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COLORADO
<br />Water Quality Assessment
<br />Department of Public
<br />P
<br />6'I Health b Environment
<br />normally apply. The chronic IWC is determined using the following equation:
<br />IWC = [Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)] X 100%
<br />The flows and corresponding IWC for the appropriate discharge point are:
<br />Permitted Feature Chronic Low Flow, Facility Design Flow IWC, (%)
<br />30E3 (cfs) (cfs)
<br />002, 011, 017, 020,
<br />021, 023, 025 0 Varies for each outfall 100
<br />The IWC for this permit is 100%, which represents a wastewater concentration of 100% effluent to 0%
<br />receiving stream. This IWC correlates to chronic WET testing. The fact sheet and the permit will contain
<br />additional information regarding the type of WET testing applicable to this facility. Even though outfall 025
<br />discharges into Deacon Gulch which does not have the full set of aquatic life standards, the downstream
<br />receiving water, the Yampa River, is a T and E segment and no assimilative capacity is available.
<br />Therefore, the IWC of 100 is based upon this segment.
<br />Agricultural Use Parameters (SAR and EC):
<br />Section 31.11(1)(a)(iv) of The Basic Standards and Methodologies for Surface Waters (Regulation No. 31)
<br />includes the narrative standard that State surface waters shall be free of substances that are harmful to the
<br />beneficial uses or toxic to humans, animals, plants, or aquatic life. The interpretation of these conditions
<br />(i.e., "no harm to plants" and "no harm to the beneficial uses") and how they were to be applied in permits
<br />were contemplated by the Division as part of an Agricultural Work Group, and culminated in the most recent
<br />policy entitled Implementing Narrative Standards in Discharge Permits for the Protection of Irrigated Crops
<br />(hereafter the Ag Policy)
<br />The outfalls discharge to zero low flow streams that discharge into Deep Cut Ditch (outfalls 017, 020, 021,
<br />023, 025) and the Yampa River (002 and 011). Based on available information, the water in Deep Cut Ditch
<br />and the Yampa River is used for irrigation water. The evaluation of the suitability (i.e., quality) of irrigation
<br />water is complex and involves the detailed understanding of the interactions of plant tolerances, soil types,
<br />and agricultural management practices. Irrigation water has two properties - salinity and sodicity - that can
<br />have concurrent impacts on the irrigated crop beneficial use. The Division has thus determined that two
<br />parameters, specifically electrical conductivity (EC) and sodium absorption ratio (SAR), are the best
<br />parameters to regulate in discharge permits to control levels of salts to minimize both the loss of irrigated
<br />crop yield and the sodium hazard.
<br />In order to establish "standards" and limits for EC and SAR, the Division must: (1) determine the most
<br />sensitive crop usually grown in the area downstream from the discharge and determine the corresponding EC
<br />of irrigation water (EC,,) threshold value for no reduction in yield below100%; and (2) determine the SAR
<br />based on the EC,, value, with consideration of existing water quality, to prevent the exceedance of the SAR.
<br />Electrical Conductivity: The electrical conductivity (EC) is also known as specific conductance, conductance,
<br />conductivity, or specific conductivity. Crops have varying sensitivity to electrical conductivity. Studies have
<br />established the maximum conductivity in the water in the root zone that will result in no reduction of crop
<br />yield. This value is referred to as the EC saturation extract or ECe. However, the EC, is not the same as the
<br />EC of the irrigation water (EC„,). The EC,, is the maximum conductivity in the irrigation water that will result
<br />in no reduction in crop yield.
<br />Common crop ECS, thresholds are reproduced from the Ag Policy and/ or Colorado State University Fact Sheet
<br />no.0.506, and are summarized in Table A -9a. Note that other EC, are listed in tables in appendixes to the
<br />Ag Policy.
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<br />4300 Cherry Creek Drive S., Denver, CO 80246-1530 303-692-2000 www.colorado.gov/cdphe/wqcd I ��
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