Nonpoint Source Success Story Georgia Proactive Approach Improves St. Marys River and Promotes Green Infrastructure in Tidal Estuary

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I NONPOINT SOURCE SUCCESS STURY

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Proactive Approach Improves St. Marys River and Promotes Green
Infrastructure in Tidal Estuary

Waterbody Improved The CityofStl Marys in Camden County, Georgia, installed green

infrastructure (Gl) in two phases to infiltrate and treat polluted
stormwater runoff flowing directly to the St. Marys River from a highly impervious urban coastal
riverfront landscape. This project demonstrated Gl effectiveness to mitigate nuisance flooding, sea
level rise, and low dissolved oxygen (DO) in the coastal environment (flat topography, shallow water
tables, sandy underlying soils, and tidal influence). A comparison of pre- and post-installation storm
sampling showed reduction in sediment, total nitrogen, and total phosphorus loads and higher
monthly DO averages for six out of 12 months, indicating a measured improvement in water quality.

Problem

The Council on Environmental Quality's draft Climate
and Economic Justice Screening Tool identifies census
tracts in the City of St. Marys as overburdened by
pollution and suffering economic loss rate to build-
ing value caused by natural hazards such as annual
flooding. The St. Marys River is a coastal river that
discharges into an ocean estuary. A six-mile segment
(Catfish Creek to Millers Branch) was placed on
Georgia's 2004 Integrated 305(b)/303(d) List of Coastal
Streams as "not supporting" the designated use of
fishing due to low DO caused by urban runoff from
impervious surfaces that decrease base flow and send
oxygen-demanding pollutants into the river. Another
six-mile segment of the St. Marys River (Millers Branch
to Burrells Creek) was moved from "supporting"
status between 2012 and 2014 and remains listed
as "assessment pending" for DO while the Georgia
Environmental Protection Division (EPD) determines
the natural DO for the area.

A total maximum daily load (TMDL) revised in August
2017 for the Catfish Creek to Millers Branch segment
imits pollutant load allocations from nonpoirit
sources, expressed as Ultimate Oxygen Demand, to
2,685 pounds (lbs) per day to help meet Georgia's
fishing DO standard of a daily average of 5.0 milligrams
per liter (mg/L) and not less than 4.0 mg/L. The 2003
water quality data from EPD Station #08011021 on the
St. Marys River at Interstate 95 (1-95) show that low DO
usually occurred during the summer months.

Figure 1. Rainwater collects in front of City Hall (on
Osborne Street) before project construction.

Story Highlights

The city received Clean Water Act section 319(h) grant
funds in 2017 and 2018 to implement Phases 1 and 2
of the "Coastal Urban Stormwater BMP Retrofits using
Gl/LID Project." The city installed bioretention systems
and permeable interlocking pavers along St. Marys and
Osborne streets, two Historic Downtown areas prone
to flooding (Figure 1).

During Phase 1, 74 parking stalls of permeable pavers
(15,885 square feet) and 18 individual bioretention
cells (6,210 square feet) captured rain draining from
2.50 acres onto St. Marys Street within 250 feet of the
St. Marys River (Figure 2). Outfalls from the Gl dis-
charging downstream of the "not supporting" segment
become "upstream" during incoming tides. Evaluating
Gl performance at St. Marys Street, a site impacted by

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coastal constraints (shallow water tables, tidal influences),
indicated the need for tide control measures (check
valves, tide fiaps) on the outfall pipes to limit the back-
flow of river water into the subsurface storage layer.

Rain draining from 0.75 acres onto Osborne Street
was captured during Phase 2 with 20 parking stalls of
permeable pavers (3,465 square feet) and two biore-
tention ceils (1,010 square feet) in front of City Hall
(Figure 3). Infiltrated stormwater was fully stored and
treated on site due to no underdrains, a feasible design
because of a deeper water table and no tidal influx.

Like the Phase 1 project area, the eventual outfall is
downstream of the "not supporting" segment but
becomes "upstream" during incoming tides. Gl at the
City Hall site proved to infiltrate and remove pollutants
at high levels, serving as a measurable example of Gl
for future development.

While not ready to adopt performance standards from
the Georgia Stormwater Management Manual Coastal
Stormwater Supplement (CSS), the city proposed to refer-
ence infiltration design and performance standards from
the CSS in future updates for new or redevelopment.
Operations and maintenance factsheets and inspection
checklists for the Gl installations were assembled for
the city's Public Works Department staff. Project part-
ners disseminated research results and lessons learned
through education and outreach activities.

Results

Before this project, only one Gl system had been
built in Camden County. Consequently, an essential
element of this project was to demonstrate, monitor,
and compare Gl performance at the two sites.

Water quality data from St. Marys Street showed Gl
performance was impacted by coastal constraints
(shallow water tabies, tidai influences) compared to
Osborne Street conditions (deeper water tables, sandy
soils) where Gl performed as an effective solution.
Combined post-instaliation hydrology and water
quality was monitored over 12 months at both sites
(October 2020-September 2021). Data showed a total
of 2.34 million gallons (66% of 3.55 million gallons) of
runoff Infiltrated. In addition, 2,419 lbs (81% of 2,989.7
lbs) of sediment, 41 lbs (77% of 53.6 ibs) of total nitro-
gen, and 5.73 Ibs (70% of 8.16 Ibs) of total phosphorus
from runoff were removed. Post-instaiiation testing of
DO during July 2020-August 2021 showed the average
monthly DO concentration was 0.73 mg/L higher
during November 2020-January 2021 and 0.47 mg/L

U.S. Environmental Protection Agency

0** Office of Water
\ Washington, DC

,EPA 841-F-22-001KK
pRol^ December 2022

Figure 2. A bioretention system at "Market on Square"
reaches capacity and overflows onto permeable
pavement (St. Marys Street).

Figure 3. A full bioretention practice near City Hall
(Osborne Street) with monitoring wells in the foreground.

higher during April-June 2021. These results indicated
water quality improvement even though DO was lower
for six out of the 12 months. The impaired segment
remains listed. Design adjustments to fit the more
challenging coastal conditions as well as wider imple-
mentation will improve future Gl performance.

Partners and Funding

The Phase I total cost of $895,190 consisted of
$397,133 In 319(h) grant funds and $498,057 in match.
Local stakeholders serving on the stakeholder com-
mittee contributed 76.5 hours of volunteer match
totaling $1,866. Phase 2 totaled $241,213 with
$103,600 in 319(h) grant funds and $137,613 in match.
Combined Phases 1 and 2 match sources included
$291,218 In state funds from a Georgia Department of
Transportation Local Maintenance and Improvement
Grant and $342,586 through the City of St. Marys
General Fund and as In-kind contributions from city
staff. U.S. Geological Survey maintained instruments at
their 1-95 gaging station for post-project monitoring of
DO in the St. Marys River.

For additional information contact:

Joy Hinkle

Georgia Environmental Protection Division
470-524-0692 • joy.hinklel@dnr.ga.gov

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