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I I I background c09centration, indicate that human practices generally have not contributed significantly to DOC in the I ground water. I ' MBAS were detected in 18 samples. MBAS concen- I trations ranged from <0.05 to 0.15 mg/L (table 2), with a median concen~ation of 0.05 mg/L. There is no USEPA maximum cont Iminant level for MBAS. All wells in , I which MBAS were detected were in the eastern part of the . Mesa, from just west of 23rd Lane to east of Baxter Road, and from just south of South Road to the northern part of the study area (fig. 4). The expected maximum back- ground concentration for MBAS was set at 0.05 mg/L, the detection limit of the method of analysis (table I). The test for MBAS is not definitive for detergents because the test Chemical or bacterial IndIcators USEPA drlnklng- water MCL or MCLG' Range 01 concentrations In Mesa water samples Samples exceeding maximum back~ ground concen- tration Number Percent Table 1. Chemic~1 and bacterial indicators, expected maximum background concentration, and range of concentrations of the indicators in Mes~ water samples [USEPA, u.s. Enviro1mental Protection Agency; <, less than; >, greater than; mg/L, milligrams per liter; IlgIL, micrograms per liter; MCL, USEPA maximum contami~ nant level; MeLG, U~EPA maximum contaminant level goal; n, constituent has no USEPA drinking-water MeL or MCLG; DOC, dissolved organic carbon; TOe, total organic carbon; %, percent; poe, particulate organic carbon; mL, milliliters; MBAS, methylene blue ~ctive substances; E. coli, Escherichia coli bacteria] I ,xpected fTIaxlmum I back- Iground concen- Itratlon < mg/L Nitrate plus nitrite, as nitrogen IOmglL (MCL) 0.07-9.2 mglL Ammonia 0.17 mgIL I <0.01-0.08 mglL DOC 3. mgIL 0.8-9.5 mglL MBAS o 05 mgIL <0.05-0.15 mglL Total o ~olonies o colonies <1->2,700 15 coliform per perl 00 mL colonies 1100 mL (MCLG) per 100 mL E. coli o ~olonies o colonies <1- 9 colonies 2 Iper perlOOmL per 100 mL 1100 mL (MCLG) Caffeine 0.04 I1glL <0.04-0.28 I1glL I , , lV.S. Environm~ntal Protection Agency, 1995 20 o 18 4 Notes 80 4 Expected maximum background concentration based on concentration of nitrate plus nitrite, as nitrogen, for Pueblo Reservoir, from which Bessemer Ditch water originates (Edelmann and others, 1991). Expected maximum background concentration based on assumption that total ammonia is approximately equal to dissolved ammonia and the maximum concentration for total ammonia for Pueblo Reser- voir, from which Bessemer Ditch water originates (Edelmann and others, 1991). Expected maximum background concentration based on maximum observed TOC values (approximately 10% POC, 90% DOC, (Thurman, 1986) for Pneblo Reservoir, from which Bessemer Ditch water origi- nates (Edelmann and others, 1991). Methylene blue active substances are detergents and natural and synthetic chemical compounds. Expected maximum background concentration is 0.05 mgIL, which is the detection limit of the method. Therefore, a detection of MBAS is consid- ered as an exceedance of the expected maximum background concentration. Expected maximum background concentrations of total coliform and E. coli bacteria are based on USEPA MCLG for those biological indicators. A detection of E. coli bacteria indicates some form of fecal contamination, whether from animal or human sources, . o 72 60 8 6 Caffeine does not occur naturally in Colorado ground or surface waters, Expected maximum background concentration is 0,04 ~glL, which is the detection limit of the laboratory method. Therefore, a detec- tion of caffeine is considered as an exceedance of the expected maximum background concentration. .