Transport of PCBs in the Environment

The information below was abstracted from the Remedial Investigation for the Allied Paper/Portage Creek/Kalamazoo River Superfund Site.

PCBs are a class of synthetic organic compounds that includes 209 individual compounds referred to as congeners. The various congeners differ in the number and arrangement of chlorine atoms on the biphenyl molecule, but they all have the same basic chemical structure and similar physical properties. In the United States, PCBs were produced for commercial purposes exclusively as mixtures of PCB congeners by Monsanto Industrial Chemicals Company under the trade name Aroclor. The average chlorine content of a particular Aroclor product is, in most cases, evident in the specific product name. For example, Aroclor 1242 is 42% chlorine by weight, while Aroclor 1254 is 54% chlorine by weight. Although most PCBs produced in the United States were used in manufacturing electrical transformers and capacitors, some Aroclors were also used in other applications, including hydraulic fluids, cutting oils, heat transfer fluids, and quench oils. From 1957 through 1971, PCBs in the form of Aroclor 1242 were used in the manufacture of carbonless copy paper.

In general, PCBs are chemically and thermally stable, fairly inert, and have low solubility in water. In general, the lower the water solubility of a chemical the more likely it is to be adsorbed onto solids. The presence of other dissolved compounds in water may affect the solubility of PCBs. For example, if certain organic compounds are dissolved in water at sufficient concentrations, the energy necessary to dissolve PCBs may be reduced. This condition is referred to as cosolvency. The solubility of PCBs in water may be increased in the presence of codissolved non-polar chemicals, such as hydrocarbons.

PCBs have very high octanol-water partitioning coefficient (Kow) values. The octanol-water partitioning coefficient represents the propensity for a chemical to partition between a polar phase (water) and a non-polar phase (octanol) similar to partitioning in the soil matrix between soil organic matter and groundwater. The Kow value is determined as the ratio of the solubility of PCBs in octanol (an organic matter surrogate) to the solubility of PCBs in water. Thus, a higher Kow value indicates a greater tendency for a chemical to sorb to organic materials (e.g., peat, humic acids, and other organic colloids) in soil.

The degree of adsorption of PCBs in soils is a function of the soil organic content and the adsorption properties of the specific PCB compounds that are present. Adsorption properties are generally characterized by an organic carbon partitioning coefficient denoted by Koc. The Koc values for PCBs are relatively high, which means that PCBs readily adsorb to organic material in media such as sediments and soils.

Taken together, the combination of low water solubility and high Kow and Koc values indicates that PCBs have a strong affinity for soils and suspended solids, especially those high in TOC. PCBs bind to the organic matter fraction decreasing the dissolved fraction and encouraging the transport of PCBs sorbed to solids (i.e. particulate transport).

Other than organic content, soil or sediment characteristics that affect the mobility of PCBs include soil density, particle size distribution, moisture content, and permeability. Meteorological and physical conditions such as amount of precipitation and the presence of organic colloids (micron-sized particles) can also affect the mobility of PCBs in the environment. PCBs that are dissolved or sorbed to mobile particulates (e.g., colloids) may also migrate with groundwater in sediments and soils. PCBs can be transported by air, either adsorbed onto airborne dust particles or in a vapor phase that results from volatilization. The chemical characteristics that control the adsorption of PCBs to dust particles are described above. The Henry's Law constant of a compound provides an indication of its tendency to volatilize, and thus provides a means for ranking the relative volatility of chemicals. Henry's Law constants are used to calculate the rate of volatilization from water and can be obtained from literature or calculated by dividing the vapor pressure value of a chemical by its water solubility. PCBs have low Henry's Law constant values relative to other organic compounds, which means that at equilibrium, the fraction of PCBs found in water will be higher than the fraction found in air.

PCBs tend to be persistent in sediments and soils, and there is no known abiotic process that significantly degrades PCBs in these media. Biodegradation of PCBs in soils under aerobic or anaerobic
conditions is slow, especially in soils with high organic carbon content. At PCB concentrations below about 50 ppm, the rate of dechlorination is often very slow or non-quantifiable. This is likely because PCBs are so tightly bound to the soil that at relatively low concentrations they are not bioavailable to the biodegrading organisms.

A number of factors may affect the migration of PCBs in groundwater, including: (1) The chemical characteristics and concentrations of the specific PCB congeners/Aroclors present at the site (as described in 5.1); (2) The characteristics of the PCB source medium (e.g., PCB-containing soils and residuals), including physical state, mass, location, geotechnical characteristics (e.g., bulk density, grain size and distribution, porosity, permeability, moisture content, organic content), heterogeneity, manner and characteristics of containment (if any), concentration and distribution of any cosolvents, and susceptibility to biological interaction or disturbance; (3) The characteristics of the other (non-PCB containing) soils above the water table, including topography, vegetation, soil composition and heterogeneity, geotechnical characteristics, and depth to groundwater; (4) The hydrogeological and hydrogeochemical conditions near the source medium, including temperature, pH, conductivity, dissolved solids, DO, alkalinity, and ORP; groundwater velocity, presence and composition of mobile colloidal particles; and presence and concentration of cosolvents; and (5) The local meteorological conditions, including daily and seasonal temperature ranges and fluctuations, annual precipitation, barometric pressure, and relative humidity. Dissolved PCBs are subject to a number of different retardation processes, including sorption and various chemical reactions.

So What Does All The Above Really Mean?

PCBs in the River - the PCBs found in carbonless paper overwhelmingly stick to solid particles in water, particularly organic particles. These particles could be cellulose waste from the recycling process, soil or sediment.  Concentrations of PCBs in the Kalamazoo River are found in the banks and areas where the water flow has deposited solid particles.  Highest concentrations are found from Kalamazoo to the dam at Allegan because this stretch is where nearly all the solid particles were deposited. Portage Creek and the Kalamazoo River are where most transported PCBs from Allied Paper ended up.

PCBs in the air are only found in significant amounts when contaminated material is at the surface and being disturbed. Once securely covered up, PCB levels in the air are negligible. There is currently no issue with PCBs in the Kalamazoo air from the Allied Landfill.

PCBs in groundwater - PCBs that are dissolved in groundwater move more slowly through aquifers than the groundwater itself moves. PCBs will preferentially stick to soil particles so to be mobile, these particles must be small enough to travel through the gaps in the soil matrix. PCBs stuck to large particles are going to stay put. Along with pumping at the site, this sticking to solids is why Kalamazoo city well fields were not been contaminated long ago. Transport of PCBs for long distances via groundwater is not a significant issue in Kalamazoo. Careful aquifer monitoring and management will ensure that it never becomes an issue.

Important Disclaimer - The author of this web page is not an expert on PCBs and their pollution in the environment. He is a scientist who has read the reports and summarized the information presented in them.

Return to the Links Page

Last Edited by JMW 05/29/10