Study On Hydrogeochemical Evolution And Genesis Analysis Of Semi-arid Area

In arid and semi-arid regions,groundwater is an important source of water for people's life and production.However,due to human disturbance and natural factors,the quality of groundwater is deteriorating,and the scarcity of water is increasingly serious.The Inner Mongolia Autonomous Region,where the population is concentrated and the industrial and agricultural production is the most developed,the water shortage has become a bottleneck restricting economic development.In recent years,due to the large-scale exploitation of groundwater,the balance of regional water resources has been seriously damaged,causing a series of environmental and geological problems,such as the draining of aquifers and the deterioration of confined water.Meantime,the area with high-fluorine and high-arsenic groundwater is widely distributed,and the problem of drinking water-type fluorine and arsenic poisoning seriously threatens the health of residents.This study takes the Tumochuan Plain as an example.Through field investigation,experiment,simulation and groundwater flow system analysis,it reveals the chemical evolution law of groundwater in semi-arid areas,the cause of high arsenic and high fluoride water,and the influence mechanism of seasonal changes on groundwater quality,providing a scientific basis for the sustainable use of water resources in semi-arid areas.This paper proposes a new view that groundwater and soil salinity has an important influence on the release of arsenic in rock and soil,the increase of salt content in groundwater and geotechnical soil could obviously promote the release of arsenic in rock and soil.Through the analysis of the groundwater sub-dry and abundance seasons,it reveals the reasons for the variation of groundwater quality in the semi-arid area with the seasons and elucidates the mechanism of seasonal changes of TDS,sulfate,bicarbonate and so on.Based on the investigation and sampling analysis of Tumochuan plain,the following characteristics of ground water can be found that,?1?Tumochuan plain is an open plain that is surrounded by mountains on three sides and water one side.The groundwater is mainly storaged in the Quaternary pore-aquifer.In the front of the plain area,the sediments of the alluvial deposits with coarse particle size are the main deposits which form a single structure submersible aquifer connected up and down.In the middle of the plain area,clay deposits of up layer of Middle Pleistocene divided the aquifer into unconfined and confined.?2?The groundwater in Tumochuan Plain is reductive and weakly alkaline generally.The average mass concentration of macro-mass ions are same in both unconfined and confined aquifers,the anion is HCO₃^->Cl^->SO₄^2-,and the cation is Na⁺>Ca²⁺>Mg²⁺>K⁺.?3?Mount Daqing and Manhan mountain piedmont plain area belong to the regional groundwater recharge area,and the quality of unconfined water is mainly controlled by water-rock interaction.Alluvial plain area of the Yellow River and Dahei River belong to the retention and drainage area of regional groundwater,and the quality of groundwater is mainly influenced by evaporation-concentration.The quality of confined water in most area of Tumochuan Plain is mainly influenced by water-rock interaction,only in the Lacustrine platform of the southeast part,the groundwater are influenced by evaporation-concentration because of large amount of soluble salts deposited under the long-term evaporation and concentration of the lake basin in the period of geological history.?4?In the recharge area of groundwater in the piedmont plain of Daqing and Manhan Mountains,the groundwater presents an oxidizing environment.The Eh value is high?>50 mV?,and the COD is low?<1 mg/L?.The nitrides in the groundwater are mainly nitrate nitrogen.The coast of Dahei River and its west part belong to the retention and drainage area of groundwater,the groundwater is in a reduction environment,Eh was lower?<-50 mV?,while COD was higher?>3 mg/L?,and the nitrogen compounds in groundwater were mainly ammonium nitrogen.Correlation analysis showed that the concentrations of nitrate nitrogen in unconfined and confined water were negatively correlated with pH and COD and positively correlated with Eh value;the concentrations of ammonium nitrogen in unconfined and confined water were positively correlated with pH value and COD,and negatively correlated with Eh value.?5?Affected by climate,sedimentary environment,hydro-geochemistry and groundwater flow,high fluorine water distributed in the Lacustrine platform in the southeast of Tumochuan Plain.The highest concentrations of fluoride in unconfined and confined water were 7.2 mg/L and 11.2 mg/L,respectively.Since the middle of the third Miocene,several basaltic eruptions in Manhan Mountain provided the source material for the formation of high fluorine water.The weak alkaline environment of groundwater,high Na⁺and low Ca²⁺make fluoride in rock strata enter groundwater and migrate with groundwater.The groundwater level of discharge area is shallow and groundwater evaporates strongly,which make fluorides further enriched in these places.?6?The high arsenic region of the unconfined water is oval distribution at the center of Hasu Sea,and the maximum concentration is 200.3?g/L around Hasu Sea.The high arsenic region of the confined water is mainly distributed in the downstream of the lacustrine plain in Dahei River,with the highest concentration of 162.3?g/L.The total salt content of soil is high in the lacustrine plain of Yellow River and Dahei River,the weak alkalinity of groundwater and the reduction environment rich in organic matter provide the conditions for the enrichment of arsenic compounds.The concentration of arsenic is further enhanced because of the slow flow of groundwater,the reductive dissolution of iron,manganese oxides,hydroxides,and the competitive adsorption of HCO₃^-.?7?Leaching test found that Weak alkaline environment with pH value between8.04 and 8.33 is greatest for the release of arsenide and excessive or low pH values?between 4 and 12?inhibited the release of arsenide.The increasing of TDS in soil and groundwater is good for the release of arsenide from soil,and salt effect is obvious especially when TDS concentration is more than 3000 mg/L.?8?The soluble salts in groundwater are significantly affected by seasonal variation.The effect of precipitation on the leaching of soluble salts and dilution of groundwater salt is the main reason that leads to the change of macro ion concentration in the wet and dry period.According to the classification of Shug Kalev and Piper three line maps,the evolution law of chemical types of groundwater in the wet and dry seasons can be clearly defined.The partial acidity of precipitation has an obvious influence on the seasonal variation of the pH value in unconfined water.The surface pollutants in rainy season can easily enter the unconfined aquifer with precipitation,which is the main reason for increasing COD and ammonia nitrogen concentration in this plain area.