Mechanism Of Groundwater Acidification In Pearl River Delta

With the rapid development of economic construction in Pearl River Delta, the environmental problems caused by the improper discharge of industrial and domestic are of concern. The groundwater pollution problems have become increasingly serious, especially the groundwater acidification problem. Research showed that the acidified groundwater exceeded the rate of 68.6% and acidic groundwater with pH< 6.5 accounted for 3/4 of the whole area. Previous studies were mostly focused on the distribution of the main causes of acid groundwater and only made a brief analysis, the significant mechanism of acidification had not been specified. Therefore, the acidification mechanism is particularly important in the study of groundwater acidification in Pearl River Delta. This study started from acidic groundwater distribution, the analysis of the various factors affecting the acidification was from the perspective of the mechanism. The study carried out in three different scales: regional scale (the entire Pearl River Delta), section scale (typical profile of the study area), and micro-scale (soil constituent components). The conclusions of groundwater acidification mechanism could give suggestions for the prevention and treatment of groundwater contamination in Pearl River Delta scientifically. The main results obtained in this study were as follows:The study at the regional scale was based on the survey of groundwater pollution in Pearl River Delta from 2005 to 2008. The sources of groundwater SO42- could be used to deduce the main factors causing groundwater acidification. By analyzing the every possible source of SO42-, the acid rain was thought to be the most significant factor. The concept of acid rain damage index (AHI) was introduced to characterize the degree of harm to acid rain. The method of principal component analysis (PCA) was used to study the hydrochemical composition and the acid rain’s impact on groundwater.The study in section scale was mainly focused on vadose zone. The change of pH, soil base cation and Al concentration were analyzed in the typical acidification profile. The vadose zone characterized by sand and gravel, good permeability, good runoff conditions, high altitude, large slope showed no significant acidification. The vadose zone characterized by clay or silty clay, poor permeability, poor runoff conditions, low-lying terrain, showed significant acidification. The analysis was based on the soil buffer system:dissolution and precipitation of free carbonate; soil base cation exchange; dissolution and precipitation of aluminosilicate minerals.The buffering capacity and critical load for acid deposition of paddy soil and red soil measured by the acid-buffer curve method were used to predict the time they reached the desired acid damages. The results of the prediction of paddy soil and red soil were 86.5 years and 45.5 years, respectively. The differences of buffering capacity of unacidified paddy soil, acidified paddy soil and red soil were studied by using sequential extraction method. Their buffering capacity order was "unacidified paddy soil> red soil> acidified paddy soil", which was determined by the variation of soil solid components.