Western Geo-Engineers

(530) 668-5300; Fax (530) 662-0273; wege@cal.net

A FULL SERVICE ENVIRONMENTAL CONSULTING COMPANY

Natural Attenuation

Natural Attenuation is the degradation or dissipation of a contaminant plume through natural means. The major agent of natural attenuation is bio-degradation, which is facilitated by native bacteria. The activity of these bacteria is controlled by the concentration of contaminants and by the presence of electron acceptors, which allows the bacteria to oxidize or reduce the contaminants. In real field conditions reduction may only occur in rare cases. If the amount of electron acceptors at a site is found to be sufficient to degrade the contamination before it can endanger health or the environment, then nature can be left to take care of the problem on its own. In order to do this safely; it is necessary to have sufficient information about the site to determine the risk.

Bio-degradation

The most effective hydrocarbon degraders (eaters) are the aerobic (oxygen using) bacteria. Usually, the factor controlling the rate that these bacteria degrade the gasoline is the amount of available dissolved oxygen.

A much slower degradation process starts when the dissolved oxygen is used. The plume begins to become anaerobic and the bacteria commence to reduce nitrate, ferric iron, and sulfate to further degrade the hydrocarbons. Eventually, as these compounds are used, the bacteria begin methogenesis, in which the hydrocarbons are slowly converted to methane.

Natural Attenuation Study

In order to determine the site potential for natural and enhanced bioremediation, groundwater is sampled for the following analyses:

• Dissolved Oxygen, O₂
• Nitrate, NO₃^-
• Sulfate, SO₄^2-
• Ferrous iron, Fe²⁺. The actual electron acceptor is ferric iron Fe³⁺ but it is insoluble, so the reaction product Fe²⁺ is measured.
• TPHg/MBTEX

 Additionally Air samples are collected and tested for O2, CO2, Methane, and TPHg/MBTEX concentrations.