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 Site Remediation News May 2001 (Vol 13 N0 1) Article 03
Guidance
on Vertical Delineation of Ground Water Contaminant PlumesBy: Erick Kinsel, Bureau of Underground
Storage Tanks Scope: This guidance is intended to apply to dissolved petroleum-derived
ground water contaminant plumes in unconsolidated formations. This guidance
should not be applied to sites where ground water occurs in bedrock, or
where the contaminants of concern are dense non-aqueous phase liquids
or inorganics. Pursuant to the Technical Requirements
for Site Remediation, specifically N.J.A.C. 7:26E-4.4 (h) 3. i., if
ground water is contaminated above the applicable remediation standards,
delineation of the horizontal and vertical extent of the contaminant plume
is required. If a party can successfully demonstrate that vertical delineation
is not necessary at a particular site, the Department can grant a variance
from the vertical delineation requirement, in accordance with N.J.A.C.
7:26E-1.6 (d). The burden of proof for providing justification why vertical
delineation is not necessary in a particular case rests with the party
conducting the remedial activity. The criteria presented below are intended to assist parties conducting
remedial activities in evaluating whether vertical delineation is necessary
for dissolved petroleum-derived contaminant plumes, and in compiling the
information that will be needed by the Department in reviewing requests
for variances from vertical delineation requirements. A. Vertical delineation shall be conducted in the following situations:
Where potable wells, supply wells, industrial wells or irrigation
wells are located within 1000 feet downgradient of the site; Where potable wells, supply wells, industrial wells or irrigation
wells are located within 500 feet sidegradient of the site; Where potable wells, supply wells, industrial wells or irrigation
wells are located within 250 feet upgradient of the site; Any other sites which, in the Department’s opinion, pose a risk to
potable wells, supply wells, industrial wells or irrigation wells; In non-aquifer use areas (no potable, supply, industrial or irrigation
wells), when gasoline additives that do not significantly biodegrade
(e.g. MTBE) are present in the source area of the plume at sustained
concentrations of 10 PPM or greater; There is a downward vertical component to the ground water flow regime
at the site; Stratigraphic information shows that hydraulic conductivity increases
significantly with depth in the first 50 feet of saturated aquifer thickness;
At sites underlain by stratified material (e.g. interbedded silts,
clays and sands) with zones of higher permeability acting as zones of
preferential ground water migration; or Other information indicates that vertical migration of contamination
may be occurring. B. Vertical delineation may not be necessary in the following situations: None of the conditions in “A” above are met and;
The age and duration of the release are known, the identity and volume
of the release are known, the source and soils have been mitigated within
one year of the release, and the ground water remediation will be completed
in less than five years from the release date; The plume constituents are only petroleum-derived compounds and gasoline
additives (e.g. MTBE, TBA), and the source area ground water concentrations
of individual non-biodegradable compounds have always been and remain
below 10 ppm; The site is on a confining unit or underlain by a confining unit
that is contiguous across the site within the first 30 feet of saturated
aquifer thickness as documented by geologic reports, on-site logs, or
data from nearby sites; The source area is adjacent to a gaining surface water body. The
gaining or losing status of the surface water body shall be determined
with seasonal (e.g., quarterly) measurements of ground water and surface
water elevations. Tidal influences shall also be evaluated; or The party conducting the remedial investigation documents that vertical
migration of a contaminant plume is unlikely based upon site-specific
factors, in accordance with N.J.A.C. 7:26E-1.6. C. Guidelines on initial vertical sampling locations:
If the dominant mechanism for vertical plume displacement is infiltration
of precipitation, then the initial vertical sampling point should be
placed at the leading edge of the plume along the plume centerline as
defined by existing horizontal monitoring points. It is recommended
that the top of the vertical sampling interval be approximately 5 to
10 feet below the bottom of the horizontal sampling point. It is also
recommended that the screened interval in each well not exceed 10 feet.
This may be adjusted to account for site-specific conditions. If a lithologic change is triggering the vertical delineation requirement,
then the initial vertical point should be screened within the lithologic
zone of interest. Locate the vertical point within the plume, rather
than at the downgradient edge of the plume. If other mechanisms are controlling the vertical displacement of
the plume, then the location(s) of the vertical delineation point(s)
should be decided using site specific conditions. The Department does not recommend that vertical profile sampling
(e.g. low flow sampling, double check-valve bailer sampling) be attempted
within an individual water table well. Instead, it is recommended that
permanent vertical monitor points be installed. Initial vertical profiling
may be conducted utilizing temporary sampling points. Temporary sampling
points provide an advantage in optimizing locations and depths for subsequent
permanent monitoring points. However, in most instances successive monitoring
events over a sustained time interval will be required, necessitating
permanent monitoring points. Permanent vertical points enable head measurements
to be obtained in order to assess the presence and magnitude of a vertical
component of ground water flow.  |
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