The Ecology of Tijuana Estuary, California: An Estuarine Profile

Table 29. Streamflow data (acre-ft) for the Tijuana
River at the United States-Mexico border. All 1983
flows were augmented by discharges from Rodriguez
Dam; flows for March-December 1983 were maximum
for the period of record (from IBWG 1983).
Month
---
January
February
March
April
May
June
July
August
September
October
November
December
1983 Flows
5.236
35.849
293,494
62,938
42,599
9.696
9,242
17.092
978
1,237
4,377
6.705
Average Flows
break. Flows exceeding 7,000 m 9er day (over 2
MGD) have occurred at various unpredictable
intervals.
Plans have been developed for a major sewage
interception system, wherein flows from four
canyons will be collected and piped to the San
Diego treatment plant (R. Donally, City of San Diego
Utilities Division, pers, cornm.). in the interim,
however, raw sewage continues to flow directly into
the estuary on a daily basis. Reasons for the long
delays in solving this problem are largely political.
The United States has no jurisdiction for preventing
the flows; government agencies can only deal with
the sewage once it crosses the international
border. Local residents and estuary managers
have called for an end to the sewage pollution, but
solutions are slow to come. Direct evidence for a
detrimental impact on the estuary and its
endangered species is lacking. Attempts to
determine if toxic materials were present in
estuarine orgariisms have been unsuccessful, in
part because so many of the fishes and
invertebrates died out in 1984 (Jorgensen, pers.
comm.). While some species of bivalve molluscs
reestablished in 1985, reasons for their absence in
1986 are unknown.
In the future, other wastewater management
practices may lead to greater continuous flows At
present the city of San Diego recelves
approximately 20 MGD from Tijuana through a
sewage ptpe that !cads !o a prlrnary treatment plant
and ocean outfall in the Unlted States As Tljuana
continues to grow and additional freshwater
supplles become available from the Colorado River
aqueduct, the volume of wastewater IS expected to
exceed 30 MGD The San Diego treatment piant
cannot handle increased volumes from Mexico, so
the alternatives are for the United States to
construct a new plant to treat Mexican wastewater
or for Mexlco to treat its own sewage.
The first alternative would allow the United
States to control the ultimate discharge point for
treated wastewater (e.g., tertiary treatment and
recycling, impoundment and timed release to
Tijuana River to minimize ecological impacts, or
disposal through an ocean outfall), but cost
estimates were over $700 million. The second
alternative, for Mexico to treat its own wastewater,
was favored by Mexico and the State Department,
and current plans suggest that initial increases in
wastewater would be routed to an intertidal outfall
several kilometers south of the border, while later
discharges (over 30 MGD) would undergo primary
treatment and be discharged to Tijuana River. It is
not surprising that sewage disposal has stimulated
international controversy over the past several
years.
For the estuary, the ecological Impacts of
wastewater influx are two-fold. reduced sallnities
and Increased nutrient concentrattons Unlike
most regions of the United States, where
wastewater Influxes are of concern primarily
because estuaries undergo eutrophication, the
greater problem in the arid southwest is altered
hydrology Riverflow is normally low and confined
to winter, wastewater discharges would change an
tntermrttently flowing stream into a permanently
flowing river A recent evaluation of how increased
streamflow would affect the estuary (Zedler et ai.
1984a.b) identified impacts on fishes,
invertebrates, vascular plants, and algae (Table 30)
and led to recommendations on how to reduce
negative impacts (Zedler et a1 1984~)
A model of estuarine salinity was developed to
predict dilution with discharges of 12.5, 100, and
200 MGD The 41-year record rncludes 10 years
of "heavy" flow (greater than 10,000 acre-ft/yr), 13
years of intermediate flow (100- 10,000 acre-ftlyr),
and 18 years of low flow (0-100 acre-ft/yr)
Monthly averages were then computed for
intermedtate-flow years Wastewater discharges of
30-35 MGD, ~ndicated In one potential plan, would
exceed these intermediate-year flows six-fold tn
winter and much more In summer The quantlty of
water that would reach Tijuana Estuary would be
sufficient to reduce water salinities and affect
marine species substantially (Zedler et al 1984b)
Measurable dilution was possible with only 12.5
MGD (Figure 70) Durlng neap t~des, salrnittes
would drop much more than during sprrng tides, so
a wrde range of conditions would result It was
predicted that the estuary would become slrghtly
brack~sh at I2 5 MGD and fresh at 200 MGD.
Revtew of the literature on satlnlty tolerances
~ndicated that most species could tolerate briefly