Fate and Transport of Sodium Chloride, Calcium Sulfate (Gypsum), Sodium Sulfate, and Boron in Soil: Implications for Guideline Development Work

Transcription

1 Fate and Transport of Sodium Chloride, Calcium Sulfate (Gypsum), Sodium Sulfate, and Boron in Soil: Implications for Guideline Development Work Presented at: PTAC Soil and Groundwater Forum Presentation by: Greg Huber, M.Sc., P.Eng, PMP Anthony Knafla, M.Sc., DABT Konstaninon Vasilakos, Ph.D. March 15, 2010

2 Why study these salts? Sodium chloride NaCl common salt in produced water chloride a good tracer ion Sodium sulfate Na 2 SO 4 common natural salt sulfates also from sulfur blocks, drilling mud, acid rain Calcium sulfate (gypsum) CaSO 4 can be used for remediating SAR mined or by-product Boron B common co-contaminant in produced water 2

3 Transport differences can be significant Chloride and boron co-plume in bedrock (case-study) conservative, relatively low boron sorption potential in bedrock boron traveled at approx 25% of chloride s velocity 500 mg/l chloride ~6% of source 0.5 mg/l boron ~1.5% of source 3

4 Leaching Columns ( Permeameters ) 4

5 Leaching columns Inlet Several types of columns, all allow: permeability testing collection of leachate 1) Compaction Mold Permeameter repacked ( disturbed ) soils can have 2 outlets for edge effects very sturdy to allow use of ASTM compaction hammer Applied head Compaction mold Leachate collection 5

6 2) Fixed wall permeameter Repacked / disturbed soils Clear walls useful for visualization Allows multiple tests on homogenized soils 6

7 3) Shelby tube permeameter Sections cut from metal or clear Shelby tubes Allows undisturbed soil structure 7

8 Soil cores Clear cores collected from several Alberta sites Cut to obtain undisturbed samples for Shelby tube permeameter Emptied and recompacted for fixed wall permeameter subsoil from below water table (~4 mbgl) subsoil from above water table (~4 mbgl) Cores generously supplied by: Orphan Well Association Petro-Canada (Suncor) 8

9 Screening soils Usually screened (sieved) before repacking Reduces aggregates, gravel, and large void spaces Disturbs (alters) soil structure 10 mesh fairly common (results in <2 mm aggregates) 9

10 Leaching Column Results 10

11 Experimental overview Eight pilots experiments (n=1 for each): #1: Boron adsorption #2: Compare sat paste vs pore water concentrations: Boron #3: Compare sat paste vs pore water concentrations: Salt #4: Compare sulfate and boron leaching rates from soil #5: Examine rate of sulfate leaching from southern Alberta soil #6: Leaching salt and boron solution through clean soil #7: Evaluate gypsum solubility #8: Spiking dry salts into soil to compare leaching 11

12 Exp #1: Boron adsorption Boric acid and artificial soil used extensively in Environment Canada protocols for toxicity testing reproducible, consistent spiked soils supplied by Environment Canada for these tests testing performed at Maxxam Analytics (in-kind support) Goal: Measure HWS and sat paste boron on spiked samples to estimate adsorption Experimental: Artificial soil composition: 70% sand 20% clay 10% peat CaCO 3 to adjust ph to Texture = fine sandy loam Saturation percentage approx 60% Soil spiked with various levels of boron: 0, 29, 38, 49, 64, 83, 108, 140 (mg/kg boron added to dry soil) 12

13 HWS boron (mg/kg) Exp #1: Boron adsorption (cont d) Sat paste boron measured for various spike levels important for both toxicity and transport performed for soils aged up to 56 days 1 hour sat paste extraction used in all cases HWS vs sat paste (average of all aged samples - 1 hr extractions) y = x K d approx 1.2 K d approx sat paste boron (mg/l) Sat paste boron ranged up to 47 mg/l at 51 mg/kg HWS Lower K d s at higher boron concentrations similar to Freundlich isotherm rather than linear K d approx 1.2 in practical HWS range (<15 mg/kg HWS) 13

14 Exp #2: Sat paste vs pore water: Boron Goal: Compare soil chemistry (sat paste) to leachate (pore water) chemistry to improve understanding of groundwater concentrations (difficult to measure groundwater concentrations (pore water) unless a monitoring well is present) Pore water concentrations generally higher than sat paste due to extra dilution when making saturated paste extract Saturated paste and pore water concentrations likely more similar for boron than for salt due to soil adsorption Tested repacked artificial soil at two spike levels: 29 mg/kg added boron 38 mg/kg added boron 14

15 Exp #2: Sat paste vs pore water: Boron (cont d) Results Saturation percentage 8.7 mg/kg HWS boron (from 29 mg/kg spiked) Water content (%) Sat paste results Pore water results 59% 28% Boron (mg/l) 3.6 ~ mg/kg HWS boron (from 38 mg/kg spiked) Water content (%) Sat paste results Pore water results 57% 28% Boron (mg/l) 5.4 ~ 6.3 Moisture content of soil in leaching column (often approx half the saturation %) Boron in pore water approx 1.2x higher than in sat paste 15

16 Exp #3: Sat paste vs pore water: Salt Goal: Compare sat paste to leachate (pore water) chemistry for salt (likely a bigger difference than for boron since minimal salt sorption) Experimental (n=1) Tested undisturbed soil core: SAR-impacted loam subsoil (4m) from below water table sulfate and chloride ~ mg/kg each Results Saturation percentage Water content (%) Sat paste results Pore water results 51% 16% Moisture content of soil in leaching column EC (ds/m) EC in pore water approx 2x higher than in sat paste similar to the ratio in water content, but not exactly possibly due to soil non-homogeneity, sulfate EC non-linearities, etc 16

17 Boron in leachate (mg/l) Sulfate in leachate (mg/l) Exp #4: Sulfate and boron leaching Goal: Compare relative leaching rates of boron and sulfate (relevant to root-zone risk, DUA risk, and remediation rates) Sample preparation (n=1) lightly recompacted loam soil in fixed-wall permeameter 22% clay, very low organic matter initially saturated from bottom then reversed flow (helps de-air) Initial soil chemistry: HWS boron ~ 1.6 mg/kg sat paste boron ~ 1.5 mg/l fairly low K d for boron (~0.6) sat paste sulfate ~ 1600 mg/l (soluble mixture of Ca/Na/Mg salts) Leachate volume (ml) boron approx 15 pore volumes (1 pore volume ~ 230 ml) Leachate volume (ml) sulf ate approx 15 pore volumes (1 pore volume ~ 230 ml)

18 Relative concentration Exp #4: Sulfate and boron leaching (cont d) Show as relative concentrations and pore volumes for comparison normalize to peak concentration = % removal shown by red dashed line 1.20 crossing points shown with black arrows relative sulfate relative boron Leachate (pore volumes) Approximately 5 pore volumes required to leach 90% of sulfate 2.5- times more water required to leach boron to same relative extent 18

19 EC in leachate (ds/m) Goal: Examine rate of sulfate leaching from southern Alberta soil Sample preparation (n=1) tested undisturbed core from near Medicine Hat, Alberta (cores supplied by Petro-Canada / Suncor) cut core, lightly repacked section to same density as undisturbed section Initial soil chemistry (sat paste): sulfate mg/kg (mix of Na/Mg/Ca) chloride mg/kg EC ds/m Results: Initial leachate EC approx 2-fold higher than sat paste EC EC drops toward ~10% of initial value in ~3-4 pore volumes Exp #5: Sulfate leaching Leachate (pore volumes) Suggests soluble sulfate salts are capable of rapid leaching from surface soils given sufficient moisture relevant to leaching of sulfates brought to surface by construction activities undisturbed recompacted 19

20 Boron in leachate (mg/l) Sulfate in leachate (mg/l) Chloride in leachate (mg/l) Exp #6: Leaching salts through clean soil Goal: Leach chloride, sulfate, and boron solution through clean (pre-leached) loam to evaluate relative transport rates Solution preparation (n=1) Ca/Na adjusted to SAR of fair Leaching solution (inlet): EC: 4.6 ds/m Chloride: 1000 mg/l Sulfate: 1000 mg/l Boron: 3.9 mg/l Calcium: 560 mg/l Sodium: 500 mg/l SAR: 5.9 Results: Leachate approaches inlet concentrations over time 2-3 pore volumes (chloride, SO 4 ) pore volumes (boron) chloride Leachate (pore volumes) sulfate Leachate (pore volumes) boron Leachate (pore volumes)

21 Exp #6: Leaching through clean soil (cont d) Relative concentration Relative concentration Chloride and sulfate leached at similar rates at these moderate concentrations Boron significantly slower than chloride or sulfate chloride sulfate boron Leachate (pore volumes) 1.2 Boron matches chloride closely if hypothetically accelerated by 5-fold (K d ~ 1) chloride boron (accelerated 5x) Leachate (pore volumes) 21

22 Exp #7: Gypsum solubility Goal: Measure gypsum (CaSO 4 ) solubility in clean water and compare to solubility in saline water (relevant to salt-impacted soil) Sulfates generally have lower solubility than chlorides common to have saturated gypsum in solution plus undissolved gypsum in soil gypsum solubility reported as approximately 2 g/l in literature (~2000 mg/l) reportedly can almost double in presence of different ions (e.g., chloride) Example from our lab of effect of other dissolved ions (n=1) excess gypsum dissolved in distilled water: calcium ~ 590 mg/l sulfate ~ 1,400 mg/l EC ~ 2.2 ds/m Approx 2000 mg/l total, similar to literature excess gypsum dissolved in 10,000 mg/l NaCl: would have been ~10 ds/m if fully soluble calcium ~ 1,200 mg/l sulfate ~ 2,400 mg/l EC ~ ds/m (from gypsum) Thus, max gypsum contribution to EC approx 2-4 ds/m 22

23 Exp #7: Gypsum solubility (cont d) This max gypsum EC applies to saturated paste AND pore water Plant roots affected mainly by pore water salt concentrations Suggests saturated paste test may overestimate gypsum toxicity: Thought experiment: Compare two soils with same measured EC s (gypsum vs NaCl): Sat paste Pore water (leachate water) gypsum: 3 ds/m?? ds/m NaCl: 3 ds/m?? ds/m Same soil EC measured in lab 23

24 Exp #7: Gypsum solubility (cont d) Plant roots affected by pore water concentrations and EC Max gypsum EC applies to saturated paste AND pore water Suggests saturated paste test may overestimate gypsum toxicity: Thought experiment: Compare two soils with same measured EC s (gypsum vs NaCl): Sat paste Pore water (leachate water) gypsum: 3 ds/m 3 ds/m Recall gypsum s EC limit NaCl: 3 ds/m ~6 ds/m Less predicted effect on plants for gypsum 24

25 Exp #8: Spiking salts into soil Goal: Compare leaching rates of CaSO 4, Na 2 SO 4, and NaCl from artificially impacted (spiked) soil Sample preparation (n=1) homogenized and screened dry loam (22% clay, some natural sulfate) spiked with matched cation equivalents of each salt lightly repacked into 3 fixed wall permeameters saturate columns from top with water, collect leachate from bottom Results: Soil EC / cation ratio Initial soil EC (ds/m) Initial leachate EC (ds/m) Original soil CaSO 4 spiked Na 2 SO 4 spiked NaCl spiked EC / cation ratio higher for chloride than sulfate here initial leachate EC fold higher than soil EC (highest for NaCl) 25

26 EC in leachate (ds/m) Exp #8: Spiking salts into soil (cont d) Different leaching behavior observed for the three salts: NaCl initially fastest to leach (in terms of pore volumes) CaSO 4 slowest to leach due to continuous dissolution of solid gypsum Na 2 SO 4 overtook NaCl after approx 1.5 pore volumes CaSO4 Na2SO4 NaCl Leachate (pore volumes) Surprising that Na 2 SO 4 behaved differently than NaCl during early leaching likely more common at high salt concentrations possibly due to EC vs cation non-linearities for sulfate, or column wetting effects Potential future directions for SO 4 vs Cl research: column wetting effects - effect of soil texture / structure solubility limits, EC non-linearities - other salts such as MgSO 4, CaCl 2 26

27 Conclusions and Implications Leaching columns useful for transport experiments variety of configurations and chemical species studied (B, Cl, SO 4, Ca, Na) understanding transport is vital for guideline development Boron often travels several-fold slower than chloride or sulfate primarily due to soil adsorption adsorption also affects pore water concentrations and plant toxicity relevant for GW guidelines and remediation Sulfate behavior may differ from chloride behavior especially at high concentrations or with calcium ions (gypsum, CaSO 4 ) Na 2 SO4 potentially less mobile than NaCl at high concentrations Na 2 SO4 potentially less effects on EC than sodium chloride at high concs Gypsum EC in sat paste and pore water is limited to approx 2-4 ds/m this limit may cause sat paste test to over-estimate plant risk from gypsum Soluble sulfates may be ~90% leached after 3-5 pore volumes relevant to sulfates brought to surface by construction practices boron may require 2 to 3-fold more water for comparable leaching

28 ACKNOWLEDGEMENTS Petroleum Technology Alliance of Canada (PTAC) Program of Energy and Resource Development (PERD) Environment Canada Petro-Canada (Suncor) Orphan Well Association Earthmaster Environmental Strategies Maxxam Analytics 28