Contamination And Countermeasures Of Santan Groundwater Source For Concentrated Exploitation In Lanzhou City

Lanzhou 'Santan' groundwater source for concentrated supply(SGSCS)located on the bank of Yellow River,covers about 16 km2,the thickness of its aquifer is ca.300m.SGSCS is the main groundwater source of Lanzhou city,and is also a reserved water source for emergency situation.From 1980's,the TDS of the groundwater in SGSCS raised distinctly,caused by the invasion of brine water from surrounded tertiary aquifer,which induced by intensive exploitation.On the other hand,the organic pollution from a petrochemical base located in upper reaches of SGSCS also threatened the water quality of SGSCS.In recent years,the exploitation of groundwater has been largely reduced to about 3 × 1 106m3/year.Upon above mentioned reason,we set the objective of this dissertation to find ways to restore the water supply ability of SGSCS?To achieve this objective,we carried out in-depth investigation for the hydrogeological condition of the study area,and brought forward the plan of restore a previous Yellow River channel located in the south of SGSCS,which was buried during passed years.By using the groundwater modeling software GMS,we modeled the effect of the restoration of south Yellow River channel,including the change of groundwater flowing field,prevent the invasion of brine groundwater from south boundary,and the invasion of organic pollution from upper reaches,and get results as following:1.Keep the present annual exploitation of groundwater,the total recharge of phreatic groundwater is 1845 x 104m3/a,total discharge of groundwater is 639x 104m3/a,and the change of groundwater storage is 1062×10m3/a,and the groundwater level rise steadily.2.The main recharge sources of the phreatic groundwater include precipitation,infiltration of agricultural irrigation,leakage of channels,infiltration of sewage,seepage of Yellow River,and the lateral recharge from south boundary,they counts 98%of the total recharge;the main discharge of groundwater is exploitation,accounts for 60%of the total discharge.3.Three groundwater recharge and pumping scenarios were designed and modeled,according to the hydrogeological condition,and the water level and water quality in the coming 10 years were predicted:(1)Keep the present groundwater exploitation.The groundwater level near Yellow River bank rise steadily,and the TDS of groundwater in the north of south Yellow River channel will descend,the TDS of groundwater in the south of south Yellow River channel will rise,and the brine groundwater in the south boundary will migrate northward slowly.(2)Partially restore the exploitation of SGSCS.The TDS of groundwater in the north of south Yellow River channel will still descend,but the interface of brine water and fresh water will move northward.On the other hand,the organic polluted groundwater move to around the Shengou fault after 10 years of exploitation.(3)Restore the south Yellow River channel,and partially resume the exploitation of SGSCS.The fresh water infiltrated from the Yellow River channel will push the brine water from south boundary back;the TDS along the river bank will descend steadily.Because the south Yellow River channel also go between the SGSCS and the organic polluted groundwater in upper reaches,and form fresh water wall to separate the organic polluted groundwater and the groundwater of SGSCS.4.The optimized SGSCS management scenario is:restore the south Yellow River channel,and the pumping well should locate in the north of south Yellow River channel.In this scenario,over 90%of the exploitation is recharged by the infiltration of Yellow River,the brine water in the south of SGSCS,and the organic pollution contaminated groundwater in the west(upper reach)of SGSCS can't contaminate the groundwater of SGSCS.Therefore,restore the south river channel is a effective route to meliorate and restore SGSCS.