USGS Summary: Physical Characterization of Groundwater
in Milwaukee’s Menomonee River Valley
5.2
Flow System Evaluation. The USGS constructed a flow model to contribute to the ongoing investigation activities by integrating data in a quantitative framework, guiding data
collection, and testing conceptual models to account for the controls on groundwater
flow and contaminant movement within the Valley. Specific information to be gained
from the modeling included identification of potential receptors and particle tracking to
evaluate travel paths and travel times for contaminate fate and transport evaluations.
A step-wise modeling approach was used in which a one-layer analytical element model
simulates the regional flow system and also furnishes boundary conditions for a local
model capable of simulating three-dimensional effects.
The analysis was conducted at three area scales. An analytic element code called
GFLOW was applied to a 200 square mile regional area that extends over most of
Milwaukee County as shown in Figure 5-1. The two zones of recharge shown in the
figure are based on a recent study of recharge rates in southeastern Wisconsin (D.
Cherkauer, 2001. Distribution of Groundwate Recharge in Southeastern Wisconsin.
Final Report to Source Water Protection Program, Wisconsin Natural Department of
Resources.). The regional one-layer model, centered on the Valley and incorporating
the entire Deep Tunnel System, is a vertically integrated representation of the
hydrogeologic system shown in Figure 5-2. The inset model, corresponding to the box
in Figure 5-1, was also centered on the Valley and incorporated approximately two-thirds
of the Deep Tunnel System. Figure 5-3 shows the model grid – each cell is 250 ft on a
side. The eastern half of the grid contains estuary water bodies that act effectively as
arms of Lake Michigan.
The local model contained more geologic detail than the regional model and
permitted simulation of three-dimensional flow paths. Figures 5-4 and 5-5 contain an
east-west and a north-south cross section, respectively, through the Valley that
correspond to a MODFLOW row and column. The four uppermost model layers in the
sections represent unlithified units while the remaining layers constitute the dolomite.
The cross sections demonstrate that the depth from land surface to the top of the
dolomite is variable below the Valley. The recharge rate to the local model is equal to
0.6 inch/yr everywhere.
Previous studies performed by researchers at the University of Wisconsin-Milwaukee
provided a range of expected values for subsurface properties in southeast Wisconsin.
Date collected at and around the Valley study area as part of the brownfields project
provided additional information on the distribution of fill/estuary deposits and the
underlying clay-rich glacial tills. Reports issued by the MMSD contained information on
the location and depth of the Deep Tunnel segments in the dolomite aquifer. A recent
study dedicated to estimating the dry-weather infiltration to the Deep Tunnel was very
helpful in constructing and calibrating the models (Rust/Harza, 2002. Internal Inspection
of the Inline Storage System. Report for the Milwaukee Metropolitan Sewerage
District ).
The models were calibrated by adjusting the hydraulic conductivity of the glacial and
dolomite material. Calibration benefited from many recent water-level measurements
that delineated the regional and local water table surface as well as the vertical gradient
within the shallow deposits. A preliminary calibration using the regional model relied on
matching observed to simulated base flow within the study prior to the construction of
the Deep Tunnel. The regional model furnished the lateral flows into and out of the grid
of the local model. Following a sensitivity analysis and repeated simulations, the local
model provided a good match to the water-level targets and the vertical gradients as well
as the infiltration flux targets along multiple segments of the Deep Tunnel. The best-fit
input values for the local model are listed in Table 5-1. Figure 5-6 plots observed versus
simulated head values. Tables 5-2, 5-3 and 5-4 demonstrate the quality of the
calibration in terms of the three distinct sets of targets: water levels at observation wells,
vertical gradients at well nests, and fluxes over segments of the Deep Tunnel. Because
the match is good across target types, the overall calibration of the local model is judged
acceptable.
The calibrated horizontal hydraulic conductivity values for the fine-grained glacial and
dolomite units fall within the expected range. The calibration indicates that the ratio of
horizontal to vertical anisotropy for the entire system is on the order of 1500 to 1.
6.2
Flow System Modeling. As discussed in Section 5.2, the USGS completed a modeling
study to determine the fate of recharge to the Valley adjacent to Lake Michigan. The
modeling efforts determined that two major receptors exist for recharge that flows
through Valley sediments: surface-water bodies and a Deep Tunnel System constructed
approximately 300 ft below land surface to store runoff. The primary objective of the
modeling was to delineate the contributing areas within the Valley for each receptor.
The step-wise modeling approach demonstrates that each receptor plays a role in
capturing recharge, but that the Deep Tunnel is the dominant sink for water recharging
the Valley.
The regional model indicates that the Deep Tunnel captures recharge from almost half
the land area at the regional scale with the remainder discharging mostly to the
Menomonee River and Lake Michigan (Figure 6-15). Over the more limited area of the
local MODFLOW model, about 73% of the land area is captured by the Deep Tunnel and
the remainder by surface water bodies (Figure 6-16). Within the Valley study area the
simulation indicates that the Deep Tunnel captures all the recharge to the Valley east of
25th Street except for limited sections along the River and Canals. West of 25th Street
the Menomonee River captures most of the recharge (Figure 6-17).
The analysis allows a quantitative estimate of the various sources of water that supply
inflow to the Deep Tunnel. Within the domain simulated by the local model, the
estimated Deep Tunnel dry-weather inflow is on the order of 1.7 million gallons per day.
Of this amount:
1)
2)
3)
approximately 81% is natural recharge from precipitation;
approximately 15% is derived from Lake Michigan; and
approximately 4% is contributed by downward flow from the surface water
bodies.
Almost all the water derived from surface water bodies originates from estuary water
bodies, and, therefore, is effectively drawn from Lake Michigan.
The model, in conjunction with the estimates for effective porosity included in Table 5-1,
can also be used to trace groundwater flow paths from the Valley water table. The
expected median time necessary for groundwater to flow horizontally and vertically from
the Valley to the Deep Tunnel through the pores in the glacial material and the cracks in
the dolomite is on the order of 250 years. Almost all of the Valley to Deep Tunnel travel
times fall between 35 to 350 years (Figure 6-17). The most rapid travel times occur in
the northern portion of the Valley where the depth to bedrock is least. The median travel
time for particles that circulate over shorter paths to the Menomonee River and to
shipping canals is 8 years, with almost all of the travel times falling between 1 and 100
years. Figure 6-18 shows typical pathlines with travel times in cross section.
Simulations suggests that the presence of utilities such as gravel-backfilled sewers
within the Valley can change the path of flow, but are not likely to provide conduits for
flow to the estuary. One point of uncertainty is the fate of water that enters the
groundwater system within approximately 125 feet (125 feet equates to one-half the
dimension of a model node) of the estuary. The local model does not have enough
detail to resolve flow patterns at this scale, but measurements from mini-piezometers
that span surface and groundwater suggest that at most locations the estuary waterways
lose to the groundwater and, therefore, act as sources of water rather than discharge
points. It is likely that recharge adjacent to these water bodies is also diverted
downward toward the Deep Tunnel.