Full Description

The San Antonio Water System (SAWS) is currently constructing an aquifer storage andrecovery (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 wellfield 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, nearthe 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 500ft to 800 ft and well yields range up to 3,500 gallons per minute (gpm). Detailed geophysical andboring log data were collected, lithologic contacts were identified, and an extensive aquifer testprogram was carried out during the construction of all the wells. Because the Carrizo aquifer isthe most productive water-bearing unit, it is the target formation for the ASR and productionwells. A MODFLOW1 groundwater model of the aquifer system was developed in the projectarea. An initial four-layer ASR model design (representing Queen City, Reklaw, Carrizo, andWilcox Formations) was ultimately expanded to six layers to represent the locally recognizedhydrogeologic divisions within the Carrizo Formation (upper Carrizo, massive Carrizo, lowerCarrizo). All of the ASR wells and are screened in the massive Carrizo, thus the three-layerCarrizo subdivision more accurately represents well field operations and well performance.A finite difference grid of 332 rows and 393 columns was constructed with 200-footsquare cells in the vicinity of the well field, expanding to 1,000-foot square cells toward the edgesof the model domain. The 200-foot spacing in the well field allows all existing and planned ASRwells to be accurately represented. Because the primary purpose of the model is to simulate ASRoperations, the calibration concentrated on replicating aquifer tests conducted during thedevelopment of ASR wells. Inflow of water to each model layer includes primary recharge at the outcrop and cross-formationalflow at depth. Outflow in each model layer may include well pumpage, discharge torivers, flow through the model boundaries, and cross-formational flow. All layers are representedsuch that flow in both the confined and unconfined (outcrop) parts of the formation is realisticallysimulated. 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 wasalso used for the top of the uppermost active layer to represent the influence of water tableconditions in the Sparta and other younger formations above the Queen City. The MODFLOWwell package was also used to represent flux boundary conditions. "Background" pumpage fromthe Carrizo within the model domain is a major discharge from the conceptual model, so itsinclusion is important for this model to represent current conditions. MODFLOW's riverpackage was used to represent the Atascosa River and San Antonio River in the outcrop of eachmodel layer. Both streams are slightly gaining in the model area, though they abut the region tothe southwest in the Frio and Nueces River basins, where losing stream conditions are prevalent. Includes figures.

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Edition: Vol. - No. Published: 06/01/2006 Number of Pages: 10File Size: 1 file , 920 KB