Correlation of structural lineaments and fracture traces to water-well yields in the Edwards Aquifer, Central Texas




Alexander, Kenneth Bower, 1961-

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Lineaments are "straight lines visible from afar on the surface of the earth". In the Austin, Texas area, lineaments reflect the structural grain of the Balcones-Ouachita fault zone and may indicate subsurface geologic phenomena such as faults, fractures, and joints. These structural features often represent discrete zones of high permeability, and thus, areas of enhanced flow of groundwater capable of transmitting greater quantities of water than surrounding, non-fractured, rock. For this study more than 900 lineaments and fracture traces, identified in aerial photographs during a previous study, were detected in the Barton Springs section of the Edwards Aquifer. The endpoints of each linear feature were digitized and tagged with a unique identification label. Rose plots, Cartesian histograms, and a series of statistical operations were utilized to illustrate regional trends in the orientation of lineaments. As an indicator of well productivity, specific capacities of 27 wells in the area were obtained. Sixty-one water samples were collected and analyzed to test for possible chemical evidence of lineament-well interactions. The orientations of lineaments and fracture traces in the study area clearly display a bimodal distribution with a primary trend of N 40 E and a secondary peak of N 50 W. A general correlation exists between increased well productivity and decreased distances to the nearest lineament, particularly within 200 feet of lineaments. Also, 10 of the 13 largest specific-capacity values are from wells located southeast of southwest-northeast trending lineaments. Nonparametric statistical methods show that direction from lineaments is a significant factor in predicting water-well yields. Lineaments provide a tool for predicting possible sites of environmental sensitivity with respect to groundwater resources. Examples include the siting of groundwater monitoring wells for point sources of pollution, predicting the likely underground flow paths of a pollution plume or locating dam sites for recharge enhancement. Awareness of the location, orientation, and density of structural lineaments will allow the water-resource manager to identify discrete groundwater flow paths, and, thus, predict contaminant plume migration.