AVI has found that many physical and
chemical parameters controlling contaminant fate and transport and cleanup are common to a
wide range of sites. For example, soil permeability and its relationship to site
stratigraphy is an important control to the rate and direction of contaminant migration,
cleanup, and risk. Capillary soil properties are another lithologically controlled
parameter that strongly influences immiscible phase and unsaturated fluid and vapor flow.
Therefore, although every site is unique, there are similar conditions to be defined.
Based on this observation, AVI developed a
menu of predefined scopes of work to measure and use these parameters to assess site
closure strategies. The scopes are organized into three levels with fixed fees. As the
levels increase, the scopes of work become more technically comprehensive and focussed on
specific site closure objectives. AVI's fixed fee technical services help clients reliably
plan their site closure programs while providing a flexible but controlled work scope.
Additional work, if needed, can be added on to the previous level with a minimum of effort
and cost. It is interesting to note that AVI's effort levels (1992) are very
analogous to the RBCA approach (1994, 1995), but were developed several years before the
publication of the ASTM risk methodology. This is the beauty of science; its
underlying logic should lead people independently to the same conclusions.
Level I tasks provide the experienced
client with specific hydrogeologic parameters derived from site testing using technically
rigorous methods. The deliverable for a Level I task is a data and parameter summary.
Hydrogeologic interpretations other than parameter derivation are left to the client. For
example, a Level I aquifer test provides calculated transmissivity, storativity and other
appropriate hydrogeologic parameters along with draw-down data and analyses. The report
briefly describes field procedures but does not correlate the results of the test to the
AVI has found that Level I work is useful
because of AVI's cost effectiveness in field procedures and computational analyses, as
well as our technically rigorous methods. We can provide top quality hydrogeologic data at
up to one-half the cost of most providers. However, because of the technical capabilities
of AVI's hydrogeologic team, the best value of this service level may be the technical
defensibility and rigor of the data and parameter derivations.
Level II tasks include the scope of Level
I tasks, but further evaluate the hydrogeologic data to bracket key remediation,
transport, risk, or site closure expectations. The evaluations assume that the bulk
formation parameters (e.g., typical aquifer or SVE test hydraulic parameters) used are
generally representative of site conditions. Although anisotropy and simple geologic
layering can be considered, refined evaluations of heterogeneity are not provided.
Likewise, general contaminant properties might be considered, such as gasoline versus
diesel, but site specific distributions of different compounds are not addressed
quantitatively. Level II calculations provide an excellent physical basis to consider
certain cleanup strategies or possible fate and transport or risk conditions.
For example, a Level II Soil Capillary
Report includes the raw capillary characteristic parameters provided in a Level I report,
but also includes calculation of specific hydrocarbon volume (volume per unit area),
vertical saturation distribution, and hydrocarbon relative permeability. Calculations are
based on the raw capillary data, free product thickness measurements provided by the
client, and fluid properties taken from literature (Figures 1and 2). The Level II Soil
Capillary Report provides technically founded free product saturation information unlike
the erroneous free product thickness exaggeration calculations that have become an
industry accepted standard. This information can be used by the client or lead consultant
to estimate free product migration and recovery targets under various remedial strategies.
A second example, a Level II Soil Vapor
Extraction (SVE) Test Report, includes the Level I field testing and reporting scope, but
also provides a quantitative basis to evaluate the likely effectiveness of the method at a
particular site. It provides analyses that can be used to determine cleanup well spacing
as a function of cleanup time (Figure 3). The Level II SVE also provides engineering data
necessary for full scale system design, such as total vapor flow from the well field and
radius of cleanup. Note that the radius of clean up is very different from the industry
standard radius of influence approach. The standard radius of influence approach is not
based on vapor and chemical flux and cannot provide a time dependent analysis of remedial
effectiveness or be used for wellfield design and optimization.
Level III Tasks
Level III tasks provide modeling analyses
of chemical fate and transport and remedial strategies. These calculations can explicitly
account for contaminant type and distribution and the effects of horizontal and vertical
heterogeneity on contaminant transport and cleanup. These calculations can provide a basis
for passive remedial closure (no action) when appropriate. For example, a Level III SVE
Test Report can evaluate different cleanup scenarios at a site with a heterogeneous
distribution of hydrocarbons and compare the results to risk-based goals (Figures 4-7). As
seen in the figures, this type of well field optimization evaluation translates to cost
savings by identifying the number of wells required to achieve time and contaminant
concentration closure goals.
Because lithologic heterogeneity can be
critical control factors in the fate and transport of contaminants, AVI has developed the
capability to objectively record lithologic variability at existing cased monitor wells
through electromagnetic surveying (Figure 8). This capability is particularly valuable at
sites where subjective geologic logs have been recorded by several geologists. This method
provides AVI with the ability to assign nonuniform hydrogeologic parameters across a site
in a cost effective and technically defensible manner, but without intrusive drilling and
soil sampling methods. This distribution is then used in quantitative evaluations.