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 At pres- and injection of large volumes of ent, however, a county policy favors the full injection of withdrawn geothermalfluids to protect against land subsidence.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