Using Stable d15N Ratios to Examine Sources of Nitrogen within a Residential Community Nora Egan Demers Department of Biological Sciences, Florida Gulf Coast University, Ft. Myers, Florida, USA

1
Figure 1. Aerial image showing surface drainage
and water flow patterns for San Carlos Park, Lee
County Florida. Approximate locations of homes
with septic tanks are indicated by blue
highlighting.
Using Stable d15N Ratios to Examine Sources of
Nitrogen within a Residential CommunityNora Egan
Demers Department of Biological Sciences,
Florida Gulf Coast University, Ft. Myers,
Florida, USA
Abstract I report on a spatial study of stable
nitrogen isotope ratios in the residential
community of San Carlos Park, Lee County, FL, USA
focusing on determining the influences of various
anthropogenic nutrients (mainly septic and
fertilizer) to the outstanding Florida waterway
that leads to the Estero Bay. The study
benchmarks the water quality of the storm-water
drainage system in this community of nearly 8000
single-family and duplex homes (developed before
most storm-water treatment requirements). The
work is the first step in a multi-year
restoration effort undertaken in partnership with
local government, civic organizations and other
participants. This project occurred during the
summer and fall of 2007, and early spring of
2008. Results support using stable nitrogen
isotopes to help distinguish anthropogenic
sources of nitrogen pollutants, but not on the
spatial scale of individual lots.
Lake at Golf course that takes effluent from
water treatment facility- note green hue to water
Littoral Islands placed to help remove nutrients
from San Carlos Park community
Sediment samples stable isotope ratios Sediment samples stable isotope ratios
location d15N
Bird bath algae -1.54
Lake surrounded by homes on sewer in Park 1.05
"Pristine" creek (Telegraph) 1.17
NE corner of Park (septic tank area) 1.87
Behind home with septic on side canal 2.5
residential community canal with no septic influence 2.69
Agricultural creek (Spanish Creek) 4.11
main flowway downstream of golf course 5.07
main flowway downstream of golf course- nearby homes on city sewer 5.37
outfall of community 7.35
Golf course lake- no effluent 7.81
Golf Course effluent lake 17.8

• Contributions to excess nutrients include
• the apparently innate American desire for green
  lawns causes us to add too many nutrients and
  herbicides to our lawns, the excess of which is
  being transported into the failing storm-water
  system, and
• over 85 of the residences use septic tanks.
• A confounding factor is that the local community
  golf course receives the treated wastewater from
  the wastewater treatment plant and uses that
  water to irrigate the golf course- (to the tune
  of greater than 50,000 gallons per day,) a common
  practice in our region.

• Stable nitrogen isotope research has been firmly
  established as an effective means to distinguish
  the various sources of nitrate contaminants in
  terrestrial soils and aquatic environments
  through examination of water, sediments, plants,
  and various consumers. Stable isotope research
  is based on the fact that the ratios of 14N and
  15N occur overall on earth in a stable and fixed
  proportion of approximately 273 14N atoms for
  each 15N atom, (gt99.6 14N) while the d15N ratio
  among specific pools (wastewater, fertilizer and
  atmospheric nitrogen) in the environment differs
  (Peterson and Fry 1987).
•  
• d15N R(sample)-R(standard)/R(standard)1000
• where R ratio of 15N/14N and R standard is the
  atmospheric standard.
•  
• Generally, Nitrogen from
• fertilizers has an d15N around 0- 4 ppt (parts
  per thousand) (Heaton, 1986),
• septic systems and animal waste has a d15N from
  8 to 22 ppt. (Heaton, 1986).
• atmospheric depositions are about 2 to 8.00 ppt
  (Kreitler et al, 1978).

• San Carlos Park
• (approximately 2,366 acres) (Section 46, Township
  25, Range 08)
• single-family, residential community constructed
  in the 1960s, prior to current storm-water
  treatment requirements.
• storm-water runoff from the San Carlos Park
  community is funneled to a single outflow system
  (Lee County monitoring site 46B-9GR), into the
  Mullock Creek headwaters.
• Mullock Creek is on the Florida Department of
  Environmental Protections impaired waters list
  for dissolved oxygen and Chlorophyll a. Mullock
  Creek flows directly into Estero Bay, an
  Outstanding Florida Water, west of U.S. 41.

Aerial image showing surface drainage and water
flow patterns for San Carlos Park, Lee County
Florida. Approximate locations of homes with
septic tanks are indicated by blue highlighting.
Canal sampling site at NE corner of community
Acknowledgements