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Corresponding Author

Gad, Mohammed

Subject Area

Civil and Environmental Engineering

Article Type

Original Study

Abstract

One characteristic of a fractured aquifer is the large contrast between the hydraulic parameters of fracture zone and that of the surrounding rock matrix. In this paper a trial to determine the aquifer constants of the Early Devonian sandstone fractured aquifer in Lange Bramke Basin, Germany was carried out applying Rorabaugh's conceptual model. This model was modified and used in estimating the hydraulic parameters of the conduit and diffuse flow systems from discharge hydrograph analysis. Daily discharge from a spring in the aquifer under consideration was continuously monitored for the period 12 June-31 Aug during 2001, 2002 and 2004. These records were used in recession hydrograph analysis. The identification different slopes in the recession hydrograph proceed to isolate each slope and to derive the ratio between transmissivity T and specific yield Sy (aquifer diffusivities T/Sy) from each slope. Two pumping and two slug tests were carried out during this study to check the estimated hydraulic parameters of the diffuse flow system. The results showed that the C.14 M. I. Gad T/Sy ratios derived from the recession hydrograph analysis ranged from 0.024 to 1.298 m2/sec with an average value of 0.438 m2/sec for conduit flow system, from 0.164 to 0.345 m2/sec with an average value of 0.222 m2/sec for intermediate-flow system and from 0.016 to 0.211 m/sec with average value of 0.107 m2/sec for diffuse flow system. Assuming Sy value for pure conduit equals 1.0, the resulted T value (0.438 m2/sec) was in order-of magnitude with T value obtained by tracer test (0.98 m2/sec). In addition, the T values for the rock matrix derived from analysis of the recession hydrograph ranged from 3.2x10-1 to 42,4 x10-4 m2/sec which is comparable with the range calculated from the aquifer tests (from 0.17x10-4 to 4.8x10-4 m2/sec). Since conduit systems typically display considerable spatial variability, it was concluded that the derived I values for the conduit flow system may need to be established at a local scale, while the values for the diffuse flow system may be applicable at a regional scale. As a result, the expensive isotope and/or tracer techniques in fractured rocks may be restricted to calibrate the more practicable and, according to the study, reliable analytical approaches for estimation the aquifer constants in fractured rocks.

Keywords

hydrogeology; Fractured rocks; Lange Bramke Basin; Aquifer diffusivity; Flow type; Hydrograph analysis; Aquifer tests

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