Use of TMDL Credits for BMP Comparisons
Florida Department of Environmental Protection (FDEP) issued total maximum daily load (TMDL) mandates in March 2009 for the Indian River Lagoon (IRL), a bay in east central Florida. (FDEP 2009). The TMDL requires communities to reduce nutrient loadings in their stormwater runoff. FDEP is currently undertaking the implementation stage of the TMDL program, called a Basin Management Action Plan (BMAP), which is enforced through National Pollutant Discharge Elimination System (NPDES) municipal separate storm sewer system (MS4) permits for entities in the IRL. The IRL drainage basin encompasses 16 cities, three counties, four military bases, eight water control districts, and two Florida Department of Transportation (FDOT) districts.
Figure 1. Project cost versus annual TP removed by type
Figure 2. TN and TP removal costs for four BMP types
For regulatory and compliance purposes, a customized version of the GIS-based Pollutant Load Simulation Model (PLSM) was developed that allowed discrete analysis of stormwater loadings from each sub-basin, as well as reductions from existing and proposed best management practices (BMPs) throughout the lagoon. PLSM generated existing pollutant loads from multiple, spatially distributed inputs such as land use, soil types, hydrologic boundaries, rainfall, runoff coefficients, event mean concentrations (ECMs), and BMP type.
Most MS4s in the IRL will struggle to attain BMAP compliance due to financial limitations and shortage of available land to construct retrofit projects of significant size, although some MS4s may have a surplus of pollutant reduction credits.
Stormwater Solutions was tasked with development of a TMDL Credit Valuation that could be used for trading pollutant removal credits among entities. Credit purchases could be used by entities to avoid costs and resources required for the process of land acquisition, construction, and maintenance of retrofit projects within their jurisdiction. Costs and pollutant removal effectiveness of 75 BMPs in the IRL basin were analyzed, and development of a common metric for credit pricing established the value of TMDL credits.
BMP Comparison Strategy
Numerous BMP comparisons and performance metrics have been developed in the past by cumulating records of historical projects in diverse locations, over varying timeframes, with mixed pollutant calculation methods. This type of data mining leads to weak data variable control, resulting in inaccurate comparison results.
Datasets were controlled in this analysis with the goal of reducing variability as much as possible from pollutant load calculations, BMP efficiencies, cost estimates, and watershed uniqueness. A strategy was devised for comparing BMP cost and performance within the context of TMDL allocations and compliance. The strategy utilized the following principles:
- All comparable BMPs were located in the IRL.
- BMPs were on FDEP’s list of BMP types that qualified for TDML credits.
- The same pollutant load model was used for all loading calculations.
- The same criteria were used for selection of all model variables.
- Consistent pollutant removal efficiencies were used for all BMPs.
- Cost estimates were developed using current construction costs.
- A consistent metric was used for comparison of BMP performance.
Types of BMPs
For TMDL treatment purposes, FDEP recognizes only a limited number of BMPs that have FDEP-documented removal efficiencies. Table 1 shows the BMPs and associated removal efficiencies that may be used to calculate load reductions for BMAP compliance. Provisional BMP values will be adjusted as additional research is compiled.
The majority of soils throughout the master plan areas are Type A or A/D, with low, flat topography. Groundwater elevations vary from 1 to 4 feet below surface elevations.
In Florida 87.8% of rainfall events have 1 inch or less of precipitation, and 75.1% of events produce less than 0.5 inch of rain. (Harper 2007). The majority of rainfall events result in low runoff volume, especially in soils with high permeability. In the IRL, annual rainfall was spatially distributed over the basin, ranging from 46.66 to 54.65 inches per year. For pollutant loadings and BMP removal efficiency calculations, FDEP uses the metric of pounds of pollutant generated on an annual basis to normalize the frequency distribution of many small storms that generate little or no runoff volume or pollutant loads. TMDL pollutant load models do not use the same rainfall distributions and frequencies that are required for new development stormwater permitting.
The BMPs selected for comparison in this study were modeled with the same PLSM model used for IRL BMAP regulation. Individual subbasins were keyholed out of the master model, resulting in consistent variables and calculation methods for all pollutant loadings. All BMP removal efficiencies were calculated using the methods from Table 1.
Estimation of BMP cost introduces potential variability to BMP comparisons. Ten different engineers will give 10 different costs estimates for a project. Unit prices vary with time, economic conditions, and contractors. In 2010, the author used consistent unit prices to generate each of the 75 project cost estimates in this study.
BMP costs for retrofit projects are highly affected by land acquisition costs. Land values can be higher than BMP construction costs and can vary widely due to many variables of zoning and location. Of the 75 projects selected, 13 had land acquisition components. To normalize cost variability, land costs were removed from all BMP cost estimates in this report.
BMP cost-comparison studies often use a common denominator of acres treated to meet a regulatory goal that has a presumptive criteria of 80% removal or a specified pollutant concentration. While this type of comparison works well for new development scenarios, available land limitations may prevent a retrofit pond from meeting the size required for new development permitting. In these situations the BMP will not provide a theoretical 80% removal. For TMDL purposes of retrofitting existing development, the number of acres treated is not a valid metric because the goal for TMDL compliance is not to design a BMP to treat a certain number of acres. Rather, the goal is to maximize the number of pounds of pollutant than can be removed at a specific site given the limitations of BMP selection and limited land size, regardless of the drainage basin size.
Figure 3. Area-weighted TP removal by BMP type
Figure 4. Interval plot of cost per pound/acre by BMP type
Annual pollutant loads removed for each parameter, PLRp, were calculated with the following equation:
where PLp is the pollutant load calculated from the PLSM model and RE is the removal efficiency from Table 1.
The size of drainage basins treated ranged from 0.88 to 549.63 acres, with a median size of 20.82 acres. Estimated project costs (without land) varied from $1,779 to $1,431,480, providing a broad range of project conditions and scales for analysis. Table 2 shows that sample sizes for BMP types ranged from 12 for DRY to 28 for FWET.
The goal of this study was to calculate a valuation of TMDL credits for MS4 trading purposes. A metric of costs per pound of pollutant removed was chosen to allow flexibility for utilization with any pollutant listed for impairment. The parameters used in this analysis were TN and TP as required in the IRL TMDL. Analysis showed land costs unreasonably skewed the dataset, so land costs were removed from BMP costing.
Initially the four types of BMPs investigated were wet detention ponds, dry retention ponds, floating vegetated wetlands, and exfiltration trenches. Exfiltration trench costs were found to be significantly higher than other BMP types, exfiltration trenches were not recommended as a comparable BMP type for this valuation analysis.
Author's Bio: Claudia Listopad, Ph.D., is president of Applied Ecology Inc. in Indian Harbour Beach, FL.
Author's Bio: Gordon England, P.E., D.WRE, is president of Stormwater Solutions in Cocoa Beach, FL.