Boundary Condition Assumptions Water Quality of Inflows
June 10, 2009
Boundary Condition Assumptions – Water Quality of Inflows
As part of TMDL development, three conditions for inflows to the river must be characterized: 2001 or “existing” condition (core model), natural conditions (No Source scenario), and TMDL allocation conditions (TMDL scenarios). The following discussion describes the process used to characterize the quality of groundwater, tributaries, municipal and industrial facilities, stormwater, and CSOs.
Groundwater
Existing Conditions - 2001
The estimates of existing quality of groundwater entering the river are documented in PSU’s report describing the core model. Average pollutant concentrations measured in wells in the vicinity of known groundwater inflows were used in the model.
Natural Conditions
As seen in the graph below, the estimated 2001 phosphorus concentrations vary by location. For the No Source scenario, all groundwater was set to the lowest average value used as input in the core model (0.006 mg/l PO4, 0.05 mg/l NH3).
Natural condition inflow/outflow of groundwater to the river is equal to the estimated 2001 flow, plus the mean 2001 point source flows. These flows are re-located to the nearest, downstream groundwater inflow. This added flow is based on an assumption that water discharged by point sources, which is pumped from the aquifer by the community, would have flowed into the river in a distributed manner under natural conditions.
TMDL Scenarios
The TMDL scenarios allocate the existing loading from groundwater. Therefore, the TMDL scenarios include groundwater at 2001 flow and quality.
Tributaries
Existing Conditions - 2001
The estimates of 2001 flow and water quality for the three tributaries (Hangman Creek, Coulee Creek, and Little Spokane River) are documented in PSU’s report describing the core model. Coulee Creek is small relative to the other two tributaries. For the core model and all TMDL analyses, the pollutant concentrations for Coulee Creek are set equal to Hangman Creek concentrations.
Natural Conditions
Pollutant concentrations in the headwaters of Hangman Creek and Little Spokane River are used as estimates of the natural water quality of these streams. Year-round measurements are available for the Scotia monitoring location near the headwaters of the Little Spokane River. These values were input directly into the No Source model setup. For Hangman Creek, only limited data are available for the Calif monitoring location near the headwaters of Hangman Creek (Jan to March, Oct to Dec). Nevertheless, these samples are the best available information. To fill the gaps in the total phosphorus data at Calif, the monthly 2001 conditions were scaled down by 60% until the winter and fall concentrations approximated the concentrations measured at the Calif site (under the assumption that human caused increases in phosphorus follow the same pattern as natural variation). This extrapolation provides a common seasonal pattern for phosphorus between the no source and 2001 conditions, which also provides a consistent baseline for TMDL allocations below.
TMDL Scenarios
The TMDL scenarios require a percent reduction in tributary concentrations which are based on Ecology’s technical analysis for the draft Hangman sediment TMDLs. To determine the average monthly concentration of human-caused pollution, the natural background concentrations are subtracted from the existing (2001) concentrations. The percent reductions are then applied to the human-caused portion of the pollution in order to identify the monthly average “allowable” human increase in pollution, expressed in units of concentration.
The equation defining the TMDL condition is the following:
TMDL conc. = (2001 conc.) – {(% reduction)*(2001 conc. – No Source conc.)}
The % reductions proposed for each tributary are the following:
Hangman/Coulee:
20% (Mar-May)
40% (June)
50% (July -Oct)
Little Spokane:
36% (Mar-Oct)
Municipal and Industrial Facilities
Existing Conditions - 2001
The core model includes time-varying, 2001 discharges from permitted point sources, based on flow and discharge quality data collected by the facilities.
Natural Conditions
Municipal and industrial discharge flows are set to zero for the No Source scenario. The mean 2001 flow from each facility is re-located to groundwater (see discussion of groundwater above)
TMDL Scenarios
The TMDL model scenarios include the municipal discharges at 2027 design flows to account for predicted future growth, while industrial sources at current design flows.
Effluent quality for the TMDL scenarios is set at the estimated “limit of technology” level for each facility and each oxygen-demanding pollutant. Except for a scenario that includes a seasonally-variable limit for city of Hayden, all pollutant concentrations are set at a constant value for the entire March through October period.
Stormwater
Existing Conditions - 2001
The core model does not include explicit stormwater discharges, due to the lack of specific information for 2001, as well as rainfall data indicating that stormwater loadings were not a significant factor in 2001 water quality conditions (compared to point sources, tributaries and groundwater). Based on the need to establish a quantified stormwater allocation in the TMDL, additional analysis has been conducted since completion of the core model (see below).
Natural Conditions
Stormwater flow is set to zero for the No Source scenario.
TMDL Scenarios
The model scenarios for the TMDL condition include the addition of explicit stormwater discharges for both Idaho and Washington. It is not feasible to identify and include all outfall pipes in the model, so these discharges are simplified as a single pipe discharge at the downstream end of the cities of Spokane and Coeur d’Alene.
The pollutant concentrations in stormwater are estimated using data supplied by the City of Spokane. The average total phosphorus concentration in this urban stormwater is estimated at 0.31 mg/l.
The proposed TMDL allocates the existing loading from stormwater. Unlike groundwater and point sources, stormwater flow is highly variable and has a unique flow pattern each year. To develop a conservative and “generic” stormwater allocation (one not tied to low flows and random events of 2001), the daily mean flow (estimated from March to October mean precipitation) was used as a constant flow input in the model. The mean is used under the assumption that an average precipitation year could occur during a low base flow year like 2001. This assumption and other technical assumptions in the calculation of the runoff are elements of the margin of safety.
Combined Sewer Overflows (CSOs)
Existing Conditions - 2001
CSOs, like stormwater, were not included in the core model, due to the lack of specific information for 2001 and a determination that stormwater loadings were not a significant factor in 2001 water quality conditions (compared to point sources, tributaries and groundwater). Nevertheless, during major storms in Spokane, combined sewers are expected to overflow and discharge to the river. Based on the need to establish a quantified CSOs allocation in the TMDL, additional analysis has been conducted since completion of the core model. This analysis relies on city of Spokane estimates of the average discharge and pollutant concentrations from CSOs.
Natural Conditions
CSO flow is set to zero for the No Source scenario.
TMDL Scenarios
The proposed TMDL allocates the existing loading from CSOs. Like stormwater, CSO flow is highly variable and has a unique flow pattern each year, and the TMDL scenario includes the daily mean flow (scaled to the March to October period from the city’s annual estimates). The mean is used under the assumption that an average precipitation year could occur during a low base flow year like 2001. This assumption is an element of the margin of safety.
Supporting information
Groundwater
–Spokane County groundwater data, PSU model development reports
Tributaries
– Ecology water quality data, PSU model development reports, spreadsheets for estimation of natural condition and TMDL condition, proposed Hangman Creek TMDL, USGS flow data
Municipal and Industrial Facilities
– Facility monitoring data and growth predictions, EPA memo on phosphorus treatability.
Stormwater
– Simple method for flow estimation(available online links), spreadsheet for flow calculations, City of Spokane discharge quality data, basin impervious area from USGS aquifer study, National Weather Service precipitation data, EPA NURP study
CSOs
– City of Spokane CSO flow and discharge quality data, National Weather Service precipitation data
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