Evaluation of Groundwater Supply Alternatives Within Bernal Subbasin

Zone 7 Water Agency – Tennis Park Test Well Summary and Production Well Design Recommendations 8 Zone 7 Water Agency December 2025 Gravel Envelope – Based on the sieve analysis of selected formation samples (Figure 7), a 6 x 9 graded gravel envelope material was selected for use in this well. The gravel chosen is 4.9 times larger than the finest formation sample (555-560 feet bgs) at the d70 (cumulative percent retained) value. The 0.080-inch screen slot size will retain over 90 percent of the gravel material. The gravel envelope will be placed from 172 to 780 feet bgs. A two-foot, fine sand transition seal will be placed from 170 to 172 feet bgs. Annular Seal – The deep annular seal will ensure that only groundwater from the targeted intervals will enter the well structure. The annular seal shall consist of “10.3-sack” sand/cement grout and shall be placed from a depth of 170 feet bgs to the ground surface. Based on recent projects of similar size and scope, LSCE’s preliminary Engineer’s Estimate for the well construction, development, and testing is approximately $2,000,000. Items such as water supply for drilling, fluid and cutting disposal, development water disposal, material and equipment storage, sound attenuation, working hours, and site security and safety that may affect the cost of well installation and testing will need to be addressed in the final specifications before bidding. Alternate Well Design Considerations Well design recommendations were based on direction to maximize well yield. The recommended well design includes screens placed opposite each zone identified in the test well borehole below a depth of 195 feet. Development of well capacity estimates assumed that each zone screened in the test well would be targeted for completion in a production well. The projected yield of the proposed Tennis Park Well constructed as recommended above is between 5,100 and 3,400 gpm, depending on long term and seasonal water level variations and a conservative well efficiency value as discussed in the Pumping Capacity TM (Appendix C). Based on dynamic flow testing conducted in the test well, the bottom three zones (560 - 760 feet) in the recommended well design will contribute 11% of the total well yield or between 561 and 374 gpm of the projected yield of the production well. In contrast, the upper four zones in the production well would contribute 89% of the well yield, or 4,539 to 3,026 gpm. The estimated cost to drill, construct, and test the production well as recommended above is $2,000,000, which equates to $2,545 per foot for the 780 foot well. The estimated cost to construct a 540 foot well that does not screen the zones between 560 and 760 feet is $1,384,560, or 31% less than a 780 foot well. Put another way, an 11% reduction in yield will result in a 31% cost savings for the construction of the production well.