HYDROSTRATIGRAPHIC ANALYSIS OF VERTICAL FLOWPATHS BETWEEN TRINITY HILL COUNTRY AND LLANO UPLIFT AQUIFERS, BLANCO COUNTY, TEXAS

Local-to-regional-scale flowpaths need to be better understood before methods are defined to measure the impact of phreatophyte removal on recharge and spring flow in Texas Hill Country watersheds. The Pedernales River watershed includes the erosional northern limit of the Trinity Hill Country aquifer in Cretaceous rocks. The Pedernales River valley is underlain by the Llano Uplift aquifer consisting of lower Paleozoic rocks. The study area in Blanco County, Texas, is between the Pedernales River and its surface-water divide with the Blanco River. Karst features would make it hard to confidently map the groundwater divide in the Trinity Hill Country aquifer, assumed to roughly coincide with the surface-water divide. Groundwater flowpaths from upland areas, where groundwater in the Trinity Hill Country aquifer is recharged, might follow local flowpaths to discharge in 4^th-5^th-order tributaries, intermediate-scale flowpaths to 3^rd-order tributaries with perennial springs, and deeper flowpaths to the Pedernales River, a 2^nd-order river and regional base level.

The dynamic pressure increment (Δp) measures departure from hydrostatic and indicates where flow of groundwater is directed downward (-Δp; recharge) or upward (+Δp; discharge). It also reflects hydrostratigraphic layers. Water-level data were obtained from the Blanco-Pedernales Groundwater Conservation District, Texas Water Development Board, and field measurements. Δp values were geographically distributed on a USGS (1 Arc second resolution) National Elevation Dataset of the study area. A significant portion of -Δp values for individual wells falls within a specific distance of hill crests. Most positive +Δp values lie within stream valleys. Sorting Δp data for analysis by stream order might provide a basis for locating local, intermediate, and regional components of flowpaths and mapping the interconnection between the Trinity Hill Country and Llano Uplift aquifers.