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A SUTRA Model
of Seawater Intrusion P.E. Misut 1, W.
Yulinsky 2, D. Cohen 3, D. St. Germain 3, C.
I. Voss 1, J. Monti Jr. 1 1 U. S. Geological Survey, USA 2 N. Y. C. Department of Environmental Protection, USA 3 Malcolm Pirnie Inc., USA INTRODUCTION: New York City obtains about 99 percent of
its freshwater from a surface-water system. The supply could run short,
however, during a prolonged drought or as a result of a system malfunction.
The aquifers of eastern New York City (Brooklyn and Queens, fig. 1) could
provide a substantial supplement for use in times of shortage, but excessive
pumping of these aquifers in the past has caused seawater intrusion,
resulting in the shutdown of muncipal supply wells throughout Brooklyn and in
several parts of Queens. The coastal-plain aquifers are recharged by abundant precipitation and
currently provide about 1 percent of the total supply for New York City. This
water is provided mainly to local communities that lack connection to the
city’s surface-water system. Rehabilitation of the existing infrastructure
could provide a supplemental supply of about 2,000 kg/s, or 3 percent of the
city’s total supply, if the surface-water supplies are diminished by drought
or system failures. In 2001, the U.S. Geological Survey, in cooperation with New York City
Department of Environmental Protection (NYCDEP; http://www.nyc.gov/dep), began a 3-year
study to develop a model to simulate a wide range of pumping scenarios and
predict the location and degree of seawater intrusion that would occur if
ground water were used to make up a deficit in the city’s supply.
Hypothetical pumping scenarios that could increase the ground-water yield to
more than 10,000 kg/s are being evaluated (Malcolm Pirnie, 1999). This
abstract summarizes the coastal-plain aquifer system, describes the model and
the initial conditions, and presents some results of interface-movement
scenarios. HYDROLOGY: The
ground-water system on western Long Island (Brooklyn and Queens) consists of
four unconsolidated aquifers and two confining units and is underlain by
gently southward sloping impermeable bedrock (fig. 2). The
bedrock crops out in the northwest, is about 300 m below sea level in the
southeast along the south shore, and about 250 m below sea level in the
southwest along the south shore. The sequence of unconsolidated deposits,
from the bedrock upward, consists of the Lloyd aquifer, the Raritan clay, and
the Magothy aquifer, all of Cretaceous age, and the Jameco aquifer, the
Gardiners Clay, and the upper glacial aquifer (with outwash and moraine
zones), all of Pleistocene age. These six units vary in thickness locally and
pinch out in some areas (Smolensky and others, 1989).
At several locations, the Gardiners Clay contains erosional holes that
provide vertical hydraulic connections. The water table is mostly in the
upper glacial aquifer. The distribution of freshwater and the position of the
freshwater/seawater interface do not conform to the Ghyben-Herzberg relation
as a result of local anisotropy and varying unit thickness. The response of
the seawater interface to the sea-level rise since the Pleistocene has been
delayed by hundreds to thousands of years in the Lloyd aquifer beneath the
Atlantic Ocean as a result of confinement by the overlying Raritan Clay (Meisler and others, 1984). Pumping during the mid-20th century caused
increased salinity in water pumped from wells screened in the upper glacial,
Jameco, and Magothy aquifers and necessitated the shutdown of all
public-supply wells in Brooklyn in 1947 (Soren, 1976). Inflows to and outflows from the aquifers are currently near
steady state. About half of the annual precipitation (1.1 m/yr) recharged the
flow system before urbanization, but the present extensive stormwater- and
sanitary-sewer systems probably decrease recharge to one-third of the annual
precipitation (Misut
and Monti, 1999) About 440 kg/s of
fresh ground water enters the aquifers system laterally from Nassau County to
the east. Natural outflow occurs mostly near the shore, but partly as
freshwater seepage to ponds and streams and as subsea discharge. The water
supply that is piped from upstate surface-water reservoirs discharges to the
sanitary-sewer system, bypassing the ground-water system. MODELING. Numerical
modeling of the Long Island ground-water-flow system began in the 1970's (Getzen, 1974, Getzen, 1977,
Gupta and Pinder, 1978). Three-dimensional transient-state MODFLOW models were
developed in the 1980's and 1990's (Reilly and Harbaugh, 1980; Buxton and
Smolensky, 1999; Misut and Monti, 1999), and cross-sectional SHARP models
were developed in the 1990's (Kontis, 1999). Development of a three-dimensional SUTRA model began in 2001. The
MODFLOW, SHARP, and SUTRA codes with supporting documentation are available
free of charge at http://water.usgs.gov/software.
The New York City MODFLOW models used fixed no-horizontal-flow boundaries in
combination with vertical leakage across confining units into constant-head
boundaries to represent seawater interfaces. The SHARP models provided only a
limited physical mechanism to represent movement of seawater interfaces in
response to pumping. The SUTRA model code (Voss, 1984) and graphical user interface (Voss and others, 1997) were chosen for this study for its ability to
simulate variable fluid density driven by solute mass and/or temperature on
three-dimensionally deformed finite-element meshes. A raster format was used
to store hydrogeologic and hydrologic data at a resolution of 30 m to
match available satellite imagery. Raster-formatted data were interpolated
onto points corresponding to finite-element meshes; these point-wise data
were then imported into a graphical user interface for SUTRA. Boundary Conditions: Specified flux was used to represent the
ground-water recharge mechanism. Five distributions of recharge were prepared
to correspond to rates under the following conditions: predevelopment, two simplified historical
stages of urbanization, current (2000) conditions, and 1962-66 drought
conditions. The predevelopment period was assigned a uniform recharge
distribution of 0.56 m/yr in inland areas. The simplified historical stages
were represented by two zones that change historically in size and shape—an
undeveloped zone, which was assigned a recharge rate of 0.56 m/yr, and an
urbanizing zone, in which recharge was decreased to 0.25 m/yr to account for
consumptive water use and conveyance of storm-water runoff to streams. The
current-conditions distribution of recharge
was based on satellite imagery. The drought-simulation period entailed
a scalar reduction in the current-conditions distribution. Hydrostatic-pressure boundaries with saltwater chloride concentration
of 19,000 mg/l were used in offshore zones. Pressures at sea-floor nodes were
calculated from NOAA bathymetric data (http://www.noaa.gov).
Hydrostatic-pressure boundaries with freshwater chloride concentration of 0
mg/l were applied at some streams and ponds and to the entire eastern side of
the model to represent the continuation of aquifers beyond the model domain.
A no-flow boundary was used to represent the model bottom (bedrock). Model Discretization: A
graphical user interface facilitated rediscretization through a one-step
interpolation process that allows the model results to be used as initial
conditions for a later run with a different mesh (fig. 3). Meshes were designed with varying grid density to represent: Initial Conditions: The
position of the seawater interface in the Lloyd aquifer and Raritan Clay still
reflects the lowered sea level of the Wisconsin glaciation, about 10,000
years ago. Exploratory SUTRA simulations were conducted to investigate the
post-glacial interface movement and to generate initial conditions for
pumpage-evaluation runs that start at the present and allow the interfaces to
move slowly landward. Runs that started with a model domain saturated
completely with freshwater and applied time-invariant boundary conditions
corresponding to current sea level did not approach inferred current
interface positions within a 10,000-yr post-glacial period; they took much
longer. Therefore, simulation of pre-glacial conditions may also be required.
Furthermore, the inferred current interface configuration was never reached
during these simulations. One explanation for this is the presence of holes
in the Gardiners Clay, which are slightly below the present sea level and
thus become exposed to seawater at some undetermined time leading up to the
present as sea level rises. The arrival of the rising sea at a Gardiners Clay
hole opens a new path of least resistance for density-driven downward flow of
seawater. The rate of sea-level rise is uncertain, however, and this process
cannot be represented by time-invariant boundary conditions in which sea level
is fixed. Finally, an analysis was done to calculate rates of interface
movement in a pre-development simulation, with current sea level, in which
the interfaces were started at their inferred current position. Only the
interfaces in the Lloyd and Raritan moved significantly and it can be assumed
that the interfaces of the Magothy, Jameco, and upper glacial aquifers are at
steady state at the current time. The urbanization period (1900-2000) was
then simulated and provided results that were satisfactory as initial
conditions for the pumpage-evaluation runs. Model Calibration.
Parameters were adjusted during a sequence of about 50 model calibration runs
to attain the best match of simulated pressures and concentrations to field
data. Final values of the most
sensitive parameters are as follows: Porosity: 15 percent Storativity: 10 -8
{kg/(m s2)}-1; Permeability: Except for the upper glacial aquifer, hydraulic properties of
hydrogeologic units are uniform, and entail extrapolation to the offshore
part of the model domain, where little data was available to calibrate
against. SIMULATION RESULTS. The
drought-emergency reactivation scenario entailed rehabilitating wellfields
that were taken out of service and replaced by surface-water supply. If a
drought were to decrease the surface-water supply, these wells would be
reactivated to produce a total of about 2,000 kg/s, about 80 percent
greater than the amount currently pumped. Each of these wells is screened in
either the Magothy, the Jameco, or the upper glacial aquifer. The location of
the seawater interface in the Lloyd aquifer is not critical for this
particular simulation, but other scenarios are being considered that involve
new wells in the Lloyd aquifer. Areas with a potential for seawater intrusion
were identified through an analysis of contours of change in solute-mass
fraction from current conditions to the simulation result after 2 yr of
pumping (fig 5 and
fig 6). The pumping period was
limited to 2 yr because longer periods resulted in landward water table
gradients from the coast to the pumping wells. A change equivalent to a 50 mg/l or greater increase in
chloride concentration was identified as an indicator of significant
intrusion potential; areas with this potential were in the nearshore areas
closest to pumping wells and closest to the hole of the Gardiners Clay. Changes
smaller than 50 mg/l may be attributable to numerical inaccuracies and/or
naturally occurring changes resulting from post-Pleistocene sea-level rise
and, therefore, warrant site-specific investigation. The shortest distance
between an area with significant intrusion potential and a pumping well
exceeds 3 km; therefore, the risk of well contamination within 2 yr of
supplemental pumping is small. Keywords: SUTRA, numerical modeling, finite element method Corresponding author: Paul Misut, Hydrologist, USGS, 2045 Rt. 112,
Coram, NY 11727. Email: pemisut@usgs.gov References ______2002, Ground-Water Software, accessed February 3, 2003 at http://water.usgs.gov/software. ______2002, New York City Department of Environmental Protection, accessed February 3, 2003 at http://www.nyc.gov/dep. Buxton, H.T., Smolensky, D.A. (1999) Simulation analysis of the development of the ground-water flow system of Long Island, New York: U.S. Geological Survey Water-Resources Investigations Report 98-4069, 57 p. Getzen, R.T. (1974) The Long Island ground-water reservoir –a case study in anisotropic flow: Urbana-Champaign, Illinois, University of Illinois, Ph.D. thesis, 130p. Getzen, R.T. (1977) Analog-model analysis
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