**[Topic List:]{.underline}**

1.  Detention Basin Treatment of Hydrocarbon Compounds in Urban
    Stormwater - Nicole Beck

2.  Preliminary TMDL Lake Tahoe Clarity Model results - Dave Roberts

**[Topic Summaries:]{.underline}**

**Detention Basin Treatment of Hydrocarbon Compounds in Urban
Stormwater** - Nicole Beck

Nicole gave a presentation on the South Tahoe Public Utility District
and Forest Service funded study which investigated the fate of
hydrocarbon contaminants in storm water runoff infiltrated by detention
basins. In particular, storm water and ground water samples were
analyzed for: methyl tert-butyl ether (MtBE), benzene, toluene, xylene,
ethyl benzene, total petroleum hydrocarbon in the gasoline range
(TPH-gas), TPH-diesel, total extractable hydrocarbons (TEH), total
extractable petroleum hydrocarbons (TEPH), and grease and oil. Two out
of 15 basins examined were selected for monitoring based on potential
for storm water hydrocarbon sources, ease of instrumentation, and lack
of known ground water contamination. The City of South Lake Tahoe's
Eloise Basin (130 acre watershed, residential and commercial land uses,
estimated 70% impervious) and Industrial Basin (53 acre watershed,
industrial and undeveloped uses, estimated 35% impervious) were selected
for monitoring. Data was collected to support basin water balances
(basin topography, basin soil core samples, and basin water elevations),
runoff event and basin storage water quality assessment (1^st^ flush
passive Thurston samplers, and grab samples from basins and at inlets),
and understanding weather event response (precipitation and air
temperature). Groundwater was monitored for elevation (continuous depth
and spot measurements) and water quality (4" elevation increases
triggered event sampling) with monitoring well networks. Dual (or
paired) monitoring wells were used in some locations to provide better
characterization through the depth of the soil profile with shorter
screen lengths. The basin's physical properties were characterized with
surface area, maximum and minimum flow paths, storage capacity, and
wetted perimeter, as well as soil stratigraphy based on soil cores.
Ground water elevation monitoring revealed localized mounding below the
basins during periods of inundation. This effect was more prominent for
the smaller Eloise Basin that was at capacity for 158 days during the
study (Oct 2003 50 Dec 2005) versus only 5 days at capacity for the
Industrial basin. The shallower screened monitoring wells in the berm of
Eloise basin actually showed that ground water elevation exceeded the
elevation of the basin bottom for several months of each water year from
mid January to May. Upgradient wells confirmed that the ground water
elevation mounding was a localized impact of the infiltration basins in
addition to the regional seasonal fluctuations during periods of
groundwater recharge. A total of nine storm events were sampled during
the study period from Oct 2003 to Dec 2005, some for ground water and
all for surface water for at total of 41 stormwater samples and 77
groundwater samples. Of the hydrocarbons analyzed, TEPH and TEP- diesel
were detected most in surface storm water, with 86% of the surface water
samples exceeding the Lahontan RWQCB standard for TPH-diesel (100 ppb).
Only trace amounts of toluene and xylene were detected in surface storm
water at a 20 % frequency, and no benzene, ethylbenzene, or fuel
oxygenates (including MtBE) were detected in stormwater or groundwater.
No petroleum hydrocarbon constituents were detected in Industrial Basin
groundwater, and only trace levels of toluene, xylene, oil and grease
were found in a cross gradient well installed for a nearby ground water
contamination case at Eloise Basin. The study conclusion is that soil
interaction is an effective means to treat low-level hydrocarbons in
stormwater, but that dry wells may pose a risk. Based on the study 2^ND^
NATURE recommends that soil/water interactions can be maximized through
shallow basins with large surface area, lengthy flow paths, long
hydraulic residence times, greater wetted perimeter, and increased depth
to groundwater. It is also recommended that future erosion control
project development utilize distributed infiltration features to avoid
excessive stormwater infiltration in one area. Some monitoring was also
done for nutrients in the basin storm water and ground water and the
data suggests that pulses of nutrients are delivered to ground water
from basins during periods of inundation when there is little to no
separation between the basin bottom and ground water elevations. The
study has been documented is a report (title same as presentation) that
was prepared for South Tahoe Public Utility District. Nicole Beck can be
contacted at: [nbeck@2ndnatureinc.com](../nbeck@2ndnatureinc.com) or
(831) 426-9119.

**Preliminary TMDL Lake Tahoe Clarity Model results** - Dave Roberts

Dave presented the preliminary Lake Tahoe Clarity Model results which
represent the culmination of five years of TMDL work by Lahontan RWQCB,
UC Davis Tahoe Environmental Research Center, and others and the final
product of Phase I (Technical TMDL) TMDL delivery phase. The clarity
model receives input from the watershed model, which has been calibrated
and validated by stream monitoring data, to determine the impacts of all
sediment and nutrient sources on the lake clarity. The previously
presented updated Lake Tahoe Pollutant Budget informs both the watershed
model directly (for upland, stream channel erosion) and the clarity
model both directly (for shoreline erosion, atmospheric deposition, and
groundwater) and indirectly (through watershed model). The clarity model
is a hydrodynamic/thermodynamic process based numerical model made up of
a series of models made to simulate processes in nature including: water
quality (ecological) model, particle fate model, and optical model.
These models take into account thermal stratifications, wind mixing,
stream inflow insertion depth, and evaporation as well as nutrient
cycling, particle dynamics, algal growth, and light absorption and
scattering. The adequacy of the clarity model has been examined with
testing against real data and by extensive peer review. Uncertainties
can be related to model parameters that are not known with sufficient
confidence. The Tahoe clarity model has 31 parameters that have been
selected and refined based on earlier calibration and validation phase.
Some of the parameters that are uncertain and possibly in error include:
lake inflow, outflow, ground water contributions, meteorology, and
atmospheric deposition. The clarity model was calibrated based on a
5-year period from1999 to 2004 and compared well to measured secchi data
for that same period. A sensitivity analysis showed that the following
parameters, in the order given, demonstrate and increasing sensitivity,
a wider range of predicted secchi depths, when increased or decreased by
50 %: light absorption, atmospheric loads, and particle settling rate.
The clarity model was then used to predict the response to various
combinations of instantaneous and incremental reductions in sediment and
nutrient loading, which lead to the preliminary finding that a 35%
reduction in all fines and nutrients from all sources (by incremental
reductions of 1.75% a year for 20 years) would achieve the water clarity
goal of 30 meters by the year 2020. While the clarity response predicted
by the model may seem too quick, actual average annual secchi depth
measurements have shown similar quick improvements of increases of 3
meters in two years. The results of the Lake Tahoe Clarity Model have
not been documented in a report yet, but you can find the Pathway 2007
Forum presentation at:
<http://www.waterboards.ca.gov/lahontan/TMDL/Tahoe/clarity_presentation2fForum_27july06.pdf>.
The next steps in Phase II of the TMDL development are to develop an
Integrated Water Quality Management Strategy, identify load reduction
possibilities, allocate pollution load reductions, and develop an
implementation and monitoring plan. You can contact Dave Roberts at:
<DRoberts@waterboards.ca.gov> or (530) 542-5469.

In Attendance:

Nancy Alvarez, USGS- WRD\*

Steve Baglin, Wood Rodgers

Nicole Beck, 2^ND^ NATURE

Scott Brown, NTCD\*

Jim Harris, USFS

Alan Heyvaert, DRI\*

Roger James, WR Management

Alfred Knotts, EDOT

Jack Landy, EPA/Lahontan RWQCB

Theresa Loupe, USFS

Chad Praul, EI

Dave Rios, NTCD\*

Dave Roberts, Lahontan

Catherine Schoen, USFS\*

Andy Smith, WET Lab\*

Chris Twomey, c2me\*

Rita Whitney, TRPA\*

Next Agenda\*: