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Daily flows from the USGS Gage Station 07133000 (Arkansas River at Lamar, CO)were obtained and the annual 30E3 <br /> tow flow was calculated using U.S. Environmental Protection Agency (EPA) DFLOW software. The output from <br /> DFLOW provides calculated chronic low flows for each month. <br /> Based on the low flow analysis described above, the upstream tow flows available to the West Farm Pit discharge <br /> were calculated and are presented in the table below. <br /> Low Flows for Arkansas River at the West Farm Pit <br /> Low Flow(cfs)l Annual I Jan I Feb I Mar I Apr I I Jun I Jul I AugOct I Nov I Dec <br /> 30E3 Chronic 1 3.9 1 3.9 1 3.9 1 4.0 1 4.2 1 5.$ 1 9.1 1 5.4 1 4.1 1 4.1 1 3.9 1 3.9 1 3.9 <br /> Therefore,the 30E3:Design Flow dilution ratio equals 1.8:1 for this discharge. <br /> Antidegradation Review <br /> The antidegradation review is not necessary as the receiving stream is designated Use Protected. <br /> Andbackslidtnct[Pick one from below] <br /> As the receiving water is designated Use-Protected, the antibacksliding requirements in Regulation 61.10 have been <br /> met. <br /> PROCESS WATER: BASIS FOR SITE SPECIFIC PARAMETERS <br /> Electrical Conductivity:Segment 01c of the Lower Arkansas Sub-basin, Arkansas Basin is designated for agricultural <br /> beneficial use, and has a ratio of the chronic low flow of the receiving stream to the design flow less than 100:1, in <br /> accordance with Water Quality Policy 24. An irrigation intake is located less than one mile downstream of the <br /> discharge. Therefore, the facility is required to monitor EC levels during this permit term to characterize EC in <br /> discharges from this industry. <br /> Potentially Dissolved Selenium:Segment 01c of the Lower Arkansas Sub-basin,Arkansas Basin is listed as impaired <br /> (Regulation No. 93)for selenium in accordance with Section 303(d)of the federal Clean Water Act. The Division has <br /> not yet developed a TMDL for selenium for this segment. Selenium has the potential to be in the discharge as <br /> wastewater discharged from sand and gravel operations typically results from the pumping of groundwater that <br /> accumulates in the pits and from the washing of rock/gravel.The selenium concentration in this wastewater is <br /> influenced by local geology, groundwater hydrology, and upgradient land use. Therefore, site-specific sampling and <br /> reporting for selenium is required. <br /> Dissolved Manganese: Segment 01c of the Lower Arkansas Sub-basin,Arkansas Basin is listed as impaired (Regulation <br /> No. 93)for manganese in accordance with Section 303(d)of the federal Clean Water Act. This parameter can increase <br /> in pollutant concentration and loading due to its presence in the dewatering environment. Therefore, monitoring for <br /> the 30 day average and daily maximum is established for dissolved manganese. <br /> Total Recoverable Uranium:Segment 01c of the Lower Arkansas Sub-basin, South Platte basin Basin is listed as <br /> impaired (Regulation No. 93)for uranium in accordance with Section 303(d)of the federal Clean Water Act. Further, <br /> the Division has not yet developed a TMDL for uranium for this segment. This parameter has the potential to be in the <br /> discharge as it may be present in groundwater dewatering discharges when natural background levels are <br /> concentrated through pumping. <br /> Determining WQBELs for Total Recoverable <br /> Because the uranium assimilative capacity is calculated based on a range of standards, The Basic Standards and <br /> Methodologies for Surface Water requires further evaluation. Specifically, the regulations state that "Control <br /> requirements, such as discharge permit effluent limitations, shall be established using the first number in the <br /> range as the ambient water quality target, provided that no effluent timitation shall require an "end-of-pipe" <br /> discharge level more restrictive than the second number in the range." Because the WQBEL for uranium has <br /> been calculated to be less than the second number in the range of standards, the second standard, 30mg/l, <br /> would instead be substituted as the WQBEL pursuant to the regulations. <br /> Determining Reasonable Potential for Total Recoverable Uranium WQBELs <br /> The West Farm Pit has submitted uranium discharge data for 5 quarters, and an analysis must be performed to <br /> determine whether to include WQBELs in the certification. This qualitative reasonable potential (RP)analysis <br /> Fact sheet Page 2 of 5 <br /> 4300 Cherry Creek Drive S.,Denver,CO 80246-1530 P 303-692-2000 www.colorado.gov/cdphe/wgcd <br /> John W. Hickentooper,Governor I Larry Wotk,MD,MSPH,Executive Director and Chief Medicat Officer �® <br />