Boise River Total Phosphorus Mass Balance Model Ben Cope EPA Region 10 Office of Environmental Assessment
• Impetus: City of Boise Offset Project • Adapted from earlier CH2M Hill model – Excel spreadsheet…simple – Removed TMDL allocation calculators – IDEQ review of data-to-model accuracy • Mainstem Boise River only – Follows south channel at Eagle Island • Mass balance of total phosphorus – Assumes no loss due to instream processes – Steady state – snapshot in time • Two periods analyzed – July 2001 – low flow (approx. 30Q10) – August 2000 – higher, more typical summer flow
Mainstem Inflows and Diversions These are flows >20 cfs in July 2001 Boise River at Boise New York, Ridenbough, Settler’s, Boise Lander WWTP Boise City Canal, Thurman Mill Total number of boundary New Dry Creek inputs = 75 West Boise WWTP North Channel Routing Thurmon Phyllis, Eureka Canyon, Caldwell High Line 15 Mile, Middleton WWTP, Star Feeder, Mill Slough, Star WWTP, Mason Drain Riverside Hartley (Combined) Sebree Indian Creek (includes Nampa WWTP) Caldwell WWTP Eureka , Lower Center Point Conway Gulch, Dixie Drain Island High Line, McConnel Island Groundwater (distributed inflow Boise River at Parma from Caldwell to Parma)
Development Process • Selection of time frames • Input data review (with IDEQ) • Flow evaluation • Total Phosphorus evaluation • Model application/scenarios • Documentation
Mainstem Flow – July 2001 Mass Balance Estimate and Measured Instream Flow Boise River on July 12, 2001 3500 Model with GW 3000 Measurement Mass Balance Model - Calculated Flow 2500 Flow (cfs) 2000 1500 1000 500 0 75.0 70.0 65.0 60.0 55.0 50.0 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 River Mile
Mainstem Flow – August 2000 Mass Balance Estimate and Measured Instream Flow Boise River on August 15, 2000 4500 Model with GW 4000 Measurement 3500 Mass Balance Model - Calculated Flow 3000 Flow (cfs) 2500 2000 1500 1000 500 0 75.0 70.0 65.0 60.0 55.0 50.0 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 River Mile
Total Phosphorus – July 2001 Estimated vs Measured Instream Phosphorus Boise River: July 2001 0.60 Instream Measurement 0.50 Mass Balance Model 0.40 Total Phosphorus 0.30 0.20 0.10 0.00 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -0.10 River Mile
Total Phosphorus – August 2000 Estimated vs Measured Instream Phosphorus Boise River: August 2000 0.50 0.45 Instream Measurement 0.40 Mass Balance Model Total Phosphorus 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 River Mile
Application City of Boise Offset Scenarios Predicted Instream Phosphorus Boise River: July 2001 Conditions Buildout Effluent Flow (39 mgd) 0.400 Existing Condition 0.342 No Offset - B45 0.316 0.300 No Offset - B70 0.304 Total Phosphorus Offset - B225 0.263 Offset - B350 0.200 0.100 0.020 0.000 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 River Mile
Key Assumptions/Uncertainties • Groundwater flow and TP concentration – Uniform flow in lower reach – Concentration set at mean drain concentration • Consistent with USGS sampling mean but limited samples • Drain TP data is “patchy” – Synoptic sampling by USGS will help ground truth • Offset assumption: TP removed from mainstem by diversions does not return to mainstem – How much comes back to mainstem: unknown – Thorny problem in past TMDL efforts
Documentation
Questions? Acknowledgements: Sherrill Doran, CH2M Hill Darcy Sharp, IDEQ Susan Beattie, IDEQ Bill Stewart, EPA
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