Modeling

The BMW Association received a large nonpoint source grant in 2005 to write a watershed plan and develop a watershed and in-reservoir model to help develop a pH TMDL and implementation plan.

2007 Modeling Effort

AECOM was hired in 2007 to model both reservoirs and the watershed. The watershed model selected was SWAT (Soil and Water Assessment Tool, developed by USDA) and the reservoir model was WASP (Water Quality Analysis Simulation Program, developed by EPA). These models were calibrated with 2003 and 2004 flow and water quality data.

Initial information from the modeling included current conditions:

– Approximately 90% of the phosphorus came from wastewater
– Internal loading was hard to quantify but it is enough to support hyper-eutrophic conditions on its own.
– Point sources would need to be reduced by 90+% in order to see any water quality changes
– Background pH is above 8.0 because of high Alkalinity
– Water quality goals that would support the pH and DO standards include maximum summer total phosphorus of 0.100 mg/L, Alkalinity of 95 as CaCO3 mg/L, and a chlorophyll-a of 25 µg/L or less.

Once the models were calibrated and current conditions understood, BMW Association came up with eight management scenarios that covered several nutrient reduction levels. The model results for these scenarios provided helpful insight to develop an implementation plan.

Management Scenarios:

Reduce wastewater phosphorus to 1.0 mg/L, 0.5 mg/L, and 0.05 mg/L
No pumping of treated effluent from Metro to Barr Lake
No wastewater at all in the watershed
Reduce wastewater phosphorus to 0.5 mg/L and reduction internal loading by 50% and 90%

Even with removing all wastewater from the watershed, it was a challenge to get total phosphorus below 0.100 mg/L in the reservoirs. There is no silver bullets and all sources of phosphorus will need to be reduced in order to achieve a 96% reduction in annual phosphorus loading.

2013 Modeling Effort

Integral Consulting was hired in 2013 to update both the SWAT and WASP models. Quality and quantity data from 2003 to 2009 were used to re-calibrate the models. Then the models were compared to the 2010 data. There were no major changes but the model was better calibrated that provided more confidence with the modeling outputs. Another set of three management scenarios were analyzed.

Management Scenarios:

Wastewater treatment that included Metro, Littleton/Engelwood, and Centennial Water and Sanitation District all treating to 0.100 mg/L, Aurora treating to 0.200 mg/L, and the rest of the smaller facilities treating to 1.0 mg/L
In-canal treatment that would reduction phosphorus from getting to Barr Lake
Wastewater treatment reductions and in-canal treatment

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