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Donald Kellar, 2009,
Identification of
Water Sources during Wet and Dry Months within Medea Creek
through Ion and Isotope Analysis: Committee Members: Dr.
Barry Hibbs (Committee Chair), Dr.
Andre Ellis, Dr. Kim Bishop
This study was conducted within
Medea Creek, located in the city of Agoura Hills, Los Angeles
County, California. Medea Creek is part of the Malibu Creek
Watershed. The study was conducted to determine water source
influences between wet and dry seasons within Medea Creek. Water
sources consisted of local groundwater and rain influence,
versus urban runoff from imported and isotopically different
“State Project Water”. The study was conducted between January
of 2008 and March of 2009 with a total of six separate sampling
events. Wet months included January through March, and dry
months included July through December. Stream samples were
analyzed for chloride, nitrate, and sulfate. Most samples were
also analyzed for oxygen and hydrogen isotopes.
The results of this study
indicate that water collected from Conifer Street, Cornell
Intersection, and Paramount Ranch (downstream stations)
originates mostly from a locally derived precipitation source,
via recharge to shallow aquifers and discharge (baseflow) to
Medea Creek. Oak Canyon Community Park (most upstream station)
appears to be sourced primarily by locally derived groundwater
baseflow during the wet months, wherein the isotopic signatures
indicate isotopically heavier (local) composition. Diluted
nitrate concentrations exist during the wet months as well. Oak
Canyon appears to have the highest correlation to State Project
Water during the dry season however, indicating falling water
tables, decreased baseflow, and larger amounts of urban runoff
that is derived from the imported “State Project” water. The
remaining stations (Conifer Street, Cornell Intersection, and
Paramount Ranch) are less influenced by State Project Water and
urban runoff, because
d18O
and
dD signatures become
progressively heavier downstream, indicating a greater input of
locally derived baseflow water- sources. |
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Alejandro Lopez, 2009, Management
of Leaking Underground Storage Tank (UST) Sites and Data
evaluation of UST Sites with Free Product in Los Angeles Area: Dr.
Barry Hibbs (Co-Chair), Dr.
Weixing Tong (Co-Chair), Dr. Kim Bishop
The Underground Storage Tank (UST) Program of the Los Angles
Regional Water Quality Control Board (LARWQCB) is a regulatory
agency for sites with underground fuel storage tanks in Los
Angeles and Ventura Counties. The UST Program protects public
health and safety and the environment from releases of petroleum
and hydrocarbons from tanks through regulation. Before 1980s
many USTs consisted of bare steel single layer tanks and pipes,
which corroded over time, and eventually resulted in leakage.
Faulty installation and inadequate handling may also cause
leaks. Cleanup of leaking tanks often involves a soil and
groundwater investigation and remediation, under the direction
of a regulatory agency. Currently, the Los Angeles UST Program
has more than 1400 open sites, of those 217 have or had
historically found free product.
Free product associated with UST sites is a Light Non Aqueous
Phase Liquid (LNAPL) that floats on the groundwater table. It
is the same for the petroleum hydrocarbons found in the soil and
groundwater. Free product migration path leaves behind a soil
adsorbed phase plume that may become volatile or that may
dissolve into groundwater.
Study sites encompass the Los Angeles-San Gabriel Hydrologic
Unit, the Santa Clara Calleguas Hydrologic Unit, the Ventura
River Hydrologic Unit, The Pitas-Point Hydrologic Unit, the
Malibu Hydrologic Unit and the Channel Islands Hydrologic Unit.
The lithology of the study sites is variable, given that
Southern California is a tectonically active setting, resulting
in lithology of the sites being very heterogeneous. Lithology
of monitoring wells in the study, indicate that high benzene
wells and free product wells are generally located down gradient
from leakage source. However grain size and hydraulic gradient
differences determines how fast and in what direction the LNAPL
will migrate. Free product migrates faster though lest
resistant (coarse soils) pathways.
Once a site has been reported as having hydrocarbon
contamination, the source of the contamination need to be found
and eliminated. UST program sites with a reported spill are a
spill of petroleum hydrocarbons such as gasoline. With regard to
groundwater contamination, the main regulatory concerns are BTEX
species: B (benzene), T (toluene), E (ethylbenzene), and X (xylene
isomers: o-X, m-X, p-X), and other oxygenates such as MTBE
(methyl tert-butyl ether) and TBA (tert-butyl alcohol) (Mackay,
2008).
In this project, data from a total of 129 open cases that
contain or historically contained free-product in the subsurface
is analyzed. The data were gathered after submission of the
second quarterly 2007 self-monitoring reports to the LARWQCB
through second quarter 2009, and include maximum benzene
concentrations, groundwater flow direction and monitoring well
geology. The data was analyzed and provides a range of benzene
concentrations that may be used as an indication of possible
presence of free product, in sites which no free product has
been encountered. I discuss lithology, soil-liquid behavior,
groundwater gradient fluctuation, advection, diffusion,
dispersion, vapor phase, dissolved and adsorbed plumes and
possible ways to remediate contamination. I also include a case
study of free product in the subsurface.
A value above the average 27,811 ppb dissolved phase benzene
concentration in groundwater is a conservative selection to use
as marker for possible free product presence near monitoring
well at sites where free product has not been discovered. Figure
24 shows the maximum reported dissolved benzene and its
corresponding lithology for 84 (excludes outliers) cases. 52%
(44 cases) are less than 20,000 ppb, 30% (25 cases) are between
20,000 to 40,000 ppb, and 18% (15 cases) are over 40,000 ppb.
Site lithology was dominated by sands with 45 cases, followed by
clays with 22 cases, and lastly silts with 15 cases. Average
benzene (43,048 ppb) concentrations are highest in silts,
decreasing to clayey sand (29,250 ppb), silty clay (27,132 ppb),
sandy silt (26,041 ppb), silty sand (25,064 ppb), sand (22,325
ppb), sandy clay (16,033 ppb), gravelly sand (12,744 ppb),
clayey silt (9,540 ppb), and groundwater in clay (4,787 ppb) is
the lowest benzene average concentration.
Eighty eight cases have benzene sampling well and free product
well as the same well (groundwater was sampled when no free
product was encountered) with a 14 ppb to 190,000 ppb range in
benzene concentration. Monitoring well that are at a distance
from free product wells have a maximum concentration of 75,000
ppb, and a maximum distance of 152 feet, between free product
well and groundwater sampling well. Down gradient groundwater
monitoring wells in close proximity to the spill source are best
use of resources when attempting to locate free product at sites
with suspected UST LNAPL leakage. |
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Angela Dickeson, 2009,
Sources of Waters and Ion Concentrations in the Conejo Creek and
Tributaries, Ventura County, California: Committee Members: Dr.
Barry Hibbs (Committee Chair), Dr. Jennifer Garrison, Matthew Kelliher
This study was conducted to examine the sources of flows and
ions in the Conejo Creek in Thousand Oaks, Ventura County,
California. The study was conducted between August 2008 and
march 2009. Samples were collected at four events and analyzed
for chloride, sulfate, and nitrate. Selected samples were also
analyzed for isotopes of oxygen and hydrogen. The sampling
events included two events after local rains and two events
which were “dry.” The results of the study indicated that
a small portion of the water present in the Conejo Creek and its
tributaries comes from state project water, infiltrated through
the soil column and introduced into the Creek through baseflow.
The shallow groundwater discharge seen in dry months, is
augmented by rainwater recharge of shallow groundwater and
subsequent discharge to the Creek in wet months. Increased
groundwater discharge to the Creek is associated with increased
chloride and sulfate concentrations.
Creek flows are also increased by runoff from developed areas
with impervious cover , which is associated with higher nitrate
concentrations. Nutrients are loaded at various rates in
different areas of the watershed, higher in the North Fork of
Conejo Creek and lower in Indian Creek. They are attenuated as
the water makes its way down the Creek channel. |
