Relationship between nutrient pollutants and suspended sediments in upper reaches of Yangtze River

* Corresponding author.

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Relationship between nutrient pollutants and suspended sediments in upper reaches of Yangtze River

Xiao-qing Wang*,Zhao-cun Liu,Ji-lun Miao,Ning Zuo

Southwest Research Institute of Water Transport Engineering,Chongqing Jiaotong University,Chongqing 400016,PR China

Received 24 December 2013; accepted 5 January 2015 Available online 15 April 2015

* Corresponding author.

Abstract

In order to study the relationship between nutrient pollutants and suspended sediments (SS)in the upper reaches of the Yangtze River and two tributaries,water samples were collected from September 1,2010 to September 30,2011 at the Zhutuo,Cuntang,Beibei,Wulong,Qingxichang,Wanxian,and Fengjie cross-sections.In the laboratory,the SS concentration and the concentration of SS whose particle size was smaller than 0.02 mm were measured.The phosphorus (P),nitrogen (N),and permanganate index (CODMn)concentrations in the natural water sample,the settled water sample,and two types of filtered water samples obtained through filter membranes with pore sizes of 0.02 mm and 0.45 mm were monitored synchronously.The results show that there are strong relationships between the P and CODMnconcentrations and the SS concentration.P mainly exists in particulate form,while N mainly exists in dissolved form.SS whose particle size is smaller than 0.02 mm accounts for a high proportion of sediments in the Yangtze River and has a strong effect on water quality.At the seven cross-sections,the amounts of P,N,and CODMnin particulate form in the wet season are higher than in the dry season and the adsorption amounts of P,N,and CODMnper unit mass of sediment are higher in the dry season than in the wet season.?2015 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:Suspended sediment; Phosphorus; Nitrogen; Permanganate index

This work was supported by the Chongqing Education Foundation (Grant No.KJ1400324)and the Advanced Research Project of Chongqing Science and Technology Commission (Grant No.cstc2013jcyjA20013).

E-mail address: xiaoqingwang2002@hotmail.com (Xiao-qing Wang).Peer review under responsibility of Hohai University.

http://dx.doi.org/10.1016/j.wse.2015.04.003

1674-2370/?2015 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Sediment particles play an important role in water quality (Lopez et al.,1996).The Yangtze River is the third longest river and the Yangtze River Basin is the ninth largest catchment in the world.The discharge of the Yangtze River is the largest to the Western Pacific Ocean and the fifth largest in the world,and its sediment load is the fourth largest in the world.Sediment particles originating from soil loss in the upper reaches of the Yangtze River have been estimated to exceed 40 million tons annually (Shi et al.,1992).The adsorption behaviour of pollutants on sediments has attracted significant interest in recent years (Kan et al.,1998; Chiou et al.,1998; Kile et al.,1999; Westall et al.,1999; Liu et al.,2001,2008; Zhao et al.,2001).Some studies have focused on the relationship between phosphorus (P)and sediment in coastal areas (Zhou et al.,2005; Wang et al.,2009a,2009b; Cao et al.,2011).Others have focused on the relationship between nitrogen (N)and sediment in the field and in laboratory experiments (Sfriso and Marcomini,1999; Stimson and Larned,2000; Gardner et al.,2001; Lu¨et al.,2005).P and N have been identified as limiting nutrient factors for eutrophication in water (Ryther and Dunstan,1971; Bizsel and Uslu,2000).A large amount of monitoring work has been conducted to investigate changes in P and N concentrations and loads in other water bodies (Bowes and House,2001; Sigua and Tweedale,2003; Bowes et al.,2003; Salvia Castellví et al.,2005; Quilbe et al.,2006; Kangur and Mols,2008).These studies indicate that there are different relationships betweennutrient pollutant concentrations and the suspended sediments (SS)concentration in different water bodies.

The aim of this study was to determine the relationship between nutrient pollutants and SS in the upper reaches of the Yangtze River,as well as to determine how sediment adsorption affects the concentrations of nutrient pollutants in the water by synchronously monitoring P,N,CODMn,and SS concentrations.

2.Materials and methods

2.1.Field sampling and monitoring

The study area is shown in Fig.1.It includes the Yangtze River and two important tributaries,the Jialing River and the Wujiang River,the inflow discharge of which accounts for 90% of the total inflow discharge from all tributaries.The study area is divided into three reaches: (1)the first reach is between the Zhutuo cross-section and the Fengjie crosssection in the Yangtze River,and includes five monitoring cross-sections (the Zhutuo,Cuntang,Qingxichang,Wanxian,and Fengjie cross-sections),of which four have monitoring stations (the Zhutuo,Cuntang,Qingxichang,and Wanxian cross-sections); (2)the second reach is between the Wulong cross-section in the Wujiang River and the Wujiang Estuary,and includes one monitoring station at the cross-section; and (3)the third reach is between the Beibei cross-section in the Jialing River and the Jialing Estuary,and includes one monitoring station at the cross-section.

The Water Environmental Monitoring Center in the upper reaches of the Yangtze River monitored the concentrations of nutrient pollutants and SS synchronously from September 1,2010 to September 30,2011 once per month.Water samples were collected using horizontal water samplers at seven crosssections.At each cross-section,three sampling verticals were set,one 50 m from the left bank,one in the middle of the river,and one 50 m from the right bank.There were three sampling sites on each sampling vertical,one 0.5 m below the water surface,one in the middle of the water depth,and one 0.5 m above the river bed.One-thousand mL water was taken from every sampling site,and a total of 9 000 mL of mixed water was collected as one water sample.Two of the same water samples were collected at each cross-section.One was for water quality measurement,and the other was for sediment measurement.In addition,discharge data were collected during the test period from Water Environment Monitoring Center gauging stations.

2.2.Sample treatment and measurement

Fig.1.Study area and cross-sections.

There were 182 water samples collected from seven crosssections in the upper reaches of the Yangtze River.After the water samples were transferred to the laboratory,they were stored in refrigerators until measurements were taken.Ninetyone water samples for sediment measurement were used to measure the SS concentration and the concentration of SS whose particle size was smaller than 0.02 mm.Each of the other 91 water samples was treated to obtain four different types of water samples: natural water samples,settled water samples,and two types of filtered water samples.The natural water sample was the original water sample from the river.The settled water sample was obtained by removing deposited particles from the original water sample after it was placed and after it remained static for half an hour.The filtered water sample A was obtained by removing sediment particles from the original water sample through filter membranes with a pore size of 0.02 mm and the filtered water sample B was obtained by removing sediment particles from the original water sample through filter membranes with a pore size of 0.45 mm.The natural water sample was used to measure the concentrations of total phosphorus (TP),total nitrogen (TN),and total CODMn.The filtered water sample B was used to measure the concentrations of dissolved phosphorus (DP),dissolved nitrogen (DN),and dissolved CODMn.The concentrations of P,N,and CODMnin particulate form were obtained by subtracting the dissolved pollutant concentrations from the total pollutant concentrations.The settled water sample was used to measure the pollutant concentrations,except for those absorbed on the deposited particles.The filtered water sample A was used to measure the pollutant concentrations in dissolved form and the portion absorbed on SS whose particle size was smaller than 0.02 mm.The adsorption amounts of P,N,and CODMnper unit mass of sediment were defined,respectively,as the P,N,and CODMnconcentrations divided by the SS concentration.

Table 1　Concentrations of P,N,and CODMnin different periods.

The P concentration was measured using the molybdate spectrophotometric method according to the Water Quality e Determination of Total Phosphorus Ammonium Molybdate Spectrophotometric Method (GB 11893e89).The N concentration was measured using the alkaline potassium persulfate digestion UV spectrophotometric method according to the Water Quality e Determination of Total Nitrogen Alkaline PotassiumPersulfate Digestion UV Spectrophotometric Method (GB 11894e89).The CODMnconcentration was measured using the potassium permanganate method according to the Water Quality e Determination of Permanganate Index (GB 11892e89).The SS concentration was measured according to the Technical Standard for Determination of Sediment Particle Size in Open Channels (SL 42e2010).

3.Results and discussion

3.1.P,N,and CODMnconcentrations and adsorption amount

In the Yangtze River Basin,the wet season is from May to October,and the dry season is from November to April of the next year.In the dry season,the average discharges of the Yangtze River,the Jialing River,and the Wujiang River are 3 420 m3/s,300 m3/s,and 399 m3/s,respectively,and in thewet season,the average discharges are 20 947 m3/s,1 759 m3/s,and 1 761 m3/s,respectively.

Table 2　Adsorption amounts of P,N,and CODMnper unit mass of sediment in different periods.

Table 1 shows that the SS concentration in the Yangtze River is higher than that in the Jialing River and the Wujiang River.P mainly exists in particulate form,which accounts for 55.6%e91.9% of TP,while N mainly exists in dissolved form,which accounts for 59.7%e89.7% of TN.

Table 2 shows that the adsorption amounts of N and CODMnper unit mass of sediment in the tributaries are higher than those in the Yangtze River.Compared with the results in published studies on the adsorption amounts of P,N,and CODMnon bed sediments in other water bodies (Lu¨et al.,2005; Quilbe et al.,2006; Kangur and Mols,2008; Wang et al.,2009a; Cao et al.,2011),the adsorption amounts of P,N,and CODMnper unit mass of sediment in the Yangtze River are high,which may be due to the serious water pollution in the upper reaches of the Yangtze River.

The relationship between the concentrations of nutrient pollutants and SS shows that,although the total adsorption amounts of nutrient pollutants on sediments increase with the SS concentration,the adsorption amounts of nutrient pollutants per unit mass of sediment decrease with the increase of the SS concentration.In the wet season,the SS concentration is 8e29 times as high as that in the dry season,but the pollution level in SS in the dry season is high.Therefore,the adsorption amounts of P,N,and CODMnper unit mass of sediment in the dry season are 3e9 times as high as those in the wet season.

3.2.Correlations between P,N,and CODMnconcentrations and SS concentration

Fig.2 shows the relationships between the DP,DN,and dissolved CODMnconcentrations and the SS concentration in the natural water sample.There is no evident correlation between the DP,DN,and dissolved CODMnconcentrations and the SS concentration.Fig.3 shows that the TP and CODMnconcentrations have a positive correlation with the SS concentration,but the TN concentration does not show a significant correlation with the SS concentration.The reason is that P is more adhesive to sediment particles and is likely to be absorbed on sediments,while N mainly exists in dissolved form.The results show that the adsorption characteristics of P are similar to those of CODMn,but different from those of N.

Figs.4 and 5 show the correlations between the P,N,and CODMnconcentrations in the natural water sample,settledwater sample,and filtered water sample A.The results show that the pollutant concentrations in the filtered water sample A have a stronger correlation with those in the settled water sample than with those in the natural water sample,and the P,N,and CODMnconcentrations in the settled water sample are lower than those in the filtered water sample A.The results from the measurement of the SS concentration show that the particle size of more than 60% of SS in the settled water sample is lower than 0.02 mm,especially in the water with a low SS concentration.This indicates that SS with a particle size smaller than 0.02 mm has a strong effect on the water quality.

Fig.2.Correlations between DP,DN,and dissolved CODMnconcentrations and SS concentration.

4.Conclusions

The concentrations of P,N,and CODMnin dissolved form and particulate form were monitored for thirteen months at seven cross-sections along the Yangtze River.The concentrations showed that there were strong relationships between nutrient pollutants and SS in the Yangtze River.

(1)The TP and CODMnconcentrations had positive correlations with the SS concentration,but the TN concentration did not have a strong correlation with the SS concentration.The N concentration in the Yangtze River was less affected by SS than the P and CODMnconcentrations were.Thus,it was concluded that P mainly exists in particulate form,which accounted for 55.6%e91.9% of TP,while N mainly exists in dissolved form,which accounted for 59.7%e89.7% of TN.

Fig.3.Correlations between TP,TN,and CODMnconcentrations and SS concentration.

Fig.4.Correlations between P,N,and CODMnconcentrations in natural water sample and in filtered water sample A.

(2)SS with a particle size smaller than 0.020 mm made up a proportion greater than 60% of the SS in the Yangtze River.This portion of the SS had strong effects on the water quality.

(3)The adsorption amounts of nutrient pollutants per unit mass of sediment decrease with the increase of the SS concentration.At seven cross-sections in the upper reaches of the Yangtze River,the SS concentration in the wet season is 8e29 times as high as that in the dry season.The adsorption amounts of nutrient pollutants per unit mass of sediment in the dry season are 3e9 times as high as those in the wet season.

Fig.5.Correlations between P,N,and CODMnconcentrations in settled water sample and in filtered water sample A.

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