Local Soil Conditions: Comfort and Brackett soil series on the Edwards Plateau — thin, stony clay loams over Edwards Limestone with 6–18 inches of soil depth on most rural parcels. Percolation rates are highly variable: karst solution cavities create zones of extremely rapid drainage (< 1 min/inch) that provide no sewage treatment, while clay-filled fissures in the same limestone produce rates of 60–120 min/inch. Urban Bexar County to the east has deeper Lewisville and Houston Black clay (Vertisols) with very slow percolation.
Water Table: Ranges dramatically with topography and geology. In the Edwards Plateau Hill Country northwest of San Antonio, the water table corresponds to the Edwards Aquifer potentiometric surface, typically 50–300 feet below ground. In eastern Bexar County on the Coastal Plain transition, water tables are 15–40 feet deep. Seasonally, Edwards Aquifer levels fluctuate 10–50 feet based on recharge from rainfall on the contributing zone.
Climate Impact: San Antonio's semi-arid subtropical climate averages 32 inches of annual rainfall with high interannual variability — rainfall can range from 15 to 55 inches depending on drought and La Nina/El Nino cycles. The bimodal rainfall pattern, with peaks in May-June and September-October, creates alternating wet and dry stress cycles on drain fields. Extreme heat — average July highs of 97°F — and periodic multi-year droughts put systems through hydraulic stress from the opposite direction: shrink-swell clay soils in eastern Bexar County crack deeply during drought, creating bypass pathways that can short-circuit treatment. The region's proximity to the Gulf of Mexico means occasional tropical moisture events can deliver 5–10 inches in 24 hours.