Risk Assessment Of Groundwater Contamination From Reclaimed Water Infiltration In The River

Reclaimed water used in water environment of lakes and rivers, may lead togroundwater contamination, so it is necessary to assess the groundwater pollution riskproducing in the process of reclaimed water use. In this paper, the intake area ofChaobai River was selected as a case study area. The rivers-the unsaturated zone–the groundwater system was integrated. Chloride, nitrate and naphthalene werechosen as pollution risk factors. The special of the process that canals rechargegroundwater through leakage and pollutant migration and transformation processeswas synthetically analyzed. To simulate the water movement and solute transportationin vadose, the Hydrus-1D software wass applied in vadose zone, and then the waterflux, solute concentration in real time between the vadose zone and saturated zonewere determined. Then the simulation results were assigned to theVisual-MODFLOW software applied in the saturated zone. The contaminant densitiesand pollution ranges of different times can be fround to reflect spatial and temporaldistribution of contaminant. Finally, pollution levels were divided and that could betaken as quantitative indicator of groundwater pollution risk level. The paper’s mainresults were as follow:(1) On the basis of analyzing the characteristics of the river infiltration, theamount of leakage was estimated with the Plotnikov’s Formula in which the influenceof silting was considered. In the different part of river, the moist soil phase durationwere19-36d. The amount of infiltration at#15,#18,#22were respectively665-144,33.4-7.26,771-167(Unit:104m3). The infiltration rate after ten years was about1%of the initial moment.(2) The Hydrus-1D software was applied to simulate migration andtransformation of water flux and solute in the vadose zone. The simulation resultsshowed: at#15and#22, the chloride quickly risen to80mg/L which was supplyconcentration of reclaimed water. And the content of nitrate through the vadose zonerose first and then fell. Polycyclic aromatic hydrocarbons, naphthalene was hard to get through the vadose zone and its concentration was at nanogram levels whennaphthalene reached the phreatic surface. At#18, after2400days, chlorine reachedthe concentration of the upper boundary supplies; Nitrate concentration was less than4μg/L; Naphthalene cannot reach at the phreatic surface.(3) The Visual-MODFLOW software was applied in the saturated zone. Thesimulation results of migration and transformation of pollutant in the aquifer showedthat in reclaimed water influence area, the concentration of chloride was focused onthe60-80mg/L; The highest concentration of Nitrate was less than8mg/L and theconcentration range was focused on2-5mg/L; Maximum concentrations ofNaphthalene was110-6mg/L and Naphthalene was detecte only for a brief small area,its concentration was focused on10-9~10-7mg/L.(4) On the basis of simulation results, pollution levels were divided, and then atdifferent times, groundwater pollution risk evaluation results showed that between1~10year, the secondary pollution risk area of groundwater were extended from9.9to24.6(unit: km2) during one year. The third pollution risk area of groundwater onlyexisted during the3thmonth to the18thmonth. The third pollution risk area was lesserand the area was biggest at9thmonth and the biggest area was0.274km2. Thepollution risk was different in different locations. At specific location ‘A’, there wasthe secondary pollution risk from reclaimed water suppling. The pullution risk levelwas three-level during the period of the125thday–the412thday and at the rest of thetime the level was the secondary pollution risk. At specific location ‘B’, pollution risklevel had been the secondary since the126thday.