Vance, David B.¹ 
(1) ARCADIS G&M, Inc, Midland, TX

ABSTRACT: The Effect of Physical, Geochemical, and Microbiological Heterogeneity on the Assessment and Remediation of Contaminated Soil and Groundwater

Groundwater systems and overlying vadose zones are generally envisioned as being relatively homogenous, with perhaps some layering, and with fluid flow the dominant active process. This representation can be modeled relatively easily and in most instances is adequate in support of groundwater production. However, it is a vision that is seriously inadequate when the issue is groundwater that has been impacted with organic or inorganic contaminants. Given the often-stringent standards for groundwater quality, extremely small quantities of contamination in isolated areas of the subsurface can have significant impact. 
Transport dynamics in the subsurface, the assessment of contamination in soil and groundwater, and remediation are all effected by subsurface heterogeneity. Physical flow heterogeneity can have significant impact on remediation systems that employ injection programs. In addition, it is not just permeable pathways and fluid flow that exhibit heterogeneity. Subsurface process dynamics dominated by heterogeneities also include spatial and temporal variation in: aqueous and solid phase geochemistry; subsurface indigenous bacterial populations; and dynamic interactions between the indigenous bacteria, site geochemistry, and the contaminants. Heterogeneity of the bacterial populations is particularly manifest among the anaerobes, which are often dominant in the natural attenuation of hydrocarbon releases and the core of newly developed in situ bioremediation programs exploiting reagent injection. 
This paper presents the basic concepts and impacts of physical, geochemical, and microbiologal heterogeneity on the dynamics of COCs in the subsurface. It includes examples of assessment and remediation from the field that illustrate the nature and magnitude of the effects.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.