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

1.  Announcemtents

2.  Wetland and Infiltration Basin Performance: 8 Questions Answered --
    Dr. Alan Heyvaert

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

**Announcements:**

1.  The NV Water Resources Association released this year's journal. It
    is called the Tahoe Edition and contains studies presented at last
    years Research Symposium at the Cal-Neva. See the web link above.

2.  Call for abstracts from the Federal Interagency
    Sedimentation/Hydrologic Modeling Conference has been extended. See
    accompanying electronic attachment.

3.  Register now for the 2005 Drinking Water Forum to be held May 5 at
    IVGID Chateau.

**Wetland and Infiltration Basin Performance: 8 Questions Answered --
Dr. Alan Heyvaert**

Alan's presentation covered a wide range of topics surrounding eight
questions that were originally formulated at a CTC workshop held at the
Tallac Vista center in 2001. Most of the information related to our
current understanding of Tahoe's pollutants of concern, effectiveness of
constructed wetlands/sedimentation basins, and design features of water
quality projects. Alan's format included a presentation of his (and many
other researcher's) work and then discussion points for each of the
eight questions. The discussion points were designed to gain feedback
about the science and stimulate conversations about the implications of
the work presented. This summary will attempt to capture just a few of
the key points from each of the eight questions and mention discussion
points that inspired conversation at the meeting.

**Question #1:** centered on sources of sediment and nutrients to the
lake. Alan clearly stated that the pollutants of concern are:

-   Sediment less than 10 microns

-   Dissolved Phosphorous

-   Dissolved Nitrogen (especially Nitrate).

He also showed an updated version of the Lake Tahoe nutrient budget.
Highlights include:

-   60% of total nitrogen is from atmospheric deposition

-   40% SRP from groundwater, 33% from atmospheric deposition

-   27% of total phosphorus from atmospheric deposition, 29% from stream
    loading, and 27% from direct runoff to the lake.

In the category of direct runoff loading to the lake, primarily from
urbanized areas, it is estimated that:

-   75% of SRP load in the "direct" runoff is from rain events, and

-   60% of Nitrate and Ammonia loads in "direct" runoff is from snow
    melt

Alan also showed simple comparison tables of nutrient and TSS
[concentrations]{.underline} by land use. Highlights include:

-   Highway, Commercial, and Residential runoff all have an order of
    magnitude more TP, TN, and TSS than in rainfall or forested runoff

-   SRP not significantly elevated from background in residential areas
    (low density)

**Q1 Discussion Points:** A) Stormwater treatment projects should
emphasize phosphorus and sediment control over nitrate control in areas
of developed land use. B) Runoff from commercial areas and highways
should be treated for soluble P removal before infiltration to
groundwater. C) Strategies are needed to help reduce local atmospheric
loading of N and P.

**Question #2:** What are target particle sizes? Here Alan presented a
very interesting chart from Ted Swift's research that shows (on average)
about 20% of light scattering/absorption (clarity reduction) is caused
by pure water, 30% is caused by organic particles, and a full 50% is
caused by inorganic particles (sediment). He also showed several charts
that illustrate the settling rates of various sized particles based on
Stokes law. The charts show that there is a exponential increase in
theoretical settling time as particles get smaller.

**Q2 Discussion Points:** a) particle sizes less than about 10 microns
are especially detrimental to Lake Tahoe clarity. B) Most water quality
basins are unlikely to remove particles much smaller than 10-20 microns,
unless they provide significant runoff volume reductions.

**Question #3:** How much pretreatment is needed for basins? Alan
recommended thinking of the answer to this question from a whole
watershed perspective because no single BMP can reduce all of our
pollutants at maximum efficiency. The presentation mentions upland
pretreatment (aka source control) and describes soil stabilization,
hydrologic source control, dispersed flow routing, and
evapotranspiration. \[Editorial: Often design discussions mention
"source control" and then rapidly move on to "treatment" BMPs. I would
like to encourage more presentations at LTIMP that delve deeply into
source control.\] Alan then moved on to describe a "treatment train"
approach to water quality BMPs. He mentioned a flow path that includes
sand traps at drainage inlets, large forebays (for sediment removal) and
finally basin or wetland treatment followed by dry meadow dispersion
where available.

**Q3 Discussion Points:** A) Opportunities to enhance evapotranspiration
should be studied to reduce runoff volumes and basin sizes. B) less
infiltration advised for downgradient areas with runoff from development
near lakeshore or SEZs. C) disperse routing to reduce stormwater flow
rates and volumes.

**Question #4:** What is the optimal basin design for fine sediment and
dissolved P removal? Alan's presentation included many recommendations
taken from the literature and his own work. Highlights include:
adjustable flow control, oval configuration, maximize flow path length
for settling, avoid short circuiting, use emergent veg. for wind
control, avoid anoxic conditions, and consider vector control issues.
Numerous texts are available for standard designs.

**Q4 Discussion Points:** A) Should large flows be bypassed? Alan showed
concentration and load graphs for a large rain event that typifies our
storms. The graphs show a first flush effect for pollutant
[concentration]{.underline} but not for load. This effect was
particularly notable for SRP in this event. This example shows the
importance of treating the entire storm if load reductions are the goal.
B) What are appropriate holding times? C) Should active or passive
controls be used on basins? D) Can we manage flow seasonally?

**Question #5:** What is the performance comparison for wet vs. dry
basins? Alan showed a chart from the International BMP Database of the %
removal for several pollutants on wet vs. dry basins. Generally the wet
systems were more effective at removing pollutants. A particularly
poignant example is that dry basins were shown to be a [net
source]{.underline} of dissolved P while wetlands removed 36% of the
dissolved P. Alan also displayed a comparison of the Tahoe City Wetlands
Treatment System (TCWS) and the International BMP database that showed
roughly similar performance. In spite of the greater concentration
reductions of wet basins, dry basins can make significant changes to
stormwater pollutants because they tend to create greater reductions in
surface runoff volumes. Alan also mentioned that Tahoe's dry climate
requires a year-round water supply to grow wetland vegetation. He
suggests that wetland systems be designed to intercept shallow ground
water as a source of year-round irrigation.

**Q5 Discussion Points:** A) when are wetlands or detention basins
preferred? B) Can more biotic components be introduced to dry basins to
enhance performance?

**Question #6:** When are wetland treatment systems effective? Alan
mentioned several factors that affect performance. Seasonal factors
(including cold winters) are not likely to have much effect on P or
sediment removal, since P and sediment removal is primarily based on
physio-chemical processes (with a small amount of vegetative uptake for
P). He stated that dissolved N removal is primarily a biotic process and
that the TCWS successfully removed N throughout the year.

**Q6 Discussion Points:** A) optimal performance requires continuous,
adjustable baseflow and bypass capability. B) Can treatment wetlands be
created by intercepting groundwater to maintain a baseflow through the
summer? C) Should natural wetlands at Tahoe be used to treat stormwater?
Could snowmelt runoff rates through wetlands be regulated to increase
nitrate removal?

**Question #7:** What are the effects of infiltration basins on
groundwater? This part of the presentation began with a chart showing
concentrations of dissolved nutrients in surface water and several types
of groundwater. It showed somewhat increased concentrations of dissolved
N and P species as land use became more urbanized. Alan also mentioned
that hydraulic conductivity of the soils is the key determiner of load.
At the TCWS, the ratio of surface to groundwater flow was greater than
30:1. He estimated that groundwater advection for N and P was \<10% of
the surface water load. Alan also mentioned work at Round Hill where the
effluent from two stormwater vaults is emptying into a well constrained
groundwater system. No phosphorus breakthrough has been observed in this
groundwater system yet, although the monitoring wells are located less
than 200 feet from the stormwater discharge point.

**Q7 Discussion Points:** A) locate basins on sites that have complete
soil-vegetation-groundwater system for optimal nutrient reduction. B)
Basin substrate amendments with natural phosphorous affinity may help
adsorb dissolved P. C) Prevent anoxic conditions from forming below or
down-gradient of infiltration areas.

**Question #8:** What is the optimal maintenance schedule? Alan started
with a slide that recommended seasonal or event based inspections,
regular clearing of forebays, and yearly repair/maintenance. The
literature shows that basins offer a 10-20 year life expectancy. He also
suggested harvesting of basin vegetation in late summer and infiltration
rate monitoring to determine if soil tilling is needed. Alan also
brought up advanced chemical treatments at that time. There are
currently several efforts in the basin, including CalTrans projects and
work by Phil Bachand.

**Q8 Discussion Points:** A) on average over the long-term, wetland
loading should not exceed 2 g of P/ sq. m./ year (N should be limited to
20 g/ sq. m/ year). B) the long detention times to settle particles of
\<10 microns suggests that we look much more strongly at source control.
C) Research has shown that there is an "irreducible" concentration level
for our pollutants. BMPs should only be used in areas where reductions
are possible. D) A standardized monitoring protocol should be developed
for Tahoe BMPs. E) Establish a funded working group or committee with
appointed members that coordinate Basin-wide performance goals,
assessment monitoring and maintenance programs.

The group made many comments on Alan's presentation and additional
discussion is warranted. Alan has offered to come to back to next
month's meeting to allow for an extended discussion. Please make an
effort to attend if you saw this presentation. Many thanks to Alan for
the significant effort he put into this useful presentation.

Attendance and next agenda on next page.

In Attendance:

Chris Anderson, MacTec\*

Nicole Beck, 2^nd^ Nature Inc.

Larry Benoit, TRPA

Ivo Bergsohn, STPUD\*

Chris Bowles, PWA

Guy Burge, NTCD

Kim Carr, CTC

Scott Cecchi, TRCD

Chris Conway, MacTec

Charlie Donohue, NDSL

Robert Erlich, Lahontan

Vanessa Gallo, NDSL

Michelle Gamble, RCI

Jon-Paul Harries, TRPA

Mary Horvath, Wood Rodgers

Roger James, WRM Inc.

Alfred Knotts, EDOT

Jack Landy, Lahontan\*

Ruth Page, Homeowner

Mark Palmer, HSWL

Chad Praul, NTCD\*

Kansas McGahan, Placer Co.

Amy Roberts, NTCD

Tim Rowe, USGS

Jane Schmidt, NRCS

Carol Schupbach, Entrix

Andy Smith, WET Lab

John Stanley, TRPA

Dan Sussman, League

Chuck Taylor, NRCS\*

Matt Vitale, NTCD

Brent Wolfe, CTC\*

Rita Witney, TRPA\*

David Zander, CTC

Drea Traeumer, Kennedy-Jenks

Leslie Allen, LTEEC

Shawn Beauduy, TRCD

Jason Drew, NTCD

Lance Bell, WET Lab

Next Agenda: