AWWA ACE63035

The San Antonio Water System (SAWS) is currently constructing an aquifer storage and recovery (ASR) well field in southern Bexar County in Texas. For advanced planning, design, and operations, an advanced groundwater modeling tool is needed to assess the effects of well field operations within the well field and in the surrounding area. The well field is located on a triangular section of land in southern Bexar County, near the three-county corner junction with Atascosa and Wilson Counties. At buildout, SAWS will have 34 ASR wells and 6 production wells. The wells range in depths from about 500 ft to 800 ft and well yields range up to 3,500 gallons per minute (gpm). Detailed geophysical and boring log data were collected, lithologic contacts were identified, and an extensive aquifer test program was carried out during the construction of all the wells. Because the Carrizo aquifer is the most productive water-bearing unit, it is the target formation for the ASR and production wells. A MODFLOW1 groundwater model of the aquifer system was developed in the project area. An initial four-layer ASR model design (representing Queen City, Reklaw, Carrizo, and Wilcox Formations) was ultimately expanded to six layers to represent the locally recognized hydrogeologic divisions within the Carrizo Formation (upper Carrizo, massive Carrizo, lower Carrizo). All of the ASR wells and are screened in the massive Carrizo, thus the three-layer Carrizo subdivision more accurately represents well field operations and well performance. A finite difference grid of 332 rows and 393 columns was constructed with 200-foot square cells in the vicinity of the well field, expanding to 1,000-foot square cells toward the edges of the model domain. The 200-foot spacing in the well field allows all existing and planned ASR wells to be accurately represented. Because the primary purpose of the model is to simulate ASR operations, the calibration concentrated on replicating aquifer tests conducted during the development of ASR wells. Inflow of water to each model layer includes primary recharge at the outcrop and cross-formational flow at depth. Outflow in each model layer may include well pumpage, discharge to rivers, flow through the model boundaries, and cross-formational flow. All layers are represented such that flow in both the confined and unconfined (outcrop) parts of the formation is realistically simulated. The General Head Boundary (GHB) MODFLOW package was selected for all layers for both easterly and westerly lateral boundaries as well as the downdip boundary. The GHB was also used for the top of the uppermost active layer to represent the influence of water table conditions in the Sparta and other younger formations above the Queen City. The MODFLOW well package was also used to represent flux boundary conditions. "Background" pumpage from the Carrizo within the model domain is a major discharge from the conceptual model, so its inclusion is important for this model to represent current conditions. MODFLOW's river package was used to represent the Atascosa River and San Antonio River in the outcrop of each model layer. Both streams are slightly gaining in the model area, though they abut the region to the southwest in the Frio and Nueces River basins, where losing stream conditions are prevalent. Includes figures.