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anecdotal evidence suggests (Kohout and Kolipinski 1967),
studies show that the modern fresh groundwater inputs into
Biscayne Bay are very small (« 1 0 % of the surface flows; Langevin 2001). In addition, the saltwater intrusion line in south
Florida has been stable or has encroached further inland over
the past two decades (Sonenshein 1995) despite efforts to
protect the water supply from saltwater intrusion, and hypersaline conditions are commonplace during droughts. Both of
these observations support the understanding that groundwater flow to Biscayne Bay is limited under current conditions.
Because of the relatively small contribution groundwater
makes to the total water budget and the limited availability of
observed data, groundwater flows were not accounted for in
this analysis. However, because of the importance of groundwater flow during the dry season and in drought conditions,
these flows could be included in the estimates of mean annual
beneficial water volume if a reliable means for quantifying
the groundwater flows to the bay existed. Work to estimate
groundwater flows may provide additional information for estimating beneficial flow volumes in future analyses.
Surface Water. Canal flow estimates are derived from the
head and tail water elevations across the coastal flow control
structures maintained by the SFWMD and are stored in its
DBHYDRO database. The observed flow data from the coastal control structures S197, S20, S20F, S20G, S21, S21A, S22,
S25B, G93, S25B, S26, S27, S28, S29, and S29Z for the time
period 1985-2005 were examined. On average, 1.14 M acreft/yr (accurate to about +/- 5%) of total surface freshwater
flows enter any part of Biscayne Bay. For just the waters entering the boundaries of Biscayne National Park (direct flows
through S20F, S20G, S21A, S21, and S123 at the northern
coastal boundary are included, as are indirect flows from S22
and G93 into Central Bay and S197 into Barnes Sound that
eventually pass through park waters), the average freshwater
flux is much less, about 475 K acre-ft/yr or 40% of the total.
Figures 6 and 7 show the volume of flow contributed by
each of the structures relative to each other. Of the annual
average of 475 K acre-ft of canal flows that are discharged to
southern Biscayne Bay from 1985-2004, 131 K acre-ft (27%
of all flows) entered directly into Biscayne National Park
through C-103 (S-20F), 107 K acre-ft (23%) through C -l
(S-21), 69 K acre-ft (15%) through C-102 (S-21A), and a minor amount through Military Canal. In addition, there were
indirect flows to the park waters through C-100 (S-123) (47
K acre-ft, or 9%), C-2 (S-22) (95 K acre-ft, 20%), and some
minor flow through C-3 (G-93). Additional freshwaters eventually enter the park through its southern boundary at the
entrance to Card Sound. The freshwater in Card Sound and
Barnes Sound section comes primarily from discharges from
the C -l 11 Canal (S-197) (26 K acre-ft, 6%) into Manatee Bay
in western Barnes Sound, with some additional unquantified
contributions from overland runoff from extensive freshwater
and coastal wetlands contiguous with the mainland shoreline
Figure 6. Distribution of total flow to Biscayne Bay by
structure.
Mean Total Flow to Biscayne Bay 1994-2004 =
S123
S25B
S197
1176.6 Kaf
Ecological and Hydrologic Targets for Western Biscayne National Park 17
Figure 7. Annual average (percent of total) canal discharges to
southern Biscayne Bay.
of these two basins. Because no other significant or quantifiable source of surface or groundwater exists, these coastal
structure flows into southern Biscayne Bay are considered in
this analysis to be the only freshwater inflows along the coast.
Salinity. The salinities present in Biscayne Bay are directly
dependent upon these freshwater fluxes. Under the current
water management scheme, large plumes of relatively freshwaters (<25ppt) extend away from the canal mouths towards
the bay axis during periods of high rainfall. These fresher waters are then mixed into the other bay waters and are subject
to partial exchange with marine waters (35 ppt) through tidal
processes. The result in a typical year is an average bay salinity less than marine (<35ppt) during the wet season and approaching or exceeding marine during the dry season, though
during years with less-than-average canal run discharges it is
common to observe hypersaline (>37ppt) conditions through
large portions of southern Biscayne Bay, including the western shoreline.
Time series of salinity data have been collected by MiamiDade Department of Environmental Resources Management
(DERM), Florida International University (FIU), and NPS at
scattered points at different intervals within Biscayne Bay for
more than the past 10 years. The salinity at a given station is
largely a function of the efficiency of tidal exchange at that
location (usually related to the distance from the ocean with
its typical salinity of 35-37 ppt), the freshwater surface flow to
the bay (mostly local but some remote influences dependent
on location), the time history of evaporation and precipitation
in the bay, the volume of intra-bay transports, and any wind
events within the past few weeks that greatly influence mixing
rates and on/offshore transports. These individual time series
offer little help in assessing the synoptic distribution of spatial
gradients within the bay, and very few are in the WBZ that is
the region of greatest interest for salinity targets due to their
ecological importance there. Taken as a whole, however, these
salinity data can help elucidate the net result of all the influences on salinities in the bay.
If these observed data are integrated over 30 days, and
grouped by their general location within the bay and their
distance from the coastline (approximating the effect of both
distance from the freshwater flows and the ocean influences),
some interesting general trends emerge when correlated
against the integrated observed flows from the coastal structures (Fig. 8). Nearshore (<2 km from shore, but more than
0.7 km from any canal mouth to avoid aliasing from any freshwater plume emanating from it) there is a dramatic decrease
in the monthly salinity with increasing flow. However, with
increasing flows there is a proportionally decreasing influence
on the salinity, with a fairly well-defined l/x" shape but with
a significant random error about the mean. Beyond a yearly
flow rate of about 300-400 K acre-ft/yr (415-550 cfs) there is
Net Effect of Surface Flow on Salinity in South Biscayne Bay
Integration time = 30 days
Figure 8. Observed south Biscayne Bay salinity data integrated
over 30 days and grouped according to their distance from the
coastline vs. flow rates expressed as K acre-ft/yr. The thick red
curve denotes the area away from canal mouths but within 2km