A Multilevel System for High-Resolution Monitoring in Rotasonic Boreholes

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Abstract

A modular multilevel system was adapted for high-resolution, depth-discrete monitoring of hydraulic head and ground water quality in rotasonic boreholes or boreholes produced with similar dual-casing drilling methods. The system accommodates up to 15 monitoring intervals within one hole and can be used to monitor overburden and/or bedrock to depths of 100 m (330 feet) or more. It is most effective where static water levels are shallower than 9 m (30 feet) below ground surface. Sand packs around each monitoring port define the monitoring interval, and bentonite seals placed above and below each sand pack isolate the intervals. Each sand and bentonite layer has a practical minimum length of 0.5 m (1.6 feet); therefore, a 15 port system can monitor, with maximum detail, a minimum vertical span of 15 m (50 feet). All system components, primarily flush joint polyvinyl chloride (PVC) casing segments, stainless steel ports, Teflon® tubing, and PVC centralizers, are commercially available and require little preconstruction. An open 6-mm (¼-inch) inner diameter tube is connected to each port for manual hydraulic head measurements and water sampling with a peristaltic pump. To assess installation and performance of the new system, nine rotasonic holes in overburden and bedrock between 20- and 30-m (65- and 100-feet) deep were monitored at two sites. This detailed vertical monitoring provided important information on hydraulic head and contaminant distributions that would have been missed with fewer monitoring intervals. The monitoring system offers unique advantages where detailed monitoring in heterogeneous settings is needed to understand ground water flow and contaminant migration or evaluate the performance of remediation efforts.

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