INNOVATIVE BASE-WIDE APPROACH TO GROUNDWATER CLASSIFICATION AND CLOSURE OF THE SHALLOW GROUNDWATER-BEARING UNIT (GWBU) AT LACKLAND AFB, TEXAS UNDER THE TEXAS RISK REDUCTION PROGRAM (TRRP)

The Air Force used a base-wide approach for corrective action and closure of the uppermost GWBU at Lackland AFB instead of the conventional approach of addressing contaminated groundwater on a site-by-site basis. The study area is the 6,700-acre Lackland AFB installation, comprised of the Lackland Main Base and Lackland Training Annex. The TRRP has criteria for classifying groundwater as Class 1, 2, or 3, based on use and quality or yield of the GWBU. Class 1 groundwater is a sustainable resource that is suitable for drinking water, while Class 3 groundwater is non-sustainable and of limited usefulness. Protective concentration limits or PCLs are the cleanup standards for groundwater and are based on the groundwater classification. Class 3 PCLs are generally 100 times higher than Class 1 or 2 PCLs, therefore significantly less effort is required for Class 3 groundwater cleanup and closure than for Class 1 or 2 groundwater. The main objectives of the Lackland Base-wide Groundwater Study were to classify the shallow GWBU and develop an installation-wide conceptual site model or CSM.

The shallow GWBU consists of a 2 to 45 ft thick layer of intermixed silt, clay, and gravel overlying the Navarro Clay. The shallow GWBU is separated from the Edwards Aquifer by an approximately 700 ft thick leaky confining layer comprised of clay, shale, and limestone. To refine the CSM and to classify the groundwater, the following activities were conducted: install and log lithology in 68 wells in the shallow GWBU; map water-bearing, drainage, and stratigraphic zones; stream gauge Leon and Medio Creeks; analyze TDS in the shallow GWBU; and conduct aquifer tests to document properties of the shallow GWBU. In October 2007, the Texas Commission on Environmental Quality (TCEQ) approved the Base-wide Groundwater Study Report, which documents that the shallow GWBU at Lackland AFB is Class 3.

A base-wide groundwater Affected Property Assessment Report is in preparation and utilizes the Base-wide Groundwater Study findings. With Class 3 PCLs, only one small area in the shallow GWBU requires action, which is likely to be limited to institutional controls, to achieve closure of all shallow groundwater at Lackland AFB. This will essentially eliminate the need for costly and unnecessary remediation of the shallow GWBU at Lackland AFB.