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i&j) NONPOINTSOIREESICEESSSTORY
Multiple Projects Improve Water Quality in Lake Rebecca
Waterbody Improvsd '3eau^^u' '_a'ce Rekecca Park Reserve offers opportunities
for outdoor fun and is a haven for wildlife, including trumpeter
swans. Elevated phosphorus in Lake Rebecca led to frequent algae blooms during the summer
months, which caused low dissolved oxygen levels, poor water clarity, and fish kills. It also affected
the lake's recreational use. As a result, the Minnesota Pollution Control Agency (MPCA) added Lake
Rebecca to the Clean Water Act (CWA) section 303(d) list in 2008. The Three Rivers Park District
(TRPD) led efforts to apply alum to the lake and implement projects to control the invasive curly-leaf
pondweed, treat stormwater from developed areas, and improve a nearby livestock operation's
management of manure and stormwater. Phosphorus levels dropped, and MPCA removed Lake
Rebecca from the impaired waters list in 2018.
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Problem
Lake Rebecca is just west of Minneapolis in the Lake
Rebecca Park Reserve, which is owned and managed
by the TRPD (Figure 1). The lake offers many recre-
ational resources such as fishing piers, boat access,
swimming area and beach areas. Lake Rebecca is also
surrounded by pedestrian hiking trails, paved and
single track biking trails, horse trails, picnic areas,
piay areas, and an off-leash dog park. The iake is of
high recreational Importance to the TRPD. The iake
is frequently monitored for chlorophyli-o (Chl-o),
total phosphorus (TP) and Secchi disk transparency.
The Lake Rebecca Park Reserve is aiso a haven for
wiidiife, Including free-flying trumpeter swans, which
can be seen In the park year-round. Several hundred
swans gather there during late fail. The Lake Rebecca
Park Reserve is part of the TRPD's Trumpeter Swan
Restoration Program.
Lake Rebecca experienced frequent algae blooms
during the summer months. This decreased oxygen
in the water column and contributed to poor water
clarity, caused fish kills, and impacted other recreation
such as swimming. In 2008, average TP concentrations
were 74 micrograms per liter (|ig/L), Chl-o was 44 |_ig/L
and Secchi depth was 1.26 meters (m). Since 1994, the
highest annual average concentrations of TP and Chl-o
and lowest transparency were all observed in 2007,
with TP concentrations recorded at 130 |ig/L, Chl-o
at 97.9 |ig/L, and Secchi at 0.6 meters. These failed to
meet the applicable state water quality standards for
nutrient/eutrophication biological indicators requiring
Figure 1. Lake Rebecca is in the South Fork Crow River
watershed in eastern Minnesota.
that TP is less than 40 |ig/L, Chl-o is less than 14 [ig/L,
and Secchi depth is greater than 1.4 m Secchi (for deep
lakes in the North Central Hardwood Forest Ecoregion
[NCHF] ecoregion). As a result, MPCA added Lake
Rebecca to the impaired waters list in 2008.
Legend
~ Lake Rebecca Drainage
HUC8 Watersheds
	| Minnesota County Boundaries
Lake Rebecca Drainage Area

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Sources of excess phosphorous loading to the lake
included landscape contributions and internal loading
from lake sediments. A 93-acre horse farm outside
of the park was identified as a significant source of
external phosphorus loading. Lake sediment core data
collected in 2010 was used to determine the mass of
potentially releasable mobile phosphorus from the
lake sediment—this was used for alum treatment
dosing calculations. Compared to other lakes in the
Twin Cities metropolitan area, the mobile phosphorous
fraction in Lake Rebecca was considered high. The
presence of invasive curly-leaf pondweed also pre-
sented challenges for management and water quality.
Story Highlights
In 2009, TRPD staff launched efforts to control the
invasive curly-ieaf pondweed. TRPD staff worked with
Hennepin County Environmental Services to improve
manure and stormwater management on the nearby
horse farm. TRPD also addressed stormwater from
the park's developed areas. The most critical project
component was the application of aluminum sulfate
(aium) to the lake to reduce internal cycling of phos-
phorus from bottom sediments. TRPD staff worked
with contractors to apply alum to the lake in fall 2010
and spring 2011 (Figure 2). In each case, the alum was
applied at a dose of approximately 740 gallons per acre
over about 220 acres of the 258-acre lake (total dosage
rate of about 1,480 gallons/acre). The Rebecca Lake
restoration initiative was largely completed in 2011.
Results
Rebecca Lake has generally met standards since
completion of the restoration effort (Table 1). Recent
summer TP averages have been 25-40 [ig/L since
2011 (Figure 3). The anticipated effects of the alum
treatment should last 10-15 years. Lake Rebecca was
removed from Minnesota's 303(d) impaired waters list
in 2018. Lake Rebecca was the first lake in the TRPD to
achieve delisting for nutrients.
Partners and Funding
Project partners included TRPD, Hennepin
County Environmental Services, Pioneer Sarah
Creek Watershed Management Commission, the
Metropolitan Council, and the Minnesota Department
of Natural Resources. TRPD was awarded a
Figure 2. Alum remains suspended in Lake Rebecca's
waters shortly after application.
Table 1. Lake Rebecca data overtime.
Description
TP
Chl-a
Sccchi
ug/L
Hg/L
m
NCHF: Trophic state
thresholds for impairment
(pre-2010 cycle)
<45
< 18
> i.i
NCHF: Aquatic recreation
use standard (Class 2B)
<40
< 14
> 1.4
1994-2005 Lake Rebecca
data (original listing dataset)
81
46.7
1.2
2012-2017 Lake Rebecca
data (delisting dataset)
33
14.8
1.8
TP and Chl-a Mean
—Secchi	
¦Chl-a Trendiine
1994 1995 1996 1999 2001 2002 2004 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
Figure 3. Lake Rebecca annual water quality results
(1994-2017).
Lessard-Sams Outdoor Heritage Fund (LSOHF) grant
of $450,000 in 2009 to implement the water quality
improvement projects for Lake Rebecca, including a
shoreline restoration project, a multi-year curly leaf
pondweed control project, and an alum treatment.
The total cost of the LSOHF project was estimated
at $480,000 with the TRPD providing an in-kind
service match valued at $30,000. LSOHF funds are
provided through the Clean Water, Land and Legacy
Amendment to the to the Minnesota Constitution.
PROl*°
2
o
U.S. Environmental Protection Agency
Office of Water
Washington, DC
EPA 841-F-21-001B
January 2021
For additional information contact:
Brian Vlach
Three Rivers Park District
763-694-7846
Eric Alms
Minnesota Pollution Control Agency
651-757-2589 • eric.alms@state.mn.us

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