I. EMPIRICAL STREAM, LAKE, and ESTUARY MODEL ASSUMPTIONS FOR CONVENTIONAL POLLUTANTS A. 7Q10 Flow Values 1. The 7Q10 flow in unregulated, natural streams is to be determined from Low-flow and Flow-Duration Characteristics of Mississippi Streams, U.S.G.S., Report 90-4087 (hereinafter "Report 90-4087") or the most recent update of this publication.2. 7Q10 value of a gage will be used directly if gaging station is at or near the point of discharge.3. 7Q10 flow coefficients (7Q10 value in CFS/drainage area in square miles) of a gaging station will be used to calculate a 7Q10 value for a point discharge if there is a gaging station on the stream or on a nearby stream. An average 7Q10 flow coefficient may be used if there is more than one nearby gaging station.4. 7Q10 flow coefficients can be taken from Report 90-4087 if no gaging station is available. The value will be assumed to be in the middle of the given range.5. A 7Q10 flow coefficient of 0.0 cfs will be used for intermittent streams or when the Report 90-4087 lists the 7Q10 flow coefficient as less than 0.01 cfs per square mile.6. The annual 7Q10 flow will be used for seasonal winter allocations, unless data is available to determine seasonal or monthly 7Q10 flows.7. Semi-annual, quarterly or monthly 7Q10 flows with their respective average maximum temperatures may be used to determine various seasonal wasteload allocations.8. In regulated streams the legally guaranteed minimum flow will be used for allocations unless otherwise provided in these regulations.9. Spatially distributed flow will be included to account for flow gained at 7Q10 from sources other than major tributaries.10. Spatial flow will be calculated between gaging stations if available.11. Spatial flows will be determined by using 7Q10 flow coefficients where sufficient gaging stations are not available.12. Spatial flow will be included at equal increments over the length of a given stream segment. B. 7Q2 Flow Values The 7Q2 flow in unregulated, natural streams will be used in conjunction with the other assumptions contained herein for establishing permit limitations for storm water permits. The 7Q2 flow will be determined from Low-Flow and Flow-Duration Characteristics of Mississippi Streams, U.S.G.S., Report 90-4087 or the most recent update of this publication. In cases in which either (1) the data is indefinite or inconclusive, or (2) the 7-day, 2-year minimum flow and/or the 7-day, 10-year minimum flow are inappropriate because of the hydrology of the area, other appropriate State and federal agencies will be consulted in establishing the applicable stream flow.
C. Temperature The criteria for temperature selection are as follows:
1. For streams for which sufficient temperature data is available, the design temperature will be the average daily maximum temperature for the months of July and August.2. For streams with insufficient or no temperature data, the following will be assumed: | Stream Size | Annual | Summer (May-Oct) | Winter (Nov-April) |
| Streams with minimum low flows 300 cfs | 30°C | 30°C | 20°C |
| Streams with minimum low flows 50 cfs and [LESS THAN] 300 cfs | 28°C | 28°C | 20°C |
| Streams with minimum low flows [LESS THAN] 50 cfs | 26°C | 26°C | 20°C |
D. Velocity Time-of-travel measurements will be used when available. Average reach velocity can be determined by completion of a dye tracer study. A minimum velocity of 0.1 fps will be used.
Estimation Procedures:
Click Here To View Image
where:
V = velocity in fps
QAct = 7Q10 + discharge flow in cfs
S = stream slope in ft./mile
QAvg = the average stream flow in cfs
E. Depth Depth will be used if accurate stream depth profiles are available as determined by measurement or available flood plain maps. For larger, slow-moving rivers, depths may also be estimated. In the WASP models, minimum depth will be one-half of the estimated or measured depth.
F. Slope 1. Stream slope determinations will be made from GIS computer software, NHD Plus values, USGS quad maps, or flood plain reports.2. Stream slope profiles will be analyzed (elevation vs. mile) to determine if the slope changes along the stream length being modeled.3. Model stream segments will correspond to noticeable stream slope changes.G. Kd (Carbonaceous Deoxygenation Rate)1. When usable field data are not available, the stream's Kd rate will be based on both the type of wastewater, type of treatment and/or the instream CBODu concentration.3. When instream CBODu values approach background conditions the Kd rate will be set to 0.15/day and the ratio of Ka/Kd = 2 or Ka = 0.3/day.4. When actual data are available, the Kd rate will be determined according to the procedures outlined in Rates, Constants, and Kinetics Formulations in Surface Water Quality Modeling (Second Edition), EPA/600/3-85/040 or most current version.5. Kd is assumed equal to Kr (overall rate of CBODu removal from the water column) in most model applications.6. Normally, a laboratory-derived "bottled" CBODu decay rate taken from the effluent only (Kl) will not be used in modeling. Typically, an instream decay rate will be used for modeling purposes.7. Kd = 0.3/day (base e) at 20°C will be used for streams receiving upgraded lagoon effluent (single or multi-cell lagoons upgraded with sand filters, artificial wetlands, etc.).8. The following clarification can be used in the estimation procedure for Kd, CBODu rate. | Type of Treatment | Instream CBODu (mg/l) | Instream CBOD5* (mg/l) | Kd (base e @ 20°C) (day-1) |
| Lagoon (CBOD5=30) | 15 | 10 | 0.6 |
| Lagoon (CBOD5=30) | [LESS THAN] 15 and 7 | [LESS THAN] 10 and 4.7 | 0.4 |
| Lagoon (CBOD5=30) | [LESS THAN] 7 | [LESS THAN] 4.7 | 0.3 |
| Mechanical (CBOD5[LESS THAN]30) | 7 | 4.7 | 0.4 |
| Mechanical (CBOD510) | [LESS THAN] 7 | [LESS THAN] 4.7 | 0.3 |
| Mechanical (CBOD5[LESS THAN]10) | - | - | 0.3 |
Note these values are estimates. If actual data are available, they should be used.
H. Kn (Nitrogenous Deoxygenation Rate) 1. Kn has been found to range from 0.3 to 1.5 per day (at 20°C) in free-flowing streams containing greater than 2 to 3 mg/l of dissolved oxygen. Impounded streams or streams with low DO levels will exhibit Kn's as low as 0.0 to 0.3 per day.2. In the absence of measured values, Kn (base e @ 20°C) will be assumed as 0.3 per day for streams with slope less than or equal to 20 ft./mile and 0.5 per day for streams with slope greater than 20 ft./mile. When actual data are available, the Kn rate will be determined according to the procedures outlined in EPA/600/3-85/040.I. Ka (Reaeration Rate) 1. For small streams in Mississippi the most appropriate formula for calculating the reaeration coefficient is the one developed by E. C. Tsivoglou. Click Here To View Image
Where:
Ka = reaeration rate, 1/day
C = escape coefficient, 1/ft.
S = slope, ft./mile
V = velocity, mile/day
2. Assume escape coefficients recommended for Mississippi.(a) C = 0.11 for stream flow less than 10 cfs(b) C = 0.0597 for greater than or equal to 10 cfs to a stream flow less than 280 cfs3. O'Conner-Dobbins equation may be used for streams with depths greater than 5 feet and where there are adequate stream depth profiles or reasonable estimates available. If stream flow is less than 280 cfs, Tsivoglou escape coefficient values should be considered. Click Here To View Image
where:
V = velocity in ft./sec.
D = depth in ft.
Ka = reaeration rate 1/day (base e @ 20°C).
4. A minimum Ka value of 0.15/day will be used except in the case mentioned under Kd where Ka/Kd is not less than 2.5. In the WASP model COVAR may be considered.J. Stream Background Conditions Assume the following stream background conditions unless data show otherwise.
1. DO = 85[CENT] of saturation at assumed stream temperature (table attached)K. Photosynthesis / Respiration 1. Input values for P and R (mg/l/day) can be determined in stream studies using the:(c) Light/Dark Bottle Method2. In the absence of field data, P and R will be assumed to be 0.0 mg/l/day. This assumption will be reevaluated for streams dominated by algae.L. Sediment Oxygen Demand 1. In the STREAM Model, sediment oxygen demand ("SOD") (mg/l/day) will be assumed to be 0.0. In WASP and other dynamic models, SOD rates may be used to calibrate the model. All values used for SOD rates will be within normal ranges found in the ecoregion being modeled.2. Where SOD rates have been determined or sludge blankets are known to exist, SOD will be incorporated in models.3. SOD rates in g/m2/day will be converted to mg/l/day according to the following equation: Click Here To View Image
where:
S = SOD rate in mg/l/day @ 20°C
B = SOD rate in g/m2/day @ 20°C
H = average reach depth in feet
M. Wastewater Inputs 1. The Department's water quality model, STREAM uses first order kinetics to characterize ultimate CBOD decay. Once effluent limits are set using this model, CBOD5 will be determined for inclusion in the permit.2. The following ultimate CBOD to CBOD5 ratios will be used when actual data are not available. | Wastewater | Ratio |
| Sanitary (mechanical secondary) | 1.5 |
| Sanitary (advanced) | 2.3 |
| Food Processing | 3.0 |
| Meat/Poultry Processing | 2.5 |
| Pulp/Paper | 5.0 |
| Tannery | 3.0 |
| Textile | 3.0 |
3. Industries will be encouraged to provide actual ultimate CBODU and NBODU values for the wastewater under evaluation. The method of choice for determining these values will be the method outlined by NCASI in Ultimate Oxygen Demand (Biochemical), NE87-03.4. The model uses first order kinetics to characterize oxidizable nitrogen or NBODU decay. Wastewater inputs/outputs are NH3-N (as nitrogen). The value is converted to oxygen demand using the factor 4.57.N. Disinfection 1. Bacteria allocations for effluents will be assigned so as to meet the State's water quality standards for the designated use of the receiving water. A background coliform concentration of 200#/100 ml will be assumed in fresh water at the low-flow condition, unless site-specific data taken from an upstream site, approved by the Department, during a low-flow event indicates that another background level should be used.2. Marine waters (recreational salt-waters) will have a background concentration of 35 colonies/100 ml at the low-flow condition, unless site-specific data taken from the water body, approved by the Department, during a low-flow event indicates that another background level should be used.3. Allocations will be derived according to the following dilution mix equation: Click Here To View Image
where:
CE = allowable effluent bacteria concentration in colonies /100ml
QE = daily average effluent flow in cfs
CH = headwater bacteria concentration of 200 colonies /100ml
QH = headwater flow (7Q10) in cfs
CT = bacteria standard after mixing (usually 200 colonies /100ml May through October or
2000 col/100ml November through April)
QT = total stream flow after mixing in cfs
4. Disinfection may be required for hydrograph controlled release (HCR) lagoons.O. Chlorine Toxicity Residual chlorine allocations for all municipal and industrial effluents will be developed so as to meet the State's water quality criteria. To properly select the final in-stream target concentration, the type of receiving water (fresh or estuarine) and the IWC* (instream waste concentration) must be known. Once this information is known, allocations will be determined using the following dilution mix equation:
Click Here To View Image
where:
CE = allowable effluent chlorine concentration in ug/l
QE = daily average effluent flow in cfs
CH = headwater chlorine concentration (usually 0.0 ug/l)
QH = headwater flow (7Q10) in cfs
CT = chlorine standard in ug/l (after mixing)
| Acute | Chronic |
| Fresh | 19 | 11 |
| Estuarine | 13 | 7.5 |
QT = total stream flow in cfs (after mixing)
P. Instream Waste Concentration The instream waste concentration (IWC) is the resulting percentage of effluent after complete mixing with the receiving water body at the headwater flow appropriate to the allocation procedure, normally the 7Q10. Acute or chronic pollutant target criteria are selected based on the resulting IWC.
Click Here To View Image
For IWC [LESS THAN] 1[CENT] use acute criteria; For IWC 1[CENT] use chronic criteria.
Q. Ammonia Toxicity Ammonia must not only be considered due to its effect on dissolved oxygen in a receiving water, but also its toxicity potential. It is recognized that effluent ammonia concentrations may be more restricted due to toxicity than due to oxidation. Consequently, the modeler of conventional pollutants must consider ammonia toxicity.
Ammonia as nitrogen (NH3-N) allocations for effluents will be developed to meet the water quality criteria given in Quality Criteria for Water, 1986, EPA 440/5-86-001. Generally, ammonia limits will be placed in permits of municipal facilities utilizing lagoon type treatment. To properly select the final in-stream target concentration, the IWC (instream waste concentration) must be known and the warm water target values used. Stream temperature and pH after mixing must also be known or assumed. For empirical modeling a pH of 7.0 and a stream temperature of 25oC are assumed limitations. Once this information is known, allocations will be determined using the following dilution mix equation:
Click Here To View Image
where:
CE = allowable effluent NH3 concentration in mg/l
QE = daily average effluent flow in cfs
CH = headwater ammonia concentration of 0.1 mg/l
QH = headwater flow (7Q10) in cfs
CT = ammonia criteria in mg/l (after mixing)
QT = total stream flow in cfs (after mixing)
Final ammonia allocations will be reported as ammonia nitrogen.
NH3-N = NH3 x 0.822
II. CONVENTIONAL WATER QUALITY MODELS The Department's freshwater quality model is a steady-state modified Streeter-Phelps dissolved oxygen sag model. The model includes the stream's carbonaceous and nitrogenous BOD ultimate demand, the stream's reaeration rate, the net photosynthetic demand and production, and the benthic oxygen demand. The model was developed in 1973 by the staff of the Civil Engineering Department at Mississippi State University. The STREAM model was updated by MSU in 2004 to work in a java and oracle computer environment. The model is used for both empirical and calibration purposes.
For salt water modeling, nutrient modeling, and highly complex hydrology, the Department will use a combination of the Environmental Fluids Dynamic Code EFDC model for the hydrology and the WASP model. Both of these models are supported by EPA and are accessible in the public domain.
The Department may utilize other models and/or documents which are approved by both the Department and EPA, are scientifically defensible and/or have been duly promulgated through the Federal Administrative Procedure Act.