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TMDL TARGET:
IN THIS TMDL DOCUMENT, THE WATER QUALITY STANDARDS ARE THE INSTREAM GOALS. THE TERM TARGET CONCENTRATION REFLECTS THE APPLICATION OF AN EXPLICIT MARGIN OF SAFETY (MOS) TO THE WATER QUALITY STANDARD. SEE TMDL SECTION 8.4 FOR AN EXPLANATION OF MOS.
THE APPLICABLE STATE OF TENNESSEE WATER QUALITY STANDARDS, CHAPTER 1200-4-3, GENERAL WATER QUALITY CRITERIA, JANUARY, 2004 FOR RECREATION USE CLASSIFICATION (MOST STRINGENT): THE CONCENTRATION OF THE E. COLI GROUP SHALL NOT EXCEED 126 COLONY FORMING UNITS PER 100 ML, AS A GEOMETRIC MEAN BASED ON A MINIMUM OF 5 SAMPLES COLLECTED FROM A GIVEN SAMPLING SITE OVER A PERIOD OF NOT MORE THAN 30 CONSECUTIVE DAYS WITH INDIVIDUAL SAMPLES BEING COLLECTED AT INTERVALS OF NOT LESS THAN 12 HOURS. FOR THE PURPOSES OF DETERMINING THE GEOMETRIC MEAN, INDIVIDUAL SAMPLES HAVING AN E. COLI CONCENTRATION OF LESS THAN 1 PER 100 ML SHALL BE CONSIDERED AS HAVING A CONCENTRATION OF 1 PER 100 ML. IN ADDITION, THE CONCENTRATION OF THE E. COLI GROUP IN ANY INDIVIDUAL SAMPLE TAKEN FROM A LAKE, RESERVOIR, STATE SCENIC RIVER, OR TIER II OR III STREAM (1200-4-3-.06) SHALL NOT EXCEED 487 COLONY FORMING UNITS PER 100 ML. THE CONCENTRATION OF THE E. COLI GROUP IN ANY INDIVIDUAL SAMPLE TAKEN FROM ANY OTHER WATERBODY SHALL NOT EXCEED 941 COLONY FORMING UNITS PER 100 ML.
IN ADDITION TO UTILIZING THE E. COLI WATER QUALITY STANDARDS (WITH MOS) AS THE TARGET, BECAUSE OF THE DEMONSTRATED HIGH CORRELATION BETWEEN E. COLI AND FECAL COLIFORM IN THIS WATERSHED, THIS TMDL ALSO UTILIZES A FECAL COLIFORM TARGET AS A SURROGATE FOR DETERMINING THE ATTAINMENT OF THE E. COLI STANDARD. IN THE STATE OF TENNESSEE, E. COLI AND FECAL COLIFORM ARE WELL CORRELATED (R = 0.902) WHEN EVALUATING ALL AVAILABLE ECOREGION DATA (623 OBSERVATIONS). FURTHERMORE, AS DESCRIBED IN TMDL SECTION 3.0, THE IMPAIRED WATERBODIES OF THE LOWER DUCK RIVER WATERSHED (HUC 06040003) LIE WITHIN LEVEL IV ECOREGIONS 71F AND 71H. THE CORRELATION BETWEEN E. COLI AND FECAL COLIFORM IN LEVEL III ECOREGION 71 IS FAIR (R = 0.669); HOWEVER, THE CORRELATIONS BETWEEN E. COLI AND FECAL COLIFORM IN LEVEL IV ECOREGIONS 71F (R = 0.983) AND 71H (R = 0.960) ARE EXCELLENT.
THIS TMDL THUS EMPLOYS BOTH THE E. COLI WATER QUALITY STANDARD AND THE SURROGATE FECAL COLIFORM BY DETERMINING THE RESULTS FROM THREE DIFFERENT METHODOLOGIES: 1) THE GEOMETRIC MEAN STANDARD FOR FECAL COLIFORM OF 200 COUNTS/100 ML IS UTILIZED, 2) THE FECAL COLIFORM SAMPLE MAXIMUM OF 1,000 COUNTS/100 ML, IS UTILIZED AND 3) THE E. COLI SAMPLE MAXIMUM OF 941 COUNTS/100 ML IS UTILIZED. THE MOST PROTECTIVE (OR HIGHEST PERCENT OF LOAD REDUCTION) OF THE THREE METHODOLOGIES WILL DETERMINE THE PERCENT REDUCTION(S) REQUIRED FOR IMPAIRED WATERBODIES.
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