12.4. Mobility in Soil

This section provides information on how a substance moves through soil and potentially reaches groundwater. Soil mobility assessment is essential for evaluating environmental fate, predicting potential contamination of groundwater resources, and determining appropriate risk management measures for terrestrial ecosystems.

Soil Mobility Overview

Soil mobility refers to the ability of a substance to move through soil and potentially reach groundwater. It is influenced by both substance properties and soil characteristics:

Adsorption: Binding of a substance to soil particles, reducing mobility
Desorption: Release of a substance from soil particles, increasing mobility
Leaching: Downward movement of a substance through soil with water
Volatilization: Transfer of a substance from soil to air
Runoff: Horizontal movement of a substance with surface water

Key Soil Mobility Parameters

Parameter	Definition	Significance
K_oc	Organic carbon-water partition coefficient; measure of adsorption to soil organic carbon	Primary indicator of soil mobility
K_d	Soil-water partition coefficient; ratio of concentration in soil to concentration in water	Soil-specific adsorption measure
K_f, 1/n	Freundlich adsorption coefficient and exponent	Non-linear adsorption parameters
Henry's Law Constant	Air-water partition coefficient	Indicator of volatilization potential from soil
GUS Index	Groundwater Ubiquity Score; combines K_oc and degradation half-life	Predictor of groundwater contamination potential

Soil Mobility Classification

Based on K_oc values, substances can be classified according to their mobility in soil:

K_oc Value	Mobility Class	Description
< 50	Very high mobility	Minimal adsorption, rapid leaching
50 - 150	High mobility	Limited adsorption, significant leaching
150 - 500	Moderate mobility	Moderate adsorption and leaching
500 - 2000	Low mobility	Strong adsorption, limited leaching
2000 - 5000	Slight mobility	Very strong adsorption, minimal leaching
> 5000	Immobile	Extremely strong adsorption, negligible leaching

The GUS Index (Groundwater Ubiquity Score) combines K_oc and degradation half-life:

GUS = log(half-life) × (4 - log(K_oc))

GUS < 1.8: Low leaching potential
1.8 ≤ GUS ≤ 2.8: Moderate leaching potential
GUS > 2.8: High leaching potential

Standard Test Methods

OECD Test Guidelines:

OECD 106: Adsorption/Desorption Using a Batch Equilibrium Method
- Determines K_d, K_oc, K_f, and 1/n
- Uses multiple soil types
OECD 121: Estimation of the Adsorption Coefficient (K_oc) on Soil and Sewage Sludge using HPLC
- Rapid screening method
- Based on retention time on HPLC column
OECD 312: Leaching in Soil Columns
- Simulates leaching under field conditions
- Provides information on mobility and transformation products

Other Methods:

US EPA OPPTS 835.1230: Adsorption/Desorption (Batch Equilibrium)
US EPA OPPTS 835.1240: Leaching Studies
Lysimeter studies: Field-scale leaching assessment

Factors Affecting Soil Mobility

Substance Properties

Water solubility: Higher solubility generally increases mobility
Hydrophobicity (log K_ow): Higher values generally decrease mobility
Ionization state: Affects adsorption to soil particles
Molecular size: Larger molecules may have reduced mobility
Volatility: Affects partitioning to soil air

Soil Properties

Organic matter content: Higher content increases adsorption of hydrophobic substances
Clay content: Higher content increases adsorption, especially for polar and ionic substances
pH: Affects ionization of substances and surface charges of soil particles
Cation exchange capacity: Higher CEC increases adsorption of cationic substances
Soil structure: Affects water flow and preferential pathways

Environmental Conditions

Rainfall/irrigation: Higher water input increases leaching
Temperature: Affects adsorption-desorption equilibria and degradation rates
Soil moisture: Affects adsorption sites availability and microbial activity
Depth to groundwater: Shorter distance increases contamination risk
Seasonal variations: Affect water table fluctuations and soil conditions

Example: Soil Mobility Data

Substance X Soil Mobility Profile:

K_oc: 2000 L/kg
K_d: 150 L/kg (average across soil types)
Freundlich parameters: K_f = 120 L/kg, 1/n = 0.85
Henry's Law Constant: 3.2 × 10^-5 Pa·m³/mol
GUS Index: 1.2
Mobility classification: Low mobility

Soil Column Leaching Test Results:

Sandy soil: 5% of applied substance detected in leachate
Clay soil: < 1% of applied substance detected in leachate
Organic soil: < 0.5% of applied substance detected in leachate

Soil Transport Characteristics

Vertical mobility: Limited due to strong adsorption to soil organic matter
Horizontal migration: Moderate, primarily with surface runoff
Surface runoff potential: Medium, especially during heavy rainfall events
Groundwater contamination risk: Low based on K_oc value and GUS index
Volatilization from soil: Negligible based on Henry's Law Constant

Despite the generally low mobility, transport may be enhanced in sandy soils with low organic matter content or under conditions of heavy rainfall. Site-specific assessment is recommended for sensitive areas.

Soil Type Effects

Soil Type	K_d (L/kg)	Mobility Characteristics
Clay soils	250-300	Strong adsorption, very limited mobility
Sandy soils	50-100	Moderate adsorption, increased mobility
Organic soils	300-400	Very strong adsorption, high retention
Loamy soils	150-200	Moderate adsorption and mobility

pH dependency: The substance shows increased mobility at low pH (< 5) due to reduced ionization and weaker adsorption to soil particles.

Environmental Factors

Temperature influence: Minimal effect on adsorption within normal environmental range
Moisture effects: Increased soil moisture generally increases mobility by reducing available adsorption sites
Microbial activity impact: Significant, as degradation reduces the amount available for transport
Seasonal variations: Higher mobility expected during wet seasons due to increased water flow
Freeze-thaw cycles: May temporarily increase mobility due to soil structure changes

Implications for Environmental Risk Assessment

Substances with high mobility (low K_oc) pose greater risks to groundwater
Substances with low mobility (high K_oc) may accumulate in topsoil
Mobility data is essential for predicting environmental concentrations (PECs)
Soil mobility affects exposure of soil organisms and terrestrial plants
Transport to surface water via runoff or drainage must be considered

Regulatory Considerations

Soil mobility data is required for substances produced or imported at ≥ 100 tonnes/year under REACH
For plant protection products, detailed leaching studies are mandatory
Highly mobile substances may be subject to groundwater monitoring requirements
Mobility data influences risk management measures and use restrictions
Substances with high groundwater contamination potential may face regulatory restrictions

Modeling Approaches

Several models are available to predict soil mobility and leaching potential:

FOCUS models: Used for regulatory assessment of plant protection products in the EU
PEARL: Pesticide Emission Assessment at Regional and Local scales
PELMO: Pesticide Leaching Model
PRZM: Pesticide Root Zone Model
MACRO: Model for preferential flow in macroporous soils
SCI-GROW: Screening Concentration In Ground Water

These models typically require input parameters such as K_oc, degradation half-life, application rate, and soil/climate data to predict environmental concentrations.

Quality Assurance Checklist

Verify that soil adsorption tests follow recognized guidelines (e.g., OECD)
Ensure that multiple soil types are used to account for variability
Check that the mobility conclusion is consistent with the K_oc data
Confirm that pH dependency of adsorption is addressed for ionizable substances
Verify that potential for groundwater contamination is assessed
Ensure that any limitations of the assessment are acknowledged