Analysis of Source and Extent of Subsurface Oil Plume, San Marcos River Seep Site, Guadalupe County, Texas

A seep on the south bank of the San Marcos River, about 5 miles west of Luling, Texas, in the Luling-Branyon Oil Field, is the discharge point of a subsurface plume of oil and saltwater. This investigation aimed to build on previous operator-sponsored attempts to identify the source of oil and map the extent of the oil plume in 2 weeks of fieldwork. This effort provided:

• Demarcation of the oil plume to an area of about 100 feet x 220 feet (~33,000 square feet) adjacent to the seep.
• An improved map of the water table for inferring the direction of oil movement, drawn on the basis of water-level measurements at 12 sites and an elevation survey.
• Installation of five new monitoring wells for water-level measurements and later use in fluid sampling and subsurface remediation.
• Collection of 10 archived subsurface cores of the oil-bearing zone.
• Analysis of total petroleum hydrocarbons (TPH) compared with 1993 data.
• Creation of a detailed map of producing, shut-in, and abandoned oil wells and monitoring wells using global positioning system (GPS) field data overlain on a digital photograph in a geographical information system (GIS).