Water Resource Under The Condition Of Controlling Land Subsidence In Tianjin

Severe land subsidence has happened in Tianjin area since 1958 with groundwater yieldgradually increasing and groundwater level lowering. It has a great significance to deal rightlywith the relation between deep groundwater exploration and the control of the land subsidenceto support the development of the society and economy. The study aims to explore the waysof sustainable development of deep groundwater resources so as to control land subsidenceand includes such working items as recognizing the cycling of deep groundwater systems andcharacteristics of groundwater resources according to deep groundwater flow fields and itshydrochemical isotopes, and computing the composition of deep groundwater resources anddetermining a "secure pumping" in consideration of physical and mechanical properties ofclay and characteristics of land subsidence to keep total settlement to a acceptable level.The deep and shallow groundwater have formed a consistent flow field under thecondition of exploitation,companied by a continual distribution of stable isotopes δ18O andδD. The content of radioactive isotopeδT in deep groundwater is over 8TU generally. The14C age of deep groundwater is also under the control of the water level. The amount of "oldwater" from clay is small in the areas of groundwater sallow-buried, so"new water" in sallowaquifers affects deep groundwater more greatly and the deep groundwater have a age of about10000aB.P. On the contrary, deep-buried groundwater has an older age which can exceed20000aB.P.,which demonstrates that the deep groundwater receives leakage recharge fromsallow-buried aquifers.According a result of a deep groundwater balance budget, the averageannul yield from the deep groundwater in the studied area is about 4.7012×108m3/a betweenthe years of 1991 and 2000, among which leakage shares 38.2%, released water from pressedclay 41.3%, literal flow 14.9% , elasticity released water 5.6% and the former two itemsconstitute the main parts of pumping.The best favorable exploitation layers for the deep groundwater are the second and thirdaquifer groups. The drainage of the clays in the second and third aquifer groups during theyears between 1991 and 2000 contributes 33.65% and 27.98% proportions of their totalwithdrawal respectively, which are less than the numbers in the fourth and fifth aquifer groups.It can be tested by the monitoring data of land subsidence and the physical and mechanicalparameters of stratum that after long intensive pumping the consolidation rate of cohesiveclays has already risen, so settlement due to groundwater pumping becomes small. Andaccording to a further result of the modeling to groundwater flow and land subsidence, if thewater levels in the second and third aquifer groups are controlled about 30-40m and 40mrespectively, little land subsidence will occur. While water levels are controlled about 35-40mand 45m in the rural regions, the consequent total settlements will be below 10mm/a.Underthe condition of keeping the water levels above-mentioned, the total exploitable resources ofgroundwater in the deep-buried aquifers is 26755×104m3/a, and average resources modulus is2.9×104m3/a?km2.Compared with the yield from the deep-buried aquifers during the yearsbetween 1991 and 2000, the total groundwater resources will reduce to the proportion of44.5% , while land subsidence will reduce to 68.3%.The deep groundwater in Tianjin plain can get recharge during exploitation. Theconsolidation rate of cohesive clays will rise after long intensive pumping. The effectivemeasure to keep groundwater pumping sustainable and control land subsidence is thatpumping with a stable rate in historical exploitation layers and keeping water levels aroundthe critical level.