TOTAL MAXIMUM DAILY LOAD (TMDL)
For
Fecal Coliforms
In
North Fork St. Lucie River
(WBID3194)
Prepared by:
US EPA Region 4
61 Forsyth Street SW
Atlanta, Georgia 30303
October 2008
vvEPA
United Slates
Environmental Protection
Agency
In compliance with the provisions of the Federal Clean Water Act, 33 U.S.C §1251 et. seq., as
amended by the Water Quality Act of 1987, P.L. 400-4, the U.S. Environmental Protection
-------
Agency is hereby establishing a Total Maximum Daily Load (TMDL) for fecal coliforms in the
North Fork St. Lucie River (WBID 3194). Subsequent actions must be consistent with this
TMDL.
James D. Giattina Date
Director
Water Management Division
11
-------
-------
TABLE OF CONTENTS
1. INTRODUCTION 1
2. PROBLEM DEFINITION 4
3. WATERSHED DESCRIPTION 4
4. WATER QUALITY STANDARD AND TARGET IDENTIFICATION 8
5. WATER QUALITY ASSESSMENT AND DEVIATION FROM TARGET 8
6. SOURCE ASSESSMENT 12
6.1. Point Sources 13
6.2. Nonpoint Sources 13
6.3. Wildlife 13
6.4. Agricultural Animals 13
6.5. Onsite Sewerage Treatment and Disposal Systems (Septic Tanks) 14
6.6. Urban Development 14
7. ANALYTICAL APPROACH 15
7.1. Percent Reduction Approach for TMDL Development 15
8. DEVELOPMENT OF TOTAL MAXIMUM DAILY LOADS 16
8.1. Critical Conditions 17
8.2. Margin of Safety 17
8.3. Determination of TMDLComponents 17
8.4. Waste Load Allocations 17
8.5. Load Allocations 18
8.6. Seasonal Variation 18
8.7. Recommendations 18
REFERENCES 19
APPENDIX A: Water Quality Data 20
-------
LIST OF TABLES
Table 1: Land Cover Distribution for WBID 3194 in acres and percentage 8
Table 2: Monitoring Stations used in the Development of this Fecal Coliform TMDL 11
Table 3. Summary of Fecal Coliform Monitoring Data in WBID 3194 11
Table 4. Livestock Inventory by County (source: USDA, 2002) 14
Table 5. County Estimates of Septic Tank Installations 14
Table 6. Summary of TMDL Components 17
Table 7: Guide to Water Quality Remark Codes (Rcode column in data tables) 21
Table 8. Water Quality Data and Percentile Calculations 21
LIST OF FIGURES
Figure 1: FDEP Group 2 River Basins 2
Figure 2: St. Lucie / Loxahatchee River Basin 3
Figure 3: North St. Lucie Planning Unit 7
Figure 4. Sampling stations in WBID 3194 10
Figure 5: Fecal Coliform and Rainfall 12
in
-------
LIST OF ABBREVIATIONS
AWT Advanced Waste Treatment
BMP Best Management Practices
BPJ Best Professional Judgment
CFS Cubic Feet per Second
CWA Clean Water Act
DMR Discharge Monitoring Report
EPA Environmental Protection Agency
F.A.C. Florida Administrative Code
GIS Geographic Information System
HUC Hydrologic Unit Code
LA Load Allocation
MGD Million Gallons per Day
MOS Margin of Safety
MPN Most Probable Number
MS4 Municipal Separate Storm Sewer Systems
NASS National Agriculture Statistics Service
NLCD National Land Cover Data
NPDES National Pollutant Discharge Elimination System
NRCS Natural Resources Conservation Service
OSTD Onsite Sewer Treatment and Disposal Systems
PLRG Pollutant Load Reduction Goal
Rf3 Reach File 3
RM River Mile
STORET STORage RETrieval database
TMDL Total Maximum Daily Load
USD A United States Department of Agriculture
USGS United States Geological Survey
WBID Water Body Identification
WLA Waste Load Allocation
WMP Water Management Plan
WWTF Wastewater Treatment Facility
IV
-------
SUMMARY SHEET
Total Maximum Daily Load (TMDL)
1. 303(d) Listed Waterbody Information
State: Florida
Major River Basin: St. Lucie River Basin
Impaired Waterbodies for TMDLs (1998 303(d) List):
WBID
3194
Segment Name and Type
North Fork St. Lucie River
River Basin
St. Lucie
County
St. Lucie
Constituent(s)
Fecal Coliform
2. TMDL Endpoints (i.e., Targets) for Class III Waters (fresh and marine):
Fecal Coliform: Not to exceed 400 MPN/lOOmL in more than 10 percent of the samples and not
to exceed 800 MPN/lOOmL in any one sample. Insufficient data were collected to evaluate the
chronic fecal coliform criterion (i.e., 200 MPN/lOOmL expressed as geometric mean of 10 or
more samples collected in a 30-day period).
3. Fecal Coliform Allocation:
WBID
3194
W L Awastewater
N/A
WLAMs4
67 percent
reduction
LA
67 percent
reduction
Margin of Safety
Implicit
TMDL
67 percent
reduction
Note: N/A is an abbreviation for "not applicable"
4 Endangered Species (yes or blank): Yes
5 EPA Lead on TMDL (EPA or blank): EPA
6. TMDL Considers Point Source, Nonpoint Source, or both: Both
7. Major NPDES Discharges to surface waters addressed in TMDLs:
NPDES Number
FLR04E013
FLR04E031
FLR04E044
FLR04E065
FLR04E001
FLR04E029
Name
Martin County
City of Stuart
Sewall's Point
Fort Pierce
Port St. Lucie
St. Lucie County
Note: MS4 permittees will only be responsible for reducing the anthropogenic loads associated
with stormwater outfalls they own or otherwise has responsible control over.
-------
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
TOTAL MAXIMUM DAILY LOAD (TMDL)
FECAL COLIFORM IN NORTH FORK ST. LUCIE RIVER (WBID 3194)
1. INTRODUCTION
Section 303(d) of the Clean Water Act (CWA) requires each state to list those waters within its
boundaries for which technology based effluent limitations are not stringent enough to protect
any water quality standard applicable to such waters. Listed waters are prioritized with respect to
designated use classifications and the severity of pollution. In accordance with this
prioritization, states are required to develop Total Maximum Daily Loads (TMDLs) for those
water bodies that are not meeting water quality standards. The TMDL process establishes the
allowable loadings of pollutants or other quantifiable parameters for a waterbody based on the
relationship between pollution sources and in-stream water quality conditions, so that states can
establish water quality based controls to reduce pollution from both point and nonpoint sources
and restore and maintain the quality of their water resources (USEPA, 1991).
The Florida Department of Environmental Protection (FDEP) developed a statewide, watershed-
based approach to water resource management. Under the watershed management approach,
water resources are managed on the basis of natural boundaries, such as river basins, rather than
political boundaries. The watershed management approach is the framework FDEP uses for
implementing TMDLs. The state's 52 basins are divided into 5 groups. Water quality is assessed
in each group on a rotating five-year cycle. The Group 2 basin is shown in Figure 1 and includes
the St. Lucie and Loxahatchee River Basin. The St. Lucie and Loxahatchee River Basin
encompasses many square miles. To provide a smaller-scale geographic basis for assessing,
reporting, and documenting water quality improvement projects, the FDEP subdivided the Group
2 area into smaller areas called planning units. Planning units help organize information and
management strategies around prominent subbasin characteristics and drainage features. To the
extent possible, planning units were chosen to reflect subbasins that had previously been defined
by the South Florida Water Management District (SFWMD). The St. Lucie and Loxahatchee
River Basin contains eight planning units: C-25/Basin 1, North St. Lucie, C-24, C-23, South St.
Lucie, C-44, Loxahatchee and Coastal. Water quality assessments were conducted on individual
waterbody segments within planning units. Each waterbody segment is assigned a unique
waterbody identification (WBID) number. Waterbody segments are the assessment units or
polygons that have historically been used by the FDEP to define waterbodies in their biannual
inventory and reporting of water quality to EPA under Section 305(b) of the federal Clean Water
Act. The same WBIDs are also the assessment units identified in the FDEP's biannual lists of
impaired waters submitted to EPA as part of their reporting under Section 303(d) of the CWA.
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Apalachicola -Chipol
Mid die St. Johnsj
Tampa Bay TributariesL
I V
| Charlotte Harbor |
St. Lucia - Loxahatehe
Figure 1: FDEP Group 2 River Basins
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
St. Lucie - Loxahatchee
Group 2 Verified Listed
Waters
• Interstates
• US Routes
Major Rivers (lines)
Major Rivers
j 1998303d
I Planning Units
County Boundaries
I WBID
TMDL by Year
| 12005
I [2010
Figure 2: St. Lucie / Loxahatchee River Basin
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
2. PROBLEM DEFINITION
Florida's final 1998 Section 303(d) list identified WBID 3194 in the St. Lucie River Basin as not
supporting water quality standards (WQS) due to coliform bacteria. After assessing all readily
available water quality data, EPA is responsible for developing a fecal coliform TMDL in WBID
3194, North St. Lucie. The location of WBID 3194 is shown in Figure 2. The TMDL addressed
in this document are being established pursuant to EPA commitments in the 1998 Consent
Decree in the Florida TMDL lawsuit (Florida Wildlife Federation, et al. v. Carol Browner, et al.,
Civil Action No. 4: 98CV356-WS, 1998).
WBID 3194 is designated as a Class III marine water. The designated use of Class III waters is
recreation, propagation and maintenance of a healthy, well-balanced population offish and
wildlife. Class III waters are further categorized based on fresh or marine waters. Water quality
criteria for fecal and total coliform do not vary between Class III fresh or marine waters.
3. WATERSHED DESCRIPTION
As discussed in the introduction, FDEP manages water resources based on river basins. The
river basins are organized from large groups of major river basins to smaller watersheds called
planning units, and finally to small waterbody polygons called WBIDs. The St. Lucie River
Basin, North St. Lucie Planning Unit, and North St. Lucie WBID are described in the following
text. The following information is from the St. Lucie and Loxahatchee Basin Status Report
(FDEP, 2003).
In the St. Lucie Basin, most of the land in the non-coastal areas is used for the production of
citrus and beef cattle. The extensive network of canals that drains these agricultural areas
transports stormwater runoff containing nutrients, sediment, bacteria, and other pollutants. These
pollutants reach the natural drainage-ways (such as the North and South Forks of the St. Lucie
River) and ultimately the St. Lucie Estuary and the South Indian River Lagoon. The St. Lucie
Canal (C-44), the inland waterway that connects Lake Okeechobee to Florida's east coast,
transports regulated releases of water from Lake Okeechobee and runoff from agricultural areas
within the C-44 basin. Other major canals also transport stormwater from inland agricultural
areas to the estuary. Canals C-23 and C-24 discharge water into the North Fork of the St. Lucie
River and the C-25 Canal discharges to the Indian River Lagoon. These canals transport loads of
nutrients and eroded sediment to the estuary and slugs of fresh water that create fluctuations in
estuarine salinity levels. Urban and residential areas continue to expand in the coastal areas, with
polluted urban stormwater runoff and seepage from septic tanks also contributing to the water
quality problems in streams and canals. As a result, parts of the St. Lucie Estuary (SLE) appear
to be impaired by nutrients, copper, and low levels of DO. Nutrient loads, salinity fluctuations,
and accumulations of sediment stress the estuarine ecology. Other evidence of impairment was
gathered for the SLE segments in a FDEP South East District biological survey (Graves et al.,
2002). Sediment accumulation, decline of sea-grasses and oysters, algal blooms, fish kills, and
low diversity of benthic macroinvertebrates in the SLE comprise this body of evidence.
WBID 3194 is in the North St. Lucie planning unit of the St. Lucie Basin. It extends from Ft.
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Pierce Inlet to the St. Lucie Inlet and westward to the C-24 Canal. Historically it has drained
naturally into the St. Lucie Estuary and includes the North Fork St. Lucie River and its main
tributaries, Tenmile Creek, and Fivemile Creek. WBID 3194 is 48 percent residential as shown
in Table 1 which is based on a 1999 coverage provided by the SFWMD. The planning unit also
includes the North St. Lucie Water Control District, located in the northern part where drainage is
to Tenmile Creek, C-25 Canal, and the C-24 Canal. The North St. Lucie planning unit is located
in eastern St. Lucie County and includes Port St. Lucie and the western half of Ft. Pierce, the
western part of Stuart, as well as Palm City, North River Shores, and Lighthouse Point. This
watershed is now greatly modified by canals. The eastern terminus of the C-24 Canal is located
in this North St. Lucie planning unit. Water from C-24 is released to the North Fork of the St.
Lucie River via the C-23A Canal. Figure 3 is a composite map of this planning unit that shows
potentially impaired waters and potential point sources of pollution.
Approximately 14 percent of this planning unit has been identified as wetland and 12 percent as
upland forests. The wetland areas are located primarily in two areas, along the North Fork of the
St. Lucie River and in the Savannas wetland. The Savannas State Reserve is an Outstanding
Florida Water (OFW). All waters in this planning unit are designated as Class III, including
canals. Straightening and channelization have significantly modified the North Fork of the St.
Lucie River, a state aquatic preserve. These modifications have reduced the river system's
ability to filter sediment and attenuate nutrients and have dramatically reduced the wetlands that
provide habitat. Sediment transported into the North Fork has been accumulating in abnormal
quantities in the river bed (FDEP, 2003). The North Fork forms the upper segment of the SLE.
Adverse ecological impacts to the estuary caused by the canal discharges of nutrients, sediment,
and fresh water are well documented. A water quality study on Tenmile Creek, the major
tributary to the North Fork, identified significant concentrations of pesticides in the water (most
notably malathion and ethion) that are apparently related to citrus farming in the Tenmile Creek
Basin (FDEP, 2003). Fish kills and the documentation of degraded biological communities in
Tenmile Creek may be attributable to the pesticide load. Sedimentation in Tenmile Creek and the
North Fork due to canal erosion in the NSLWCD has also been documented as a concern (FDEP,
2003).
A significant portion of the planning unit is categorized as agricultural, primarily citrus
production. Individual citrus growers are participating in the BMP program to reduce pollutant
loadings to stormwater. Several programs supported by the St. Lucie River Issues Team are
focused on reducing irrigation volumes that directly affect the volume of polluted runoff and the
magnitude of transported sediment from irrigated citrus groves. In the Citrus Irrigation
Conversion project supported by NRCS, cost-share contributing growers in the North St. Lucie
planning unit are converting to low-volume irrigation equipment to help reduce discharges.
Currently, stormwater transported by canals C-23 and C-24 enters directly into the North Fork St.
Lucie River through tidal structures. The IRL South Feasibility Study includes the northern
diversion component that will result in a significant improvement to the quality and better
regulation of water discharged to the North Fork. Under this component, stormwater from the
C-23 and C-24 Canals will be diverted into one of two reservoirs to be constructed in the along
the eastern boundary of the C-24 and C-23 basins (C-23/24 North and South Reservoirs). Water
from these reservoirs could be returned to the canals to equalize storage, to supply water, or to be
diverted to the C-23/24 stormwater treatment area (STA), located in the northwestern part of this
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
planning unit, where it would be treated. From the STA, the treated water would be routed via a
bypass canal to Tenmile Creek and into the North Fork. The northern diversion component will
improve the quality of water and the timing of fresh water being delivered to the North Fork and
the SLE. Hydrologic models predict that it can come close to achieving pre-drainage distribution
flows (quantity) to the North Fork. The Feasibility Study also includes a significant project to
restore the natural hydrology of the North Fork by reconnecting river floodplains and oxbows
and returning the river to a condition similar to its historic path. The North Fork Floodplain
Restoration component will increase the capacity of the river to accommodate flows and improve
water quality and habitat. This component is also a St. Lucie River Issues Team project. The
North Fork Floodplain Restoration project is already underway. It was one of the numerous
water quality improvement projects sponsored by the St. Lucie River Issues Team. Other Issues
Team projects in this planning unit that are funded and underway include the following:
• The Tenmile Creek Restoration (a Central and Southern Florida [C&SF] Ecosystem Restoration
Critical Project that includes construction of a temporary/seasonal stormwater basin to provide
treatment and flow equalization of water in Tenmile Creek);
• NSLR Canal Retrofits and NSLWCD Bank Restoration projects, under the Issues Team
umbrella, addressing soil erosion and sediment transported by canals;
• The Platt's Creek restoration project that also provides treatment of water entering the North
Fork in St. Lucie County; and
• Several urban stormwater retrofit projects benefiting the North Fork and SLE (FDEP, 2003).
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
• JOTieatoandinduat'aJ
wasluwaldr tudliiy
discharging !o surface water
A Dsynosrc and indust71*!
,•:.-. •-.••.'!- ' . il
di6cr\a-t]ing ;o ground walar
O Wastswa:er facility greater
than 0,1 MGD
+ AcTwelandtilllCbssI II. III. C*Dj
D aiate-"ufK:«d "a^a-atius naiia Ma
• Supe^urtc rutzardwis waste she
n SFWMD ilrjdur«
J1 ,': Arna ct knnw grojnd wiilnr
oofi*.amlnal!cin
No* S(. Lucic Ranning Unit
Wa:erbcdy ide":rtica:io- boundary
O. .'•:-;:--: -p. i"hr -;.i V/.IT-.,
3D3a listed wataiDody seg-Tie-"1;
Potc-"lialr>' irrpaired wafcrtwdy SBgrrent based on
Imraircd Waters Rule siraluatlon
n
aturcs fKjm In
larf /ig ^nlt map
eir^ present.
aw* ah tNs
Kf. -eported as
ST.UJCIE CO.
-MARTIN OS;
Figure 3: North St. Lucie Planning Unit
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Table 1: Land Cover Distribution for WBID 3194 in acres and percentage
Land Use Category
Residential (1100-1390)
Commercial, Industrial, Public (1400, 1500, 1800)
Agriculture (2000 series)
Rangeland (3000 series)
Forest (4000 series)
Water (5000 series)
Wetlands (6000 series)
Barren & Extractive (7000, 1600)
Transportation & Utilities (8000 series)
TOTAL (acres)
Area (acres)
17367
2319
1682
3402
5716
1507
3140
274
974
36381
Percentage
48%
6%
5%
9%
16%
4%
9%
1%
3%
4. WATER QUALITY STANDARD AND TARGET IDENTIFICATION
The water quality criteria for protection of Class III waters are established by the State of Florida
in the Florida Administrative Code (F.A.C.), Section 62-302.530. The individual criteria should
be considered in conjunction with other provisions in water quality standards, including Section
62-302.500 F.A.C. [Surface Waters: Minimum Criteria, General Criteria] that apply to all waters
unless alternative or more stringent criteria are specified in F.A.C. Section 62-302.530.
Fecal coliforms are a subset of the total coliform group and indicate the presence of fecal
material from warm-blooded animals. Total coliform bacteria generally indicate the presence of
soil-associated bacteria and result from natural influences on a water body such as rainfall runoff
as well as sewage inflows. The most probable number (MPN) or membrane filter (MF) counts
per 100 milliliter (mL) of fecal coliform bacteria shall not exceed a monthly geometric mean of
200, nor exceed 400 in 10 percent of the samples, nor exceed 800 on any one day. Monthly
geometric means are based on a minimum of 10 samples taken over a 30-day period. The
geometric mean criteria reflect chronic or long-term water quality conditions whereas the 400
and 800 values reflect acute or short-term conditions. The target for this TMDL is the daily 800
MPN/100 mL and the "not to exceed 400 in 10 percent of the samples" criteria, since insufficient
monthly data were collected to evaluate the monthly average 200 criteria.
5. WATER QUALITY ASSESSMENT AND DEVIATION FROM TARGET
To determine the status of surface water quality in Florida, three categories of data, chemistry
data, biological data, and fish consumption advisories, were evaluated to determine potential
impairments. The level of impairment is defined in the Identification of Impaired Surface Waters
Rule (IWR), Section 62-303 of the Florida Administrative Code (F.A.C.). The IWR defines
FDEP's threshold for identifying water quality limited WBIDs to be included on the State's 303
(d) list. In addition, all waters on the 1998 303 (d) list that were not delisted remain on the
current 303 (d) list and require TMDLs. The North Fork St. Lucie River is on FDEP's planning
list for fecal coliform bacteria. EPA assessed the data and concluded the WBID is impaired for
fecal coliform and a TMDL must be developed.
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
FDEP maintains ambient monitoring stations throughout the basin. Data collected at monitoring
stations within the impaired WBID are reported in IWR Run 32, which were used in the analysis
for the TMDL. Table 2 provides a list of the monitoring stations, which are displayed spatially in
Figure 4. provides a summary of the fecal coliform monitoring data collected at each station.
According to Table 3,15 percent of the observations exceed the 400 MPN/100 mL criterion and
7 percent of the samples exceed the 800 MPN/lOOmL criterion. Most violations of the acute
criteria occur at stations located near urban areas (see Figure 4). For additional information, the
water quality data are tabulated in Appendix A. Results having laboratory data qualifiers were
not used in the TMDL analysis due to the uncertainty associated with the concentrations reported
reflecting actual conditions.
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Figure 4. Sampling stations in WBID 3194
10
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Table 2: Monitoring Stations used in the Development of this Fecal Coliform TMDL
Station ID
21FLA 28010009
21FLBRA3194-B
21FLBRA3194-C
21FLBRA3194-D
21FLBRA3194-E
21FLBRA3194-A
21FLWPB 28010010
21FLWPB 28010405
21FLWPB 28010573
21 FLWPB 2801 0879
Station Name
NO FORK ST LUCIE RIV SR 712
North St. Lucie - River bend
North St. Lucie - St. Lucie River - Downstream (1)
North St. Lucie - St. Lucie River downstream (2)
North St. Lucie - St. Lucie River - downstream (3)
North St. Lucie - Prima Vista Boat Ramp
NO FORK ST LUCIE PRIMA VISTA BR
Canal @ Midport Rd PSL near Lyngate Park
North St. Lucie River downstream of creek mouth
NORTH FORK ST LUCIE RIV AT PORT ST LUCIE BLVD
Number of
Observations
18
4
4
4
1
5
10
6
5
11
Table 3. Summary of Fecal Coliform Monitoring Data in WBID 3194
Station ID
21FLA 28010009
21FLBRA3194-B
21FLBRA3194-C
21FLBRA3194-D
21FLBRA3194-E
21FLBRA3194-A
21FLWPB 28010010
21FLWPB 28010405
21FLWPB 28010573
21FLWPB 28010879
ALL STATIONS
% Samples >
400
(MPN/lOOml)
22%
0%
0%
0%
0%
20%
0%
50%
0%
18%
15%
% Samples >
800
(MPN/lOOml)
17%
0%
0%
0%
0%
20%
0%
0%
0%
9%
7%
Minimum
Concentration
(MPN/lOOml)
1
11
12
19
19
16
1
30
1
32
1
Maximum
Concentration
(MPN/lOOml)
2200
34
170
120
19
2000
400
680
100
2000
2200
Notes: Insufficient data collected to evaluate geometric mean criterion.
Violations of the fecal coliform criteria often occur in response to rainfall events. Precipitation
data collected at stations near the impaired WBID was plotted with the fecal coliform results to
identify conditions when violations occurred (see Figure 5). In most instances, exceedances of
the criteria occur in response to rain events while at other times exceedances occur during dry
conditions. Implementation of this TMDL should address controlling nonpoint sources during
both wet and dry weather conditions. Rainfall data used in this analysis was from two sources,
the St. Lucie County Airport NOAA station and the Ft. Pierce station of the Florida Automated
Weather Network (FAWN).
11
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Fecal Coliform WBID 3194
• sta 28010009
x sta 3194-D
sta3194-A
sta 28010879
sta3194-B
+ Sta 28010405
sta3194-C
- Rainfall
o
o
S
u
o
O
10000 -ri
1000
100
1/1/1997 8/24/1998 4/15/2000 12/6/2001 7/29/2003 3/20/2005 11/10/2006
Figure 5: Fecal Coliform and Rainfall
6. SOURCE ASSESSMENT
An important part of the TMDL analysis is the identification of source categories, source
subcategories, or individual sources of coliform bacteria in the watershed and the amount of
pollutant loading contributed by each of these sources. Sources are broadly classified as either
point or nonpoint sources.
A point source is defined as a discernable, confined, and discrete conveyance from which
pollutants are or may be discharged to surface waters. Point source discharges of industrial
wastewater and treated sanitary wastewater must be authorized by National Pollutant Discharge
Elimination System (NPDES) permits. NPDES permitted facilities discharging treated sanitary
wastewater or stormwater (i.e., Phase I or II MS4 discharges) are considered primary point
sources of coliform.
Nonpoint sources of coliform are diffuse sources that cannot be identified as entering a
waterbody through a discrete conveyance at a single location. These sources generally, but not
always, involve accumulation of bacteria on land surfaces and wash off as a result of storm
events. Typical nonpoint sources of coliform include:
• Wildlife
• Agricultural animals
• Onsite Sewer Treatment and Disposal Systems (septic tanks)
• Urban development (outside of Phase I or IIMS4 discharges)
A geographic information system (GIS) tool was used to display, analyze, and compile available
information to characterize potential bacteria sources in the impaired WBID. This information
12
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
includes land use, point source dischargers, soil types and characteristics, population data
(human and livestock), and stream characteristics.
6.1. Point Sources
There are 36 permitted wastewater treatment facilities in the North St. Lucie planning unit. None
of these facilities are permitted to discharge directly to surface water that would impact water
quality in WBID 3194. Figure 3 shows the location of permitted wastewater treatment facilities,
landfills, and delineated areas in the North St. Lucie River planning unit.
Municipal Separate Storm Sewer Systems (MS4s) may also discharge bacteria to water-bodies in
response to storm events. Large, medium, and small MS4s serving populations greater than
50,000 people, or with an overall population density of 1,000 people per square mile, are
required to obtain a NPDES storm water permit. There are six MS4 permits that have the
potential to impact water quality in WBID 3194: Martin County (FLR04E013), City of Stuart
(FLR04E031), Sewall's Point (FLR04E044), Fort Pierce (FLR04E065), Port St. Lucie
(FLR04E001) and St. Lucie County (FLR04E029).
6.2. Nonpoint Sources
Runoff from urban and agricultural areas impacts water quality in the North Fork St. Lucie River
and its tributaries. Urban (48 percent residential and 6 percent commercial) and forest (16
percent of WBID) are the predominant land uses in the WBID. Medium and high density
residential development makes up the largest percentage of urban land in the WBID.
6.3. Wildlife
Wildlife deposit bacteria in their feces onto land surfaces where it can be transported during
storm events to nearby streams. Bacteria load from wildlife is assumed background, since the
contribution from this source is small relative to the load from urban and agricultural areas.
Water fowl (e.g., egrets, ducks, wood storks, herons) often frequent stormwater ponds.
Depending on the number of birds, the contributions of fecal coliform could result in stream
concentrations above the criteria.
6.4. Agricultural Animals
Agricultural animals are the source of several types of coliform loadings to streams, that impact
water quality. This source includes agriculture runoff from pastureland and cattle in streams.
The land use within the impaired WBID is 14 percent agricultural and rangeland (Table 1), so
this landuse likely discharges a significant amount of the bacteria load.
The USDA National Agricultural Statistics Service (NASS) compiles Census of Agriculture data
by county for virtually every facet of U.S. agriculture (USDA, 2002). The "Census of
Agriculture Act of 1997" (Title 7, United States Code, Section 2204g) directs the Secretary of
Agriculture to conduct a census of agriculture on a 5-year cycle collecting data for the years
ending in 2 and 7. Livestock inventory from the 2002 Census of Agriculture reports for St. Lucie
and Martin Counties are listed in Table 4. Cattle and calves are the predominate livestock.
Confined Animal Feeding Operations (CAFOs) are not known to operate in either St. Lucie or
13
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Martin County. In 2002, NASS reported 221,537 acres of farmland in St. Lucie County and
206,198 acres of farmland in Martin County.
Table 4. Livestock Inventory by County (source: USDA, 2002)
Livestock
(inventory)
Cattle and calves
Hogs and Pigs
St. Lucie
31,944
394
Martin
27,279
439
6.5. Onsite Sewerage Treatment and Disposal Systems (Septic Tanks)
Onsite sewage treatment and disposal systems (OSTDs) including septic tanks are commonly
used where providing central sewer is not cost effective or practical. When properly sited,
designed, constructed, maintained, and operated, OSTDs are a safe means of disposing of
domestic waste. The effluent from a well-functioning OSTD is comparable to secondarily
treated wastewater from a sewage treatment plant. When not functioning properly, OSTDs can
be a source of nutrient (nitrogen and phosphorus), pathogens, and other pollutants to both ground
water and surface water. The State of Florida Department of Health publishes septic tanks data
on a county basis (www.doh.state.fl.us/environment/ostds/statistics/ostdsstatistics.htm). Table 5
summarizes the cumulative number of septic systems installed since the 1970 census. The data
does not reflect septic tanks removed from service.
Table 5. County Estimates of Septic Tank Installations
County
St. Lucie
Martin
Number Septic Tanks
(1970-2002)
43,022
27,284
6.6. Urban Development
Fecal coliform loading from urban areas is attributable to multiple sources including stormwater
runoff, leaks and overflows from sanitary sewer systems, illicit discharges of sanitary waste,
runoff from improper disposal of waste materials, leaking septic systems, and domestic animals.
In 1982, Florida became the first state in the country to implement statewide regulations to
address the issue of nonpoint source pollution by requiring new development and redevelopment
to treat stormwater before it is discharged. The Stormwater Rule, as outlined in Chapter 403
Florida Statutes (F.S.), was established as a technology-based program that relies upon the
implementation of BMPs that are designed to achieve a specific level of treatment (i.e.,
performance standards) as set forth in Chapter 62-40, F.A.C. Florida's stormwater program is
unique in having a performance standard for older stormwater systems that were built before the
implementation of the Stormwater Rule in 1982. This rule states: "the pollutant loading from
older stormwater management systems shall be reduced as needed to restore or maintain the
beneficial uses of water" (Section 62-4-.432 (5) (c), F.A.C.).
14
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Nonstructural and structural BMPs are an integral part of the State's stormwater programs.
Nonstructural BMPs, often referred to as "source controls", are those that can be used to prevent
the generation of NFS pollutants or to limit their transport off-site. Typical nonstructural BMPs
include public education, land use management, preservation of wetlands and floodplains, and
minimizing impervious surfaces. Technology-based structural BMPs are used to mitigate the
increased stormwater peak discharge rate, volume, and pollutant loadings that accompany
urbanization.
7. ANALYTICAL APPROACH
The approach for calculating coliform TMDLs depends on the number of water quality samples
and the availability of flow data. When long-term records of water quality and flow data are not
available, the TMDL is expressed as a percent reduction. Load duration curves are used to
develop TMDLs when significant data are available to develop a relationship between flow and
concentration. Flow measurements are not available in WBID 3194, nor were sufficient
information available to estimate flow; therefore, this TMDL is expressed as a percent reduction.
7.1. Percent Reduction Approach for TMDL Development
The percent reduction needed to meet the applicable criterion is calculated based on a percentile
of all measured concentrations. The (p X 100) percentile is the value with the cumulative
probability of p. For example, the 90th percentile has a cumulative probability of 0.90. The 90th
percentile is also called the 10 percent exceedance event because it will be exceeded with the
probability of 0.10. Therefore, considering a set of water quality data, 90 percent of the
measured values are lower than the 90th percentile concentration and 10 percent are higher. Since
the water quality standard states the fecal coliform concentration shall not exceed 400 in 10
percent of the samples, 400 should be targeted with a percentile slightly larger than 90 to ensure
less than 10 percent of the values exceed 400. A 95th percentile of 400 counts per 100 ml was
initially selected as the target for this TMDL. This would meet the water quality standard and
provide a margin of safety by ensuring that only 5 percent of the data exceed a concentration of
400. There are many formulas for calculating percentiles and these can be found in text books
on statistics. The Hazen formula was used to calculate the 95th percentile, as it is recommended
in Hunter's Applied Microbiology (2002) article on bacteria in water. Application of the Hazen
formula to data collected in WBID 3194 is provided in Appendix A.
The percent reduction was also calculated using the maximum concentration measured in the
WBID and the 800 MPN/lOOml criterion. The larger of the two percent reduction values was
selected as the TMDL. The TMDL percent reduction required to meet the coliform criteria is
based on the following equation:
Percent Reduction = (existing 95th percentile concentration - criteria) / existing 95th percentile
concentration x 100 (Equation 2)
In WBID 3194 the 95th percentile concentration of the existing data is 1220 MPN/lOOml, and a
67 percent reduction is necessary to meet the water quality target of 400 MPN/100 ml 95 percent
of the time. The "not to exceed 800" criterion results in a reduction of about 64 percent.
15
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
8. DEVELOPMENT OF TOTAL MAXIMUM DAILY LOADS
The TMDL process quantifies the amount of a pollutant that can be assimilated in a waterbody,
identifies the sources of the pollutant, and recommends regulatory or other actions to be taken to
achieve compliance with applicable water quality standards based on the relationship between
pollution sources and in-stream water quality conditions. A TMDL can be expressed as the sum
of all point source loads (Waste Load Allocations (WLA)), nonpoint source loads (Load
Allocations (LA)), and an appropriate margin of safety (MOS), which takes into account any
uncertainty concerning the relationship between effluent limitations and water quality:
TMDL = E WLAs + E LAs + MOS
As discussed earlier, the WLA is broken out into separate subcategories for wastewater
discharges and stormwater discharges regulated under the NPDES Program:
TMDL = X WLAswasteWater+ Z WLASNPDES Stormwater + Z LAs + MOS
The various components of the TMDL equation may not sum up to the value of the TMDL
because: a) the WLA for NPDES stormwater is typically based on the percent reduction needed
for nonpoint sources and is also accounted for within the LA; and b) TMDL components can be
expressed in different terms (e.g.,, the WLA for stormwater is typically expressed as a percent
reduction, and the WLA for wastewater is typically expressed as mass per day).
WLAs for stormwater discharges are typically expressed as "percent reduction" because it is
very difficult to quantify the loads from MS4s (given the numerous discharge points) and to
distinguish loads from MS4s from other nonpoint sources (given the nature of stormwater
transport). The permitting of stormwater discharges also differs from the permitting of most
wastewater point sources. Because stormwater discharges cannot be centrally collected,
monitored, and treated, they are not subject to the same types of effluent limitations as
wastewater facilities, and instead are required to meet a performance standard of providing
treatment to the "maximum extent practical" through the implementation of BMPs. This
approach is consistent with federal regulations (40 CFR § 130.2(i)), which state that TMDLs can
be expressed in terms of mass per time (e.g., pounds per day), toxicity, or other appropriate
measure. The TMDL for the North Fork St. Lucie River is expressed as a percent reduction.
The TMDL is expressed as a daily load by multiplying the water quality target by an estimate of
flow in the WBID. The North Fork St. Lucie River is an ungaged waterbody and therefore it is
not possible to estimate flow with the available data. However, it is recommended that flow be
measured at the time of sampling to ensure compliance with the TMDL. The maximum one day
load the creek can transport in any 30-day period and maintain water quality standards is
calculated by multiplying 800 MPN/100 ml times the flow (in cubic feet per second) and a
conversion factor to obtain units of fecal coliform counts/day.
16
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
8.1. Critical Conditions
The critical condition for nonpoint source coliform loading is an extended dry period followed
by a rainfall runoff event. During the dry weather period, coliforms build up on the land surface,
and are washed off by rainfall. The critical condition for point source loading occurs during
periods of low stream flow when dilution is minimized. Water quality data have been collected
during both time periods. Most violations occur in response to rain events. Critical conditions
are accounted for in the analyses by using the entire period of record of water quality data
available for the WBID.
8.2. Margin of Safety
TMDLs shall include a margin of safety (MOS) that takes into account any lack of knowledge
about the pollutant loading and in-stream water quality. In this case the lack of knowledge
concerns the data, how well it represents the true water quality, and the estimation of the
exceedance probability. There are two methods for incorporating a MOS in the analysis: 1)
implicitly incorporate the MOS using conservative model assumptions to develop allocations; or
2) explicitly specify a portion of the TMDL as the MOS and use the remainder for allocations. In
this TMDL, an implicit MOS was used by targeting reductions that will result in no more than 5
percent of the samples exceeding a concentration of 400 counts per 100 ml even though the
standard requires less than 10 percent exceedance.
8.3. Determination of TMDLComponents
The TMDL values represent the maximum daily load the stream can assimilate and maintain
water quality standards. The TMDLs are based on the "not to exceed 400 in 10 percent of the
samples" criterion and are expressed in units of MPN per day. TMDL components for the
impaired waterbodies required to achieve the numerical criterion are summarized in Table 6.
Table 6. Summary of TMDL Components
Stream Name /
WBID
North Fork St.
Lucie River
(3194)
Parameter
Fecal
Coliform
WLA
Wastewater
N/A
MS4
67%
reduction
LA
67%
reduction
TMDL
67%
reduction
8.4. Waste Load Allocations
There are no wastewater facilities discharging in WBID 3194; however, there are MS4s
contributing pollutants. The MS4s are assigned a percent reduction equivalent to the TMDL
reduction. MS4 permittees will only be responsible for reducing the anthropogenic loads
associated with stormwater outfalls they own or otherwise has responsible control over.
17
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
8.5. Load Allocations
There are two modes of transport for nonpoint source coliform bacteria loading into the stream.
First, fecal coliform loading from failing septic systems and animals in the stream are considered
direct sources of coliform to the stream, since they are independent of precipitation. The second
mode involves coliform loadings resulting from accumulation on land surfaces transported to
streams during storm events. Data from this WBID shows violations during wet and dry weather
conditions, so both direct and indirect sources should be targeted by the reductions.
8.6. Seasonal Variation
Seasonality was addressed by using all water quality data associated with the impaired WBID,
which was collected during multiple seasons over several years.
8.7. Recommendations
It is recommended that flow be measured at the time of sampling so that loads can be calculated.
Determining the source of bacteria in waterbodies is the initial step to implementing a coliform
TMDL. FDEP employs the Basin Management Action Plan (B-MAP) as the mechanism for
developing strategies to accomplish the necessary load reductions. Components of a B-MAP are:
• Allocations among stakeholders
• Listing of specific activities to achieve reductions
• Project initiation and completion timeliness
• Identification of funding opportunities
• Agreements
• Local ordinances
• Local water quality standards and permits
• Follow-up monitoring
18
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
REFERENCES
Cleland, Bruce, 2003. TMDL development from the "bottom up " - Part III: Duration curves
and wet-weather assessments. America's Clean Water Foundation, Washington, DC. September
15,2003.
Florida Administrative Code (F. A.C.). Chapter 62-302, Surface Water Quality Standards.
Florida Department of Environmental Protection (FDEP), 2003. Basin Status Report, St. Lucie
and Loxahatchee Basin, FDEP Division of Water Resource Management, Group 2 Basin,
February 2003.
Hunter, PR. 2002. The Society for Applied Microbiology, Letters in Applied Microbiology. 34.
283-286.
USD A, 2002. 2002 Census of Agriculture, Volume 1, Geographic Area Series, Part 9, U.S.
Department of Agriculture, National Agricultural Statistics Service. AC02-A-9, June 2004.
USDA, 1997. 7997 Census of Agriculture, Volume 1, Geographic Area Series, Part ¥2, U.S.
Department of Agriculture, National Agricultural Statistics Service. AC97-A-42, March 1999.
USEPA, 1991. Guidance for Water Quality -based Decisions: The TMDL Process. U.S.
Environmental Protection Agency, Office of Water, Washington, DC. EPA-440/4-91-001, April
1991.
19
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
APPENDIX A: WATER QUALITY DATA
20
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Table 7: Guide to Water Quality Remark Codes (Rcode column in data tables)
Remark
Code
A
B
E
I
J
K
L
Q
T
U
<
z
Definition
Value reported is mean of two or more samples
Result based on colony counts outside the acceptable range
Extra sample taken in compositing process
The value reported is less than the practical quantification limit and greater than or equal to
method detection limit.
the
Estimated. Value shown is not a result of analytical measurement.
Off-scale low. Actual value not known, but known to be less than value shown
Off-scale high. Actual value not known, but known to be greater than value shown
Sample held beyond normal holding time
Value reported is less than the criteria of detection
Material was analyzed for but not detected. Value stored is the limit of detection.
NAWQA - actual value is known to be less than the value shown
Too many colonies were present to count (TNTC), the numeric value represents the filtration
volume
Table 8. Water Quality Data and Percentile Calculations
Date
4/12/2006
5/22/2006
9/27/2006
11/1/2006
2/26/2007
10/15/2007
11/6/2007
6/30/2003
5/22/2006
9/27/2006
11/1/2006
4/26/2007
11/9/2007
11/19/2007
7/31/2005
8/2/2005
8/30/2005
10/8/2005
3/14/2006
4/12/2006
5/22/2006
6/21/2006
9/6/2006
10/31/2006
7/8/1997
8/8/1997
10/1/1996
Station
21 FLWPB 2801 0009
21 FLWPB 2801 0009
21 FLWPB 2801 0009
21 FLWPB 2801 0009
21 FLWPB 2801 0009
21 FLWPB 2801 0009
21 FLWPB 2801 0009
21FLGW 18845
21 FLWPB 2801 0010
21 FLWPB 2801 0010
21 FLWPB 2801 0010
21 FLWPB 2801 0010
21 FLWPB 2801 0010
21 FLWPB 2801 0573
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLA 28010009
21 FLA 28010009
21 FLA 28010009
Result
757
712
1387
1182
155
134
3400
2
99
171
200
99
24
152
20
60
30
109
46
80
20
20
58
58
210
260
800
Rcode
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
J
J
L
T, . Percentile by
Rank TT ,, J ,
Hazen method
Data with remark codes
are not used in the
analysis due to the
uncertainty associated
with the sample
collected on 1/27/2000.
21
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Date
1/27/2000
8/11/2003
8/19/2003
10/15/2007
11/6/2007
11/6/2007
4/26/2007
11/9/2007
11/19/2007
6/24/2006
11/16/2006
8/12/2006
8/12/2006
8/12/2006
6/21/2006
3/1/2007
6/24/2006
4/16/2007
9/16/2006
6/14/2007
11/16/2006
5/13/1999
6/21/2006
4/16/2007
9/16/2006
6/21/2006
6/17/2007
2/23/2007
8/12/2006
11/16/2006
2/23/2007
11/16/2006
4/23/2007
7/19/2005
11/22/2007
10/11/2007
10/11/2007
10/18/2007
3/14/2006
8/19/2007
8/17/2005
3/4/1997
8/16/2007
8/6/2007
8/1/1996
8/6/2007
6/2/1998
9/16/2006
3/14/2006
9/16/2006
Station
21 FLWPB 2801 0405
21FLGW 20004
21FLGW 20030
21 FLWPB 2801 0009
21 FLWPB 2801 0009
21 FLWPB 2801 0009
21 FLWPB 2801 0010
21 FLWPB 2801 0010
21 FLWPB 2801 0573
21FLBRA3194-B
21FLBRA3194-C
21FLBRA3194-A
21FLBRA3194-B
21FLBRA3194-D
21FLBRA3194-E
21 FLWPB 2801 0010
21FLBRA3194-A
21 FLWPB 2801 0573
21FLBRA3194-A
21 FLWPB 2801 0009
21FLBRA3194-B
21 FLWPB 2801 0405
21FLBRA3194-C
21 FLWPB 2801 0879
21FLBRA3194-B
21FLBRA3194-D
21 FLWPB 2801 0010
21 FLWPB 2801 0879
21FLBRA3194-C
21FLBRA3194-A
21 FLWPB 2801 0573
21FLBRA3194-D
21 FLWPB 2801 0009
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0879
21 FLWPB 2801 0573
21 FLWPB 2801 0010
21 FLWPB 2801 0010
21 FLWPB 2801 0010
21 FLWPB 2801 0879
21 FLA 28010009
21 FLWPB 2801 0009
21 FLWPB 2801 0879
21 FLA 28010009
21 FLWPB 2801 0573
21 FLA 28010009
21FLBRA3194-D
21 FLWPB 2801 0009
21FLBRA3194-C
Result
250000
3400
2000
1
1
1
1
1
1
11
12
16
19
19
19
22
22.5
23
25
27
27
30
31
32
34
35
35
38
39
42
42
47
49
50
50
52
64
68
80
80
80
90
98
98
100
100
120
120
148
170
Rcode
L
Q
Q
Rank
Percentile by
Hazen method
See above note
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
1%
2%
4%
5%
7%
8%
10%
11%
13%
14%
15%
17%
18%
20%
21%
23%
24%
26%
27%
29%
30%
32%
33%
35%
36%
38%
39%
40%
42%
43%
45%
46%
48%
49%
51%
52%
54%
55%
57%
58%
60%
61%
63%
64%
65%
67%
68%
22
-------
Fecal Coliform TMDL for North Fork St. Lucie River (WBID 3194)
September 2008
Date
9/13/2006
11/20/1996
8/22/1 999
12/14/2005
1/25/1998
4/12/2006
11/5/1997
6/27/2006
4/10/1996
3/15/2000
9/6/2006
6/27/2006
9/30/1 999
6/11/2000
7/16/2005
6/29/2000
8/4/1998
2/15/1996
12/7/2006
12/11/2005
9/9/2006
Station
21 FLWPB 2801 0879
21 FLA 28010009
21 FLWPB 2801 0405
21 FLWPB 2801 0879
21 FLA 28010009
21 FLWPB 2801 0010
21 FLA 28010009
21 FLWPB 2801 0010
21 FLA 28010009
21 FLWPB 2801 0405
21 FLWPB 2801 0010
21 FLWPB 2801 0009
21 FLWPB 2801 0405
21 FLWPB 2801 0405
21 FLWPB 2801 0879
21 FLWPB 2801 0405
21 FLA 28010009
21 FLA 28010009
21FLBRA3194-A
21 FLWPB 2801 0879
21 FLWPB 2801 0009
Result
176
180
190
220
250
260
305
340
350
370
400
490
500
550
630
680
1060
1220
2000
2000
2200
Rcode
Rank
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
Percentile by
Hazen method
70%
71%
73%
74%
76%
77%
79%
80%
82%
83%
85%
86%
88%
89%
90%
92%
93%
95%
96%
98%
99%
In this TMDL the Hazen formula was used since it is recommended in Hunter's Applied
Microbiology (2002) article concerning bacteria in water. To use the Hazen formula to calculate
the percentile associated with the sample concentrations, the data are first sorted by
concentration, lowest to highest. A ranking is assigned to each sample, with the lowest
concentration having a rank of 1 and the highest concentration having a rank equivalent to the
total number of samples collected. The percentile is calculated as follows:
Percentile = (Rank - 0.5)/ (total number of samples collected)
For example, on June 29, 2006 a fecal coliform concentration of 680 MPN/lOOml was measured
at station 21FLWPB 28010405. This concentration ranks number 63 out of 68 samples collected
in WBID 3194 without data qualifiers (i.e., rcodes). The associated percentile is calculated as:
Percentile = (63-0.5)768 = 0.92 = 92%
This implies that 92 percent of the time the concentration is less than 680 MPN/lOOml.
23
------- |