## Third Creek STREAMDO Model
Overview STREAMDO was the model chosen to predict the DO levels in Third Creek resulting from oxygen demanding loads. STREAMDO is a model that is capable of modeling a wide range of processes that affect water quality. In additio n, STREAMDO has been used to model Segment 15 of the Platte River and used widely throughout the U.S. Environmental Protection Agency (EPA) Region VIII.
The basis for model construction is data collected as part of the loading based sampling events. The re mainder of this appendix discusses model construction, calibration, and validation.



## Model Construction STREAMDO is a spreadsheet model employed to determine steady state DO concentrations as well as ammonia toxicity. The format is very convenient for modif ying model structure and inserting additional processes. The following are key construction components of the model:

 Streamflow

 Stream velocity

 Stream temperature

 Observed instream DO concentrations

 Minimum and maximum daily DO concentrations

 CBOD

 Organic nitrogen 5

 Total ammonia

 Un

- ionized ammonia

 Nitrate and nitrite

 pH

 Sediment Oxygen Demand (SOD)

 Stream depth

 Chlorophyll

- a
STREAMDO calculates water quality conditions in specified computational elements or "segments" throughout the water body of interest. The model requires that data be entered for each segment entering the water body and upstream conditions. Observed water quality parameter values are entered for the conditions at the end of each model segment for output comparison. The model is constructe d to simulate the mainstem of Third Creek starting from below the C+ pond, WQTC01, to where Third Creek meets Tower Road, WQTC06. This section of the creek that the model focuses upon is approximately 4 miles in length. The mainstem of Third Creek has been separated into four uniform segments, each representing a 1

- mile reach as shown in the following figure.

## Model Calibration and Validation The results of receiving water simulations are more meaningful when some sort of confirmatory analysis accompanies them. Calibration involves minimization of deviation between measured field conditions and model output by adjusting parameters of the model. Data required for this step are a set of known input values along with corresponding field observation results. T he results of the calibration effort provide information as to which parameters have the greatest effect on output. Validation involves the use of a second set of independent information to check the model calibration.
During Loading Events 1 and 2, data w as collected on two occasions; April 12, 2001 (Event 1) and April 23, 2001 (Event 2). The second sampling event was chosen for model calibration because of its typical characteristics; flow, loading, and deicing operations at the airport. During the first sampling event, the airport was shut down for a period creating atypical loading conditions in the creek.
Due to the variation in events at DIA, it was appropriate to also calibrate to a mid

- winter cold event where runoff volume is relatively small compare d to typical fall and spring storms that generate more runoff. After comparing data collected during LB3 with LB4 it was determined that data from LB4 should be used for calibration purposes although results from an attempt to calibrate to LB3 conditions a re also contained within this appendix. Both events occurred during low flow and freezing conditions.
The major calibration target is DO concentrations in Third Creek. For LB4, the travel time in Third Creek during the events was used to assess which DO co ncentrations to set as calibration targets in the model. The travel time of Third Creek was estimated at 3.9 miles per day based on an average measured stream velocity of 0.24 feet/second during the fourth event. The actual travel time is most likely less than the estimated value due to freezing on Third Creek during portions of the day during the sampling period. Based on the estimated travel time, it is anticipated that the de

- icing load from the precipitation event should have passed by WQTC04 by January 26, 2002. Therefore, instantaneous DO concentrations measured in the field during LB4 were utilized for calibration purposes. 

## Input Parameters The parameters inputted to STREAMDO prior to calibration of model coefficients are: flow, velocity, depth, temp erature, DO, DeltaDO, CBOD 5 Upstream #1 WQTC02 #2 WQTC03 #3 WQTC04 #4 WQTC06

4 3 2 1 0 Mile: WQTC01 WQTC05

, organic nitrogen, ammonia, Model Segmentationun

- ionized ammonia, nitrate

- nitrite, pH, and chlorophyll

- a . These values were measured or calculated based on data collected in the field.


### Loading Based Events One and Two The April 23, 2001 loading e vent (LB2) sampling data was chosen for model calibration for spring conditions. Two flow values were used in model input. One was an initial flow of 14.68 cfs plus tributary flow. Tributary flow was calculated using two methods. The first method was to us e the measured tributary flow 0.07 cfs at WQTC05. The second method was part of the calibration procedure where the difference in observed mainstem flow was considered to be the tributary flow contribution. During LB2, the mainstem flow rates at WQTC01 and WQTC02 were 14.68 and 16.63 cfs respectively. Thus, the tributary flow, the difference in mainstem flows, was calculated to be 1.95 cfs.
The velocities used in the model were based upon data collected during loading sampling LB2. These were the actual ve locities observed in the creek. This assumption can be contrasted by using the flow contributions in the mainstem from the upstream reach and tributary to back calculate the velocity in each reach, thus keeping continuity in the mainstem. The velocity in t he mainstem was calculated as the average velocity from the sampling event data at each location. This computation takes the cross

sectional area and divides it by total flow. The average depth in the channel was calculated at each location using the cross

- sectional data collected during the sampling event. The mean depth is calculated by the cross

- sectional area divided by the width.
The temperature, DO, and pH data was obtained from the field measurements taken at the sampling event by an instream monitor ing device. This data can be found in the Section 3 of this report.
The model parameter DeltaDO is the difference between the daily maximum DO concentration and the minimum DO concentration. The DeltaDO is used to estimate the amount of DO used in daily ph otosynthesis activities and used to estimate the total DO deficit in the stream. This amount is estimated from data collected by two in situ DO sampling devices located in the creek. During this second sampling event, the DeltaDO is estimated to be 1.25 mg /L.

CBOD 5

The SOD was estimated from literature values. Typi cal SOD values for sandy bottom streams ranged from 0.2 to 1.0 g/m data were obtained from LB2 sampling results (see Section 3). The concentrations for ammonia, un

- ionized ammonia, nitrate

- nitrite, and chlorophyll

- a were also obtained from LB2 sampling results.

2 /d. In the temperature range 0 to 5 **°** C, SOD rates approach zero (Chapra 1997). Due to winter, conditions and temperatures observed during the loading sampling event an SOD value of 0.1 g/m 2 / d was used as a model input.

### Loading Based Events Three and Four During both the third and fourth events, some of the sites on Third Creek were iced over, which made flow measurement difficult. Flow was determined by sites, which were mostly free of ice o r where ice was easily broken making flow measurement possible. The following sites were used to determine average flow in Third Creek during LB3: WQTC01, WQTC02, WQTC05, WQTC06. A total flow of 0.4 cfs was used for modeling purposes and a tributary flow b ase of 0.04 cfs was assumed based on measurements at WQTC05. During the fourth event the following sites were used to determine average flow in Third Creek: WQTC01, WQTC02, and WQTC03. A total flow of 0.22 cfs was used for modeling purposes. Since flow wa s obstructed at WQTC05 due to construction, a tributary flow base of zero was assumed. Average velocities and depths were calculated using the stations designated for each event. Calculations for flow can be found at the end of this appendix.

Observed tem perature, DO, CBOD 5



## Calibration Parameters , organic nitrogen, ammonia, un

- ionized ammonia, nitrate

- nitrite, pH, and chlorophyll

- a were based on observed data from field data collection. DeltaDO was calculated using the Sonde located downstream of WQTC01. The average of the daily DeltaDO for the period of data collection was used for calculation purposes. DeltaDO is used in STREAMDO for calculating the photosynthesis coefficient used in the model.

Rate coefficients representing reaeration, BOD, nitrification , and SOD are utilized by STREAMDO to predict instream DO levels. Following is a description of these parameters:

 K1 is the carbonaceous deoxygenation rate coefficient (l/day)

 K2 is the reaeration rate coefficient (l/day)

 K3 is the ammonia nitrification ra te coefficient; reflects nitrogenous oxygen demand due to conversion of NH 3 to (NO 2 + NO 3

 K4 is the ammonification rate coefficient; reflects the conversion of organic nitrogen to ammonia nitrogen (l/day) ) (l/day)

 K5 is the ammonia loss rate coefficient; r eflects non

- DO consuming activities, which result in a loss if ammonia such as ammonia volatilization and direct assimilation to aquatic life and bacteria (l/day)

 K6 is the nitrate/nitrite loss coefficient; reflects non

- DO consuming activities, which resul t in loss of nitrate/nitrite including denitrification and direct assimilation to aquatic plants and bacteria (l/day)

 S Benthic demand coefficient; reflects the DO burden caused by the benthic utilization, otherwise known as SOD (g/m2/day)The typical rang es for the STREAMDO parameters as stated in the QUAL2E Users Manual (EPA/600/3

- 87/007, 1987), EPA's Rates, Constants, and Kinetics Formulations in Surface Water Quality Modeling (EPA/600/3

- 85/040, 1985) and Chapra (1997) are shown in the following table.

P ublished Parameter Ranges Coefficient Range CBOD 5 coeff K 1 0.02

- 3.4 (1/day) Reaeration coeff K 2 0

- 100 (1/day) NH 3 coeff K 3 0.1

- 2.0 (1/day) Organic N coeff K 4 0.02

- 0.4 (1/day) NH 3 Loss K 5 (a) (1/day) NO 2 + NO 3 K 6 0

- 1.0 (1/day) Benthic Dema nd S (g/m 2/day) 0.2

- 1.0

(a) No published ranges available, assumed low due to lower pH observed during data collection activities.

The calibration effort showed that K1 and K2 had the most effect on DO concentrations. K2 was inputted into the model prior to modifying K1 concentrations as K2 was calculated using the Owens and Gibbs formula:

where: U is the mean velocity in fps H is the mean depth in feet

Due to LB2 conditions, the ammonia nitrification rate coefficient, K 3 , was determined to be at t he lower end of published values due to winter conditions, indicating a slow conversion from ammonia to nitrate and nitrite. This slow reaction rate was also seen in the parameters for ammonification rate (K 4 ), ammonia loss rate (K 5 ), and benthic oxygen de mand (S). The nitrate/nitrite loss rate (K 6

The model was calibrated using Event 2 data in two ways. First, the model was calibrated with observed data assuming no additional loading or flows. Second, the model was calibrated again with observed data but with an additional CBOD ) was calibrated using published values as a guide, but there is a sudden loss in concentration in the observed values at stations TC04 and TC06 indicating a higher than normal loss rate in the third and fourth se gments.

5 loading (fl ow and concentration) in the first segment forcing the CBOD 5

The concentration of CBOD conditions to reflect the concentrations observed in Third Creek at WQTC02. Currently, the model does not include any additional groundwater or lateral inflows due to additional drainage area oth er than the tributary (WQTC05) entering Segment 1.

5 85

. 1 67

. 0

2 

## 6

. 21 H U k **=**

in the first segment of the model was calculated by the mixing of upstream conditions and the loading from the tributary draining the northern part of Runway 34/16, WQTC05. The c oncentrations of the upstream and tributary sections are 267 mg/L and 212 mg/L respectively for Event 2. The loading of tributaryitself does not increase the CBOD 5 concentrations to the observed levels downstream at WQTC02 of 433 mg/L. It is likely that t he timing of Event 2 sampling "missed" the loading in the upstream and tributary sections and captured these loadings downstream. Regardless of cause of increase in CBOD 5

In t he forcing model, the CBOD , this increase was accounted for in the "forcing," or second calibration method.

5 concentrations for the tributary contributions from the northern portion of Runway 34/16, WQTC05, were increased from the observed concentrations of 212 mg/L to 1,689 mg/L. The tributary concentration is calculated using the fol lowing formula:

where Q is the flow in cfs C is the concentration in mg/L
This forcing of the CBOD 5 concentration is due to an increase in concentration along the creek. The incremental increase in CBOD 5

The DO in Third Creek is a function of the carbonaceous deoxygenation rate, reaeration rate, photosynthesis/respiration, and the SOD. These rates are calibrated by using the data co llected during Event 2 as discussed in the previous section. concentration was forced by increasing the tribu tary concentration and calculated using the mass balance described above.

When the CBOD 5 loading is increased because of adjusting the CBOD 5

Figures for the aforementioned CBOD loading from the tributary, there is additional oxygen demand. This oxygen demand depletes the available DO levels observed in the creek. The effect is seen where the DO concentrations in Third Creek are lower than the nonforced model.

5 The sampling event data in the mainstem of Third Creek during LB2 shows an unexplained decrease in nitrate/nitrite at WQTC04. The concentrations of nitrate/nitrite observed at WQTC03 drop from 0.38 mg/L to 0.21 mg/L and 0.11 mg/L at WQTC04 and WQTC06 respectively. The model was not able to capture this observed shar p decline in the creek without an extremely high nitrate/nitrite loss coefficient of 50 (1/day) in Segments 3 and 4. Currently, there is no basis for this assumption. These low nitrate/nitrite levels likely do not have much affect on DO and calibrating to such a fine increment is not warranted. and DO can be found at the end of this appendix.

For the third and fourth events, the Owens and Gibbs formula was appropriate for use; however, based on the velocities, depths, and ice observed in the field, the original values were multiplied by 0.5 and 0.2, respe ctively, to account for less reaeration due to ice cover. The Owens and Gibbs formula should also be used for depths greater than 0.4 

## ( **)**

**Tributary TC**  **TC TC**  **TC Tributary**  

### Q **C Q**  **C Q**  **C** 01 01 02 02 

### * *

**−**

**=**feet and less than 11 feet and for velocities greater than 0.1 feet per second and 5 feet per second. The average depth ob served in the field was below this range slightly. The average depth used in the STREAMDO model was 0.26 feet during LB3 and 0.25 feet for LB4, and the average velocity 0.22 and 0.24 feet per second, respectively. A calculated K2 value of 109 was found for LB4. The calculated K2 is thus based on values slightly outside of the appropriate range for the equation. This is one of few equations available to calculate K2 for small streams such as Third Creek. As discussed previously, the calculated reaeration rat e was multiplied by an "ice cover factor." Recommended factors range from 0.05 for complete ice cover to 1.0 for no ice cover. Depending on the extent of cover, reaeration values can be greatly reduced (EPA 1600/3

- 87/007, 1987).

K1, K3, K4, K5, and K6 were modified to match observed data assuming that instream concentrations vary slightly even after deicing loads pass through Third Creek. Following are the coefficient values used in the calibration effort for LB3 and LB4:

 K1

– 1.0 for LB3 and 2.3 and 1.3 f or LB4

 K3

- 2.0

 K4

- 0.2

 K5

- 0.01

 K6

- 0.2
K1 was assumed higher for LB4 during the first segment because easily consumable materials could be broken down more rapidly than at the downstream locations. K3 and K4 were selected on the upper end of the re commended range based on observed data. K5 was assumed low based on lower pH and water temperatures noted during data collection. The K6 value selected is on the lower end of the recommended range based on instream temperatures and observed nitrate concent rations.
SOD was calculated using the following formula based on temperature (Chapra 1997):



## S = S 20 **θ**

where: S = 0.5 **θ** = 1.065 (T

- 20)



## Discussion This appendix contains the STREAMDO model output files for the two events (LB2 and LB4) that were chosen to brac ket the seasonal aspects of runoff in Third Creek related to deicing activities. For LB2, the model observed versus model predictions were considered adequate, although the flow conditions were "forced" to match observed conditions. For LB4, the calibratio n effort was not as robust and observed CBOD 5 concentrations were not matched sufficiently predicted CBOD 5 levels, although observed DO concentrations matched predicted DO concentrations. The results of this modeling analysis suggests that a steady

- state m odel is difficult to apply to the Third Creek system which experiences very dynamic runoff conditions and pollutant loads due to variability in conditions experienced throughout deicing season.