. 36, no. 2,
p. 253-277.
References Cited 21
-------
APPENDIX
-------
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island.
A. Well cluster MW07-03S,D 27
B. Well cluster MW07-05S,D,R 28
C. Well cluster MW07-09D,R 29
D. Well cluster MW07-1 OS,D 30
E. WellMW07-llD 31
F. WellMW07-12D 32
G. Well cluster MW07-13S,D 33
H. Well cluster MW07-16D,R 34
I. WellMW07-18D 35
J. Well cluster MW07-19S,D 36
K. Well cluster MW07-20S,D 37
L. Well cluster MW07-21S,D,R 38
M. Well cluster MW07-22S,D 39
N. Well cluster MW07-23S,D 40
O. Well cluster MW07-24S,D 41
P. Well MW07-26S 42
Q. WellMW07-27D 43
R. WellMW07-28D 44
S. WellMW07-29D 45
T. WellMW07-30D 46
U. Well MW07-31S 47
Appendix 25
-------
A. WELL CLUSTER MW07-03S,D
Sea Level -
LU
UJ
-30
LU
Q
LAND
. SURFACE
UPPER
SAND
SILT
[ TILL
BEDROCK -
-90
WELL CLUSTER
MW07-03S.D LITHOLOGY
AND LOCATIONS
OF SCREENS
50 100 150 200 250 0
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0 10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island.
-a
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to
a.
to
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-------
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CO
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-------
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- -
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SAND
LAND
SURFACE
SILT
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SAND
TILL
BEDROCK -
I
WELL CLUSTER LITHOLOGY
MW07-09D.R
AND LOCATIONS
OF SCREENS
50 100 150 200 250
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0 10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
TJ
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to
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WELL CLUSTER LITHOLOGY NATURAL GAMMA, IN EM CONDUCTIVITY, IN EM RESISTIVITY,
MW07-10S.D COUNTS PER SECOND MILLISIEMENS PER METER IN OHM-METERS
AND LOCATIONS
OF SCREENS
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Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
-------
E. WELLMW07-11D
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-20
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LU
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-
-
-
LITHOLOGY
I
50 100 150 200
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0 10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
-------
F. WELLMW07-12D
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COUNTS PER SECOND MILLISIEMENS PER METER IN OHM-METERS
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Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
m
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-------
G. WELL CLUSTER MW07-13S,D
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SAND
SILT
-
TILL
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-
-
-
-
LITHOLOGY
AND LOCATIONS
OF SCREENS
I
50 100 150 200 250 0
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0 10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
•o
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H. WELL CLUSTER MW07-16D,R
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LOCATIONS
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-
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-
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"
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100 20(
NATURAL GAMMA, IN
COUNTS PER SECOND
1,000 0
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
10 20 30 40
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
-------
/. WELLMW07-18D
£L\J
10
Sea Level
-10
-20
LU
LU
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-
-
-
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-
-
-
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SURFACE -
-
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SAND
-
-
SILT
-
-
-
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- SAND
-
TILL-, -
-
BEDROCK -
MWOT^ISD LITHOLOGY
AND LOCATION
OF SCREEN
50 100 150 200 250
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0
10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
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1
Q.
x'
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
U
Ol
-------
J. WELL CLUSTER MW07-19S,D
CT
C
o
9L
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K. WELL CLUSTER MW07-20S,D
£\1
10
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-20
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-60
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-
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-
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-
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~
TILL ;
_
BEDROCK ;
WELL CLUSTER L|THOLOGY
MW07-20S.D
AND LOCATIONS
OF SCREENS
50 100 150 200
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
10 20 30 40
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
•o
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a
x
-------
L. MW07-21S,D,R
q
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Q
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LAND
SURFACE
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SAND
TILL
BEDROCK -
-90
WELL CLUSTER
MW07-21S,D,R LITHOLOGY
AND LOCATIONS
OF SCREENS
i ,
50 100 150 200 250 0
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0 10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
o
a
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
i
-------
M. WELL CLUSTER MW07-22S,D
10
Sea Level
-10
-20
UJ
LJJ
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LJJ
Q
|-40
-50
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-70
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-
-
-
-
-
-
-
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-
-
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-
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WELL CLUSTER LITHOLOGY
MW07-22S.D
AND LOCATIONS
OF SCREENS
NATURAL GAMMA, IN
COUNTS PER SECOND
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
1
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Q
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-70
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_ SILT
LAND
SURFACE
UPPER
SAND
TILL
BEDROCK
WELL CLUSTER LITHOLOGY
MW07-24S.D
AND LOCATIONS
OF SCREENS
50 100 150 200 250 0
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 BOO
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0 10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
•o
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Q.
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-------
P. WELL MW07-26S
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-50
-60
-70
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.
-
- -
_
-
-
-
-
-
-
- -
-
-
-
WELL
MW07-27D
LAND ;
- SURFACE -
UPPER -
SAND ;
SILT ;
-
TILL ;
-
-
- BEDROCK -
-
-
-
LITHOLOGY
AND LOCATION
OF SCREEN
50 100 150 200 250 0
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0
I I I
i I i
10 20 30 40
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
T3
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to
IS
Q.
-------
R WELL MW07-28D
q
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W
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5'
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LU
-30
LU
Q
-90
LAND
SURFACE
UPPER
SAND
SILT
TILL
BEDROCK
WELL
MW07-28D
AND LOCATION
OF SCREEN
LITHOLOGY
50 100 150 200
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
30
40 50
EM RESISTIVITY,
IN OHM-METERS
o
01
J5"
JO
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
-------
S. WELL MW07-29D
10
Sea Level
-10
-20
LJJ
LJJ
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LU
Q
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5
-50
-60
-70
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-
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i :
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i i i
WELL
MW07-29D
AND LOCATION
OF SCREEN
LITHOLOGY
50 100 150 200 250 0
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
i i ,
, i i
1,000 0 10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
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3
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20
Sea Level
m
LLI
-30
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LAND
SURFACE
UPPER
SAND
SILT
SILT
LOWER
SAND
- TILL
BEDROCK -
WELL
MW07-30D LITHOLOGY
AND LOCATION
OF SCREEN
50 100 150 200 250 0
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
1,000 0 10 20 30 40 50
EM RESISTIVITY,
IN OHM-METERS
TJ
O
5'
D
01
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33
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
-------
a
01
u
20
10
Sea Level
-10
-20
LU
LU
--30
LU
Q
j5 -40
P
-50
-60
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WELLMW07-31S
;
-
-
-
-
•
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_
-
-
- -
-
;
-
-
-
-
-
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SURFACE .
:
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SAND ;
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SILT
-
TILL
-
'•
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-
-
-
-
NOT LOGGED
i
i
i
i
NOT LOGGED
MW07-31S LITHOLOGY
AND LOCATION
OF SCREEN
NATURAL GAMMA, IN
COUNTS PER SECOND
200 400 600 800
EM CONDUCTIVITY, IN
MILLISIEMENS PER METER
10 20 30 40
EM RESISTIVITY,
IN OHM-METERS
50
Appendix 1. Lithologic and borehole geophysical logs at Calf Pasture Point, Davisville, Rhode Island—Continued.
•O
1
Q.
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-------
|