Borehole Flows, Hydraulic Heads, and Fracture-Zone Connection in The Potsdam Sandstone at The Altona Flat Rock Research Site in Northern New York

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Numerous borehole-geophysical methods are available for the characterization of fractures penetrated by boreholes but relatively few of these methods help to characterize the connections between fractures in the rock surrounding boreholes. Unfortunately, most studies in fracture-rock hydrology conclude that the characteristics of connections between locally transmissive fractures have a greater influence over aquifer-scale permeability than the local aperture of individual fractures (Long and others, 1996; LeBorgne and others, 1996; Shapiro and others, 2007). The study described here focused on the application of borehole-flow methods in the characterization of possible hydraulic connections between fracture zones penetrated by open bedrock boreholes. The Potsdam sandstone of Cambrian age forms an extensive fractured-rock aquifer in northern New York. The hydraulics of three interconnected boreholes that penetrated multiple fracture zones in the Potsdam sandstone aquifer were studied at the Plattsburgh State University Flat Rock research site near Altona, New York. Televiewer and natural-gamma logs were used to correlate fracture zones penetrated by two shallow boreholes and a deep borehole at the Flat Rock site. Vertical flows in the multi-zone boreholes were measured with heat-pulse or electromagnetic flowmeters under quasisteady-state ambient and pumped conditions. Flows also were measured under transient cross-borehole pumped or injection and recovery conditions. Composite hydraulic heads, which reflect the transmissivity-weighted average of the fracture-zone heads, were measured daily in the deep and one of the shallow boreholes in 2002-2003. The transmissivity, hydraulic head, and connection geometry of the penetrated fracture zones were determined through singleand cross-borehole flow-modeling methods described by Paillet (1998 and 2000). In 2000, shallow boreholes 500A (USGS Cl-215) and 500B (USGS Cl-216) were drilled 20 ft apart and penetrated the Potsdam sandstone from 680 to 550 ft above sea level. In 2002, deep borehole 102 (USGS Cl-214) was drilled 400 ft northeast of boreholes 500A and 500B and penetrated the Potsdam sandstone from 670 to 535 ft above sea level. The lower part of borehole 102 penetrated Altona shale and siltstone and Precambrian crystalline bedrock. The composite hydraulic head in borehole 500B dropped more than 10 ft during air-hammer drilling of borehole 102 (fig. 1). Following drilling, the head in borehole 500B recovered for 3 days to within 3 ft of the pre-drilling trend before continuing on the seasonal recession. The composite hydraulics heads in boreholes 500B and 102 showed very similar seasonal fluctuations in their timing and magnitude during 2002-2003.