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Site investigation work at Forsmark has recently terminated with the completion of two large-scale tracer tests. The main purpose of the tests was to confirm the final hydrogeological and transport models developed from earlier investigations at the site. A second purpose was to determine in-situ transport parameters for comparison with laboratory data and finally, to compare parameters derived from multiple-hole tracer test with parameters from single-hole tracer tests. The latter type of test has been used in several boreholes at the site. Similar tracer tests are planned at the Laxemar-Simpevarp site during spring and summer 2008.
The first tracer test was focused on retention parameters and therefore included both non-sorbing and sorbing tracers. The test was performed at a depth of about 420 m within the major gently dipping fracture zone A2. Two cored boreholes, KFM02A and KFM02B, about 50 m apart, were used as source and sink for the test. The test sequence included tracer dilution tests to determine flow responses from the pumping, a pre-test using a single non-sorbing dye tracer (Rhodamine WT), and finally the main test. Only a limited number of sorbing cation tracers (Cs, Rb and Li) could be used due to high background concentrations in the native saline groundwater. The non-sorbing dye tracer Uranine was used as reference. Large efforts were made to keep a constant chemical environment during the test e.g. by using a synthetic groundwater and keeping an inert atmosphere for the tracer solution. All injected tracers were recovered in the withdrawal well and significant retardation of the sorbing species could be observed. Numerical inverse modelling of the test was performed and showed retardation parameters similar to the ones derived from single-hole tracer tests performed at the same location in KFM02A.
The second tracer test focused on the connectivity of the fracture system at Forsmark. Non-sorbing tracers were injected at six locations at distances ranging between 70 to 500 metres from the sink borehole. The sink borehole HFM14 is located in the central part of the site and the borehole is inferred to intersect the superficial part of zone A2. The tracer injections included five rare-earth metal complexes (Tb, Gd, Dy, Ho and Eu-DTPA) and perrehnate (ReO4), all previously verified as being non-sorbing. The results of the test verified the hydrogeological model, indicating good connectivity along fracture zone A2, and all injected tracers were recovered in the withdrawal well. The test also verified less good connectivity with some of the other hydraulic features at the site.
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