General
Approach for Modeling Bacterial TMDLs on the
Stephanie L. Johnson, CRWR
TABLE OF CONTENTS
Research
is currently being conducted at the Center for Research in Water Resources to
develop a general method for modeling bacterial total maximum daily loads (TMDLs) along the Middle Bay System of the
Figure
1: Six
The TMDL
program is based on regulations stated under Section 303 of the U.S.
Environmental Protection Agency's (EPA) Clean Water Act. Under these
regulations, states are required to list waterbodies
that do not meet water quality standards; for each of these waterbodies
a TMDL must be developed. The TMDL is an allocation of pollutant loading
(to all contributors within a watershed) based on the calculated total amount
of pollutant that a waterbody can receive
while still meeting water quality standards. The state agency in charge
of TMDLs in
CRWR's previous work on this topic has
concentrated on the
Figure 2:
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Load duration curves are one method that has been used to successfully develop TMDLs for bacterial impairments. A load duration curve is a graph depicting the percentage of time that a given load (CFU/day of bacteria, for example) is equaled or exceeded. The pollutant load at a given point in a stream is calculated by multiplying the flow by the pollutant concentration and a conversion factor; the result is a pollutant load (typically load per day). Target loading curves are created by combining a flow duration curve with the water quality standard for the parameter of interest. Measured pollutant concentrations are then converted to loads and added to the curve. A point falling above the target curve is out of compliance; a point falling below the curve is in compliance. An example load duration curve is shown in Figure 3.
Figure 3: Sample Load Duration Curve
TCEQ
has recently started using the load duration curve method for calculating
bacterial TMDLs. This method is attractive
because it effectively integrates flow regimes into TMDL analyses, clearly
communicates data through a method that is understandable to the general public
and has historically been acceptable to the EPA. A webscraper
tool has been developed within the Excel software to automatically compute
duration curves for a given water quality monitoring site within the state of
Figure 4: Load Duration Curve Tool
Future
work on this topic will be to extend load duration curve modeling from a riverine analysis to include bacterial loadings to the
bays. Methods will be developed to account for bay hydraulics and tidal
exchanges, while keeping the analysis simple enough for use in preliminary TMDL
development. Modeling methods that have proven successful in the
For more information on this project, please contact:
Stephanie L. Johnson, P.E.
Ph.D.
Candidate
Center for Research in Water Resources
University of Texas at Austin
email: sljohnson@mail.utexas.edu
These materials may be used for study, research, and education, but
please credit the authors and the Center for Research in Water Resources, The