Jean-Philippe Nicot, Principal Investigator;
Alan Dutton (now at The University of Texas at San Antonio), H. Seay Nance, Bridget R. Scanlon, and Robert
C. Reedy; assisted by Katherine Kier and Thandar Phyu.
The Queen City and Sparta aquifers are locally important water resources in Texas and are classified as
minor aquifers. They are part of several large depositional systems of Paleocene-Eocene age prograding on
the Gulf coast. The Queen City and Sparta Formations overlie the Carrizo Formation, separated by a leaky
aquitard. The Carrizo Formation represents the upper part of a major Texas aquifer system, the Carrizo-Wilcox
aquifer. Three overlapping, quasi-three-dimensional numerical models of the Carrizo-Wilcox aquifer were
previously constructed and calibrated. The project involved adding three layers over the five main layers
already represented in the three Carrizo-Wilcox Groundwater Availability Modeling programs (GAMs), the Queen
City and Sparta Formations, as well as the intermediate aquitard.
The first step of the project consisted of determining the structure of the added layers, mainly the top
and bottom of the formations, collecting data to infer hydrologic properties, and obtaining insight into
recharge to the aquifers. Structure work was done by correlating stratigraphic boundaries throughout the study
area at approximately 220 well locations. Collecting more than 1,000 specific-capacity data helped determine
hydrologic properties. A reasonable range of recharge was estimated by relating chloride concentrations in
precipitation water to those of shallow reaches of the aquifers. Water-quality analysis confirmed the current
conceptual model of the aquifers.
The second step of the project consisted of calibrating the groundwater model. Modeled heads and stream-flow
values were adjusted to match field measurements by varying and fine-tuning hydrologic parameters. Hydrologic
parameters were regionally varied according to our conceptual models but never only locally in order to avoid
the trap of overcalibration. After calibration, the numerical model was used as an evaluative and predictive
tool, and simulations were made to help us understand future water-level changes with assumed periods of normal
and drought-of-record precipitation. The project was funded by the Texas Water Development Board as part of its
GAM program, and the Bureau was a subcontractor to Intera.