Five Total Maximum Daily Loads for Indicator Bacteria in Four Austin Streams
EXECUTIVE SUMMARY: This document describes total maximum daily loads (TMDLs) for four Austin streams and their tributaries in which concentrations of indicator bacteria exceed the criteria used to evaluate attainment of the contact recreation use. The Texas Commission on Environmental Quality (TCEQ) first identified the impairments to the Spicewood Tributary to Shoal Creek (Segment 1403J) and Taylor Slough South (1403K) in the 2002 State of Texas Clean Water Act Section 303(d) List (TCEQ, 2002), adding Waller Creek (1429C) and Walnut Creek (1428B) when the list was updated in 2006. The impaired segments and corresponding assessment units (AUs) are: Spicewood Tributary to Shoal Creek (1403J_01); ? Taylor Slough South (1403K_01); ? Walnut Creek (1428B_05); ? Waller Creek (1429C_02, 1429C_03);
Together these four freshwater streams total approximately 31.6 miles in length with watersheds covering 63.465 square miles. They are almost entirely within the City of Austin full purpose, planning, or exterritorial jurisdiction. They are almost entirely within Travis County, except that the Walnut Creek watershed includes a very small portion of Williamson County. Currently, there are not any permitted domestic wastewater discharges within the watersheds of any of these streams. The Walnut Creek Wastewater Treatment Facility (WWTF), operated by the City of Austin, discharges its effluent directly into the Colorado River instead of Walnut Creek. There are not any permitted industrial bacteria discharges within the watersheds. The Freescale Semiconductor WWTF only discharges bacteria-free process water. The primary loads are from various nonpoint sources that enter the streams via stormwater. The Spicewood Tributary to Shoal Creek is an intermittent freshwater stream approximately 1.4 mile in length from MOPAC/Loop-1 upstream to its headwaters near Spicewood Springs Road and Mesa Drive. The watershed is about 0.650 square miles and is entirely located in the City of Austin. There are no regulated wastewater discharges within this watershed. Taylor Slough South is a perennial freshwater stream approximately 1.1 mile in length from Lake Austin upstream to its headwaters near West 35th Street and MOPAC/Loop-1. The watershed is 0.650 square miles and is entirely located in the City of Austin. There are no regulated wastewater discharges within this watershed. Waller Creek is a perennial freshwater stream approximately 6.7 miles in length from its confluence with Lady Bird Lake upstream to its headwaters near Northcrest Boulevard and West St. Johns Avenue. The watershed is 5.648 square miles and is entirely located in the City of Austin. There are no regulated domestic wastewater discharges within this watershed. Walnut Creek is a perennial freshwater stream approximately 22.4 miles in length from its confluence with the Colorado River upstream to its headwaters near McNeil Drive and Parmer Lane. The watershed is approximately 56.517 square miles and is mostly in the City of Austin full purpose jurisdiction. However portions are in the planning, or exterritorial jurisdictions. Currently, there is only one industrial wastewater discharge located within its watershed, the Freescale Semiconductor plant, which only discharges bacteria-free process water into Walnut Creek AU 1428B_01. Escherichia coli (E. coli) are the preferred indicator bacteria for assessing the contact recreation use in freshwater, and were used for development of the TMDLs, with one exception. Fecal coliform bacteria were used for assessment of Walnut Creek AU 1428B_02 because it was the standard when data were collected in 1999. E. coli data are not currently available, but will be collected in the future. The criteria for assessing attainment of the contact recreation use are expressed as the number (or “counts”) of bacteria. The primary contact recreation use is not supported when the geometric mean of E. coli samples exceeds 126 most probable number (MPN) per 100 milliliters (mL), or the geometric mean of fecal coliform samples exceeds 200 MPN per 100 mL. For the 2012 assessment period, the geometric means of all AUs examined exceeded 126 MPN/100 mL E. coli or 200 MPN/100 mL fecal coliform, indicating non-support of primary contact recreation. Possible sources of indicator bacteria within the watersheds of the impaired AUs are stormwater runoff from regulated storm sewers, illicit discharges from storm sewers, sanitary sewer overflows (SSOs), and unregulated sources such as wildlife, unmanaged feral animals, and pets. Load duration curve (LDC) analyses of instream flows were used to estimate allowable pollutant loads and specific TMDL allocations. Because bacteria loads are usually highest at high flow, the very high flow regime was used as the critical flow for determining the TMDL. Predictions of future growth of existing or new domestic point sources were not necessary. The City of Austin has informed TCEQ that it intends to accommodate all growth with its central wastewater treatment system, which discharges directly into the Colorado River instead of these watersheds. The wasteload allocation (WLA) for regulated stormwater was based on the percentage of each watershed regulated under a Phase I or Phase II Texas Pollutant Discharge Elimination System (TPDES) stormwater permit. Compliance with these TMDLs is based on keeping indicator bacteria concentrations in the selected waters below the geometric mean criterion of E. coli less than 126 MPN/100 mL or fecal coliform less than 200 MPN/100 mL.