Event Mean Concentrations & Florida EMC Database

Stormwater Quality Series  |  Florida Environmental Resource Permitting  |  Compiled from source slides 3–15

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EMC Definition and Calculation

Section 1  ·  Source slides: 4, 5

The Event Mean Concentration (EMC) is the foundational metric used to characterize the pollutant content of stormwater runoff from a single storm event. It represents the flow-weighted average concentration of a pollutant across the full duration of a runoff event, rather than a simple snapshot at any one moment.

Core Formula

This flow-weighted approach is critical because pollutant concentrations typically fluctuate substantially within a single storm — often peaking early in the event (the “first flush” effect) and declining as the storm progresses. A simple grab sample at any single point in time would not capture this variability and would produce an unreliable estimate of total pollutant loading.

Deriving an Annual EMC

A single event EMC is useful for characterizing one storm, but regulatory and planning applications — including TMDL development and pre/post loading analyses — require an annual EMC that represents the typical pollutant concentration across a full year of runoff. The annual EMC is derived by collecting and averaging EMC values from multiple sampled events.

As additional events are sampled, a running average of EMC values is computed. Stability in that running average — where the addition of new events no longer causes meaningful shifts in the mean — is the practical indicator that sufficient data have been collected and the annual EMC estimate has converged.

From Annual EMC to Annual Mass Loading

Once a reliable annual EMC has been established for a pollutant and land use type, annual pollutant mass loading can be estimated using a straightforward relationship:

• Annual runoff volume is estimated from rainfall records and land-use-specific runoff coefficients or curve numbers.
• Annual EMC values are drawn from monitored studies or, where site-specific data are unavailable, from published databases such as the Florida EMC Database.
• Mass loading results feed directly into TMDL allocation calculations and ERP stormwater treatment evaluations.

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Runoff Variability and Characteristics

Section 2  ·  Source slides: 3, 4

Stormwater runoff quality is among the most variable phenomena encountered in environmental monitoring. Understanding the sources and magnitude of this variability is essential for designing monitoring programs that produce defensible EMC values and for interpreting published EMC data correctly.

Two Dimensions of Variability

Drivers of Runoff Quality Variability

• Rainfall intensity: Higher-intensity events mobilize more particulate-bound pollutants through erosive energy; low-intensity events may carry higher concentrations of dissolved constituents flushed from accumulated surface deposits.
• Rainfall frequency and antecedent dry period: Longer dry periods allow greater accumulation of atmospheric deposition, traffic-related debris, and biological material on impervious surfaces, elevating first-flush concentrations.
• Soil type: Soils with lower infiltration capacity generate more overland flow more quickly, increasing the contact time between runoff and surface pollutant sources and raising overall loading potential.
• Land use and impervious cover: Urban land uses with high impervious cover generate flashier, more pollutant-concentrated runoff than undeveloped or low-density areas where infiltration attenuates both volume and concentration.

Implications for Data Collection and Use

A monitoring protocol intended to produce a valid annual EMC must be specifically designed to capture the full range of variability — sampling events across wet and dry seasons, a spectrum of storm sizes, and varying antecedent conditions. Without this breadth of coverage, the resulting EMC will systematically over- or under-represent true annual pollutant loading, undermining its use in TMDL development and pre/post impact analyses.

The applications that depend on annual EMC data — including watershed-scale TMDL load allocations and the comparison of pre-development versus post-development pollutant loads required under Florida ERP — demand the highest defensible level of data quality. Shortcutting the monitoring design invariably propagates error into regulatory decisions.

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Florida EMC Database History

Section 3  ·  Source slides: 6, 7, 11, 12

Florida’s stormwater quality regulatory framework relies on a curated, state-specific database of EMC values rather than national datasets, because Florida’s subtropical climate, flat topography, sandy soils, and land use patterns produce runoff characteristics that differ substantially from conditions in other regions of the United States.

Origins: Tampa Bay SWIM Study (1990)

The Florida EMC Database was originally compiled by the Environmental Research & Design (ERD) consulting firm in 1990 as part of the Tampa Bay Surface Water Improvement and Management (SWIM) program. The SWIM initiative required a reliable basis for estimating pollutant loads entering Tampa Bay from surrounding urban and agricultural watersheds, and no Florida-specific compiled database existed at the time.

Study Selection Criteria

Rigorous screening criteria were applied to ensure that only studies producing defensible, representative EMC values were incorporated. A study had to meet all of the following minimum standards:

• Single land use: The monitored catchment had to represent a single, relatively homogeneous land use category. Mixed-use catchments introduce confounding that makes it impossible to attribute EMC values to a specific land use type.
• Duration of at least one year: A full annual cycle of monitoring was required to capture seasonal variation in rainfall, pollutant loading, and vegetation conditions.
• Minimum of five sampled events: At least five storm events had to have been sampled to provide a minimal basis for averaging, though the preferred threshold was higher.

1994 Expansion

A follow-on publication in 1994 extended the original database by adding dedicated subcategories for two land use types that had been underrepresented in the initial compilation:

• Highway runoff: Roads and transportation corridors generate a distinctive pollutant signature — elevated metals (zinc, copper, lead from traffic), hydrocarbons, and suspended sediments — that warranted a separate characterization rather than lumping with general commercial or industrial categories.
• Agricultural subcategories: Florida’s diverse agricultural sector (cattle operations, row crops, citrus groves) produces runoff with markedly different pollutant profiles across commodity types, making a single “agriculture” EMC inadequate for load estimation purposes.

EMC Calculation Method

Database Updates and Current Reference

The Florida EMC Database has been updated incrementally as new Florida-specific monitoring studies have become available. Each revision has been evaluated against the same selection criteria used in the original compilation to maintain consistency. The most current EMC values for regulatory use are published in Table 9.2 of the Applicant’s Handbook (AH), which is the authoritative reference for ERP stormwater quality analyses in Florida.

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Land Use Category Definitions

Section 4  ·  Source slides: 7, 10

The Florida EMC Database is organized around a structured set of land use categories that are defined by their expected runoff quality characteristics rather than by legal, zoning, or mapping classification systems. This distinction is fundamental: the categories exist to group land uses that produce statistically similar EMC values, not to mirror any particular regulatory or cartographic classification.

Relationship to FLUCCS

The 13 Land Use Categories

The database encompasses 13 land use categories spanning pre-development (undisturbed) and post-development (managed or urbanized) conditions. The following table summarizes the primary categories and their defining characteristics:

Category	Abbreviation	Description
Low-Density Residential	LDR	Single-family housing at densities typically below 2 units/acre; moderate impervious cover; significant landscaped areas.
Single-Family Residential	SFR	Conventional single-family subdivisions; moderate to higher impervious cover; standard residential drainage patterns.
Multi-Family Residential	MFR	Apartments, condominiums, and townhomes; generally higher impervious cover than SFR; shared infrastructure.
Low-Intensity Commercial	LIC	Strip malls, office parks, neighborhood commercial; moderate impervious cover; lower traffic volumes than HIC.
High-Intensity Commercial	HIC	Regional shopping centers, big-box retail, dense commercial corridors; high impervious cover; high traffic and pollutant loading.
Industrial	—	Manufacturing, warehousing, and light industrial; pollutant profile varies widely with type of operations.
Highway	—	Roads, interchanges, and transportation corridors; distinctive metals and hydrocarbon signature from traffic.
Agriculture — Cattle	—	Improved pasture with cattle grazing; elevated nutrients (particularly nitrogen and phosphorus from manure).
Agriculture — Row Crops	—	Annual crops with tillage; elevated sediment, nutrients, and pesticide potential.
Agriculture — Citrus	—	Citrus groves; characteristically different drainage and chemical inputs than row crops or pasture.
Recreation / Open Space	—	Parks, athletic fields, undeveloped open space; excludes golf courses, which are classified separately due to intensive chemical inputs.
Pre-Development (Undisturbed)	—	Native vegetation, wetlands, and natural areas with minimal human disturbance; baseline reference condition.
Other / Miscellaneous	—	Land uses not fitting cleanly into other categories; applied with caution and professional judgment.

Why Category Definitions Matter

Correct assignment of a project site’s land use to the appropriate EMC category is the first and most consequential step in any loading analysis. Misclassification — for example, treating a high-intensity commercial development as low-intensity commercial, or including a golf course within a recreation/open space category — will systematically bias loading estimates and potentially lead to under-designed stormwater treatment systems or inaccurate TMDL allocations.

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Runoff Data by Land Use

Section 5  ·  Source slides: 8, 9, 13, 14, 15

This section surveys the monitoring data underlying several of the most significant land use categories in the Florida EMC Database, highlighting data volume, key pollutant trends, and important caveats associated with each category’s dataset.

Single-Family Residential (SFR)

The SFR category is supported by one of the largest individual datasets in the database, with 17 separate SFR studies contributing monitoring data. This breadth provides a relatively robust statistical basis for the SFR EMC values compared to some other categories.

Commercial Categories: Low vs. High Intensity

The database distinguishes between low-intensity commercial (LIC) and high-intensity commercial (HIC) land uses, a distinction that is borne out by the underlying monitoring data. Comparative analysis of the two datasets reveals:

• HIC sites consistently show higher concentrations of traffic-related pollutants — particularly zinc, copper, and total suspended solids — reflecting greater vehicle density, more extensive impervious pavement, and higher pollutant accumulation rates.
• Nutrient concentrations (total nitrogen, total phosphorus) are elevated in both categories relative to residential uses, driven by atmospheric deposition onto large impervious areas and limited uptake by sparse vegetation.
• LIC runoff EMC values are generally intermediate between SFR and HIC, consistent with the intermediate impervious cover and traffic intensities characterizing this land use type.

Highway Runoff

Highway runoff is supported by 15 studies in the Florida database, making it one of the better-characterized categories. The highway dataset exhibits a notable temporal trend:

Pollutant Data Availability Across Categories

Data availability is not uniform across pollutants and land use categories. The database is most robust for the following parameter groups:

• Nutrients (Total Nitrogen, Total Phosphorus): Consistently measured across virtually all studies; among the most reliable parameters in the database across all land use categories.
• Solids (Total Suspended Solids, Volatile Suspended Solids): Widely measured; good data coverage across urban categories; somewhat less consistent in agricultural datasets.
• Metals (Zinc, Copper, Lead, Cadmium): Strong data availability, particularly for urban and highway categories; lead data must be interpreted with awareness of the pre/post leaded gasoline era of collection.
• Biological oxygen demand, oil and grease, pesticides: Less consistently measured; data gaps exist across multiple land use categories, limiting statistical reliability of EMC values for these parameters.

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Topic 6: Applicant’s Handbook EMC Values

Section 6 of 9 · Florida Stormwater EMC Series · Module 4

Table 9.2 of the Florida Environmental Resource Permit (ERP) Applicant’s Handbook provides the official Event Mean Concentration values used across all ERP permit applications statewide. These values cover 23 distinct Florida land use categories and serve as the regulatory baseline for stormwater quality calculations submitted to the water management districts and FDEP.

Table 9.2 Land Use Categories and Coverage

The 23 land use categories in Table 9.2 span the full spectrum of Florida development types — from low-density residential and commercial corridors to agricultural operations, transportation infrastructure, and open natural areas. Each category carries associated EMC values for Total Nitrogen (TN), Total Phosphorus (TP), and other regulated constituents. Because ERP applications are required to demonstrate that post-development loading meets specified criteria, accurate EMC assignment to each land use parcel directly affects permit outcomes.

TN Values: Lower Than the Prior Statewide Database

One of the most significant findings embedded in Table 9.2 is that the Total Nitrogen EMC values are lower than those found in the predecessor statewide EMC database for most land use categories. This downward revision reflects improved monitoring methods, larger sample sizes, and more rigorous quality assurance protocols applied to the data underlying the Applicant’s Handbook values. For permit applicants, lower TN EMC values generally produce lower calculated mass loadings, which can ease compliance with receiving water nitrogen load allocations.

Engineers and environmental consultants preparing ERP applications should not import TN values from older databases or published research without verifying alignment with the current Table 9.2 values, as discrepancies can introduce errors into loading calculations and trigger agency review comments.

TP Values: Broadly Consistent with Prior Data

In contrast to TN, Total Phosphorus concentrations in Table 9.2 are broadly similar to those in the prior statewide EMC database. This consistency indicates that the historical TP monitoring data were robust and that the revised compilation did not substantially shift the phosphorus picture for Florida land uses. Practitioners can have greater confidence that TP-based loading estimates have been stable across database generations, though the Table 9.2 values remain the required reference for all current ERP applications.

BMPFast Software and Mass Loading Calculations

BMPFast is the FDEP-supported software platform that implements Table 9.2 EMC values in automated mass loading calculations. The tool is designed to streamline permit preparation by integrating land use area inputs, EMC assignments, rainfall data, and BMP performance factors into a unified computational workflow. BMPFast supports the full suite of 23 Table 9.2 land use categories and applies the correct EMC values automatically when a FLUCCS or equivalent land use code is entered.

The user-defined input pathway in BMPFast preserves the software’s utility for projects in unique land use contexts not well-represented by the 23 standard categories, or for sites where long-term monitoring has established that local concentrations differ materially from the handbook values. Documentation supporting any user-defined EMC input should be retained in the project file for agency review.

Feature	Table 9.2 Default	User-Defined Option
EMC source	Applicant’s Handbook Table 9.2	Site-specific monitoring data
Agency approval required	No — standard pathway	Yes — prior coordination required
Land use categories	23 standard FLUCCS-based categories	Any, as justified by data
TN values vs. prior database	Generally lower	Site-dependent
TP values vs. prior database	Broadly similar	Site-dependent
BMPFast integration	Automatic assignment	Manual entry with documentation

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Topic 7: Development Impact on Stormwater Quality

Section 7 of 9 · Florida Stormwater EMC Series · Module 4

One of the clearest findings from Florida’s stormwater quality data is that land development substantially elevates pollutant concentrations in runoff. Comparing EMC values across natural, agricultural, and urban land use categories reveals consistent and often dramatic increases in both nitrogen and phosphorus — with important differences in the magnitude of impact between the two constituents.

Total Nitrogen: Moderate but Widespread Increases

Developed land uses in Florida produce Total Nitrogen concentrations that are generally 1 to 3 times higher than those measured in natural area runoff. This range reflects the diversity of nitrogen sources associated with development: atmospheric deposition on impervious surfaces, lawn fertilization, pet waste, septic system contributions, and vehicle exhaust products. While the multiplier is moderate compared to phosphorus, the geographic extent of developed land means that nitrogen loading increases affect a large proportion of Florida’s receiving water network.

Total Phosphorus: Substantial Amplification

The development impact on Total Phosphorus is considerably more severe. Developed land TP concentrations are 3 to 10 times higher than natural area levels — a range that reflects both the phosphorus-rich soils in many parts of Florida and the array of phosphorus sources introduced by human activity. Construction site erosion, fertilizer application, atmospheric deposition on hard surfaces, and urban pet and wildlife waste all contribute to this amplification.

The 3–10× range is not uniform across development types. High-intensity commercial and transportation land uses tend toward the upper end of the multiplier, while low-density residential development with substantial vegetative cover may fall closer to the lower end. Agricultural operations, particularly those involving row crop production or intensive animal operations, also drive TP concentrations toward the higher end of the range.

All Land Uses Exceed Average Natural Area EMCs

A foundational principle of Florida’s EMC framework is that every developed or agricultural land use category produces runoff that exceeds average natural area concentrations for both TN and TP. There is no category of human land use in the Table 9.2 dataset that consistently matches or falls below natural area EMC benchmarks. This finding has important implications for stormwater management policy: any conversion of natural land to human use necessarily increases pollutant loading to receiving waters, making BMP implementation and retention of natural buffers essential components of water quality protection.

Agricultural and Urban Areas: Highest Observed Increases

Within the spectrum of developed land uses, agricultural operations and high-intensity urban areas consistently show the largest departures from natural area EMC baselines. Agricultural contributions reflect fertilizer application, tillage-induced soil disturbance, and in many cases direct discharge of irrigation return flows and animal waste. Urban contributions reflect the concentration of impervious surface, which increases both the volume and velocity of runoff while reducing the natural filtration and uptake functions that vegetated soil provides.

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Topic 8: Natural Area EMC Monitoring Project

Section 8 of 9 · Florida Stormwater EMC Series · Module 4

To establish credible baseline EMC values for undeveloped natural lands in Florida, FDEP funded a targeted monitoring study specifically designed to characterize the quality of stormwater runoff from undisturbed upland natural communities. The resulting dataset provides the empirical foundation for the natural area EMC benchmarks used in the Applicant’s Handbook and in comparative analyses of development impacts.

Study Design and Site Selection

The monitoring project deployed 33 automated sampling stations distributed across 10 Florida State Parks located throughout the state. State Parks were selected because they represent some of the most intact remaining examples of Florida’s natural land cover, with minimal human disturbance and well-documented vegetation communities. The statewide distribution of sites was designed to capture geographic and climatic variability across Florida’s diverse landscape regions — from the panhandle’s temperate communities to the subtropical ecosystems of South Florida.

Monitoring Period and Seasonal Coverage

Data collection ran for 14 months from July 2007 through August 2008. This duration was intentionally designed to span multiple seasonal cycles, capturing both the wet season (June through September) when the majority of Florida’s rainfall and associated runoff occurs, and the dry season when storm events are less frequent but can produce concentrated first-flush pollutant pulses. The 318 samples collected across this period provide a statistically robust characterization of runoff quality under varied antecedent moisture conditions, storm intensities, and seasonal vegetation states.

Natural Community Types Represented

Nine natural community types were included in the monitoring network, selected to represent the full range of upland cover types prevalent across Florida. Together, these nine community types account for approximately 92% of Florida’s upland natural land cover — meaning the dataset provides near-comprehensive representation of the state’s undeveloped upland landscape. The community types span a gradient from fire-maintained scrub and flatwoods to mesic hammocks, reflecting the ecological diversity of Florida’s terrestrial systems.

Iron and Fecal Coliform: Notable Exceedances

Among the monitored constituents, two stood out for exceeding Florida Class III water quality criteria at multiple natural area monitoring sites: iron and fecal coliform. These exceedances carry important implications for regulatory interpretation and permit review.

• Iron: Iron concentrations in natural area runoff exceeded Class III criteria at multiple sites. This finding reflects Florida’s naturally iron-rich soils — particularly in flatwoods and other poorly drained upland communities where organic acids mobilize iron from soil profiles during rainfall events. The exceedances are attributable to natural geochemical processes rather than human activity, underscoring the importance of using natural area baselines rather than water quality standards as benchmarks when evaluating the marginal impact of development on iron loading.
• Fecal coliform: Fecal coliform concentrations also exceeded Class III criteria at multiple natural sites. Natural sources of fecal coliform in undeveloped Florida landscapes include wildlife populations (particularly deer, raccoons, and birds), and soil bacteria capable of producing coliform indicator organisms. These results reinforce that receiving water bacteria exceedances in Florida watersheds cannot be attributed solely to human land use — background natural contributions must be considered in any total maximum daily load (TMDL) or source assessment analysis.

Study Significance for EMC Development

Prior to this study, natural area EMC values used in Florida permitting were largely estimated from limited local monitoring or adapted from out-of-state databases that did not reflect Florida’s unique hydrologic, pedologic, and ecological conditions. The FDEP-funded Natural Area EMC Monitoring Project provided the first systematic, statewide, Florida-specific dataset for undeveloped upland runoff quality. Its results directly informed the natural area reference values embedded in the Applicant’s Handbook and continue to serve as the foundational baseline for assessing development impacts on receiving water quality.

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Topic 9: Natural Community Classification Systems

Section 9 of 9 · Florida Stormwater EMC Series · Module 4

Assigning accurate EMC values to Florida land uses requires a consistent and well-defined classification system that can be applied across permit applications, monitoring datasets, and GIS-based planning tools. Florida currently uses several overlapping classification frameworks for natural and developed land cover, and understanding how these systems relate to each other is essential for practitioners working with stormwater quality data and ERP applications.

Florida Vegetation and Land Cover (FFWCC)

The Florida Fish and Wildlife Conservation Commission (FFWCC) produces the Florida Vegetation and Land Cover classification, which is derived from aerial and satellite photography interpreted through standardized photo-interpretation protocols. The FFWCC system provides broad coverage of both developed and natural land categories and is updated periodically to reflect land use change across the state. Its aerial photography basis makes it well-suited for landscape-scale assessments and regional stormwater planning.

Florida Natural Areas Inventory (FNAI)

The Florida Natural Areas Inventory provides a more ecologically detailed classification specifically designed for natural community characterization. FNAI classifications are based on field-verified vegetation surveys and are the standard system used by Florida State Parks — which explains their use as the primary classification framework in the Natural Area EMC Monitoring Project described in Topic 8. FNAI community types carry detailed ecological descriptions, associated species lists, and management implications that make them particularly valuable for assessing natural area runoff quality.

Florida Cooperative Land Cover Map

Recognizing the complementary strengths of the FFWCC and FNAI systems, the Florida Cooperative Land Cover Map was developed to integrate both frameworks into a single unified product. This combined map provides statewide coverage with the broad applicability of the FFWCC remote sensing approach and the ecological precision of FNAI field-verified community types. The Cooperative Land Cover Map is available in GIS format and serves as a key data source for statewide stormwater planning, TMDL development, and natural area EMC assignments.

FLUCCS Codes and EMC Integration

The Florida Land Use, Cover and Forms Classification System (FLUCCS), developed by the Florida Department of Transportation, provides the dominant land use classification framework used in ERP permit applications and environmental impact assessments. FLUCCS codes organize land use into a hierarchical numbering system that covers both developed and natural land categories at varying levels of specificity.

Within the stormwater EMC framework, FLUCCS codes serve two important functions:

• Conversion to general EMC categories in BMPFast: BMPFast converts FLUCCS codes to the general EMC land use categories defined in Table 9.2 of the Applicant’s Handbook. This conversion allows practitioners to enter FLUCCS-coded land use maps directly into the BMPFast workflow without manually cross-referencing the EMC category assignments for each parcel.
• Expansion to include natural community indices: Work is ongoing to expand FLUCCS codes to incorporate natural community indices — essentially embedding FNAI-level ecological detail within the FLUCCS hierarchical structure. This expansion would allow the FLUCCS system to carry the fine-grained community-type information needed to assign natural area EMCs with greater precision, improving the accuracy of loading calculations for projects with significant undeveloped land cover.

Classification System Relationships: Summary

System	Basis	Primary User	EMC Application	GIS Available
FFWCC Vegetation and Land Cover	Aerial/satellite photography	FFWCC; landscape planners	Broad natural and developed category mapping	Yes — statewide
Florida Natural Areas Inventory (FNAI)	Field-verified vegetation surveys	State Parks; conservation managers	Natural area EMC baseline classification	Yes — statewide
Florida Cooperative Land Cover Map	Combined FFWCC + FNAI	Statewide planners; TMDL programs	Unified land cover for EMC assignments	Yes — statewide
FLUCCS	Hierarchical land use codes	ERP applicants; FDOT; water managers	Direct BMPFast EMC category conversion	Yes — statewide

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Appendix — Quick Reference Cards

Condensed reference summaries for field use, permit preparation, and training review.

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