Preliminary Research On Effects And Controls Of Irrigation Water On Heavy Metal Pollution Of Farmlands

This article investigated the contents of Cd,As and other pollutants in the up-to-standard surface water and other irrigation sources and the surrounding agricultural land irrigated in the Northwest River Delta,explored the water quality of irrigation water,the input of heavy metals from irrigation sources,the impact of irrigation water on the distribution and redistribution of cadmium and arsenic in agricultural land,and the mechanism and method of prevention and remediation of cadmium and arsenic in irrigation water and agricultural soil.Specifically,we selected the agricultural land that can represent the irrigation mode of each region from the upstream to the downstream of the Northwest River Delta as the object,and preliminary explored the water environment status of the entire agricultural land irrigation system in the agricultural slack period-background content,and each irrigation mode And the distribution characteristics of typical pollutants in the whole area.From August to November 2019?agricultural busy period?,select Foshan research area as the representative,explored the characteristics of heavy metal content and comprehensive water quality in potential irrigation water sources;explored the accumulation of cadmium and arsenic pollution in agricultural land on river beaches,including the input-output of irrigation and its impact on the accumulation process;and explored the horizontal direction of cadmium and arsenic in irrigation water Occurrence status during migration and distribution characteristics of main irrigation structures,and corresponding pollution prevention measures were proposed.Combined with the research on the migration law of cadmium and arsenic and the adsorption and desorption/quantitative fitting of paddy soil,initially explored the control ideas and process flow of Cd and As input of agricultural land soil irrigation sources.The results are as follows:1.Agricultural slack period,the background research on irrigation water in the Northwest River Delta found that most of the pollutants in the water environment of the irrigation system are in a relatively stable state-the allowable content of the basic ecological environment that can self-purify.The influence of irrigation water along the path shows that there is a certain difference between the cross-sectional dynamic index and the water source index.Sediment has a greater impact on the irrigation channel backwater,especially nitrogen,phosphorus and suspended particles.Rainwater scouring will significantly affect the redistribution of pollutants in rivers and lakes.It will also have a certain impact on the irrigation canal and its surrounding water environment.According to the CV coefficient,it can be seen that Cd,As,Cu,and total phosphorus are at a medium variability level,and there are no obvious abnormal values.Among them,the background value of Cd and As is not high,which meets the requirements of irrigation.2.Farming busy season,research on potential irrigation water sources in Foshan research area found that irrigation water sources research found that there is slight pollution of heavy metal elements of category 1 to 2 in the local area.The high regional background is an important reference for risk aberrations.Anthropogenic sources/anthropogenic activities are the main factors leading to high regionalization of heavy metal pollution,and the main driving force for the release of fixed forms.Reducing human impact is an important measure to effectively control agricultural environmental risks.As is at a poor level,and Cd is at a relatively low level that is relatively safe.Farming busy season,research of the river in the Foshan research area found that irrigation may be an important way for Cd and As from other sources?Agricultural materials,atmospheric subsidence and rainwater run-off,etc.?to return to agricultural land after environmental self purification and drainage.The ecological apparent?activity?risk of local Cd is relatively greater than that of As.River water scouring is one of the main driving forces for the redistribution of exogenous large-scale Cd and As along the water flow.Farming busy season,A study on input sources of Cd and As in agricultural land in the Foshan research area found that irrigation water has the largest impact on the external input of Cd in agricultural land,followed by agricultural material and atmospheric deposition.Agricultural material has the largest impact on the external input of agricultural land As,followed by atmospheric deposition and irrigation water.The local Cd and As problem is affected by the environment and agriculture.It is recommended to increase the risk monitoring of irrigation water Cd and agricultural substance As.Farming busy season,research on the horizontal migration of Cd and As in the river water environment of Foshan research area found that the distribution of Cd and As content in river water sediment,low tide beach farmland and high tide beach farmland is as follows:average Cd content:Low tide beach>River water sediment>High tide beach;average As content:River water sediment>Low tide beach>High tide beach.The distribution of Cd and As content in river water,reservoir sediment and agricultural land in dike is as follows.The average Cd content:River water sediment>Agricultural surface soil in dike>Reservoir bottom mud;the average As content:River water sediment>Reservoir sediment>Agricultural surface soil in dike.3.Through laboratory tests on leaching soil columns with different concentrations of Cd and As,it was found that Neutral high-purity water leaching rice acid soil shows better leaching ability,Almost all of the soil columns that have been leached for 20 years under simulated irrigation have leaching,which may be related to the slow release of free-form iron ions and weak acid groups in paddy soil,and stabilized by destroying the elements that make up the soil spatial structure,accelerate leaching and migration.The longitudinal migration of Cd in TZ_kb1,TZ₁,TZ₂,TZ₃ and TZ₅found that with the increase of water injection?irrigation years?,the Cd in the upper and lower layers of the soil column all lost to varying degrees.With the longitudinal leaching of irrigation water,when the soil adsorbed After the capacity is greater than the water injection and rinsing capacity,the trend of accumulation?TZ₄-TZ₆?further appears.The vertical migration results of As in TZ_kb1,TZ₁,TZ₂,TZ₃ and TZ₅ found that As mainly accumulates in the upper layer of the column.Without considering other output channels,the cumulative amount of 20 years has reached the pollution risk screening value.Cd has stronger binding ability to acid paddy soil,and its cumulative efficiency is greater than Cd in 20 years.On the contrary,with time,TZ₃and TZ5,the cumulative efficiency of Cd in acid paddy soil may be greater than As.As a whole,As is less affected by irrigation water on vertical leaching of the soil column compared with Cd.It was found in the experiment of paddy soil particle size screening,the adsorption kinetics of cadmium in soil is characterized by a rapid increase in the amount of adsorption in 0 to 3 min,and then the adsorption rate slows down until equilibrium is reached.The isotherm adsorption of cadmium ion in paddy soil was fitted by Langmuir equation,and the judgment coefficient was greater than0.94.Through the comparative analysis of A_f,B_f and R²_adj,it can be seen that the S-type function model?Logistic model?has a better fit for the adsorption and desorption,and the adsorption and desorption of paddy soil is more in line with Logistic growth.Paddy soil has a good adsorption and retention effect,and the adsorption effect of large-scale particles is relatively better.A review of the rapid purification matrix found that filtration rate is an important index,and it is of great practical value to explore the combination,ratio,landfill form,and water passing method of the large-scale matrix adsorption performance.A comprehensive study of irrigation water purification at home and abroad found that the combined process has good purification effect,but the construction area is relatively large,which is suitable for intensive production of agricultural land.At present,small agricultural land and areas with tight land use are more suitable for supporting rapid purification devices.In the northwest river delta area,which belongs to the storage and extraction irrigation area,it can be combined with water storage structures for purification treatment.A multi-level vertical filtration coupling of irrigation water of U-shaped overflow weir has been designed reservoir integrating deep purification function and water storage function.