Effects Of Typical Heavy Metals On Soil And Plants In Bioretention Facilities

Bioretention facilities are commonly used in the construction of Sponge Cities because they serve multiple functions,such as reducing rainwater runoff and purifying rainwater.Since they are often exposed to the human living environment,there are still doubts whether the soil removes the pollutants of rainwater runoff while making the heavy metal content in its body exceed the soil environmental standard value,thus harming the human body and whether it affects the growth of plants and harms aesthetics.The study aims to assess whether heavy metals in rainwater runoff adversely affect plants in the biological retention system based on the rainfall characteristics of Beijing.This ranges from the migration and accumulation law of heavy metals in "rainwater runoff-planting matrix-plants",the accumulation of typical heavy metals in rainwater runoff and the enrichment of various plants in the biological retention system and the migration capacity,and correlation analysis between soil urease and typical heavy metals in the biochemical index of the planting matrix.The effects of typical heavy metal pollutants on the photosynthetic physiological indices of different plants,biochemical indices such as peroxidase and soluble sugars,and the migration and transformation of typical heavy metals between "plant substrate" finally,through the comprehensive analysis of the above contents,the suitability of typical plants as vegetation layers of biological retention systems and the evaluation of soil heavy metal pollution levels and risk assessments were concluded:(1)Based on the soil control value of construction land in soil environmental quality,the heavy metals of rainwater runoff in bioretention facilities have not reached the point of affecting human health and need to be controlled and remediated.However,according to the soil background value of Beijing city,the accumulation index method and the evaluation of potential environmental hazards are carried out.The heavy metals of rainwater runoff can pollute the soil Cd to varying degrees,and there is a certain potential threat to the soil environment.(2)Rainwater runoff promotes soil urease in the unit where Iris lactea Pall.and Iris tectorum Maxim.grows.Adding humus indirectly promotes the growth of soil urease.The four heavy metals showed a coordinating effect on the changes of each heavy metal in plant leaves;an appropriate increase in the effluent concentration would strengthen the correlation between plants and soil heavy metals.In contrast,too high concentrations would weaken the correlation between plants and soil heavy metals.(3)Among the three plant species,soil and Ophiopogon japonicus group plants have the lowest accumulation ability for the four heavy metals,while Iris tectorum Maxim.has the highest accumulation ability.Among the four typical heavy metals in rainwater runoff,Cd is more easily accumulated by the soil and plants,while Pb is less easily accumulated.Adding humus in various fillers had a favourable effect on the accumulation of heavy metals in soil and inhibited the accumulation of heavy metals in plants.(4)Heavy metals promote the formation of chlorophyll in Iris lactea Pall.and Ophiopogon japonicus but have a certain inhibitory effect on the chlorophyll content in Iris tectorum Maxim.The effects on the net photosynthetic rate of different plants;Iris lactea Pall.and Iris tectorum Maxim.are the inhibitory effect while promoting Ophiopogon japonicus.The peroxidase content in the plant is increased,and its peroxidase activity sequence is Iris tectorum Maxim.>Iris lactea Pall.> Ophiopogon japonicus.The effect on soluble sugars in Iris lactea Pall.is: the role of soluble sugars in Iris tectorum Maxim can be transformed at different stages.The promoting or inhibiting effect of iris soluble sugar is different under different filling conditions,and the addition of humus is unconducive to the production of plant soluble sugar.