FIELD COMPARISON OF METHODS FOR COLLECTING HYDRAULIC HEAD PROFILES ACROSS AN AQUITARD

Evaluating the distribution of hydraulic head across and within aquitards is important for assessing aquitard integrity and for understanding the potential for recharge and/or contaminant transport to adjacent aquifers and wells. We are comparing three methods for acquiring head profiles across a buried shale aquitard in south-central Wisconsin: lightweight straddle packers, buried pressure transducers, and a flexible liner multiport (FLUTe) system. The aquitard consists of shale and mudstone within the Cambrian Eau Claire Formation. This aquitard is about 8 ft thick, occurs about 300 ft below the land surface, and overlies a regional sandstone aquifer. The downward vertical hydraulic gradient across the aquitard exceeds 3 and internally is as high as 9. The gradient is steepest over less than 1 ft at the base of the shale.

The three techniques use 4- or 6-in diameter boreholes to access the aquitard and adjacent aquifers. A lightweight packer assembly with a 2.2-ft open straddle was used to collect 57 sequential head and hydraulic conductivity (K) measurements over 200 ft of 4-in borehole, with 11 measurements inside the aquitard itself. Nine vibrating-wire pressure transducers are sealed in a 6-in borehole using bentonite chips, with two transducers inside the aquitard. The multiport system consists of four and six ports in two adjacent boreholes, with four ports inside the aquitard.

All three systems produce excellent detailed profiles of hydraulic head. Of the three methods, the packer system is probably the most economical and effective system for characterizing an aquitard if sufficient time is available for conducting the tests; an advantage of this method is that it leaves an open borehole for future work. The multiport and buried transducer systems are more useful where longer-term monitoring or water sampling is required. However, the transducer system requires the sacrifice of a borehole, and failed transducers cannot be repaired. The multiport system is relatively expensive, but easy to install. It provides the added advantage of allowing the collection of discrete water samples, is suitable for long-term monitoring, and is removable.