Numerical Simulation Of Groundwater Pollutant Transport In Phosphogypsum Dump

Phosphorus chemical industry is one of the traditional industries in China.It developed rapidly in the 1960s and has gradually reached a mature level.With the continuous development of phosphogypsum chemical industry,a large number of phosphogypsum waste residues are produced.Because of its low comprehensive utilization rate,the main treatment method is stacking.The acidity and other harmful substances contained in phosphogypsum waste residue stored in open air are easy to pollute the surrounding environment,especially in the absence of artificial seepage control or seepage control system damage,after rainwater leaching,groundwater is easily polluted.In order to find out the influence of phosphogypsum dump on groundwater under natural impervious conditions,this paper chooses a phosphogypsum dump in Chongqing and uses the groundwater numerical simulation software visual MODFLOW to carry out hydrogeological survey in the study area.The groundwater flow field in the study area is simulated and the solute transport of the two main leaching pollutants,total phosphorus and fluoride,is simulated and predicted.The following conclusions are drawn:（1）The groundwater quality in the study area is evaluated by single factor evaluation method,conventional Nemerow index method,modified Nemerow index method and fuzzy comprehensive evaluation method.The evaluation results show that the water quality in the upstream and boundary areas of the study area is better than that in the downstream areas,the upstream and boundary water quality in the study area are mainly of type III and below,the downstream water quality is mainly of type IV,HCO₃^--Ca is the main hydrochemical type in the study area,and SO₄^2--Ca is the main hydrochemical type affected by dump in the downstream.According to the evaluation results,it is speculated that the existence of phosphogypsum slag field may have some influence on groundwater quality.（2）Toxic leaching experiment of the new and old residues phosphogypsum waste residue were carried out separately.And the results showed that the pH of the new residue was lower than that of the old residue,and the contents of total phosphorus,phosphate and fluoride were higher than that of the old residue,the content range of characteristic pollutants of new and old slag in the dump was obtained.The leaching experiment was carried out on the phosphogypsum waste residue,and the pH of the leachate increased with the leaching time.The concentration of the three pollutants decreased with time.The thicker the residue,the lower the pH,the higher the concentration of the three pollutants in the leachate;through the analysis of the soil upstream of the study area,the characteristic pollutants in the soil upstream of the study area,and the concentration of the pollutants is not negligible.The groundwater monitoring results get the background concentration of the relevant pollutants.（3）Using visual MODFLOW to simulate the groundwater flow field of phosphogypsum dump the simulation results are in good agreement with the actual groundwater flow field,the fitting correlation between simulated water level and actual observed water level is 0.993.Underground water flows from the southwest side of the dump to the northeast side of the dump.（4）Based on MT3DMS module,the solute transport model of groundwater with two characteristic pollutants of total phosphorus and fluoride was established.The simulation results show that with the increase of time,the range of pollution transport is wider.The pollutants in phosphogypsum dump are strong as pollution sources.The pollutant concentration in the new slag area is the highest,and the pollutants gradually diffuse outward.The main migration direction is along the downstream direction of groundwater flow field.Fifteen years after the establishment of phosphogypsum dump,the maximum transport distance of total phosphorus reached 209.6 m,the influence area was 19042.0 m²,the maximum fluoride transport distance was 150.7 m,the influence area was 11160.5 m².After 20 years,the maximum migration distance of total phosphorus level was 234.5 m,the influence area was 21244.3 m²,the horizontal maximum over-standard distance of fluoride was 163.7 m,the influence area was13911.2 m².