v 7-1
DRAFT FINAL REPORT FOR USEPA APPROVAL
Section 7
Model Development for Vandalia Lake
7.1 Basis for pH TMDL
The relationships between pH, chlorophyll "a," and phosphorus were discussed in
Section 5.1.5.1.2. Figure 7-1 shows the relationship between chlorophyll "a" and pH in
Vandalia Lake. As explained in Section 5.1.5.1.2, the figure is expected to show an
increase with pH as chlorophyll "a" increases. The relationship between chlorophyll
"a" and phosphorus in Vandalia Lake is shown in Figure 7-2. Likewise, this figure is
expected to show a direct relationship between the constituents. The relationships
presented in Figures 7-1 and 7-2 provide general trends between model constituents
and represent the data available from sampling. The lack of a well-defined relationship
for Figure 7-2 confirms the need for a larger data set. The general relationships shown
in Figures 7-1 and 7-2 suggest that controlling total phosphorus will decrease
chlorophyll "a" concentrations, which will in turn bring pH into the range required for
compliance with water quality standards. The TMDL will be based on the existing
relationships with the knowledge that a larger data set would result in a more robust
TMDL. It is therefore recommended that a TMDL endpoint of 0.05 mg/L for total
phosphorus for Vandalia Lake be utilized so that the pH standard is achieved.
7.2 Model Overview
The models used for the TMDL analysis of Vandalia Lake were GWLF and
BATHTUB. These models require input from several sources including online
databases, GIS-compatible data, and hardcopy data from various
agencies. This section describes the existing data reviewed for
model development, model inputs, and model calibration and
verification.
Schematic 1 shows how the GWLF model and BATHTUB
model is utilized in calculating the TMDL. The GWLF model
predicts phosphorus loads from the watershed. These loads are
then inputted in the BATHTUB model to assess resulting
phosphorus concentrations. The GWLF model outlined in
Schematic 2 shows how
GWLF predicts phosphorus
loads from the watershed.
The transport block of the
GWLF model uses the
Universal Soil Loss
Equation to determine
erosion in the watershed.
The transport block also calculates runoff based
on the SCS Curve Number equation. The nutrient
block allows the model user to input
Schematic 1
Models used for
Vandalia Lake
TMDL calculation.
GWLF
BATHTUB
TMDL
CALCULATIONS
NUTRIENT
BLOCK
TRANSPORT
BLOCK
WEATHER
BLOCK
GWLF
OUTPUT
Schematic 2
GWLF Model.
