Formation of the River Water Chemistry in the Middle Section of Dousitu River , China

On basis of hydrogeology of the study area, the characteristics of chemistry of river water and groundwater were analyzed. Analysis results of three river water samples (B3, B4 and B5) collected in the middle section of Dousitu River show the TDS increases from B3 to B4 and decreases from B4 to B5. The concentrations of Cl, Na, K, Mg and HCO3 have a similar change with TDS, but the concentrations of Ca and SO4 2increase steadily along flow path. The chemical types of the river water change from HCO3•Cl-Na to Cl•SO4•HCO3-Na, and finally to SO4•Cl-Na. The causes of these changes are analyzed and studied in depth using various methods. Results indicate river water evaporation, dissolution/precipitation of minerals, cation exchange and mixing of different waters all play important roles in the formation of the river water chemistry. But in different sections of the river, the main processes are different. The study shows that when a reservoir is built in arid areas, the loss of water resources due to evaporation is huge, which can lead to the degradation of water qualities. Hence, great care should be taken to build a reservoir in arid or semi-arid regions.


Introduction
The formation of river water chemistry has always been the hot topic in hydrology and hydrogeology.It is of great importance to study the formation and evolution of river water chemistry for the purpose of river water quality protection and river ecology protection.Many hydrologists and hydrogeologists around the world have done some significant studies in this interesting and important field.Ronald J. Gibbs 1 studied the water chemistry data of the Amazon River in 1972.In his study, he recalculated the mean dissolved salts concentration.The calculated results showed that the mean dissolved salts concentration is 11 percent lower than previous calculations for South American rivers and 2.6 percent lower than previous calculations for the rivers of the world.Rothwell et al. 2 studied the spatial and seasonal patterns of river water chemistry at approximately 800 sites in Northwest England and found that there is a clear distinction between the uplands and lowlands in water quality.Upland waters are acidic and have low concentrations of base cations, explained by background geological sources and land cover, while in the lowlands, both past and present human activities have a major impact on river water chemistry.Hein H. van Kleef et al. 3 studied the changes caused by reduced acidifying deposition in 68 Dutch moorland pools in 2010.Smolders et al 4 revealed the intra-annual variability of the major ion composition of the Pilcomayo River, and its relationship to discharge.There are many more scientists [5][6][7][8][9] paying their attention to the river chemistry study from various aspects all over the world.
Dousitu River is one of the tributaries of the Yellow River.The source of the river is near Etuok County of Inner Mongolia, China.It flows from east to west and intersects the Yellow River near Xinjian Yidui (Figure 1).The length of the river is 166 km, and the drainage area is 4160 km 2 .The average annual 10 runoff of the river is 1264×10 4 m.Location of the study area and the sampling position The Dousitu River drainage basin is situated in an arid area of north-western China.The main characteristics of the climate are a long and cold winter as well as a short and warm summer.According to the monitoring data in Etuok County, the annual temperature in the area changes from 5.3 to 8.9 o C and has an average value of 6.9 o C. The precipitation mainly occurs between June and August.The annual precipitation ranges from 125.2 mm to 611.6 mm, and has an average value of 267.5 mm.Evaporation is intensive, and the intensity is between 1947.7 and 2494.9 mm.
In the middle section of Dousitu River (B 3 to B 5 in Figure 1), the changes of the chemistry of river water are interesting.The TDS of the river water increases from B 3 to B 4 , but decreases from B 4 to B 5 .The concentration of Cl -, Na + , K + , Mg 2+ and HCO 3 -has a similar change with TDS, but the concentration of Ca 2+ and SO 4 2-increases steadily from B 3 to B 5 .Accordingly, the chemical type of river water changes from HCO    What are the reasons for the changes of the chemistry of the river water?What recharge and discharge relationship between river water and groundwater can be drawn from these changes?This paper gives a detailed analysis.

Geology and Hydrogeology
In recent years, much research work has been done in Ordos Cretaceous Artesan Basin (CAB) [11][12][13][14] .Based on the hydrogeolology, recharge and discharge characteristics of groundwater, CAB can be divided into 5 groundwater flow systems 15,16 .Dousitu River drainage basin is one of them.According to the lithology and geological age of the strata, the aquifers in the Dousitu River drainage basin can be divided into four groups [16][17][18] .From the top down, they are the Cenozoic group, the Luohandong group, the Huanhe group and the Luohe group, respectively.There is no continuous aquitard between these aquifers 18,19 .
The Cenozoic group is composed of Tertiary and Quaternary strata.The lithology of the Quaternary stratum, which covers the whole studied area, is mainly composed of fine sand of wind, flood or river deposition, with a light yellow or grey color.The lithology of the Tertiary strata is mainly sandstone and mudstone, rich in gypsum and other easily dissolving salts.It occurs primarily in the western part of the studied area (Figure 1).
The Luohandong group occurs mainly in the western part of the studied area (Figure 1).In most parts of the area, it is overlain by Tertiary stratum.It is a set of continental clastic rocks of early Cretaceous age.The maximum thickness of the formation in the region is more than 200 m.The lithology is primarily sandstone, coarse sandstone interbeded by fine sandstone, conglomerate, and sandy gravel.
The Huanhe group exists in the whole studied area (Figure 1).In the eastern part, apart from a few outcrops, the group is overlain by Quaternary stratum.In the western part, the Huanhe group is overlain by the Luohandong group.The Huanhe group is also a set of continental clastic rock formed in the early Cretaceous period.The maximum thickness of the group in the region is more than 500 m.The lithology is primarily conglomerate, sandy gravel, sandstone and becomes finer downward.
The Luohe group does not outcrop in the studied area.It is overlain by the Huanhe group in the whole region.The lithology of the group is mainly continental clastic rock of sandstone, mudstone, conglomerate and sandy gravel formed in the early Cretaceous period.The maximum thickness of the group in the region is more than 300 m.
In the Dousitu River drainage basin, groundwater is mainly recharged by precipitation.In the horizontal direction, the groundwater in all the aquifers of the north bank flows from north-east to south-west, and the groundwater in all the aquifers of south bank flows from south-east to north-west 20 .In the vertical direction, the level of shallow groundwater in eastern part of the study area is higher than that of deep groundwater, so the water flows downward, whereas the level of deep groundwater in the western part of the study area is higher than that of shallow groundwater and the water flows upward.The discharge of groundwater in the region is mainly to the Dousitu River, pumping for irrigation, and drinking as well as the outflow to western boundary.

Chemical characteristics of river water and groundwater
To study the chemistry of river water and groundwater, 22 water samples including 5 river water samples and 17 groundwater samples were collected (in September 2004) for chemical analysis.The sampling location of these water samples is shown in Figure 1.The water samples in Table 1 are put along the flow direction.For example, the water samples before B 1 in Table 1 are in the upper part of B 1 , the water samples between B 2 and B 3 are in the upper part of B 3 and lower part of B 2 .
Figure 2 shows the changes of TDS and concentrations of major species in river water along the flow path.It can be seen from the figure that the TDS and the concentrations of major species in river water remain basically unchanged from B 1 to B 3 .From B 3 to B 4 , except the concentration of Ca 2+ , TDS and concentrations of other species increase greatly.From B 4 to B 5 , except the concentrations of Ca 2+ and SO 4 2-, TDS and concentrations of other species decrease to some extent.3 shows the changes of concentration of major cations and anions in meq% along the flow path.Similarly, from B 1 to B 3 , the concentration of all the species in meq% is relatively steady.The concentration of Na + in meq% is greater than 75% for all the river waters and is obviously the primary species of all the cations.By contrast, the changes of the concentration of anions in meq% are relatively large.From B 1 to B 5 , HCO 3 -in meq% decreases while Cl -and SO Changes of concentration of major species in meq% along flow path Figure 4 shows the changes of the saturation indices of calcite, dolomite and gypsum along the flow direction.It can be seen that all the river waters is undersaturated with respect to gypsum, although its saturation index increases steadily along the flow path.For calcite and dolomite, except dolomite is slightly undersaturated in B 5 , all the other river waters are supersaturated with respect to the minerals.From the above discussion we know the major changes of the chemistry of river water occurred between B 3 to B 5 , which is in the middle section of the Dousitu River.So the river in this section is divided into two parts for the study, which are B 3 -B 4 and B 4 -B 5 , respectively.

Water chemistry changes from B 3 to B 4
From B 3 to B 4 , the TDS and concentration of major species in river water increase to different degrees.According to field investigation, a dam was built in B 4 in 1978.Though the dam is only 20 m high, it forms an open water surface of about 4.5 km 2 .The direct result is the intensive evaporation of river water.Furthermore, the samples collected on 8~9 September, 2004 has experienced intensive evaporation during the whole summer.So the increase of TDS and concentration in B 4 is understandable.Further analysis of the chemistry of river water from B 3 to B 4 indicates the increase is different for different species.The concentrations of Cl -, SO 4 2-, Na + , Mg 2+ , K + and TDS in B 4 are 5.04, 5.05, 4.04, 3.6, 3.39 and 3.87 times of that in B 3 , respectively.While the increases of Ca 2+ and HCO 3 -are relatively small, and their concentrations in B 4 are only 1.19 and 2.51 times of that in B 3 .
The river water in B 4 is mainly recharged by upstream river water and groundwater.Because the chemistry of river water and groundwater in the upper part of B 4 are similar, the river water in B 3 is used as the recharging water to analyze the formation of the chemistry of river water in B 4 .

Estimation of evaporation proportion
If there were no effects of chemical reactions, the increase rate of the concentrations of various species should be same under the sole influence of evaporation.The above-mentioned different concentration increase rates for different species indicates the formation of the river water chemistry in B 4 has been affected by some chemical reactions.For the following reasons, the main reactions are the dissolution of gypsum and halite, as well as the precipitation of calcite.
(1) The study area is located in the arid region, the Tertiary stratum scattered in the area contains large amount of gypsum and halite.The river water in the area is all undersaturated with respect to these minerals.(2) River water in B 3 and B 4 is supersaturated with respect to calcite, and the concentration increase rates of Ca 2+ and HCO 3 -are apparently smaller than that of Cl -, SO 4 2-, Na + , Mg 2+ and K + .Where have the Ca 2+ and HCO 3 -gone during the process of evaporation?Precipitation of calcite is the natural answer.To estimate the evaporation proportion during the formation of river water chemistry in B 4 , the effects of chemical reactions to the changes of the concentrations of various species should be analyzed first.Because of the dissolution of halite, the concentrations of Cl -and Na + will increase on the basis of evaporation.So under the sole influence of evaporation, the concentration increase rates of Cl -and Na + from B 3 to B 4 should be smaller than the present values of 5.04 and 4.04.In other words, evaporation proportion estimated by the concentration increase rates of Cl -and Na + will be larger than the real value, although we do not know this value up to now.Because of the dissolution of gypsum, evaporation proportion estimated by the concentration increase rate of SO 4 2-will also be too large.Compared with the above species, estimates based on the increase rates of Ca 2+ and HCO 3 -(1.19 and 2.51, respectively) are smaller than the real value because of the precipitation of calcite.Since the precipitation of calcite is mainly caused by the dissolution of gypsum, the contribution of dissolution of gypsum and halite to the increase of TDS is lager than the decrease caused by the precipitation of calcite.Therefore the evaporation proportion estimated by the increase rate of TDS (3.87) is also larger than the real value.Through the above analysis, the increase rate caused solely by evaporation should be in the range of 2.51 to 3.87.Considering the increase rates of Mg 2+ and K + (3.6 and 3.39, respectively), the average value of their increase rates is used to estimate the effects of sole evaporation.That is to say, in the process of formation of the chemistry of river water from B 3 to B 4 , the concentration increase rate caused solely by evaporation is 3.5, or the evaporation proportion in B 4 is 71.43%.

Reactions forming the chemistry of river water in B 4
The chemistry of river water in B 3 , B 4 and the residual water after evaporation (WAE) by the above proportion are compared in Table 2.It is clear although the chemistry of WAE is close to the river water in B 4 , but the two have distinct differences.These differences are mainly caused by chemical reactions.According to the saturation indices in Figure 4, halite and gypsum are under saturated in B 3 and B 4 , while calcite is supersaturated.So the main reactions forming the chemistry of B 4 are thought to be the dissolution of halite and gypsum, the precipitation of calcite and degassing of CO 2 , the later is the common reaction accompanying the precipitation of calcite.(4) At the same time, because the dissolution of halite can not solely explain the increase of Na + in B 4 and the concentration of Mg 2+ increased in B 4 compared with B 3 .Considering the flow rate of river water decreased a lot in B 4 because of the building of the reservoir and a thick layer of fine deposits formed in the reservoir bottom.So the above changes of the cation concentration are explained by the following reactions.
K + + NaX = Na + +KX (5) Ca 2+ + 2NaX = 2Na + +CaX 2 (6) Ca 2+ +MgX = Mg 2+ + CaX (7) The method of mass balance is used to calculate the amount of chemical reactions (1) ~ (7)  during the formation of the chemistry of river water in B 4 from WAE.Table 3 lists the calculated results.In the table, the amounts of chemical reactions (5) ~ ( 7) are denoted by adsorbed amount of K + , Ca 2+ and Ca 2+ , respectively.The calculated results in Table 3 confirmed the qualitative guess.That is during the formation of chemistry of river water in B 4 from WAE, the dissolution of halite and gypsum, the precipitation of calcite and the degassing of CO 2 are the main reactions.Cation exchanges between K + and Na + , Ca 2+ and Na + and Ca 2+ and Mg 2+ also play certain roles.
(5) (6) (7) 5.39 2.35 -3.33 -3.09 -0.01 (K + ) -0.24 (Ca 2+ ) -0.05 (Ca 2+ ) In fact, during the processes of the formation of the chemistry of river water in B 4 , mineral dissolution and precipitation, cation exchange and evaporation occur at the same time.But evaporation is the main process, which caused the subsequent chemical reactions.The two is separated for the convenience of analysis in our discussion.G 15 , G 16 and their average value, the mixing proportion of groundwater in river water from B 4 to B 5 can be calculated by equation (8).Table 4 lists the calculated mixing proportion and the corresponding composition of simply-mixed water.In the table, row B 4 -G 15 , B 4 -G 16 and B 4 -G avg are the mixed results of B 4 with G 15 , G 16 , and G avg , respectively.One thing should be noted is that G avg is the simple half-and-half mixing of G 15 and G 16 .4 shows when the mixing proportion based on mass balance of Cl -is used, the calculated concentration of Na + +K + for the 3 sets of mixed water equals approximately to that of river water in B 5 .This is because the property of Na + +K + is similar to Cl -and they all belong to the relatively non-reactive ions.But for the reactive species of Ca 2+ , Mg 2+ , SO 4 2and HCO 3 -, it is obviously improper to explain the chemistry of river water in B 5 only by the simple process of mixing.Considering the hydrogeology and calculated saturation indices of minerals, the deviations between the simply-mixed water and B 5 will be explained by the dissolution of gypsum and the precipitation of calcite and dolomite.

Dissolution of gypsum
According to the calculated results in Table 4, the concentration of SO 4 2-in B 5 should be 412.77~433.45mg/L, if it were formed by the simple mixing.The actual concentration of SO 4 2-in B 5 is 780.5 mg/L.Since the river water in B 4 , B 5 and the groundwater between them are all undersaturated with respect to gypsum (Table 1) and the Cenozoic group in the area is rich in gypsum, it is reasonable to think the extra SO 4 2-comes from the dissolution of gypsum through the following reaction: Using the calculated results in Table 4, when the amounts of gypsum dissolved by the mixed water B 4 -G 15 , B 4 -G 16 and B 4 -G avg are 3.615 (= (780.5-433.45)/96),3.831 and 3.725 mmol/L, respectively, the final results will be the concentration of SO 4 2-in B 5 .

Precipitation of calcite and dolomite
The dissolution of gypsum introduced Ca 2+ into the water, for the mixed water B 4 -G

Restored results
In this paper, the restored concentration is defined as the concentration of mixed water without chemical reactions or before chemical reactions which can get the concentration of B 5 after the chemical reactions happens.According to the above calculation, the restored concentration of HCO 3 -is a little lager but close to the simply-mixed (Table 4).The reason for this is the precipitation of calcite may not completely follow the stoichiometric relationship of equation ( 10), some CO 2 produced in reaction (10) may dissolve into water and not release to the atmosphere.So the decrease of HCO 3 -calculated using the stoichiometric relationship of equation ( 10) may be a little larger, which leads to the larger calculated concentration of HCO 3 -.
Here the restored B 5 (represented by B 5 ') based on B 4 -G avg is taken as an example to show the results.In Figure 7, the restored concentration of HCO 3 -is drawn against TDS.In this case, B 5 ' has already on the line drawn through G 15 , G 16   The chemistry of river water in B 4 is formed by the conjunctive processes of evaporation and chemical reaction.Evaporation is the main process, which makes the concentration of species increase by about 3.5 times.The main chemical reactions forming the chemistry of river water in B 4 are dissolution of halite and gypsum, the precipitation of calcite and degassing of CO 2 .Cation exchanges between K + -Na + , Ca 2+ -Na + and Ca 2+ -Mg 2+ also play some roles.
The concentrations of non-reactive species in B 5 can be explained by the simple mixing process.But for the reactive species, chemical reactions occurred between B 4 and B 5 have distinct effects on their concentrations.Mass balance of Cl -shows the river gets more than 50% of groundwater recharge between B 4 and B 5 .On the basis of mixing, the dissolution of gypsum, precipitation of calcite and dolomite and the degassing of CO 2 also play important roles in forming the chemistry of river water in B 5 .
The formation processes of the chemistry of river water in B 4 indicate that when a reservoir is built in an arid area, the loss of water resources due to evaporation is huge, and making things worse, the evaporation can also lead to the degradation of water qualities.Hence, great care should be taken to build a reservoir in arid and semi-arid regions.

Figure 1 .
Figure 1.Location of the study area and the sampling positionThe Dousitu River drainage basin is situated in an arid area of north-western China.The main characteristics of the climate are a long and cold winter as well as a short and warm summer.According to the monitoring data in Etuok County, the annual temperature in the area changes from 5.3 to 8.9 o C and has an average value of 6.9 o C. The precipitation mainly occurs between June and August.The annual precipitation ranges from 125.2 mm to 611.6 mm, and has an average value of 267.5 mm.Evaporation is intensive, and the intensity is between 1947.7 and 2494.9 mm.In the middle section of Dousitu River (B 3 to B 5 in Figure1), the changes of the chemistry of river water are interesting.The TDS of the river water increases from B 3 to B 4 , but decreases from B 4 to B 5 .The concentration of Cl -, Na + , K + , Mg 2+ and HCO 3 -has a similar change with TDS, but the concentration of Ca 2+ and SO 4 2-increases steadily from B 3 to B 5 .Accordingly, the chemical type of river water changes from HCO 3 •Cl-Na to SO 4 •Cl -Na.

Figure 2 .
Figure 2. Changes of TDS and concentrations of major species along flow pathFigure3shows the changes of concentration of major cations and anions in meq% along the flow path.Similarly, from B 1 to B 3 , the concentration of all the species in meq% is relatively steady.The concentration of Na + in meq% is greater than 75% for all the river waters and is obviously the primary species of all the cations.By contrast, the changes of the concentration of anions in meq% are relatively large.From B 1 to B 5 , HCO 3 -in meq% decreases while Cl -and SO 4 2-in meq% increase.The main anion in B 1 , B 2 and B 3 is HCO 3 -, Cl -and SO 4 2-rank second and third respectively.From B 4 to B 5 , the main anions change from HCO 3 -to Cl -and SO 4 2-.So the chemical type of river water in B 1 , B 2 , and B 3 is HCO 3 -Na or HCO 3 •Cl-Na, in B 4 it changes to Cl•SO 4 •HCO 3 -Na, in B 5 , it becomes SO 4 •Cl -Na.

Figure 4 .
Figure 4. Changes of saturation indices of major minerals along flow path Figure 5 is the Piper diagram for all the waters samples in the study area, it is obvious that groundwater is mainly HCO 3 •Cl-Na (Ca, Mg), except G 11 and G 12 are SO 4 •Cl-Na, and SO 4 -Na•Ca, respectively.The TDS of groundwater has an increasing trend from the upper reaches to the lower reaches.As has been said before, in the upper reaches, the river water is mainly HCO 3 •Cl-Na, towards the lower reaches, it changes to Cl•SO 4 •HCO 3 -Na and SO 4 •Cl-Na.

Figure 5 .
Figure 5. Piper diagram for the water samples in the study area ▲ Groundwater • River water and B 4 , which indicates the mixing of B 4 with the average mixture of G 15 and G 16 can form the chemistry of river water in B 5 through the following processes: (1) mixing of 44.77% B 4 and 55.23% G avg ; (2) the 3.725 mmol/L dissolution of gypsum;(3) the precipitation of calcite and dolomite by the amounts of 346.22 and 113.36 mg/L, respectively.This process also shows from B 4 to B 5 , at least 55% of river water is from groundwater.

Table 1
lists the chemical analysis results.In the table, G 1 ~G17 are groundwater, B 1 ~B5 are river water.SI cal, SI gyp , and SI dol are the saturation indices of calcite, gypsum and dolomite, respectively.

Table 1 .
Chemical analysis results of Dousitu river water and nearby groundwater (Unit: mg/L) 4  2-in meq% increase.The main anion in B 1 , B 2 and B 3 is HCO 3 -, Cl -and SO 4 2-rank second and third respectively.From B 4 to B 5 , the main anions change from HCO 3 -to Cl -and SO 4 2-.So the chemical type of river water in B 1 , B 2 , and B 3 is HCO 3 -Na or HCO 3 •Cl-Na, in B 4 it changes to Cl•SO 4 •HCO 3 -Na, in B 5 , it becomes SO 4 •Cl -Na.

Table 2
Comparison of the water chemistry of B 4, B 3 and WAE (Unit: mg/L)

Table 4 .
Calculated mixing proportion and the corresponding concentrations of mixed This is also in accordance with the calculated saturation indices of calcite in B 4 and B 5 .It can be seen from equation(10)that with 1 mol precipitation of Ca 2+ , there will be 2 mol decrease of HCO 3 -in the solution.Therefore, for the mixed water B 4 -G 15 , B 4 -G 16 , and B 4 -G avg , the concentration decreases of HCO 3 -caused by the precipitation of calcite are 304.31(=2×61×(165.87-66.10)/40),386.53 and 346.22 mg/L, respectively.At the same time, compared with 15.6 mg/L of Mg 2+ in B 5 , the concentration of Mg 2+ in the mixed water is obviously too large.It will precipitate by dolomite just as Ca 2+ in the manner of calcite.Using the similar method, the concentration decrease of HCO 3 -caused by the precipitation of dolomite for the mixed water B 4 -G 15 , B 4 -G 16 and B 4 -G avg are 94.86 (=2×61×(34.26-15.60)/24),131.10 and 113.36 mg/L, respectively.It can be seen from the above discussion that the concentration of HCO 3 -in the mixed water of B 4 -G 15 , B 4 -G 16 and B 4 -G avg should be 570.07(170.90+304.31+94.86),688.53 and 630.48 mg/L, respectively if the dissolution of gypsum and the precipitation of calcite and dolomite were not happened after the mixing of B 4 with groundwater.