ensc lecture 5

Fate of substances in the environment 

• Overtime, have transfer between “environmental compartments”

  • Can have individual behaviour of molecules or toxicants in mixtures 

  • May also have chemical biological “transformation” when moving 

• Lifetime: average time a toxicant spends in a particular compartment (AKA residency time)

  • Related to half-life 

• Persistence: tendency of a toxicant to remain in a particular compartment 

  • Toxicant with a longer lifetime is more persistent 

• Persistent organic pollutants (POPs): organic molecule, toxicants featuring lifetime (or half-life) greater than one year 

  • Didn't break down 

  • Accumulate in the environment and last forever 

• Persistence of a toxicant is related to how far it will travel in the environment 

• Bioaccumulation: describes the fate of toxicants in the “biological organic compartment”

  • Characterized by accumulation in food web

  • Correlate with persistence in organism tissues 

  • Depends on chemical properties of toxicant 

  • Is the combination of two factors 

    • Bioconcentration - partition of toxicant into biological organism 

    • Biomagnification- concentration amplified in the food web 

• Bioconcentration

  • How does the toxin partition 

  • Partition equilibrium between water and a second phase 

  • Related to the hydrophobicity of toxicant molecule 

  • Modelled by chemical partition constant between octanol and water (Kow)

  • Octanol is surrogate for fatty (lipid) tissue in organism “lipophilocity”

• Bioconcentration - practical measures 

  • Biocentration factor (BCF): measured with biological (real) organisms 

    • Laboratory- based measurement

    • One toxicant molecule, controlled exposure 

    • Partition of toxicant into organism or tissue from external environment 

    • Related to hydrophobicity or lipophilicity (Kow)

  • Ctissue is the level in the organism (or lipid tissue) 

  • Cwater is the level of water  

• Bioconcentration factors in fish 

  • Bioconcentration factor (BCF): laboratory experiment - fish placed in contaminated water

    • No food provided - only gill exposure 

    • Measure levels in fish (steady state)

    • Measure levels in water 

BCF and long Kow 

• Log BCF strongly correlated with log Kow for a series of toxicants 

Limitations in BCF

• Real water phase has multiple compartments

• “Dissolved” toxicant is true water compartment 

• Some toxicants “absorbed” onto surfaced or into other phases 

• Other phases include dissolved organic carbon and particulate organic carbon 

Bioaccumulation of organic contaminants 

•  Biomagnification: refers to the amplification of concentration in an organism if it consumed contaminated organism 

  • Assumes almost all contamination in the prey (consumed) is retained by the predator (consumer) organism - linked to persistence 

Bioaccumulation in fish/ aquatic systems 

• Remember bioconcentration factor (BCF): laboratory experiment - fish placed in contaminated water with no food 

• Bioaccumulation factor (BAF): includes all exposure routes - included uptake from food (biomagnification) and from gills (BCF)

Historical - Bioaccumulation of DDD

• Case study for DDD- example from silent spring (carson) 

• Clear lake, califoria- breeding site for western grebe (large water fowl) 

  • In late 40s, 50s, DDD is used to control clear lake gnat 

  • 1949 spraying DDD in water at 14 ppb (no adverse effects noted) 

  • Gnats disappeared, but later recovered 

  • Sprayed again in 1954, DDD in water - 20 ppb 

  • Some grebe deaths noted, no connection made 

  • In 1957 more gnats (resistance developing?), spray again, significant grebe death (1000 pairs before 1949, now down to 30 pairs) 

  • Decision to analyze system for DDD is made 

  • Water level of DDD are still in the ppb range, but fish levels are 40-300

  • Grebe levels at 1600 ppm in fatty tissue 

DDT bioaccumulation 

• DDT bioaccumulation in the food chain (or food web) 

  • See increase with “trophic level” or with age at one level 

Bioaccumulation of PCBs 

• PCBs- hydrophobic organic compounds 

• Additional example with great lake data 

• Persistence - 4 years in biological tissue, - 8 years in sediment 

Fate and bioaccumulation summary 

• Understanding the relationship between bioaccumulation and chemical properties helps predict effects of contaminant 

  • For new contaminants or situations can use Kow to estimate BAF

  • Important for risk assessment (e.g can predict outcomes of hypothetical situations) 

• Bioaccumulation of persistent compounds is as important as inherent hazard (toxicity)

• For compounds with low toxicity present at low concentrations, may observe unexpected toxicity in real ecosystems