Characteristics of Internal Ammonium Loading from Long-Term Polluted Sediments by Rural Domestic Wastewater

International Journal of Environmental Research and Public Health

Article Characteristics of Internal Ammonium Loading from Long-Term Polluted Sediments by Rural Domestic Wastewater

Xiang Luo 1,2, Yungui Li 1,2,* , Qingsong Wu 1,2, Zifei Wei 1,2, Qingqing Li 3,* , Liang Wei 1,2, Yi Shen 4 and Rong Wang 5

1 Department of Environmental Engineering, Southwest University of Science and Technology, Mianyang 621010, China; [email protected] (X.L.); [email protected] (Q.W.); [email protected] (Z.W.); [email protected] (L.W.) 2 Sichuan Provincial Sci-Tech Cooperation Base of Low-cost Wastewater Treatment Technology, Southwest University of Science and Technology, Mianyang 621010, China 3 Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA 4 Department Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou 310032, China; [email protected] 5 School of National Defense Science and Technology, Southwest University of Science and Technology, Mianyang 621010, China; [email protected] * Correspondence: [email protected] (Y.L.); [email protected] (Q.L.)

Received: 10 October 2019; Accepted: 18 November 2019; Published: 22 November 2019

Abstract: Given long-term decentralized and centralized rural domestic wastewater (RDW) discharge, nitrogen is continuously depositing in sediments. RDW discharge is assumed to be an important source of ammonium in surface water; however, the eﬀect of long-term RDW discharge on nitrogen pollution in sediments remains unknown. Batch incubations were conducted to investigate the characteristics of internal ammonium loading from long-term polluted sediments by RDW discharge. Four sediments were demonstrated to be heavily polluted by long-term RDW discharge, with total nitrogen (TN) values of 5350, 8080, 2730, and 2000 mg kg 1, respectively. The internal ammonium · − release from sediment was a slow and long process, and the risk of ammonium release from sediment during the dry season was signiﬁcantly greater than that during the wet season. Though all selected sediments were heavily polluted by long-term RDW discharge, the relative contribution of internal ammonium loading from sediments was generally lower than that of external pollution. Hence, dredging is not suggested for RDW-polluted sediments except in response to an emergency. The excessive ammonium in the selected catchment was mainly from untreated and centralized black water in RDW. Centralized black waters in rural communities are highlighted to be separately treated or reused to maintain ammonium content at a safe level.

Keywords: ammonium; sediment; dredging; rural domestic wastewater; small basin

1. Introduction The nitrogen cycle is among the most signiﬁcant of earth’s biogeochemical cycles. It has been radically disrupted by the excessive increase in anthropogenic nitrogen discharge, which totals approximately 2.7 times the estimated safe nitrogen discharge threshold in China [1]. Along with tightly controlled point-source pollution and urban nonpoint source (NPS) pollution in China, rural NPS pollution is increasing, and rural domestic wastewater (RDW) discharge is responsible for 8% of the nitrogen discharge [1–3]. Pervasive RDW is the key sources of surface water pollution in rural areas [4,5], where a large amount of highly concentrated nitrogen is directly discharged into nearby

Int. J. Environ. Res. Public Health 2019, 16, 4657; doi:10.3390/ijerph16234657 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2019, 16, 4657 2 of 15 surface water bodies [6,7]. Most RDW is decentralized discharge because of the lack of wastewater collection systems. With the rapid development of the rural economy, rural households are more centralized within rural communities, where wastewater collection systems are generally designed and built as encouraged by the policy of local governments, resulting in the relatively centralized RDW discharge in China. Given long-term decentralized and centralized RDW discharge, nitrogen is continuously accumulating in the sediments of small basins in rural areas, such as low-lying lands, ponds, and small water catchments. However, the effect of long-term decentralized and centralized RDW discharge on nitrogen pollution in the sediments of small basins remains unknown. Sediments can serve as both a sink and source of nitrogen [8,9]. Notably, sediment release (also as NPS pollution) is another important source for aqueous nitrogen pollution [10–16]. Much attention has been paid to water bodies in which sediments have been polluted by integrated sources, including both point-source and NPS pollutions [1,11,17–20]. For example, Dianchi Lake [8], Chaohu Lake [21], and Erhai lake in China [22], and the Mississippi River in the United States [23]. However, the internal loading of nitrogen from sediments that have been chronically contaminated by decentralized and centralized RDW discharge (NPS pollution) in the small basins of rural areas (ponds or wastewater catchments) remains unknown. Furthermore, the role of internal loading from sediments and RDW discharge in the aqueous nitrogen balance in rural small basins has not been evaluated. In the present study, two ponds and two small water catchments that have been subject to long-term pollution by RDW discharge (decentralized and centralized) were selected to assess the effect of this discharge on nitrogen pollution in sediments. Ammonium was chosen as the representative reactive nitrogen pollutant. Batch incubation in ultrapure and sample water was conducted to first investigate characteristics of internal ammonium loading from long-term polluted sediments by RDW [24,25].

2. Materials and Methods

2.1. Sample Collection Four sediments (S1–S4) and related sample waters (SW1–SW4) were collected from four small basins (B1–B4), all of which were polluted by RDW. The site information of B1–B4 is listed in Table1. RDW mainly includes black water (sewage from toilets) and gray water (other domestic wastewater except that from toilets, including from cooking, dishwashing, and baths) [26]. S1 and SW1 were sampled at a small ﬁsh pond (B1) near farmland which has been polluted by decentralized untreated RDW (gray water) discharge for more than 30 years. S2 and SW2 were sampled from a landscape pond (B2) with treated centralized RDW (both black and gray water) input over the most recent 20 years. S3/SW3 and S4/SW4 were sampled from two small water catchments (B3 and B4) around small scale rural communities. B3 has been polluted by centralized RDW discharged from a 12-household community for more than 20 years but only by gray water. B4 has been polluted by centralized RDW containing both gray and black water discharged from an eight-household community for nearly 10 years. Int. J. Environ. Res. Public Health 2019, 16, 4657 3 of 15

Table 1. Site information for the four small basins polluted by RDW (rural domestic wastewater).

Sediment Water 2 RDW Discharge Small Type Geographic Water Area Basin Location 1 Samples Sample /(m2) Scale Type Treated History Collection System N 31 54’56.42”, 30 B1 Pond ◦ S1 SW1 446 4-households Gray water No No E 104◦69’88.56” years Both black N 31 53’64.94”, 20 B2 Pond ◦ S2 SW2 781 >10-households water and Yes Yes E 104 70’01.99” years ◦ gray water N 31 57’79.18”, 20 B3 Tributary ◦ S3 SW3 490 12-households Gray water No No E 104◦70’08.96” years Both black N 31 57’86.28”, 10 B4 Tributary ◦ S4 SW4 223 8-households water and No Yes E 104 66’76.84” years ◦ gray water Notes: 1 Geographic Location was determined using a Global Positioning System (GPS) in Mianyang, Sichuan Province, Southwest China. 2 The water area was calculated using ArcGIS software (Environmental Systems Research Institute, California, USA).

A multiple mixture sampling method was used to collect sample water (SW1–SW4) from the surface layer of B1–B4 for determination of ammonium and incubation experiments. Each sediment sample (S1–S4) was collected from ﬁve random spots along the shore of the selected basin with a sampling grab at a depth of 0–10 cm and then mixed homogenously. Larger objects such as gravel, leaves, and organic debris were removed from fresh sediments. Some wet and fresh sediments were used for the incubation experiments. Other sediments were freeze dried after being transported to the laboratory in sealed plastic bags placed in iceboxes. After freeze drying, sediment was milled and sieved through a 100 mm screen. The samples were sealed and stored for characterization.

2.2. Sample Characterization Ammonium concentrations in four sample waters (SW1–SW4) were determined after sampling in these small basins. The pH values of sediments (S1–S4) were measured using a pH meter (PHS-2C Precision Acidity Meter, Shanghai Jingke Leici, Shanghai, China) by mixing the sediments with ultrapure water at a sediment-to-water ratio of 1:2.5. The sediment water content was measured using the drying method. The total organic carbon (TOC) contents in S1–S4 were measured using a total organic carbon analyzer (Elementar Analysensyteme GmbH, Hanau, Germany). The sediment nitrogen content (TN) was determined using a CHNS elemental analyzer (Elementar Analysensyteme GmbH, Hanau, Germany). The exchangeable nitrogen (TNexc) contents in the sediments, which includes extractable ammonium (NH3-Nexc), exactable nitrate (NO3-Nexc), and nitrite (NO2-Nexc) were extracted by potassium sulfate (K2SO4) and measured. Ammonium (HJ535-2009), nitrite (GB7493-87), and nitrate (HJ/T346-2007) were determined according to the Chinese Standard using an ultraviolet spectrophotometer (UV-1600 Shanghai Mapada Instruments Co., Ltd., Shanghai, China). The detection limit was 0.025 mg L 1 for ammonium, 0.003 mg L 1 for nitrite, and 0.08 mg L 1 for nitrate, respectively. · − · − · − 2.3. Sediments Incubation The ammonium release characteristics from sediments were investigated by simulated laboratory batch incubation in ultrapure water. The role of internal loading from long-term RDW-polluted sediments and the RDW discharge on ammonium balance was illustrated by batch incubation in sample water. The experimental design is shown in Table2. The incubation experiment was performed in a 15.6 L polyethylene barrel. During all batch tests, the initial sample water volume was maintained at 10.0 L and the weight of the wet sediment was adjusted according to the designed solid-to-liquid ratio. The tested sediment was spread at the bottom of a barrel and the liquid phase was gradually introduced from the top of the sediment using a siphoning tube. The sediment was evened out allowing particulate settling prior to the experiments. During the experiment, the outside of the barrel was wrapped with black bags and only daylight was allowed to contact the tested system from the top to mimic the sediment environment. Incubation was controlled at a natural temperature and under open-air conditions. Int. J. Environ. Res. Public Health 2019, 16, 4657 4 of 15

Table 2. The experimental design for incubation of long-term RDW-polluted sediments.

Sediment-to- Ammonium Research Objective Sediment 1 Water 2 Water Ratio Concentration 4

S1 PW 1:20 Cspw, t Eﬀect of the S1 PW 1:10 Cspw, t sediment-to-water S1 PW 1:05 Cspw, t Ammonium release ratio S1 PW 1:01 Cspw, t 3 characteristics of - PW - Cpw, t RDW-polluted S1 PW 1:20 C sediments spw, t Diversity of the four S2 PW 1:20 Cspw, t heavily polluted S3 PW 1:20 Cspw, t sediments S4 PW 1:20 Cspw, t 3 - PW - Cpw, t

S1 SW1 1:20 Cssw, t Eﬀect of the S1 SW1 1:10 Cssw, t sediment-to-water S1 SW1 1:05 Cssw, t ratio S1 SW1 1:01 Cssw, t - 3 SW1 - C The role of internal sw, t loading and external S1 SW1 1:20 Cssw, t discharge on S2 SW2 1:20 Cssw, t ammonium balance S3 SW3 1:20 C Diversity of the four ssw, t S4 SW4 1:20 Cssw, t heavily polluted 3 - SW1 - Csw, t sediments 3 - SW2 - Csw, t 3 - SW3 - Csw, t 3 - SW4 - Csw, t Notes: 1 S1–S4 are the selected long-term RDW-polluted sediment samples from small basins (B1–B4); 2 PW is the ultrapure water (Milli-Q), and SW1–SW4 represent the selected sample waters from B1-B4; 3 this is the natural 4 incubation of water without sediment. Cspw, t is the ammonium release from sediment incubation in ultrapure water; Cpw, t is the ammonium incubation in ultrapure water without sediments; Cssw, t is the ammonium with sediment incubation in sample water; and Csw, t is the ammonium incubation in the sample water without sediments.

The study of different sediment-to-water ratios on the influence of ammonium release from sediment was conducted using the S1 sediment. Sediment-to-water ratios of 1:20, 1:10, 1:5, and 1:1 were used. In the studies of the ammonium evolution in the sample water of sediments from different waterbodies, the sediment-to-water ratio was maintained at a 1:20 ratio, designed to mimic the hydrologic condition. In the study of each sediment, four barrels were used in parallel as follows: (1) sediment and 10.0 L of sample water from a studied waterbody; (2) sediment with 10.0 L of pure water (Milli-Q) with the same sediment-to-water ratio as that in (1); (3) 10.0 L of ultrapure water alone to monitor the cleanliness of the atmosphere; and (4) 10.0 L of sample water alone to monitor the natural evolution of ammonium in the sample water without sediment. At a given incubation time, an aliquot of 50.0 mL of supernatant sample water was taken from each barrel and the water samples were filtered through a 0.45 µm filter. Their ammonium concentrations were determined and termed Cssw, t, Cspw, t, Cpw, t, and Csw, t, respectively. A duplicate sample was periodically analyzed for ammonium.

3. Results and Discussion

3.1. Ammonium Pollution of Sample Waters Caused by RDW The classiﬁcation of aqueous ammonium in small basins (SW1–SW4) according to the Chinese environmental quality standards for surface water is shown in Figure1. The ammonium concentrations were relatively low in SW1–SW3 at 0.49 mg L 1, 0.72 mg L 1, and 0.71 mg L 1, respectively. The water · − · − · − quality of SW1, SW2, and SW3 was determined to be Class II, Class III, and Class III, respectively; Int. J. Environ. Res. Public Health 2019, 16, 4657 5 of 15

1 1 1 where Class II 0.5 mg L− and Class III 1.0 mg L− . The ammonium levels were 42.0 mg L− in ≤ · ≤ · 1 · Int.SW4, J. Environ. which Res. signiﬁcantly Public Health 2019 breached, 16, x the Class V surface water-quality standard of 2.0 mg L .5 of 15 ≤ · −

43 42 41 40

Ⅴclass -1 2.0

Ⅳ class 1.5 -N/mg·L 3 Ⅲ class NH 1.0

Ⅱclass 0.5 Ⅰclass 0.0 SW1 SW2 SW3 SW4

FigureFigure 1. 1. ClassificationClassiﬁcation of of aqueous aqueous ammonium ammonium in in small small basin basin (sample (sample water, water, SW1–SW4) SW1–SW4) according according to to thethe Chinese Chinese environmental environmental quality quality stan standardsdards for surface water (GB3838-2002).

3.2.3.2. Characterizations Characterizations of of RDW-Polluted RDW-Polluted Sediments Sediments SelectedSelected physicochemical physicochemical properties properties of of the the four four tested tested sediments sediments (S1–S4) (S1–S4) are are listed listed in in Table Table 3.3. pHpH values values of of the the four surface sediments were were slightly alkaline alkaline (ranging (ranging from from 7.60 7.60 to 9.07). The The total total organicorganic carbon carbon (TOC) (TOC) contents contents of of S1–S4 S1–S4 were were much much different. diﬀerent. Compared Compared to the to theTOC TOC contents contents of the of 1 1 twothe twoponds ponds (S1 = (S124.9= g·kg24.9−1 gandkg −S2 and= 55.6 S2 g·kg= 55.6−1), the g kg TOC− ), thecontents TOC of contents the two of catchments the two catchments (S3 = 14.5 1 · 1 · g·kg(S3 =−1 14.5and gS4kg =− 14.1and g·kg S4 =−1)14.1 were g kgmuch− ) werelower. much Total lower. nitrogen Total content nitrogen (TN) content is a useful (TN) isindex a useful for · · assessingindex for sediment assessing nitrogen sediment pollution, nitrogen pollution,which is defined which isas deﬁnedheavily aspolluted heavily if pollutedTN is greater if TN than is greater 1500 1 1 mg·kgthan 1500−1 [27]. mg Thekg− TN[27 of]. S1–S4 The TN was of 5350, S1–S4 8080, was 5350,2730, 8080,and 2000 2730, mg·kg and 2000−1, respectively, mg kg− , respectively, all indicating all · · heavyindicating pollution heavy and pollution necessary and dredging. necessary Notably, dredging. TN Notably, was positively TN was linearly positively related linearly to TOC related (R2 to= 0.87),TOC (suggestingR2 = 0.87), that suggesting the nitrogen that the and nitrogen carbon andloadings carbon in loadingsthe sediment in the were sediment simultaneous were simultaneous processes ofprocesses natural biomass of natural input biomass and inputexternal and water external pollution water (Figure pollution S1(A)). (Figure S1A). The risk of ammonium release from sediment mainly results from exchangeable nitrogen. Table 3. Physicochemical properties of the four tested sediments. Considered as releasable nitrogen [28], the exchangeable nitrogen (TNexc) contents ranged from 90.1 −1 to 243 mg·kg , accountingWater for 3.0% toTOC 4.7%2 of theDOC TN3 in the TNsediments.4 TN The proportions5 NH -N of 6the Sediment pH exc 3 exc exchangeable ammoniacalContent nitrogen1 /% (NH/(g kg3-N1)exc) to/(mg TNkgexc 1ranged) /(mg fromkg 1 )48.4%/(mg tokg 88.7%,-1) /(mgwhichkg were1) · − · − · − · · − muchS1 higher 9.07values than 66.6those of the 24.9exchangeable 8.3 nitrite and 5350 nitrate nitrogen. 189 NH3-Nexc 169 greatly variedS2 in the 8.23order of S1 85.1(169 mg·kg−1) 55.6 > S2 (158 mg·kg 7.9−1) > S3 8080 (71.8 mg·kg− 2431) > S4 (43.6 158mg·kg−1), consistentS3 with 8.37 the order of 38.3 TN (Figure 14.5S1 (B)). 7.5 2730 130 71.8 S4 7.60 37.3 14.1 7.3 2000 90.1 43.6 Notes: 1 The water contentsTable 3. of Physicochemical sediments were measured properties using of the the drying four method.tested sediments.2 TOC is the total organic carbon 3 4 5 content, DOC is the dissolved organic carbon content, TN is the total nitrogen content, TNexc is the content of exchangeable nitrogen, and 6 NH -N is the exchangeTOC 2 ammoniumDOC 3 nitrogen content.TN 4 TNexc 5 NH3-Nexc 6 Sediment pH Water Content3 1 exc/% /(g·kg−1) /(mg·kg−1) /(mg·kg−1) /(mg·kg-1) /(mg·kg−1) S1 9.07 66.6 24.9 8.3 5350 189 169 S2The risk8.23 of ammonium 85.1 release from 55.6 sediment mainly 7.9 results 8080 from exchangeable 243 nitrogen. 158 ConsideredS3 as8.37 releasable nitrogen 38.3 [28], the exchangeable 14.5 7.5 nitrogen (TN 2730exc ) contents 130 ranged from 71.8 90.1 S4 7.601 37.3 14.1 7.3 2000 90.1 43.6 to 243 mg kg− , accounting for 3.0% to 4.7% of the TN in the sediments. The proportions of the ·1 2 exchangeableNotes: The ammoniacal water contents nitrogen of sediments (NH 3were-Nexc measured) to TNexc usingranged the drying from method. 48.4% to TOC 88.7%, is the which total were organic carbon content, 3 DOC is the dissolved organic carbon content, 4 TN is the total nitrogen much higher values than those of the exchangeable nitrite and nitrate nitrogen. NH3-Nexc greatly variedcontent, in the 5 orderTNexc is of the S1 content (169 mg of kgexch1angeable) > S2 (158 nitrogen, mg kg and1) >6 NHS33 (71.8-Nexc is mg thekg exchange1) > S4 ammonium (43.6 mg kg 1), · − · − · − · − consistentnitrogen with content. the order of TN (Figure S1B).

3.3.3.3. Ammonium Ammonium Release Release Characteristics Characteristics of of RDW-Polluted RDW-Polluted Sediments Sediments TheThe fishpond ﬁshpond sediment sediment (S1) (S1) was was selected selected to to investigate investigate the the effect eﬀect of of the the sediment-to-water sediment-to-water ratio ratio onon internal internal ammonium release from sediment incubation in ultrapureultrapure waterwater (Figure(Figure2 ).2). Cspw, t t represents the ammonium concentration with time in the ultrapure water–sediment incubation −1 −1 system (mg·L ), while Cpw, t (mg·L ) is the ammonium level with time in ultrapure water without sediments, which was near zero during the trial period. Therefore, the intrinsic internal ammonium ’ release (Cspw, t = Cspw, t– Cpw, t ) was nearly equal to Cspw, t . The kinetics of Cspw, t were used to illustrate the ammonium release potential from four RDW-polluted sediments and the effect of the Int. J. Environ. Res. Public Health 2019, 16, x 6 of 15 sediment-to-water ratio during the incubations. Ammonium can be released from the sediment by a number of processes such as diffusion, desorption, and transformation.

3.3.1. Effect of the Sediment-to-Water Ratio

According to Figure 2, the kinetics of Cspw, t were divided into three stages: A rapid increase stage, a slow decrease stage, and a stable stage. During the first stage, Cspw, t rapidly increased over 3–10 days which was mainly because of the internal release of the RDW-polluted sediments. During the second stage, Cspw, t gradually decreased via natural reduction processes over approximately 10 days, such as nitrification during the incubation process. During the final stable stage, Cspw, t remained at a relatively low concentration (0.23–0.36 mg·L−1) for the four different sediment-to-water ratios. Despite the similar concentrations during the final stable stage, the maximum Cspw, t (Cspw, m) values were highly influenced by the sediment-to-water ratios during the incubations (Figure S2(A)).

The time to reach the maximum Cspw, t (Cspw, m ) gradually increased with an increase in the Int.sediment-to-water J. Environ. Res. Public ratios, Health ranging2019, 16, 4657 from 3–10 days (Figure 2). 6 of 15 Cspw, m is a useful index for an internal ammonium release risk comparison. During S1 incubation, it increased with an increase in the sediment-to-water ratio (i.e., 1.42, 1.99, 2.07, and 3.36 representsmg·L−1, at sediment-to-water the ammonium concentration ratios of 1:20, with 1:10, time 1:5, in and the ultrapure1:1, respectively), water–sediment all of which incubation breached system the 1 1 (mg L ), while Cpw, t (mg L ) is the ammonium level with time in ultrapure water without sediments, Class· −Ⅲ surface water-quality· − standard in China (Figure 2). This result suggests that the risk of whichammonium was nearrelease zero from during sediment the trial during period. the Therefore, dry season the (November intrinsic internal to April ammonium in Mianyang) release is = (significantlyCspw,0 t Cspw, greater t –C thanpw, t) that was during nearly equalthe wet to seasonCspw, t. (May The kinetics to October of C inspw, Mianyang). t were used The to illustrateammonium the ammoniumconcentration release during potential the dry from season four was RDW-polluted also found sedimentsto be greater and than the ethatﬀect during of thesediment-to-water the wet season in ratiothe same during hydrological the incubations. years Ammonium[29]. Therefore, can we be releasedshould pay from much the sediment more attention by a number to the ofammonium processes suchconcentration as diﬀusion, during desorption, the dry season and transformation. which should be constantly monitored and strictly controlled.

3d 13d 4d 16d 2.0 3.0 (A) s:l=1:20 (B) s:l=1:10 C spw, t Cspw, t 1.6 C 2.5 pw, t Cpw, t Ⅳ class 2.0 1.2 Ⅴ class -1 -1 1.5 Ⅲ class 0.8 Ⅳ class -N/mg.L -N/mg.L 3 3 1.0 Ⅲ class NH 0.4 Ⅱ class NH 0.5 Ⅱ class 0.0 Ⅰ class 0.0 Ⅰ class 0 5 10 15 20 25 0 5 10 15 20 25 t/d t/d

7d 16d 10d 20d 3.0 (D) s:l=1:1 (C) s:l=1:5 C 3.0 spw, t Cspw, t Cpw, t 2.4 Cpw, t

1.8 Ⅴ class 2.0 1 - -1

L Ⅴ class

Ⅳ class mg. 1.2 Ⅳ class N/ - -N/mg.L 3 3 1.0 Ⅲ class

NH Ⅲ class NH 0.6 Ⅱ class Ⅱ class 0.0 0.0 Ⅰ class Ⅰ class

0 5 10 15 20 25 0 5 10 15 20 25 t/d t/d Figure 2. Kinetics of ammonium ammonium release release fr fromom sediment sediment (S1) incubation incubation at at different diﬀerent sediment-to-water ratios in ultrapureultrapure waterwater ((Cspw, t t).). The The sediment-to-water sediment-to-water ratios ratios for for ( (AA),), ( (BB),), ( (CC),), and and ( (DD)) were were 1:20, 1:10, 1:5, and 1:1,1:1, respectively.respectively. TheThe controlscontrols were thethe kineticskinetics ofof ammoniumammonium incubation in ultrapure

water (C(Cpw, t t)) without without sediments. sediments.

3.3.1. Eﬀect of the Sediment-to-Water Ratio

According to Figure2, the kinetics of Cspw, t were divided into three stages: A rapid increase stage, a slow decrease stage, and a stable stage. During the first stage, Cspw, t rapidly increased over 3–10 days which was mainly because of the internal release of the RDW-polluted sediments. During the second stage, Cspw, t gradually decreased via natural reduction processes over approximately 10 days, such as nitrification during the incubation process. During the final stable stage, Cspw, t remained at a relatively low concentration (0.23–0.36 mg L 1) for the four different sediment-to-water ratios. · − Despite the similar concentrations during the final stable stage, the maximum Cspw, t (Cspw, m) values were highly influenced by the sediment-to-water ratios during the incubations (Figure S2A). The time to reach the maximum Cspw, t (Cspw, m) gradually increased with an increase in the sediment-to-water ratios, ranging from 3–10 days (Figure2). Cspw, m is a useful index for an internal ammonium release risk comparison. During S1 incubation, it increased with an increase in the sediment-to-water ratio (i.e., 1.42, 1.99, 2.07, and 3.36 mg L 1, at · − sediment-to-water ratios of 1:20, 1:10, 1:5, and 1:1, respectively), all of which breached the Class III Int. J. Environ. Res. Public Health 2019, 16, 4657 7 of 15 surface water-quality standard in China (Figure2). This result suggests that the risk of ammonium release from sediment during the dry season (November to April in Mianyang) is significantly greater than that during the wet season (May to October in Mianyang). The ammonium concentration during the dry season was also found to be greater than that during the wet season in the same hydrological years [29]. Therefore, we should pay much more attention to the ammonium concentration during the Int. J. Environ. Res. Public Health 2019, 16, x 7 of 15 dry season which should be constantly monitored and strictly controlled. 3.3.2. Variations in the Ammonium Release of Four Heavily Polluted Sediments 3.3.2. Variations in the Ammonium Release of Four Heavily Polluted Sediments The kinetics of ammonium release from the S1−S4 incubation in ultrapure water is shown in The kinetics of ammonium release from the S1 S4 incubation in ultrapure water is shown in Figure 3. The ammonium release kinetics of S1−S4 in the− ultrapure water–sediment incubation system Figure3. The ammonium release kinetics of S1 S4 in the ultrapure water–sediment incubation system was similar to 3.3.1, including three stages. −Despite the similar balance concentration in the final was similar to 3.3.1, including three stages. Despite the similar balance concentration in the final stable stable stage, the maximum of Cspw, t (Cspw, m) was much different among the four RDW-polluted stage, the maximum of Cspw, t (Cspw, m) was much different among the four RDW-polluted sediments. sediments. The time to reach the maximum of C (C ) was similar for S1−S4 (Figure 3). The time to reach the maximum of Cspw, t (Cspw, mspw,) was t similarspw, m for S1 S4 (Figure3). Though the Though the four chosen sediments were all heavily polluted (TN > 15001 − mg·kg−1), the C values four chosen sediments were all heavily polluted (TN > 1500 mg kg ), the Cspw, m valuesspw, m of S1–S4 · − wereof S1–S4 relatively were lowrelatively when thelow sediment-to-water when the sediment-to-water ratio was 1:20, ratio in thewas order 1:20, ofinS1 the (1.42 order mg ofL S11, Class(1.42 · − IV)mg·L> −S31, Class (0.69 mgⅣ) L> S31, (0.69 Class mg·L III) >−1S4, Class (0.55 Ⅲ mg) >L S41 ,(0.55 Class mg·L III) >−1, S2Class (0.50 Ⅲ mg) > LS2 1(0.50, Class mg·L II).− Comparing1, Class Ⅱ). · − · − · − theComparingCspw, m ofthe S1–S4 Cspw, tom theof S1–S4 ammonium to the ammonium concentration concentration in SW1–SW4, in SW1–SW4, we found thatwe found the Cspw, that m theof 1 −1 S1Cspw, was m much of S1 was higher much than higher that than of SW1 that (0.49of SW1 mg (0.49L ),mg·L and the), andCspw, the mCspw,of S2–S3 m of S2–S3 was similarwas similar to the to · − ammoniumthe ammonium in SW2–SW3. in SW2–SW3. The TheCspw, Cspw, m mvalue value of of S4 S4 was was dramatically dramatically less less than thanthe the ammoniumammonium inin SW4.SW4. This demonstrated that the important ammonium in SW1–SW3 waswas from internalinternal loading,loading, butbut thethe ammoniumammonium inin SW4SW4 waswas mainlymainly attributedattributed toto thethe centralizedcentralized blackblack waterwater discharge.discharge. TheThe lowerlower ammoniumammonium concentrationconcentration inin SW1SW1 (0.49(0.49 mgmg·LL −11) was because the real sediment-to-water ratioratio inin pondpond · − B1B1 waswas lessless thanthan 1:20.1:20. It has been suggested that black waterswaters inin centralizedcentralized RDWRDW bebe separatelyseparately collectedcollected forfor furtherfurther treatmenttreatment oror resourceresource cyclecycle [[1].1].

2.0 (A)S1 s:l=1:20 (B)S2 s:l=1:20 0.6 Cspw, t Cspw, t C Cpw, t 1.6 pw, t Ⅳ class Ⅱ class 0.4

1.2 -1 -1

Ⅲ class 0.8 -N/mg.L -N/mg.L 3 3 0.2 NH NH 0.4 Ⅱ class Ⅰ class

0.0 0.0 Ⅰ class

0481216 0481216 t/d t/d

(C)S3 s:l=1:20 (D)S4 s:l=1:20 1.0 1.0 C C spw, t Ⅲ class spw, t Ⅲ class

Cpw, t Cpw, t 0.8 0.8 1 -1 - 0.6 0.6

Ⅱ class Ⅱ class

-N/mg.L 0.4

-N/mg.L 0.4 3 3 NH NH 0.2 0.2 Ⅰ class Ⅰ class 0.0 0.0

0 4 8 12 16 0481216 t/d t/d

FigureFigure 3.3. Kinetics of ammoniumammonium release fromfrom fourfour sedimentssediments (S1–S4)(S1–S4) incubatedincubated inin ultrapureultrapure waterwater (C ). The sediment-to-water ratio was 1:20 for the four sediments. The controls were the kinetics (Cspw,spw, t ). The sediment-to-water ratio was 1:20 for the four sediments. The controls were the kinetics of of ammonium incubation in ultrapure water (C ) without sediments. ammonium incubation in ultrapure water (Cpw,pw, t) without t sediments.

3.3.3. The Difference of Ammonium Release between Theoretical Calculation and Testing

Notably, the order of Cspw, m (S1 > S3 > S4 > S2) was much different from that of TN and NH3- Nexc (i.e., S2 > S1 > S3 > S4). This suggests that it is difficult to use TN or NH3-Nexc to directly evaluate the ammonium release risk from polluted sediments. Assuming that all ammonium in a sediment is c released into 10.0 L of ultrapure water, its concentration in such a system (Cspw) can be expressed by Equation (1) as follows, and as shown in Figure S3. Int. J. Environ. Res. Public Health 2019, 16, 4657 8 of 15

3.3.3. The Diﬀerence of Ammonium Release between Theoretical Calculation and Testing

Notably, the order of Cspw, m (S1 > S3 > S4 > S2) was much diﬀerent from that of TN and NH3-Nexc (i.e., S2 > S1 > S3 > S4). This suggests that it is diﬃcult to use TN or NH3-Nexc to directly evaluate the ammonium release risk from polluted sediments. Assuming that all ammonium in a sediment is c released into 10.0 L of ultrapure water, its concentration in such a system (Cspw) can be expressed by Equation (1) as follows, and as shown in Figure S3.

w (1 M%) m Cc = × − × (1) spw V where Cc (mg L 1) is the theoretical ammonium concentration in an ultrapure water–sediment spw · − incubation system, w (mg kg 1) is the exchange ammonium nitrogen content in a dry sediment, M (%) · − is the water content of the tested sediments, m (kg) is the mass of the experimental sample, and V (L) is c the volume of the water. The Cspw values were found to be much greater than those of Cspw, t, and this gap was more distinctive when the incubation was at a higher sediment-to-water ratio. For example, Cc of S1 reached 56.3 mg L 1 when the sediment-to-water ratio was 1:1, which was 17-fold higher spw · − than the Cspw, m of S1. This indicated that the internal ammonium release from sediment was a slow and long process and that the risk of ammonium release can be overestimated by the theoretical calculation using NH3-Nexc, particularly during the dry season.

3.4. Characteristics of the RDW-Polluted Sediments Incubated in Sample Water

3.4.1. The Source and Sink Effect of Sediment on Ammonium Balance The kinetics of ammonium with S1 incubation at different sediment-to-water ratios in sample water are shown in Figure4 and those of the S1–S4 incubation in sample water are shown in Figure5. Sediments can act as source or sink in the aquatic ecosystem cycle and are important in regulating the ammonium balance [30]. The intrinsic contributions from sediments to aqueous ammonium in the sample water–sediment incubation system (Cssw,0 t) were calculated using Equation (2) and are shown in Figures S4 and S5: C0 = Cssw, t Csw, t (2) ssw, t − 1 where Cssw, t (mg L ) is the ammonium concentration with time in the sample water–sediment · − incubation. Sediment was a source of ammonium when Cssw,0 t > 0, while it was a sink with the dominant ammonium sorption onto sediment when Cssw,0 t < 0. The calculated intrinsic contributions from sediments (Cssw,0 t) in the sample water were generally slightly lower than those (Cspw,0 t) in ultrapure water (except as shown in Figure S4C), mainly given the stronger natural ammonium reduction in sample water with abundant nitrifying bacteria. Similar to the kinetics of Cspw, t in the ultrapure water–sediments incubations, the kinetics of Cssw, t also included three stages (i.e., first a rapid increase, then a slow decrease, and finally a stable stage at a relatively low concentration, except for S4 as shown in Figure5D). These three stages were reached within approximately 13–20 days. The kinetics of Cssw, t in the S4 incubation with sample water was markedly different from that of the other three RDW-polluted sediments (i.e., Cssw, t 1 directly decreased to 1.80 mg L within 16 days). The distinctive kinetic of Cssw, t in the S4 incubation · − was mainly attributed to the remarkably high initial aqueous ammonium concentration in the SW4 1 (42.0 mg L ). The sharp decline of Cssw, t in the S4 incubation was quite similar to that of Csw, t in the · − SW4 incubation, suggesting that the decline in Cssw, t was actually the result of the natural ammonium reduction in SW4. However, the natural reduction of ammonium in the surface water contributed by the highly concentrated RDW discharge required a long time, which was among the major reasons contributing to water eutrophication and black odorous river [8,31,32]. Generally, the Cssw, t value was higher than that of the Csw, t, indicating the source effect of sediments on ammonium in the surface water. Taking the most polluted S1 as an example, the maximum Cspw,0 t reached 1.12, 1.57, Int. J. Environ. Res. Public Health 2019, 16, 4657 9 of 15

2.58, and 3.09 mg L 1 with sediment-to-water ratios of 1:20, 1:10, 1:5, and 1:1, respectively. And all of · − the maximum Cspw,0 t breached the Class III surface water-quality standard in China (Figure S4). However, the source and sink eﬀect of sediment on ammonium was constantly changing during the incubation process and was obviously inﬂuenced by the sediment-to-water ratio and the initial SW (aqueous ammonium in small basins) concentration. (1) When the sediment-to-water ratio was

1:1,Int. J. S1Environ. served Res. onlyPublic asHealth a source 2019, 16 of, x ammonium in the entire incubation but demonstrated a “source9 of 15 sink source” effect during the incubation at sediment-to-water ratios of 1:20, 1:10, and 1:5, → → respectively. (2) (2) When the initial concentration of of the the discharged discharged RDW RDW was was high, high, S4 S4 was was a sink for pollutants in the RDW during the the first first five five days days (i.e., (i.e., Cspw, tt was slightly lower than than Cpw, t t).). Notably, Notably, the contribution of the sink effect effect from sediment on surface water quality can be negligible when the ammonium concentrationconcentration in in the the RDW RDW discharge discharge is extremely is extremely high. high. (3) The (3) constantlyThe constantly changing changing effects effectsof sediments of sediments on regulating on regulating the ammonium the ammonium balance were balance caused were by variationscaused by in variations natural temperature in natural temperatureand ammonium and reductionammonium in reduction sample water. in sample Forexample, water. For when example, the aqueous when the ammonium aqueous ammonium gradually graduallydecreased viadecreased natural via reduction natural processes, reduction the processe exchangeables, the exchangeable ammonium was ammonium considered was to beconsidered desorbed from sediments to the sample water resulting in a “sink source” effect.→ to be desorbed from sediments to the sample water resulting→ in a “sink source” effect.

3d 10d 3d 13d 3.0 2.5 (A) s:l=1:20 (B) s:l=1:10 C ssw, t Cssw, t C 2.5 sw, t Csw, t 2.0 Ⅴ class 2.0 Ⅴ class -1 1.5 -1 Ⅳ class 1.5 Ⅳ class -N/mg.L -N/mg.L 3 1.0 3 Ⅲ class 1.0 NH NH Ⅲ class 0.5 0.5 Ⅱ class Ⅱ class 0.0 0.0 0 5 10 15 20 25 0 5 10 15 20 25 t/d t/d

10d 16d 7d 13d 4.0 (C) s:l=1:5 (D) s:l=1:1 3.2 C ssw, t Cssw, t

Csw, t C 3.0 sw, t 2.4 1 - -1 L Ⅴ class 2.0 1.6 mg. Ⅴ class N/ - -N/mg.L 3 3 Ⅳ class

NH Ⅳ class NH 1.0 0.8 Ⅲ class Ⅲ class

Ⅱ class Ⅱ class 0.0 0.0 0 5 10 15 20 25 0 5 10 15 20 25 t/d t/d

Figure 4. Kinetics of ammonium withwith sedimentsediment (S1) (S1) incubation incubation at at di differentﬀerent sediment-to-water sediment-to-water ratios ratios in insample sample water water (C ssw,(Cssw, t). t The). The sediment-to-water sediment-to-water ratios ratios for for (A ),(A (B),), (B (C),), (C and), and (D) ( wereD) were 1:20, 1:20, 1:10, 1:10, 1:5, and1:5, 1:1,and respectively.1:1, respectively. The controlsThe controls were were the kinetics the kineti of ammoniumcs of ammonium incubation incubation in the samplein the sample water (Cwatersw, t) (withoutCsw, t) without sediments. sediments. Int. J. Environ. Res. Public Health 2019, 16, 4657 10 of 15 Int. J. Environ. Res. Public Health 2019, 16, x 10 of 15

(A)S1 s:l=1:20 1.8 (B)S2 s:l=1:20 2.4 C Cssw, t ssw, t C Csw, t sw, t 2.0 1.5 Ⅴ class Ⅳ class -1 -1 1.6 1.2 Ⅳ class -N/mg.L

-N/mg.L 1.2 3 3 0.9 Ⅲ class NH NH Ⅲ class 0.8 0.6

0.4 Ⅱ class Ⅱ class 0.3 0481216 0481216 t/d t/d 48.0 1.8 (C)S3 s:l=1:20 (D)S4 s:l=1:20 Cssw, t Cssw, t Csw, t Csw, t 1.5 36.0 Ⅳ class 1 -1 - 1.2 24.0 -N/mg.L -N/mg.L 3 3 0.9 Ⅲ class NH

NH 12.0

0.6 0.0 Ⅱ class Ⅴ class 0.3 0 4 8 12 16 0 4 8 12 16 t/d t/d Figure 5. Kinetics of ammonium for four sediments (S1–S4) incubation in sample water (C ). The Figure 5. Kinetics of ammonium for four sediments (S1–S4) incubation in sample water (Cssw,ssw, t). The sediment-to-watersediment-to-water ratio was 1:20 for the four sediments. The The controls controls were were the the kinetics kinetics of of ammonium ammonium incubation in sample water (C ) without sediments. incubation in sample water (Csw,sw, t t) without sediments.

3.4.2. Relative Relative contribution contribution of of internal internal loading loading and and external discharge

The relativerelative contribution contribution of of internal internal loading loading from from sediment sediment (Rs) and (Rs) external and external pollution pollution (Rsw, sample (Rsw, samplewater) towater) the aqueous to the aqueous ammonium ammonium distribution distribution were further were further calculated calculated using Equations using Equation (3) and (3) (4), and as Equationshown in (4), Figures as shown6 and 7in: Figures 6 and 7: Csw Rsw% = Csw 100 %, (3) Rsw% =C ×100× %, (3) Cssw C0 ssw’ Rs% = Cssw 100 %, (4) Rs% =Cssw×100× %, (4) Cssw where R (%) is the relative contribution of sample water (external pollution) to ammonium and R (%) where Rsw (%) is the relative contribution of sample water (external pollution) to ammonium ands is the relativesw apparent contribution of sediment to ammonium in the sample water–sediment system. R (%) is the relative apparent contribution of sediment to ammonium in the sample water–sediment Ass shown in Figure6, the relative contribution to aqueous ammonium was mainly from the sample system. As shown in Figure 6, the relative contribution to aqueous ammonium was mainly from the water during the early stage, then mostly from the internal release of S1, and then by the sample water sample water during the early stage, then mostly from the internal release of S1, and then by the during the ﬁnal stage. R from S1 increased with an increase in the sediment-to-water ratio and the sample water during the sfinal stage. 𝑅 from S1 increased with an increase in the sediment-to-water time for sediment to control water qualitys was highly related to this ratio. As shown in Figure7, R ratio and the time for sediment to control water quality was highly related to this ratio. As shown ins values from four of the heavily polluted sediments by long-term RDW discharge (S1–S4) were mostly Figure 7, Rs values from four of the heavily polluted sediments by long-term RDW discharge (S1–S4) less than those of Rsw with the incubation of sediments (despite the low ammonium concentration in were mostly less than those of Rsw with the incubation of sediments (despite the low ammonium SW1–SW3). concentration in SW1–SW3). 3.4.3. Water Quality Management Suggestions for Rural Areas Dredging was a common restoration method to reduce internal loading of sediment and was extensively used during recent decades, having been applied since the 1970s [11,33–35]. However, as previously described, either the absolute internal ammonium loading or the relative contribution from long-term and heavily polluted sediments by RDW discharge was relatively low (except during the dry season). Moreover, sediments also play an important role in regulating the pollution balance Int. J. Environ. Res. Public Health 2019, 16, 4657 11 of 15

by acting as a sink for external pollution in the aquatic ecosystem cycle, such as for heavy metals and POPs (persistent organic pollutants) [30,36–38]. Hence, dredging is not believed to be the best choice for RDW-polluted sediments for aqueous ammonium risk control. The key point to maintain aqueous ammonium at a safe level is to manage the major source of external pollution (i.e., mainly RDW-discharge and particularly centralized black water). Centralized RDW with black water should be strictly treated before discharge into surface water bodies. The small-scale sewage treatment technology for RDW is relatively mature in China and the most important and difficult process is the related operational management [39]. Considering the significant cost of regular RDW treatment is unacceptable for developing countries, centralized rural communities are highlighted, which offer the possibility of separate collection, treatment, or reuse of black water. Dredging is not suggested except in response to an emergency. Int. J. Environ. Res. Public Health 2019, 16, x 11 of 15

(A)s:l=1:20 R (B)s:l=1:10 sw 180 Rsw 180 R s Rs

120 120

60 60 % % R R

0 0

-60 -60

-120 0 5 10 15 20 25 0 5 10 15 20 25 t/d t/d

120 (C)s:l=1:5 R 180 sw (D)s:l=1:1 Rsw R s 100 Rs

120 80

60 60 % % R R 40 0

-60 0

0 5 10 15 20 25 0 5 10 15 20 25 t/d t/d

FigureFigure 6. 6. RelativeRelative contribution contribution of of intern internalal sediment sediment loading loading (S1) (S1) and and ex externalternal pollution pollution (sample (sample water) water) toto aqueous ammoniumammonium during during incubation incubation at at di ﬀdiffeerentrent sediment-to-water sediment-to-water ratios. ratios. The sediment-to-waterThe sediment-to- waterratios ratios for (A for), ((BA),), ((CB),), and(C), and (D) ( wereD) were 1:20, 1:20, 1:10, 1:10, 1:5, 1:5, and and 1:1, 1:1, respectively. respectively. RRswsw (%) (%) isis the relative relative contributioncontribution of the sample water toto ammoniumammonium andand R𝑅s (%) (%) is is the the relative relative contribution contribution of of sediment sediment to toammonium ammonium in in the the sample sample water–sediment water–sediment system. system.

(A)S1 180 (B)S2 180

120 120 s:l=1:20 s:l=1:20 60 60 Rsw Rsw % R

R% R s Rs 0 0

-60 -60

-120 -120 0481216 0481216 t/d t/d

120 (C)S3 (D)S4 s:l=1:20 120 Rsw

90 Rs 80

s:l=1:20 60 40 Rsw % % R Rs R 0 30

-40 0

-80 0481216 0481216 t/d t/d Figure 7. Relative contribution of internal sediment loading and external pollution (sample water) to aqueous ammonium during the four sediments’ incubation (S1–S4). The sediment-to-water ratio was Int. J. Environ. Res. Public Health 2019, 16, x 11 of 15

(A)s:l=1:20 R (B)s:l=1:10 sw 180 Rsw 180 R s Rs

120 120

60 60 % % R R

0 0

-60 -60

-120 0 5 10 15 20 25 0 5 10 15 20 25 t/d t/d

120 (C)s:l=1:5 R 180 sw (D)s:l=1:1 Rsw R s 100 Rs

120 80

60 60 % % R R 40 0

-60 0

0 5 10 15 20 25 0 5 10 15 20 25 t/d t/d Figure 6. Relative contribution of internal sediment loading (S1) and external pollution (sample water) to aqueous ammonium during incubation at different sediment-to-water ratios. The sediment-to-

water ratios for (A), (B), (C), and (D) were 1:20, 1:10, 1:5, and 1:1, respectively. Rsw (%) is the relative Int. J.contribution Environ. Res. Publicof the Health sample2019 water, 16, 4657 to ammonium and 𝑅 (%) is the relative contribution of sediment 12 of 15 to ammonium in the sample water–sediment system.

(A)S1 180 (B)S2 180

120 120 s:l=1:20 s:l=1:20 60 60 Rsw Rsw % R

R% R s Rs 0 0

-60 -60

-120 -120 0481216 0481216 t/d t/d

120 (C)S3 (D)S4 s:l=1:20 120 Rsw

90 Rs 80

s:l=1:20 60 40 Rsw % % R Rs R 0 30

-40 0

-80 0481216 0481216 t/d t/d

FigureFigure 7. 7. RelativeRelative contribution contribution of of internal internal sediment sediment loading loading and and external external polluti pollutionon (sample (sample water) water) to aqueousto aqueous ammonium ammonium during during the four the four sediments’ sediments’ incubation incubation (S1–S4). (S1–S4). The sediment-to-water The sediment-to-water ratio ratiowas was 1:20 for the four sediments (S1–S4). Rsw (%) is the relative contribution of sample water to ammonium and Rs (%) is the relative apparent contribution of sediment to ammonium in the sample water–sediment system.

4. Conclusions The risk of ammonium release from sediment during the dry season was found to be signiﬁcantly greater than that during the wet season in this study. Internal ammonium release from sediment was a slow and long process and the risk of ammonium release from sediment could be overestimated by the theoretical calculation using tested exchangeable nitrogen. Sediments can serve as source or sink, regulating the ammonium balance in an aquatic ecosystem cycle such as the “sink source” eﬀect of → a high concentration of RDW discharge. Though all of the selected sediments were heavily polluted as a result of long-term RDW discharge, their relative contribution of internal loading was generally less than that of external pollution. Hence, dredging was not suggested for RDW-polluted sediments except in response to emergency. The excessive ammonium in the selected catchment was mainly a result of discharge of untreated and centralized black water in RDW. The centralized black waters in rural communities were highlighted for separate treatment or reuse to maintain aqueous ammonium at a safe level. We call for more studies on the nitrogen cycle contributed by the RDW discharge and proper treatment technology for centralized black waters.

Supplementary Materials: The following are available online at http://www.mdpi.com/1660-4601/16/23/4657/s1, Figure S1: Relationship between TOC (A) and NH3-Nexc (B) of the four selected sediments and TN in the sediments. TN is the total nitrogen content. TOC is the total organic carbon content. NH3-Nexc is the exchangeable ammonium nitrogen content; Figure S2: The kinetics of ammonium with sediment (S1) incubation at different sediment-to-water ratios in ultrapure water (Cspw, t) and sample water (Cssw, t). The controls were the kinetics of ammonium incubation in ultrapure water (Cpw, t) and sample water (Csw, t) without sediments; Figure S3: Ammonium concentration in ultrapure water–sediment incubation system at different sediment-to-water ratios (A) and four sediments (B) by calculating; Figure S4: The intrinsic kinetics of ammonium in the sediment (S1) incubation at different sediment-to-water ratios in ultrapure water (Cspw,0 t) and sample water (Cssw,0 t). The Int. J. Environ. Res. Public Health 2019, 16, 4657 13 of 15

sediment-to-water ratios for (A), (B), (C), and (D) were 1:20, 1:10, 1:5, and 1:1, respectively. Cspw,0 t and Cssw,0 t are the difference between Cspw, t and Cpw, t, Cssw, t and Csw, t, respectively; Figure S5: The intrinsic kinetics of ammonium in the four sediments incubation in ultrapure water (Cspw,0 t) and sample water (Cssw,0 t). The sediment-to-water ratio was 1:20 for four sediments (S1–S4). Cspw,0 t and Cssw,0 t are the difference between Cspw, t and Cpw, t,Cssw, t and Csw, t, respectively. Author Contributions: Y.L. and X.L. conceived and designed the experiments; X.L., Q.W., and Z.W. performed the experiments and analyzed the data; Y.L. and X.L. wrote the paper; and Q.L., L.W., Y.S., and R.W. revised the paper. All authors have read and approved the final manuscript. Funding: This research was funded by the International Cooperation Project of Sichuan provincial science and Technology Department, grant number 2017HH0042. Acknowledgments: This work was supported by the International Cooperation Project of Sichuan Provincial Science and Technology Department (2017HH0042). The authors appreciate the testing assistance of MianYang KaiLe Testing Co., Ltd. The authors are thankful for the useful suggestions and comments from Yuwei Chen and Neslon Belzile (Laurentian University, Canada). Conflicts of Interest: The authors declare no conflict of interest.

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