Issues with BC CSR Salt Standards in Peat Soils Related
to Saline Produced Water
Mark Hugdahl,
Technical Director, ALS Environmental (Canada)
WaterTECH 2013
Outline of Presentation
Context - Environmental Impacts of Saline Produced
Waters on Peatlands
Introduction to BC Salt Standards
Lab Testing for Na & Cl in Peat Soils for BC CSR
• Saturated Paste Extraction Method
Units Issue with BC Salt Standards for Peat Soils
• Problems created by CSR units (mg/kg dry weight)
• Evidence to support Solution Based Salt Standards (mg/L)
• Predictions of environmentally protective Salt Standards
(Solution Standards, as mg/L)
Context: Environmental Impacts of Saline
Produced Water in BC Peatlands
Majority of BC Oil and Gas development occurs in
Peatlands of NE BC, north of Fort St. John.
Saline produced water releases are fairly common
• Pipeline ruptures (produced water or emulsion).
• Releases from well-site activities (sumps, flare pits, drilling waste, etc).
Primary environmental concern is usually toxicity to plants
by Na+ and Cl- ions
• Reduced crop yields due to saline soils is well known.
• Osmotic stress limits plant water update (also specific ion effects).
• Plant sensitivity to salinity varies widely by species.
• Salts can also impact soil invertebrates, but plants are generally
more sensitive to NaCl (Bright & Addison).
Introduction to BC Salt Standards
•
Schedule 5 Soil Matrix Standards for Na+ & Cl–
•
Primary Drivers:
–
–
•
Introduced Jan 1, 2009.
Road Salt – e.g. MOT road salt storage facilities.
Produced water releases at NE BC Oil and Gas lease sites.
Standards Derived by Bright & Addison (2002)
–
–
First toxicity based specific ion salt standards in Canada.
Different approach from Alberta Salt Guidelines (Soil EC & SAR).
•
Units: [Na+] & [Cl-] as mg/kg dry weight of soil
•
Lab method: Saturated Paste
–
Same as EC & SAR, but EC & SAR are derived from mg/L concentrations.
BC CSR Schedule 5 Chloride Standards
Units: mg/kg (dry weight)
BC CSR Schedule 5 Sodium Standards
Units: mg/kg (dry weight)
Physical Characteristics of Peat Soils
• High Total Organic Carbon
– Often near 100% Organic Matter
– Average TOC ~40-50%
• High Moisture Content &
Water Holding Capacity
– Extremely high water holding
capacity
– Usually saturated or near saturated
in the field
– Average Moisture ~87%
• Very Low Dry Bulk Density
– 0.15 grams/cm3
– ~12x less dense than sand
• Vastly different properties than
Mineral Soils!
Lab Analysis of Na & Cl – “Saturated Paste”
• BC Salt Stds Require “Sat Paste” for Na & Cl
– “Sat Paste” procedure extracts ionic components from soils.
– Labs measure mg/L concentrations of Sat Paste extract.
– Results are converted to mg/kg for evaluation of Salt Stds.
• Why Saturated Paste?
– The standard international method for soil salinity analyses.
– Used for agricultural soil fertility assessments since 1939.
– Extract concentration correlates with dissolved ionic concentrations that plant
roots are exposed to in soil pore waters.
– Main Advantage: Automatically accounts for Water Holding Capacities of
different soil types (as mg/L units).
Traditional Sat Paste applications use results in mg/L.
Saturated Paste Extraction Procedure
•
•
•
•
•
Dry, & grind or disaggregate the soil
Weigh a portion of dry soil (typically ~ 200 g)
Add water & mix - continue until a paste forms
Equilibrate ≥ 4 hrs
Collect extract and analyze
• Calculate Saturation %:
Sat % = [(Wt water @ saturation) / (Soil dry wt)] * 100%
Alternatively, samples can be pasted “as-received” without drying.
Saturated Paste Example – Mineral Soil
At Saturation:
Soil glistens & flows
slightly
Will slide cleanly from
spatula
A trench carved in
surface will close if
container is jarred
Saturated Paste Example – Peat Soil
Visible cues to
saturation are
more difficult to
identify than for
mineral soils.
Separating the Extract from the Paste…
Vacuum Filtration
Filter Press
Instrumental Analysis of Na & Cl
The Sat Paste Extract is analyzed, with
initial results in mg/L:
– Sodium is normally
analyzed by ICP.
– Chloride is normally
analyzed by Ion
Chromatography or by
Colourimetry.
BC Salt Stds for Peat Soils – The Units Problem
• BC Salt Stds were derived for Mineral Soils.
– Physical properties of minerals soils are vastly different from peat soils!
• Conversion from mg/L soil solution concentrations to mg/kg
differs radically between Mineral Soils & Peat Soils.
– Difference in conversion factors is about 10-20x !
• Toxic effects of Na & Cl almost certainly correlate better with
mg/L concentrations than mg/kg.
– Na & Cl ions are highly soluble, tend to exist in dissolved phase in soil pore
waters (especially for road salt & produced water sources).
– This differs from most metals & organics, which tend to adsorb to soils.
– Salinity effects on plants definitely correlate with soil EC across soil types (Soil
EC is measured directly from Sat Paste extract).
Effect of Sat% on Na & Cl as mg/kg
Sat% = (Wt Water @ saturation / Dry Wt Soil) x 100%
[Na or Cl] as mg/kg = mg/L x [Sat% / 100%]
With identical pore water concentrations, mg/kg concentrations for a
Peat Soil will be 10-20x higher than for a typical Mineral Soil.
Soil EC Correlations with [Chloride]
•
Soil EC correlates well with [Chloride] as mg/L (left)
–
–
•
Mineral Soils data from Salt Standards Derivation studies (Bright & Addison 2002).
Peat soils data is from PTAC Phase 2 field study (Doug Bright, 2009).
No correlation at all between Soil EC and [Chloride] as mg/kg for
different soil types.
–
–
Same data plotted right and left in different units.
Slope of EC / Chloride (as mg/kg) is inversely proportional to Saturation %, and can
vary by > 30x as a function of soil type.
Effect of Soil Type on BC Chloride Standard
Soil Pore Water Chloride
= 1,100 mg/L
Soil Pore Water
Chloride = 52 mg/L
MINERAL SOIL coarse
if Sat% = 32%
PEAT SOIL
if Sat% = 670%
Chloride as mg/kg
= 1,100 mg/L x 0.32
= 350 mg/kg
Chloride as mg/kg
= 52 mg/L x 6.7
= 350 mg/kg
Therefore, the 350 mg/kg BC CSR Std for Chloride is equivalent to:
– 1,100 mg/L Soil Pore Water [Chloride] for Mineral Soil w/ Sat% of 32%.
– 52 mg/L Soil Pore Water [Chloride] for Peat Soil w/ Sat% of 670%.
– 21x difference due solely to difference in water holding capacities!
Observations About Salt Std Derivations
Plant Toxicity Data*
– Used for “Toxicity to Soil Invertebrates and
Plants” Standards.
• Sodium 200 mg/kg, & Chloride 350 mg/kg
• Plants were more sensitive to salt than soil
invertebrates.
• Data was from mineral soils.
– Tox threshold used to derive this standard was
initially expressed as EC 4.6 dS/m (50% loss of
yield in 25% of plant species).
– Data was re-expressed in mg/kg Na+ & Clconcentrations to derive final standards.
– The EC 4.6 dS/m tox threshold is relevant to all
soil types, but after conversion to mg/kg units,
the final standard is directly relevant only to
coarse mineral soils.
Salt-impacted Peatland Vegetation
* from Bright and Addison (2002)
Observations About Salt Std Derivations
Soil Invertebrate Toxicity Data*
– Only Mineral Soils were used for soil
invertebrate toxicity tests.
– Soils used had only 10-30% moisture.
– Fine grained soils with highest moistures had
least toxic effects (as mg/kg NaCl):
• “The soil type had a profound influence on
toxicity”.
• e.g. EC50 for reproductive success in F. candida
was 7x higher for OECD soil (fine grained, 10%
organic) than Clinton soil (coarse grained).
F. Candida
(photo: Vrije Universiteit Amsterdam)
• I predict that EC50 concentrations would
correlate far better across soil types using mg/L
Saturated Paste salt concentrations.
* from Bright and Addison (2002)
Effect of Soil Type Assumption on CCST GW Model
•
BC Groundwater Stds derived from CSST GW Transport Model
–
–
–
•
e.g. Groundwater Flow to Surface Water used by Aquatic Life (550 mg/kg Chloride).
Soil / Leachate Partitioning Model converts soil conc (mg/kg) to leachate (mg/L).
Model assumes Fraser River sand – vastly different from peat.
How would GW Stds differ if Peat Soil was assumed?
–
Applying Peat Soil physical constants would give 19x higher GW stds (as mg/kg)!
Soil / Leachate Partitioning
Model for Chloride:
CS = CL { Kd + nu/Pb ]
CS = Concentration, Soil
CL = Concentration, Leachate
Differences due to soil type would not exist if the Salt Standards were expressed
as mg/L (soil solution) instead of mg/kg (soil dry weight).
CSST = Contaminated Sites Soil Task group
Recommended Changes to BC CSR Salt Stds
• Re-Derive Salt Stds as mg/L Soil Solution Concentrations
– Use Sat Paste method for unsaturated soils.
– For 100% field saturated soils, sample pore-waters directly if possible (e.g. by
piezometer).
– Eliminate variability across soil types due to water holding capacity differences.
• Evaluate Need for Independent Salt Stds for Peatlands
– Requires new controlled toxicity data for peatland ecosystem receptors
(vegetation and soil invertebrates).
• A Proposal to fund required toxicity research is before CAPP.
– Proposal developed by Doug Bright (Hemmera).
– BC MOE has committed to address this issue if tox testing can be funded.
Predictions of a Soil Chloride Standard (as mg/L) for
Protection of Plants & Soil Invertebrates
• Plant Toxicity Threshold (Bright & Addison 2002): EC 4.6 dS/m
– EC 4.6 correlates to ~ 1,480 mg/L Chloride as Sat Paste (CRC Handbook)
• Alberta Environment Salt Guidelines
– EC 2–4 = 625–1,280 mg/L Chloride (Sat Paste)
– EC 3–5 = 950–1,610 mg/L Chloride (Sat Paste)
A.Env. Soil Quality Guidelines - Unrestricted Land Use
Salinity as EC (dS/m)
Topsoil
Subsoil
• PTAC Peatland Salinity Field Study
– Vegetative threshold (peatland plants) ~ EC 3.0 - 4.0?
– Correlates to 950 - 1,280 mg/L Chloride (Sat Paste)
"Good"
<2
<3
"Fair"
2 to 4
3 to 5
PTAC Peatland Salinity Field Study
•
Field Observation Study of Salinity Impacted Peatlands
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–
•
Doug Bright, primary author (2009)
9 Produced Water release sites in Alberta and BC
Soil Invertebrate Observations
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Mites = Most prevalent invertebrates found
No relationship found between salinity and soil invertebrate
abundance or composition
Mite
•
Plant Species Sensitivity Distributions vs Soil EC
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–
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Correlated species presence-absence data with Soil EC
Species Sensitivity Distributions compiled for 31 common peatland plant species
Salinity Tolerance Ranges and Extirpation Thresholds were estimated (as EC)
for each species
99
98
Fireweed (Epilobium angustifolium )
95
ta
xo
n
Sphagnum magellanicum
Water sedge (Carex aquatilis )
fo
r
80
ea
ch
90
ut
io
n
70
60
50
40
30
is
tri
b
Labrador tea (Ledum groenlandicum )
of
d
EC 4
Sphagnum angustifolium
Sphagnum girgenshohnii
95
th
%
ile
Percentile of Ranked Data
(individual taxon extripation estimates)
PTAC Study - Vegetative Salinity Threshold ~ EC 3-4 ?
20
Black spruce (Picea mariana)
10
5
Rough bentgrass (Agrostis scabra )
2
1
2.0
3.0
10
20
30
Soil EC (mS/cm)
A “Vegetative Threshold” for Peatland Plants of ~ EC 3-4 could be proposed based on
estimated extirpation thresholds shown above.
Graphics Courtesy of Doug Bright, Hemmera
Relevance & Conclusions…
•
A large proportion of the 2,150 abandoned well-sites in BC are
affected by this issue.
•
Overly conservative peatland standards are counter-productive.
–
–
–
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BC CSR Salt Standards should be converted to mg/L Solution
Standards.
–
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Peat-forming wetlands (Bogs and Fens) take up to 10,000 years to form.
Excavation/Landfill remediations permanently destroy Wetland ecosystems!
Best practice remediation techniques are “minimal disturbance”.
Peatland / Wetland specific standards may or may not be necessary.
Several lines of evidence point to an ecologically protective
threshold for Chloride of ~ 600 - 1,500 mg/L .
–
–
For protection of plants & soil invertebrates.
New Tox data for peatland ecosystems should allow changes to proceed.
Questions?