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Cl & VA TMDLs 0257 density of water use within the alluvial conidor and surrounding High Plains Aquifer ensures that the river will he a losing stream from Syracuse eastward. SUIl'ounding groundwater has typically had sulfate levels under 500 mgfl. The lack of fresh water inflow from the surrounding aquifer has left the alluvial aquifer subject to elevated sulfate levels as river water has been induced downward into the unconsolidated deposits. . hltp:/Iwww.kdhe.state.ks.uslondlluastream . Background Levels: Sulfate has certainly been elevated within the river for dccades and it is likely that natural levels contributed from the interaction of the Arkansas River with gypsum deposits in the Pierre Shale in eastern Colorado would elevate sulfate concentrations above thc water quahty eliteria for domestic water supply or livestock. However, the pattern of irrigation return flows bas increased the sulfate concentrations through evapotranspiration and extcndcd inundation of gypsum soils to aggrevate the current impairments, 4. ALLOCA TION OF POLLUTION REDUCTION RESPONSIBILITY The nature of sulfate loading along the Arkansas River has been rooted in decades of natural contributions aggravated by patterns of irrigation water use and reuse, Therefore, short term reduction in sulfate loads crossing the stateline will be negligible, Flows of improved water quality over the long tern1 may gradually bring about a lowering in ambient concentrations of sulfate seen throughout these stream reaches, As such. widespread application of this TMDL and its desired endpoints is prcmaturc, Therefore. establishment of background levels is appropriate and allocations relative to point and non-point sources are to be made in light of those elevated levels and current contributions" Point Sources: Unless point sources act to concentrate salts through reuse and evaporation. they will tend to discharge water that is similar in sulfate content to that within their water supplies, Garden City's wastewater is an order of magnitude lower in sulfate than the river condition. However, evaporative action through cooling towers at power plants can create effluent with high sulfate levels, The Wasteload Allocation recognizes that the 7QIO for the Arkansas River at Garden City results in a flow at Picrceville that is effectively zero. Therefore, any flow seen at the monitoring site at Pierceville will be either be a fraction of thc design flows from the Garden City area or water passing through Gardcn City at higher !lows, Based on the relationship between Coolidge flow and flow at Garden City. it would seem that the chief contlibution to low flows is potentially the Garden City wastewater. As Garden City effluent dominates downstream flow more and more, watcr quality in the liver near Pierceville should improve relative to background levels, because of the lower sulfate levels in the municipal wastewater. Assuming 70% of the wastewater is lost through evaporation, resulting sulfate concentrations will rise to the 1800-2000 mg/I at Pierceville. Assuming Garden City upgrades to a design flow of 6 MGD and the power plant holds its design flow constant at 0.477 MGD, the Wasteload Allocation of 13,6 Tons per day. should reduce sulfate concentrations below the current average of 1875 mg/l at flows at or below 10 cfs (Figure 9). Given the city will discharge up to 6 MGD and that its sulfate levels are below 400 mgll 85% of the time, Gardcn City should be allocated 10 T/D of sulfate, The power plant has a small amount of effluent which is high (avg = 2460 mg/I) in sulfate, If the plant effluent is held to a level below the cUlTent ambient average of 1875 mg/l, the resulting allocation to the plant should be ),6 T/D at 0,477 MGD and 1800 mglL Figure II indicates the resulting conccntrations of sulfate at Garden City and Pierceville under varying rates of Garden City discharge, Unless the power plant dischargcs at its design flow, sulfate Icvcls should be below cUIl'ent avcragcs if Garden City discharges at least 3 MGD, thereby achieving the endpoint of this TMDL. Reduction of power plant discharges would also improve downstrcam watcr quality conditions. The quality of the effluent established by this ,111"'01 "'I.n'