1. Total Maximum Daily Loads (TMDLs): An Overview of TMDLs in Texas
20th Annual Lower Rio Grande Valley Water Quality Management and Planning Conference
May 17, 2018
Roger Miranda, P.G.
Total Maximum Daily Load Program
Texas Commission on Environmental Quality
Good afternoon. I’m going to be presenting a brief overview of the TCEQ’s Total Maximum Daily Load program. TMDLs are a deceptively simple concept, but the level of complexity associated with some TMDLs can be a bit overwhelming at times. I will do my best today to keep things as simple as possible by providing simplified examples of the concept and its application in Texas water bodies.

2. The Law requires us to ask…
What are the problems?
How bad are they?
How can they be corrected?
The federal Clean Water Act requires all states to monitor water quality and compare it to criteria set up to protect everything from aquatic plants and animals to people. The criteria are commonly referred to as Surface Water Quality Standards. The monitoring conducted by the states covers many pollutants, from specific toxic chemicals to temperature to pH to bacteria. The clean water act ultimately also requires states to address pollution in surface waters.

3. Texas TMDL Program
Texas is required under Section 303(d) of the federal Clean Water Act to list impaired waters and to take action to restore them.
Impaired waters are identified every two years on the Texas Integrated Report of Surface Water Quality.
A surface water body is considered impaired if it does not meet the criteria for support of one or more of its beneficial uses, as defined in the Texas Surface Water Quality Standards ( TAC).
As do other states, Texas must identify water bodies that do not meet Texas’ Surface Water Quality Standards, a.k.a. impaired water bodies, and to compile and submit lists of impaired water bodies to the USEPA. The list of impaired water bodies is called the 303(d) list, named after the section of the CWA that requires it and it is part of Texas biennial Integrated Report of Surface Water Quality.
Impairment example:
Bacteria causing non-support of the contact recreation use.
Information on impaired water bodies in Texas can be found on the TCEQ website on the Integrated Report page. If you Google “Texas Integrated Report” the page will pop up as your first choice.

4. TMDL: Total Maximum Daily Load
How bad is the problem?
Where is it coming from?
How much is too much?
The third requirement of the Clean Water Act regarding impaired water bodies, and most relevant for this presentation, is the requirement that states develop TMDLs for each pollutant that contributes to the impairment of a listed waterbody. States must assess the severity of the impairment, identify the sources of the problem and determine the assimilative capacity of the water body to the pollutant causing the impairment or impairments.

5. Texas TMDL Program TMDL – Total Maximum Daily Load
CFR Title 40 Section 130.7
Determines the maximum amount (load) of a pollutant a water body can receive and still maintain uses (expressed in load/day)
Allocates this load to broad categories of sources in the watershed
Adopted by TCEQ
Approved by EPA
As described under the implementing regulations of the Clean Water Act. (40 CFR 130.7), a TMDL is like a pollution budget that is based on the amount of a particular pollutant that a water body can receive and still meet its applicable water quality standards. That budget is then allocated among broad categories of pollutant sources. These pollutant “loads” have legal standing. They are formally adopted by the TCEQ and must be approved by the USEPA.
Many of the examples I’ll discuss today are associated with bacteria impairments, which are among our most numerous impairment listings. Indicator bacteria, such as E. coli and Enterococcus, are used to determine if a stream is being contaminated by fecal matter, which can impair a water body due to potential pathogens. A TMDL is a way of calculating the highest allowable amount of all pollutants in surface waters, including indicator bacteria. It’s a pollutant loading limit for meeting water quality standards.

6. TMDLs in Texas As of May 2018 TMDLs Segments* Assessment Units
Stream Miles
Watershed Area (Mi2)
281
198
416
2,463
25,892
The TCEQ’s TMDL Program is in its 20th year and, in that time, the program has developed 281 TMDLs in 198 water quality segments, which include water bodies such as streams, lakes, estuaries, bays and beaches.
*All water body types, including streams, lakes, estuaries, bays and beaches

7. TMDLs and TMDL Allocations
The maximum amount (load) of a pollutant a water body can receive and still maintain its uses.
E. Coli example:
TMDL (MPN/day) = Criterion (MPN/100ml) * flow (cfs or cms) * conversion factor
Expressed in units per day LA
WLA
As we have mentioned, TMDLs are the best possible estimates of assimilative capacity of a waterbody for a specific pollutant under consideration. Commonly expressed as a load with units of mass per day, TMDLs may also be expressed in other ways such as concentration, toxicity or even heat.
A simple TMDL example would be bacteria counts per day (often a statistical value like Most Probable Number per day). In this example we start with the acceptable bacteria criterion instream and simply multiply it by a representative, or a protective, flow value (for example the median of high-range flows).
Using conversion factors, we can express the loading limit in terms of numbers of bacteria per day. This equation is then used to allocate that maximum load into two broad categories of pollution sources, regulated sources (such as wastewater treatment facilities) and unregulated sources (such as agricultural runoff), plus a margin of safety.
Who or what exactly receives pollutant load “allocations” ? I’ll try to get into that in a little more detail later in this presentation.
TMDL = Σ WLA + Σ LA + MOS

8. TMDL Allocations TMDL = Σ WLA + Σ LA + Σ MOS + FG
Waste Load Allocations (WLA) – Regulated Sources
Load Allocations (LA) – Unregulated Sources
Margin of Safety (MOS) – Explicit or Implicit
Future Growth (FG) – Potential Regulated Sources
So let me explain what some of the terms in the TMDL equation mean. The waste load allocation, or WLA, is the sum of loads from regulated sources; think - activities that require a permit like municipal wastewater discharges, stormwater runoff and industrial wastewater discharges. The load allocation, or LA, is the sum of pollutant loading from all unregulated sources – think agricultural runoff or background sources.
A margin of safety is required of all TMDLs; it is used to address uncertainty in the TMDL analysis and it can be expressed explicitly or implicitly – (provide an explanation). And we’ve introduced a new in the factor in the TMDL equation future growth. Allocations for future growth are not required by federal regulations. When feasible, however, incorporating this factor into the equation affords a certatin level of sustainability. Most of our TMDLs, do in fact, contain an allocation for future growth.

9. Σ WLA = Σ WLAWWTF + Σ WLAPSW
TMDL Allocations
TMDL = Σ WLA + Σ LA + Σ MOS + FG
Σ WLA = Σ WLAWWTF + Σ WLAPSW
WLAWWTF – Waste Load Allocation for Wastewater Treatment Facility Discharges
WLAPSW – Waste Load Allocation for Permitted Stormwater Discharges
Expressed in Units Per Day
So, I’m going to make things a little complicated now by fleshing out the WLA term (the waste load allocation term) a little more. There was a time, early in the TCEQ’s TMDL program, in which the waste load allocation referred strictly to the portion of the TMDL allocated to point source discharges, such as wastewater treatment facilities. However, with the advent of storm water regulations and the establishment of the TCEQ’s stormwater program, we started incorporating the TMDL load allocation for regulated storm water into the overall waste load allocation, in essence treating it as a point source pollutant load.

10. Σ WLA = Σ WLAWWTF + Σ WLAPSW
TMDL Allocations
Σ WLA = Σ WLAWWTF + Σ WLAPSW
WLAWWTF
Individual facilities receive load allocations which can also be expressed in terms of effluent limits (i.e. effluent flows and concentrations)
Based on full permitted flow
Based on permissible end point concentrations (for example E. coli or Enterococci to support Primary Contact Recreation)
Updated through the TCEQ’s Water Quality Management Plan*
* Updated quarterly
For the WLAWWTF, the TMDL allocation is made by combining the allocations made to permitted wastewater treatment facilities. The allocation is typically the sum of individual waste loads for existing WWTFs, calculated as their full permitted flow times the permitted effluent concentrations, which, in the case of bacteria TMDLs, is the instream water quality criterion (or, in the case of an explicit MOS, a high percentage of that criterion; I’ll explain this statement in a minute).
The criteria associated with primary contact recreation is 35 counts per 100 mL Enterococci and 126 counts per 100 mL for E. coli. WWTFs discharging to the TMDL segments are assigned an effluent limit based on the TMDL allocation, which is itself often based on the instream criterion. The individual allocations for each WWTF in the watershed are specified in the TMDL document
Changes in WWTFs (a new permit, increase or reductions in permitted flow) are handled through updates to the TCEQ’s water quality management plan, which is updated quarterly. These could involve drawing from the future growth component or redistribution of the load allocations between WLA and LA.

11. TMDL Allocations Σ WLA = Σ WLAWWTF + Σ WLAPSW
Σ WLAPSW = (TMDL – Σ WLAWWTF – FG – MOS) * FDAPSW
Σ WLAPSW = Total Runoff Load * FDAPSW
Σ WLAPSW – Sum of all Regulated Stormwater Loads
TMDL – Total Maximum Daily Load
Σ WLAWWTF – Sum of all WWTF Loads
FG – Sum of Future Growth Loads for Potential Permitted Facilities
MOS – Margin of Safety
FDAPSW – Fractional Portion of Drainage Area under Jurisdiction of Stormwater Permits
To calculate WLAPSW (that is, the wasteload load allocation for permitted stormwater), the fractional portion of the total drainage area under the jurisdiction of stormwater permits (FDAPSW) must be determined to estimate the portion of the overall runoff that should be allocated to WLAPSW.
Once this fraction is determined, it is applied to the total runoff load, which is what is left of the TMDL once all other allocations are made (including, WLAWWTF, FG and MOS). WLAPSW is part of the set of Total Maximum Daily Load allocations, but it is difficult to think of it as a daily load because of the intermittent and variable nature of stormwater flows.

12. TMDL Allocations FDAPSW Σ WLAPSW = Total Runoff Load * FDAPSW
100 sq. miles
20 sq. miles
FDAPSW
Urbanized Area
Subwatershed Area =
20 mi2
100 mi2 = =
20% or 0.2
To show you how FDAPSW is calculated, I want to illustrate how it is derived from the combined area under the jurisdiction of regulated stormwater permits and the total drainage area of the watershed. This figure shows how FDAPSW is calculated. It is simply the ratio or percentage of the stormwater jurisdictional area in the TMDL watershed to the total area in the TMDL watershed. For Phase 1 MS4 permits the jurisdictional area is defined by the city limits. For Phase II MS4 permits, the jurisdictional area is defined by the Urbanized Areas described in the 2010 census.
In this example, the combined stormwater jurisdictional area comprised 20% of the TMDL watershed, hence the FDApsw is 0.2.

13. Σ WLA = Σ WLAWWTF + Σ WLAPSW
TMDL Allocations
Σ WLAPSW = (TMDL – Σ WLAWWTF – FG – MOS) * FDAPSW
WLAPSW
Single aggregate load allocation
For municipal, industrial, and construction stormwater permits combined
Loads are not assigned to individual permits
Determination of the aggregate WLAPSW is based on the proportion of urbanized area in the TMDL watershed
The WLAPSW is the product of the FDApsw and the load allocation remaining after the waste load for waster and the margin of safety and future growth allocations are subtracted from the TMDL. It is expressed as a single aggregate load for all permitted stormwater in the TMDL watershed and it subsumes all land permitted under municipal, industrial, and construction stormwater permits. The TCEQ does not specify allocations for individual stormwater permitees in its TMDLs. Remember, WLApsw is a component of the overall Waste Load Allocation (ΣWLA); that is ΣWLAWWTF + ΣWLApsw = ΣWLA.
Σ WLA = Σ WLAWWTF + Σ WLAPSW

14. TMDL Allocations TMDL = Σ WLA + Σ LA + Σ MOS + FG
Σ LA = TMDL – Σ WLA – FG – MOS
Σ LA – Allowable loads from unregulated sources
TMDL – Total Maximum Daily Load
Σ WLA – Sum of all WWTF Loads and all other regulated sources (includes WLAPSW)
FG – Sum of Future Growth Loads (i.e., Potential Permitted Facilities)
MOS – Margin of Safety
Unregulated sources are generally nonpoint sources and typically enter the impaired segment through diffuse, often temporally inconsistent, locations.
As mentioned previously, future growth addresses the sustainably advantageous appeal to account for future loadings that may occur due to population growth and the need to predict commensurate changes in community infrastructure and development.

15. Load Duration Curve Example
Flow Duration Curve
To illustrate how these allocations are typically made, I’d like to use a simple example using load duration curves. But, first I feel I should give a brief explanation of load duration curves. Load duration curves start out as flow duration curves, which with some of you may be familiar. Flow duration curves are simply a frequency distribution of gaged flow values. Flow values are plotted with respect to their frequency of occurrence or duration. The values on the y-axis are flow values and the values on the x-axis are frequency percentiles. The median flow is at the 50% mark.
The median flow is found at the 50th percentile.

16. Load Duration Curve Example
Flow Duration Curve
Highest
Flow
Mid-range
Lowest
For use in bacteria TMDLs, flow duration curves are often subdivided into flow ranges. It is the practice of the TCEQ’s TMDL program to calculate TMDLs using the median flow value in the highest flow range on the curve.

17. Load Duration Curve Example
Highest
Flow
Mid-range
Lowest
Load
Load (MPN/day)
Percent of days load exceeded
Flow duration curves can be converted into load duration curves by multiplying the gaged flow values by the instream water quality criterion; in this case a bacteria criterion. Notice that the y-axis now shows units of MPN/day because we have also used conversion factors to align our flow volumes with our concentrations (i.e., deciliters with cubic feet or cubic meters and seconds with days).
Flow (cfs or cms) * criterion (MPN/100ml) * conversion factor

18. Load Duration Curve Explicit 5% MOS 126 MPN * 0.95 = 120 MPN
If we have bacteria data, we can plot it on the load duration curve using the date of sample collection to pair the flow on the day of collection with the actual bacteria concentration measured on the same day; multiplying the two yields an instantaneous load that can be plotted on the curve. We also often apply the margin of safety for the TMDL upfront by using a fraction of the criterion (1-MOS * criterion), rather than the actual criterion, to convert the FDC into the LDC.

19. Load Duration Curve Allocation to Runoff (LA + Σ WLAPSW)
Σ WLA + FG
Allocation to Runoff
(LA + Σ WLAPSW)
Allocation to WWTFs
(Σ WLAWWTF + FG)
With Load Duration Curves, the curve itself represents the instream TMDL, so we can think of area under the curve as the allocatable load. The load from WWTFs is fairly constant under most conditions so it can be represented as a straight horizontal line. Adding an allocation for future growth to the WWTF load allocation leaves only the load attributed to runoff (hatched). It is this hatched portion of the curve that is apportioned into LA and to WLAPSW using the FDAPSW.

20. Load Duration Curve Allocation to Runoff (LA + Σ WLAPSW)
Allocation to WWTFs
(Σ WLAWWTF + FG)
LA+
Σ WLA
PSW
But, since we’re setting the TMDL at the highest flow regime, in the 10% exceedance range, were looking at only this portion of the graph.
Σ WLA
+ FG

21. TMDL Allocations Final TMDLAU
TMDL
WLAWWTF
WLAPSW LA FG
MOS
0666_01
100.0
10.0
16.0
64.0
5.0
Final TMDL
In this example, let’s assume our units are in billion MPN/day Enterococci
WLAWWTF = 10% of total load
FG = Population increase of 50% (5 billion counts)
Explicit MOS = 5% (5 billion counts)
Runoff load (LA+WLASWP) is 80% of total load
FDAPSW = 20%
This is a table you would typically see in one of our TMDLs, showing a TMDL and load allocations for Assessment Unit 0666_01. In this example I have chosen simple round numbers to represent WLAWWTF, WLAPSW, LA, FG and MOS to simplify the math.
Let’s assume our TMDL is an even 100 billion MPN/day (for example from the median flow value of the high flow range of our flow duration curve). Let’s also assume that the sum of bacteria loads from our wastewater treatment facilities at full permitted flow is 10% of the TMDL and that we estimate our future growth to increase that wastewater load by 50% or an additional 5 billion counts per day (recall the straight line across the bottom of the area of our load duration curve). A 5% explicit margin of safety takes another 5 billion counts from our 100 billion count TMDL, leaving us only 80 billion counts per day for our LA and WLA. The stormwater jurisdictional area within our TMDL watershed comprises 20% of the TMDL watershed area, which strips 16 billion counts from the 80 billion daily counts remaining after our wastewater, margin of safety and future growth are allocated (from the original100 billion count TMDL). This leaves 64 billion daily counts for our load allocation (LA).
New WWTF permit or increased discharge results in a shift in allocations from Future growth to WLAWWTF.
Changes in areas under MS4 results in a shift in allocations between WLAPSW and LA.

22. TMDL Implementation Plans (I-Plans)
Designed to satisfy the TMDL regulatory requirements for “reasonable assurance” of implementation
Determines what will be required to manage the loads from all sources, including stormwater
Describes the activities that will be implemented over a specific time period by the stakeholders to improve water quality
I want to talk a little about TMDL Implementation Plans also known as I-Plans.
They are designed to satisfy the TMDL regulatory requirements for “reasonable assurance” of implementation
An I-plan determines what will be required to manage the loads from all sources, including stormwater
They describe the activities that will be implemented over a specific time period by the stakeholders to improve water quality
I-Plans typically described two categories of activities, Control Actions (which are basically regulatory controls, such as lowering discharge limits for wastewater treatment facilities) and Management Measures (which typically refer to best management practices [or BMPs], the vast majority of which are designed to be implemented voluntarily). BMPs, by the way can be Structural (things such as LID, detention basins, filter strips, porous pavement, etc.) or they can be Nonstructural (such as ordinances, financial incentives, training, education, outreach and even recognition programs).

23. I-Plans in Texas Area = 25,892 Mi2 AUs = 416 Segments = 198 May 2018
Here’s a map of Texas showing the location of watersheds where TMDLs and I-Plans have been implemented or are under development.
They comprise a total area of 25,892 Mi2 and include 198 Segments and 416 Assessment Units.
The watersheds include not only urban areas but also large portions of rural areas. As you can see it’s a significant amount of area in Texas; a little under 10% of the total land area of the state (9.6%).

24. Education & Outreach
I mentioned education and outreach efforts. We see education and outreach not just as a good BMP, but also as one of our most important tools for stakeholder involvement in TMDLs and I-Plans and for the success of our program. Meeting and event attendees often include elected officials and other local decision-makers. We typically rely on our local partners to identify and recruit stakeholders. Local entities, including institutions of higher education, councils of government, river authorities and non-profits, provide education and outreach and assist us by facilitating and organizing public meetings. These groups have provided a variety of workshops and events available to the public, including the Watershed Stewards Program, feral hogs workshops, the Texas Well Owner Network, and the riparian & stream ecosystem workshops. They continue to host these events throughout Texas in addition to one-on-one meetings for stakeholder education and input.

25. Water Quality Educational Events & Exhibits
We are also improving stakeholder participation through water quality educational events and exhibits sponsored directly by the TCEQ or through one of our local partners. Here’s an example of outreach and education for our youngsters of all ages, a wetland planting event. Earth Day Bay Day.

26. Our website provides a lot of the general information I have presented and I want to take this opportunity to mention that our program is not limited to developing TMDLs and I-Plans. With the establishment, recently by the EPA, of the 303(d) Vision Strategy nation-wide, our program can also work to develop alternatives to TMDLs, such as watershed based plans or watershed protection plans, as we call them. So, we are excited about having this flexibility to restore and protect water quality in new and innovative ways.

27. For More Information on TMDLs
TCEQ TMDL Program Website:
Roger Miranda
(512)
Here is the web address, but you can also feel free to contact me anytime for information.
And, I’ll be glad to answer any questions you may have right now.