STATE WATER RESOURCES CONTROL BOARD
WORKSHOP SESSION - DIVISION OF CLEAN WATER PROGRAMS
November 3, 1999
PETITION OF UNOCAL CORPORATION FOR REVIEW OF DENIAL OF PETROLEUM UNDERGROUND
STORAGE TANK SITE CLOSURE AT 1665 PACIFIC AVENUE, STOCKTON, CALIFORNIA
San Joaquin County
Health and Safety Code section 25299.39.2, subdivision (b) provides that a petroleum underground storage tank (UST) owner or operator who believes that the corrective action plan for the owner’s or operator’s site has been satisfactorily implemented may petition the manager of the UST Cleanup Fund (Fund) for review of the owner’s or operator’s case. Health and Safety Code section 25297.1, subdivision (d) provides that responsible parties may seek review of a local oversight agency decision. UNOCAL Corporation (Petitioner) filed petitions pursuant to these provisions for review of the decision of the San Joaquin County Environmental Health Division (County) not to close this case.
Petitioner contends that its case should be closed because the concentrations of petroleum hydrocarbons at its site do not pose a threat to human health and safety, or the environment.
The Fund manager has reviewed petitioner’s case and has concluded the petitioner’s contention has merit. Accordingly, the proposed order concludes that the detectable concentrations of residual petroleum at petitioner’s site do not pose a threat to human health and safety, or the environment, and do not affect, or threaten to affect, current or probable future beneficial uses of water. However, because both the County and the Central Valley Regional Water Quality Control Board do not concur with the Fund manager’s conclusions, the proposed order requires petitioner to monitor conditions at the site for one year and directs the County not to require any additional work at the site until the results of the year’s monitoring have been evaluated. The order provides that at the end of the one-year monitoring period, the State Water Resources Control Board may reconsider closure of the case.
Should the State Water Resources Control Board adopt the proposed order?
Central Valley Regional Water Quality Control Board
None
That the State Water Resources Control Board adopt the proposed order.
STATE WATER RESOURCES
CONTROL BOARD
ORDER: WQ 99 -
UST
UNOCAL
CORPORATION
for Review of Denial of
Petroleum Underground
Storage Tank Site Closure
at
1665 Pacific Avenue,
Stockton, California
BY THE BOARD:
UNOCAL Corporation (Petitioner) seeks review of the
decision of the San Joaquin County Public Environmental Health Division
(County) not to close Petitioner’s case involving an unauthorized release of
petroleum at its site located at 1665 Pacific Avenue, Stockton,
California. For the reasons set forth
below, this order determines that Petitioner should monitor its site for one
year before the State Water Resources Control Board (SWRCB) will consider
closure. The order also directs that
the County not require any additional work other than monitoring during this
one-year period.
Tank owners and operators who are eligible for
reimbursement from the Underground Storage Tank (UST) Cleanup Fund can petition
the Fund Manager for a review of their case if they feel the corrective action
plan for their site has been satisfactorily implemented, but closure has not
been granted. (Health and Saf. Code, § 25299.39.2, subd. (b)).[1]
The site is part of the SWRCB underground storage tank
local agency oversight program.
Responsible parties such as Petitioner may seek review of local agency
decisions made under this program. (Health and Saf. Code § 25297.1, subd (d)).
Several statutory and regulatory provisions provide the
SWRCB, Regional Water Quality Control Boards (RWQCBs), and local agencies with
broad authority to require responsible parties to clean up a release from a
petroleum UST. (e.g., Health & Saf. Code, § 25299.37; Wat. Code, § 13304,
subd. (a)). The County has been
designated as an agency to participate in the local oversight program for the
abatement of, and oversight of the abatement of, unauthorized releases of
hazardous substances from USTs. (Health & Saf. Code, § 25297.1). The SWRCB has promulgated regulations
specifying corrective action requirements for petroleum UST cases. (Cal. Code
of Regs., tit. 23, §§ 2720-2728). The
regulations define corrective action as “any activity necessary to investigate
and analyze the effects of an unauthorized release, propose a cost-effective
plan to adequately protect human health, safety and the environment and to
restore or protect current and potential beneficial uses of water, and
implement and evaluate the effectiveness of the activity(ies).” (Cal. Code
Regs., tit. 23, § 2720).
Corrective action consists of one or more of the following phases: (1) preliminary site investigation, (2)
soil and water investigation, (3) corrective action plan implementation, and
(4) verification monitoring. (Cal. Code Regs., tit. 23, § 2722,
subd. (a)).
The preliminary site assessment phase includes initial
site investigation, initial abatement actions, initial site characterization,
and any interim remedial action. (Cal. Code Regs., tit. 23, § 2723,
subd. (a)). Corrective action is
complete at the conclusion of the preliminary site assessment phase, unless
conditions warrant a soil and water investigation. A soil and water investigation is required if any of the
following conditions exists: (1) There
is evidence that surface water or groundwater has been or may be affected by
the unauthorized release; (2) Free
product is found at the site where the unauthorized release occurred or in the
surrounding area; (3) There is evidence
that contaminated soils are or may be in contact with surface water or
groundwater; or (4) The regulatory agency requests an investigation,
based on the actual or potential effects of contaminated soil or groundwater on
nearby surface water or groundwater resources or based on the increased risk of
fire or explosion. (Cal. Code Regs., tit. 23, § 2724).
The purpose of a soil and water investigation is "to
assess the nature and vertical and lateral extent of the unauthorized release
and to determine a cost-effective method of cleanup." (Cal. Code of Regs.,
tit. 23, § 2725, subd. (a)).
SWRCB Resolution No. 92-49, Policies and Procedures for Investigation and Cleanup and Abatement of
Discharges Under Water Code Section 13304, also applies to petroleum UST
cases. Resolution No. 92-49 directs
that water affected by an unauthorized release attain either background water
quality or the best water quality which is reasonable if background water
quality cannot be restored (SWRCB Resolution No. 92-49, III.G). Any alternative level of water quality less
stringent than background must be consistent with the maximum benefit to the
people of the state, not unreasonably affect current and anticipated beneficial
use of water, and not result in water quality less than that prescribed in the
water quality control plan for the basin within which the site is located
(hereafter Basin Plan). (Ibid.)
Resolution No. 92-49 does not require, however, that the
requisite level of water quality be met at the time of site closure. Even if the requisite level of water quality
has not yet been attained, a site may be closed if the level will be attained
within a reasonable period (SWRCB Resolution No. 92-49, III.A).
The Central Valley RWQCB’s Basin Plan designates existing
and potential beneficial uses of groundwater in the San Joaquin Valley
groundwater basin as municipal and domestic supply (MUN), agricultural supply
(AGR), industrial service supply (IND), and industrial process supply (PRO)
Central Valley Regional Water Quality Control Board & SWRCB, Water Quality
Control Plan, Central Valley Region (1994) at p.II-3). The Basin Plan specifies a narrative taste
and odor water quality objective as follows:
“Ground waters shall not contain taste or odor-producing substances in
concentrations that cause nuisance or adversely affect beneficial uses.” (Id.
at p. III-10). The Basin Plan also
contains the following narrative water quality objective for organic
chemicals: “...ground waters designated
for use as domestic or municipal supply (MUN) shall not contain concentrations
of chemical constituents in excess of maximum contaminant levels (MCLs)
specified in...Title 22 of the California Code of Regulations...” (Id. at
III-9).
With regard to the water quality objectives for organic
chemicals, the State Department of Health Services (DHS) has set MCLs for
benzene, toluene, ethylbenzene, and xylene (BTEX) in drinking water of 1 ppb,
100 ppb, 680 ppb, and 1,750 ppb, respectively. (Cal. Code of Regs., tit.
22, § 64444). Although DHS has not yet set an MCL for
methyl-tertiary-butyl-ether (MTBE), DHS has set an interim action level of
35 ppb (DHS Memorandum from Joseph P. Brown, Ph.D., Acting Chief, Water Toxicology
Unit to Alexis M. Milea, P.E., Acting Supervisor, Standards and Technology
Unit, Office of Drinking Water (February 19, 1991) at p. 2). DHS has adopted a 5 ppb MTBE
concentration as a secondary drinking water standard for taste and odor. The threshold odor concentration of
commercial gasoline (measured as total petroleum hydrocarbon gasoline, or
TPH-g) in water is commonly accepted to be 5 ppb, with 10 ppb giving a strong
odor.
The following is a brief historical summary of
Petitioner’s site at 1665 Pacific Avenue in the City of Stockton. The site is located in a
commercial/residential area of the city and was an operating service station
from about 1947 to 1988; gasoline was dispensed from two 10,000 gallon
capacity USTs. Since demolition of the
station and removal of the USTs in 1988, the site has remained vacant.
The site is underlain by flood plain sediments (clay,
silt and sand) of the Pliestocene Victor Formation. In the vicinity of the site, groundwater in the more permeable
strata of the Victor Formation is generally of inferior quality (concentrations
of nitrate greater than 45,000 ppb, chloride greater than 300,000 ppb, and
TDS greater than 1,000,000 ppb) and is not viewed by local water purveyors as a
source of drinking water. Underlying
the Victor Formation are the alluvial sediments (clay, silt, sand, and gravel)
of the Plio-Pliestocene Laguna Formation.
Groundwater in the Laguna Formation is used extensively throughout the
area for municipal, industrial, and agricultural supply. There are no water supply wells within 800
feet of the site and those municipal wells located in the general area are
generally constructed with annular seals which extend to depths of 150 to 200
feet so as to exclude the shallower poor quality groundwater in the Victor
Formation. The nearest surface water, a
canal that extends easterly from the Port of Stockton, is about 2,000 feet to
the west. Shallow groundwater at the
site flows in a general easterly direction with a gradient of about 0.002.
In January 1988, analyses of soil samples from five soil
borings drilled to depths of 20 to 30 feet indicated that a release had
occurred at the site. In February 1988,
three groundwater monitor wells were installed to assess site groundwater conditions. The initial groundwater samples from the
three wells revealed an absence of gasoline constituents in groundwater near
the location of the two gasoline USTs, high concentrations (99,000 ppb TPH-g
and 5,700 ppb benzene) in the vicinity of the easterly pump island, and low
concentrations (170 ppb TPH-g and 1 ppb benzene) in the vicinity of the waste
oil UST. Data gained from the well
installations also indicated that the groundwater was confined and that it
flowed in an easterly direction.
In March 1988, the two 10,000-gallon capacity gasoline
USTs, the waste oil UST, and the dispensers and associated piping were removed
from the site. Visual inspection of the
bottom and side-walls of the gasoline UST excavation revealed that soil was
affected by a release prompting petitioner to over-excavate an estimated 850
cubic yards of soil to a final depth of 28 to 30 feet.[2] Soil samples collected from the bottom and
sidewall of the excavation indicated TPH-g and benzene concentrations ranged
from 3 to 18 ppm and less than 0.05 to 1.7 ppm, respectively.
In August 1989, two additional groundwater monitor wells
were constructed off‑site approximately 100 feet east and northeasterly
of the former pump island where high constituent concentrations had previously
been detected in groundwater. Analyses
of groundwater samples from these wells revealed low concentrations of TPH-g
(150 and 240 ppb), xylene (1.4 and 1.9 ppb), and ethylbenzene (less than
0.5 and 8.1 ppb); benzene and toluene were not detected.
In February 1992, three vapor extraction wells and two
more groundwater monitor well were installed.
Two of the vapor extraction wells were located where the easterly pump
island had been while the third well was positioned at the former location of
the site’s second, westerly pump island.
One of the new groundwater monitor wells was constructed about 300 feet
down-gradient of the site while the other was positioned on-site and
up-gradient, about 60 feet southwest of the pair of vapor extraction
wells. Analyses of soil samples
collected during the drilling of these wells indicated an absence of gasoline
constituents in the vicinity of the westerly pump island and at the locations
of the two newly installed groundwater monitor wells. However, in the vicinity of the easterly pump island, TPH-g and
benzene were detected at concentrations as high as 7,300 ppm and 3.4 ppm,
respectively, at a depth of 26 feet; the data indicated that concentrations
diminished to 120 ppm and 0.083 ppm, respectively, at a depth of 51 feet.
In July 1994, two more groundwater monitor wells were
installed southeast and southwest of the site to complete definition of the
plume. Water quality data from the
array of nine groundwater monitor wells showed that the dissolved phase benzene
plume, centered at the location of the easterly pump island, extended
down-gradient about 120 feet, no more than 50 to 60 feet off-site.
In January 1996, recovery of residual petroleum
hydrocarbons in the vadose zone was initiated using Soil Vapor Extraction (SVE)
technology. The SVE system was operational
through November 1996 and recovered an estimated 400 gallons of TPH-g,
including about two gallons of benzene.
Beginning in May 1996, MTBE was analyzed in quarterly
groundwater samples from site monitor wells.
To date, analyses (EPA Method 8260) have detected low concentration of
MTBE, generally 2 to 10 ppb, in groundwater samples collected during the winter
of 1998/1999 from up-gradient, cross-gradient, and down-gradient monitor
wells. These occurrences of MTBE are
consistent with United States Geological Survey findings that atmospheric
“washout” can result in concentrations of MTBE in excess of 20 ppb in shallow
groundwater.
By letter to Petitioner dated January 9, 1997, County
staff concluded that “...the assessment of the lateral extent of groundwater
contamination is complete, the assessment of soil contamination is complete,
and the plume appears to be stable.”
The County recommended that a feasibility study and corrective action
plan (CAP) be submitted, quarterly groundwater monitoring continue, and data in
support of the presence of natural attenuation be collected.
By letter to the County dated February 7, 1997,
Petitioner summarized the extent of remedial activities undertaken at the site
and the findings of its investigative efforts. Petitioner then concluded that the site was suitable for closure
as a low-risk groundwater case and that further site assessment or corrective
action was not necessary.
By letter to Petitioner dated March 3, 1997, County staff
concluded that the plume of dissolved phase petroleum hydrocarbons was about
130 feet long and 60 feet wide, that the average benzene concentration within
the limits of the plume was 32 ppb, and that the plume was stable and
declining. However, the County also
concluded that site closure at that time was not appropriate without additional
information that demonstrated that natural attenuation was occurring and an
explanation that natural attenuation would restore the beneficial use of
groundwater.
By letter to the UST Cleanup Fund Manager dated June 13,
1997, Petitioner requested that its case be reviewed for closure. Petitioner contended that petroleum
hydrocarbon sources (USTs and piping) had been removed from the site, impacted
soil had been excavated, SVE had removed additional residual petroleum
hydrocarbons in soil, free product had never been observed, MTBE had never been
detected in groundwater, the nearest public water supply well is approximately
800 feet from the site, and the County had determined that assessment of the extent
of affected soil and groundwater was complete.
By letter to the UST Cleanup Fund dated January 16, 1998,
County staff expressed concern regarding Petitioner’s failure to define the
extent of affected soil and groundwater and to document the volume of affected
soil excavated at the site.
In March 1998, responding to the County’s request for
information demonstrating that natural attenuation was occurring at the site,
Petitioner analyzed groundwater samples from site wells for selected inorganic
constituents which are indicative of microbial metabolism of petroleum
hydrocarbons. These analyses, in
addition to showing that very active biodegradation was occurring within the
dissolved phase hydrocarbon plume, also showed that in shallow groundwater, background
concentrations of nitrate (about 70 ppm) and Total Dissolved Solids (1,000 to
1,200 ppm) exceeded State MCLs (45 ppm and 500 ppm respectively).
In January 1999, responding to the County’s demand that
further delineation of affected soil and groundwater was required, Petitioner
collected soil samples to a depth of 90 feet and installed two more monitor
wells in the vicinity of the release.
Analyses of the soil samples showed that soil affected with TPH-g and
BTEX was present in the depth interval of 40 to 60 feet, that only trace
concentrations (less than 0.02 ppm) of toluene and xylene were present at 70
feet, and that all gasoline constituents were not detectable in the samples
from the 80 and 90 foot depths.
Concentrations of MTBE were not detectable in any of the samples
collected.
The two new wells were constructed to monitor groundwater
at the depth intervals of 50 to 60 feet and 85 to 90 feet. These wells, in conjunction with other
monitor wells, provided a means of assessing the vertical hydraulic gradient
and the vertical extent of the dissolved phase hydrocarbon plume at the
site. The initial sampling of
groundwater from the two new wells indicated that groundwater at 50 to 60 feet
and 85 to 90 below grade had benzene concentrations of 14,000 ppb and 210 ppb[3],
respectively. The hydraulic gradient
between the two zones monitored was about 0.02 upward.
On May 4, 1999 the County issued a directive requiring
Petitioner to submit a workplan to define the lateral and vertical extent of
offsite, down-gradient soil and groundwater contamination, and to install wells
onsite for feasibility studies of viable remedial alternatives. On June 3, 1999, Petitioner filed a petition
pursuant to Health and Safety Code §25297.1 seeking review of the May 4, 1999
directive and the County’s failure to close the site. Petitioner also asked the SWRCB to issue a stay of the May 4th
directive pending SWRCB action on Petitioner’s request for closure. However, since this order considers the
petitions on their merits
the issuance of a stay order is now
moot. Because both petitions raise similar issues, we have consolidated them
and will deal with both in this order.
By
letter dated July 22, 1999, the Chief of the Division of Clean Water Programs,
State Water Resources Control Board, informed the County that sufficient data
were available to conclude that further assessment of site conditions was not
warranted and that he would recommend closure if the results of four additional
quarters of sampling remained consistent with the historic data. In responding to the letter, the County
expressed disagreement with the Division Chief’s conclusions and contended that
declining concentrations of dissolved phase hydrocarbons in site groundwater
was a consequence of natural attenuation via dispersion and dilution and that
the occurrence of biologic degradation was not supported by available
data. The County also contended that
down-gradient monitor wells do not monitor the depth interval where the highest
concentrations of dissolved phase hydrocarbons were detected at the site, hence
the need for continued investigative activities.
Contention: Petitioner contends its case should be
closed because soil and groundwater assessment is complete and corrective
actions conducted have diminished the presence of residual petroleum
hydrocarbon constituents to a point where they pose a “low risk” to public
health and safety, the environment, and to current or probable future
beneficial uses of water.
The County concedes that natural attenuation is occurring
at the site but it argues that biologic degradation is not a factor in the
observed decline of petroleum hydrocarbon constituents in groundwater and
contends that down-gradient monitor wells do not extend to the depth interval
where the highest documented concentrations of dissolved phase hydrocarbons
have been detected at the site.
Findings: Petitioner’s contention has merit. As explained below, the facts in the record
support the finding that additional soil and groundwater investigation or
additional remediation is not necessary, and that residual petroleum
hydrocarbon constituents at Petitioner’s site do not pose a threat to human
health and safety, or the environment, and do not adversely affect, or threaten
to affect, current or anticipated future beneficial uses of water. In addition, the level of site cleanup is
consistent with the maximum benefit to the people of the state and will meet
the applicable objectives in the Central Valley RWQCB Basin Plan within a
reasonable time frame.
The County’s contentions are not supported by the facts
in the record. As explained below, the
facts show that both aerobic and anaerobic biologic degradation of residual
petroleum hydrocarbon constituents is occurring at the site. The facts further show that all
down-gradient monitor wells extend to and below the depth interval where the
highest documented concentrations of dissolved phase hydrocarbons have been
detected at the site.
The primary sources of the release (the USTs, dispenser,
and associated piping) have been removed as well as a substantial volume of
affected soil from the immediate vicinity of the former USTs. Free product has never been observed in any
site monitor wells and SVE has removed a substantial mass of residual petroleum
hydrocarbons in the vicinity of the former pump island.
With regard to affected groundwater and its potential to
adversely impact current or anticipated future beneficial uses, substantial
evidence in the record indicates that concentrations of dissolved phase
hydrocarbons are decreasing, the plume is stable, MTBE is not present as a
consequence of the release, and attenuation and biologic degradation of
residual petroleum hydrocarbons in site soil and groundwater is occurring. Further, available data indicate that the
affected groundwater monitored by site wells, in addition to being of inferior
quality, is not hydraulically connected to the deeper groundwater aquifers that
are used locally for municipal, industrial or agricultural supply.
The highest documented concentrations of petroleum
hydrocarbons detected in site groundwater are in samples from monitor well MW-2
(this well is completed to a depth of 50 feet at the location of the
former pump island where the release occurred). Concentrations of TPH-g and benzene typically ranged from 100,000
to 200,000 ppb and 6,000 to 25,000 ppb, respectively, from 1988 through 1994;
concentrations of these constituents have declined to about 80,000 and 2,400
ppb, respectively, as demonstrated by the monitoring results of the past
year. Reported concentrations of TPH-g
and benzene in groundwater samples from the monitor well MW-4 (immediately down‑gradient
of MW-2 and completed to a depth of 54 feet) have ranged from
69 to 460 ppb and “non-detect” to 2.0 ppb, respectively[4]
(32 sampling events over nine years).
Other down-gradient wells, MW-5 (completed to a depth of 55 feet) and MW‑7 (completed
to a depth of 60 feet), have infrequently detected low concentrations of
benzene (0.85 to 2.2 ppb), toluene (0.74 to 4 ppb), ethylbenzene (0.63 ppb),
and xylene (0.52 to 4.8 ppb) since 1994 (during 7 of 42 sampling events). Concentrations of TPH-g and benzene in
groundwater samples from wells peripheral to the former pump island have declined
from relatively high concentrations (3,300 to 110,000 ppb and 160 to
330 ppb respectively) reported through 1995, to very low concentrations
(“non-detect” for TPH-g and 1 ppb to “non-detect” for benzene) since 1997
(the past nine quarterly monitoring events).
Similarly, concentrations of toluene, ethylbenzene, and xylene in
groundwater samples from well MW-2 have historically ranged from 25,000 to
82,000 ppb, 2,700 to 9,900 ppb, and 14,000 to 37,000 ppb, respectively. However, concentrations of these
constituents are less than MCLs (generally “non detect” to less than 10 ppb) in
groundwater from all other site monitor wells.
Thus the data clearly show the plume is stable, concentrations of
residual petroleum hydrocarbons in groundwater are decreasing, and that
residual concentrations in excess of Basin Plan objectives are limited to less
than 60 feet off-site with respect to toluene, ethylbenzene, and xylene, about
70 feet with respect to benzene, and an estimated 150 feet with regard to
TPH-g.
The introduction of petroleum hydrocarbons into the
subsurface causes rapid changes in the prevailing groundwater geochemistry due
to microbially mediated reactions between the hydrocarbons and naturally
occurring inorganic compounds. Aerobic
respiration, denitrification, Fe (III) reduction, sulfate reduction, and
methanogenesis are largely responsible for these changes, and each leaves
behind a unique biogeochemical signature.
Analyses of the groundwater samples collected from site wells in March
1998 show that concentrations of reduced iron (ferrous) are greater, and
nitrate and sulfate concentrations are significantly less, in samples from
wells within the plume when compared to samples from “background” wells. For example, groundwater samples from
up-gradient and cross-gradient wells have ferrous iron concentrations of less
than 0.1 ppm while groundwater samples within the plume have reported
concentrations ranging from 0.5 to 3.4 ppm.
Up-gradient concentrations of nitrate and sulfate are about 70 ppm and
170 ppm respectively while concentrations of these constituents at the core of
the plume are “non detect” and in down-gradient groundwater nitrate
concentrations are “non detect” while sulfate concentrations are only 10 to 20
ppm. Thus, the data demonstrate that
the dissolved phase hydrocarbon plume in site groundwater is undergoing
biodegradation via iron, sulfate and nitrate reduction (i.e., anaerobic
biodegradation).
Measurements of dissolved oxygen in groundwater samples
collected from site wells in February 1999 provide the basis to conclude that
aerobic biodegradation is also acting to attenuate the petroleum hydrocarbons
present in site soil and groundwater.
Concentrations of dissolved oxygen in groundwater from wells MW-1, MW-2,
MW-3, and MW-6 (the four shallow wells that have historically defined the core
of the site’s groundwater plume) were less than 0.5 ppm. Concentrations of dissolved oxygen in
groundwater from up-gradient, cross-gradient, and down-gradient monitor wells
ranged from 2.24 to 5.49 ppm. Thus the
low dissolved oxygen concentration associated with the location of the release
relative to the higher concentrations at distance further demonstrates the
on-going biodegradation and attenuation of the hydrocarbon plume.
In considering possible impacts to current and future
beneficial uses of groundwater, three factors are relevant: 1) according to
Department of Water Resources (DWR) Bulletin 146 (1967) and Petitioner’s
March 1998 sample analyses, shallow groundwater in the vicinity of the site is
of “inferior quality,” 2) the groundwaters monitored by site wells are not in
direct hydraulic communication with deeper groundwater aquifers or local supply
wells, and 3) as discussed above, the plume is stable and concentrations are
decreasing as a consequence of natural attenuation. A review of DWR Well Drillers’ Reports for water supply wells
located in the vicinity of the site show that these wells are constructed with
sanitary seals, which extend to depths of 150 to 200 feet below ground
surface. The purpose of this method of
well construction is to exclude poor quality shallow groundwater (which
generally contains elevated concentrations of nitrate, chloride, and TDS). The presence of this water in the Victor
Formation is a consequence of the easterly intrusion of poor quality water
underlying the Delta and the Stockton Deep Water Ship Channel over the past 50
years or so. An analysis of water level
elevation data for site wells also shows an absence of the seasonal
fluctuations one would expect if the groundwater monitored was in direct
hydraulic communication with local production wells. Hence, the localized volume of affected water is not currently
being used and, due to its inferior quality, is not likely to be used in the
future. Furthermore, historic
groundwater monitoring data, in addition to showing plume stability and
decreasing constituent concentrations, shows that the poor quality of the
shallow groundwater enhances the on-going bioremediation, consequently reducing
the residual petroleum hydrocarbon mass in the subsurface. Because constituent concentrations will
continue to decline as the mass of residual petroleum hydrocarbons decrease
over time, continued monitoring of site groundwater would serve no useful
purpose.
The only way to ensure complete removal of lingering,
residual, detectable concentrations of petroleum constituents in site
groundwater would be to excavate an additional several thousand cubic yards of
affected soil to depths in excess of 50 feet.
This would entail a substantial disruption of streets, businesses,
sewers and other utilities while producing little or no benefit to current or
anticipated beneficial uses of the minimal area of groundwater not meeting
Basin Plan objectives.
Furthermore, if complete removal of detectable traces of
petroleum constituents becomes the standard for UST corrective actions, the
statewide technical and economic implications will be enormous. For example, disposal of soils from comparable
areas of excavation throughout the state would greatly impact already limited
landfill space. In light of the minimal
if any benefit to be gained, the evidence of continuing attenuation of residual
petroleum concentrations, and the precedent that would be set by requiring
additional excavation at this site, attaining background water quality in this
limited area is not feasible. While it
is impossible to determine the precise level of water quality that will be
attained given the residual petroleum constituents that remain at the site, in
light of all the factors discussed above, a level of water quality will be
attained that is consistent with the maximum benefit to the people of the state[5].
The final step in determining whether cleanup to a level
of water quality less stringent than background is appropriate for this site
requires a determination that the alternative level of water quality will not
result in water quality less than that prescribed in the relevant Basin
Plan. Pursuant to SWRCB Resolution No.
92-49, a site may be closed if the Basin Plan requirements will be met within a
reasonable time frame.
In this particular case, as discussed above, TPH-g and
BTEX in the shallow groundwater in immediate contact with the limited residual
petroleum hydrocarbon constituents adsorbed to soils will likely remain above,
and thus violate, the Basin Plan’s objectives in a localized volume of
surrounding groundwater for a significant period of time. This time period could be anywhere from a
few decades for BTEX to degrade below MCLs to hundreds of years for that
limited volume of groundwater in immediate contact with longer chain, immobile
residual petroleum constituents adsorbed to soils to meet the commonly accepted
5 ppb taste and odor threshold.
Nonetheless, during this time these residual
concentrations in excess of Basin Plan objectives will not pose a threat to
current or future beneficial uses. It
is highly unlikely that petroleum hydrocarbon constituents detected in
localized areas in the immediate area of the release will migrate substantially
beyond current limited spatial extent.
Though the longer chain hydrocarbons comprising weathered TPH-g
biodegrade more slowly than certain petroleum constituents, such as benzene,
they are also more recalcitrant (i.e., less volatile, less soluble and highly
absorbent) and much less mobile. It is
also highly unlikely that this particular very limited pocket of shallow
groundwater will be used directly as a source of drinking water. Thus, the significant period of time that it
will take for water quality in this limited area to meet all Basin Plan
objectives is a reasonable time frame.
Closure of the site, given the facts in this particular case, is
appropriate. Nevertheless, both the
County and CVRWQCB do not agree that residual diminishing concentrations of
petroleum hydrocarbons remaining at the site do not threaten the beneficial use
of underlying groundwater. It would
therefore be appropriate to monitor site groundwater for dissolved phase BTEX, TPH-g,
and oxygen four additional quarters to validate more convincingly the above
findings that are based on the currently available information.
1. There is no
evidence of MTBE in groundwater as a consequence of the release at this
site. Corrective actions including soil
vapor extraction and removal of an estimated 850 cubic yards of contaminated
soil have removed a significant mass of residual petroleum hydrocarbons at
Petitioner’s site.
2. The plume of
dissolved phase hydrocarbons is stable, concentrations of constituents are
declining, and naturally occurring biodegradation of residual petroleum
hydrocarbons adsorbed to soil and present in groundwater is occurring.
3. Ten years
after the release was stopped, groundwater meets Basin Plan objectives with
respect to BTEX constituents within about 70 feet of the site; with respect to
TPH-g, groundwater meets Basin Plan objectives within 150 feet of the site.
4. Petitioner’s
site is located in a commercial/residential area. No water supply wells are located within 800 feet and the nearest
surface water is about 2,000 feet from the site.
5. Current water
supply wells in the vicinity of the site are constructed with sanitary seals
which generally extend to depths of 150 to 200 feet below ground surface for
the purpose of excluding poor quality shallow groundwater. Any future supply wells which may be
constructed in the vicinity of the site will in all likelihood be similarly
constructed.
6. Additional soil and water remediation at Petitioner’s site is
not necessary.
7. The level of site cleanup is
consistent with the maximum benefit to the people of the state.
8. Given the
adverse technical and economic implications statewide if further corrective
action other than monitoring was required, and the minimal benefits, if any,
that would be gained by further corrective action, it is not feasible to attain
background water quality at Petitioner’s site.
9. Detectable
concentrations of BTEX in shallow groundwater in contact with the limited
residual petroleum hydrocarbons adsorbed to soil particles may remain above
MCLs and thus violate the Basin Plan objectives in a very localized, small
volume of surrounding groundwater for decades.
10. Detectable
concentrations of TPH-g in shallow groundwater in contact with the limited
residual petroleum hydrocarbons adsorbed to soil particles will likely remain
above 5 ppb (the commonly accepted odor threshold for drinking water) and thus
violate the Basin Plan’s narrative odor objective in a very localized, small
volume of surrounding groundwater for anywhere from decades to hundreds of
years.
11. The
determination as to what constitutes a reasonable period to attain water
quality objectives must be based on evaluation of all relevant factors,
including but not limited to the extent and gravity of any threat to public
health and the environment during the period required to meet Basin Plan
objectives. Although the time required
to attain objectives in this case is lengthy, it is a reasonable period
considering the facts of this particular case, including that there are no
known drinking water wells within 800 feet of the site and that affected water
is of inferior quality and will likely be excluded from any well which may be
constructed in the future. It is highly
unlikely that petroleum constituents detected in the immediate area of the
discharge will migrate substantially beyond the current limited spatial extent,
and it is highly unlikely that this particular very limited pocket of shallow
groundwater will be used directly as a source of drinking water in the
foreseeable future.
12. Both the
County and the Regional Board do not agree that residual diminishing
concentrations of petroleum hydrocarbons remaining at the site do not threaten
the beneficial use of underlying groundwater.
Therefore, it would be appropriate to monitor site groundwater for
dissolved phase BTEX, TPH-g, and oxygen for four additional quarters to
validate more convincingly staff’s site conceptual model.
13. Therefore, no
further corrective action other than monitoring is needed at this site.
14. The above
conclusions are based on the site-specific information relative to this
particular case.
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IV.
ORDER
IT
IS Therefore ORDERED THAT:
1.
Petitioner’s
request for closure is denied at this time.
2.
No
further action be required until the groundwater monitoring as described in
this order is completed.
3.
If
the results of the additional four quarters of monitoring remain consistent
with the historic data and further validate staff’s site conceptual model, this
matter is to be scheduled for consideration for closure by the SWRCB at its
November 2000 workshop and meeting.
CERTIFICATION
The undersigned,
Administrative Assistant to the Board, does hereby certify that the foregoing
is a full, true, and correct copy of a resolution duly and regularly adopted at
a meeting of the State Water Resources Control Board held on November 18, 1999.
AYE:
NO:
ABSENT:
ABSTAIN:
______________________________
Maureen
Marché
Administrative
Assistant to the Board
[1] To the extent that the SWRCB may lack authority to review this petition pursuant to Health and Safety Code section 25299.39.2, subdivision (b) because the petitioner did not submit a corrective action plan for the site, the petition is being reviewed pursuant to Health and Safety Code section 25297.1, subdivision (d) and SWRCB Resolution No. 88-23.
[2] The volume of soil excavated is a point of contention. Petitioner indicates in its Site Closure Summary report that an estimated 1,100 cubic yards of soil were excavated while the County contends that only 350 cubic yards are documented to have been excavated. However, in a March 12, 1991 letter to petitioner, the County states that;“...approximately 1300-1400 cubic yards of contaminated soil...” were excavated. Based on the reported dimensions of the excavation, we have estimated 850 cubic yards.
[3] Concentrations of dissolved phase petroleum hydrocarbons detected in groundwater from this deeper zone are an artifact of the well installation process, i.e., contamination introduced at the time the well was being drilled and constructed. Subsequent sampling, in March and May 1999, have demonstrated a steady decline in the concentrations of all gasoline constituents. i.e., 150 ppb benzene in March, 43 ppb in May.
[4] One sample, collected in August 1992, had a reported TPH-g concentration of 3,000 ppb.
[5] In approving an alternative level of water quality less stringent than background, the SWRCB has also considered the factors contained in California Code of Regulations, title 23, section 2550.4, subdivision (d). As discussed earlier, the adverse effect on shallow groundwater will be minimal and localized, and there will be no adverse effect on the groundwater contained in deeper aquifers, given the physical and chemical characteristics of petroleum constituents; the hydrogeological characteristics of the site and surrounding land; and the quantity of the groundwater and direction of the groundwater flow. In addition, the potential for adverse effects on beneficial uses of groundwater is low, in light of the proximity of groundwater supply wells; the current and potential future uses of groundwater in the area; the existing quality of groundwater; the potential for health risks caused by human exposure; the potential damage to wildlife, crops, vegetation, and physical structures; and the persistence and permanence of potential effects.
Finally, a level of water quality less stringent than background is unlikely to have any impact on surface water quality, in light of the volume and physical and chemical characteristics of petroleum constituents; the hydrogeological characteristics of the site and surrounding land; the quantity and quality of groundwater and the direction of groundwater flow; the patterns of precipitation in the region, and the proximity of residual petroleum to surface waters.