Geochemistry and sediment in the main stream of the Ca River basin,Vietnam:weathering process,solute-discharge relationships,and reservoir impact

Ho Thi Phuong·Kenji Okubo·Md.Azhar Uddin

Abstract In this study, we investigated the chemical composition of dissolved solids in the Ca River basin,North-Central Vietnam. Water samples were collected from August 2017 to July 2018 at three hydrological stations located in the main stream of the Ca River.Carbonate weathering was found as the dominant process controlling the water chemistry in that area.The average concentrations of dissolved solids generally decreased from upstream to downstream,resulting in low concentrations of the major ions in the downstream basin.Variations in the concentrations of major chemical ions and suspended solids at discharge were also investigated.Major chemical weathering products were found to behave chemostatically with increasing discharges upstream.However,dilution behaviors of solutes were shown in both midstream and downstream. Primary evidence shows that water storage in reservoirs impacts a variety of suspended solids and dissolved solids in the Ca River.

Keywords Ca River·Dissolved solids·Geochemistry·Carbonate weathering·Suspended solids

1 Introduction

The natural chemical compositions and transport fluxes in rivers depend on multiple environmental factors such as sources(lithosphere,atmosphere,biosphere),sinks(vegetation uptake,settling),rate-controlling factors(temperature,water circulation),and drainage basin area(Meybeck 1994；Milliman and Farnsworth 2011).Atmospheric pollution and human activity can have significant effects on the natural geochemistry of river basins(Chetelat et al.2008；Li et al.2009；Li and Zhang 2008；Roy et al.1999).Detailed geochemical studies have quantified major ion compositions(Li and Zhang 2008；Maharana et al.2015),weathering processes(Chetelat et al.2008；Sarin et al.1989),long-term fluxes(Negrel et al.2007；Sarin et al.1989),and controlling factors in solute exports from various scales of basins(Godsey et al.2009；Musolff et al.2015).Some have researched natural river geochemistry,where basins with minimal human activity—such as hilly headwater basins(Bruijnzeel 1983)and unpolluted or lesspolluted river basins(Meybeck 1994)—were considered.However,establishing natural background values is challenging because most major rivers are already polluted or exposed to long-range transport of atmospheric pollutants(Meybeck and Helmer 1989).

Rivers in Vietnam,like many other rivers around the world,have been impacted by economic development.Reservoirs of various sizes have been constructed along the rivers for power generation,water supply,and flood control(Amos et al. 2017). In addition, other anthropogenic activities(e.g.,intensive agriculture,land-use change,and industrial development)can have significant impacts on how natural river materials move.However,geochemical data for Vietnamese rivers are sparse.Thus,we investigated the geochemistry of the Ca River,one of the large basins in north-central Vietnam, covering an area of 27,200 km2.Major ion chemistries were determined at sites upstream of the reservoirs,which were primarily under forest cover,and at sites mid-and downstream,below the reservoirs.Weathering processes controlling the major geochemistry were also determined.Additionally,variations in the concentrations of major chemical ions and suspended solids in the discharge were investigated.

2 Study area

The Ca River is an international river located between 18°15′00′′N to 20°10′30′′N and 103°45′20′′E to 105°15′20′′E (Fig.1). The basin covers 27,200 km2,including 17,730 km2in Vietnam's territory and 9470 km2in Laos.The main river originates from Mt.Muong Khut and Muong Lap (1800—2000 m) in Laos, flows from northwest to southeast,enters into the Nghe An,Thanh Hoa,and Ha Tinh provinces of Vietnam,and flows out to the Eastern Sea at the Hoi estuary.Total river length is 531 km,of which 170 km runs through Laos and 361 km is in Vietnam.Forests in the Ca River basin are largely located upstream of three Laos provinces(Bolikhamxay,Xieng Khouang,and Houaphanh).In Vietnam,forests are concentrated north,northwest,and southwest of the basin at elevations of 150—1500 m(IWRP 2012).The catchment is covered primarily by forest(44%)and agricultural crops(18%)(Chikamori et al.2012).Area soils are formed from parent rocks,Ferralsol in particular(83.51%)(IWRP 2012；Nauditt and Ribbe 2017).Other soil types are Fluvisol and Acrisol.The Ca River basin is located in a monsoon climate,and rainfall is distributed over the year,which has two distinct seasons:the dry season and the rainy season.In the upper reaches of the river,the rainy season is from May to October,but downstream,it is from June to November.Average annual precipitation in the basin is 1100—2500 mm.

Fig.1 Map of the Ca River basin

Multiple reservoirs have been constructed in the Ca River basin,namely Ban Ve,Khe Bo,Thac Muoi,and Sao reservoirs.These multi-purpose reservoirs are used for hydropower generation,water supply,irrigation,and flood and drought control.Ban Ve is the largest among them and is located at approximately 100 m elevation.Its gross capacity is 1835×106m3,and its effective capacity is 1383×106m3.

Samples were obtained from the main stream of the Ca River at My Ly (104°18′54′′E and 19°36′51′′N), Dua(105°02′20′′E and 18°59′20′′N), and Yen Thuong(105°23′00′′E and 18°41′10′′N). My Ly is located approximately 215 m above the Ban Ve reservoir(70 km).It covers 1190 km2or 4.4%of the total drainage basin.Located in the high mountains and covered primarily by forest,the My Ly basin is sparsely populated and is subject to little human impact.Both Dua and Yen Thuong are downstream of all reservoirs,covering 20,800 km2and 23,000 km2(76.5% and 84.6% of the river basin),respectively. Both are strongly influenced by human activity(e.g.,agriculture and mining activities).Additionally,water storage and reservoir operations impact the Ca River and its resources.

3 Sampling and analytical methods

From August 2017 to July 2018,121 water samples were collected from three hydrological stations in the main stream of the Ca River basin.During the flood season,water samples were collected four to six times monthly,and during the dry season,they were collected one to four times monthly.Samples were representative of the range of discharge rates.

Each sample of 2 L,collected from an average depth of 10 cm at the river bank,was placed in high-density polyethylene containers then immediately cooled and maintained at a low temperature until analysis.Concentrations of the major cations(Ca2＋,Mg2＋,Na＋,and K＋),anions,and dissolved silica(SiO2)were determined in the laboratory.Cation and most anion concentrations were found using ion chromatography(Shimadzu,Japan).Dissolved silica and phosphate were measured using a DR900 (HACH) colorimeter. Water samples were filtered through pre-washed 0.45-μm Millipore membrane filters before ion concentrations were determined.Additionally,the concentration of suspended solids was determined by filtering 100 mL sample through a 0.45-μm membrane filter(Whatman).Finally,the total concentration of all dissolved solids was the sum of the major elements plus dissolved silica.

4 Results and discussion

4.1 Contributions of chemical compositions

The concentration of each major chemical in the main stream of the Ca River basin is given in Table 1.Total dissolved solids (TDS) varied from 77 to 205 mg/L,averaging 144 mg/L, higher than the world average,100 mg/L(Milliman and Farnsworth 2011).The average value for TDS is comparable to that reported for the Hong(176 mg/L)in Vietnam,but it is lower than that reported for the Son in India(227 mg/L)and the Upper Han River in China(248 mg/L)(Li and Zhang 2008；Maharana et al.2015；Moon et al.2007).Compared with the rivers draining in areas dominated by silicate rock,such as the upper Ganjiang(63 mg/L)and rivers in the Southeast Coastal Region of China(75.2 mg/L),TDS in the Ca River is much higher(Liu et al.2018；Ji and Jiang 2012).However,it is much lower compared to the Huanghe(557 mg/L)and Tarim(1000 mg/L),both of which drain in areas dominated by evaporite dissolution(Fan et al.2014；Xiao et al.2012).Bedrock lithology plays a critical role in controlling the character and quantity of the total delivered(Meybeck 1987；Milliman and Farnsworth 2011).Human activity also significantly influences TDS values[e.g.,in the Liao River of China(400 mg/L)and several European rivers(Ding et al.2016；Milliman and Farnsworth 2011)].

The total cationic charge (Tz＋=2Ca2＋＋2Mg2＋＋K＋＋Na＋) ranged from 1145—2981 μeq/L, averaging 2119 μeq/L. The total anionic chargeranged from 811 to 2271 μeq/L,averaging 1567 μeq/L.The extent of Tz＋-Tz-charge imbalance,characterized by the normalized inorganic charge balance[NICB= （T z＋-Tz-）/Tz＋],is related to the contributions of other anions(Li et al.2009；Ji and Jiang 2012).

Calcium was the dominant cation,ranging in concentration from 381 to 1005 μM,accounting for an average of 62.0%of the total cation charge.Magnesium followed,ranging from 102 to 368 μM,accounting for an average of 23.2%of the total cation charge,then sodium(12.3%)and potassium(2.4%)followed.Bicarbonate was the dominant anion,ranging from 600 to 2039 μM,accounting for 84.4%of the total anion charge.Chloride and sulfate together comprised 12.6%of the total anion charge in nearly equal proportions.Nitrate and phosphate contributed negligible proportions(2.7 and 0.3%,respectively).

Dissolved Si ranged in concentration from 133 to 250 μM,averaging 202 μM,comprising an average of 8.6%of the TDS.It ranked third in abundance after HCO-3(55.8%)and Ca2＋(18.3%).Compared to other major ions,dissolved Si was relatively independent of lithology,remaining constant between 80 and 160 μmol/L SiO2(except in streams draining volcanic rocks,where it reaches 350 μmol/L SiO2)(Meybeck 1987).Dissolved Si content is,however,controlled by the climate(temperature and rainfall)(White and Blum 1995).

4.2 Spatial and seasonal variations of major solutes and suspended solids

The total concentrations of major solutes at My Ly ranged from 142 to 205 mg/L,averaging 168 mg/L.At Dua,TDS varied from 85 to 171 mg/L,averaging 138 mg/L.On the other hand,the TDS value at Yen Thuong ranged from 77 to 167 mg/L,averaging 128 mg/L.Generally,the total solute concentration decreased from upstream to downstream,consistent with decreased concentrations of the major ions Ca2＋,Mg2＋,Na＋,Cl-,and SiO2in the downstream basin.The observed spatial variations in solute concentrations in the main stream of the Ca River could be related to the effects of tributary inflows,resulting in dilution.A downstream trend in K＋concentration was not significant,reflecting its conservative behavior in the basin.An increase inconcentration in the downstream basin might have been from anthropogenic sources such as agricultural activity and untreated sewage(Maharana et al.2015).

The study period was divided into two sub-periods:the wet season(June to November for Dua and Yen Thuong and May to October for My Ly)and the dry season(December to May and November to April, respectively)(Fig.2).Trends in seasonal solute variations were similar between Dua and Yen Thuong.The concentrations of almost all tested ions increased from the wet to dry seasons,indicating the dilution effect of greater atmospheric precipitation during the rainy season.For that reason,at Dua and Yen Thuong, the total solute concentration increased from the wet to dry seasons.At My Ly,however,TDS decreased slightly from the wet to dry seasons,a result of decreased concentration in the major ionOther elements remained constant when comparing the two seasons. Phosphate concentration increased in the wet season at all stations,indicating the contribution of organic matter degradation(at My Ly)or human activity(at Dua and Yen Thuong).

Normally, suspended sediment concentration (SSC)decreases from upstream to downstream and from the wet to dry seasons(Fig.2).In our case,it greatly varied frommonth to month(Fig.3),showing the opposite trend of that seen for solute concentration, except at My Ly (for example,in June and July).The highest SSCs occurred during September,October,and July at Dua and Yen Thuong(135—317 mg/L).Peak SSC occurred in June at My Ly(1534 mg/L),indicating significant erosion in the upper basin due to heavy rain.

Table 1 Chemical compositions of the rivers in the Ca River basin

Table 1 continued

Table 1 continued

Fig.2 Seasonal variations in the concentration of major ions(μM),TDS,and SS(mg/L)

Fig.3 Monthly variations in the concentration of TDS and SS(mg/L)

4.3 Weathering processes controlling the major ion chemistry

Fig.4 The Gibbs graph of Ca River between the ratio of Na/(Na+Ca)and total dissolved solids

The three primary lithologies undergoing chemical weathering were silicates, carbonates, and evaporites(Gaillardet et al.1997).Carbonate weathering(calcite,dolomites) produces Ca2＋, Mg2＋, and. Silicate weathering results in,Na＋,K＋,Mg2＋,Ca2＋,and SiO2.Evaporite dissolution results in Na＋,K＋,Cl-,and(Han and Liu 2001；Meybeck 1987；Sarin et al.1989). The relationships between(Ca2＋＋Mg2＋)/total cations,and(Na＋＋K＋)/total cations for the Ca River were determined to evaluate the contribution of rock weathering.The Ca2＋andfor most samples fell along a 1:1 equiline(Fig.5a),implying that carbonates dissolution dominates in this drainage basin(Roy et al.1999).Theis slightly more enriched than Ca2＋at My Ly,possibly consistent with a silicate weathering source for some of this anion(Holland 1978).The scatter plot (Fig.5b) of （Ca2＋＋Mg2＋) against total cations indicates a significant contribution of these two cations(approximately 85%of the total).Any deviation of（Ca2＋＋Mg2＋)from the 1:0.85 line was attributed to increasing proportions of Na＋＋K＋)(Fig.5b,c),implying contributions from silicate weathering or evaporite dissolution(Li and Zhang 2008；Maharana et al.2015).The correlations among geochemistry parameters and their correlations with suspended solids were investigated for each station (Table 2). The significant relationships between Ca2＋/Mg2＋, Ca2＋/, Mg2＋/,Ca2＋/TDS,Mg2＋/TDS,and/TDS at all stations support the finding of carbonate weathering dominance in the Ca River basin.Significant correlations between SiO2and Na＋,K＋,orwere not found,implying minimal silicate weathering. However, moderate correlations between Cl-,, Na＋, K＋, and TDS were found,indicating that evaporite weathering was the source(Li and Zhang 2008).

The Piper diagram is widely applied to determine the classification of water on the basis of its chemical character(Maharana et al.2015；Negrel et al.2007；Ji and Jiang 2012).The triangular cationic fields of the Piper diagram(Fig.6)revealed that most of the water samples fell into the Ca2＋field,whereas in the anion triangle,the majority fell into thefield.Chemical data plotted on the diamond-shaped central field revealed the dominance of the Ca2＋-type.Therefore,Ca2＋-is the dominant hydrogeochemical species in the Ca River basin.The Piper diagram for our Ca River samples is similar to that for the Son River(Maharana et al.2015)and the upstream Huanghe River(Fan et al.2014),dominated by carbonate weathering.However,it differs from the midand downstream sections of the Huanghe River(Fan et al.2014),where evaporite dissolution dominates.They also differ from the Liao River,Daling River,Hun-Tai River(Ding et al.2016),Ou River,and Min River(Liu et al.2018), where silicate weathering dominates. In many watersheds around the world,carbonate weathering play an important role in controlling river water chemistry because carbonate is more susceptible to weathering than silicate(Fan et al.2014；Roy et al.1999).

Fig.5 Scatter plots between a Ca2+and ,b Ca2+,Mg2+,and total cations,and c Na+,K+,and total cations

4.4 Variations in major solute concentrations at discharge

The relationships between solute concentration and its discharge were investigated at each hydrological station.Concentrations of the weathering-derived solutes were plotted against instantaneous discharge on logarithmic axes(Fig.7),yielding the approximate relationships between concentration and discharge using a power law(Baronas et al.2017；Herndon et al.2015；Musolff et al.2015):

where C is solute concentration,Q is discharge,and a and b are constants.The exponent b is the slope of the C-Q relationship on logarithmic axes.When concentration does not change with changing discharge,the relationship is said to be chemostatic(Moatar et al.2017；Musolff et al.2015).When this is the case,b is between-0.1 and 0(Herndon et al.2015；Hunsaker and Johnson 2017).When discharge increases,solute concentrations can either increase(enrichment behavior,b ＞0)or decrease(dilution behavior,b＜-0.1),and the relationship is said to be chemodynamic(Herndon et al.2015).

Our results show that the upstream catchment(My Ly)behaved chemostatically for the major chemical weathering products except for Na＋(b=-0.11),(b=0.11),and(b=0.15).The large store/high production rate of weathering products can lead to chemostatic behavior for major ions(Musolff et al.2015).Nutrient increases with increasing discharge at My Ly indicated the source was organic matter degradation in the forest area.Similar trends for the major ions at discharge were seen at Dua and Yen Thuong.Negative slopes were found for Ca2＋,Mg2＋,and(b=-0.12 to-0.28)with low variability(R2≥0.5)and for Na＋and(b=-0.13 to-0.22)with moderate variability(R2≥0.3).The concentrations ofandwere constant or increasing,indicating enrichment in nutrient sources in the basin(Musolff et al.2015).Meanwhile,K＋and Cl-behaved chemostatically,increasing in the discharge at all basins, indicating important biogeochemical influences(Saunders and Lewis 1989) or exogenous sources (atmospheric deposition)(Musolff et al.2015).For the major solutes(Ca2＋,Mg2＋,Na＋,and),the dilution slopes at the downstream station were greater than those found midstream,indicating that catchments with higher water yields were more likely to express dilution(Moatar et al.2017).The negative correlation between major elements and runoff increased(higher R2values)with decreasing elevation(Torres et al.2015).Less-reactive elements(Ca2＋,Mg2＋,andexhibited strong correlations with discharge and indicated various degrees of dilution with increasing runoff(Baronas et al.2017).However,low correlations found forCl-,and K＋reflected biological processes,atmospheric input, or anthropogenic impacts (i.e., fertilizer application)(Bluth and Kump 1994；Moatar et al.2017).

Fig.6 Piper trilinear diagram of Ca River water in comparison with other river basins(data from Ding et al.2016；Fan et al.2014；Liu et al.2018；Maharana et al.2015)

4.5 Primary evidence of reservoir impact on concentrations of suspended solids and dissolved solids

Located above the largest reservoir of the Ca River basin,My Ly is significantly influenced by the operation of the Ban Ve reservoir.The impact of water storage at the Ban Ve reservoir on the correlation between SSC and discharge at My Ly is shown in Fig.8a.In August and September(the rainy season),SSC increased with increasing discharge.Water storage starts at the end of the rainy season(October),leading to increased water levels behind the reservoir.The water level was raised until it was over the My Ly station.Consequently,runoff was delayed during November and December.As a result,the SSC gradually decreased as suspended sediment settled,and sediment suspension was deferred during this period.The reservoir opened at the beginning of January,but SSC remained low until March because it was the dry season.Rain in April and May can wash out the settled sediment surrounding the upper part of My Ly,resulting in the steep increases in SSC.Thereafter,SSC increases with increasing discharge in June and July,consistent with increases seen in August and September.

Dua and Yen Thuong,located in the lower part of the main reservoirs in the Ca River basin,showed similar SSC behaviors in their discharges(Fig.8b,c):decreasing SSCs with increasing discharge when comparing the rainy to dry seasons.This reflects the impact of suspended sediment settling in the reservoirs.Upstream sediment erosion also influenced that seen downstream；this is implied by the high SSC found in May.Generally,SSCs are positively correlated with streamflow(Moore and Anderholm 2002).However,reservoir operations lead to a variety of SSC behaviors in the discharge.

Fig.7 Variation of river elements concentration with discharge at My Ly(a,b),Dua(c,d),and Yen Thuong(e,f)

Dam closure not only affects suspended sediment but also river water quality(Muigai et al.2010；Castilla-Hernandez et al.2014).Variations in solute concentrations with discharge at My Ly were quite different from the fluxes with discharge at Dua and Yen Thuong.Differences could be the results of the impacts of the reservoir.During the wet season,when water is stored at My Ly,most ion concentrations were less variated than in other periods.When the reservoir was open,ion concentrations increased with discharge during the dry season until the highest concentration was reached at the first flash flood on 17 June 2018.Since then,ion concentrations decreased due to dilution.However,inverse correlations between ion concentrations and discharges were seen at Dua and Yen Thuong,indicating dilution effects(Zhang et al.2015).When discharge exceeded 1000 m3/s,ion concentrations decreased sharply,for example,on 17 December 2017,8—12 October 2017,and 22 July 2018.The correlations between TSS and geochemical parameters are shown in Table 2 and indicate that suspended solids are negatively correlated with all ions(exceptand)and TDS at Dua and Yen Thuong.This is common with increases in sediment, whereas chemical ions become diluted with increasing discharge. In contrast, positive correlations between TSS andand TDS were identified at My Ly.These results imply a significant geochemical impact of water storage in the My Ly basin.When the water level at My Ly rises,My Ly can act as a reservoir,resulting in sediment suspension and sediment settling.Dam closures lead to increased water residence time, causing increased interaction between sediment particles and solutes.Dam closures also change the environmental water conditions(low oxygen content and high temperature),favoring biogeochemical processes(Moatar et al.2017).Muigai et al.(2010)indicated that the reservoir leads to decreases in dissolved oxygen content due to eutrophication and increases the TDS value.Moore and Anderholm(2002)found that,in downstream reservoirs,TDS concentrations increase from evapotranspiration while nutrient concentrations decrease from settling and nutrient uptake,and SSCs decrease due to settling.In this study,it was not easy to assess the impact of the reservoir on the downstream basin because TDS values showed small seasonal variations. Moreover, the TDS concentration and composition were significantly influenced by multiple factors such as human activity,groundwater discharge,and tributary inflow.Therefore,the impact of dam closure on TDS in the downstream basin requires further investigation.

5 Conclusions

From 121 Ca River water samples collected from August 2017 to July 2018 at three stations,cations(Ca2＋,Mg2＋,Na＋,and K＋),anions(,Cl-,anddissolved silica,SSCs,and TDS values showed that carbonate weathering is dominant.Bicarbonate and calcium are the prevailing chemical species,accounting for 84.4 and 62.0%of the total anionic and cationic charges,respectively.The TDS values varied from 77 to 205 mg/L,averaging 144 mg/L and decreasing in the downstream basin with decreases in major solute concentrations.

The relationships between solute concentrations and discharge show that the upstream catchment (My Ly)behaves chemostatically for the major chemical weathering products.Similar negative trends for Ca2＋,Mg2＋,Na＋,andwith discharge at Dua and Yen Thuong indicate dilution behaviors for those ions.The constant or increasing ion concentrations ofandin the three basins reflect enrichment of nutrients from organic matter degradation or human activity.

Relationships between SSC and discharge show that the reservoirs along the Ca River trigger sediment suspension and sediment settling at the My Ly station and decreasing SSCs at the Dua and Yen Thuong stations.Delays in water flow can lead to greater interactions between sediment and solutes and can allow biogeochemical processes that result in unusual changes in the concentrations of major ions and TDS with increasing discharges at My Ly.

Fig.8 Variation of suspended sediment concentration with discharge at My Ly(a),Dua(b)and Yen Thuong(c)

AcknowledgementsThis research was financially supported by the Japan Society for the Promotion of Science(Grant no.R11604).The authors would like to express their deep appreciation to Dr.Mitsuyo Saito of Graduate School Environmental and Life Science,Okayama University for her helpful comments on the manuscript and kind support through the research process.The authors also thank staffs of North-Central Hydro-meteorological Centre,Vietnam for allowing us access to their gauging records.An anonymous reviewer is thanked for critically reading the manuscript and suggesting substantial improvements.