HYDROGEOCHEMICAL AND HYDROGEOLOGICAL EVIDENCE FOR BLANCO RIVER RECHARGE OF JACOB'S WELL, A KARST SPRING IN HAYS COUNTY, TEXAS

Jacob's Well is a karst spring in Hays County, Texas, that has shown a significant decrease in flow over the past 10 years, which has been assumed to be caused by over-pumping in the Trinity aquifer. However, the source of water in Jacob's Well has never been demonstrated. Speculated sources of recharge to the spring are the Blanco River, the Trinity aquifer system, or a mixture of water from the two sources. Water chemistry, potentiometric surface, and stream flow analyses indicate the Blanco River as the primary source water for Jacob's Well. Aqueous geochemistry data from the spring, the Blanco River, and wells surrounding the area were collected and analyzed for field parameters, dissolved metals, and anions. Water level data were collected from residential wells using e-line and chalk-tape methods. Stream flow was measured along the Blanco River with an acoustic current meter. The geochemistry of Jacobs's Well is consistent with the upstream reach of the Blanco River within the study area and is inconsistent with the Trinity aquifer system (table). Blanco River discharge decreased by 5.8 cfs (0.16 m³/s) over a highly fractured, 1000 ft (305 m) long section. Water table elevation and PHREEQC geochemical modeling results suggest that this section and other losing sections of the Blanco River contribute significantly to the recharge of Jacob's Well.

Table:  Representative water analyses.  Concentrations are in ppm.

	ToC	pH	[Cl]	[Ca]	[Mg]	[Sr]	[SO4]	Alkalinity as [HCO3]
Trinity Aquifer	25.6	6.98	16.2	67.8	39.7	5.45	65.3	331
Blanco River	22.6	7.41	10.83	73.8	17.4	0.527	23.4	247
Jacob's Well	20.67	7.17	11.28	86.26	18.4	0.46	20.49	329