Short-term geochemical investigation and assessment of dissolved elements from simulated ash reclaimed soil into groundwater

Highlights

• Ash reclaimed soil is a source for the transfer of toxic elements into groundwater.

• We estimate 83.055% of the total variance may be associated with five factors.

• As, Sb, Cd, Pb and Ni were the dominant contaminants.

• As, Mn, Cd, Sb, Co and V are the largest contributors to health risks.

• Sampling pointes belong to 3 groups, reflecting influences extent from leaching.

Abstract

A soil column migration trough was used to study the leaching behavior and geochemical partitioning of fifteen elements Al, As, Cr, Cu, Fe, Mg, Sn, Sb, Zn, V, Co, Mn, Pb, Ni and Cd in simulated ash reclaimed soil. According to the results of cluster analysis for the sampling stations, there were three clusters: Cluster 1 of 7 wells with relative good groundwater quality originated from the background control area, Cluster 2 of 9 wells with worst groundwater quality in the downstream parts of the simulated ash reclaimed soil, and Cluster 3 of 2 wells with representative of samples influenced by the combined effect of injection of leaching solution and the main current. Statistical analysis identified five factor types that accounted for 83.055% of the total variance, which declined in the order: ash-soil rate > leaching intensity > water depths > flow velocity > leaching time. As, Sb, Cd, Pb and Ni were the dominant contaminants. The water around ash reclaimed soil was unsuitable for drinking. As, Mn, Cd, Sb, Co and V were the largest contributors to health risks. Soils reclaimed with fly ash can consequently be a long-time source for the transfer of toxic elements into groundwater.

Introduction

Solid waste such as fly ash, slag or sinter are frequently used in soil improvement to reclaim land which has recently been damaged (Cetin et al., 2010; Tastan et al., 2012). Every year, a large number of fly ash, cinder, construction waste and slag were produced in China (Fu et al., 2016). Fly-ash is a large amount of solid wastes produced from a power plant (Schwartz et al., 2016), taking up a great amount of land and causing seriously environment pollution. Large quantities of fly ash are used to modify pavement subgrades (Ettler and Johan, 2014), build compound mixing additives (Piatak et al., 2015) and compound fertilizers (Ram and Masto, 2014), so the conceivable release of potentially toxic elements can be expected (Ettler and Johan, 2014). There are a few evidences that some elements have harmful effects on plants, organisms and aquatic environments via the resource utilization of fly ash as fertilizer of reclaimed soil (Karbassi et al., 2014; Minkina et al., 2014; Alves et al., 2014; Nshimiyimana et al., 2016), which causes the potentially toxic elements pollution of deeper soil layers, surface runoff and groundwater by leaching (Mbugua et al., 2014; Nyale et al., 2014; Augustsson et al., 2016). However, there are also some evidences that the concentration of toxicants leached from alkaline ash are lower (Ram et al., 2000; Ram et al., 2007; Ram et al., 2011; Ram et al., 2015), which is less of a concern when those ash are used in agriculture and silviculture, based on the finding that there are no adverse health effects on human when related agricultural products are consumed (Kandlakunta et al., 2009). It is still need to enhance further understanding of the impact of ash reclaimed soil leaching processes on groundwater by the release of elements, in order to find out more strategic analysis of short-term and actual long-term prognosis. In this study, the elements contamination degree and latent ecological risk in reclaimed soil filled with fly ash were investigated by the method of sampling analysis, leaching experiment, migration monitoring experiment and evaluation of ecological risk.

A soil column test is a useful tool to determine the release rates of different elements from fly ash in reclaimed soil. And a migration experiment is an effective means to determine corresponding migration rates of those elements in soil and groundwater (Panteleit et al., 2011). Both of them are helpful for the predictability of practical results under various conditions, except laboratory results (Joris et al., 2014; Pokrovsky et al., 2016). They are also potentially valuable instruments to enhance further understanding of the impact of short- and long-term leaching processes on groundwater pollution and relative mechanisms (Spielmeyer et al., 2017). As above, based on the results of indoor simulation experiment, the material exchange and distribution patterns of elements between the solid-liquid phases have been revealed. In addition, groundwater samples in different layer were collected, and the content distribution of elements in different layer was researched. A visual simulation of digital contour map is used to study the migration characteristics of elements in fly ash and the space-time distribution feature of elements is analyzed by Kriging interpolation method. Source identification of elements has been analyzed by multivariate statistical method, and site-relevant risk and exposure assessments of drinking water on recipients around ash reclaimed soil have been conducted.

The objectives of this study were (1) to explore the effects of leaching time, ash-soil rate, leaching intensity and flow velocity on the leaching of different elements in soil and groundwater by modeling; (2) to assess the potential risks associated with different elements in groundwater following consumption of drinking water by residents living near the ash reclaimed soil; (3) to research the elements concentration characteristics of soil and groundwater in different layer; (4) to reveal the mobility of different elements in the interactive system of ‘soil and groundwater’. The research systematically discussed the transformation character of some elements by the multivariate statistical analysis, as well as the contribution of different elements to the water quality of surrounding groundwater. The research and analysis method in this work are also useful to the effective removal of toxic elements in ‘soil and groundwater’ system. We can increase, modify, and drop treatment requirements for some elements based on importance, interest, new developments, and the time or space change of different elements during the process of leaching.