Mapping fracture flow paths with a nanoscale zero-valent iron tracer test and a flowmeter test

Abstract

The detection of preferential flow paths and the characterization of their hydraulic properties are important for the development of hydrogeological conceptual models in fractured-rock aquifers. In this study, nanoscale zerovalent iron (nZVI) particles were used as tracers to characterize fracture connectivity between two boreholes in fractured rock. A magnet array was installed vertically in the observation well to attract arriving nZVI particles and identify the location of the incoming tracer. Heat-pulse flowmeter tests were conducted to delineate the permeable fractures in the two wells for the design of the tracer test. The nZVI slurry was released in the screened injection well. The arrival of the slurry in the observation well was detected by an increase in electrical conductivity, while the depth of the connected fracture was identified by the distribution of nZVI particles attracted to the magnet array. The position where the maximum weight of attracted nZVI particles was observed coincides with the depth of a permeable fracture zone delineated by the heat-pulse flowmeter. In addition, a saline tracer test produced comparable results with the nZVI tracer test. Numerical simulation was performed using MODFLOW with MT3DMS to estimate the hydraulic properties of the connected fracture zones between the two wells. The study results indicate that the nZVI particle could be a promising tracer for the characterization of flow paths in fractured rock.

Comparison of Hydraulic and Tracer Tomography for Discrete Fracture Network Inversion

“…Improved tomograms can be achieved by increasing the source-receiver density, by optimal and full usage of information of measured data, instead of, for example, utilizing the trends and characteristics of response data only, and by combining different measurements that deliver complementary data. The combined use of salt tracer and electrical resistivity tomography [17], of salt tracer and ground penetrating radar [41], as well as of nanoscale zero-valent iron tracer and flowmeter testing [42] have been proposed for better delineation of fracture flow paths. However, combining different methods also raises efforts and the cost of field investigation campaigns.…”

Experimental Study of the Dominant Flow Paths and Analysis of the Influence Factors Through Fracture Networks

“…Gong and Rossen [16] focused on the influence of fracture aperture distribution, and found that even a well-connected fracture network could behave like a much sparser network when the aperture distribution was broad enough. Chuang et al [17] used nanoscale zero-valent iron (nZVI) particles as tracers to characterize fracture connectivity between two boreholes in fractured rock. The position where the maximum weight of attracted nZVI particles was observed coincides with the depth of a permeable fracture zone delineated by the heat-pulse flowmeter.…”