The Chemical and Biological Groundwater Quality of the Edwards Aquifer in the Sink Creek Drainage Basin, Hays County, Texas




Kolbe, Christine M.

Journal Title

Journal ISSN

Volume Title



<p>A study of chemical and biological groundwater quality was conducted in an area of the Edwards Aquifer recharge zone, the Sink Creek Drainage in the Guadalupe River Basin. This study is based on evidence of bacterial contamination of groundwater found in the San Marcos area during a hydrogeological study of the spring and groundwater flow (Ogden, Quick, Rothermal and Lunsford, 1986). The study revealed high levels of fecal and total coliforms in several test wells following a large rainfall. To expand on these findings a study was developed in an effort to determine the possible sources and frequency of bacterial contamination of local water wells.</p> <p>The study site was the Country Estates Subdivision west of San Marcos. The subdivision is located adjacent to Sink Creek and the Williamson- Freeman Flood/Control Recharge Structure. The flood control/recharge structure, built over Sink Creek, is located on the Freeman Ranch. The flood plain is used for grazing cattle. The fecal material from these cattle was thought to be the primary source for bacterial contamination of the wells in the area.</p> <p>Twenty-two wells were sampled before, during and after rainfall events for a period of one year from July 1985 to June 1986. Samples collected for bacterial and chemical analysis were run using U.S. Environmental Protection Agency (1984) approved methods. Bacterial analysis for fecal and total coliforms was done using the membrane filter technique. These indicator organisms were used to detect the possible presence of pathogens in the water. Chemical analysis was done to find any patterns or changes in the water chemistry before, during and after rainfall events.</p> <p>The largest rainfall occurred in late November 1985 (sample days 130 to 132) and May/June 1986 (sample days 316 to 335). Although rainfall did occur in September and October 1985, these rainfalls were less important than the others because they followed a long period of drought. The two largest rainfalls corresponded to changes in both the concentration of chemical constituents and the levels of bacterial contamination. Decreases in chemical concentrations were found following the two largest rainfalls. This was due to the rapid recharge of surface runoff in the area of the contaminated wells. The chemical concentrations in surface water were lower and mixing with the groundwater caused a dilution effect within the groundwater. The uncontaminated wells showed no distinct changes following rainfall events.</p> <p>Fecal and total coliforms showed similar peaks following the two major rainfall events in November 1985 and May/June 1986. Increases in the coliform levels corresponded to the decreased concentrations of the chemical constituents. The presence of coliforms during other periods of rainfall was evident, but not in the same order of magnitude. Rainfall which occurred over a longer period of time had less impact on water quality. Only heavy rainfall occurring over a short period of time caused counts greater than 200 colonies/100 ml in the test wells. To aid in determining possible sources of contamination, fecal coliform/fecal streptococcus (FC/FS) ratios were calculated for ten of the most heavily contaminated wells. The major source indicated by the FC/FS ratios was found to be nonhuman.</p> <p>Bacteriological contamination occurred in 68.2% of the wells sampled with 40% of those wells having coliform counts > 200 colonies /100 ml. Contamination had a direct relation to the turbidity in wells following heavy rainfall.</p>



Edwards Aquifer, Groundwater quality, Sink Creek, Water chemistry


Kolbe, C. M. (1988). The chemical and biological groundwater quality of the Edwards Aquifer in the Sink Creek Drainage Basin, Hays County, Texas (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.


Rights Holder

Rights License

Rights URI