Impacts of water quality variation and rainfall runoff on Jinpen Reservoir, in Northwest China

Zi-zhen Zhou,Ting-lin Huang*,Wei-xing Ma,Yang Li,Kang Zeng

School of Environmental and Municipal Engineering,Xi'an University of Architecture and Technology,Xi'an 710055,PR China

Impacts of water quality variation and rainfall runoff on Jinpen Reservoir, in Northwest China

Zi-zhen Zhou,Ting-lin Huang*,Wei-xing Ma,Yang Li,Kang Zeng

School of Environmental and Municipal Engineering,Xi'an University of Architecture and Technology,Xi'an 710055,PR China

The seasonal variation characteristics of the water quality of the Jinpen Reservoir and the impacts of rainfall runoff on the reservoir were investigated.Water quality monitoring results indicated that,during the stable stratifcation period,the maximum concentrations of total nitrogen,total phosphorus,ammonia nitrogen,total organic carbon,iron ion,and manganese ion in the water at the reservoir bottom on September 6 reached 2.5 mg/L,0.12 mg/L,0.58 mg/L,3.2 mg/L,0.97 mg/L,and 0.32 mg/L,respectively.Only heavy storm runoff can affect the main reservoir and cause the water quality to seriously deteriorate.During heavy storms,the stratifcation of the reservoir was destroyed,and the reservoir water quality consequently deteriorated due to the high-turbidity particulate phosphorus and organic matter in runoff.The turbidity and concentrations of total phosphorus and total organic carbon in the main reservoir increased to 265 NTU,0.224 mg/L,and 3.9 mg/L,respectively. Potential methods of dealing with the water problems in the Jinpen Reservoir are proposed.Both in stratifcation and in storm periods,the use of measures such as adjusting intake height,storing clean water,and releasing turbid fow can be helpful to safeguarding the quality of water supplied to the water treatment plants. ?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/).

Water quality;Seasonal variation;Rainfall;Impact of storm runoff;Intake height adjustment

1.Introduction

Numerous reservoirs have been built for drinking water supply,power generation,agricultural irrigation,and food control(Casamitjana et al.,2003;Liu et al.,2011).In recent years,most of them,particularly deep-water reservoirs,have faced eutrophication and seasonal water shortage(Yu and Wang,2011).In deep-water reservoirs,seasonal stratifcation prevents the transport of dissolved oxygen from surface water to bottom water,which results in an anaerobic environment (McGinnis and Little,2002;Beutel et al.,2007).Under long-duration anaerobic conditions,pollutants are released from sediment into the overlying water.When the reservoir water from different depths mixes due to the decrease of surface water temperature,pollutants in the reservoir may lead to water quality deterioration(McGinnis and Little,2002;Beutel et al.,2007;Wang et al.,2012a).

With climate change all over the world,many studies on the infuence of rainfall and runoff on reservoirs and rivers have been reported(Wang et al.,2012b;Wu et al.,2012;Liu et al., 2014).Rainfall events cause disturbances to water bodies by changing the hydrological conditions and infuencing the thermalstructureofreservoirs(Huangetal.,2014).Moreover,large amounts of particulate pollutants carried by runoff are brought into reservoirs.This causes serious exogenous pollution,which can conversely promote the overproduction of algae(Wang et al.,2001;Vaze and Chiew,2004;Zhang et al.,2006).

During the storm period,reservoir water can be seriously polluted,increasing the cost of water treatment.Therefore,theinvestigation of water quality change regularities and the effects of runoff on reservoirs are highly signifcant to reservoir management.The aim of this study was to investigate the seasonal variations of the Jinpen Reservoir water quality and the impacts of different rainfall events on the water quality of the reservoir.

2.Materials and methods

2.1.Sampling sites

The Jinpen Reservoir(at latitudes from 34°13′N to 34°42′N andlongitudesfrom107°43′Eto108°24′E)islocatedinnorthof theQinlingMountainsanditis90kmawayfromXi'anCity.The reservoir is the main water source of Xi'an City.It is a canyonshaped deep-water reservoir(Ma et al.,2015),and its main reservoir length is 3.5 km.The total capacity of the reservoir is 2.0×108m3and the effective capacity is 1.8×108m3.The main function of the Jinpen Reservoir is urban water supply. Agricultural irrigation,power generation,and food control are its accessory functions.

The Heihe River,which originates from the Qinling Mountains,is the main water supply for the Jinpen Reservoir (Fig.1).This river is 91.2 km long with a catchment area of 1 418 km2,and the catchment is largely undeveloped,consisting primarily of mountains covered with forest.The arrangement of water monitoring sites is as follows:S1 is located in the upstream area;S2,S3,and S4 are located in the transition area;S5 is located in the deep-water area of the main reservoir;and S6 is located near the intake tower for the drinking water plants.In recent years,water quality problems have increased dramatically due to release of pollutants from sediment and storm runoff,leading to serious water quality problems in Xi'an City.

Fig.1.Water sampling sites in Jinpen Reservoir and Heihe River.

2.2.Field observation

Under normal water quality monitoring conditions,water samples were collected weekly in the Jinpen Reservoir from November 1,2013 to October 31,2014.The dissolved oxygen (DO)concentration,water temperature,turbidity,and pH value were monitored in situ at sites S5 and S6 using a Hydrolab DS5 multi-probe sonde(Hach,USA)at 1-m water depth intervals.Water samples from sites S5 and S6 were taken every 5 m from the surface to the bottom using precleaned high-density polyethylene bottles with preservative already added.All samples were immediately cooled and stored at 4°C before analysis.

During rainfall,the sampling interval was changed to once a day.The DO concentration,water temperature,turbidity,and pH value were determined vertically in situ from S1 to S6 at 1-m water depth intervals.Water samples were also collected vertically at these sites every 5 m from the surface to the bottom.

For the water samples,the concentrations of total nitrogen (TN),ammonia nitrogen(NH+4-N),total phosphorus(TP), total organic carbon(TOC),permanganate index(CODMn), iron ion(Fe3+),and manganese ion(Mn7+),as well as the algal density and algal species,were measured.Concentrations of TN and NH+4-N were determined with a SEAL AA3 HR AutoAnalyzer(SEAL,Germany).TP concentration was measured with ultraviolet-visible spectrophotometry after potassium persulfate digestion at 121°C for 30 min.TOC concentration was determined with a TOC-L total organic carbon analyzer(Shimadzu,Japan).CODMnconcentration was determined with potassium permanganate oxidation and then titrated with sodium oxalate.Concentrations of metal ions were measured through inductively coupled plasma mass spectrometry(ICP-MS)after fltration through 0.22 μm flter membranes.Algal density and algal species were identifed with the method of microscopic examination.

3.Results and discussion

3.1.Water quality variations of Jinpen Reservoir

Table 1 shows the mean values of concentrations of TN,TP,TOC,Fe3+,and Mn7+in surface(0-0.5 m),middle (30-35 m),and bottom(60-70 m)water at four measurement times in one month in the main reservoir.The stratifed period was from April to November,and the remaining time of the year was the mixing period(Huang et al.,2010,2014).

During the research period,TN concentrations varied from 1.3 mg/L to 2.5 mg/L.Relatively low concentrations of TN commonly appeared in the mixing period.High concentrations of TOC in hot seasons(from June to September)were caused by the overproduction of algae in the surface water.At the same time,the DO in the bottom water was consumed. The TN concentration of the bottom water showed a tendency to increase,and it reached a maximum value of 2.5 mg/L before the storm period(usually in the middle of September). The TP concentration of the bottom water reached 0.12 mg/Lin September.The NH+4-N and TOC concentrations of the bottom water reached 0.52 mg/L and 3.5 mg/L,respectively. The high concentrations of TN,TP,NH+4-N,and TOC were due to release of pollutants from sediment under anaerobic conditions(Beutel et al.,2007;Wang et al.,2012a;Yang et al.,2012).

Table 1 Concentrations of TN,TP,NH+4-N,TOC,Fe3+,and Mn7+in surface,middle,and bottom water of Jinpen Reservoir

During the storm period in September,the quality of the bottom water changed with the storm runoff fowing into the reservoir.As shown in Table 1,TN,Fe3+,and Mn7+concentrations decreased from 2.5 mg/L,0.97 mg/L,and 0.34 mg/L to 1.7 mg/L,0.37 mg/L,and 0.12 mg/L,respectively.However, the concentrations of TP,NH+4-N,and TOC increased to 0.16 mg/L,0.65 mg/L,and 3.8 mg/L,respectively.

Fig.2 shows the microscopic examination results of algae, which are the mean values at four measurement times in one month in the main reservoir.Algal density started to increase from April,with the increase of water temperature and illumination intensity.Amaximumalgaldensity,2.5×106cells/L, occurred in September,and green algae accounted for 75%. After the storm period,the algal density experienced a sharp decrease.From November to April of the next year,the algal density decreased to 0.5×106cells/L,and diatoms were the dominant species.The proportion of diatoms started to decrease from 68%in December to 15%in September of the next year.Meanwhile,the proportion of green algae started to increase from December and reached a maximum of 75%in September of the following year.

3.2.Effects of different rainfall events on water quality

Field work was conducted on fve different rainfall events of 22 mm,17 mm,19 mm,107 mm,and 42 mm.Different rainfall amounts and infows corresponded to different undercurrent thickness(Table 2).The normal infow to the reservoir was below 10 m3/s.The largest rainfall of the monitoring year occurred on September 9,when the infow reached 579 m3/s,and the maximum undercurrent thickness reached 40 m.With high infow continuously fowing into thereservoir,the undercurrent thickness further expanded to 50 m on September 17.

Fig.2.Algal density and species proportions of Jinpen Reservoir.

Table 2 Five different rainfall events and corresponding infows to Jinpen Reservoir and undercurrent thickness.

The results of measurement of concentrations of pollutants at different monitoring sites are shown in Fig.3.Under normal runoff conditions,the turbidity was quite small. With rainfall of about 20 mm,the turbidity of the upstream water at S1 increased to about 160 NTU.However,under storm runoff conditions,the turbidity rapidly increased to 359 NTU at S1.Throughout the process of runoff fowing into the reservoir,the water turbidity gradually decreased. During the three small rainfall events on May 11,July 11, and August 7,the maximum TOC,TP,and NH+4-N concentrations at S1 in the upstream water were 5.1 mg/L, 0.072 mg/L,and 0.72 mg/L,respectively.With the infow to the reservoir,the concentrations of pollutants decreased gradually.At S5 in the main reservoir,the maximum TOC, TP,and NH+4-N concentrations decreased to 4.2 mg/L, 0.036 mg/L,and 0.56 mg/L,respectively.These concentrations of pollutants and the turbidity at S5 were at the normal levels.The main reservoir near the intake tower was nearly unaffected.

Under storm runoff conditions,the infow to the reservoir reached 579 m3/s on September 9,and,at the same time,the turbidity in the upstream water increased to 359 NTU.The upstream infow water quality also deteriorated.The concentrations of TOC,TP,and NH+4-N upstream increased to 7.2 mg/L,0.2 mg/L,and 1.2 mg/L,respectively.The storm runoff signifcantly infuenced the intake tower area,where the corresponding turbidity at S6 was 139 NTU,and the concentrations of TOC,TP,and NH+4-N increased to 4.5 mg/L, 0.076 mg/L,and 0.62 mg/L,respectively.

The source water of the Jinpen Reservoir is the rain fowing through mountains,jungles,and farmland.During the storm, large amounts of pollutants carried by the storm runoff fowed into the river and eventually into the reservoir.The reservoir water quality is greatly infuenced by the storm runoff.

Fig.3.Pollutant concentrations at different sites under different runoff conditions.

3.3.Effects of storm runoff on water quality

3.3.1.Effects of storm runoff on stratifcation

Before the storm on September 6,the Jinpen Reservoir was still in a stable stratifcation state.As shown in Fig.4,the temperature difference between the surface and bottom water wasabout12°C.Verticalturbiditywasmaintainedat4-6NTU. The bottom water was in an anaerobic state.

In general,the temperature of infowing water was higher than that of the bottom water and lower than that of the surface water of the main reservoir,so the infowing water formed an undercurrent.If the amount of the undercurrent was small,it would mix with the water in the river before fowing into the reservoir.On September 9,the rainfall was 107 mm,and the infow was 579 m3/s.The undercurrent that fowed into the main reservoir was 40 m thick(Table 2).The vertical water column in the reservoir started to mix with the undercurrent, because the undercurrent speed was slowed down.As shown in Fig.4,the vertical water temperature difference was rapidly compressed.In the process of mixing,the water temperature difference continued to decrease.On September 27,the temperature difference was reduced to 4°C,and the water column was in a state of mixing.Thus,the stratifed structure of the main reservoir was damaged.

3.3.2.Effects of storm runoff on DO concentration, turbidity,and pH value

After the storm,the DO concentration of the bottom water (at a depth of 70 m)in the main reservoir quickly increased to 9.2 mg/L on September 9.This was because of the high concentration of DO in the undercurrent;it reached nearly 10 mg/L on September 17.Due to the high concentration of TOC(Fig.3),the DO concentration of the bottom water decayed rapidly.DO was quickly consumed by the easily decomposed organic matter from mountains and jungles.On September 27,the DO concentration of the bottom water decayed to 6.6 mg/L.

After the storm,the subsurface fow with a high turbidity of 264NTUstartedtomixwiththereservoirwater,andtheaverage turbidity of the water column increased from 5 NTU on September6to131NTUonSeptember9.OnSeptember17,the mixing process of the undercurrent and the reservoir water was nearlycompleted,andtheaverageturbidityofthewatercolumn increased to 65 NTU.However,on September 27,the average turbidity of the water column decreased to 30 NTU.

After the storm,the pH value of the bottom water experienced an increase from 7.29 on September 6 to 8.62 on September 17,and then decreased to 8.46 on September 27, showing the same trend as the DO concentration and turbidity.

Fig.4.Vertical variations of water temperature,turbidity,DO concentration,and pH value in main reservoir.

3.3.3.Effects of storm runoff on nutrient salts,organic matter,and metals

In the main reservoir(S5),pollutant concentrations at different depths were examined before the storm on September 6 and during the storm on September 9(Table 3).

The average concentrations of TOC at sampling depths below 40 m increased to 3.75 mg/L.The dissolved organic carbon (DOC)concentration ofthe bottom wateron September 9 was also measured,and the value 3.85 mg/L was almost the same as that of TOC(3.90 mg/L).It was obvious that soluble small-molecule organic matter accounted for most of the TOC.This fact provided an explanation for the rapid consumption of DO in the bottom water after the storm.

The concentrations of Fe3+and Mn7+in the bottom water decreased from 0.97 mg/L and 0.32 mg/L on September 6 to 0.38 mg/L and 0.18 mg/L on September 9,respectively.Since there was no artifcial metal pollution source in the upstream region of the Jinpen Reservoir,the metal concentrations of the storm runoff were quite low.Undercurrent with low metal concentrations mixed with the water with high metal concentrations,causing the decrease of the concentrations of Fe3+and Mn7+.The metal ions that were released from the sediment were in a reduction state.These ions were further oxidized by the undercurrent with a high DO concentration. Thus,sedimentation of oxides was also a reason for the decrease.

3.3.4.Effects of storm runoff on algae

Before and during the two light rains on July 11 and August 7 and the storm on September 9,the algal density was analyzed through themethod ofmicroscopicexamination,andtheresults are shown in Fig.5.After the rains on July 11 and August 7, the algal density decreased from 1.9× 106cells/L and 2.0×106cells/L to 1.4×106cells/L and 1.6×106cells/L, respectively.A slight increase of diatoms and a small decrease of green algae were observed.The colder infow from the upstream and the weak illumination intensity might be the main reasons.

Fig.5.Algal density and algal species proportions before and after rainfalls.

However,the decreased amplitude of algal density during the storm reached 51.3%,which was greater than that during the light rainfalls.As demonstrated in Table 4,after the storm,the quality of surface water was improved.The concentrations of nutrient salts and organic matter were quite low. This might explain why algal blooms in the surface water were not observed in the next ten days.After the storm,the proportion of green algae was still above 50%,accounting for the majority of the algae.

4.Solutions to water problems

4.1.Seasonal water quality problems

In the stratifcation months,especially from June to September,the overproduction of algae in the surface water increases the cost of the water treatment plants.The bottom water is severely polluted by the pollutants released from the sediment and the sink of suspended organic particles.The intake height adjustment is used as a method to avoidsupplying the surface water and bottom water.Another method is to destroy the water stratifcation and thus inhibit the release of pollutants from sediment.Awater-lifting aerator is an effective technology,solving the seasonal water quality problems for deep-water reservoir.During the water-lifting aerator operation,a strong disturbance of water will destroy the stratifcation and the DO concentration in the bottom water will increase,thus inhibiting release of pollutants from sediment.Furthermore,algae production will be controlled,as algae are pushed into the water below(Miu and Yin,2007; Cong et al.,2009,2011).

Table 3 Pollutant concentrations at different depths before and during storm in main reservoir.

Table 4 Quality of surface water after storm on September 9.

4.2.Storm runoff problems

During the storm period,the turbidity of the outfow reached levels as high as 130 NTU,which signifcantly exceeded the turbidity limitation provided inEnvironmental Quality Standards for Surface Water(GB3838-2002)of China.On the other hand,the source water with high turbidity greatly increased the cost of water treatment.The water quality of the effuent of the water treatment plant was also unstable and worse under the storm runoff conditions than under the normal runoff conditions.

Storing clean water,releasing turbid fow,and dredging are considered effective methods of solving storm runoff problems (Wang and Hu,2009).The strategy of storing clean water has been effectively applied to the Sanmenxia Reservoir(Wang et al.,2005).Releasing turbid fow has become the main strategy of controlling sedimentation in the Xiaolangdi Reservoir.The strategy of dredging has been used in the Hengshan Reservoir and Zhuwo Reservoir(Wang and Hu, 2009).Nonetheless,the fundamental method of preventing soil erosion in upstream areas and the surrounding environment is improving vegetation coverage,changing farming practices,changing land use patterns,and cultivating plants with well-developed roots(Wilkinson,1999;Reubens et al., 2007;Li et al.,2011;Nunes et al.,2011).

5.Conclusions

Long-duration anaerobic conditions in the bottom water caused by stratifcation can lead to seasonal variation of water quality in the Jinpen Reservoir.During the stratifcation period,the bottom water is polluted by the pollutants released from the sediment.However,as a canyon-type reservoir,the Jinpen Reservoir is sensitive to storm runoff.Under normal runoff conditions,the water quality of the infow is better than that of the main reservoir.However,during storm periods,the stratifcation is destroyed and the water quality is severely polluted by the storm runoff.Particulate nitrogen,phosphorus, and soluble organic molecules are brought into the reservoir by the infow,leading to higher concentrations of TN,TP,and TOC under storm runoff conditions than under normal runoff conditions.Understanding the variation of water quality and the impact of rainfall runoff on reservoir water quality can provide theoretical guidance for water managers to ensure outfow water quality during the fooding season.Thus,for reservoir managers,it is extremely important to control the water quality at all times during the storm.

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Received 14 May 2015;accepted 25 September 2015

Available online 17 December 2015

This work was supported by the National Science and Technology Pillar Program(Grant No.2012BAC04B02)and the National Natural Science Foundation of China(Grant No.51478378).

*Corresponding author.

E-mail address:huangtinglin@xauat.edu.cn(Ting-lin Huang).

Peer review under responsibility of Hohai University.

http://dx.doi.org/10.1016/j.wse.2015.12.003

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/).