ENVIRONMENT
ceeded over much of the valley by
the year 2010 (Fig. 2).2 This means
that powerplants in the Salton Sea
and Brawley resource areas would
have to reduce emissions by ap-
proximately 82%. Odor poses
another problem (even if ambient
levels of hydrogen sulfide are kept
below the California standard)
since 50% of the population can
detect hydrogensulfide at concentrations of 4 to 12 ug/m>
Hydrogen sulfide does have a
beneficial fertilizing effect on crops
grown in the valley but only at
levels exceeding the ambientair
quality standard.
Water quality
The extensive use of agricultural
wastewater for power plant cooling
or injection to geothermal reser-
voirs will lower the Salton Sea’s
elevation and increaseits salinity.
These changes have contradictory
ecological effects. In the mediumgrowth scenario, reliance on
wastewater to support geothermal
development would arrest the rising elevation of the sea, which has
been flooding riparian habitats in
recent years. Increases in salinity,
on the other hand, will have a
negative impact on fish reproduc-
tion when salinities exceed
40 000 ppm total dissolved solids.
Our analyses indicate that, with the
medium-growth scenario and normal hydrologic conditions, toxic
salinities would appear between the
years 1985 and 1990.° Without
development, toxic levels are not
likely to appear until early in the
United States, and the extraction
sole source of cooling water for
flashed-steam powerplants. At pres- and injection of large volumes of
ent, however, a county policy
favors the full injection of
withdrawn geothermalfluids to
protect against land subsidence.
Thus, external sources of waterwill
probably have to be used.
Subsidence and seismicity
The Imperial Valley is one of the
most tectonically active areas of the
Salinity — 10° ppm total dissolved solids
sulfide emissions are not abated to
below 30 g/MWh, the California air
quality standard (42 ug/m? averaged over one hour) will be ex-
120-7] 7791
110 t— Reference case and
medium energy
—
90 KH Conservation case,
no geothermal
development
_|
100
growth
80 |—
pmen
Reference case,
70 F—_ no geothermal
development
60
50
the fracture pressure of rocks).
zoL_1
|
1
1980
1990
|
2000
201 0
Year
il
Qio
Salinity of the Salton
Sea predicted for
three cases: existing irrigation efficiencies (reference case), improved efficiencies (conservation
case), and a reference case with
growth in geothermal energygenerating capacity of 100 MW/yr
and the use of agricultural waste
waters for cooling. Predictions are
based on normal hydrologic conditions and average evapotranspira-
tion from crops. Toxic effects on fish
reproduction are expected when the
concentration of total dissolved
solids exceeds 40 000 ppm (dashed
line).
Large-scale use of wastewater
would not be necessary if steam
condensate were used as the
21
TTS TD
Nevertheless, we recommend that
40
1990s (Fig.3).
7
4
geothermal fluids could alter
naturally occurring subsidence and
seismicity. Our preliminary anaiysis
indicates that the extraction of
geothermalfluids could result in
aquifer compaction within the
reservoir, and this could eventually
alter surface elevations.” Changes
in the slope of the land could, in
tum, hinder the irrigation of crops.
We recommendthat data on both
subsurface compaction and
changes in surface elevations
(Fig. 4) be collected by the responsible agencies with a view to
developing the ability to predict
possible land deformation.
The injection of large volumes of
spent geothermal fluids could alter
seismicity. However, webelieve that
this is unlikely becausefluids will be
injected at low pressures(i.e., below
the U.S. Geological Survey continue its monitoring of earthquakes
as geothermal energy production
proceeds to determine whether
changes in seismic activity occur.It
may be difficult to distinguish between natural and induced events
in this seismically active region.
Data on the location, frequency,
and focal depths of earthquakes
near geothermal fields will have to