A Study Of The Relationship Between River And Groundwater And Groundwater Quality Characteristics Near River In Zhongmu County

Groundwater over-pumping and water pollution have become serious environmental problems with the rapid development of China's society and economy.Groundwater level has been declining continueously due to excess exploitation of groundwater resources in some areas of the country.In these areas,pollutants migrate into groundwater with surface water as the latter is recharged by the former.It is important to study the relationship between river and groundwater to find out spatiotemporal variations of both water quantity and quality for the purposes of water resources assessment,water pollution control,and protection of the ecological and geological environment.It is for these reasons that we selected the Zhongmu county in Henan Province of China as the study area because of its dense population,fast industrial and agricultural development,and large water demand,and serious water pollution in this county.The aims of the study are to investigate the relationship between Jialu river and nearby shallow groundwater and to analyze the causes of groundwater level decline,based on which to evaluate shallow groundwater pollution.Numerical simulation of groundwater flow in the Zhongmu county was conducted to study the influence of shallow groundwater pumping and the relationship between the river and groundwater.First,a field geological investigation and hydrogeological tests were conducted to find out the distribution of the geological formations and aquifer,and to evaluate the permeability of the vadose zone and aquifer.Results showed that the vertical hydraulic conductivity of the vadose zone in this area is 0.79 m/d,and the upper phreatic aquifer mainly consists of silt and sand which are highly permeable with a hydraulic conductivity range of 1.1 to 1.7 m/d.Then,both the river stage and groundwater level were monitored to evaluate the relationship between river and groundwater.Results indicated that the river stage was generally higher than the groundwater level and the river recharges groundwater mainly due to groundwater pumping.An average water flux per unit width is 2.04 m2/d.Finally,based on the analytical results of the river water and groundwater samples,the quality of the river water and groundwater were evaluated,and their characteristics and causes of pollution were further analyzed.Results showed that the river is seriously polluted by NH3 and COD and their concentrations exceed the standard for surface water class III,and about 16.7%of the samples their NO3-N exceeds the standard.Groundwater pollution by NH3 near river was serious due to the river recharge:56.3%of the samples exceed the standard.The maximum concentration of NH3-N was 2.94 mg/L,which is 14.7 times of the national standard of groundwater class III.The NH3 concentration in groundwater gradually decreases away from the river due to the nitrification and adsorption.The NO3-N concentration in groundwater exceed the standard only at No.3 well and the NO3-N concentration in the groundwater gradually increases away from the river due to nitrification and infiltration of the nitrogen fertilizer applied at the land-surface.The conceptual and numerical model of the shallow groundwater flow in the study area were established based on the hydrogeological conditions.The simulation software MODFLOW was used to carry out the simulation.The model was calibrated again the measured water levels in the observation wells.Results showed that a cone of depression has formed due to groundwater pumping in the south-central part of the county.The groundwater flow field without pumping in the study area was simulated to compare with the current situation.Results showed that the relationship between river and groundwater in some river sections has reversed from that groundwater recharges river to river recharges groundwater.The groundwater levels with no pumping generally rise relative to the current situation.The groundwater level rises greatly in the southern and central parts of the county with more than 10 meters while in other parts rise 1?4 meters.The hydrological budget shows that river recharges groundwater with a rate of 1.12×108 m3/a under pumping conditions while groundwater recharges river with a rate of 0.9×108 m3/a under no pumping scenario.