| Groundwater is an important resource for human survival and development,and its stable recoverable reserves and good water quality are the basic guarantees for social and economic development of a region,particularly for the northern China.Drinking water of residents and the industrial and agricultural production in Hohhot,Inner Mongolia,China mainly depend on groundwater.Due to natural factors including climate changes and influences of human activities,the environmental quality of groundwater changes and groundwater is polluted in varying degrees.Moreover,a large area of depression cones appears in the concentrated exploitation areas.Studying the change laws and influences of groundwater quality is significant for the scientific planning and management as well as sustainable utilization of regional groundwater resources.This study analyzes years of monitoring data of groundwater in Hohhot region by using the statistical method.On this basis,this research comprehensively evaluates the environmental quality of groundwater and determines the main pollution factors for groundwater quality.Furthermore,this study investigates the change characteristics of groundwater quality and main pollutants,summarizes the change laws of groundwater levels and explores the main factors influencing environmental quality of groundwater and influence degrees.The main conclusions are shown as follows:(1)The phreatic water demonstrates a poor quality and is generally polluted.More than 50%of water is at grades IV and V and factors consisting of total hardness.The phreatic water quality in the research area in 2000 was the poorest after slowly decreasing from 1990 to 2000,while the quality significantly changed and became better from 2000 to 2015.Water quality at grades IV and V mainly distributes in suburban centralized residential areas and agricultural planting areas.NO3-,NO2-and NH3-N mainly affect the phreatic water with quality at grades IV and V in the research area,showing more than 20%of influence degrees.However,the confined water shows a good quality and is polluted slightly.The good and excellent rate of water quality of centralized water supply wells reaches more than 85%,while other water source wells are polluted in varying degrees.The quality of confined water in the research area was poor in 1995 and 2005 and showed periodic changes on the whole.Furthermore,the water quality of centralized water supply wells was stable and the water quality at grade IV only appeared in No.l well in the south row of the north waterwork,while water quality of other water source wells changed randomly in different years,influencing quality of regional confined water.The main factors affecting the quality of confined groundwater include NO3-,NO2-,NH3-N,COD and total hardness.(2)The pollution factors of groundwater in the research area show certain change laws in time and spatial scales.The total hardness and concentration of pheartic water gradually increased and the areas where the total hardness and concentration surpassed standard limits of grades IV and V expanded year by year,seriously influencing water quality.In 2005,the maximum concentration of pollutants was 1,460.9 mg/L and the measuring points where the water quality is at or surpasses grade IV accounted for 53%.The concentration of NO3-and NO2-changed periodically and increased year by year from 1990 to 2005,while decreased gradually from 2005 to 2015.The maximum concentrations of NO3-and NO2-reached 217.44 mg/L and 3.98 mg/L,respectively.The change of the area where the concentrations of NO3-and NO2-exceeded standard limits of grades IV and V were basically consistent with that of concentration.NO3-and NO2-seriously polluted the phreatic water.The concentration of NH4+ reduced year by year,which had little influence on the quality of phreatic water.The concentration of arsenic ions has not surpassed the standard limit of grade III since 2000.(3)The concentration of NO2-in the confined groundwater reached the largest,11.21 mg/L in 2005 and then decreased year by year,and the area of polluted regions reduced.However,NO2-still severely pollutes the confined water and should not be ignored.From 1990 to 2015,NO3-concentration decreased from 68.12 mg/L to 27.76 mg/L,reducing the influences on groundwater quality.In 1990～2005,the maximum NH4+ concentration rose from 1.13 mg/L to 12.34 mg/L and the area with water quality surpassing standard limits increased year by year.It is found that the water quality has not gone beyond the standard limit of grade III even since 2010 and tended to be stable.The total hardness exceeded the standard limit in 1995 and 2015,showing a little influence.In 1990,2000 and 2005,the confined water was polluted by arsenic ions whose maximum concentration was 0.049 mg/L.The concentration of arsenic ions has not surpassed the standard limit of grade III since 2010.(4)Based on the statistical analysis of observation data of groundwater levels in the research area,the phreatic water level dropped slowly,and the average water level decreased by 3.2 m for the past ten years with an average decline rate of 0.32 m/a from 2006 to 2014.The regions with declining water levels were mainly distributed in the northwestern and eastern agricultural areas of the research area,while the annual water level changed slightly in the urban area in the centre.The confined water level reduced year by year to 74.35 m,the lowest in 2015.The average groundwater level declined by 8.4 m at a decline rate of 0.84 m/a on average.The research demonstrates that the continuous decrease of confined water levels forms a depression cone centered in the north,west and southwest exploration areas and the area gradually expands.The results indicate that groundwater in the research area is seriously over-exploited.Under the influence of human activities,the groundwater flow in the study area has changed obviously,which shows that the regional groundwater level has been declining as a whole.As a result,the shallow aquifer has been drained,the flow area has narrowed,the shallow water flows,Pressure groundwater and other environmental problems.(5)Human factors are the main reasons for the changes of groundwater quality in the research area.By using the grey correlation analysis method,the correlations of main influence factors with groundwater quality,water level and pollution factors are analyzed.The results show that groundwater exploitation and changes of land use have the greatest impacts on groundwater quality.The sewage discharge significantly influences phreatic water in urban and rural residential areas.Furthermore,the fertilizer use presents great impacts in agricultural planting areas.Therefore,fertilizer use and sewage discharge are the main factors that causing phreatic water quality in the research area being greatly influenced by nitrate. |
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