Groundwater flow conditions in a low-angle fracture zone at Finnsjön, Sweden

Abstract

The performance and results of a series of hydraulic head measurements and differently designed tracer tests are described. A combined interpretation of the results is utilized to estimate the natural groundwater flow conditions in a low-angle major fracture zone (Zone 2) investigated at the Finnsjön study site, central eastern Sweden. The transport of tracer-labelled flushing water used during drilling of a cored borehole was utilized to determine hydraulic connections and estimations of hydraulic parameters. The breakthrough of tracer was monitored at a distance of 440 m from the injection point with a mean residence time of 37 days. In order to determine transport parameters of the fracture zone, a tracer test was performed in conjunction with an interference test. The hydraulic fracture conductivity, flow porosity, equivalent fracture width and dispersivity were calculated for three different tracer routes. Tracer was also used to detect any bypass around the packers during the interference test. A series of groundwater flow determinations are also described, which used the point dilution technique in packed-off borehole sections, carried out under natural gradient conditions. The groundwater head measurements indicated that a pressure gradient from high to low exists between the adjacent rocks and Zone 2; where Zone 2 is located closer to the ground surface, the opposite situation prevails. The upper highly conductive part of the zone was found to have high natural flow rates, whereas in the lower highly conductive part, the groundwater flow rates were very low, close to the rate of molecular diffusion.