Second Quarterly Progress Report September 2004
Highland Silver Lake Watershed (ROZA)
Limno-Tech, Inc. Page 2
characteristics include a watershed with predominantly agricultural land use, and
a lake impaired by manganese, total phosphorus, low dissolved oxygen, aldrin and
• Management objectives: These objectives consist of the specific questions to be
addressed by the model. For this application, the management objective is to
define a credible TMDL.
• Available resources: This corresponds to the amount and time and data available
to support TMDL development. Water quality data currently exist for Highland
Silver Lake. One aspect of this work is to define whether or not the existing data
are sufficient to allow development of a credible TMDL.
Several modeling frameworks potentially applicable for developing TMDLs were
identified, spanning a range of detail from simple to complex. Selection of a specific
modeling framework is complicated by the fact that the definition of a “credible” TMDL
depends upon the level of detail to be contained in the implementation plan. If the goal of
the TMDL implementation plan is to define the primary sources of impairment and
quickly identify the general level of reduction required, relatively simple models can be
used to develop a credible TMDL. If the goal of the TMDL implementation plan is to
explicitly define the specific levels of controls required, more detailed models (and
additional data) are required to develop a credible TMDL. Specific recommendations are
provided which correspond to the level of detail provided in other Illinois TMDL
implementation plans conducted to date.
The recommended approach consists of using the GWLF and BATHTUB models to
address total phosphorus, dissolved oxygen, and manganese problems in Highland Silver
Lake. Specifically, GWLF will be applied to calculate phosphorus loads to Highland
Silver Lake over a time scale consistent with the nutrient residence time of Highland
Silver Lake. BATHTUB will then be used to predict the relationship between
phosphorus load and resulting in-lake phosphorus and dissolved oxygen concentrations.
This relationship will be used to define the dominant sources of phosphorus to the lake,
and the extent to which they must be controlled to attain water quality standards. Simple
empirical spreadsheet methods would be used to define the expected system response
time to attain aldrin and chlordane standards. Application of these models will require no
additional data collection.
Two alternative approaches are also provided. The first alternative approach would not
include any watershed modeling for phosphorus, but would focus only on determining
the pollutant loading capacity of the lake. This approach would be used to determine
existing loading sources, prioritize restoration alternatives and support development of a
voluntary implementation plan that includes both accountability and the potential for
adaptive management. Other alternative model frameworks are also provided in the
event that more detailed implementation plans are desired. These frameworks have
significantly greater data requirements, and their use would require additional data
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