Attachment G – 

WQBEL Technical Support Document 
1

Technical Support Document 

Derivation of the Water Quality Based Effluent Limit (WQBEL) 

 for Phosphorus in Discharges to the Everglades Protection Area 

United States Environmental Protection Agency 

August 24, 2010 

This report documents the United States Environmental Protection Agency’s 

(USEPA) derivation of the Water Quality Based Effluent Limit (WQBEL) to be applied in 

National Pollutant Discharge Elimination System (NPDES) permits for discharges from the 

Stormwater Treatment Areas (STAs) into the Everglades Protection Area (EPA). The 

WQBEL is calculated to protect the designated use1

 of the Everglades by ensuring the State 

of Florida water quality criterion for total phosphorus of 10 parts per billion (ppb) as a longterm geometric mean is not exceeded in the receiving waters of the Everglades. 

The WQBEL for STAs 1E, 1W, 2, 3/4, 5, 6, and other future STAs, has two components, 

both of which must be met. TP concentrations in the discharge from each STA may not 

exceed either: 

Part 1: 10 ppb as an annual geometric mean in more than two consecutive years; or 

Part 2: 18 ppb as an annual flow-weighted mean 

Both parts of the WQBEL are necessary to assure that the long-term criterion of 10 ppb is 

met at each STA. The background and statistical approach for derivation of each WQBEL 

component is described below. 

I. The Total Phosphorus Criterion 

Since the WQBEL is derived from and must comply with the underlying water quality 

standard, it is important to understand the underlying phosphorus criterion for the Everglades 

Protection Area. In 2005, the Florida Department of Environmental Protection (FDEP) 

adopted and USEPA approved a numeric water quality criterion for Total Phosphorus (TP) 

for the EPA. The TP criterion is a long-term geometric mean of 10 parts per billion (ppb, or 

micrograms per liter) in surface water (Rule 62-302.540(4)(a), Florida Administrative Code 

(F.A.C.)). The criterion applies throughout all of the EPA, which includes the Arthur R. 

Marshall Loxahatchee National Wildlife Refuge (Refuge), Water Conservation Areas 

(WCAs) 2 and 3, and Everglades National Park (Park). The 10 ppb geometric mean criterion 

is the numeric interpretation of the FDEP narrative nutrient criterion, which states, “In no 

case shall nutrient concentrations of a body of water be altered so as to cause an imbalance in 

natural populations of aquatic flora or fauna” (Rule 62-302.530(47)(b), (F.A.C.). The FDEP 

TP Rule is consistent with the State of Florida’s Everglades Forever Act that specifies, “In no 

case shall such phosphorus criterion allow waters in the Everglades Protection Area to be 

 

1

 The designated use of Everglades Protection Area is "Recreation, propagation and maintenance of a healthy, 

well-balanced population of fish and wildlife" (Rule 62-302.540(4)(a), F.A.C., Rule 62-302.400(1)). 
2

altered so as to cause an imbalance in the natural populations of aquatic flora or fauna” 

(Florida Statutes 373.4592(4)(e)2). 

Surface water TP at reference sites in the Everglades varies spatially across water quality 

sampling stations and temporally at any given station. TP has been measured monthly since 

the 1970s at reference2

 locations within the Everglades marsh that are 10-20 miles 

downstream of stormwater inflows. TP concentrations at these unimpacted Everglades 

locations experience natural fluctuations such that annual geometric mean TP can exceed 10 

ppb, with individual sample readings even higher (FDEP 2000, 2003, 2010a). In deriving the 

TP criterion, FDEP used data from 9 reference sites in the Refuge and WCA2A from 1994-

2001, along with data from an additional site from 1978-2001, for a total of 55 “stationyears” of data (FDEP 2003). Subsequent data verify the appropriateness of the derived 

criterion - from 1994 to 2008, the arithmetic mean of annual geometric means3 at the five 

reference sites within WCA2A that were used to develop the TP criterion was: E5 7.3 ppb; 

F5 10.0 ppb; U1 8.0 ppb, U2 7.9 ppb; U3 8.0 ppb (Figure 1). Although annual geometric 

means varied at an individual site by a factor of two, and approached 13 ppb to 14 ppb at 

reference sites F5 and U3, the long-term average did not exceed 10 ppb at each site. These 

sites remain unimpacted (FDEP 2010a). 

Figure 1. Natural variation in annual geometric mean TP concentration at five marsh 

reference sites within WCA2A, where the long-term 10 ppb criterion is met. These stations 

were used to derive the TP criterion. 

 

2

 A reference station is a location that identifies the background level of water quality in the Everglades. These 

sites exhibit the unaltered ecosystem structure and function that are typical of a location with no evidence of 

phosphorus impacts or imbalance. Reference sites are routinely used to develop water quality criteria. 

3

 The arithmetic mean of annual geometric means at marsh stations is the calculation method that was used to 

derive the 10 ppb long-term geometric mean TP criterion (FDEP 2003). It is also the calculation method used 

to determine compliance with the TP criterion (FDEP 2006). 

4

5

6

7

8

9

1 0

1 1

1 2

1 3

1 4

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Marsh TP ppb

E5

F5

U1

U2

U3

10 ppb
3

II. The Total Phosphorus Criterion Assessment Methodology for the Everglades 

Protection Area 

The TP criterion rule for the EPA, approved by the USEPA, includes a four-part assessment 

methodology (four-part test) developed by FDEP (FDEP 2003, 2005a). The purpose of this 

test is to serve as an assessment method to determine whether the long-term (decades) 10 ppb 

geometric mean criterion is being achieved on a short-term (annual) basis. The four-part test 

places upper limits on TP concentration in the Everglades marsh using certain temporal and 

spatial scales to limit the variability in measured TP concentrations throughout the 

Everglades to the variability observed at the unimpacted marsh reference sites. USEPA’s 

evaluations of this test have concluded that attaining these four concentration requirements 

together assures that the long-term TP concentration does not exceed the 10 ppb geometric 

mean criterion throughout the entire marsh (USEPA 2005, Walker 2004, 2005a). 

The TP criterion is expressed as a geometric mean at ambient marsh stations. The geometric 

mean is often used to represent the central tendency of environmental data that are lognormally distributed and/or have a positive skew, as do phosphorus levels in the Everglades. 

The geometric mean has been used since the 1990s in the Everglades to determine 

compliance with phosphorus levels within the Refuge using data from 14 marsh stations 

(SFWMD 1992a; U. S. District Court, 2001). 

The first three parts of the four-part test allow concentrations to be averaged across the 

network of marsh monitoring sites within the Everglades waterbody to address the natural 

spatial and temporal variability within the EPA. The fourth part of the test provides an 

annual maximum concentration not to be exceeded at any individual site (FDEP 2005a). The 

TP criterion was derived using data from Everglades freshwater open water sloughs, and, 

therefore, the four-part test is applied to marsh stations in these same aquatic habitats (62-

302.540 (4)(d)1. FAC). Below are the four parts of the TP criterion assessment 

methodology. 

Part one: the five-year average of annual geometric means is not to exceed 10 

ppb, averaged across the network of marsh monitoring stations within the 

Everglades. This criterion was derived based on the central tendency and upper 

95% confidence interval of the annual geometric mean TP concentrations 

measured at a group of reference sites in WCA2A and the Refuge over six to 

eight year periods of record (FDEP 2003, 2005a). This part of the test is 

supplemented by three additional parts which limit the allowable spatial and 

temporal variability. 

Part two: the annual geometric mean, when averaged across stations, is less than 

or equal to 11 ppb each year. This value was derived as the upper 95th percent 

confidence limit using a pooled value of spatially averaged annual variances 

from individual sites within reference areas, and is intended to limit the temporal 

range of annual concentrations across the network of monitoring stations to the 

range exhibited by the network of reference sites. 
4

Part three: the annual geometric mean, when averaged across stations, is less 

than or equal to 10 ppb for three of five years. This is intended to limit the 

allowable temporal variability on a frequency of exceedance basis. 

Part four: the annual geometric mean at all individual stations is less than or 

equal to 15 ppb. This value is the 95th percentile of long-term annual geometric 

means estimated from annual values at five WCA2A reference sites (47 stationyears of data, FDEP 2003, 2005a). The inclusion of an additional 25 stationyears of data from five reference sites in the Refuge resulted in the same 15 ppb 

limit (FDEP 2003). FDEP concluded that an annual geometric mean above 15 

ppb has less than a 5% chance (type I error) of occurring within the expected 

range of annual values from a long-term reference distribution of phosphorus 

data which is centered at 10 ppb (FDEP 2005a). In other words, if the annual 

geometric mean exceeds 15 ppb, then it is highly unlikely (<5% chance) that the 

long-term criterion is met. This part of the four-part test provides an upper 

bound for allowable annual geometric means at individual monitoring stations 

every year and is necessary to ensure that averaging across stations alone does 

not mask problems at individual sites, thereby allowing areas of the EPA to 

become impacted without detection. 

Independent evaluation of the four-part test developed by FDEP found it to be an 

effective and protective methodology for determining attainment of the long-term 

phosphorus criterion on a short-term basis (Walker 2004, 2005a; USEPA 2005). 

III. NPDES requirements 

Where technology-based permit effluent limits are not adequate to attain the water quality 

criterion of a waterbody, the CWA and implementing regulations for the CWA’s National 

Pollutant Discharge Elimination System (NPDES) program require that discharge limits in 

NPDES permits be set at levels to meet water quality standards (see CWA section 

301(b)(1)(C) and 40 CFR § 122.44(d)(1) and (5)). These limits are known as WQBELs. 

The NPDES regulations further require that WQBELs “derive from and comply with” all 

applicable water quality standards (see 40 CFR § 122.44(d)(1)(vii)(A)). USEPA 

recommends the use of statistical procedures that translate underlying water quality criteria 

into defensible, enforceable and protective WQBELs (USEPA 1996). Based on this 

underlying regulation and as a result of the translation, WQBELs may differ from the 

underlying water quality criterion. WQBELs are often expressed as both a long-term value 

and a short-term value. The simultaneous use of both values ensures that a given facility’s 

long-term performance is maintained, while still allowing for some short-term variability 

(USEPA 1991). 

NPDES regulations require that a WQBEL be expressed as maximum daily and monthly 

averages, unless impractical (see 40 CFR § 122.45(d)). However, as a technical matter, 

USEPA understands that expressing permit effluent limitations for nutrients like phosphorus 
5

over shorter terms (for example, as a daily maximum, weekly average, or monthly average) 

may be impractical and supports expression as annual permit limits since the effects of 

nutrients are expressed far afield and over longer time periods. Recent USEPA guidance for 

Chesapeake Bay permitting recognized this issue (USEPA 2004). USEPA is following the 

same approach for the WQBEL in this situation. Given the weekly and monthly variability 

in biological wetland treatment systems such as the STAs, monthly and daily limits are not 

appropriate. 

IV. The two-part WQBEL for the Everglades Protection Area and its derivation 

The WQBEL for STA 1E, 1W, 2, 3/4, 5, and 6 has two components, both of which must be 

met. Compliance with the WQBEL is determined on an annual basis. TP concentrations in 

the discharge from each STA may not exceed either: 

Part 1: 10 ppb as an annual geometric mean in more than two consecutive years; or 

Part 2: 18 ppb as an annual flow-weighted mean 

The WQBEL was derived by using the statistical methods described below to apply the 10 

ppb long-term geometric mean (GM) TP criterion directly to the discharge for each STA. 

The objective of the WQBEL is to determine whether the long-term (decades) 10 ppb 

criterion is met at the point of discharge on a short-term (annual) basis. If the WQBEL is met 

at each STA discharge, phosphorus will not be discharged into the Everglades at 

concentrations that will result in exceeding the criterion over the long-term (decades), nor 

will short-term exceedances be allowed that would result in local degradation of the 

receiving waters near the discharge points. The two-part WQBEL allows for expected yearto-year variability in the STA discharge TP concentration, as observed at the marsh reference 

sites used to develop the TP criterion, while attaining the long-term TP criterion. 

A. Assumptions 

The WQBEL derivation is based on several assumptions. 

1) The two-part WQBEL will be applied to each STA on an annual water year (WY) basis 

(May – April).4

 In the case of multiple discharge structures from a STA, the maximum 

annual FWM limit (18 ppb) will be applied to the FWM TP concentration across all 

discharge points and pump stations sampled as identified in the permit, resulting in one 

annual FWM reported for the STA. The annual GM limit (10 ppb) will be applied to the 

GM of grab samples collected at each outflow structure on days when discharge is 

occurring, resulting in one annual GM reported for the STA. The present number of 

 

4

 A Water Year is defined as May through April: WY 2010 is May of 2009 through April 2010. This reflects 

the annual hydrologic cycle in south Florida with the initiation of the wet season about May. This WY 

designation is used to calculate the annual marsh geometric mean TP values used to assess compliance with the 

TP criterion in the EPA. The May to April WY is also used to measure compliance with the EAA best 

management practice (BMP) regulatory rule and the C-139 Basin regulatory rule. The May to April WY also 

has been used to assess compliance with permit effluent limits for the STAs since 1996. 
6

outflow structures for each STA is as follows: 1- STA1E; 2- STA1W; 1- STA2; 6 - 

STA3/4; 6- STA5; and 3- STA6. 

2) The WQBEL is applied directly at the discharge from each STA. Changes in surface 

water TP concentration between the STA discharge point and the location where the 

discharge enters the marshes of the EPA are ignored for several reasons. For example, 

little water exists in the marshes during the dry season. Additionally, USEPA interprets 

Florida water quality standards to not allow mixing zones or credit for dilution for 

nutrients. Also, STA1E and STA1W discharge into a canal that flows unimpeded 

directly into the Refuge rim canal and marsh. At present, the other STAs discharge into 

canals which flow several miles before the water can enter EPA marshes. After 

hydropattern restoration efforts are in place, STA2 and STA3/4 will also discharge 

directly into EPA marshes. An STA discharge into a canal may be mixed with other 

water with a higher or lower TP concentration, and may also be subject to 

biogeochemical processes which could either increase (e.g., reflux of phosphorus from 

the sediment) or decrease (e.g., assimilation or adsorption) the TP concentration in the 

water prior to entry in the Everglades marsh. These potential changes are not considered 

in the application of the WQBEL. Thus, USEPA is neither considering the loss of, or the 

addition of, phosphorus between the discharge point and the Everglades marsh itself. 

3) The derivation of the 18 ppb FWM WQBEL annual limit uses the 90th percentile of the 

data distribution for STA discharge data that are adjusted to simulate an STA facility 

discharging at the long-term TP criterion of 10 ppb. This approach is consistent with 

NPDES program guidance (USEPA 1991) and with the derivation of previous discharge 

limits in the Everglades (Walker 2000; Nearhoof et al. 2005; Goforth et al. 2007; FDEP 

2010b). Since the 18 ppb annual limit was derived from weekly TP data, it is assumed 

that future calculations of STA annual average TP will use weekly data. 

4) It is assumed that the Everglades marsh does not have a long-term net assimilative 

capacity above the TP criterion.   This is consistent with the assumptions made to develop 

and adopt the 10 ppb long-term geometric mean criterion and previous determinations by 

USEPA and FDEP that a 10 ppb criterion is sufficiently protective and is not overly 

protective (USEPA 1999, 2005; FDEP 2003). 

B. Database Development 

Derivation of the 18 ppb FWM annual discharge limit requires a statistical model that 

accounts for the variability in TP at the STA discharge, and it requires determining the 

relationship between the annual geometric mean TP and flow-weighted mean TP at STA 

discharges. Each STA has daily flow data. There are two sets of TP concentration data at 

each STA discharge. TP concentrations were determined for water samples collected weekly 

in two ways: grab samples and flow-proportional samples. Annual geometric mean outflow 

TP concentrations were calculated over the period of record for each STA, using weekly TP 

data from grab samples collected at each outflow structure or pump on days when discharge 

was occurring. This approach is consistent with the grab-sampling methodology used in 

deriving the phosphorus criterion and testing compliance at marsh sites. Weekly TP data 
7

from flow-weighted composite samples were used along with discharge data to compute 

annual flow-weighted TP means. SFWMD provided annual flows and flow-weighted-mean 

discharge concentrations over the period of record for each STA. These were computed by 

SFWMD using the same datasets and procedures used in reporting compliance with the 

existing discharge limits (SFWMD, 2010a). 

USEPA uses over 90% of the available “STA-years” of data generated from May 1995 

through April 2010 for the seven STAs. This data set includes data from the Everglades 

Nutrient Removal Project (ENRP - a 3700-acre prototype STA which was operated with 

nearly uniform TP and hydraulic loading and later was expanded to become STA1W), 

STA1W itself, STA1E, STA2, STA3/4, STA5, and STA6 (Appendix 1). The performance of 

some STAs has been adversely influenced by excessive phosphorus loadings, extreme 

droughts (2001, 2007-2008), repair of damage sustained in severe hurricanes (STA1W 2005-

2007) and initial construction problems (STA1E 2010). Performance has also been 

temporarily affected by partial operation due to implementation of measures designed to 

optimize performance, such as internal levee construction, vegetation management, and 

accompanying stabilization. The above factors are likely to have increased the variability of 

these STA discharges relative to that expected under long-term operations, although 

occasional future disturbances would be expected due to maintenance and vegetation 

management practices. However, in order not to have extreme values in the dataset which 

are not representative of stable STA operation, the data used for WQBEL derivation were 

screened in the following manner: 

(1) STA startup periods were excluded; 

(2) Data were excluded for STA1W for WY2005-2007 when performance was 

significantly impaired due to phosphorus overloading, hurricane damage, and 

construction associated with repair and optimization; and 

 

(3) Data were excluded for STA1E in WY2010 because of vegetation loss and 

deterioration in performance due to construction problems resulting in 

excessive water depths. 

Data were included from the initial phase of STA-1W (ENRP WY 1996-2000), which was 

operated at relatively steady flows that may not be representative of all future STAs. Data 

also were included for STA5 in WY2007 and STA6 in WY 2007-2008 when the coincidence 

of extreme drought, construction, and implementation of optimization measures contributed 

to elevated TP concentrations in the discharges. Retaining these data from STA1W, STA5 

and STA6 in the data set used for WQBEL derivation provide a broader basis for calibration 

and a range of variability to account for future performance and disturbances. These retained 

data are indicative of conditions that USEPA reasonably expects to recur. The resulting 

period of record STA data used to develop the WQBEL (50 STA-years) is sufficient 

compared with the number of station-years of data used to derive the TP criterion (55) and 

the four-part test (72). The STA outflow data that were used for WQBEL derivation are 

shown in Appendix 1. 
8

C. The requirement not to exceed a geometric mean of 10 ppb in more than two consecutive 

years. 

The first part of the WQBEL requires that 10 ppb as an annual geometric mean (GM) is not 

exceeded in more than two consecutive years. This part of the test is based directly upon the 

long-term TP criterion and is independent of the historical STA discharge data. This part of 

the WQBEL is expressed as a geometric mean because the criterion is expressed as a 

geometric mean. As noted above, the geometric mean is often used to represent the central 

tendency of environmental data that are log-normally distributed and/or have a positive skew, 

as do phosphorus levels in the Everglades. The annual geometric mean will be computed 

using weekly grab samples collected at each discharge structure on days when there is flow. 

The use of the weekly grab samples to compute the annual geometric mean is consistent with 

how marsh data were used to derive the TP criterion and are used to assess compliance with 

the four-part test. 

This two consecutive year requirement is non-parametric and is insensitive to the variability 

in the STA discharge concentration since it is based on the central tendency (the geometric 

mean) of the long-term criterion. Three years of data are required to determine compliance 

with this WQBEL requirement. A WQBEL test based on three years of data makes it 

possible to determine compliance with a criterion that is expressed as a long-term value 

(decades) in a shorter time period (three years), thereby providing protection against nonattainment of the water quality criterion in advance of a longer term measurement period 

such as five years. 

Assuming random and statistically independent variations in the annual GMs, if an STA is 

discharging at the long-term criterion of 10 ppb, the probability of exceeding 10 ppb in 3 

(more than 2) successive years would be 0.13 (= 0.5N where N = number of years). This 

type I error rate of 0.13 is consistent with the 0.10 type I error rate (90th percentile) used to 

derive the annual limit (see page 21). The requirement of not exceeding a limit in more than 

two consecutive years is already used in the TP control regulatory rules for the EAA basin 

and the C-139 basin (SFWMD 2010b). 

 

D. Derivation of the requirement not to exceed 18 ppb flow-weighted annual average in any 

year 

1) Use of the flow-weighted mean 

The flow-weighted mean (FWM) is the most commonly used statistical expression of the 

central tendency to account for total phosphorus at discharge structures in the Everglades. 

All of the phosphorus limits and water quality standards applied at water discharge structures 

in the Everglades use FWM TP values. For the Park, FDEP’s phosphorus rule specifies that 

achievement of the criterion will use phosphorus limits that are applied as FWM TP values at 

structures that discharge into the Park (Rule 62-302.540(4)(c), F.A.C.; SFWMD 1992a; 

Walker 1999; U. S. District Court 2001). The FWM concentration is the average 
9

concentration of phosphorus in the water, weighted proportionally for the volume of flow 

during the time of sampling: 

∑

∑ × = Flow

Flow Concentration

[FWM ]

Phosphorus data are collected weekly at STA discharge structures, and water flow rate is 

measured daily. Phosphorus samples taken during weeks when the flow is high are given 

proportionally greater weight in the calculation of the flow-weighted mean than are samples 

taken when the flow is low. Acute, high flow events result in a higher load of phosphorus 

discharged into the Everglades, as compared to weeks with the same concentration but less 

flow. Other statistical expressions of the central tendency or mean do not consider the flow 

rate when the TP concentration is sampled. Without the consideration of flow, mass loading 

of phosphorus is absent. Concentration alone without flow does not account for flow events 

that can affect the cumulative delivery of phosphorus to the receiving water body. 

For the Everglades marsh, the loading of nutrients such as phosphorus is an important 

consideration in evaluating and addressing the impacts on the waterbody. The naturally 

nutrient-poor marshes of the Everglades are affected by both the concentration and the load 

of phosphorus. Once phosphorus is discharged into the Everglades, this mass or load of TP 

cycles within the Everglades marsh where it can continue to impact Everglades flora and 

fauna. The higher total load received from higher flows may accumulate in the marsh and 

affect the long-term observed concentrations in the marsh.5

 

Stormwater Treatment Area effluent TP and water volume tend to vary seasonally because of 

the seasonal nature of rainfall and stormwater runoff. Water volume and TP concentration 

also vary annually because of variation in rainfall and stormwater runoff across years. For 

example, summary data provided by South Florida Water Management District (SFWMD 

2010a) indicate that from 2005-2009 the total volume of water treated each year by the six 

STAs ranged from about 774,000 acre-feet in water year 2008 to 1,556,000 acre-feet in 2005. 

Outflow FWM TP for individual STAs ranged from 13 ppb (STA 3/4 2009) to 193 ppb 

(STA5 2007). 

Expressing the WQBEL as a FWM TP concentration includes consideration of not only 

concentration but load as well, and therefore protects the downstream Everglades from 

phosphorus loads. Loading rate (mass per time) is calculated as flow rate (volume per time) 

multiplied by concentration (mass per volume). For each STA, TP is sampled weekly via 

grab samples taken at the downstream side of each discharge structure. Flow-proportional 

composite samples are also taken weekly and analyzed for TP at these same locations to 

better account for the variation in discharge volume within the week. Weeks with high TP 

concentration and high discharge rate are of concern because of their potential to impact the 

Everglades. During the week of February 16, 2006 (week 1), STA1W discharged 127 ppb of 

 

5

 Excess phosphorus causes systemic ecological responses in the Everglades such as changed algal (periphyton) 

communities, loss of water column dissolved oxygen, changes to invertebrate communities and the food web, 

and expansion of cattail with a decline in native sawgrass, sloughs and wet prairies (SFWMD 1992b; 

McCormick et al. 1999, 2002; FDEP 2000, 2001a, 2001b). 
10

TP at a flow rate of 61 cubic feet per second (cfs) (weekly STA data from FDEP 2010). 

During the week of September 28, 2004 (week 2), this STA also discharged at 127 ppb, but 

at a much higher flow rate of 3115 cfs. The phosphorus load discharged during week two 

was about 50 times higher than during week one due to the much higher discharge rate, 

although the TP concentrations were identical. During the week of December 10, 2002 

(week 3) STA1W discharged 63.5 ppb at a flow rate of 1240 cfs. Although the concentration 

discharged during week one was two times the concentration discharged during week three, 

the TP load discharged into the Everglades during week three was over ten times higher due 

to the much higher discharge rate. Only expressing the WQBEL as the geometric mean of 

TP at STA outflows does not consider the flow volume from the STA and, therefore, 

overlooks the discharge of high phosphorus loads when flows are high. 

2) The relationship between the GM and FWM at STA discharges. 

For each STA, the annual GM and FWM TP concentrations were calculated for each STA 

using the procedure described above. Figure 2 shows the correlation between the flowweighted means and geometric means on a yearly basis, as well as for the total period of 

record by STA. The FWMs are approximately 20% higher than the GMs, both on an annual 

and long-term basis (LTFWM = 1.23 x LTGM, R2

 = 0.92). These results indicate that based 

on the same data, at the STA discharge the long-term TP geometric mean of 10 ppb (the 

criterion) is equivalent to a long-term TP flow-weighted mean of 12 ppb. The high 

percentage of explained variance (R2 = 0.92) indicates a very strong relationship, one that can 

be reliably employed in this WQBEL derivation. Previous investigators also independently 

concluded that a long-term geometric mean of 10 ppb is equivalent to 12 ppb flow-weighted 

for STA discharges (FDEP 2010b). 
11

Figure 2. Relationship between Flow-Weighted Mean (FWM) and Geometric Mean (GM) 

Total Phosphorus Concentrations in STA Discharges. Data are not rescaled.

Historical monitoring data from each STA (not rescaled). Blue diamonds (♦) are the results 

for each STA-year; red squares (■) are the period of record result for each STA. The dotted 

line depicts the average ratio of the FWM to the GM across STAs and years (y = 1.23x). The 

FWMs of STA discharges typically exceed GMs by 23% on both an annual basis and a longterm basis. Long-term is the period of record for each STA, the longest of which is 12 years 

for STA6 (Appendix 1). This relationship between flow-weighted means and geometric 

means reflects the flow and concentration dynamics associated with STA discharges. The 

relationship is observed over a wide range of discharges and concentrations. 

STA6

STA5

STA34

STA2

STA1E

STA1W

ENRP

10

100

10 100

Geometric Mean ppb

Flow‐Weighted Mean ppb

y = 1.23 x, R2 = 0.92
12

3) Data rescaling 

Deriving the annual maximum WQBEL requires consideration of the variability of TP 

concentration at the STA discharge. Since none of the STAs are currently discharging at a 

long-term geometric mean of 10 ppb, the existing data could not be directly used to 

understand and address year-to-year variability. Therefore the historical data from each STA 

have been statistically adjusted or rescaled to simulate a STA discharge at the criterion (a 

long-term geometric mean of 10 ppb). The rescaling factor for each STA is computed as the 

ratio of the 10 ppb criterion divided by the arithmetic mean of the annual geometric mean TP 

concentrations at the point of discharge. This calculation procedure is consistent with the 

procedure used in the phosphorus water quality standards for computing long-term geometric 

mean concentrations at marsh sites under the four-part test compliance methodology (FDEP 

2006). The summary of results by STA is provided in Appendix 3. The original and 

rescaled annual average STA discharge TP data are shown in Figure 3 (geometric means) and 

Figure 4 (flow-weighted means). Similar to reference sites in the Everglades marsh (Figure 

1), annual average TP at STA discharges may vary while still attaining the long-term 

criterion. 
13

Figure 3. Period of record annual geometric mean TP concentration for each STA discharge. 

10

100

1000

ENRP_1996ENRP_1997ENRP_1998ENRP_1999ENRP_2000

STA1W_2001STA1W_2002STA1W_2003STA1W_2004STA1W_2005STA1W_2006STA1W_2007STA1W_2008STA1W_2009STA1W_2010

STA1E_2008STA1E_2009STA1E_2010

STA2_2002STA2_2003STA2_2004STA2_2005STA2_2006STA2_2007STA2_2008STA2_2009STA2_2010

STA34_2006STA34_2007STA34_2008STA34_2009STA34_2010

STA5_2001STA5_2002STA5_2003STA5_2004STA5_2005STA5_2006STA5_2007STA5_2008STA5_2009STA5_2010STA6_1999STA6_2000STA6_2001STA6_2002STA6_2003STA6_2004STA6_2005STA6_2006STA6_2007STA6_2008STA6_2009STA6_2010

Historical GM ppb

0

5

10

15

20

25

30

35

ENRP_1996ENRP_1997ENRP_1998ENRP_1999ENRP_2000

STA1W_2001STA1W_2002STA1W_2003STA1W_2004STA1W_2005STA1W_2006STA1W_2007STA1W_2008STA1W_2009STA1W_2010

STA1E_2008STA1E_2009STA1E_2010

STA2_2002STA2_2003STA2_2004STA2_2005STA2_2006STA2_2007STA2_2008STA2_2009STA2_2010

STA34_2006STA34_2007STA34_2008STA34_2009STA34_2010

STA5_2001STA5_2002STA5_2003STA5_2004STA5_2005STA5_2006STA5_2007STA5_2008STA5_2009STA5_2010STA6_1999STA6_2000STA6_2001STA6_2002STA6_2003STA6_2004STA6_2005STA6_2006STA6_2007STA6_2008STA6_2009STA6_2010

Rescaled GM ppb

Data

15 ppb = 4PT Marsh Limit

10 ppp = Criterion

Top: Annual geometric means. Red bars are the long-term geometric mean for each STA. 

Bottom: Annual geometric means rescaled so that each STA discharge has a discharge at the 

long-term geometric mean criterion of 10 ppb. Data outliers that were excluded from the 

WQBEL derivation because they do not represent STAs in stable operation include STA-1W 

in WY 2005-2007 (overloading, hurricane damage, and repair) and STA-1E in WY 2010 

(vegetation loss and deterioration in performance due to construction problems resulting in 

excessive water depths).
14

Figure 4. Period of record annual flow-weighted mean TP concentration for each STA. 

10

100

1000

ENRP_1996ENRP_1997ENRP_1998ENRP_1999ENRP_2000

STA1W_2001STA1W_2002STA1W_2003STA1W_2004STA1W_2005STA1W_2006STA1W_2007STA1W_2008STA1W_2009STA1W_2010

STA1E_2008STA1E_2009STA1E_2010

STA2_2002STA2_2003STA2_2004STA2_2005STA2_2006STA2_2007STA2_2008STA2_2009STA2_2010

STA34_2006STA34_2007STA34_2008STA34_2009STA34_2010

STA5_2001STA5_2002STA5_2003STA5_2004STA5_2005STA5_2006STA5_2007STA5_2008STA5_2009STA5_2010STA6_1999STA6_2000STA6_2001STA6_2002STA6_2003STA6_2004STA6_2005STA6_2006STA6_2007STA6_2008STA6_2009STA6_2010

Historical FWM ppb

0

10

20

30

40

50

60

ENRP_1996ENRP_1997ENRP_1998ENRP_1999ENRP_2000

STA1W_2001STA1W_2002STA1W_2003STA1W_2004STA1W_2005STA1W_2006STA1W_2007STA1W_2008STA1W_2009STA1W_2010

STA1E_2008STA1E_2009STA1E_2010

STA2_2002STA2_2003STA2_2004STA2_2005STA2_2006STA2_2007STA2_2008STA2_2009STA2_2010

STA34_2006STA34_2007STA34_2008STA34_2009STA34_2010

STA5_2001STA5_2002STA5_2003STA5_2004STA5_2005STA5_2006STA5_2007STA5_2008STA5_2009STA5_2010STA6_1999STA6_2000STA6_2001STA6_2002STA6_2003STA6_2004STA6_2005STA6_2006STA6_2007STA6_2008STA6_2009STA6_2010

Rescaled FWM ppb

Data

QBEL = 18.1

LTFWM = 12.0

Top: Annual flow-weighted means. Red bars are the long-term FWM mean for each STA. 

Bottom: Annual FWMs rescaled so that each STA discharge has a long-term flow-weighted 

mean of 12 ppb, which is equivalent to a geometric mean of 10 ppb. Data outliers that were 

excluded from the WQBEL derivation because they do not represent STAs in stable 

operation include STA-1W in WY 2005-2007 (overloading, hurricane damage, and repair) 

and STA-1E in WY 2010 (vegetation loss and deterioration in performance due to 

construction problems resulting in excessive water depths).
15

4) Derivation of the Annual Maximum Discharge Limit 

A maximum discharge limit is needed to assess compliance annually and to assure that STA 

discharges do not cause exceedances of the TP criterion in the EPA. The methodology used 

to derive the annual maximum discharge limit is similar to that used in previous derivations 

of the interim STA limit of 50 ppb (Walker 1996; Nearhoof et al., 2005) and in previous 

WQBEL derivations (FDEP 2005b; 2008; 2010b; Walker 2005b). In translating the 10 ppb 

long-term geometric mean (LTGM) criterion into the annual FWM discharge limit, USEPA 

considered the expected year-to-year variability in TP concentrations, as observed both at 

marsh monitoring sites and in the STA discharges, and differences between the geometric 

and flow-weighted annual mean values observed at STA discharges. 

The statistical derivation (Appendix 2) involves fitting a log-normal distribution to the annual 

data, adjusted to reflect the expected variability in an STA FWM discharge that is in 

compliance with the 10 ppb long-term geometric mean criterion. Given that some individual 

STAs have only three or five years of data, data for all STAs were pooled. The data from 

each STA have been rescaled as described above to a long-term geometric mean of 10 ppb. 

Figure 5 presents frequency distributions of annual geometric and flow-weighted means for 

STA discharges in compliance with the phosphorus criterion. The annual maximum 

WQBEL is set at the 90th percentile of the frequency distribution of annual FWM values. An 

annual FWM above the 90th percentile would have only a 10% chance of occurring within 

the expected range of annual values from a long-term reference distribution of phosphorus 

discharge data which is centered at the TP criterion. The statistical result (18.2 +/- 0.5 ppb) 

is rounded to the nearest ppb to derive the 18 ppb FWM limit. This 18 ppb result is identical 

to the annual limit derived by FDEP (2010b) using STA data collected through April 2009. 

A second method confirms the annual maximum WQBEL. The TP criterion four-part test 

limit that must be met every year at each marsh station is 15 ppb as a geometric mean. This 

is the marsh counterpart to the annual limit that must be met every year at each STA. As 

indicated above, annual FWMs in STA discharges average 23% higher than GMs (Figure 2). 

Simply translating the 15 ppb GM marsh limit directly into a FWM limit (FWM = 1.23 x 

GM) yields a WQBEL estimate of 18.45 ppb, which when rounded to 18 ppb is identical 

with the result obtained from the frequency distribution of the rescaled STA data. Together 

these results indicate that the available data and methodologies provide a WQBEL annual 

maximum limit that is consistent with the marsh maximum limit, and is consistent with 

achieving compliance with the TP criterion throughout the downstream marsh. A 

comparison of equivalent long-term and annual geometric and flow-weighted means is 

provided in Table 1. 

Table 1. Comparison of Equivalent Annual and Long-Term Geometric Mean and Flow 

Weighted Mean TP Concentration Values. WQBEL requirements are in bold. 

 Geometric Mean Flow-weighted Mean 

Long-term Limit 10 ppb 12 ppb 

Annual Limit 15 ppb 18 ppb 
16

Figure 5. Frequency Distributions of Annual Geometric and Flow-Weighted Means for STA 

Discharges in Compliance with the P Criterion. 

Variations in annual FWMs (red) and GMs (blue) expected if the discharge had a long-term 

GM of 10 ppb or the equivalent long-term FWM of 12 ppb (12.0, Standard Error = 0.5 ppb). 

The difference between the distributions reflects the fact that at STA discharges FWMs are 

on average 23% higher than the GMs of STA Discharges (Figure 2). The solid line 

represents the statistical fit of rescaled STA data (log-normal distribution). 

The 18 ppb WQBEL to be met each year is found at the intersection of the FWM distribution 

with the 90th percentile (18.2 with an estimated uncertainty of +/- 0.5 ppb). 

The distribution of geometric means is similar to the distribution at marsh sites used to derive 

the phosphorus criterion. The vertical blue line indicates the 90th percentile of the GM, 

which is the 15 ppb annual limit for compliance with the phosphorus criterion at marsh sites. 

SE = Standard Error, approximate uncertainty in the LTFWM and WQBEL estimates. 

 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

4 6 8 10 12 14 16 18 20 22 24

Yearly TP ppb

Frequency

Flow‐Weighted

Geometric TP Criterion = 10 ppb

WQBEL = 18 ppb

4PT Marsh Limit = 15 ppb
17

The concept of higher permit limits for shorter compliance periods is scientifically sound 

provided that the resulting WQBEL attains the water quality criterion (USEPA 1991). 

Shorter term compliance periods are also consistent with the four-part assessment test for the 

criterion, which includes a short-term requirement that the annual geometric mean of 15 ppb 

not be exceeded at each individual station. The corresponding STA effluent short-term 

annual limit is 18 ppb (FWM). 

Requiring a one-part WQBEL with only the requirement not to exceed 18 ppb (FWM) as an 

annual maximum limit is inadequate for assuring that the criterion is met at the STA effluent 

over the long-term. If the future variability in TP at the discharges is less than the variability 

assumed in the derivation, and the discharges are near 18 ppb, it is possible that STAs could 

discharge in a range that does not achieve compliance with the TP criterion (i.e. exceed a 

LTGM of 10 ppb or the equivalent LTFWM of 12 ppb) and still be in compliance with the 

annual FWM discharge limit of 18 ppb. The facility could be found to be “in compliance” 

while the long-term concentration at the effluent could be as high as 18.4 ppb (18 ppb after 

rounding), well above the LTFWM of 12 ppb. Moreover, USEPA (2005) and FDEP (2003) 

rejected a proposal for a long-term geometric mean TP criterion of 15.6 ppb as not being 

protective within the Everglades marsh, and concluded that based on nutrient threshold 

research, at a long-term criterion of 15.6 ppb imbalance has already occurred. FDEP 

concluded that “all of the information evaluated by the Department indicates that the 

proposed phosphorus criterion of 10 ppb would be protective of the natural flora and fauna in 

the EPA without being overly protective or below natural background values” (FDEP 2002, 

2003). USEPA previously reviewed the scientific basis for the 10 ppb TP criterion and 

concurred with FDEP’s conclusion during the 2005 Clean Water Act review of the TP rule 

(USEPA 2005). Therefore, an STA discharging at 18 ppb over the long-term would not meet 

the protective criterion. 

The discharge must meet both requirements to be in compliance with the WQBEL. In 

essence, neither levels of TP in the discharge that would result in an exceedance of the water 

quality criterion on a long-term basis (10 ppb as a GM) nor levels that would exceed the 

single station maximum (15 ppb as an annual geometric mean) are allowed. The annual 

FWMs will be computed from STA outflow volume and concentration data using the 

methodology currently used by SFWMD in reporting compliance with the existing STA 

discharge permits. Compliance is evaluated based upon data collected in each Water Year 

(May - April). For compliance purposes the WQBEL and annual results will be rounded to 

the nearest whole number. 
18

Figure 6 presents the year to year variability in marsh and STA discharge concentrations. 

The variability of TP observed at the STA discharges to date (0.33) is slightly higher than the 

variability observed at unimpacted marsh stations and STA3/4, the best performing STA 

(0.22). However, the variability observed at the ENRP was less than what occurs at 

Everglades marsh sites while the variability observed at STA 3/4 was comparable to marsh 

sites. 

Figure 6. Year-to-Year Variability in Marsh and STA Discharge Concentrations 

Everglades Marsh Sites, Geometric Means,  POR 1995‐2010, >= 10 Years / Site STAs Geo Means, >= 3 Yrs STAs FW Means

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

3ANMESO

3ASMESO

CA215

CA27CA29

CA311CA315CA316CA317CA318

CA32CA33CA34CA35CA38

EP

LOX10LOX11LOX12LOX13LOX14LOX15LOX16

LOX3LOX4LOX5LOX6LOX7LOX8LOX9

NE1

NP201

P33P34P35P36P37

S345B6

TSB

WCA2F4

Geometri

ENRP

STA1W

STA2

STA34

STA5STA6

All

Flow‐Wtd

ENRP

STA1W

STA2

STA34

STA5STA6

All

Pooled Estimate for STA FWMs

Pooled Estimate Marsh

Year-to-year standard deviations in log-transformed TP concentrations. The y-axis is the 

natural logarithm of the standard deviation. Light blue bars represent marsh sites with at 

least 10 years of data. Geometric means (dark blue bars) and flow-weighted means (red bars) 

are shown at STA outflows with at least three years of data. 
19

Figure 7 presents the STA data used to derive the WQBEL rescaled to the long-term TP 

criterion (FWMs top, GMs bottom). Annual average TP concentration at the STA discharge 

may vary while still attaining the WQBEL. 

Figure 7. The two-part WQBEL. STA data used to derive the WQBEL are shown rescaled 

to the long-term TP criterion. 

0

5

10

15

20

25

ENRP_1996ENRP_1997ENRP_1998ENRP_1999ENRP_2000

STA1W_2001STA1W_2002STA1W_2003STA1W_2004STA1W_2008STA1W_2009STA1W_2010

STA1E_2008STA1E_2009

STA2_2002STA2_2003STA2_2004STA2_2005STA2_2006STA2_2007STA2_2008STA2_2009STA2_2010

STA34_2006STA34_2007STA34_2008STA34_2009STA34_2010

STA5_2001STA5_2002STA5_2003STA5_2004STA5_2005STA5_2006STA5_2007STA5_2008STA5_2009STA5_2010STA6_1999STA6_2000STA6_2001STA6_2002STA6_2003STA6_2004STA6_2005STA6_2006STA6_2007STA6_2008STA6_2009STA6_2010

Rescaled Yearly FWMppb

1‐Yr Limit = 18 ppb

Long‐Term FWM = 12 ppb

0

5

1 0

1 5

2 0

2 5

ENRP_1996ENRP_1997ENRP_1998ENRP_1999ENRP_2000

STA1W_2001STA1W_2002STA1W_2003STA1W_2004STA1W_2008STA1W_2009STA1W_2010

STA1E_2008STA1E_2009

STA2_2002STA2_2003STA2_2004STA2_2005STA2_2006STA2_2007STA2_2008STA2_2009STA2_2010

STA34_2006STA34_2007STA34_2008STA34_2009STA34_2010

STA5_2001STA5_2002STA5_2003STA5_2004STA5_2005STA5_2006STA5_2007STA5_2008STA5_2009STA5_2010STA6_1999STA6_2000STA6_2001STA6_2002STA6_2003STA6_2004STA6_2005STA6_2006STA6_2007STA6_2008STA6_2009STA6_2010

Rescaled GM ppb

Rescale d Da ta

15 ppb = 4PT M ars h Lim it

10 ppp = C riterio n

L T Geo Mean = 1 0 ppb

4PT M arsh

Lim i t = 1 5 ppb
20

Figure 8 presents the WQBEL compared to the long-term GM and FWM effluent TP for the 

seven individual STAs. Data from the periods of record for each of the STAs were rescaled to 

meet the 12 ppb LTFWM limit (bottom) and the annual 18 ppb maximum limit (top). Although 

there is some variation in the effluent concentrations across STAs due to their performance to 

date, the confidence limits overlap for all seven STAs and the concentrations are statistically 

indistinguishable. Therefore, USEPA appropriately pooled the STA data when deriving the 

WQBEL. 

Figure 8. Long-term FWMs & QBELs estimated for individual STAs rescaled to a long-term 

flow-weighted mean of 12 ppb. 

Rescaled Long‐Term F low‐Weighted Means and 90th Percentiles (WQBELs) for pooled da ta set and each STA.

Red lines a re results for pooled dataset.  Error bars are approximate 80% confidence intervals.

WQBEL = 18.2 +/- 0.6 ppb 

0

5

10

15

20

25

ALL

ENRP

STA1W

STA1E

STA2

STA34

STA5STA6

WQBEL ppb

LTFWM = 12 +/- 0.5 ppb 

0

2

4

6

8

10

12

14

16

ALL

ENRP

STA1W

STA1E

STA2

STA34

STA5STA6

Long-Term FWM ppb
21

E) Excursion frequencies and type 1 error. 

The application of any statistically derived WQBEL involves an assessment of the risk 

that a facility will be found to exceed the WQBEL when in fact the facility is actually 

discharging at or below the long-term criterion. This is referred to as a false positive 

rate (in statistical terms, this is a type I error). The false positive rate assumed for the 

development of the WQBEL is 10%. The opposite risk, that a facility will not be 

found to exceed the WQBEL when it is in fact discharging above the long-term 

criterion, is referred to as a false negative rate (in statistical terms, this is a type II 

error). The risks of type I and type II error cannot be avoided because of the inherent 

variability in the data. 

The derivation of the marsh TP criterion for the Refuge and WCAs 2 and 3 used a 

significance level of 5% (FDEP 2003, 2005a). Compared to a 5% significance level, a 

significance level of 10% has two times the risk that the facility will be found to be out 

of compliance when it is meeting the long-term criterion. The phosphorus criterion 

limits for inflows to the Park used a significance level of 10% (SFWMD 1992a). 

Previous STA permit effluent limits used a significance level of 10% (Nearhoof et al. 

2005; Goforth et al. 2007; FDEP 2010b). The statistical approaches that were used to 

develop the BMP program compliance test for the C-139 basin and for the EAA basin 

also used a significance level of 10% (SFWMD 2010b). 

Figure 9 shows the expected annual excursion frequencies for each component of the test for 

STAs operating with long-term GM values ranging from about 6 to 14 ppb, as compared with 

the long-term GM of 10 ppb required to achieve compliance with the P criterion. Results are 

based upon 50,000 years of simulated discharge concentrations drawn from the log-normal 

distribution with the parameters calibrated to the rescaled historical monitoring data used to 

derive the annual limit (natural logarithm mean = 2.47, standard deviation = 0.33, Figure 6). 

Requiring more than one WQBEL test simultaneously results in a higher type I error rate 

(excursion frequency) than a single test because each individual test has a type I error. The 

resulting type I error for the two-part WQBEL assuming the current variance at STA 

discharges is estimated to be about 15% (Figure 9 top, intersection of either excursion 

frequency with 10 ppb LTGM). This variability in TP at the discharge is expected to 

decrease in the future as the STAs are designed and operated to achieve compliance with the 

criterion. The lower chart shows the excursion frequencies that would occur if the actual 

discharge variability were similar to marsh data or to STAs operating in the low 

concentration range (natural logarithm standard deviation = 0.22, Figure 6). STA 3/4 has 

operated with a TP variance at the discharge comparable to the Everglades marsh, while the 

ENRP had a variance lower than the Everglades marsh (Figure 6). With future operations, as 

STA discharges operate at a lower concentration range with the variance observed at STA 

3/4 and the ENRP, the excursion frequency is projected to decrease to less than 10% (Figure 

9 bottom, intersection of either excursion frequency with 10 ppb LTGM). As noted above, a 

10% type I error or excursion frequency is consistent with previous regulatory requirements 

in the Everglades. It also provides a level of protectiveness for the Everglades. 
22

Figure 9. Simulated Excursion Frequencies versus long-term GM. 

Top: Variance estimated from all historical STA data: 

Bottom: Variance estimated from marsh data and STA’s in low concentration range (STA 

3/4): 

X Axis: LTFWM of STA discharge. An STA operating with a LTGM < 10 ppb would be 

in compliance with the phosphorus criterion. Y-Axis: Predicted excursion frequencies for 

each component of the two-part test. The red line depicts the probability that either of the 

tests will be exceeded in any year. 

Log StdDev  = 0.22

0.0

0.1

0.2

0.3

0.4

0.5

6 7 8 9 10 11 12 13 14

Actual Long‐Term Geo Mean ppb

Excursion Freq

1 Yr FWM > 18 ppb

> 2 Yrs GM > 10 ppb

Either

Log StdDev  = 0.33

0.0

0.1

0.2

0.3

0.4

0.5

6 7 8 9 10 11 12 13 14

Actual Long‐Term Geo Mean ppb

Excursion Freq

1 Yr FWM > 18 ppb

> 2 Yrs GM > 10 ppb

Either
23

V. Summary 

The WQBEL presented here for the STA discharges is derived from and complies with the 

10 ppb geometric mean long-term TP criterion for the Everglades. The WQBEL for STA 

1E, 1W, 2, 3/4, 5, and 6 and other future STAs that discharge into the EPA has two parts, 

both of which must be met. TP concentrations in the discharge from each STA may not 

exceed either: 

Part 1: 10 ppb as an annual geometric mean in more than two consecutive years; or 

Part 2: 18 ppb as an annual flow-weighted mean 

The WQBEL is directly derived from and is equivalent to the 10 ppb geometric mean longterm TP criterion and it is directly applied to each STA outflow. Compliance with the 

WQBEL is determined on an annual basis. In the case of multiple discharge structures from 

a STA, the maximum annual FWM limit (18 ppb) will be applied to the FWM TP 

concentration across all discharge points and pump stations sampled as identified in the 

permit, resulting in one annual FWM reported for the STA. The annual GM limit (10 ppb) 

will be applied to the GM of grab samples collected at each outflow structure on days when 

discharge is occurring, resulting in one annual GM reported for the STA. For compliance 

purposes the WQBEL and annual results will be rounded to the nearest whole number. 

Requiring both parts of the WQBEL provides greater certainty that attainment of the 

WQBEL will result in attainment of the underlying 10 ppb long-term geometric mean TP 

criterion. 

Inherent in this WQBEL derivation are the following conservative assumptions: 

- There is no long-term net assimilative capacity in the canal between the STA 

discharge point and the Everglades receiving marsh. 

- The criterion applies throughout the Everglades Protection Area, including 

the Everglades marsh at the edge of the waterbody. It is assumed that there is 

no long-term net assimilative capacity within these marshes above the 

criterion that would not trigger an imbalance. 

- A significance level of 10% is used to establish the 18 ppb annual limit. 

- The 18 ppb annual limit to be met every year is expressed as a flow-weighted 

mean, providing a limit that addresses TP loads. 

- A two-part WQBEL is established, providing assurance that the long-term 

criterion with be attained at each STA. The requirement that the 10 ppb 

(GM) long-term criterion is not exceeded more than two consecutive years 

provides a statistical component that is robust to the inter-annual variability 

in TP at the STA discharge. 

 

These factors together provide a margin of safety that will assure that compliance with the 

WQBEL will result in attainment of the TP criterion within the EPA. 
24

VI. References 

Florida Department of Environmental Protection. 2000. Everglades Phosphorus Criterion 

Technical Support Document. Staff Draft Part I: Water Conservation Area 2. Florida 

Department of Environmental Protection, Everglades Technical Support Section. Grover 

Payne, Temperince Bennett, Kenneth Weaver and Frank Nearhoof. Revised November 

2000. Tallahassee, Florida. 

Florida Department of Environmental Protection. 2001a. Everglades phosphorus criterion 

technical support document. Staff Draft Part II: Water Conservation Area 1. Florida 

Department of Environmental Protection, Everglades Technical Support Section, Bureau of 

Water Resource Management. Grover Payne, Temperince Bennett, Kenneth Weaver and 

Frank Nearhoof. Revised January 2001. Tallahassee, Florida. 

Florida Department of Environmental Protection. 2001b. Everglades Phosphorus Criterion 

Technical Support Document. Staff Draft Part III: Water Conservation Area 3 and 

Everglades National Park. Florida Department of Environmental Protection, Everglades 

Technical Support Section. Grover Payne, Temperince Bennett, Kenneth Weaver and Frank 

Nearhoof. October 2001. Tallahassee, Florida. 

Florida Department of Environmental Protection. 2003. Development of a numeric 

phosphorus criterion for the Everglades Protection Area. Payne, Grover, Kenneth Weaver 

and Temperince Bennett. Chapter 5 in 2003 Everglades Consolidated Report. South 

Florida Water Management District and Florida Department of Environmental Protection. 

West Palm Beach, Florida. 

Florida Department of Environmental Protection. 2005a. Technical and regulatory 

information in support of the State of Florida’s adopted phosphorus criterion contained in 

Rule 62-302.540 Florida Administrative Code. Submitted to USEPA January 12, 2005. 

Florida Department of Environmental Protection. 2005b. Derivation of the water quality 

based effluent limit (WQBEL) for phosphorus in discharges to the Everglades Protection 

Area. Payne, Garry, Kenneth Weaver and Frank Nearhoof. Florida Department of 

Environmental Protection. Tallahassee, Florida. 

Florida Department of Environmental Protection. 2006. Calculation of annual and 5-year 

geometric mean total phosphorus concentrations to assess achievement of the phosphorus 

criteria for the Everglades Protection Area. July 2006. Tallahassee, Florida. 

Florida Department of Environmental Protection. 2008. Technical support document: 

derivation of the water quality based effluent limit (WQBEL) for phosphorus in discharges to 

the Everglades Protection Area. Payne, Garry, Kenneth Weaver and Frank Nearhoof. 

October 15, 2008. Florida Department of Environmental Protection. Tallahassee, Florida. 
25

Florida Department of Environmental Protection. 2010a. Annual Total Phosphorus Criteria 

Compliance Assessment for Water Year 2005 through Water Year 2009. Appendix 3A-6 in 

South Florida Environmental Report – Volume I.

Florida Department of Environmental Protection. 2010b. Technical support document: 

derivation of the water quality based effluent limit for total phosphorus in discharges to the 

Everglades Protection Area. May 3, 2010. Payne, Garry, Kenneth Weaver, Frank Nearhoof 

and Katie Hallas. Florida Department of Environmental Protection. Tallahassee, Florida. 

Florida Department of Environmental Protection. 2010. FDEP WQBEL calculation 

spreadsheet. STA data used to derive WQBEL. Posted on the Technical Oversight 

Committee web site May 5, 2010. File name: fdep_wqbel_calc_95-09_ggp_04-11-10.xlsx 

Goforth, Gary, Nenad Iricanin, Steve Hill, Shi Kui Xue, Tracey Piccone, Frank Nearhoof and 

Ken Weaver. 2007. Technical support document for the STA-1W TBEL. October 12, 2007. 

McCormick, Paul, Susan Newman, Shili Miao, Ramesh Reddy, Dale Gawlik, Carl Fitz, Tom 

Fontaine and Darlene Marley. 1999. Ecological needs of the Everglades. Chapter 3 in 

Everglades Interim Report. South Florida Water Management District. West Palm Beach, 

Florida. 

McCormick, Paul V., Susan Newman, ShiLi Miao, Dale E. Gawlik, Darlene Marley, K. 

Ramesh Reddy and Tom D. Fontaine. 2002. Effects of anthropogenic phosphorus inputs on 

the Everglades. Chapter 3 (pages 123-146) in The Everglades, Florida Bay and Coral Reefs 

of the Florida Keys: An Ecosystem Sourcebook. James W. Porter and Karen G. Porter, 

editors. CRC Press. Boca Raton, Florida. 

Nearhoof, F., K. Weaver, G. Goforth and S. Xue. 2005. Test for determining achievement 

of 50 part per billion phosphorus initial TBEL for Everglades stormwater treatment areas. 

August 2005. Florida Department of Environmental Protection and South Florida Water 

Management District. Tallahassee, Florida. 

Pietro, K., G. Germain, R. Bearzotti and N. Iricanin. 2010. Performance and optimization 

of stormwater treatment areas. Chapter 5 in South Florida Environmental Report – Volume 

I. South Florida Water Management District, West Palm Beach, FL. 

South Florida Water Management District. 1992a. Surface Water Improvement and 

Management Plan for the Everglades. Appendix E. Derivation of phosphorus limits for 

Everglades National Park. West Palm Beach, Florida. 

South Florida Water Management District. 1992b. Surface Water Improvement and 

Management Plan for the Everglades. Supporting Information Document. Derivation of 

phosphorus limits for Everglades National Park. West Palm Beach, Florida. 

South Florida Water Management District. 2010a. STA period of record data. Excel file 

provided to USEPA via e-mail June 17, 2010. File name: STA-POR-WY2010.xlsx. 
26

South Florida Water Management District. 2010b. Phosphorus source controls for the south 

Florida environment. Chapter 4 in “South Florida Environmental Report – Volume I”. 

Edited by S. Van Horn and P. Wade. South Florida Water Management District, West Palm 

Beach, FL. 

United States District Court. 2001. United States vs. South Florida Water Management 

District et al, Case no. 88-1886-Civ-Hoeveler (2001 S.D. FL). Settlement Agreement. 

Appendix B. 

United States Environmental Protection Agency. 1991. Technical support document for 

water quality-based toxics control. March 1991. Second printing. EPA/505/2-90-001. 

Office of Water. Washington, D. C. 

United States Environmental Protection Agency. 1996. NPDES Permit Writers' Manual. 

December 1996. EPA-833-B-96-003. Office of Water. Washington, D. C. 

United States Environmental Protection Agency. 1999. Approval of water quality standards 

for surface waters of the Miccosukee Tribe of Indians of Florida. May 23, 1999. Letter from 

John H. Hankinson, Jr., Regional Administrator, Region 4, USEPA to Billy Cypress, 

Chairman, Miccosukee Tribe of Indians of Florida. 

United States Environmental Protection Agency, 2004. Annual Permit Limits for Nitrogen 

and Phosphorus for Permits Designed to Protect Chesapeake Bay and its tidal tributaries 

from Excess Nutrient Loading under the National Pollutant Discharge Elimination System. 

Memorandum from James A. Hanlon, Director, Office of Wastewater Management, to Jon 

Capacasa, Director, Water Permits Division, EPA Region 3 and Rebecca Hammer, Director, 

Chesapeake Bay Program Office. May 3, 2004. 

United States Environmental Protection Agency, 2005. Approval of water quality standards 

for phosphorus within the Everglades Protection Area. Letter from J. I. Palmer, Jr., Regional 

Administrator, Region 4, USEPA to Mimi Drew, Director, Division of Water Resource 

Management, Florida Department of Environmental Protection. January 24, 2005. 

Walker, William W., Jr. 1996. Test for evaluating performance of Stormwater Treatment 

Areas. Draft. Prepared for U. S. Department of Interior. January 3, 1996. 

Walker, William W., Jr. 1999. Long-term water quality trends in the Everglades. Pp. 447-

466 in Phosphorus biogeochemistry of subtropical ecosystems. K. R. Reddy, G. A. OConner and C. L. Schelske, editors. CRC Press, Inc. Boca Raton, Florida. 

Walker, William W. Jr. 2000. Interim phosphorus standards for the Everglades. Pages B-34 

to B-46 in Nutrient Criteria Technical Guidance Manual. Lakes and Reservoirs. First 

Edition. United States Environmental Protection Agency. Office of Water, Office of 

Science and Technology. EPA 822-B00-001. April 2000. Washington, D. C. 
27

Walker, William W. 2004. Evaluation of 1-year 15 part per billion limit. 

Walker, William. 2005a. Evaluation of a 4-part test for TP compliance: ENP and LNWR. 

Walker, William. 2005b. Estimation of water quality based effluent limits for measuring 

compliance with the Everglades phosphorus criterion. Draft for discussion purposes. For the 

U. S. Department of Interior. November 8, 2005. 

 
28

 

Appendix 1. Historical STA Outflow Data Used for WQBEL Derivation 

Flows, FWMs, Status from SFWMD (2010); GM = geometric means of grab samples from outlet structures on 

days with flow; data are from SFWMD DBHYDRO database. 

STA Water Yr FWM ppb Flow kac‐ft Grab Samples Geo Mean ppb Status(SFWMD) ENRP 1996 24 172.4 47 24

1997 19 119 43 20

1998 21 81 48 21

1999 19 86 51 19

2000 25 121 83 19

STA1W 2001 39 91 40 25

2002 37 268 36 27

2003 53 596 78 46

2004 47 298 48 39 Cell 2o ff‐line for tus suck removal and Cell 4 off‐line for vegetation rehabilitation (4 months)

2005 98 383 60 68 Western Flowway off‐line fo r LTP enhanc, Northern flowway off‐line for rehab/hu rricane repairs (variable times)

2006 113 138 49 84 ""

2007 119 126 52 76 Northern, Western and Eastern Flowways off‐line fo r LTP Enhancements(variable times)

2008 53 117 24 42 Eastern Flowway off‐line for LTP Enhancements and Western Flowway off‐line for Rehabilitation (4 months)

2009 36 187 31 27

2010 40 221 40 36

STA1E 2008 20 125 38 18 Eastern Flowway passed startup Sept 200 7and on‐line but with flow restrictions due to PSTA project

2009 21 149 26 19 Eastern Flowway on‐line but with flow restrictions due to PSTA projec t

2010 94 89 31 55 Western Flowway out of commis sion

STA2 2002 16 241 29 17

2003 18 308 39 15

2004 14 285 51 11

2005 20 371 51 15

2006 21 322 51 18 Begin  load diversion from S5A Basin (G341 Clo sure)

2007 41 218 27 23

2008 22 227 30 18 Cell 4pas sed startup 9/21/07

2009 18 291 22 17

2010 37 371 36 20

STA34 2006 23 736 252 18 Western Flowway off‐line fo r LTP Enhancements (6 months)

2007 22 355 83 18 Western Flowway off‐line fo r LTP Enhancements (1 month)

2008 20 296 117 19

2009 13 459 143 12

2010 15 637 246 14

STA5 2001 99 40 35 75

2002 83 126 78 65

2003 134 161 102 1 13

2004 97 136 96 90

2005 82 121 77 62 Cell 1B off‐line for LTP Enhancemen ts

2006 95 201 92 1 00 Cells 2A and 2B o ff‐line for LTP Enhancements

2007 192 54 24 1 31 Cell 2B off‐line for LTP Enhancemen ts

2008 96 7 7 76

2009 56 106 57 36 Cell 1A of f‐line for Rehabilitation;Sou thern Flowway passed startup 8/28/0 8

2010 51 97 86 41

STA6 1999 22 24 19 14

2000 15 59 30 13

2001 36 26 52 20

2002 16 22 67 13

2003 26 26 77 14

2004 12 29 68 10

2005 19 16 59 14

2006 25 23 47 26

2007 45 12 41 21 Section 1o ff‐line 2 months fo r LTP Enhancements

2008 36 2 8 40 Section 2pas sed startup 8/14/07

2009 27 7 24 32 Section 1DataOnly (Section 2 Not Stabilized, No C139 Annex Pump)

2010 29 14 56 22 ""
29

Appendix 2. Statistical Methodology 

The objective is to estimate the two parameters of the log-normal distribution (mean, 

standard deviation) of early FWMs for a hypothetical STA in compliance with the 

phosphorus criterion (Long-Term Geometric Mean of 10 ppb). Computations are 

applied to log-transformed time series of yearly FWM concentrations for each STA, 

each rescaled to LTGM = 10 ppb. 

 yij = ln ( Cij) 

 m = y N

k

i

n

j

ij

i

/

1 1

∑∑= =

 sy

2

 = [ ( ) ]/( )

1 1

2

y y N k

k

i

n

j

ij i

i

∑ ∑ − −

= =

 df = N - k

 Lp = exp (m + sy ·tp) 

where, 

 Cij= FWM for year j and STA i, rescaled to LTGM = 10 ppb. 

yij= ln [ Cij ] 

i y = mean Ln (FWM) for STA i across years 

N = total number of STA-years 

k = number of STAs 

 m = Mean of log-normal distribution across STAs and years (rescaled FWM) 

sy

2

 = pooled year-to-year variance across STAs and years 

sy = pooled year-to-year standard deviation 

df = degrees of freedom in s 

Lp = limit FWM concentration with exceedance probability p

tp = 1-tailed t statistic, significance level p and degrees of freedom df 

p = 0.10

(adapted from FDEP, 2010b) 
30

Appendix 3. Summary of Data & Calculations 

 

Flow Historical ppb Historical ppb

STA Years kac‐ft/yr LTFWM SE GM SE

ENRP 5 116 22.0 1.3 20.4 0.8

STA1W 7 254 45.4 3.5 34.6 3.1

STA1E 2 137 20.7 0.4 18.6 0.5

STA2 9 293 22.9 3.0 17.1 1.1

STA34 5 497 18.8 2.3 16.5 1.4

STA5 10 105 95.5 10.4 78.9 9.5

STA6 12 22 22.2 2.9 19.9 2.6

Rescaled Flow‐Weighted Mean ppb (a) Discharge Limits ppb

Dataset LTFWM SE Ln Mean Ln SD Limit SE (b)

ALL 12.0 0.4 2.470 0.333 18.2 0.5

ENRP 10.8 1.0 2.352 0.130 16.2 1.5

STA1W 13.1 1.0 2.518 0.168 19.1 1.6

STA1E 11.1 1.6 2.409 0.028 17.1 2.5

STA2 13.4 0.9 2.540 0.356 19.5 1.4

STA34 11.4 1.1 2.408 0.259 17.1 1.6

STA5 12.1 0.8 2.457 0.382 18.0 1.2

STA6 11.2 0.7 2.484 0.401 18.5 1.0

Summary of Rescaled Data Adjusted Std

Statistic Mean Std Dev Std Dev (b ) Error

ln (GM) 2.26 0.302 0.322 0.05

ln (FWM) 2.47 0.312 0.333 0.05

ln (FWM/ GM ) 0.21 0.196 0.209 0.03

LT Flow‐Wtd Mean 12.0 0.982 1.048 0.15

LT Geometric Mean 10.0

Parameter Value Formula

Number of STAs 7 NSTAS

Number of STA‐Years 50 NYEARS

Degrees of Freedom 43 DOF =  NYEARS ‐ NSTAS

LTFWM for LTGM = 10 ppb 12.0 Average of Rescaled FWMs across STAs and Years

Std Error of LTFWM 0.40 Standard Error of Rescaled FWMs across STAs and Years

Log Mean FWM 2.47 m = Mean ( Ln Rescaled FWM)

Log Std Dev FWM 0.333 s =  Standare Deviation  (ln Rescaled FWM )

Assumed Tail Probability 0.1 p

Students‐t 1.30 t = F ( p, DOF),  1 Tailed

FWM Limit 18.2 Exp (  m + s  t )

Std Error of FWM Limit 0.50 SE of LTFWM x  Limit / LTFWM

(a) Rescaled FWM = Observed FWM x  10 /   Average of Yearly Geometric Means;

(b ) LTFWM is flow‐weighted mean of rescaled yearly FWMs for each STA

(c) SE for individual STAs uses pooled estimate of year‐to‐year standard deviation (0.33) in Ln FWM

(d ) Adjusted Standard Deviation Using  DOF instead of  NYEARS ‐ 1