Phytoremediation management of selenium-laden drainage sediments in the San Luis Drain: a greenhouse feasibility study

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Ecotoxicology and Environmental Safety 62 (2005) 309–316 www.elsevier.com/locate/ecoenv

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Phytoremediation management of selenium-laden drainage sediments in the San Luis Drain: a greenhouse feasibility study G.S. Ban˜uelosa,, Z.-Q. Linb a

USDA-Agricultural Research Service-Water Management Research Laboratory, 9611 S. Riverbend Avenue, Parlier, CA 93648, USA Southern Illinois University, Environmental Sciences Program and Department of Biological Sciences, Edwardsville, IL 62026-1651, USA

b

Received 31 December 2003; received in revised form 23 October 2004; accepted 27 October 2004 Available online 15 December 2004

Abstract An estimated 100,000 m3 selenium (Se)-laden drainage sediment resides in the San Luis Drain (SLD) of Central California. This greenhouse study was undertaken to evaluate the feasibility of growing salt- and boron-tolerant plant species in sediment for reduction of Se content by plant extraction. Drainage sediment was collected from the SLD and mixed with control soil (i.e., uncontaminated soil) to the following ratios (sediment:control soil) by volume: 0:3 (i.e., control soil only), 1:2 (i.e., 1/3 sediment and 2/3 control soil), 2:1 (i.e., 2/3 sediment and 1/3 control soil), and 3:0 (i.e., sediment only). Salt-tolerant plant species consisted of canola (Brassica napus var. Hyola 420), tall fescue (Festuca arundinacea var. Au Triumph), salado grass (Sporobulus airoides), and cordgrass (Spartina patens var. Flageo). Increased ratios of sediment:soil resulted in decreased dry matter production for all tested plant species; especially at ratios of sediment:soil greater than 1:2. Plant Se concentrations (mg kg 1 DM) ranged as follows for plant species at all ratios of sediment:soil: canola (51–72), tall fescue (16–36), and cordgrass and salado grass (9–14). Total Se concentrations in the soil were at least 20% lower at postharvest compared to preplant concentrations for all plant species at each ratio of sediment:soil. In contrast, water-extractable Se concentrations in the soil were at least three times higher at postharvest than at preplant for all plant species, irrespective of the ratio of sediment:soil. Leaching of Se occurred in irrigated bare pots from each respective ratio of sediment:soil over a duration of 60 days. Based upon the downward movement of Se in bare pots of sediment:soil, it may be more prudent to leave the drainage sediment in the SLD, incorporate clean soil, and then grow low maintenance salttolerant plants (e.g., cordgrass, salado grass) in the concrete-lined canal. By this means, possible contamination of groundwater with soluble Se will be eliminated, while phytoremediation slowly reduces Se content in the drainage sediment. Published by Elsevier Inc. Keywords: Phytoremediation; Selenium; Drainage sediment

1. Introduction The grasslands bypass project in the western San Joaquin Valley of California in the late 1970s was conceived as a means of diverting selenium (Se)contaminated agricultural drainage water around a 32,000 ha wetland complex prior to discharge to the San Joaquin River. The drain water was channeled via the San Luis Drain, small sloughs, and the San Joaquin Corresponding author. Fax: +1 559 596 2851.

E-mail address: [email protected] (G.S. Ban˜uelos). 0147-6513/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.ecoenv.2004.10.013

River toward the San Francisco Bay Delta. Interim use of a concrete-lined 45 km long channel for transporting the drain water resulted in an accumulation of Se- and boron (B)-rich drainage sediment. The prohibitive cost of removing and remediating the approximately 100,000 m3 of highly contaminated residual sediment estimated at 1.1–3.4 million dollars led to the decision to leave the sediment in place. Because the biological accumulation of Se in the Kesterson Reservoir marshes in central California resulted in extensive deformities and death of numerous fish and aquatic bird species (Ohlendorf et al., 1986), the USEPA and local public

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communities have expressed serious concerns regarding the disposal of the Se-laden drainage sediment residing in the San Luis Drain (Quinn, 1998). In a recent study on land disposal of drainage sediment, Zawislanski et al. (2003) indicated that longterm Se oxidation rates were slow under field conditions and that overall Se remained largely water-insoluble in soils. They concluded that the low solubility of Se would limit its movement to the groundwater. This is important information because selenium’s potential toxicity to wildlife is highly dependent on the bioavailability of Se in the drainage sediment. In this regard, selenium’s solubility depends on its chemical speciation and partitioning in sediment, which are controlled by sediment pH, redox potential, complexion ability of soluble and solid ligands, microbial interactions, and oxidation/reduction kinetics of Se in sediment (McNeal and Balistrieri, 1989). Selenium exists primarily as two soluble species, i.e., selenate (SeO24 ) and selenite (SeO23 ), in most seleniferous soils and agricultural drainage water. In an oxidized environment selenate salts are water-soluble and easily taken up by plants and/or leached from soil (McNeal and Balistrieri, 1989). Selenite has a strong affinity of sorption, especially by iron oxides, amorphous iron hydroxide, Al sesquioxides, and organic matter (Balistrieri and Chao, 1987). Soluble selenate or selenite can be transformed biologically into nonwater-soluble elemental Se under reducing environmental conditions. Furthermore, through microbial activity, elemental Se can also be oxidized to selenite or selenate by a diverse group of microorganisms (Sarathchandra and Watkinson, 1981). When disposing of Se-laden drainage sediment on land, strategies for managing the high Se content of the drainage sediment-enriched soil need to be fully explored. The bioremediation technology is one such strategy that uses microbial activity in conjunction with plants to stabilize, extract, and volatilize Se from the soil/water environment (Frankenberger and Karlson, 1994; Losi and Frankenberger, 1997; Lin et al., 2000; Ban˜uelos et al., 1997; Terry and Zayed, 1994, 1998). Based upon these biological approaches, Ban˜uelos et al. (2003) and Terry and Chang (2003) hypothesized that growing selected plants as a vegetative cap over drainage sediment disposed of on field sites may manage Se via the processes of extraction and volatilization. However, due to the extremely high levels of total Se (up to 75 mg kg 1 DM), extractable B (25–50 mg L 1), and salinity (EC, 50–75 dS m 1) commonly found in drainage sediment residing in the San Luis Drain, it is uncertain whether any plant species could tolerate the high levels of soluble B and high salinity. In addition, the high levels of sulfate in drainage sediment (2000–7000 mg SO4 L 1) would also compete with selenate for plant uptake (Mikkelsen et al., 1989; Bell et al.,

1992) and negatively affect Se volatilization (Zayed and Terry, 1994; Terry et al., 2000). Currently, the USDA-ARS in Fresno, CA has a 0.5 ha field test plot on which drainage sediment transported from the San Luis Drain was disposed. Different plant species are under consideration for planting to manage Se via accumulation and volatilization. This greenhouse feasibility study was undertaken to evaluate the feasibility of growing known salt- and B-tolerant plant species to manage Se in drainage sediment that has been applied to field sites. Our specific objectives of this greenhouse study were: (1) identify or screen for plant species tolerant of drainage sediment conditions; (2) determine the extent of Se phytoextraction; and (3) examine the potential effects of vegetation on soil/sediment chemistry of Se, B, and salinity.

2. Materials and methods Different plant species recognized to grow under adverse saline conditions (Ban˜uelos, 2000; Lin et al., 2002) were evaluated for their ability to survive and accumulate Se when grown in different ratios of drainage sediment to control soil under greenhouse conditions at Parlier, CA. The drainage sediment used in the greenhouse study was collected from the San Luis Drain, Mendota, CA, at a depth of 0–25 cm and control soil (a Hanford fine sandy loam) was collected from Parlier, CA, at a depth of 0–25 cm. After the sediment was homogenized in a cement mixer, the following ratios of sediment to control soil (V/V) were mixed to encourage both plant survival and biological activity: (1) no sediment and 100% control soil (0:3), (2) 1/3 sediment and 2/3 control soil (1:2), (3) 2/3 sediment and 1/3 control soil (2:1), and (4) 100% sediment and no control soil (3:0). Preplant soil samples were collected from each ratio of sediment:soil and analyzed as described later. Plant species consisted of canola (Brassica napus var. Hyola 420), tall fescue (Festuca arundinacea var. Au Triumph), salado grass (Sporobulus airoides), and cordgrass (Spartina patens var. Flageo). Plants were grown from both seed and transplants in 18-L plastic pots filled with a total of 10 kg of sediment and soil. Both modes of planting were initially investigated to determine if one form of planting enhanced the plants’ ability to survive in the respective ratio of sediment:soil. The experimental design for all studies was completely randomized with six replicates for each plant species planted as seed and as transplant, respectively, and for each ratio of sediment:soil. Canola was planted as 10 seeds per pot (and thinned later to three plants) or as three 14-day-old transplants per pot, while both tall fescue and salado grass were planted by seed at a rate of 2.0 and 0.7 g per pot, respectively, or planted as four

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4-week-old 3 clumped transplants per pot. Cordgrass provided by the USDA-NRSC Plant Introduction Station, Lockeford, CA, was planted only as five 6-week-old transplants per pot. Irrespective of mode of planting, a 20-cm perforated cup was placed 4-cm deep into each pot and was used as a receptacle for irrigation water. Plants were irrigated with a slightly saline water (ECo1 dS m 1) based upon approximated evapotranspiration losses (determined by weight losses) for each ratio of sediment:soil to minimize leachate losses. Plants were grown under controlled greenhouse conditions at 2472 1C with an average photosynthetic active radiation (PAR) of 450 mmol m 2 s 1. Drainage trays were placed under planted pots to capture any leachate, which if produced, was carefully reapplied. In addition, leachate was intentionally produced from three bare pots (without plants) for each respective ratio of sediment:soil for 60 days to determine the effect of irrigation on leaching of soluble salts from drainage sediment. For this purpose, water (ECo1 dS m 1) was applied in excess of soil evaporative losses (400 ml per pot) to each pot. Leachate (50 ml) was collected in 10-day intervals from the respective ratio of sediment: soil, refrigerated, and analyzed for Se, B, and salinity (EC) within 48 h as described later. Canola and tall fescue were harvested after 120 days of growth at 2–5 cm above the soil surface. Salado and cordgrass were clipped and then allowed to continue growing for another 120 days until they went dormant. For all plant species, above- and below-ground plant materials were removed from pots at the end of experiment. Root samples were not collected for Se analyses, due to difficulty in completely removing fine sediment particles from the roots. Harvested and clipped plant materials were washed with deionized water, oven dried at 50 1C for 7 days, weighed, and ground in a stainless-steel Wiley mill equipped with a 1-mm screen. After plant harvest, sediment cleaned of residual plant material was thoroughly mixed from each pot, and a 500-g sediment sample was collected from each pot and passed through a 2-mm sieve. Each sediment sample was dried at 50 1C for 7 days and ground to pass an 850-mm sieve. A low drying temperature of 50 1C was selected because it reduces any potential loss of Se through volatilization during sample dehydration. Water-soluble fractions of soil Se and B, and EC were determined from a sediment:soil water extract of approximately 1:1. Total sediment Se was determined in a 500-mg ground sediment sample after wet acid digestion with nitric acid, hydrogen peroxide, and hydrochloric acid (Ban˜uelos and Meek, 1990). Plant samples were acid-digested with nitric acid, hydrogen peroxide, and hydrochloric acid as described by Ban˜uelos and Akohoue (1994). Selenium in soil and plant samples was analyzed by an atomic absorption spectrophotometer (Thermo Jarrell Ash, Smith Hieftje 1000, Franklin, MA) with an

311

automatic vapor accessory (AVA 880) and B and S were analyzed by inductively coupled plasma spectrometer (Perkin-Elmer Plasma 2000 Emission spectrometer, Norwalk, CT). The National Institute of Standards and Technology (NIST) Wheat Flour (SRM 1567; Se content of 1.170.2 mg g 1 DM, 94% recovery) and internal soil standards (sediment collected from Kesterson Reservoir, CA) with a total Se content of 7.5 and 25 mg kg 1, 94% recovery, respectively, were used as an external quality control standards for soils. Soil extracts and plant tissues samples were acidified with nitric acid and extracted with 2% acetic acid, respectively, and analyzed for chloride by potentiometric titration with silver nitrate. The Model 160 conductivity/ salinity meter measured soil EC. Statistical Analysis System (SAS) Version 6.03 was used for the data analyses (SAS Inst., 1988), and Duncan’s multiple range test was applied to treatment means at the Po0:05 level of significance (Gomez and Gomez, 1984).

3. Results 3.1. Plant biomass production Among the plant species, total dry matter yields of cordgrass and salado grass were highest for all tested ratios of sediment and soil. Increased ratios of sediment:soil resulted in decreased dry matter production for all tested plant species (Table 1). Generally dry matter yields from all plant species grown in a ratio of sediment:soil greater than 1:2 were at least 3-fold lower compared to yields attained from control soils. Because there were no significant differences in dry matter yield between the same plant species planted either from seed or as transplants, data presented in Table 1 are from plants grown from both seeds and transplants for each respective ratio of sediment:soil. 3.2. Plant elemental concentrations Compared to other plant species, canola accumulated the greatest amount of Se for all ratios of sediment:soil (Table 1). The order of Se bioconcentration by the tested plant species is canola4tall fescue4cordgrass4salado grass. Selenium concentrations ranged from a low of 13 mg kg 1 DM in cordgrass and salado grass to a high of 72 mg kg 1 DM in canola. Selenium concentrations did not significantly increase for any plant species with increasing ratios of sediment:soil greater than 2:1. Among the tested plants, canola accumulated the greatest mass of Se (Se concentration  dry matter yield), irrespective of the ratio of sediment:soil (Table 1). The mass of accumulated Se was greatest at the sediment:soil ratio of 1:2 and the lowest at 3:0 for all plant species. For other selected elements, canola leaves

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Table 1 Dry matter yields, Se mass, and B, Cl, and S concentrations in several plant species grown in different ratios of sediment:soil Plant species

Sediment:soil ratio

DM (g plant 1) b,c

Se Massa (mg plant 1)

Se (mg kg

1

DM)

B (mg kg

1

DM)

Cl (mg kg

1

DM)

S (mg kg

1

DM)

Cordgrass

0:3 (control) 1:2 2:1 3:0 (sediment)

38(2)a 14(1)bc 5(.3)d 4(.3)d

12(.6)f 140(8)c 65(4)e 56(3)e

0.3(0)f 10(.4)e 13(.6)de 14(.5)de

13(.4)hi 37(2)g 49(3)f 66(3)e

7673(287)gh 14517(464)f 15791(540)f 20319(794)e

5679(240)f 7355(259)e 9931(395)d 14347(488)c

Canola

0:3 (control) 1:2 2:1 3:0 (sediment)

16(.4)b 13(.2)c 1.6(.011)e 1.4(.01)e

48(3)e 663(32)a 110(7)d 101(7)d

0.3(0)f 51(2)b 69(3)a 72(3)a

21(.7)h 171(9)c 227(16)b 311(11)a

5781(237)h 25781(575)d 40827(785)b 53908(812)a

4746(222)g 16855(530)c 21459(802)b 28903(963)a

Tall fescue

0:3 (control) 1:2 2:1 3:0 (sediment)

17(.4)b 13(.2)c 5(.04)d 1.6(.03)e

2(0)f 208(10)b 145(11)c 58(4)e

0.1(0)f 16(.5)d 29(1)c 36(2)c

10(.4)i 204(11)b 224(14)b 307(16)a

8047(288)g 17675(529)f 35320(1251)c 43048(1498)b

1075(60)k 1372(69)j 1507(72)ij 1873(92)i

Salado grass

0:3 (control) 1:2 2:1 3:0 (sediment)

36(2)a 17(.3)b 5(.2)d 4(.2)d

7(.1)f 153(8)c 55(3)e 52(4)e

0.2(0)f 9(.2)e 11(.3)e 13(.3)de

16(.4)h 51(3)ef 63(4)e 85(6)d

8543(448)g 8862(317)g 15318(499)f 15669(531)f

4925(228)g 3162(154)h 3094(142)h 3020(163)h

a

Mass of Se removed by the plant (Se concentration  biomass yield). Values are the mean from a minimum of six replications followed by the standard error in parenthesis. c Within a column, means followed by the same letters are not significantly different at Po0:05 using Duncan’s multiple range test. b

accumulated the greatest amount of S, Cl, and B among the plant species grown in the three ratios of sediment: soil. Both cordgrass and salado grass accumulated the lowest amount of Cl and B among the plant species, while tall fescue accumulated the lowest amount of S.

concentrations of salts, Se, and B were measured in the leachate produced from increased ratios of sediment: soil. Additionally, Table 3 shows that concentrations of salts, Se, and B in leachate were greatest during the first 10 days of the 60-day experiment, and then decreased with subsequent irrigations in 10-day intervals.

3.3. Soil chemistry Water-extractable Se concentrations at postharvest were at least 3-fold greater than preplant concentrations for all plant species at each respective ratio of sediment:soil (Table 2). Extractable concentrations of Se at postharvest increased proportionally to the ratio of sediment:soil in the pot. Conversely, total Se concentrations at postharvest decreased proportionally to the amount of sediment in the pot for all plant species (Table 2). Among the other parameters examined, soil salinity and extractable S increased at postharvest compared to preplant concentrations, while extractable B slightly decreased (Table 2). Soil pH was not significantly different among the different ratios of sediment:soil at preplant and/or at postharvest, except pH values were slightly lower throughout the study in control soils (Table 2). 3.4. Leaching of Se, B, and salts Irrigation of the different ratios of bare sediment:soil (without plants) leached soluble salts and waterextractable Se and B into the leachate (Table 3). Higher

4. Discussion The results obtained from this greenhouse study clearly show that all of the tested plant species exhibited significant reduction in growth when grown directly in 100% drainage sediment. Plant dry biomass production decreased for all plant species grown with increasing ratios of sediment:soil. Among the ratios of sediment: soil (excluding control soil), all tested species grew best at the ratio of 1:2 sediment:soil, while salado grass and cordgrass were the greatest yielding plant species grown in the ratio of sediment:soil greater than 1:2. Decreases in plant biomass production of all tested plant species are likely a result of the high salinity level in the contaminated drainage sediment. The high tissue Cl concentrations measured in all species with increasing sediment:soil ratios are evidence of this—excessively high levels of Cl are detrimental to plant growth (Wu et al., 1995). Similarly, Maas and Grattan (1999) describe the effect of salinity and chloride on reducing crop yields under saline conditions. Clipping perennial species (e.g., salado and cordgrass) on a regular basis

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Table 2 Selected soil chemical parameters in different ratios of sediment:soil after harvesting plants Sediment:soil ratio

Total Se (mg kg 1)

Extractable Se (mg L 1)

B (mg L 1)

S (mg L 1)

pH

EC (dS m 1)

Pre-plant 0:3 (control) 1:2 2:1 3:0 (sediment)c

0.1(0)ha,b 15(2)f 37(5)c 57(6)a

0(0) 1.0(o.1)f 2.5(.1)e 5.4(.3)d

o1(0)e 7(.2)d 22(.6)b 46(5)a

169(8)h 2467(110)f 5071(229)d 8020(348)b

7.5(.1)b 8.0(.1)a 8.2(.1)a 8.3(.1)a

1.2(.2)h 15(.7)g 30(1)e 45(3)b

Tall fescue(post-harvest) 0:3 (control) 1:2 2:1 3:0 (sediment)

0.1(0)h 12(1)g 35(2)c 52(5)ab

0(0)f 2.8(.1)e 10(.5)c 19(.8)a

o 1(0)e 5(.1)d 20(.35)bc 42(.75)a

287(9)g 3473(121)e 6045(223)cd 9940(297)a

7.7(.1)a 8.1(.1)a 8.2(.1)a 8.2(.1)a

1.5(o.1)h 17(.6)g 33(1)d 48(5)ab

Canola (post-harvest) 0:3 (control) 1:2 2:1 3:0 (sediment)

0.1(0)h 12(1)g 32(2)d 51(3)ab

0(0)f 3.0(.1)e 12(1)bc 16(1)ab

o 1(0)e 6(.2)d 18(1)bc 43(2)a

175(10)h 3291(108)e 6234(227)c 8977(278)a

7.4(.1)b 7.9(.1)a 8.0(.1)a 8.1(.1)a

1.6(.1)h 17(1)g 36(2)c 48(3)ab

Salado grass (post-harvest) 0:3 (control) 0.1(0)h 1:2 11(1)g 2:1 27(1)e 3:0 (sediment) 49(3)b

0(0)f 3.3(.1)de 11(.9)c 19(.8)a

o 1 (0)e 6(.2)d 14(.5)bc 48(.7)a

163(8)h 3073(121)ef 6384(211)c 9628(327)a

7.5(.1)b 7.9(.1)a 8.0(.1)a 8.2(.1)a

1.8(.1)h 18(1)fg 33(2)cd 49(3)ab

Cord grass (post-harvest) 0:3 (control) 1:2 2:1 3:0 (sediment)

0(0)f 3.5(.1)de 12(1)bc 20(.9)a

o 1(0)e 5(.2)d 15(1)c 47(.9)a

160(8)h 3944(91)e 6251(196)c 9062(286)a

7.3(.1)b 8.0(.1)a 8.1(.1)a 8.2(.1)a

1.9(.1)h 20(1)f 34(2)cd 51(3)a

0.1(0)h 10(1)g 28(2)e 47(3)b

a

Values are the means from a minimum of six replications followed by the standard error in parenthesis. Within a column, means followed by the same letter are not significantly different at Po0:05 using Duncan’s multiple range test. c Other characteristics of pure drainage sediment are: cation exchange capacity of 29 meq/(100 g) DW, 2.6% organic matter, 19% sand, 55% silt, and 26% clay. b

Table 3 Composition of leachate produced in 10-day intervals from irrigating different ratios of bare sediment:soila Sediment:soil ratio

Sampling (days)

Extractable Se (mg L 1)

Extractable B (mg L 1)

EC (dS m 1)

1:2

10 20 30 40 50 60

5(.3)aa,b 4(.3)ab 2(.1)bc 1(0)c o1(0)c o1(0)c

8(.3)a 4(.1)b 4(.1)b 3(.1)b 2(0)b 2(0)b

14(1)a 8(.3)b 7(.2)bc 5(.1)cd 4(.1)d 4(.1)d

2:1

10 20 30 40 50 60

14(.8)a 12(.7)a 8(.5)b 7(.3)b 4(.2)c 3(.1)c

26(1)a 16(.6)b 14(.6)b 13(.3)b 13(.3)b 8(.2)c

46(3)a 26(2)b 22(2)cd 19(.8)de 18(.7)e 17(.6)e

3:1

10 20 30 40 50 60

24(1)a 22(.7)ab 21(.5)bc 18(.3)c 5(.1)d 5(.1)d

36(1)a 35(1)a 30(.7)b 25(.6)c 24(.3)c 20(.2)d

68(3)a 57(2)b 52(2)c 47(2)d 32(1)e 26(1)f

a

Values are the means from a minimum of six replicates and standard error in parenthesis. Within a column, means followed by the same letter are not significantly different at Po0.05 using Duncan’s multiple range test for each respective ratio of sediment:soil. b

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may help maintain lower concentrations of Cl in plant tissue by removing Cl from the plant system, and thereby enhance the plant’s ability to survive in salty conditions. The range of extractable B present in the different ratios of sediment:soil was from 7 to 56 mg L 1, which is generally toxic to most plant species (Maas and Grattan, 1999). Based upon the relatively low tissue B concentrations for all tested plant species (o307 mg kg 1) in light of the highly extractable B concentrations in sediment, boron appeared, however, to have exerted a minimal effect on decreasing biomass production. Presumably leaf transpiration and plant water uptake were lower due to the high salinity levels with increasing ratios of sediment:soil, and thereby minimized movement of soluble B toward the plant roots and its subsequent uptake by the plants (Shannon et al., 1997, 1999; Grattan et al., 2003). Biomass production plays an important role in a general phytoremediation strategy for Se, if plant extraction and plant accumulation of Se are to be considered as the principle pathways for removal of Se from drainage sediment. Based upon the decreases in biomass yields with increasing ratios of sediment:soil, mixing sediment with clean soil will be a necessary step for enhancing biomass production of the tested plant species under field conditions. However, applying clean soil as a cover soil may be more practical if growing plants directly in the drainage sediment in the SLD is considered. More studies are needed to evaluate the effectiveness of applying clean soil on top of sediment as a cover soil compared to mixing clean soil with sediment. The dramatic decrease in biomass production for all tested plant species grown in a ratio of sediment: soil41:2, and the relatively low range of Se recovered in all plant tissues (9–72 mg kg 1 DM), clearly indicated under greenhouse conditions that phytoextraction with the tested plant species will not play an immediate role in Se removal from drainage sediment. Although canola accumulated the greatest amount of Se among the plant species, as an annual species it would have to be repeatedly planted in contrast to the lower maintenance perennial species. Because there are no studies to our knowledge in which the tested plant species have been grown in drainage sediment, a field study conducted by van Mantgem et al. (1996) at Kesterson Reservoir showed lower Se concentrations in plants grown in a drainage-like sediment containing 5–10 mg Se kg 1 DM compared to 12–52 mg Se kg 1 DM in this greenhouse study. Owing to the high concentrations of soluble S in the sediment and its inhibitive effect on the uptake of selenate (Mikkelsen et al., 1989), dry matter yields were not affected by the tissue Se concentrations measured for any plant species in this study. In this regard, other lab studies have shown higher concentrations of Se in plant species grown under lower concentrations of

soluble S in the growing medium (Zayed and Terry, 1992). When plants are exposed to high concentrations of Se under lab conditions, then it may be possible to observe symptoms of injury (Terry et al., 2000) depending on the plant’s ability to accumulate Se. There are, however, significant differences between Se-hyperaccumulator plants, e.g., Astragalus species, that can easily accumulate Se at nontoxic concentrations greater than 4000 mg kg 1 DM compared to nonaccumulators, e.g., white clover, that may show toxic symptoms at 300 mg kg 1 DM in its plant tissue. Clearly, the increased concentrations of soluble S measured with increasing ratios of sediment:soil reduced the uptake of Se by all plant species, despite the higher concentrations of soluble Se in the soil amended with the drainage sediment. With rhizosphere microbes in vegetated drainage sediment, elemental S can be oxidized and microbial decomposition of organic matter can lead to soluble SO4 (Zhao et al., 1996). Even the addition of irrigation water to the growing pots may have also dissolved some of the S-complex salts in the drainage sediment and contributed to higher soluble S in the soil. As a result of both microbial and physical transformations of S and S root exudation processes releasing S-laden amino acids into the rhizosphere environment, more extractable S was recovered in postharvest soils compared to those levels measured at preplant. There is a competition of S with Se for uptake and metabolism in plant tissues because sulfate and selenate are chemical analogs of one another. With high levels of S in the drainage sediment, S or sulfate in plant tissues will compete with selenate for the enzymes of the S-assimilation pathways (Zayed and Terry, 1992) and reduce the phytotoxicity potential of Se. The development of Se volatilization as a more effective strategy for remediation of Se-contaminated sediment is an innovative and environmentally sound way for Se cleanup. A recent volatilization study in constructed wetlands treating Se-laden agricultural drainage water showed that Se volatilization can represent a significant pathway of Se removal (Lin and Terry, 2003). Biogenic volatilization employs plants and plant-associated microbes to absorb Se from sediment and metabolize it to volatile Se compounds (Frankenberger and Karlson, 1994; Zayed et al., 2000). Although the drainage sediment in our study contains very high levels of sulfate that could adversely affect the uptake and volatilization of selenate-Se by plants (Zayed and Terry, 1994), our preliminary volatilization study under these greenhouse conditions showed high rates of Se volatilization (e.g., 75 mg m 2 day 1) in drainage sediment pots (1:2) with the plant species. This finding suggests that biological volatilization may be a feasible remediation component to consider for the management of Se in the drainage sediment from the San Luis Drain. To this end, a comprehensive volatilization study is

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currently in progress for the same plant species grown in drainage sediment under field conditions (Ban˜uelos et al.; unpublished). The analyses of leachate produced from irrigating bare sediment pots clearly indicated that soluble Se (most likely in the form of selenate) is susceptible to leaching and highly bioavailable. The EC and extractable Se levels decreased in the leachate over time, whereas extractable B levels clearly decreased to a lesser degree in the bare pots, as well as in the sediment:soil of planted pots at postharvest [(B requires 3–5 times more water than soluble salts to leach (Grattan and Rhoades, 1990)]. Although the amount of Se by leaching was not determined in the planted pots (leachate was reapplied), the increased concentrations of extractable Se measured in soils at postharvest for all planted pots suggest that plants and irrigation water and their influence on biological and physical processes contributed to an increased production of soluble Se at postharvest. Soluble Se would presumably be available for leaching under field conditions. If the plants are not able to extract the newly available soluble Se, then this form of Se is susceptible to leaching (as observed in the irrigated bare pots). Although Zawislanski et al. (2003) did not observe more than 2.5% increase in soluble Se annually below 15 cm in low permeable soils with applied drainage sediment under field conditions, they did report a fairly rapid Se oxidation during the first few months after field application (it is important to note that average moisture content of the sediment was higher during the first few months). Similarly Tokunaga et al. (1996) have also reported an initial rapid Se oxidation in formerly ponded and dried out sediments. Zawislanski et al. (2003) suggested that the predominance of organic Se forms may explain the initial release of soluble Se since oxidation rates for organic Se can be an order of magnitude faster than the oxidation of abiotic Se (0) (Martens and Suarez, 1997). Our preliminary speciation analysis by X-ray absorption spectroscopy (XAS) on limited soil samples indicated that the major chemical forms of Se in the vegetated sediment:soil (1:2) of this study were in the following percentages of the total Se: selenomethionine (40%), selenite (36%), elemental Se (20%), and selenate (4%) (Lin et al., limited data not shown).

5. Conclusion The higher concentrations of soluble Se measured at postharvest in our vegetated greenhouse soils infer that it may be more environmentally prudent to grow low maintenance perennial and salt-tolerant plants (e.g., salado and cordgrass), directly into the drainage canal for slowly reducing Se content in drainage sediment. Managing Se content in drainage sediment by plant

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extraction is then only plausible after mixing drainage sediment with uncontaminated soil; otherwise biomass production is strongly reduced at excessively high salinity levels. Planting directly in the SLD may provide not only the surface of the drainage sediment with a type of vegetative cap to reduce the effects caused by wind erosion, but the plants also provide a favorable habitat for microorganisms to volatilize Se from sediments to the atmosphere. Importantly, the possible impact on wildlife, e.g., birds and mammals, should be monitored. The concrete-lined bottom of the canal, which is below the residing drainage sediment, would prevent any leaching of Se from occurring to the surrounding environment. It appears that phytoextraction of Se may not be as effective at high salt and Se levels present in the drainage sediment. Hence, the additional role of biological volatilization of Se should be further investigated.

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