Nutrient Dynamics in the Salton Basin-­ Implications from Calcium, Uranium, Molybdenum, and Selenium Roy A. Schroeder, U. S. Geological Survey, , and Willian H. Orem, U.S. Geological Survey, Reston, Virginia H31B-02

ABSTRACT: The 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 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 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 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. of calciumsulfatemaybetakingplace. very closetozerosuggeststhatformation likely intheSaltonSea,whereasavalue deposition ofcalciumcarbonateisvery that precipitationcanoccur)indicates that which thermodynamicequilibriumpredicts much greaterthanzero(thevalueabove two majorrivers. Colorado Riverandindischargefromthe Highline Canal)suppliedbydiversionofthe the highhardnessinirrigationwater(East divided bythesolubilityproduct,illustrates measured activity("concentration")product Saturation index,definedasthelogarithmof 2

SATURATION INDEX -2 0 2 East HighlineCanal Alamo River Calcite Gypsum A calculatedindexthatis Salton Sea New River the depositionandper 1998, exceptatshallowdepths.Suchconditionsfavor and bottomwaterdevoidofdissolved oxygen(DO)inJuly in bottomsediment. sensitive contaminants,suchasnitr W DEPTH, IN FEET BELOW SEA SURFACE ater 50 40 30 20 10 50 40 30 20 10 0 0 0 RESIDUE-ON-EVAPORATION, ING/L -quality profilesindicatestrongthermal stratification 2 40 4 South Basin(Site1) OXYGEN, INMG/L 6 41 8 10 42 12 14 16 43 manent sequestrationofr 18 20 50 40 30 20 10 50 40 30 20 10 0 0 7.6 24 7.8 26 TEMPERATURE, IN Nor ogen andselenium, 8.0 28 th Basin(Site9) pH 8.2 30 8.4 ° C 32 8.6 edox­ 8.8 34 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. Monitoring on the Alamo River at Drop 3, a few miles upstream from the river’s outlet, since about 1970 shows that the 1988-89 period used in subsequent calculations is representative of longer term concentrations for many constituents. (Only data for nitrogen, which exhibits much greater variation than most constituents, are illustrated herein.) Nitrogen delivered to the Salton Sea is mostly in the form of nitrate, although virtually all of the nitrogen in the Salton Sea itself consists of the reduced species, ammonia and organic nitrogen. 5 CONCENTRATIONS IN MEAN RIVER ELEMENT MASS RATIOS NEW AND ALAMO RIVERS CONCENTRATIONS (parts per million) (1988-89) (discharge weighted) RIVER (1988-89) SEDIMENT (1988-89) CONSTITUENT ALAMO NEW n Nitrogen 9.3 mg/L 1,476 N/Se 1,153±334 Discharge 60% 40% Selenium 6.3 µg/L 2.51 U/Se 2.43±0.8 Uranium 17 14 1 Uranium 15.8 µg/L 1.95 Mo/Se 2.51±1.6 Molybdenum 13.3±1.9 10.8±2.5 10 Selenium 7.6±2.0 4.3±0.5 15 Molybdenum 12.3 µg/L 1,651 TOC/Se 7,522±2,984 Organic Carbon 10.4 mg/L NO2 + NO3-N 8.1±1.6 4.9±0.8 15 CONCLUSIONS: NH4-N 1.5±1.3 2.3±0.9 15 Organic-N 1.0 0.2 1 Nitrogen Reduction within the Sea = 22% Total N 10.6 7.6 Autochthonous Organic Production is >78% Total Organic Carbon 9.3 12 1

Element concentrations and mass ratios are from the 1988-89 river relative proportions (mass ratios) for all four of the above elements data and from the 1998 Salton Sea sediment data. Nonvolatile are about the same in river water and in lake sediment indicates that elements, such as molybdenum (Mo) and uranium (U), also exist as nearly all the N and Se discharged by the rivers is retained within soluble oxyanions in the oxygenated river water and as insoluble species the sediment accumulating beneath the Salton Sea. Extending in reducing bottom sediment beneath the Salton Sea. Nitrogen (N) and similar calculation methods reveals that at least 78% of organic selenium (Se) have geochemical properties similar to those of U and Mo, carbon is derived from autochthonous (within lake) production and except that Se and N can potentially be lost as gases. The fact that that no more than 22% is derived from outside the Sea.

Mecca Mecca North North Shore TOTAL NITROGEN Shore URANIUM 0.27 (in percent of 5.3 (parts per million) 2.7 0.51 6.2 14 11 dry sediment) 27 MOLYBDENUM 0.26 SELENIUM 5 (parts per million) 1.8 (parts per million) 2.8 Desert Desert 0.97 20 Shores 6.5 Shores 75 Salton Sea 111 Salton Sea 111 Beach 275 Bombay Beach 275 Bombay Beach Beach 270 270 0.66 14 265 8.0 265 31

86 86 275 275 0.90 14 8.8 36 0.79 0.06 14 2.4 Relative site depths 5.8 0.58 34 0.73 Shallow 0.26 3 Medium 1.5 2 Deep 0.10 260 255 2.7 260 255 Surface-water gaging station 0.9 1.4 250 245 250 275 Depth, in feet below sea level 0.11 2.5 245 Wildlife refuge 1.0 235 1.4 235 78 78 240 240 6 LAB SIMULATION AMMONIA PROFILE AT NORTH BASIN SITE

OF NUTRIENT REMOBILIZATION 0 35

AMMONIA 5 Water Column ORGANIC 30 NITRITE NITRATE 10 IN METERS WATER DEPTH, 25

5 Pore Water 20

10

15 15 CORE DEPTH,

10 IN CENTIMETERS 20

IN MILLIGRAMS N PER LITER 25 DISSOLVED NITROGEN SPECIES, 5 0 5 10 15 AMMONIA, IN MILLIGRAMS N PER LITER

0 Much higher ammonia concentrations in sediment pore water than in the overlying bottom water at 0 10 20 30 40 50 60 70 80 site 9 also confirm the potential importance of the diffusive mechanism. These results for N hint at DAY similar possibilities for Se remobilization under Potential mechanisms for reintroduction of sedimentary N suitable environmental conditions. into the overlying water column include mineralization (oxidation to nitrate) in areas where high-DO bottom water is present, and diffusion of ammonia from sediment into the overlying water. Results from a laboratory simulation, using a mixture of sediment and water from the Salton Sea infused with oxygen by bubbling air into the mixture, indicates that potential for oxidative mineralization exists. 7 PHOSPHOROUS AND SEDIMENT IN RIVERS 5

NEW RIVER AT INTERNATIONAL BOUNDARY 4 ALAMO RIVER AT DROP 3

3

2

1

0 TOTAL PHOSPHORUS, IN MILLIGRAMS PER LITER 0 200 400 600 800 1,000 1,200 1,400 1,600 SUSPENDED SEDIMENT, IN MILLIGRAMS PER LITER

Phosphorus (P) differs from the above redox-sensitive elements in that a significant proportion of its transport to the Salton Sea is associated with attachment to suspended sediment. Agricultural tail water and resuspension from the riverbed are sources of suspended sediment in the rivers. Positive correlation between P and suspended sediment is illustrated by data for the Alamo River at Drop 3. Discharge of wastewater from Mexicali, immediately upstream from Calexico, results in a negative correlation (decrease in P with increase in suspended sediment) indicative of dilution from the point source. However, as the New River traverses the Imperial Valley to its outlet at the Salton Sea, it increasingly acquires characteristics similar to those of the Alamo River. 8 NITROGEN AND PHOSPHOROUS, IN MILLIGRAMS PER LITER 0.10 0.15 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.05 0.60 0.20 there hasbeennodiscernableincreaseinphosphorus. has increasedseveral-foldsincethemid-1950’ suggests thatdissolved-nitrogenconcentrationin the Sea Nevertheless, analysisofbottomwater(notshown) nitrogen (50to75%today)ispresentasorganicnitrogen. of strongseasonalvariation,andbecausemostthe trations intheSaltonSeaisdifficulttointerpretbecause Comparison withhistoricaldissolved-nutrientconcen­ 0 S SAL 1954 O TON SEANUTRIENTCONCENTRA N D J NH PO NO NH F 4 3 4 4 -P (Surface) -N (Surface) -N (Surface) -N (Bottom) M A M 1955 J J A S O Calif F rom FishBulletin#113, N Fish andGame or D nia Depar J F M tment of s, while TIONS 1956 , 1961. A M J J has therebykeptdissolved Plevelsnearlyconstantforseveraldecades? the SaltonSeaashighlyinsolubleapatite(calcium phosphate)minerals,and skeletal materialfromdeadfishissequesteringP within sedimentbeneath thereby enhancingthelake’ that recyclesPfromhighlyreducingsedimentback intotheoverlyingwater Biologically productivelakescommonlydisplaya "feed-back mechanism" river this element’ modest 30%increaseinconcentrationnearthe surface, These resultscontrastsharplywiththoseofP, whichshowsonlya younger sedimentnearthesurface,asdoesprofileforN(notshown). basin (site9)in1999exhibitsalarge10-foldincreaseconcentrationfor The organiccarbonprofileinacorecollectedfromthecenterofnorth -suspended sediment.

CORE DEPTH, IN CENTIMETERS s com-parativelyhighnaturaloccurrenceinbasin soil andin 60 50 40 30 20 10 0 0.00 0 TOTAL PHOSPHORUSINCORES Carbon Or ganic 0.02 TOTAL PHOSPHORUS,INPERCENT 2 ORGANIC CARBONAND ORGANIC CARBON,INPERCENT s rateofeutrophication. 0.04 4 0.06 6 Phosphor T otal 0.08 8 Is itpossiblethat ous consistentwith 0.10 10 , 9 GEOCHEMICAL RECORD OF SEA'S FORMATION SELENIUM PROFILE 0 0

10 10

20 20 Salton Sea formed (1905-07) Salton Sea formed (1905-07)

30 ALUMINUM 30 MOLYBDENUM CORE DEPTH, IN CENTIMETERS CORE DEPTH, IN CENTIMETERS CALCIUM SELENIUM (X10)

SULFUR URANIUM Bottom of core Bottom of core 40 40 0 4 8 12 16 0 50 100 150 MAJOR ELEMENTS, IN PERCENT DRY WEIGHT TRACE ELEMENTS, IN MILLIGRAM PER KILOGRAM DRY WEIGHT

Geochemical trends for other elements also are revealed The Se profile rises through a maximum at 10-12 cm, by additional cores from site 9. Efflorescent minerals and then declines with increasing depth to levels found that accumulated on the dry lakebed prior to flooding in in Imperial Valley soils. The maximum might indicate 1905-07 cause the enrichment in calcium sulfate and dissolved Se in the Lower Colorado River was about depletion in aluminosilicate content at 22-24 cm. 50 percent higher in 1940 than today. Post depositional diagenesis and historical shifts in redox character of the Salton Sea are other possible explanations for the profile.