EVALUATING COMPOSITE HYDROLOGIC HEADS IN A STACKED AQUIFER SYSTEM USING A MULTIPORT WELL, TRINITY AQUIFER, CENTRAL TEXAS

Water supply wells often penetrate multiple aquifers or hydrostratigraphic units yet serve as the primary source of hydraulic head data for aquifer characterization and modeling. This study investigates the effects on head data from zones open across hydrologic units through the use of a multiport (Westbay) monitor well.

An 857-foot deep multiport monitor well (ID 57-64-613) with 14 zones was utilized to isolate and monitor specific aquifer units within (stratigraphically lowest to highest) the Cow Creek limestone (primary aquifer), Hensel (aquitard), Lower Glen Rose limestone (secondary aquifer), and Upper Glen Rose. By selectively opening and closing pumping ports, the impact of different aquifer units on the primary aquifer head was assessed over a 19-month period (starting August 2020).

Composite head measurements in the multiport generally reflect the overall trend of the Middle Trinity Aquifer, influenced primarily by higher transmissivity units. Zone 3 (Cow Creek) dominates head elevation, with Zone 8 (transmissive upper reef interval of Lower Glen Rose) had an influence on the Cow Creek heads by shifting composite head higher by 5-7 feet. Other zones had minimal impact, even when their head values were significantly higher.

A weighted average of head values, based on transmissivity, provided a reasonable estimate of the composite head measured in the study. However, local variations in transmissivity and measurement methods of transmissivity can lead to deviations from this estimate.

This study emphasizes the importance of understanding well completion details and the relative transmissivities of aquifer units when interpreting head data for aquifer studies. The approach of weighting heads by transmissivity offers a practical method for estimating composite heads in complex aquifer systems with good hydrogeologic data. A better understanding of these factors is crucial for accurate groundwater evaluations.