Study On The Formation Mechanism Of High-iron-manganese Groundwater In Kashgar Prefecture,Xinjiang

Iron and manganese are essential trace elements for the human body,but long-term intake of surface and groundwater that have been polluted by iron and manganese can cause chronic poisoning,and pose treat to human health.In China,high-iron-manganese groundwater accounts for 20%of the total groundwater resources and it's mainly located in high densely population alluvial plains and inland basins.High-iron-manganese groundwater is widely distributed in the Tarim Basin of Xinjiang.Therefore the high-iron-manganese groundwater in the area increases the hidden danger of drinking water safety and reduces the use value of groundwater,leading to"water quality"type water shortage problems in southern Xinjiang.In order to study the formation mechanism of high-iron-manganese groundwater in Kashgar,Xinjiang,based on 171 sets of groundwater sample from Kashgar Prefecture,draw spatial distribution maps of iron and manganese content in groundwater in the study area through GIS software and SURFER software,and explore their distribution characteristics.Tri-leaner graph of Piper method and SPSS 19.0 software multivariate statistical analysis methods are used to analyze the hydrochemical characteristics of different enrichment types of iron and manganese;using hydrogeochemical knowledge and hydrogeology reverse simulation methods to study the influencing factors of high-iron and manganese groundwater and its occurrence environment;the PHREEQC software is used to analyze the Fe and Mn element composition forms of groundwater in Kashgar prefecture,the iron and manganese mineralization SI and the law of migration and evolution of iron and manganese in the groundwater on typical hydrogeological sections.The main conclusions are as follows:(1)In the study area,the over-standard rate of Fe in unconfined groundwater,shallow confined groundwater and deep confined groundwater are 38.6%,40.6%and 40.7%,respectively;Whereas the over-standard rate of Mn in unconfined groundwater,shallow confined groundwater and deep confined groundwater are 54.6%,72.5%and 48.1%,respectively.The Fe and Mn content in unconfined groundwater is generally higher in the north and lower in the south of the study area.In shallow confined groundwater they are generally higher in the north and middle,and lower in the south;whereas in deep confined groundwater they are generally higher in the northwest and lower in the southeast of the study area.Vertically,the distribution pattern of high iron groundwater fellows the order of deep confined groundwater>shallow confined groundwater>unconfined groundwater,and high manganese groundwater fellows shallow confined groundwater>unconfined groundwater>deep confined groundwater;high-iron-manganese groundwater is mainly distributed in the western part of Kashgar and the northeastern part of Kashgar.(2)The Fe and Mn in the groundwater mainly come from the iron and manganese minerals in the original stratigraphic environment in the study area.Through rock reduction and dissolution of reducing substances,weathering and water-rock interaction the Fe and Mn transported and enriched in groundwater by underground runoff and they are the main sources of Fe and Mn in the groundwater in the study area.Also the surface water is one of Fe and Mn sources for groundwater;high-iron-manganese groundwater is mostly concentrated in Grade V quality groundwater,high-salinity and poor quality groundwater is conducive to the formation of high-iron-manganese groundwater's enrichment,and mainly conformed in SO₄·Cl-Na·Ca·Mg,SO₄·Cl-Na·Mg and SO₄·Cl-Na·Ca types water.Enhanced evaporation and concentration effect is also conducive to the formation of high-iron-manganese groundwater,and its concentration is affected by the reducing environment,and high-iron groundwater is easily affected by human activities.High-iron-manganese groundwater in the study area mainly occurs in groundwater with Na⁺content of 100?1000 mg/L,Ca²⁺content of 100?600 mg/L,Cl^-content of 100?600 mg/L,SO₄^2-content of 80?2500 mg/L,HCO₃^-content of 100?600 mg/L,NO₃^-content of 0.02?2.00 mg/L,NO₂^-content of 0.02?0.1 mg/L,Eh range from-50?250 m V,and p H value of 7.0?8.0;High Na⁺,Ca²⁺,Cl^-,SO₄^2-and HCO₃^-in groundwater environment is conducive to enrichment of Mn.Whereas high NO₃^-in groundwater is not conducive enrichment of Mn,and high NO₂^-groundwater environment is conducive to Fe enrichment.(3)Unconfined groundwater TFe in the study area is dominated by Fe(?),accounting for 87.09%of the TFe.TFe in the shallow confined groundwater is dominated by Fe(?),accounting for 75.22%of the TFe,and Fe(?)accounts for 24.78%of the TFe.TFe in deep confined aquifers is dominated by Fe(?)and Fe(?),accounting for 49.40%and 51.60%of the TFe respectively.The groundwater TMn is almost all consist of Mn(?)in the study area.According to the PHREEQC software modeling result,the main source of iron in groundwater is limonite in the primary strata,and siderite and other iron-bearing minerals also contributed.The main source of manganese is manganese-bearing minerals such as hausmannite,pyrolusite and partial rhodochrosite.In a typical hydrogeological profile,arsenic and hydrated iron oxides(Fe OOH and Fe(OH)₃)in unconfined groundwater undergo a reduction and dissolution reaction to form Fe(?),the Mn in salinity groundwater in confined aquifer is continues to undergo cations exchange reaction.And even with the depth of the aquifer increases,the hausmannite,siderite and pyrolusite in groundwater has not reached the saturation point,therefore continued to dissolve.