2018-03-01_HYDROLOGY - C1981010 ‎(2)‎

• Permit No: C1981010
• IBM Index Class Name: Hydrology
• Doc Date: 3/1/2018
• Doc Name: CDPS/NPDES Permit
• From: Department of Public Health & Environment
• To: DRMS
• Permit Index Doc Type: NPDES
• Email Name: RAR
• Media Type: D
• Archive: No

-°F COLORADO
Department of Public
�: Bib rf Health b Environment
Water Quality Assessment
Table A-9
Maximum EC,, That Will Not Reduce The 100% Yield of Selected Irrigated Crops
Common Colorado Crops
Irrigation Water Electrical Conductivity (ECW)*
Beans
0.7
Onion
0.8
Corn (grain)
1.1
Potato
1.1
Corn (silage)
1,2
Alfalfa
1.3
Wheat
4.0
Sugarbeet
4.7
Barley
5.3
wwrauo xate university Fact Sheet no.0.506
The EC,, that is used in the development of permit limits is determined based on the most sensitive of the
ECW's for the crops grown in the area. Based on available information from the local water commissioner,
for waters originating from the Yampa River and Deep Cut Ditch and used for crop irrigation, alfalfa was
determined to be the most sensitive crop.
In accordance with the Ag Policy, because the Deep Cut Ditch, which is unclassified state waters, returns
diverted flow to the Yampa River, which is classified surface waters, the EC limits are calculated for the
Yampa River. The EC limit is calculated using the mass balance equation found at the beginning of Section
IV of this analysis. The data used and the resulting calculations of the EC limit, M2, are set forth in the table
below. Note that in accordance with the Ag Policy, the EC limit will be imposed as a chronic (30 -day average)
limit and therefore chronic low flows were used together with 85th percentile EC concentrations when
calculating the limit.
Table A-10
Calculation of EC Limits for the Yampa River
Outfalls 002, 011, 017, 020, 021, 023, and 025
Parameter Q, WS) Q2 WS)Q3 (cfs) mg/I) (mg/1 M2 (Frig/!)
EC, dS/m 113.00 20.0 1 133.0 1 0.38 1.3 6.5
Note that in Figure A-2 at an EC value of 0.36 or less, the SAR must be 0. In order to achieve a 0 SAR, any
treatment process would have to eliminate all sodium, which is virtually impossible. Therefore, a minimum
EC at 0.36 will be considered.
SAR - SAR means Sodium Adsorption Ratio, which is a representation of the relative proportion of sodium
cations to calcium and magnesium cations (also known as the "sodium hazard"). The equation for SAR
follows:
SAR = Na +
Ca++ + Mg++
2
The values for sodium (Na'), calcium (Ca—) and magnesium (Mg") in this equation are expressed in units of
milliequivalents per liter (meq/l). Generally, data for sodium, calcium and magnesium are reported in terms
of mg/l, which must then be converted to calculate the SAR. The conversions are:
Pae 26 of 40 .
4300 Cherry Creek Drive S., Denver, CO 80246-1530 303-692-2000 www.cotorado.gov/cdphe/wqcd ( ®�