Nutrient Dynamics in the Salton Basin-­ Implications from Calcium, Uranium, Molybdenum, and Selenium Roy A. Schroeder, U. S. Geological Survey, San Diego, California and Willian H. Orem, U.S. Geological Survey, Reston, Virginia H31B-02 ABSTRACT: The Salton Sea has been accumulating chemical constituents delivered by its tributary streams for nearly 100 years because it has no outlet. The buildup of chemicals that are highly soluble and unreactive, such as chloride, has resulted in the development of a quasi- marine lake. In contrast, chemicals that react to form insoluble phases ultimately enter the sediment that accumulates on the floor of the Sea. Solubility properties are especially relevant for two important contaminants, selenium and nitrogen. The selenium is contained in Colorado River water used for irrigation, and nitrogen is derived mostly from chemical fertilizer in agricultural runoff. Both are delivered to the Salton Sea as highly soluble oxyanions by the Alamo and New Rivers, which are relatively high in dissolved oxygen at their outlets to the Salton Sea, but are removed as reduced species in anoxic sediment on the Sea’s floor. Without this removal mechanism, selenium concentration would presently be about 400 micrograms per liter and nitrogen would be about 100 milligrams per liter in the Salton Sea’s water, rather than the observed concentrations of only about 1 microgram per liter and 5 milligrams per liter, respectively. Ironically, anoxic conditions responsible for producing the noxious odors and low oxygen conditions that lead to periodic dieoffs of large numbers of fish in the Salton Sea have prevented aqueous selenium and nitrogen from reaching levels that could indeed pose an extreme environmental hazard. Does all the selenium and nitrogen ever discharged to the Salton Sea still reside in its sediment, or has some been lost? It is well known that certain bacteria are capable of converting both elements into gases that can then be volatilized to the atmosphere. By comparing concentrations of selenium and nitrogen with those of molybdenum and uranium, elements with similar geochemical properties, this study concludes that there is now little, if any, selenium and nitrogen loss to the atmosphere. It is important that any engineering changes made to the Salton Sea that are intended to remediate for environmental effects do not result in reintroduction of these contaminants from sediment into the overlying water. Dissolved nitrogen concentration in the Salton Sea is apparently several times higher today than it was in the mid-1950’s; yet dissolved phosphorus concentration has changed little, if at all. Why have phosphorus levels not seen a similar increase? One possible explanation is that phosphate is efficiently removed from the water column by incorporation with calcium as apatite minerals--the material that composes bone. If so, attempts to slow or reverse excessive biological productivity (eutrophication) through large-scale harvesting of fish may not result in lowering the dissolved phosphorus concentration that would thereby improve the trophic status of the Salton Sea. The selenium profile in a core from the Salton Sea was found to increase from about 9 at the sediment-water interface to as high as 15 micrograms per kilogram, before decreasing with increasing depth to the sub- microgram-per-gram level in material believed to predate the formation of the Salton Sea. The low concentrations in sediment that predates the lake are about the same as those for clay-rich soils in the Imperial Valley at the south end of the Sea. A possible explanation for the maximum at mid-depth is that the highest concentration corresponds to a period when agricultural development in the upper basin was resulting in greater input to the Colorado River from leaching of selenium-rich soluble salts than today. Presented at Spring 2000 Annual Meeting, American Geophysical Union, Washington, D.C. 1 The Salton Basin is an important economic, environmental, and Mecca SAMPLE SITE LOCATIONS international resource located in an arid area that receives less North Relative site depths than 70 mm of rainfall annually. The basin contains almost Shore 11 Shallow 400,000 hectares of intensively irrigated farmland, located 10 Medium mostly south of the Salton Sea; the city of Mexicali, a large Deep urban area of 1 million people, also is located in the basin. Surface-water gaging station 275 Depth, in feet below sea level 8 Wildlife refuge 215 Palm C Study Springs a Desert l i Area 9 San f 10 Francisco o Shores r n i 111 a River Salton Sea 275 Bombay Riverside Co Beach Los Angeles Colorado San Diego Co Imperial Co Beach 15 San Diego 270 7 Salton 265 Sea Drop 3 Alamo 5 86 Brawley 275 River The Salton 1 8 El Centro Sea is a New San quasi-marine River Calexico All American Canal 2 3 Diego UNITED STATES Yuma terminal lake, Mexicali Tijuana MEXICO originally formed 6 by accidental flooding of the 4 260 255 Colorado River nearly 100 years ago, now largely sustained by agricultural runoff. 250 Sediment grab samples were collected from 11 diverse sites in the Salton Sea in 5 245 235 July 1998, and chemical data from these samples are used, along with historical 78 aqueous data from the two major rivers, to ascertain the fundamental behavior of 240 nutrients and selenium, important contaminants in the basin and the Sea. South Basin (Site 1) North Basin (Site 9) 2 2 0 0 Calcite 10 10 20 20 0 30 30 40 40 SATURATION INDEX 50 50 Gypsum 0 2 4 6 8 10 12 14 16 18 20 7.6 7.8 8.0 8.2 8.4 8.6 8.8 -2 OXYGEN, IN MG/L pH East Highline Canal New River 0 0 Alamo River Salton Sea 10 10 Saturation index, defined as the logarithm of measured activity ("concentration") product 20 20 divided by the solubility product, illustrates 30 30 the high hardness in irrigation water (East DEPTH, IN FEET BELOW SEA SURFACE Highline Canal) supplied by diversion of the 40 40 Colorado River and in discharge from the two major rivers. A calculated index that is 50 50 40 41 42 43 24 26 28 30 32 34 much greater than zero (the value above RESIDUE-ON-EVAPORATION, IN G/L TEMPERATURE, IN °C which thermodynamic equilibrium predicts Water-quality profiles indicate strong thermal stratification that precipitation can occur) indicates that and bottom water devoid of dissolved oxygen (DO) in July deposition of calcium carbonate is very 1998, except at shallow depths. Such conditions favor likely in the Salton Sea, whereas a value the deposition and permanent sequestration of redox­ very close to zero suggests that formation sensitive contaminants, such as nitrogen and selenium, of calcium sulfate may be taking place. in bottom sediment. 3 Mecca FINES Mecca ORGANIC CARBON North North Shore (in percent Shore (in percent of 97 2.1 99 of particles 4.3 dry sediment) <0.062mm in 67 diameter) 1.2 Desert 94 Desert 8.4 Shores Shores Salton Sea 111 Salton Sea 111 Bombay Beach 275 Beach 275 Bombay Beach Beach 270 3 Mecca FINES Mecca ORGANIC CARBON Mecca INORGANIC CARBON 83 270 5.4 North North North 265 265 Shore (in percent Shore (in percent of Shore (in percent as 97 2.1 7 86 86 275 275 99 of particles 4.3 dry sediment) 23 calcium carbonate) 99 6.9 <0.062mm in 85 1 6.1 0.20 67 diameter) 27 Relative site depths 1.2 Shallow 66 1.9 Desert Desert Medium 94 Desert 8.4 25 Deep Shores Shores Shores 260 Surface-water gaging station 47 255 0.50 260 255 111 111 111 275 Depth, in feet below sea level Salton Sea Salton Sea Salton Sea 250 245 Bombay 245 250 Beach 275 Beach 275 Bombay Beach 275 Bombay Wildlife refuge 24 0.57 Beach Beach Beach 235 235 270 270 270 78 78 83 5.4 21 240 240 265 265 265 86 86 86 275 275 275 99 6.9 15 As expected, sand content increases toward the shore and in Total organic carbon in sediment shows a the deltas formed from deposition by large rivers and silt plus strong correlation with grain size, as do many clay content increases in deeper parts of the Salton Sea. trace elements and organic compounds. Relative site depths 85 1 6.1 0.20 20 9 Shallow Medium 66 1.9 14 Mecca INORGANIC CARBON Deep North 260 Surface-water gaging station 47 255 0.50 260 255 13 260 255 Shore (in percent as 275 Depth, in feet below sea level 7 250 245 245 250 250 245 23 calcium carbonate) Wildlife refuge 24 0.57 10 235 235 235 78 78 78 240 240 240 27 Desert High calcium carbonate content in deeper parts As expected, sand content increases toward the shore and in Total organic carbon in sediment shows a High calcium carbonate content in deeper parts 25 Shores of the Salton Sea confirms expectations of the deltas formed from deposition by large rivers and silt plus strong correlation with grain size, as do many of the Salton Sea confirms expectations of 111 deposition based on saturation calculations. clay content increases in deeper parts of the Salton Sea. trace elements and organic compounds. deposition based on saturation calculations. Salton Sea Beach 275 Bombay Beach 270 21 265 86 275 15 20 9 14 13 260 255 250 10 245 235 78 240 4 REDUCED NITROGEN AND OXIDIZED NITROGEN IN ALAMO RIVER AT DROP 3 14 14 ORGANIC 12 NITROGEN NITRATE 12 + AMMONIA + NITRITE 10 10 8 8 6 6 4 4 IN MILLIGRAMS N PER LITER ORGANIC NITROGEN + AMMONIA, 2 2 0 0 1970 1980 1990 2000 NITRATE + NITRITE, IN MILLIGRAMS N PER LITER YEAR, FROM 1970 TO 2000 The USGS monitored a broad suite of constituents at the New and Alamo River outlets to the Salton Sea monthly for a period of 1 year in 1988-89.