Mechanism Study On Contaminant Transport In River-Groundwater System During Riverside Pumping

In recent years, with the rapid development of industrialized economies and increase ofpopulation, some non-standard industrial waster and domestic sewage have been dischargedinto rivers, it caused more and more serious river contamination. Besides that, some abruptwater pollution events happen frequently. All of these not only aggravate the local waterresource crisis, but also seriously affected the economic development and social stabilization,as well as was comprehensively paid attention by international society. There is closerelationship between river and groundwater, because river contaminants enter subsurfaceaquifers under concentration gradient and/or hydraulic gradient and result in groundwatercontamination. Riverside groundwater source field play a non-substituent role on city supplywater especially drinking water supply in the northern regions of China. At the meanwhile,the increase of riverside groundwater exploitation intensity will accelerate transportation ofriver contaminants in groundwater. So how to ensure drinking water safety along river banksand social stabilization is a sensitive and urgent problem once river pollution and abruptwater pollution events exist. Mechanism researches of the influence of polluted river ongroundwater and water quality contamination of riverside groundwater source field, is notonly an urgent problem to solve exactly, but also a national goal and significant science andtechnology remand to ensure city drinking water safety in the northern regions.This paper, relying on the national natural science funding “simulation of influence ofriver pollution on groundwater”, designing a sand trough to simulate the relationshipbetween river and groundwater, choosing one section of a linear river and typicalcontaminants of one kind of conservative matters named NaCl and another kind ofun-conservative those named nitrobenzene, and considering influence factors of contaminanttransportation: river shape, river level, river bed media, structure and discharge level, baseon saturated and un-saturated flow theory, systemically stimulate transportation rules ofcontaminants during the evolution process between river and groundwater, construct themathematic model of coupling groundwater flow with contaminants transportation duringthe process. And it also predicts the influence extent under different groundwater pumpingintensity by the model, analyzes influence factors of contaminants transportation. And then it discusses transportation mechanism of contaminants during the evolution process of therelationship of river and groundwater. Finally it puts forward to the measurements of watersupply safety of riverside groundwater source field.Some conclusion has achieved from this study:1. Once groundwater withdrawal is larger than the maximum recharge of a river, water levelbetween river and groundwater will disjoint during exploitation of a riverside groundwatersource field. The related factors, which affect the evolution process of relationship betweenriver and groundwater, are lithology and structure of aquifers, shape and width of riverbed,water deepness and discharge level of a river, etc. These factors affect not only hydraulicconnection between river and groundwater, but also pollution transportation.2Under drive of pumping groundwater, with decline of groundwater level, not only therive-groundwater aquifer system, shape of soakage curves, recharge, recharge pattern of theriver and leakage paths have changed, but also transportation style of river pollutions haschanged and has obvious zonation. The convection zone, transition zone and dispersion zonecenter on river, enlarging and moving along river flow direction.3No matter how the relationship between river and groundwater changes, rivercontaminants will diffuse in groundwater and pollute groundwater. The closer distance from ariver, the more easily groundwater is polluted. To pumping wells along rivers, it takespollutants shorter time to reach these wells before water level between river and groundwaterdisjointed, however, after the water level between them disjointed, it takes pollutants a longertime to reach these wells with decline of exploitation water level.4Under the same hydraulic gradient, pollutant diffusion distance increases with theincrease of time, and tends to a stable value, however, pollutant diffusion velocity decreases,and tends to zero. Velocity of the front pollution diffusion is faster than that of pollution peak.Under different hydraulic gradient, the bigger hydraulic gradient is, the farther pollutantdiffusion distance is before level disjointed between river and groundwater. Then thepollutant diffusion distance is the farthest when discharge water level has access to its criticaldisjointed water level. After disjointed between them, the bigger hydraulic gradient is, theless pollutant diffusion areas reduce, then keeps stable. The bigger hydraulic gradient is, thefaster velocity of the front pollutant diffusion, but the more slowly speed of the pollution migration peak.5. During the evolution process of relationship between river level and groundwater level, ifhydraulic gradient of river-aquifer is smaller than0.15, wells will be in dispersion zones; if itshydraulic gradient is smaller than0.65, wells will be in transition zones; while if its hydraulicgradient is bigger than0.65, wells will be in advection zones. So wells should be indispersion zones as possible to ensure better water quality during exploitation of watersources near a river.6. During explorations of riverside groundwater source field, comprehensively consideringwater quality, quantity and economic efficiency, exploitation level may reach the criticaldisjointed level between a river and groundwater to stimulate the maximum recharge if waterquality of its middle and upperstream sections is better. However, exploitation level should behigher to ensure wells are always in dispersion zones if water quality of its middle andlowerstream sections is worse.