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<br /> Background <br /> The City of San Marcos is located in the eastern border of the Balcones Fault <br /> Zone Edwards Aquifer - San Antonio Region. The San Marcos Springs, which have <br /> an average flow of 166 cfs, is the lowest natural discharge point for this portion of the <br /> aquifer. All local groundwater flows in the Edwards converged at these Springs. The <br /> cities five water supply wells are located near the Springs and share with them the <br /> same zone of contribution (ZOC). Currently, the City totally depends on the Edwards <br /> for its water supply source and pumps an average of 5.56 MGD (1985-1990) to serve <br /> more than 35,000 people. According to the San Marcos Chamber of Commerce <br /> economic profile, the City houses more than 24 major manufacturers and industries. <br /> Since the Edwards is a "Karst" limestone aquifer, the vulnerability of public <br /> drinking water supply wells to contamination is two fold. First, with a conduit type <br /> transmissivity, contaminants can travel a great distance away from a pollution source <br /> toward wells before being degraded or diluted. Second, sinkholes and caves and <br /> other recharge features serve as possible locations for introducing potential pollutants <br /> into the aquifer. The aquifer's ability to retard and disperse the contaminants is low. <br /> Dye trace results conducted by the Edwards Aquifer Research and Data Center <br /> (EARDC), Southwest Texas State University (SWT), revealed that caves in the San <br /> Marcos area were connected to the Springs. In one case where fluorescein was <br /> injected into a cave 2 miles southwest of the Springs, the dye was detected at a SWT <br /> well, a City well, and a spring orifice within 9, 10, and 11 days respectively. The same <br /> study indicated that dye can travel to the Springs from a cave as far as 5.5 miles away. <br /> Among the possible incidents of contamination, spills and releases from <br /> industrial parks and pretreatment sources at manufacturing sites should receive <br /> particular consideration. Industrial and commercial areas are scattered throughout the <br /> City and ET J (Extraterritorial Jurisdiction) creating the possibility for accidental or <br /> planned discharges to Purgatory Creek and Sink Creek and their tributaries where <br /> cracked river-beds, sinkholes and caves are located. Some of these areas are directly <br /> over the Edwards Aquifer Recharge Zone or are located on tributaries crossing <br /> recharge area <br />