Urish layout I - Rhode Island Sea Grant - University of Rhode Island

This whitepaper series is the result of a collaborative research effort among Rhode Island Sea Grant, the City of
Warwick, and the R.I. Department of Environmental Management to determine specific means to restore
water quality in Greenwich Bay.
The proper citation for this report is:
Urish, D.W. and A.L. Gomez. 2004. Groundwater Discharge to Greenwich Bay. Paper No. 3 in:
M. Schwartz (ed.) Restoring Water Quality in Greenwich Bay: A Whitepaper Series. Rhode Island Sea Grant,
Narragansett, R.I. 8pp.
Additional papers in Restoring Water Quality in Greenwich Bay: A Whitepaper Series*
Responding to the Crisis: Formation of the Greenwich Bay Partnership (In prep.)
Greenwich Bay and Its Watershed: Spatial Data for Planning and Environmental Management (2000)
Stormwater Discharges to Greenwich Bay (In prep.)
Circulation and Flushing Dynamics of Greenwich Bay and Its Coves (In prep.)
An Assessment of Eutrophication in Greenwich Bay (2000)
Education and Community Outreach for Greenwich Bay (In prep.)
*Availability of whitepapers and ordering information can be found at: http://seagrant.gso.uri.edu/G_Bay/
Additional copies of this publication are available from the Rhode Island Sea Grant Communications Office,
University of Rhode Island Bay Campus, Narragansett, RI 02882-1197. Order P1689. $3.
Loan copies of this publication are available from the National Sea Grant Depository, Pell Library Building,
University of Rhode Island Bay Campus, Narragansett, RI 02882-1197. Order RIU-T-04-001.
This publication is sponsored in part by Rhode Island Sea Grant, under NOAA Grant No. NA86RG0076. The
views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its
sub-agencies. The U.S. Government is authorized to produce and distribute reprints for governmental purposes
notwithstanding any copyright notation that may appear hereon.
Cover photo courtesy of the Providence Journal Company. Reprinted with permission.
Rhode Island

Groundwater Discharge to Greenwich Bay
DANIEL W. URISH AND ANTHONY L. GOMEZ
DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING
UNIVERSITY OF RHODE ISLAND
KINGSTON, RI 02881
INTRODUCTION
Groundwater enters Greenwich Bay from two
primary sources: 1) approximately 75 percent as dry
weather flow from streams and 2) approximately 25
percent as direct seepage along the coastlines of the
bay and its contiguous coves. To a much lesser
extent, groundwater may enter as unintentional point
discharge from subsurface pipes intended to carry
only stormwater drainage. Groundwater represents
60 to 70 percent of the quantity of freshwater entering
the bay from a terrestrial origin. The remainder
enters as stream and stormwater discharge directly
from rainfall events on the watershed and direct
deposition on the bay itself.
The quality of the groundwater is highly important
to the bay since it carries with it a wide range of
anthropogenic contaminants from cultural sources
(Giblin and Gaines, 1990; Nixon, 1995), which are
disposed of directly in the ground or leached into the
ground from surface sources. The discharge of
contaminants in the poorly flushed regions of
Greenwich Bay is of special concern. From a
groundwater source, the contaminant of primary
concern is nitrogen, due to its potential to cause
eutrophication. Additionally, biological contamination
may occur from failed septic systems if there is
little opportunity for subsurface filtration and die off
of harmful pathogens.
Nitrogen in groundwater is of particular concern
since, in high concentrations, it may be a serious
health problem in drinking water, but at much lower
levels it can have serious adverse implications for
receiving waters. Nitrogen is usually the limiting
nutrient in seawater (Ryther and Dunstan, 1971); and
in poorly flushed coves and along the shoreline,
nitrogen can cause eutrophication, a condition in
which excessive growth of aquatic plant life and
algae occurs with subsequent decay and depletion of
oxygen (Richardson and Jorgensen, 1996). This can
not only cause bad odors, but will kill all oxygendependent
life in the water. Nitrogen is a major
component of sewage effluent (Canter and Knox,
1985). Existing nitrogen levels in groundwater in the
Greenwich Bay area are frequently in excess of 6
milligrams per liter (mg/l). It takes less than 1 mg/l in
poorly flushed coastal waters to cause eutrophication.
PURPOSE
The purpose of this groundwater study was to
provide information for the evaluation of the quantity,
quality, and location of groundwater discharge
into Greenwich Bay and its contiguous coves. The
information presented here focuses on three areas:
1) The delineation of source and discharge
locations for groundwater entering the bay. This task
included the delineation of sub-basins within the
watershed and the employment of thermal infrared
aerial photography
2) Water budgets for groundwater discharge to
the bay
3) Nitrogen budgets for groundwater discharge
to the bay
DELINEATION OF SUB-BASINS
In order to define the probable sources of
groundwater recharge and its discharge locations to
Greenwich Bay, the watershed was subdivided into
23 sub-basins as shown in Figure 1. These subbasins
were defined on the basis of surface topography
and watershed divide delineation. While it is
recognized that surface water topographic delineation
is not the same as groundwater sub-basin
delineation, it is a reasonable approximation and best
available given the lack of detailed groundwater
elevations (Cambareri and Eichner, 1998). These
sub-basin areas are tabulated as to size and contaminant
source significance in Table 1. It is noted that
the sub-basins of Hardig Brook and Maskerchugg
are by far the largest sub-basins, as well as the
largest unsewered areas.
1

Figure 1. Greenwich Bay watershed with sub-basin delineation
THERMAL INFRARED IMAGERY
Thermal infrared aerial imagery has been
employed to identify specific areas of concentrated
groundwater discharge along the shoreline. This
method uses the difference in thermal spectral
response to measure temperature contrast of water
along the coastal margin (Banks et al., 1996). Since
groundwater, which discharges at a temperature of
about 10 to 18 C (50 to 60 F), is colder than the bay
water in the summer, which is about 22 to 25 C (70
to 74 F), discharging fresh groundwater can be
identified from the thermal infrared response.
Additionally, when fresh groundwater enters a
saltwater body it will tend to float on top because of
its lighter density. This property enhances the ability
of the thermal infrared method to detect the colder
fresh groundwater. The discharge of coastal groundwater
takes place most prolifically over a period of
about two hours during low tide. Since coastal
groundwater discharge takes place in a highly nonuniform
manner, this delineation of fresh groundwater
discharge is important so that the most affected
shoreline can be identified and the most efficient
groundwater quality sampling locations can be
determined.
Thermal infrared surveys of Greenwich Bay and
its coastal areas were accomplished in September
1995 and again in August 1997. In order to optimize
conditions for high-quality results, the flights were
made in the early evening during low tide when
temperature contrast was high, but direct solar
effects were negligible. Graphs showing conditions
of tide and water temperature for the August 24,
1997, survey are provided as Figure 2. The photo
runs were made during the time period of 1900 to
2100 hours, shortly after sunset, to minimize the
2

Table 1. Estimated average annual groundwater discharge into Greenwich Bay from sub-basin
Sub-basin Sub-basin Linear Groundwater Groundwater Stream
Name Area Feet of Discharge Discharge Discharge
(ft 2 ) Shoreline into Bay into Bay into Bay
(lf) (ft 3 /day/lf) (ft 3 /day) (ft 3 /day)
Apponaug (APG) 5,434,358 (SL) 2,606 7.14 18,607
Arnold (ARD) 2,204,427 (SL) 6,054 1.25 7,568
Baker (BKR) 9,512,274 (ST) 32,576
Bayside (BYS) 19,234,952 (SL) 25,636 2.57 65,885
Chepiwanoxet (CHX) 30,377,773 (SL) 23,557 4.42 104,122
Cove Beach (CVB) 971,904 (SL) 5,290 0.63 3,333
East Greenwich (EGN) 11,588,590 (SL) 7,370 5.38 39,651
Goddard (GDD) 7,571,457 (SL) 19,161 1.35 25,867
Hardig (HRD) 170,058,425 (ST) 582,392
Lockwood (LKD) 5,506,024 (ST) 18,856
Long (LNG) 4,927,101 (SL) 2,606 6.47 16,861
Longmeadow (LMW) 9,483,510 (ST) 32,478
Maskerchugg (MKG) 166,460,915 (ST) 570,072
Nausauket (NKT) 3,157,196 (SL) 4,422 2.45 10,834
North Brushneck (NBN) 2,653,048 (SL) 6,054 1.50 9,081
North Buttonwoods (NBW) 7,543,626 (ST) 25,834
Oakland (OKL) 13,507,254 (SL) 16,897 2.74 46,298
Pottowomut (PTW) 12,758,790 (SL) 25,030 1.75 43,803
South Brushneck (SBN) 426,248 (SL) 3,895 0.37 1,441
South Buttonwoods (SBW) 10,022,151 (SL) 12,055 2.85 34,357
Tuscatucket (TSK) 55,876,639 (ST) 191,358
Warwick Neck North (WNN) 11,682,521 (SL) 19,516 2.05 40,008
Warwick Neck South (WNS) 15,549,097 (SL) 16,239 3.28 53,264
TOTALS 576,508,280 520,977 1,453,566
Notes:
1) In the column titled “Sub-basin Area,” SL indicates groundwater shoreline discharge, and ST indicates dry weather groundwater
discharge into a stream that subsequently discharges into the bay.
2) Groundwater discharges are estimated average daily values based on 15 inches of groundwater recharge per year.
3) Where the sub-basin is contiguous to the shoreline, the linear feet of shoreline through which the groundwater discharges are
indicated as well as the quantity per linear foot.
4) Where the sub-basin groundwater discharges into a stream, the estimated single value of that discharge is indicated.
5) As a first estimate, it is assumed that all residents get imported water supply at the rate of 170 liters per person per day and
that, for unsewered houses, this water becomes groundwater recharge.
6) As a first estimate, it is assumed that, because of large quantities of groundwater well pumping, there is no groundwater
discharge into lower Greenwich Cove from the Hunt River Basin.
effect of solar reflection. The fresh groundwater-bay
water temperature contrast at that time was approximately
14 F. The thermal infrared images show areas
of major groundwater contribution in Warwick Cove,
Brushneck Cove, Apponaug Cove, and Greenwich
Cove.
As an example, the results of the thermal infrared
imagery for Brushneck Cove are presented as
Figure 3. In this figure, the colder fresh groundwater
is defined by the dark plumes emanating from the
shoreline. The plumes are most evident along the
west shore and the central part of the cove on the
east shore.
WATER BUDGET
The detailed results of an annual water budget
analysis of each of the Greenwich Bay sub-basins is
shown in Table 1. It is to be noted that some subbasins
discharge into streams, while others discharge
directly into the bay or a cove along the shoreline.
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NITROGEN BUDGET
A nitrogen budget for Greenwich Bay was developed
for each sub-basin using the contribution input detailed in
Table 2. The values include contributions from road and
roof runoff and lawn and atmospheric sources based on the
“Cape Cod Model.” Nitrogen was evaluated as the nutrient of
primary interest because of its potential for contribution to
eutrophication in shallow and poorly flushed coastal regions
of the bay. The calculations are conservative, assuming that
no loss or alternation of nitrogen takes place after the entry
into the groundwater system. The on-site sewage disposal
contributions are estimated based on unsewered areas defined
by Warwick, R.I., area sewer maps and the number of
unsewered homes obtained from the U.S. Census Report of
1992. This information was integrated with the sub-basin
delineation using the ARCINFO Geographic Information
System (GIS).
Figure 2. Tide level and water temperature
conditions during the thermal infrared survey
of August 24, 1997
Where discharge is along the shoreline, it
is quantified as discharge in cubic feet per
day per linear foot of shoreline. Where
the discharge is in the stream, which
subsequently flows to the bay, the quantity
is described as cubic feet per day into
the bay.
Based on area climatological data, the
values assume an average annual groundwater
discharge of 38 centimeters (15
inches) over the watershed. In any case
the quantity represents average daily dry
weather discharge, and does not include
additional contribution from storm
surface run off. Also, the values do not
reflect seasonable variation, which does
occur; for example, summer dry weather
groundwater flow, when the water table is
low, will be less than spring groundwater
flow where the water table is higher
(Milham and Howes, 1994).
Figure 3. Thermal infrared image of Brushneck Cove taken
August 24, 1997
4

Table 2. Total estimated nitrogen loading for the Greenwich Bay watershed by sub-basin
Unsewered Total
Residences: Residences:
Wastewater Lawn Atmospheric Road runoff Roof runoff Total
nitrogen nitrogen nitrogen nitrogen nitrogen nitrogen
Sub-basin loading loading loading loading loading loading
Name (g/day) (g/day) (g/day) (g/day) (g/day) (g/day)
Apponaug 1,201 466 26 71 26 1,790
Arnold 520 198 11 29 11 769
Baker 8,407 1,040 46 125 58 9,676
Bayside 15,430 2,509 93 252 141 18,425
Chepiwanoxet 8,828 2,006 147 398 113 11,492
Cove Beach 689 68 5 13 4 779
East Greenwich 2,257 2,370 56 152 133 4,968
Goddard 1,461 189 37 99 11 1,797
Hardig 31,082 12,988 825 2,227 729 47,851
Lockwood 5,294 697 27 72 39 6,129
Long 4,192 499 24 65 28 4,808
Longmeadow 5,715 639 46 124 36 6,560
Maskerchugg 26,982 7,386 807 2,180 415 37,770
Nausauket 2,586 294 15 41 17 2,953
North Brushneck 1,805 184 13 35 10 2,047
North Buttonwoods 5,370 557 37 99 31 6,094
Oakland 15,484 2,337 66 177 131 18,195
Pottowomut 2,547 336 62 167 19 3,131
South Brushneck 2,968 306 2 6 17 3,299
South Buttonwoods 5,822 742 49 131 42 6,786
Tuscatucket 46,183 6,780 271 732 381 54,347
Warwick Neck North 7,214 735 57 153 41 8,200
Warwick Neck South 1,729 273 75 204 15 2,296
Sub-basin Totals 203,766 43,599 2,796 7,548 2,448 260,157
Note: Nitrogen loadings are based on the Cape Cod Commission Technical Bulletin 91-001 (Dec. 1991).
The total estimated nitrogen loading from
groundwater to Greenwich Bay is about 260 kilograms
per day (kg/day) with about 65 to 75 percent
of this originating from on-site sewage disposal
systems. The three sub-basins contributing the
largest loads of nitrogen are:
1) Tuscatucket (54.3 kg/day) discharging into
Brushneck Cove;
2) Hardig (47.9 kg/day) discharging into
Apponaug Cove; and
3) Maskerchugg (37.8 kg/day) discharging into
Greenwich Cove.
CONCLUSIONS
The contributions of groundwater and nitrogen
loading are summarized in Table 2 by receiving
water and the contributing sub-basins. The receiving
water areas receiving the largest influx of nitrogen
are Brushneck Cove and Apponaug Cove, with
Greenwich Cove a close third in ranking. These are
also the largest groundwater and dry weather stream
flow contributors. The estimated nitrogen and
groundwater discharge quantities were also prepared
for use as data inputs to the Greenwich Bay receiving
5

Table 3. Estimated nitrogen (N) loading into box model sections of Greenwich Bay and contiguous coves due
to sewering project
Present First-year Second-year Third-year
total N total N total N total N
Box Model Contributing loading loading loading loading
Designation Sub-basins (g/day) (g/day) (g/day) (g/day)
1 Bayside BYS * 32,816 18,791 13,181 4,766
Longmeadow LMW
Warwick Neck North WNN
2 Oakland OKL * 84,029 48,090 33,714 12,150
Tuscatucket TSK
North Buttonwoods NBW
North Brushneck NBN
South Brushneck SBN
Cove Beach CVB
3 Long LNG 61,309 40,177 31,725 19,046
Lockwood LKD
Apponaug APG
Arnold ARD *
Hardig HDG
4 Nausauket NKT * 8,679 5,303 3,952 1,926
Arnold ARD *
Chipiwanoxet CHX *
5 Chipiwanoxet CHX * 48,209 31,448 24,743 14,686
East Greenwich EGN
Maskerchugg MKG
Goddard GDD
6 Potowomut PTW * 15,805 8,990 6,263 2,174
Nausauket NKT *
Baker BKR
South Buttonwoods SBW *
7 South Buttonwoods SBW * 15,855 9,236 6,589 2,617
Potowomut PTW *
Oakland OKL *
Bayside BYS *
Warwick Neck South WNS
Total 266,702 162,035 120,167 57,365
*Indicates a percentage of area in more than one box.
6

water model. This input is summarized for segments
of Greenwich Bay identified as “box model” areas,
delineated in Figure 1.
The estimated loadings to Greenwich Bay were
also evaluated using future area sewering scenarios
for full sewer buildout. Based on information from
the Warwick City Planning Department, it is envisioned
that at the end of the first year after sewering
begins, 50 percent of the households will be hooked
up; at the end of the second year, 70 percent; and at
the end of the third year, 100 percent. Using this
concept, estimated future loadings to the bay were
calculated and are summarized in Table 3 on the
previous page. The results are also shown graphically
in Figure 4 below. As indicated, full sewering
would result in a major reduction of approximately
72 percent of the nitrogen input to the bay from
groundwater sources.
It is evident from inspection of Figure 3 that the
discharge of fresh groundwater is highly irregular
along the shoreline, but occurs in very discrete
plumes, and even then, occurs only during low tide
periods. Direct shoreline sampling for salinity and
nitrogen levels during low tide in the zones of
significant shoreline discharge provided direct
confirmation of these plumes and of high levels of
nitrogen entering the bay. An example of the results
is shown in Figure 5 (page 8) for a fresh groundwater
plume discharging along the shoreline of Oakland
Beach in the Brushneck Cove area, located as shown
in Figure 3 (labeled “Beach Survey Site A”). Figure 5
shows a profile of salinity and nitrogen concentrations
in groundwater flowing through the beach at
low tide. Concentrations of nitrogen average about
6 mg/l, but go up as high as 12 mg/l, more than
enough to cause serious problems from eutrophication
in poorly flushed
regions of the bay. It is
also apparent that there is
a direct correlation
between the freshness of
the discharge and the
nitrogen content, indicating
that the primary
source of the nitrogen is
the fresh groundwater.
Figure 4. Estimated future nitrogen loads to Greenwich Bay from
groundwater based on planned sewering
ACKNOWLEDGMENTS
The primary support for
this work has been
provided by the city of
Warwick and by the
Rhode Island Sea Grant
Program, with additional
graduate student support
by the U.S. Department of
Defense. The cooperation
of the various departments
of the city of Warwick and
of the many private
individuals who provided
information is also greatly
appreciated.
7

20
18
20
18
Figure 5. Salinity-nitrogen
profile for discharging groundwater
along the Oakland Beach
shoreline
16
16
14
Oakland Beach/Brushneck Cove
Samples Taken: May 21, 1998
14
NO3-N
Salinity
NO3-N Concentration (mg/L)
12
10
8
12
10
8
Salinity (ppt)
6
6
4
2
N
O
S
A
M
P
L
E
4
2
0
0 10 30 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170
Distance (ft)
0
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Using thermal infrared imagery to delineate
groundwater discharge. Ground Water
34(3):434–443.
Cambareri, T.C. and E.M. Eichner. 1998. Watershed
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Canter, L.W. and R.C. Knox. 1985. Septic Tank
System Effects on Groundwater Quality.
Lewis Publishers, Inc., pp. 75–79.
Giblin, A.E. and A.G. Gaines. 1990. Nitrogen inputs
to a marine embayment: The importance of
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Milham, N.P. and B.L. Howes. 1994. Freshwater
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Nixon, S.W. 1995. Coastal marine eutrophication: A
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Richardson, K. and B.B. Jorgensen. 1996. Eutrophication:
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Eutrophication in Coastal Marine Ecosystems,
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Ryther, J.H. and W.M. Dunstan. 1971. Nitrogen,
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1013.
Valiela, I., G. Collins, L. Kremer, K. Lajtha, M.
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