Study Of The Chronology Of Aquifer Strata And The Geochemical Behavior Of Uranium Of The Underground Brine Along The Southern Coast Of Laizhou Bay

Transgressions which happened three times at the southern coast of Laizhou Bay are the most important geological events in the depositional history which began from late Pleistocene. The alternative appearance of sea facies strata and continental facies strata have also been discovered in the Quaternary depositional strata. There are three brine-rich-aquifers exist in the sea facies strata 80 meters under the ground, which contain a large quantity of Quaternary coastal underground brine (about 7.4 billion cubic meters). The dissolved uranium concentration of underground brine is so high that the brine can be regarded as a liquid mini-uranium-mine with many advantages such as imbedded not so deep, large quantity of reserves, not far from the sea, convenient for exploitation. Therefore, the mine has an extensive perspect of exploitation. It is very important to study the chronology and to reconstruct the paleoenvironment of the sea facies strata and continental facies strata for learning the processes and mechanisms of the underground brine's formation. Studies of the range, distribution and formation causes of high level dissolved uranium in the brine can provide credible gist for the exploiting and recharging of underground brine, which has important application value.In this study, the sea facies strata and continental facies strata of the core ZK32-1 located at southern coast of Laizhou Bay were divided according to the lithologic character and the geological information of the nearby cores. The information of the foraminifer distribution was also used to double check the result. The AMS14C method and total sample dissolution method (TSD) were used to determine the boundary age of the sea facies strata and continental facies strata directly. Thus the isotopic time scale of the aquifer strata at 100 thousand years'range was established combined with data of literatures. The paleoenvironment of deposition was reconstructed through the ecological combination information of foraminifer. Dissolved uranium concentration range and distribution characters were obtained through measuring 51 samples of underground brine with anαspectrometer. The causes of high level of U concentration in the brines were analyzed from the following aspects, evaporation of seawater, infiltration of surface water, release from aquifer sediment, deoxidization of Fe-Mn oxide, mineralization of algae, influence of Sulfate-reducing bacteria and so on. The main conclusions are as followed:1) The strata of the core ZK32-1 located at the southern coast of Laizhou Bay are divided as: The third sea facies stratum is from -39 meters to -56 meters. The third continental facies stratum is from -33 meters to -39 meters. The second sea facies stratum is from -20 meters to -33 meters. The second continental facies stratum is from -9 meters to -20 meters. The first sea facies stratum is from -3 meters to -9 meters. The first continental facies stratum is from surface to -3 meters.2) The third sea facies stratum of the southern coast of Laizhou Bay has been formed from 103.50±11.56 ka B.P. to 72.16±7.41 ka B.P.. The third continental facies stratum has been formed from 72.16±7.41 ka B.P. to 45.34±5.35ka B.P.. The second sea facies stratum has been formed from 45.34±5.35 ka B.P. to 23.93±2.84 ka B.P.. The second continental facies stratum has been formed from 23.93±2.84 ka B.P. to 11.25±0.16 ka B.P.. The first sea facies stratum has been formed from 11.25±0.16 ka B.P. to 6.50±0.03ka B.P.. The first continental facies stratum has been formed after 6.50±0.03ka B.P..3) The paleoenvironment of deposition of the first sea facies stratum was shore shallow water or estuary environment. And that of the second and third sea facies strata were all estuary or tideland or shore shallow sea environment.4) The geochemical anomaly phenomenon of high dissolved uranium concentration is found in the underground brines along the southern coast of Laizhou Bay. The dissolved uranium concentrations are 3.43-95.98μg/L with the median value of 44.88μg/L, and that of more than 80% of samples measured exceed 30μg/L. Along the vertical direction of seaside, the dissolved uranium concentrations change as low-high-low. Along the horizontal direction of seaside, the distribution of dissolved uranium concentration has no obvious trend.5) The causes of high dissolved uranium in the underground brine along southern coast of Laizhou Bay are evaporation of seawater, deoxidization of Fe-Mn oxide and mineralization of algae. Because the hydrological and water-quality conditions of the underground brine are not suitable for sulfate-reducing bacteria to exist, the uranium deposition resulted by biological reduction could be avoided. Thus the dissolved uranium with high concentration can be preserved.