Environ. Sci. Technol. 1997, 31, 831-836
Trace Element (Se, As, Mo, B) Contamination of Evaporites in Hypersaline Agricultural Evaporation Ponds
disposal option is that elevated levels of trace elements, such as Se, As, Mo, and B, will accumulate in the pond waters due to evapoconcentration and subsequently pose an environmental threat to wildlife attracted to the ponds (3, 5). Due to the environmental impact that Se had on waterfowl at Kesterson Reservoir, evaporation ponds have been closely monitored with respect to chemical, biological, and hydrological properties (4, 6).
C O L I N G . O N G , * ,† M I T C H E L L J . H E R B E L , RANDY A. DAHLGREN, AND KENNETH K. TANJI Department of Land, Air & Water Resources, University of California, Davis, California 95616
The anionic oxyanions of Se, As, B, and Mo are of particular concern in the western San Joaquin Valley because they occur at elevated concentration levels in agricultural drainage waters owing to their abundance in soils derived from the marine sediments and their appreciable mobility in oxidizing environments. Once these trace elements enter the evaporation pond, they may accumulate in the water column by evapoconcentration (7) and biological accumulation (8) or be attenuated by several processes, including volatilization of Se (9, 10), biological uptake and subsequent accumulation in detrital organic matter in bottom sediments (11), reduction/ precipitation (12), adsorption to pond sediments (11), and precipitation, coprecipitation, or adsorption with evaporite minerals (13, 14). Previous studies examining evapoconcentration of trace oxyanions in evaporation ponds (14-16) showed that, relative to Cl, B exhibited conservative behavior (i.e., proportional concentration changes) while Se, As, and Mo displayed nonconservative behavior. In spite of the immobilization of some of these oxyanions from the water column, Se, As, and B were shown to approach or exceed current water quality criteria for hazardous waste.
Disposal of saline drainage waters containing elevated concentrations of the trace elements Se, As, B, and Mo poses a severe environmental hazard in the western part of the San Joaquin Valley of California. This study investigated the partitioning of these trace elements into evaporite minerals formed in agricultural evaporation ponds to determine trace element behavior during the evaporite formation stage and to provide information for development of management strategies to minimize environmental hazards. The trace elements were largely excluded from the evaporite minerals (∼100 times depleted relative to the solution phase), resulting in comparatively low trace element concentrations in these minerals. The affinity of trace element partitioning to the solid phase follows: Se ≈ B > As ≈ Mo. No differences in trace element partitioning were evident between summer and winter seasons, the type of evaporite formed, or whether the evaporites were collected wet or dry. Since trace elements accumulate in the solution phase, highly evapoconcentrated waters may exceed threshold levels for hazardous waste classification. Isolating these trace element-rich solutions during the latter stages of evapoconcentration into specialized treatment and handling facilities may provide an effective management strategy that minimizes both exposure to waterfowl and the amount of hazardous solid waste generated.
Saline agricultural drainage waters containing elevated concentrations of trace elements pose a severe environmental hazard in the western San Joaquin Valley of California (1, 2). Since natural drainage is limited in this region, agricultural evaporation ponds have become a common option for the disposal of these saline drainage waters. These ponds act principally as central containment areas for waters from surrounding farmland, receiving new inflow mainly during peak drainage periods. The pond waters are subsequently evaporated, leaving behind evaporite residues. Approximately 2900 ha of evaporation pond facilities are operational in the San Joaquin Valley (3) receiving 39.5 × 106 m3 (32 000 acre-ft) of subsurface drainage and 800 000 Mg of salts annually from 23 000 ha of tile-drained croplands (4). One of the major concerns associated with the evaporation pond
Evaporative concentration of the major ions composing saline solutions has been closely studied in relation to evaporite formation (17-22). However, little is known about the trace element content of evaporites (23). Information about the fate of the trace elements As, Mo, Se, and B during the evaporite salt formation stage has not been investigated fully since the evaporation ponds have only reached this latter depositional stage recently (6). Laboratory simulations of evaporite precipitation using synthetic pond waters show that Mo exhibits nonconservative behavior resulting from precipitation as CaMoO4 and/or coprecipitation with evaporite minerals (11). Minor incorporation of B was found with synthetic mirabilite (Na2SO4‚10H2O), thenardite (Na2SO4), and halite (NaCl), the dominant evaporites found in evaporation ponds of the San Joaquin Valley (13). Boron retention by these evaporites appears to occur primarily by adsorption. Selenium substitution in sulfate minerals such as thenardite, mirabilite, and bloedite (Na2Mg(SO4)2‚4H2O) has been previously suggested (24); however, this was based on the analysis of only one sample of thenardite, a salt efflorescent sample from Kesterson Reservoir (25). The results of that study suggested that most of the Se was contained in algae intermixed with the thenardite. Oxyanionic trace element concentrations in 50 efflorescent crusts from central and southern Alberta showed appreciable incorporation of B (20300 mg/kg) and low levels of As (1-10 mg/kg), Se (0.6-2.6 mg/kg), and Mo (0.4-5.9 mg/kg) (26). With efflorescent crusts, the chemistry of the resulting evaporite minerals is dictated primarily by the composition of the groundwater and not by partitioning of trace elements into evaporites at near-equilibrium conditions. Recent studies also show that carbonate minerals are subject to crystal surface incorporation of selenate (27, 28) and borate (29).
* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (415) 725-3162. † Present address: Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, CA 943054020.
The chemical environment (e.g., pH, Eh), element speciation, type of evaporite mineral, rate of crystal formation, and presence of dissolved and particulate organic matter all affect trace element incorporation into evaporite minerals. The possible incorporation mechanisms include coprecipi-
Introduction
S0013-936X(96)00531-7 CCC: $14.00
1997 American Chemical Society
VOL. 31, NO. 3, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
831
tation, adsorption, and occlusion (30). Substitution or coprecipitation may occur within the crystal lattice of evaporites and also along dislocations and grain boundaries (31). Substitution of SO4 in mirabilite or thenardite crystal lattices by Se oxyanions, especially SeO4, is possible since element periodicity would suggest similar chemical properties. Incorporation of trace elements by fluid inclusion or occlusion is also a possibility and may occur in cavities or along grain boundaries housing a thin solution film. Adsorption may play a major role as a step toward either coprecipitation or surface sorption. In addition, any salt coexisting in a solution containing these trace elements may be expected to contain some level of the trace elements due to surface wetness. Accumulation of trace elements in evaporites creates a major concern over the ultimate disposal of salts from evaporation ponds since evaporites contaminated with trace elements may be classified as hazardous solid wastes and thus have low to no commercial value, which leaves burial as the only disposal option (32). Lethal dose data indicate that Se anion-substituted evaporites are far more toxic than the pure evaporite. Halite (NaCl), with a LD50 of 3750 mg/kg orally in rats (33), is considered relatively nontoxic in comparison to sodium selenate (Na2SeO4), which has a LD100 of 4 mg/kg orally in rabbits (34), and sodium selenite (Na2SeO3), which has a LD50 of 7 mg/kg orally in rats (35). The objective of this study was to determine As, B, Mo, and Se concentrations in evaporites and associated solutions from seven representative evaporation ponds in the San Joaquin Valley of California. Partitioning of trace elements into the evaporite phase from the solution is examined from the point of view of enrichment or depletion relative to the macrocomponents. Trace element partitioning between the solid and solution phases has important ramifications for long-term management strategies of evaporation ponds and for assessing techniques for the ultimate disposal of accumulated evaporite minerals.
Experimental Section Sampling and Analysis. Evaporite minerals and their associated pond waters were collected from seven representative evaporation ponds in the San Joaquin Valley. Forty-five evaporite samples were collected in August 1990 while another 10 were collected in February 1991, of which 27 were in contact with mother liquors. Additionally, 28 evaporites were collected from evaporation pond cells that had recently dried. The evaporation ponds typically yielded minerals of high crystallinity and often large crystal size. Samples included large slabs formed on the pond bottom, crystals suspended within the water column, and thin films at the water surface that precipitate and dissolve with the diurnal temperature fluctuations. Pond water samples in contact with the evaporite minerals were collected in 1 L polyethylene bottles and stored at 3 °C prior to analysis. Field measurements of pond water pH, Eh, and temperature were made at the time of sampling. Solution temperatures ranged from 25 to 41 °C during the August collection, and from 6 to 15 °C in the February collection. Evaporite minerals were placed in glass vials with airtight seals and maintained at 100% relative humidity under ambient temperature conditions prior to analyses within one week of collection. Evaporite crystals were gentle crushed and mixed to obtain a representative subsample for analyses. Evaporite minerals were identified by X-ray diffraction (XRD) using a random powder mount scanned between 2 and 60° 2θ. A fast scan rate of 10 min was employed to minimize dehydration effects on hydrated phases during analysis. While we attempted to minimize alterations in evaporite mineralogy during sample storage and analysis, some changes may have occurred since several evaporites are highly transient in
832
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 3, 1997
nature, with their characteristics and even existence varying in time. The chemical composition of the evaporites was determined by dissolving ∼1 g subsamples in 1 L of distilleddeionized water. Pond water samples were warmed and shaken to dissolve all evaporite minerals that formed following refrigeration. The dissolved evaporite and pond water samples were filtered through 0.45 µm membrane filters and analyzed for cations (Na, Ca, Mg) using an inductively coupled plasma (ICP) spectrophotometer and anions (SO4, Cl) using ion chromatography with conductivity detection. Samples were analyzed for total Se by hydride vapor generation-atomic absorption spectrophotometry (HVG-AAS) (36). Preparative sample digestion for total Se was performed using a nitric/ perchloric acid digestion (37). Boron was measured by the azomethine-H colorimetric method (38) using a UV-visible spectrophotometer (13). Concentrations of As and Mo were determined using ICP and graphite furnace atomic absorption spectrophotometry (GFAAS), respectively. All standards for the trace elements were prepared in matrix-matched solutions. The water content of evaporite samples was determined by drying known weights of sample in an oven at 180 °C for 24 h. The water content was calculated on a weight/weight percentage basis. Insoluble matter content was determined by first weighing a known amount of sample in a weighing dish. The sample was mixed with 10 mL of distilleddeionized water to dissolve the soluble portion. The mixture was filtered through a preweighed 0.45 µm membrane filter and rinsed thoroughly with additional water to leach all soluble constituents. The filter was dried in an oven at 60 °C for 6 h and the insoluble matter content calculated on a weight/weight percentage basis. Trace Element Distribution. Distribution coefficients (D) are calculated on a mole fraction basis described by Driessens (39):
msolution x
Dsolution ) x
∑
msolution y
+
y)cations
Dsolid x
∑
(1) msolution z
z)anions
msolid x
(2)
)
∑
y)cations
msolid + y
∑
msolid z
z)anions
where x ) Mo, As, Se, or B. The distribution coefficients describe the proportion of all measured components represented by the trace element. The solid-solution partition coefficient (Kx) is given by
/Dsolution Kx ) Dsolid x x
(3)
Results and Discussion Evaporite Characterization. The evaporites collected in August 1990 contained 12 mirabilite-dominated, 21 thenardite-dominated, and 11 halite-dominated samples. The remaining sample in this group was a mixture of the aforementioned minerals. Evaporites collected in February 1991 contained six mirabilite-, one thenardite-, and three halitedominated samples. Based on the chemical analysis of these same samples, other evaporites commonly associated with evaporation ponds, such as calcite (CaCO3), gypsum (CaSO4‚2H2O), and bloedite, may have been present at concentrations below the detection limit of the XRD technique (generally 5-10% by weight). Morphologically, halite crystals were characterized by cubic morphology (0.5-1 cm), and mirabilite by elongated crystals (5-20 cm long, 1 cm wide, 0.1-0.3 cm thick). Thenardite does not have a characteristic macrocrystal
TABLE 1. Trace Element Concentrations in Evaporation Pond Solution Phase and Hazardous Waste Criteria concn range (mg/L)
waste criteriaa (mg/L)
0.008-8 ndb-12 3-200 1-282
1.0 5.0 70 350.0
TABLE 2. Macrocomponent Concentrations of Evaporation Pond Evaporite Samples mass % of total solid
Se As B Mo
a For Se, As, and Mo: hazardous waste criteria (threshold limit concentration) for soluble species according to Title 22, California Administrative Code. For B: Designated waste criteria for soluble species according to Title 23, California Administrative Code. b Not detected.
form in evaporation ponds; it tends to appear as small opaque grains when wet and as a fine, white powder when dry. Appreciable insoluble fractions were associated with salt efflorescence occurring on soil surfaces associated with either dried pond cells or shoreline deposits. The insoluble fraction consisted primarily of phyllosilicate clay contaminants from the mineral soil or organic matter sedimented from the water column. Pond Water Macrocomponents. From the analysis of evaporite-associated pond waters collected in August 1990, pond I water is characterized as Na-Cl, indicating the relatively similar amounts of these dominating macrosolutes (Table 2). Further, the water type at ponds IV-VI is characterized by Na-SO4 dominance. Pond III waters fell into the Na-Cl-SO4 and Na-SO4 type categories, while pond II waters were found as a mixture of all three categories. These drainage waters tend to reflect the relative solubility of evaporite minerals in the soil environment. In irrigated fields, Ca, CO3, and to a lesser degree SO4 remain in the soil profile where they precipitate as calcite and gypsum. In contrast, the highly soluble Na and Cl are readily leached from the soil profile and comprise the dominant ions in the drainage waters (40). The high concentrations of Cl and SO4 reflect the origin of these waters from marine sedimentary rock (41). Water content was high in chloride and many sulfate salts and may be attributed to sampling of submersed material, moist brines, or hydrated mineral phases. Insoluble material is primarily associated with samples collected in August 1990. Water content thus represents, in many cases, a significant proportion of the mass of the solids to be cleared from the evaporation basins. The mass of solids in removal efforts could be greatly reduced by on-site separation of the water component, perhaps also saving on bulk. The drainage waters initially discharged into the ponds have EC values ranging from 7 to 30 dS/m (16). Ponds waters containing mirabilite or thenardite typically were found to have EC values ranging between 90 and 100 dS/m while pond waters containing halite commonly were found to have EC values greater than 165 dS/m. The mirabilite form of Na2SO4 is favored in pond waters with temperatures generally less than 32 °C while thenardite is the stable form at temperatures greater than ∼32 °C (42). This stability relationship explains, in part, the greater occurrence of thenardite in the September sampling period. Trace Elements. The evaporation pond waters contained appreciably concentrations of B and Mo, with lesser concentrations of Se and As. Arsenic concentrations were often below detection limits. California hazardous liquid waste criteria are lowest for Se and As reflecting their apparent toxicity to aquatic and terrestrial organisms (Table 1). Maximum dissolved concentrations of Se, As, and B were in excess of California’s hazardous liquid waste criteria in several pond waters. The greatest excess occurred in the case of Se, which was found in concentrations up to 8 mg/L, which is 8 times greater than the waste water criterion. These potentially hazardous trace element concentrations occur
Na
Ca
Mg
SO4
Cl
water
insoluble
Pond Site I summer 1990 1 2 3 4 5 6 7 winter 1991 1 2 3
22 36 32 13 31 32 24
1 0 1 0 1 1 1
0 0 0 11 0 0 0
5 1 4 18 3 3 6
34 59 50 11 49 51 37
33 7 13 57 16 14 29
0 0 0 1 0 0 0
34 30 29
0 0 0
0 0 0
1 1 2
53 48 44
9 12 16
0 0 0
Pond Site II summer 1990 1 2 3 4 5 6 7 8 9 winter 1991 1 2 3
18 25 17 18 16 13 19 16 16
1 3 6 5 5 2 2 6 6
0 0 1 0 1 6 0 1 0
6 33 53 53 50 27 10 55 52
29 26 3 4 4 5 27 3 4
44 7 9 11 15 58 42 14 15
0 0 0 0 0 0 0 0 0
15 8 13
5 1 0
0 6 0
46 20 28
6 1 2
23 63 58
0 4 0
4 8 2 5 7
2 2 2 2 4
0 0 3 16 30
4 2 2 1
7 5 2 0
64 13 6 0
1 2 1
8 11 9
2 8 5
Pond Site III summer 1990 1 2 3 4 5
30 32 29 24 20
0 0 0 0 0
summer 1990 1 2 3 4
9 26 28 32
0 0 0 0
1 1 6 9 13
58 57 61 45 34
Pond Site IV 0 1 6 0
13 51 57 68
Pond Site V summer 1990 1 2 3
24 22 24
2 2 1
4 11 12
58 53 55
Pond Site VI summer 1990 1 2 3 4 5 6 7 8 9 10 11 winter 1991 1 2
26 14 12 9 20 13 14 14 12 12 18
1 1 4 2 1 1 0 0 1 1 1
6 23 0 2 1 0 0 0 1 4 20
57 40 37 22 42 27 28 29 26 27 43
1 2 1 3 1 2 0 0 1 1 2
9 16 37 61 32 58 57 50 59 57 13
3 26 0 1 0 0 0 1 0 2 18
13 12
6 0
0 0
50 28
1 0
20 59
0 0
Pond Site VII summer 1990 1 2 3 4 5 6 winter 1991 1 2
12 34 31 32 14 31
4 1 0 0 0 0
0 0 5 2 0 0
33 7 65 65 29 62
8 50 0 0 0 1
36 6 0 0 57 7
0 0 1 0 0 0
30 13
0 0
0 1
62 27
0 0
5 62
0 0
primarily in the hypersaline cells that are approaching dryness. The potentially adverse ramifications of these elevated trace element concentrations on waterfowl has lead to the closure
VOL. 31, NO. 3, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
833
of one evaporation pond facility and the close monitoring of other highly evapoconcentrated ponds. Trace element concentrations in evaporite samples ranged from less than detection to nearly 1 g/kg and showed the following distribution: B . Mo > As ≈ Se (Table 3). In several evaporites, As concentrations were below detection limits. Maximum trace element concentrations were 3-10 times greater than comparable values from efflorescent crusts in Alberta (26). Trace element affinity for a particular salt type (Cl dominant, SO4 dominant, Cl-SO4) was not apparent, nor were there differences between the September and February samples. Similarly, no discernible differences occurred between dried evaporites and those in contact with the pond waters. Maximum trace element concentrations were markedly below the California hazardous waste criteria for solids (Table 3). Maximum concentrations of Se, As, B, and Mo were 3, 58, 7, and 37 times lower than the hazardous waste criteria, respectively. Thus, the primary environmental concern associated with trace elements in evaporation ponds is elevated trace element concentrations occurring in the hypersaline solutions as the pond waters become highly evapoconcentrated. Partitioning. Figure 1 contains plots of Dsoln versus x Dsolid for the four trace elements. The partitioning of trace x elements could not be differentiated on the basis of sample collection dates (therefore, data points for different sample collection dates are grouped together in Figure 1). The range of mole fractions for As and Mo are comparatively narrow ( As ≈ Mo (in order of decreasing mean Kx). Distribution coefficients for Se ranged from one sample with a 103 enrichment in the solid phase of a mirabilite sample to a 103 enrichment in the solution phase. Most samples fell within a factor of 10 from the unity line with a tendency for Se enrichment in the solution phase. While not measured specifically for these solution samples, previous analyses from these same ponds showed that selenate (SeO4) is the dominant form of soluble Se (43). This is consistent with the pH and Eh relationship for typical pond waters (14, 44). Lesser concentrations of organically bound Se can be present with trace levels of selenite (SeO3). The most likely incorporation of SeO4 into evaporite minerals is substitution for SO4. However, SeO4 appears to become equally incorporated into Cl-dominated evaporites as well. Distribution coefficients for B range from near unity to a 103 enrichment within the solution phase. The preference of B for the solution phase results in the elevated B concentrations observed in most pond waters. Previous studies on the chemical speciation of B in hypersaline evaporation ponds indicate that borate is the likely dominant form of B (22) with the possibility of polymeric forms at higher B concentrations. In laboratory studies of B incorporation into halite, mirabilite, and thenardite, minor retention of B by these evaporites was found with adsorption being the primary retention mechanism (13). Arsenic distribution coefficients generally ranged from 101 to 103 enrichment in the solution phase. A number of evaporites had less than detectable concentrations of As in the solid phase, resulting in only a limited number of samples presented in Figure 1. In spite of the preference of As for the solution phase, pond water As concentrations were generally low, possibly reflecting immobilization following reduction in the upper layers of the pond water sediment (12). Based
834
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 3, 1997
TABLE 3. Trace Element Concentrations in Evaporation Pond Solid Phase and Hazardous Waste Criteria dominating anion in salt
solid phase concn (mg/kg)
Se
As
B
Mo
Pond Site I summer 1990 1a 2 3 4 5a 6a 7a winter 1991 1a 2a 3a
Cl Cl Cl Cl-SO4 Cl Cl Cl
0.675 0.067 0.612 0.324 0.387 0.521 0.927
ndb nd nd nd nd nd nd
525 114 822 741 731 975 922
27.59 2.14 17.30 12.85 13.14 13.18 29.09
Cl Cl Cl
0.166 nd 0.649
nd nd nd
328 294 225
4.38 3.99 3.99
Pond Site II summer 1990 1a 2a 3 4 5 6 7a 8 9 winter 1991 1a 2 3a
Cl Cl-SO4 SO4 SO4 SO4 Cl-SO4 Cl SO4 SO4
0.121 0.133 0.106 0.334 0.339 0.143 0.218 1.759 0.223
0.138 0.134 0.400 0.402 0.557 0.425 0.102 0.504 0.403
495 86 20 88 64 165 554 56 50
7.00 1.53 0.57 1.68 1.09 1.70 7.74 0.73 1.07
Cl-SO4 SO4 SO4
0.019 0.389 nd
0.680 0.913 0.049
43 43 18
0.94 0.87 0.34
8.092 8.589 2.867 3.421 3.179
74 92 44 180 192
7.77 11.69 30.87 20.40 32.00
0.767 1.146 nd nd
196 244 83 16
15.51 17.06 nd 5.96
0.879 0.133 nd
115 173 88
93.58 8.37 nd
Pond Site III summer 1990 1 2 3 4 5
SO4 Cl-SO4 SO4 SO4 Cl-SO4
nd 0.045 0.287 0.244 0.550
Pond Site IV summer 1990 1 2 3 4
Cl-SO4 SO4 SO4 SO4
0.721 0.377 0.344 0.113
Pond Site V summer 1990 1 2 3
SO4 SO4 SO4
0.325 0.651 nd
Pond Site VI summer 1990 1 2 3a 4a 5a 6a 7a 8a 9a 10 11 winter 1991 1a 2a
SO4 SO4 SO4 SO4 SO4 SO4 SO4 SO4 SO4 SO4 SO4
0.318 0.329 0.077 0.100 0.330 9.248 0.004 0.038 0.030 0.354 0.114
0.502 1.692 0.537 0.827 0.171 0.276 0.055 0.058 0.054 0.124 0.292
141 184 191 288 97 123 46 44 90 104 239
1.87 3.21 4.11 4.33 2.00 1.70 0.59 0.78 1.05 1.41 11.64
SO4 SO4
nd nd
1.126 0.129
117 20
1.62 0.90
Pond Site VII summer 1990 1a 2a 3 4 5a 6a winter 1991 1a 2a
Cl-SO4 Cl SO4 SO4 SO4 SO4
33.216 5.491 11.516 11.029 5.221 2.972
nd nd nd nd nd nd
326 60 nd 26 15 13
3.55 0.79 0.13 0.96 0.25 0.14
SO4 SO4
6.051 8.193
nd nd
3 1
0.61 0.23
100
500
7000
3500
hazardous waste criteriac a
Evaporite sample was in contact with pond water. b nd, not detected. c For Se, As, and Mo: hazardous waste criteria (threshold limit concentration) for total species according to Title 22, California Administrative Code. For B: Designated waste criteria for total species according to Title 23, California Administrative Code.
FIGURE 1. Distribution of trace elements, including As, B, Mo, and Se, in solids vs contact solutions. Halite minerals (chloride salts) are indicated by solid symbols; thenardite and mirabilite minerals (sulfate salts) are indicated by open symbols. Ponds I, II, III, and VII are indicated respectively by the symbol shapes 0, 4, ], and O. on speciation data and previous analytical analyses, the dominant form of As in pond waters is arsenate (AsO4) (45). The retention of arsenate by evaporite probably proceeds by weak adsorption and occlusion in interstitial pores. Molybdenum distribution coefficients indicate a 101-103 enrichment in the solution phase. Pond water concentrations of Mo accumulate to relative high levels owing to this low affinity for incorporation into the evaporite minerals. Laboratory simulations of evaporite precipitation using synthetic pond waters showed that Mo was removed from solution by precipitation as CaMoO4 and possibly by coprecipitation with other evaporite minerals (11). Speciation data indicate that molybdate (MoO4) is the dominant form of Mo under the chemical conditions typical of evaporation pond waters (14, 44). Evaporite Deposition Ramifications. The large-scale formation of evaporite deposits in evaporation pond facilities signals a new maturity in their design lifetime of 15-20 years. In terms of optimum evaporation conditions, it is fortunate that the major solute compositions result principally in the deposition of higher solubility salts so that evaporation rates are not severely hindered by surface salt crusts. The option of salt harvesting, that is, the removal of the evaporite byproduct of evaporative concentration, could be used since the bulk of salt deposits do not contain hazardous levels of trace elements, such as As, B, Mo, and Se. However, association of trace elements with deposits formed late in the desiccation process, although a small proportion of the overall deposit, could require isolating these deposits and associated mother liquor in order to avoid toxic exposure. The formation of an environmentally hazardous mother liquor is occasionally further aggravated by the deposition of
mirabilite, which absorbs large molar quantities of water during the precipitation process, thus rapidly enriching the toxicants in solution. Isolation of these trace element-rich solutions during the latter stages of evapoconcentration into specialized treatment and handling facilities, coupled with predictive hypersaline hydrogeochemical modeling for anticipating evaporite deposition, may provide a management strategy that minimizes both exposure to waterfowl and the amount of hazardous solid waste generated.
Acknowledgments This research was partially supported by a Joseph G. Prosser Dissertation Fellowship from the University of California Water Resources Center to C.G.O. The authors also acknowledge Doug Peters for field sampling assistance, and Ann Quek, Cindy Bergens, and Suduan Gao for chemical analysis work.
Literature Cited (1) Tanji, K.; La¨uchli, A.; Meyer, J. Environment 1986, 28, 34. (2) Tanji, K. K. In Agricultural Salinity Assessment and Management; Tanji, K. K., Ed.; American Society of Civil Engineers: New York, 1990; Chapter 1. (3) Chilcott, J. E.; Westcot, D. W.; Toto, A. L.; Enos, C. A. Water quality in evaporation basins used for the disposal of agricultural subsurface drainage water in the San Joaquin Valley, California. 1988 and 1989; California Regional Water Quality Control Board: Sacramento CA, 1990. (4) Tanji, K. K.; Ong, C. G. H.; Dahlgren, R. A.; Herbel, M. J. Calif. Agric. 1992, 46, 18. (5) Bradford, G. R.; Bakhtar, D.; Westcot, D. J. Environ. Qual. 1990, 19, 105.
VOL. 31, NO. 3, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
835
(6) Tanji, K. K.; Grismer, M. E.; Hansen, B. R. Calif. Agric. 1985, 39, 10. (7) Presser, T. S.; Barnes, I. Dissolved constituents including selenium in waters in the vicinity of Kesterson National Wildlife Refuge and the West Grassland, Fresno and Merced Counties, California. Water Resources Investigations Report 85-4220, U.S. Geological Survey: Menlo Park, CA, 1985. (8) Ohlendorf, H. M.; Hoffman, D.; Saiki, M. K.; Aldrich, T. W. Sci. Total Environ. 1986, 52, 49. (9) Thompson-Eagle, E. T.; Frankenberger, W. T. Environ. Toxicol. Chem. 1990, 9, 1453. (10) Gao, S.; Tanji, K. K. J. Environ. Qual. 1995, 24, 191. (11) Levy, D. B.; Amrhein, C.; Anderson, M. A. J. Environ. Qual. 1994, 23, 944. (12) Amrhein, C.; Mosher, P. A; Brown, A. D. Soil Sci. 1993, 155, 249. (13) Herbel, M. J. M.S. Thesis, University of California, Davis, CA, 1991. (14) Tanji, K. K. Transactions, 14th International Congress of Soil Science; 1990; Vol. VII, p 180. (15) Ong, C. G; Tanji, K. K. J. Agric. Food Chem. 1993, 41, 1507. (16) Ong, C. G.; Tanji, K. K.; Dahlgren, R. A.; Smith, G. R.; Quek, A. F. J. Agric. Food Chem. 1995, 43, 1941. (17) Turk, L. J. Water Resour. Res. 1970, 6, 1209. (18) Hardie, L. A.; Eugster, H. P. Mineral. Soc. Am. Spec. Publ. 1970, 3, 273. (19) Harvie, C. E., Weare, J. H.; Hardie, L. A.; Eugster, H. P. Science 1980, 208, 498. (20) Felmy, A. R.; Weare, J. H. Geochim Cosmochim. Acta 1986, 50, 2771. (21) Miyamoto, S; Pingitore, N. E. Soil Sci. Soc. Am. J. 1992, 56, 1767. (22) Smith, G. R.; Tanji, K. K.; Jurinak, J. J.; Burau, R. G. In Chemical Equilibrium and Reaction Models; Loeppert, R. H., Schwab, A. P., Goldberg, S., Eds.; Soil Science Society of America Special Publication 42; Soil Science Society of America: Madison WI, 1995; Chapter 15. (23) Hem, J. D. U.S. Geol. Surv. Water-Supply Pap. 1985, No. 2252. (24) White, A.; Benson, S. M.; Yee, A. W.; Wollenberg, H. A., Jr.; Flexser, S. Water Resour. Res. 1991, 27, 1085. (25) Presser, T. S.; Barnes, I. Selenium concentrations in waters tributary to and in the vicinity of the Kesterson National Wildlife Refuge, Fresno and Merced counties, California. Water Resources Investigations Report 84-4122, U.S. Geological Survey: Menlo Park, CA, 1984. (26) Kohut, C. K.; Dudas, M. J. Can. J. Soil Sci. 1993, 73, 399. (27) Reeder, R. J.; Lamble, G. M.; Lee, J.; Staudt, W. J. Geochim. Cosmochim. Acta 1994, 58, 5639. (28) Staudt, W. J.; Reeder, R. J.; Schoonen, M. A. A. Geochim. Cosmochim. Acta 1994, 58, 2087.
836
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 3, 1997
(29) Hemming, N. G.; Reeder, R. J.; Hanson, G. N. Geochim. Cosmochim. Acta 1995, 59, 371. (30) Walton, A. G. The formation and properties of precipitates; Robert E. Krieger Publishing Co., Inc.: Huntington, NY, 1967. (31) Moretto, R. Geochim. Cosmochim. Acta 1988, 52, 2809. (32) Ford, S. Investigation of evaporation ponds for the disposal of agricultural drainage in the San Joaquin Valley. Draft report, California Dept. of Water Resources: Sacramento, CA, 1988. (33) Boyd, E. M.; Shanas, M. N. Arch. Int. Pharmacodyn. Ther. 1963, 144, 86. (34) Handbook of Toxicology; Spector, W. S., Ed.; Saunders: Philadelphia, PA, 1956; Vol. 1, p 278. (35) Cummins, L. M.; Kimura, E. T. Toxicol. Appl. Pharmacol. 1971, 20, 89. (36) Fio, J. L.; Fujii, R. Soil Sci. Am. J. 1990, 54, 363. (37) Ogle, R. S.; Knight, A. W. Environ. Contam. Toxicol. 1989, 18, 795. (38) John, M. K.; Chuah, H. H.; Neufeld, J. H. Anal. Lett. 1975, 8, 559. (39) Driessens, F. C. M. In Geochemical Processes at Mineral Surfaces; Davis, J. A., Hayes, K. F., Eds.; ACS Symposium Series 323; American Chemical Society: Washington, DC, 1986; pp 524560. (40) Eghbal, M. K.; Southard, R. J.; Whittig, L. D. Soil Sci. Soc. Am. J. 1989, 53, 898. (41) Biggar, J. W.; Rolston, D. E; Nielsen, D. R. Calif. Agric. 1984, 38, 10. (42) Linke, W. F. Solubilities. Inorganic and metal-organic compounds, 4th ed.; American Chemical Society: Washington, DC, 1965. (43) Kang, Y.; Tanji, K. K. University of California, Davis, unpublished results. (44) Fujii, R.; Swain, W. C. Areal distribution of selected trace elements, salinity, and major ions in shallow groundwater, Tulare Basin, Southern San Joaquin Valley, California. USGS Water Resources Investigations Report 95-4048, U.S. Geological Survey: Sacramento, CA, 1995. (45) Gao, S.; Tani, K. K. University of California, Davis, unpublished results.
Received for review June 18, 1996. Revised manuscript received October 28, 1996. Accepted October 31, 1996.X ES960531G X
Abstract published in Advance ACS Abstracts, January 15, 1997.
