Anthropogenic Organic Pollutants in Groundwater Increase Releases of Fe and Mn from Aquifer Sediments: Impacts of Pollution Degree, Mineral Content, and pH

: In many aquifers around the world, there exists the issue of abnormal concentrations of Fe and Mn in groundwater. Although it has been recognized that the main source of this issue is the release of Fe and Mn from aquifer sediments into groundwater under natural environmental conditions, there lacks enough reliable scientiﬁc evidence to illustrate whether the pollutants imported from anthropogenic activities, such as organics, can increase this natural release. On the basis of time series analysis and comparative analysis, the existence of an increasing effect was veriﬁed through laboratorial leaching test, and the impacts of aquatic chemical environment conditions, such as pH, on the effect were also identiﬁed. The results showed that the increase of organics in groundwater made the release of Fe and Mn more thorough, which was favorable for the increase of groundwater concentrations of Fe and Mn. The higher the contents of Fe- and Mn-bearing minerals in aquifer sediments, the higher the concentrations of Fe and Mn in groundwater after the release reaches kinetic equilibrium. Lower pH can make the leaching more thorough, but the neutral environment also increases the amount of Mn. It can be deduced that the pollutants such as organics imported by anthropogenic activities can indeed increase the releases of Fe and Mn from aquifer sediments into groundwater, thus worsening the issue of groundwater Fe and Mn pollution. The ﬁndings provide a deeper insight into the geochemical effects of Fe and Mn in the natural environment, especially in the groundwater system. enhancing effects of organic pollution in groundwater on release of Fe and Mn from aquifer sediments; (2) to identify impacts of aquatic environmental conditions, such as pH, on the enhancing effects; (3) to reveal the mechanisms behind these effects and impacts.


Introduction
Iron and manganese elements are widely distributed in the strata, including aquifer sediments, and they can enter the groundwater through leaching and become the common hydrochemical components [1]. In aquifers around the world, the concentrations of Fe and Mn in groundwater vary very widely in space, from a few micrograms per liter [2,3] to tens of micrograms per liter or more [4]. Although this difference is related to the spatial distribution of Fe and Mn in the porous medium of the strata [5,6], it is also greatly related to the high complexities of the geochemistry processes of them [7,8], which are controlled by the hydrogeochemical environment [9,10] and have not yet been well understood [11]. When the concentrations of Fe and Mn in groundwater reach certain levels, the water intake facilities, such as pumping wells, can be clogged [11,12], thus greatly shortening the service life of the water intake project [13]. In addition, although Fe and Mn are essential elements of human bodies [14], high concentrations will not only affect the sense of groundwater as drinking water (such as color and iron smell), but also can affect human body health if drinking over a long period of time [15,16]. Therefore, the groundwater with too high This study will further the understanding of the roles of pollutants mainly introduced by anthropogenic activities, such as organics, on the geochemical processes of Fe and Mn naturally occurring in environment, especially in the groundwater system. In addition, we also sincerely hope that our efforts in this regard will be helpful for more effective control of groundwater pollution.

In Situ Sediments Sampling and Characterization
The in situ aquifer sediments were sampled in Northeast China (Figure 1a), specifically at a point (45 • 45 52.58 N, 126 • 30 12.05 E) located east of the Songnen Plain (Figure 1b), which belongs to the Songhua River Basin. The sampling site is located on the southern alluvial flat of the Songhua River (Figure 1c), which was formed by the river alluvial deposits, specifically, coarse sand and gravel of the Quaternary Holocene, which are relatively loose. There is strong hydraulic connection between the groundwater and river. In the rainy season, the river can recharge the groundwater [45]. On the contrary, groundwater can recharge the river in other periods [18]. Controlled by the river-groundwater interaction, the depth of the groundwater level around the sampling site ranges from 3 to 5 m [46,47]. In many parts of the river basin, especially in the regions close to the river and the tributaries, the concentrations of Fe can reach tens of milligrams per liter, while Mn and COD can reach several milligrams per liter in the groundwater, which has had a significant impact on the production and life of the local residents [48]. More details about the hydrogeological conditions of the sampling site can be referred to in the study of Zhu et al. (2020) [18].
Water 2021, 13, x FOR PEER REVIEW 3 of 15 Mn from aquifer sediments; (2) to identify impacts of aquatic environmental conditions, such as pH, on the enhancing effects; (3) to reveal the mechanisms behind these effects and impacts. This study will further the understanding of the roles of pollutants mainly introduced by anthropogenic activities, such as organics, on the geochemical processes of Fe and Mn naturally occurring in environment, especially in the groundwater system. In addition, we also sincerely hope that our efforts in this regard will be helpful for more effective control of groundwater pollution.

In Situ Sediments Sampling and Characterization
The in situ aquifer sediments were sampled in Northeast China (Figure 1a), specifically at a point (45°45′52.58″ N, 126°30′12.05″ E) located east of the Songnen Plain ( Figure  1b), which belongs to the Songhua River Basin. The sampling site is located on the southern alluvial flat of the Songhua River (Figure 1c), which was formed by the river alluvial deposits, specifically, coarse sand and gravel of the Quaternary Holocene, which are relatively loose. There is strong hydraulic connection between the groundwater and river. In the rainy season, the river can recharge the groundwater [45]. On the contrary, groundwater can recharge the river in other periods [18]. Controlled by the river-groundwater interaction, the depth of the groundwater level around the sampling site ranges from 3 to 5 m [46,47]. In many parts of the river basin, especially in the regions close to the river and the tributaries, the concentrations of Fe can reach tens of milligrams per liter, while Mn and COD can reach several milligrams per liter in the groundwater, which has had a significant impact on the production and life of the local residents [48]. More details about the hydrogeological conditions of the sampling site can be referred to in the study of Zhu et al. (2020) [18]. The samples of sediments were manually drilled in the phreatic aquifer using a stainless-steel hand drill with long drill pipe. To facilitate comparative analysis, samples were collected from three boreholes, distributed in a straight line perpendicular to the river. The sampling depth was two meters underground, which is in the unsaturated zone, with the thickness of four meters. In order to make the samples less disturbed, the samples The samples of sediments were manually drilled in the phreatic aquifer using a stainless-steel hand drill with long drill pipe. To facilitate comparative analysis, samples were collected from three boreholes, distributed in a straight line perpendicular to the river. The sampling depth was two meters underground, which is in the unsaturated zone, with the thickness of four meters. In order to make the samples less disturbed, the samples were immediately put into impervious and opaque bags after being drilled out, sealed, and subsequently, saved in portable refrigerator.
All sediment samples were transported to the lab immediately after sampling, followed by determination of the physiochemical properties of them in the lab (Table 1). Before the physicochemical analysis, each sample was thoroughly stirred and mixed to make it as uniform as possible. The particle sizes of the samples were determined by the MS2000 laser particle size analyzer. The lithological characteristics of the sampling points were grayish black, with good sorting and good grinding. pH was determined by the potentiometric method; NO 3 -N and NO 2 -N were determined by the potassium chloride solution extraction-spectrophotometry; organic matter was determined by potassium dichromate volumetric method by the electric furnace (1000 w) and characterized by TOC; and total Fe and total Mn were determined by the ICP-AES (PerkinElmer Optima 8000, Waltham, MA, USA) method.

Artificial Polluted Groundwater with Organics
To avoid the interference of other chemical components, the groundwater polluted with organics used for the leaching test was manually made in the lab with ultrapure water and grade pure potassium hydrogen phthalate. Potassium hydrogen phthalate was chosen as the organic substance because it is a standard solution with COD, which makes our experimental results more universal and repeatable. In detail, according to the test needs, potassium hydrogen phthalate solutions of 10, 20, 40, 50, 60, 80, and 100 mg/L were prepared for use. In addition, the potassium hydrogen phthalate solution of 50 mg/L was divided into eight parts for use, pH of which was adjusted to 3, 4, 5, 6, 7, 8, 9, and 10 through adding HCl or NaOH, according to the needs. Ultrapure water was used for the blank test instead of the polluted groundwater.
All glassware used were carefully handled before use to ensure the quality of samples. All used glassware was soaked in 10% nitric acid lotion for 4 h, then rinsed with ultrapure water thrice, and dried for 4 h at 180 • C. The bottle caps and gaskets were cleaned using an ultrasonic cleaner for 30 min with 20% anhydrous ethanol aqueous solution, then rinsed with ultrapure water thrice, and dried naturally.

Leaching Test Procedures
The leaching tests were achieved by the oscillation tests in the conical flasks carried out in the lab ( Figure 2). The ultrapure water (or organic solution) and the sediment samples for leaching were added into the conical flasks according to the ratio of liquid to soil of 10:1 in dark condition. Immediately after the addition, the conical flasks were sealed with sealing film and put into a constant temperature oscillator. The conical flasks were transferred to the centrifuge after oscillation, and centrifuged at 5000 r/min for 15 min. After centrifugation, the supernatant was taken out and filtered with a 0.45 µm water filter to remove interferences such as suspension, from which the leachate samples were obtained for measurements. To ensure the data quality, parallel samples were made at each sampling point.
Water 2021, 13, x FOR PEER REVIEW 5 of 15 of 10:1 in dark condition. Immediately after the addition, the conical flasks were sealed with sealing film and put into a constant temperature oscillator. The conical flasks were transferred to the centrifuge after oscillation, and centrifuged at 5000 r/min for 15 min. After centrifugation, the supernatant was taken out and filtered with a 0.45 μm water filter to remove interferences such as suspension, from which the leachate samples were obtained for measurements. To ensure the data quality, parallel samples were made at each sampling point. To achieve the study aims, four groups of tests (named A, B, C, and D) with different conditions were carried out ( Table 2). In each group, three sediment samples were tested independently. In group A, 16 conical flasks were used to test each sediment sample, corresponding to 16 sampling times. Each test of group A lasted 3600 min to ensure that the leaching processes were fully performed and the physicochemical processes in the sediment-water system reached dynamic equilibrium. The concentration of organics in the ultrapure water used in group A was 50 mg/L, and the test temperature was controlled at 20 °C. In the groups B, C, and D, the leaching processes were also fully performed, all of which lasted 3600 min, and this duration was based on the results of group A. The concentrations of organics in the ultrapure water used in group B were 10, 20, 40, 60, 80, and 100 mg/L, respectively, and the test temperature was also controlled at 20 °C. The To achieve the study aims, four groups of tests (named A, B, C, and D) with different conditions were carried out ( Table 2). In each group, three sediment samples were tested independently. In group A, 16 conical flasks were used to test each sediment sample, corresponding to 16 sampling times. Each test of group A lasted 3600 min to ensure that the leaching processes were fully performed and the physicochemical processes in the sediment-water system reached dynamic equilibrium. The concentration of organics in the ultrapure water used in group A was 50 mg/L, and the test temperature was controlled at 20 • C. In the groups B, C, and D, the leaching processes were also fully performed, all of which lasted 3600 min, and this duration was based on the results of group A. The concentrations of organics in the ultrapure water used in group B were 10, 20, 40, 60, 80, and 100 mg/L, respectively, and the test temperature was also controlled at 20 • C. The concentration of organics in the ultrapure water used in the group C was also 50 mg/L, the test temperature was controlled at 20 • C, while pH of the solution was 3, 4, 5, 6, 7, 8, 9, and 10, respectively.
For the purpose of comparative analysis, all the above tests were accompanied by blank tests, that is, the solution for leaching was ultrapure water instead of the polluted groundwater.

Leachate Measurements
The leachate samples were immediately tested for hydrochemistry after preparation. In consideration of the possible physiochemical changes in the leaching processes, the  (Table 3). Table 3. Items, detection methods, and statistical information of monitoring indicators.

Item (Unit) Detection Method Level of Detection (Unit)
Fe ( COD was determined using potassium dichromate volumetric method. Fe 2+ was determined using O-phenanthroline spectrophotometry. Total Fe, total Mn, K, Na, Ca, and Mg were determined using the ICP-AES (PerkinElmer Optima 8000). NO − 3 -N, Cl − , and SO 2− 4 were determined using ion chromatography (Thermo ICS-2100, Waltham, MA, USA). pH was determined using portable multi-parameter rapid water quality analyzer (HANNA-HI9828, Beijing, China). An indicative standard sample was tested every 10 samples with an error of less than 10%. All the analytical procedures are in conformance with quality requirements. The error was less than 10%, and the pass rate was 100%.

Changes of Fe/Mn with Sediments
The three samples of the aquifer sediments mainly consisted of fine sand and sand ( Figure 3) including quartz, sodium feldspar, and potassium feldspar. The Fe-bearing minerals in the three samples were mainly hematite and bixbyite; and the Mn-bearing minerals were mainly pyrolusite and bixbyite. The three samples were weak acidic with pH of 5.87-6.24 (Table 1). NO 3 -N and NO 2 -N of the samples were 22.6-27.5 mg/kg and 0.105-0.134 mg/kg, and TOC was 1550-7980 mg/kg. Total Fe was 21,360-27,749 mg/kg, and total Mn was 484-506 mg/kg, which is about one fiftieth of that of total Fe. For the convenience of discussion, the three samples are arranged in ascending order according to the Fe contents in the samples, and named I, II, and III (Table 1), respectively. Coincidentally, from sample I to III, the Mn contents are also in ascending order.  After reaching dynamic equilibrium in the organic solution systems of the three sediment samples, the concentrations of Fe of I, II, and III samples were 2.00, 2.44, and 3.89 mg/L, respectively, and the values of Mn were 0.039, 0.041, and 0.051 mg/L, respectively. That is to say, the higher the content in the sediment, the higher the corresponding concentration in the leachate after reaching equilibrium ( Figure 4). Specifically, the concentration of Fe in the leachate increased by 2%-37% for every 1000 mg/kg increase in the contents of Fe-bearing minerals in the aquifer sediment. Mn's corresponding value was 7%-200% for every 10 mg/kg increase in the contents of Mn-bearing minerals in the aquifer sediment.

Changes of Fe/Mn with Organics
The equilibrium concentrations of Fe and Mn in the leachate increased linearly with the increase of organics concentration of the solution for leaching ( Figure 5), that is, the higher the concentration of organics of the solution, the higher the equilibrium concentration of Fe and Mn in the leachate. For the three samples, the concentration of Fe in the leachate increased by 0.3-2.0 mg/L for every 10 mg/L increase in the concentration of organics of the solution. The corresponding value of Mn was 0.003-0.05 mg/L. What is more, compared with the blank tests, Fe and Mn concentration in the experiment tests was After reaching dynamic equilibrium in the organic solution systems of the three sediment samples, the concentrations of Fe of I, II, and III samples were 2.00, 2.44, and 3.89 mg/L, respectively, and the values of Mn were 0.039, 0.041, and 0.051 mg/L, respectively. That is to say, the higher the content in the sediment, the higher the corresponding concentration in the leachate after reaching equilibrium ( Figure 4). Specifically, the concentration of Fe in the leachate increased by 2-37% for every 1000 mg/kg increase in the contents of Fe-bearing minerals in the aquifer sediment. Mn's corresponding value was 7-200% for every 10 mg/kg increase in the contents of Mn-bearing minerals in the aquifer sediment.  That is to say, the higher the content in the sediment, the higher the corresponding concentration in the leachate after reaching equilibrium ( Figure 4). Specifically, the concentration of Fe in the leachate increased by 2%-37% for every 1000 mg/kg increase in the contents of Fe-bearing minerals in the aquifer sediment. Mn's corresponding value was 7%-200% for every 10 mg/kg increase in the contents of Mn-bearing minerals in the aquifer sediment.

Changes of Fe/Mn with Organics
The equilibrium concentrations of Fe and Mn in the leachate increased linearly with the increase of organics concentration of the solution for leaching ( Figure 5), that is, the higher the concentration of organics of the solution, the higher the equilibrium concentra-

Changes of Fe/Mn with Organics
The equilibrium concentrations of Fe and Mn in the leachate increased linearly with the increase of organics concentration of the solution for leaching ( Figure 5), that is, the higher the concentration of organics of the solution, the higher the equilibrium concentration of Fe and Mn in the leachate. For the three samples, the concentration of Fe in the leachate increased by 0.3-2.0 mg/L for every 10 mg/L increase in the concentration of organics of the solution. The corresponding value of Mn was 0.003-0.05 mg/L. What is more, compared with the blank tests, Fe and Mn concentration in the experiment tests was significantly higher, for example, even in the test that had only 10 mg/L organic solution added, the concentration was one and a half times that in blank test.
Water 2021, 13, x FOR PEER REVIEW 8 of 15 significantly higher, for example, even in the test that had only 10 mg/L organic solution added, the concentration was one and a half times that in blank test.

Changes of Fe/Mn with Time
In the early period (within 90 min from the beginning) of the leaching tests (group A in Table 1), concentrations of Fe and Mn in the leachate of the three sediment samples increased sharply; in the followed period, generally from 90th minute to 1800th minute, the concentrations still increased on the whole, but the speed slowed down obviously; while from 1800th minute to the end (3600th minute) of the tests, the concentrations were almost unchanged, which could be divided into the final period (the third period; Figure  6). In the whole period, the concentration curves of the three sediment samples showed a similar change trend. The concentration curves of the blank tests also showed a similar trend. The curves of Fe and Mn showed a similar trend. The concentration of Fe of the three sediment samples (I, II, and III), in which 50 mg/L COD had been added, after reaching dynamic equilibrium was 6.20, 8.00, and 9.50 mg/L, respectively, while the corresponding values of Mn were 0.072, 0.18, and 0.25 mg/L, respectively. As for the blank tests, the corresponding values of Fe were 2.00, 2.50, and 3.90 mg/L, respectively, while the values of Mn were 0.039, 0.041, and 0.051 mg/L, respectively. It is easy to see that the values varied with the sediments, elements, and the degree of organic pollution of water. All in all, the curves of experiment tests and blank tests show the similar change law. The difference between them is the amplitude of Fe and Mn concentration changes. The range of Fe and Mn concentrations in experiment tests is more dramatic, which will be three times that of the blank tests.

Changes of Fe/Mn with pH
The equilibrium concentrations of Fe and Mn in the leachate decreased linearly with the increase of pH (Figure 7), that is, the higher the pH of the solution for leaching, the lower the equilibrium concentration of Fe and Mn in the leachate. For the three samples, the concentration of Fe in the leachate decreased by 0.2-0.6 mg/L for every one pH

Changes of Fe/Mn with Time
In the early period (within 90 min from the beginning) of the leaching tests (group A in Table 1), concentrations of Fe and Mn in the leachate of the three sediment samples increased sharply; in the followed period, generally from 90th minute to 1800th minute, the concentrations still increased on the whole, but the speed slowed down obviously; while from 1800th minute to the end (3600th minute) of the tests, the concentrations were almost unchanged, which could be divided into the final period (the third period; Figure 6). In the whole period, the concentration curves of the three sediment samples showed a similar change trend. The concentration curves of the blank tests also showed a similar trend. The curves of Fe and Mn showed a similar trend. The concentration of Fe of the three sediment samples (I, II, and III), in which 50 mg/L COD had been added, after reaching dynamic equilibrium was 6.20, 8.00, and 9.50 mg/L, respectively, while the corresponding values of Mn were 0.072, 0.18, and 0.25 mg/L, respectively. As for the blank tests, the corresponding values of Fe were 2.00, 2.50, and 3.90 mg/L, respectively, while the values of Mn were 0.039, 0.041, and 0.051 mg/L, respectively. It is easy to see that the values varied with the sediments, elements, and the degree of organic pollution of water. All in all, the curves of experiment tests and blank tests show the similar change law. The difference between them is the amplitude of Fe and Mn concentration changes. The range of Fe and Mn concentrations in experiment tests is more dramatic, which will be three times that of the blank tests.
Water 2021, 13, x FOR PEER REVIEW 8 of 15 significantly higher, for example, even in the test that had only 10 mg/L organic solution added, the concentration was one and a half times that in blank test.

Changes of Fe/Mn with Time
In the early period (within 90 min from the beginning) of the leaching tests (group A in Table 1), concentrations of Fe and Mn in the leachate of the three sediment samples increased sharply; in the followed period, generally from 90th minute to 1800th minute, the concentrations still increased on the whole, but the speed slowed down obviously; while from 1800th minute to the end (3600th minute) of the tests, the concentrations were almost unchanged, which could be divided into the final period (the third period; Figure  6). In the whole period, the concentration curves of the three sediment samples showed a similar change trend. The concentration curves of the blank tests also showed a similar trend. The curves of Fe and Mn showed a similar trend. The concentration of Fe of the three sediment samples (I, II, and III), in which 50 mg/L COD had been added, after reaching dynamic equilibrium was 6.20, 8.00, and 9.50 mg/L, respectively, while the corresponding values of Mn were 0.072, 0.18, and 0.25 mg/L, respectively. As for the blank tests, the corresponding values of Fe were 2.00, 2.50, and 3.90 mg/L, respectively, while the values of Mn were 0.039, 0.041, and 0.051 mg/L, respectively. It is easy to see that the values varied with the sediments, elements, and the degree of organic pollution of water. All in all, the curves of experiment tests and blank tests show the similar change law. The difference between them is the amplitude of Fe and Mn concentration changes. The range of Fe and Mn concentrations in experiment tests is more dramatic, which will be three times that of the blank tests.

Changes of Fe/Mn with pH
The equilibrium concentrations of Fe and Mn in the leachate decreased linearly with the increase of pH (Figure 7), that is, the higher the pH of the solution for leaching, the lower the equilibrium concentration of Fe and Mn in the leachate. For the three samples, the concentration of Fe in the leachate decreased by 0.2-0.6 mg/L for every one pH

Changes of Fe/Mn with pH
The equilibrium concentrations of Fe and Mn in the leachate decreased linearly with the increase of pH (Figure 7), that is, the higher the pH of the solution for leaching, the lower the equilibrium concentration of Fe and Mn in the leachate. For the three samples, the concentration of Fe in the leachate decreased by 0.2-0.6 mg/L for every one pH increase of the solution. The corresponding value of Mn was 0.003-0.02 mg/L. Specifically, the concentration of Fe in the leachate decreased by 10-60% for every 1% pH increase of the solution. And the average decrease was 45%. The corresponding value of Mn was 11-80%, and the average decrease was 36%. increase of the solution. The corresponding value of Mn was 0.003-0.02 mg/L. Specifically, the concentration of Fe in the leachate decreased by 10%-60% for every 1% pH increase of the solution. And the average decrease was 45%. The corresponding value of Mn was 11%-80%, and the average decrease was 36%.

Input of Organics Increase Fe/Mn Release
This set of tests revealed the influences of different physicochemical conditions, such as Fe and Mn contents in the aquifer sediment, organics concentration of the solution for leaching, duration of the leaching, and pH, on the leaching process of Fe and Mn from the sediment to the leachate. All these conditions have an impact on the concentrations of Fe and Mn in the leachate. In a certain period, the longer the duration, the more fully the leaching, and the higher the concentrations of Fe and Mn in the leachate, until they reach a dynamic equilibrium between the leachate and the sediment. The previous study indicated that the average concentration of Fe and Mn in the studied area under natural conditions is 3.5 mg/L and 0.06 mg/L, respectively [18]. The average concentration comparing with our study is a little higher. The concentration of Fe and Mn in whole Songnen Plain is relatively high, and the alluvial flat is particularly high in the region. The alluvial flat is a hyporheic zone with a strong exchange, accumulation of organic matter, and human influence, which may be why Fe and Mn content in the alluvial flat is particularly high in the region [25]. The increases of Fe and Mn contents in the sediment and the organics concentration in the solution for leaching is favorable to the increases of Fe and Mn concentrations in the leachate. When the Fe and Mn contents in the sediment were 21,360-27,749 mg/kg and 484-506 mg/kg, respectively, and the reaction temperature was set at 20 °C, the equilibrium concentrations of Fe and Mn increased by 0.3-2.0 mg/L and 0.003-0.05 mg/L, respectively, for each 10 mg/L increase of organics concentration of the solution for leaching.
In the existing environmental hydrogeological investigations, organic pollution of groundwater has been found to enhance the release of Fe and Mn from aquifer sediments into groundwater [49], especially in those aquifers with abundant Fe and Mn contents. This enhancement, induced by the groundwater pollution with organics, increases the concentrations of Fe and Mn in groundwater to different degrees, and even makes groundwater in some regions unpotable considering Fe and Mn. For example, McMahon et al. (2019) [2] revealed that the concentration of Mn was related to DOC from the soils in USA. Broclawik et al. (2020) [50] found that the presence of methane contributes to the depletion of the sediment in ferric iron compounds, on the other words, increasing the iron ions in the solution.

Cause of Enhanced Releases of Fe and Mn
It can be seen from the results that the leaching of solution with pH of 7 can also release Fe and Mn from sediment to water, which can be mainly attributed to the leaching

Input of Organics Increase Fe/Mn Release
This set of tests revealed the influences of different physicochemical conditions, such as Fe and Mn contents in the aquifer sediment, organics concentration of the solution for leaching, duration of the leaching, and pH, on the leaching process of Fe and Mn from the sediment to the leachate. All these conditions have an impact on the concentrations of Fe and Mn in the leachate. In a certain period, the longer the duration, the more fully the leaching, and the higher the concentrations of Fe and Mn in the leachate, until they reach a dynamic equilibrium between the leachate and the sediment. The previous study indicated that the average concentration of Fe and Mn in the studied area under natural conditions is 3.5 mg/L and 0.06 mg/L, respectively [18]. The average concentration comparing with our study is a little higher. The concentration of Fe and Mn in whole Songnen Plain is relatively high, and the alluvial flat is particularly high in the region. The alluvial flat is a hyporheic zone with a strong exchange, accumulation of organic matter, and human influence, which may be why Fe and Mn content in the alluvial flat is particularly high in the region [25]. The increases of Fe and Mn contents in the sediment and the organics concentration in the solution for leaching is favorable to the increases of Fe and Mn concentrations in the leachate. When the Fe and Mn contents in the sediment were 21,360-27,749 mg/kg and 484-506 mg/kg, respectively, and the reaction temperature was set at 20 • C, the equilibrium concentrations of Fe and Mn increased by 0.3-2.0 mg/L and 0.003-0.05 mg/L, respectively, for each 10 mg/L increase of organics concentration of the solution for leaching.
In the existing environmental hydrogeological investigations, organic pollution of groundwater has been found to enhance the release of Fe and Mn from aquifer sediments into groundwater [49], especially in those aquifers with abundant Fe and Mn contents. This enhancement, induced by the groundwater pollution with organics, increases the concentrations of Fe and Mn in groundwater to different degrees, and even makes groundwater in some regions unpotable considering Fe and Mn. For example, McMahon et al. (2019) [2] revealed that the concentration of Mn was related to DOC from the soils in USA. Broclawik et al. (2020) [50] found that the presence of methane contributes to the depletion of the sediment in ferric iron compounds, on the other words, increasing the iron ions in the solution.

Cause of Enhanced Releases of Fe and Mn
It can be seen from the results that the leaching of solution with pH of 7 can also release Fe and Mn from sediment to water, which can be mainly attributed to the leaching and dissolution function of the water [50]. The common physicochemical conditions for groundwater leaching and chemical reactions are as follows. First, the existence of soluble components is the decisive factor. There are easily soluble Fe and Mn components in the sediment, and the higher the content of the component, the more conducive it is to leaching.
Second, data from this study demonstrate that acidic conditions are more conducive to the leaching. The lower pH gives rise to more complete leaching, for example, the acid pickling and rust removing. Third, time is an essential factor during the leaching and chemical reactions. The sufficiency of leaching and reaction is directly proportional to the time. Therefore, we could conclude that leaching occurs with minerals. This explains why Fe and Mn are detected in some natural groundwater without impacts of anthropogenic activities [51].
Previous studies have shown that organic matter can redox [52] and complex with metal minerals [34] or co-precipitate with metal elements [44]. Specifically, it reduces the redox potential in groundwater, which creates an environment more suitable for the presence of reductive substances [53,54]. In other words, the high-valent oxidation of Fe and Mn in the sediments transfers to the lower reduced state by the effects of organics matter. As for our study, small amounts of K, Ca, Na, and Mg were also detected in the leachate along with the release of Fe and Mn from the sediment (Figure 8), which indicated that there were water-sediment interactions occurring. In addition, the reason of the unusually high concentration of K was the adding of the potassium hydrogen phthalate. The redox was supposed the major reaction occurring [55]. Potassium hydrogen phthalate is the reducing agent, providing electrons to promote the reduction and dissolution of minerals [34]. Noteworthy, the complex associates with Fe to improve the stable upper limit of Fe(III) [56], which leads to higher concentration of Fe(III) in the solution [57,58]. In addition, organic-associated Fe and Mn are more stable than dissolved Fe and Mn ions [44], which can also be beneficial to the metal oxides dissolution (Figure 9). Therefore, potassium hydrogen phthalate will promote the Fe and Mn release. and dissolution function of the water [50]. The common physicochemical conditions for groundwater leaching and chemical reactions are as follows. First, the existence of soluble components is the decisive factor. There are easily soluble Fe and Mn components in the sediment, and the higher the content of the component, the more conducive it is to leaching. Second, data from this study demonstrate that acidic conditions are more conducive to the leaching. The lower pH gives rise to more complete leaching, for example, the acid pickling and rust removing. Third, time is an essential factor during the leaching and chemical reactions. The sufficiency of leaching and reaction is directly proportional to the time. Therefore, we could conclude that leaching occurs with minerals. This explains why Fe and Mn are detected in some natural groundwater without impacts of anthropogenic activities [51].
Previous studies have shown that organic matter can redox [52] and complex with metal minerals [34] or co-precipitate with metal elements [44]. Specifically, it reduces the redox potential in groundwater, which creates an environment more suitable for the presence of reductive substances [53,54]. In other words, the high-valent oxidation of Fe and Mn in the sediments transfers to the lower reduced state by the effects of organics matter. As for our study, small amounts of K, Ca, Na, and Mg were also detected in the leachate along with the release of Fe and Mn from the sediment (Figure 8), which indicated that there were water-sediment interactions occurring. In addition, the reason of the unusually high concentration of K was the adding of the potassium hydrogen phthalate. The redox was supposed the major reaction occurring [55]. Potassium hydrogen phthalate is the reducing agent, providing electrons to promote the reduction and dissolution of minerals [34]. Noteworthy, the complex associates with Fe to improve the stable upper limit of Fe(III) [56], which leads to higher concentration of Fe(III) in the solution [57,58]. In addition, organic-associated Fe and Mn are more stable than dissolved Fe and Mn ions [44], which can also be beneficial to the metal oxides dissolution ( Figure 9). Therefore, potassium hydrogen phthalate will promote the Fe and Mn release.  Results signify that pH influences the release of Fe and Mn. Lower pH results in further adsorption, causing more complexation and more release, which is consistent with our experimental results. Specifically, when the pH is below the PZC (the point of zero charge), there are electrostatic interactions between the positively charged metal oxides and negatively charged phthalic, which leads to adsorption. On the contrary, when the pH is above the PZC [59], they will repel each other, causing desorption. Noteworthy, adsorption can accelerate the complexation reaction, and the product desorbs little as the pH increases.
After analyzing the data, we find that there is liner relationship between the pH and the concentrations of Fe and Mn. The formulas show that the pH has more significant impact.

Implications for Pollution Control
With the rapid social and economic development, anthropogenic activities make the total amount of activated organics on the earth increase dramatically [60,61]. The artificial input of organic pollutants will not only lead to organic pollution of groundwater, but will also potentially worsen other water quality indicators and the surrounding environment of groundwater by changing the natural cycle of geochemical elements [62] and temperature, such as demonstrated with Fe and Mn in our study. Therefore, the implications of controlling organic pollution are not limited to organic pollution itself. Some other elements including Fe and Mn would be affected by the organic pollution and then influence the environment [63].
To monitoring organic pollution, improving the methods for assessing groundwater organic contamination is necessary. There are some methods that have been proposed recently [64,65]. Based on our study, we suggest more relevant indicators should be brought into the test list, such as Fe and Mn, and some recent field studies have taken them into the indicators [66]. In recent years, the number of studies on regional Fe and Mn investigation have increased worldwide [2]. It will be the guidance for the groundwater remediation and also should be put in the pollutant inventory. In addition, there has been a considerable amount of research into the effects of biogeochemical processes on organics [67][68][69]. The relationship between organic pollution and microbial communities in groundwater also will be a focus for future research, which will, in addition, have positive effects on finding the source of pollution. Results signify that pH influences the release of Fe and Mn. Lower pH results in further adsorption, causing more complexation and more release, which is consistent with our experimental results. Specifically, when the pH is below the PZC (the point of zero charge), there are electrostatic interactions between the positively charged metal oxides and negatively charged phthalic, which leads to adsorption. On the contrary, when the pH is above the PZC [59], they will repel each other, causing desorption. Noteworthy, adsorption can accelerate the complexation reaction, and the product desorbs little as the pH increases.
After analyzing the data, we find that there is liner relationship between the pH and the concentrations of Fe and Mn. The formulas show that the pH has more significant impact.

Implications for Pollution Control
With the rapid social and economic development, anthropogenic activities make the total amount of activated organics on the earth increase dramatically [60,61]. The artificial input of organic pollutants will not only lead to organic pollution of groundwater, but will also potentially worsen other water quality indicators and the surrounding environment of groundwater by changing the natural cycle of geochemical elements [62] and temperature, such as demonstrated with Fe and Mn in our study. Therefore, the implications of controlling organic pollution are not limited to organic pollution itself. Some other elements including Fe and Mn would be affected by the organic pollution and then influence the environment [63].
To monitoring organic pollution, improving the methods for assessing groundwater organic contamination is necessary. There are some methods that have been proposed recently [64,65]. Based on our study, we suggest more relevant indicators should be brought into the test list, such as Fe and Mn, and some recent field studies have taken them into the indicators [66]. In recent years, the number of studies on regional Fe and Mn investigation have increased worldwide [2]. It will be the guidance for the groundwater remediation and also should be put in the pollutant inventory. In addition, there has been a considerable amount of research into the effects of biogeochemical processes on organics [67][68][69]. The relationship between organic pollution and microbial communities in groundwater also will be a focus for future research, which will, in addition, have positive effects on finding the source of pollution.
Although we innovate techniques constantly, the remediation process itself still has a negative impact on the groundwater environment. Minimizing this negative impact will be a focus in the future, so remediating groundwater by controlling the biogeochemical reactions between minerals, microbes, and different elements in groundwater becomes more and more valued [70]. All in all, because of the active chemical reactions in the environment, using the interaction between organic pollution microbial and primary chemical components of the environment, such as Fe and Mn, to conduct environmental remediation may be a worthy direction for future environmental remediation, which has actually caught the attention of scientists in recent years [31,59]. Our study can be a reference for related research. The factors affecting Fe and Mn and organics have been studied in our research, which may become the basis of future feasible solutions.

Conclusions
This study reveals that Fe and Mn can release from Fe-and Mn-bearing minerals of aquifer sediments through groundwater leaching. It also further proves the hypothesis that the input of pollutant such as organics into groundwater through anthropogenic activities indeed can considerably enhance the leaching, which can cause the concentrations of Fe and Mn to increase exponentially, far more than those only obtained from natural groundwatersediments interactions. The higher the contents of Fe-and Mn-bearing minerals in aquifer sediments, the higher the concentrations of Fe and Mn in groundwater after reaching dynamic equilibrium between the sediment and the groundwater. Acidification of the groundwater environment can make the leaching more thorough.
The study explains well why the groundwater concentrations of Fe and Mn present great spatial differences in the natural environment worldwide. It is also deduced that the abnormal increase of groundwater concentrations of Fe and Mn in some aquifers can be mainly attributed to the anthropogenic activities, especially those related to organics emissions. In addition, the acidification of the water environment, which is mainly caused by anthropogenic activities, should be also responsible for the abnormal increase. Thus, groundwater pollution control may be not only effective to those pollutants discharged directly by anthropogenic activities, but may also be related to those species existing in the natural environment. Increased attention should be paid to the significance of organics on the geochemical processes of Fe and Mn in the natural environment, especially in the groundwater-sediment system, which will also be favorable for the further development of groundwater pollution control measures.