EVALUATION OF A REGIONAL KARSTIC FLOW SYSTEM IN A SEMIARID CLIMATE

A large regional karstic system in Trans-Pecos Texas flows in Cretaceous and Permian rocks to discharge at the Balmorhea Springs (San Solomon, Phantom Lake, Giffin, East Sandia, West Sandia, and Saragosa - in general order of largest to smallest historic discharge). Spring flows have declined since the late 19^th century and Phantom Lake Spring essentially no longer flows. The nearby Davis Mountains are the adjacent topographic high but recharge from them is normally limited by clay deposits under the alluvial stream channels. The head of the flow system is the basin fill deposits west of the Davis Mountains (Ryan, Eagle, and Wild Horse flats), but modeling indicates that this flow provides only about 10% of the current spring discharge. Spring water chemistry is consistently brackish (TDS ~ 2000 mg/L) Na-Cl-SO4 hydrochemical facies with an estimated age of ~8000 ybp. Recharge in the carbonate rocks of the Apache and Delaware Mountains is the logical source of the additional flow, but the flow paths, inferred from water chemistry, are not well delineated. Occasional freshening events occur when very heavy precipitation in the Davis Mountains causes streams to infiltrate in the overlying alluvium near the springs and can contribute up to 70 % of the spring flow. Minor short-term variations in water chemistry that may be related to leakage from nearby canals, local precipitation, or contributions from different karstic conduits. Dye tracing indicates that even nearby springs (e.g., San Solomon and Giffin) discharge from different conduits. Long-term preservation of these springs will require more precise delineation of the flow paths and recharge source areas as well as assessment of the effects of both past and future climatic conditions.