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CI & UA TMDLs 025f) . hUp:/Iwww.kdhe.slale.ks.uslundVuasrrearr . Population projections indicate substantial growth for Garden City (22.5%) to the year 2020. According to projections of future water use and resulting wastewater. Garden City MWWTP looks to have sufficient treatment capacity available, The plant is looking to upgrade to treat up to 6 MGD in the future. Sampling of the elTluent over 1998-1999 reveals a monthly average of 335 mg/I of sulfate in the wastewater. Under dry flow conditions. the diluent from the city may constitute the streamllow of the Arkansas Rivcr sampled at Pierceville, The electric power plant, located in west Garden City. uses cooling towers and consequently has concentrations of sulfate which exceed 2000 mgll routinely, A vemge concentration of sulfate in its wastewater reaching the river is 2460 mg/I. Loss of water through cvaporation while the wastewater Ilows down liver between Garden City and Pierceville can elevate sulfate levels found in downstream samples, Irrigation Return Flow: As noted in the analysis of the cUl1'ent situation, large concentrations of sulfate enter the state at Coolidge. Looking at coincident 1990-1993 data, average concentrations of sulfate below John Martin Dam were 1034 mg/I. Concentrations rose to 1908 mgll at Lamar. 21 miles downstream, Avemges at Coolidge for the same period were 2170 mgll. This increase in concentration occurs as a result of the pallern of water use and reuse interacting with the natural sulfate sources present in the geology and soils of Eastern Colorado, Water leaving John Martin enters the irrigation ditch system which is prevalent on thc Colorado plains, Flows at John Martin for the 1990-1993 drought time period averaged 316 cfs, This dropped to 14 cfs at Lamar, then rose to 640 cfs at Coolidge, Evaluating wel conditions by including flows from 1994 and 1995, maintains the pattern: 367 cfs at John Martin; 146 at Lamar, 885 cfs at Coolidge, It would appear that the Arkansas River conveys ground water seepage from mounded water tahles under irrigated lands and tailwater leaving one ditch system to be divel1ed hy another downstream, Most Ilow is directed through the ditch systems with flow delivery at the stateline constituting tailwater and retum flows, Hence, the elevated sulfate levels over naturally high concentrations arc chielly a consequence of the water use pattern in eastern Colorado. Within Kansas, there are currently six active ditches upstream of Garden City that use Arkansas River water: Frontier, Amazon, Great Eastern, Southside, Farmers and Garden City canals (Figure 10), Two ditches, the FL Aubrey and Alamo ditches, are no longer active. The Frontier Ditch diverts water in Colorado and retums a portion of unused water to the liver above Syracuse, The Amazon and Great Eastern Ditch divert water from the same headgate cast of Kendall and convey water to thc Lake McKinney area, Water for the Great Eastem Ditch moves through Lake McKinney to irrigate eastern Kearny County, The Amazon Ditch continues northeastward to the northeast pOltion of thc county, Occasionally, these canals will return 5-10 cfs to the river for one to two days in order to clear the canals of dehlis at the upstream end, The Southside Ditch diverts water a few miles downstream from the Amazon/Great Eastem head gate and is commonly used as an alternative conveyance system for the Farmers and Garden City ditch selvice areas to avoid significant transit losses in the river between Kendall and Deerfield, Such water returns to the river just west of Deeltield and is divelted at the Farnlers/Garden City headgate downstream of Deertield to irrigate areas near Holcomb, Indications from the Division of Water Resources indicates that lillle tailwater returns from these ditches, An average of 59,516 af/yr have been diverted from sUlface water in Hamilton, Kearney and Finney counties since 1990. given a 120 day inigation period, this would equate to about 250 cfs. Ground water irrigation stUlts in earnest south of Lakin in Kearny County and hecomcs the prevalent practice in Finney County, This marks the area where the liver overlies the High Plains Aquifer. Flows at Garden City reflect this usage as extended periods of low or no flow are recorded at the gage on the liver south of town. Very little return now comes from lands irrigated by wells, with tailwater control requiremcnts pan of watcr lights overseen by the Division of Water Resources and Groundwater Management District No.3, Given that certain reaches of the river have histOlically had large transit losses through infiltration to the surrounding ground water. additional influence on the stream from the high MI11n1 7 O.