| Extensive coal mining in the Huainan Coal Mines has resulted in extensive landsubsidence and submergence around the mines with landscapes of pondsk, lakes andwetlands. As a result, it has produced vast water resource potencial with significantimpact on sustainability of local ecnomy and environment. This research focused onthe eutrophication in the aquatic zones around the Huainan “Panxie” coal minesubsidence areas. Two research scientific problems were proposed based on localunique sedimentary environments：Firstly, could phosphorus (P) migrate from theagricultural soil column to the surface sediment or water column? Secondly, could theheavy external pollution load impact the P behavior of transforms and transports inthe water-sediment interface, enhance P-releasing potential and changed the nutrientlevel and structure in the water bodies furthermore?Three representative sites, including PJ site in the east part, GQ site in the centralpart, and XQ site in the west part of the studied mining areas, were selected to addressrelated research work. The research included the following five parts:(1) Waterquality investigation and analysis: Annual investigations on water quality parameterswere conducted in one year period of four seasons for eutrofication status evaluation,analysis of nutrient contents and ratio, and characterization of regional waterchemistry.(2) Sediment P fractionation: P contents and fractions (NH4Cl-P, BD-P,NaOH-P and OP) in the sediments of three sites from subsided land were analyzed bythe Pesnner method for P extraction to obtain the database to determine P migrationpotential.(3) Sediment P adsorption characteristics in the sediments: P adsorptionbehavior was conducted in the sediments considering the impact of regional waterchemistry in order to clarify the role of surrounding environmental solutions on Preleasing.(4) Simulation of P transforms and transport processes after agriculturalsoils inundation: Simulated experiments were designed to observe the processes ofredox and P migration in inundated agricultural soils. The goal was to obtain theprocesses and mechanisms of P transform and transport from inside soils tosoil-overlying water interface.(5) Modelling P behavior in the aquatic zones: Differentrelationship between subsidence waters and local rivers were classified, in cases ofseparated collapse lakes, lakes of permanently and seasonal connecting to rivers,respectively. Accordingly the features for the regulation of lake storage capacity andpollution loading were characterized. The water quality model of completely mixed tanks was employed to simulate phosphorus response in the water column to differenttypes for water volume filling and pollution loading within the three sites of PJ, GQand XQ.The main results and fruits include the following five parts:(1) Eutrophication characterization: The aquatic zone at XQ site, showed “lighteutrophic” states, indicating a start point for local eutrophciton processed. Whereasthe waters at the other two sits of PJ and XQ, have been processed to“meso-eutrophic” level due to heavier pollution from external inputs, where TP andTN concentrations reached at the level of0.1mg/L and2.0mg/L, respectively. All testsamples of the two mining areas presented features of rather high nitrogen tophosphorus ration (N:P) and with low soluble P concentrations, presenting Plimitation potential in the studied aquatic zones.(2) It was found that the chemical properties and P fractionation showed theobvious difference between layers along the depth, and vertical profiles of OM, TN,TP and Feowere decreasing with the deeper layers of the sediments. The range of OMconsents was0.7%to1.8%at PJ site,0.8%to1.7%at GQ site,0.6%to1.6%at XQsite; it was496.6mg/kg to1069.6mg/kg at PJ site,689.6mg/kg to1437.6mg/kg atGQ site,592.0mg/kg to1300.8mg/kg at XQ site for TN contents; and it was201.4mg/kg to348.2mg/kg at PJ site,217.8mg/kg to401.2mg/kg at GQ site,159.8mg/kgto351.0mg/kg at XQ site, respectively. Fe/Al combined P (BD-P plus NaOH-P) hastaken the greatest proportion of TP in surface sediments.(3) It was found that regional water chemistry can impact P adsorption greatly.Usually, Ca2+can enhance P adsorption on sediment surfaces while weakly alkalineconditions caused by bicarbonates are unfavorable for its adsorption. As acomprehensive effect, the positive effect of the former is greater than the negativeeffect of the latter. The site of PJ presented greater P releasing potential than those inthe GQ and XQ sites, probably due to its higher nutrient level. Overall, P releasingrisks at present in the researched sediments are weaker than those in eutrophic lakes,while they are very similar to lakes with lower trophic levels, because of their uniquesedimentary environments from inundated agricultural soils with abundant Fe oxides.(4) Characterization of P transport and transforms processes in the sediments andevaluation of its eclogical and envrionmental effects: The research about simulationexperiment of agricultural soils inundation showed that enrichment of Fe-(hydr)oxidesin surface sediments was verified to be the main reason for limitations in regional P availability in water bodies. An increasing upward gradient in the contents of OM, TN,TP, and different P fractions was caused by spatial heterogeneity in soil properties. Fe,but not its bound P, moved upward from the submerged soil column to the surface. Pwas unable to migrate upwards towards the surface sediments as envisioned, becauseof complex secondary reactions within soil minerals. On decadal or longer time scales,BD-P has the highest likelihood of being transformed into OP, which originated froma high C:P ratio in soil organic matter.(5) Modelling P behavior and giving the theoritial explantion of P level andtrends in the aquatic zones: the simulated result matched well with the observedphosphorus levels in the studied lakes, which can be confined to different levelsmeeting national water criteria such as Class III of TP concentration less than0.05mg/L, Class IV of TP concentration less than0.10mg/L if the right or properwater reconstruction or management practices have been done.The answers to the scientific questions have been given as following: more than50%BD-P along with NaOH-P to TP presented great P releasing potential based onsediment chemical properties. However, reactive Fe oxides and their capacities for Padsorption could lead to P limitation in the aquatic zones. However, it has seriousinternal risks if given the right environments such as high organic matters loadingfrom external sources. Therefore, a key point for future ecological rehabilitation andconservation may depend on the control of P release rather than N. |
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