HYDROSTRATIGRAPHY OF THE TRINITY AQUIFER, SOUTH-CENTRAL TEXAS: A GEOLOGIC MAPPER'S PERSPECTIVE

Studies of an aquifer’s hydrostratigraphy are critically important for characterizing its surface and subsurface hydrogeology and for predicting its subsurface hydraulic budgets and chemical signatures. Hydrostratigraphic mapping can also help to understand the spatial distribution and hydraulic connectivity of an aquifer’s permeable zones. Unfortunately, there’s a multitude of, and sometimes contrasting, meanings for much of the hydrostratigraphic terminology in the current literature. A hydrostratigraphic unit may simply imply a part of an aquifer, a select part of a groundwater or assessment model, or a geologic unit with distinct hydraulic properties. One of the reasons for this ambiguity is that the hydrostratigraphic unit, commonly termed “HSU”, is a term that never gained formal status within the stratigraphic community. The usage of hydrostratigraphic terminology is also inconsistent even among geologists, and the various meanings can become divergent depending on whether subsurface, bedrock, or surficial geology is involved.

Hydrostratigraphic mapping of Trinity aquifer geology is being conducted in south-central Texas just north and northwest of San Antonio. Here, the Trinity aquifer consists of three hydrologic zones: (1) An upper zone that includes the upper member of the Glen Rose Limestone, (2) a middle zone represented by the lower member of the Glen Rose Limestone and Bexar Shale and Cow Creek Limestone Members of the Pearsall Formation, and (3) a lower zone composed of the Hosston and Sligo Formations. Six informal hydrostratigraphic members proposed by Parsons Engineering Science™ (Austin, TX) for the lower member of the Glen Rose Formation have been modified based on subsurface comparisons of two well cores against 112 well logs. The Lower Glen Rose hydrostratigraphic scheme is providing the framework for a 16-layer 3-D EarthVision™ model of the Camp Stanley Storage Activity military base, Boerne, Texas, and is being tested at the nearby Guadalupe State Park and Honey Creek Natural Area to determine whether the proposed hydrostratigraphic members are mappable and not facies controlled. From a geologic mapper’s perspective, if and when hydrostratigraphic nomenclature is formalized, HSU’s must be defined as mappable and laterally continuous.