Present Situation Of Shallow Groundwater Pollution In A Natural Gas Treatment Plant In Northwest China And Its Prevention And Control Measures

In the process of natural gas development,due to the lack of understanding of the long-term,complex,concealed and difficult to recover groundwater pollution and imperfect environmental protection measures,groundwater pollution has become increasingly serious,and has become a hot spot of social concern in recent years.Therefore,in order to strengthen the understanding of the status quo of groundwater environment in natural gas fields,and to grasp the degree of influence on the groundwater environment during the development of gas fields,the groundwater pollution prevention measures are proposed in a targeted manner.This paper selects Su Lige natural gas treatment plantfor comprehensive systematic groundwater investigation and analysis.The pollution characteristics of shallow groundwater in the development area of natural gas field and its impact on the environment are mainly studied.At the same time,the time-space variation of pipeline leakage pollution in subsurface fluids was studied by using the simulation prediction method.The main results are as follows:（1）The shallow groundwater aquifer of the treatment plant is mainly the fourth-series wind-sand aquifer.The permeability coefficient of shallow groundwater aquifer,the maximum is 5.68m/d,the minimum is 5.55m/d,and the average is5.61m/d.They are obtained by the stable flow pumping test carried out by the pumping test hole in the factory area.During the survey,the groundwater of the natural gas treatment plant was buried from 1.95m to 3.08m,and the water level was1290.371～1291.245m.The northwest side of the factory is larger,while the southeast side is shallow,the plant terrain is flat,the overall buried depth difference is small.The north side of the factory is higher than the south side,the groundwater flow is generally from north to south,hydraulic gradient is 1.25‰～3.70‰,the average hydraulic gradient is 2.38‰,and the flow is more flat.（2）The evaluation results of groundwater environmental quality show that the groundwater standard index is greater than 1 in this test index,such as p H,COD_Mn,ammonia nitrogen and petroleum,and the remaining detection indexes are not exceeded,which satisfies the standard of groundwater qualityⅢ.The excess rate was from large to small in order of COD_Mn（20%）and ammonia nitrogen（7%）,Petroleum（7%）and p H（7%）,of which the excess of oil was 0.13.Combining the results of water quality analysis and testing,the production and operation status of natural gas treatment plant and the hydrogeological conditions of the factory area,the groundwater environment in the factory area is polluted to a certain extent,especially in the sewage treatment station inside the factory site condensate oil stabilizer zone.（3）From the prediction results:once there is a sewage leakage accident,resulting in the production of wastewater directly into the submersible aquifer,can cause groundwater pollution,pollution degree and aquifer penetration performance,source strength concentration,waste water leakage time and so on.It is predicted that without considering the background value of groundwater,the pipeline has been leaking continuously for 30 days,the excess range of petroleum pollutants in the aquifer can spread to 16m downstream of the leakage point,and after 100 days of continuous leakage,the pollution exceeding the scope spread to the downstream 30m,the pollutant leakage for 1000 days,Groundwater pollution exceeding the range is about 129m downstream of the leak point.（4）In accordance with the"Source control,Zoning prevention,Pollution monitoring,Emergency response",focusing on the principle of drinking water quality safety determination,in view of the pollution characteristics of the project,from the generation of pollutants,infiltration pathways,diffusion process,emergency response,the control countermeasures of groundwater are put forward.