Impacts of river impoundment on the riverine water chemistry composition and their response to chemical weathering rate

Abstract

Currently, most rivers worldwide have been intensively impounded. River damming becomes a big problem, not only in inducing the physical obstruction between upstream and downstream, but also in destroying the natural continuity of river. But the discontinuity of water quality was often neglected, which presents a challenge to traditional river geochemistry research. To understand the changes in basic chemistry of water upstream and downstream of the dam, we investigated the Miaotiao River reservoirs in series in the Wujiang River Basin, and the Hongjiadu, Dongfeng Reservoir on the upper reaches of the Wujiang River. Chemical weathering rates were calculated using the water chemistry data of the reservoir surface and downstream of the dam, in each reservoir, respectively. The results showed that the difference between the chemical weathering rates calculated from reservoir surface water and water downstream of the dam was greater in reservoirs with a longer water retention time. In Hongjiadu Reservoir with the longest water retention time among the studied reservoirs, this difference reaches 9%. As a result, the influence of river damming, especially the influence of reservoirs in series, should be taken into account when calculating the chemical weathering rate of a river basin.

Keywords

reservoirs in series / chemical weathering rate / Wujiang

Introduction

During the past century, river impoundment has become a popular hydrological landscape imposed on rivers worldwide. Globally, there are more than 50 thousand big dams (height of dam over 15 m, or storage over 3.0 × 10⁶ m³), and 100 thousand small reservoirs with storage over 10⁵ m³, and millions of smaller reservoirs (storage less than 10⁵ m³). The effective storage of the global reservoir is about 4 × 10¹² m³, and about 7.3% of the annual discharge of global rivers. The total area of reservoirs in the world is 5 × 10⁵ km², about 1/3 that of natural lakes. In China, till 2007, more than 86,000 reservoirs have been constructed with a total storage of 6.9 × 10¹¹ m³ (Jia et al., 2010).

A natural stream is a continuous system which connects the headwater of a river to the river mouth or coastal zone. However, the natural properties and biogeochemical processes of rivers have been seriously disturbed by human activities, of which river damming can be regarded as the most important of human affairs imposed on a river system. Numerous dams on a river are not only the physical obstruction between water upstream and downstream the dam, but they also can destroy the natural continuity of the river. As a result, water quality as well as the hydrological property of river are often very different upstream and downstream of the dam. Rivers, hence, show a trend to be lacustrine and fragmental. At present, reservoir effect has been observed in many studies on rivers worldwide, such as the Danube and Nile (Humborg et al., 1997). Sedimentary and geomorphic properties of river channels downstream of dams changed substantially (Yang et al., 2007; Walter and Merritts, 2008; Kummu et al., 2010).The impacts of river damming on hydrological structure, sediment deposition, fish migration and the potential earthquake have long been of concern (Humborg et al., 1997; Dumas et al., 2010; Musil et al., 2012; Zhang et al., 2012). Recently, the changes in river ecosystems, due to the dam construction, were also reported (Dynesius and Nilsson, 1994). The impacts of dam construction on the riverine biogeochemical processes and the induced ecological issues have been confirmed (Mao et al., 2005; Qi and Ruan, 2005). The relationship between reservoir and ecosystem evolution also has become a great concern (Cai et al., 2003) reduced reservoir reduces the suspended sediment deposition in the downstream reach because upper stream suspended sediment was trapped by the reservoir (Wang et al., 2012). The construction of dams can intercept upstream sediment and fundamentally change the fluvial hydrology (Kiss et al., 2008; Draut et al., 2011). Undoubtedly, large-scale river damming can significantly change the riverine material field, energy field, chemical field, and biological field, as well as the hydrodynamic regime.

In addition to the above environmental impacts, geochemical processes happening in reservoirs also can change the river water chemistry. For instance, it was reported that carbon could be obviously retained within a reservoir due to river impoundment, and riverine water chemistry characteristics also had a significant change (Li et al., 2009). However this variation of water chemistry gradually challenges the accurate evaluation of chemical weathering rate (CWR). In traditional chemical weathering research, changes in water chemical composition within a river channel were seldom considered (Brink et al., 2007; Wu et al., 2008; Moquet et al., 2011), which may bring an uncertain deviation of the CWR results. In this study, the reservoirs in series in the Miaotiao River and the Hongjiadu Reservoir, the Dongfeng Reservoir on the upper reaches of the Wujiang River were investigated. The main objectives are to quantify the changes in basic water chemistry upstream and downstream of the dam, and to understand their influence on CWR calculation.

Study area

The Wujiang River, originating from north-west of Guizhou Province, China, is an important tributary of Changjiang. The Maotiao River, developed in a karstic area, is an important tributary of the Wujiang River. The Maotiao River has a total length of 180 km, and drains an area of 3,195 km². The middle and lower reaches of the Wujiang River have developed into deep canyons, which have always been exploited for hydropower generation.

The altitudes of the Wujiang River watershed are about 1,500 m in its upper reach and about 500 m in its lower reach. Precipitation in this basin has a yearly average of 1,225 mm.

Since the construction of the Wujiangdu Reservoir, twelve hydropower plants in series were built on the Wujiang River successively, of which, six reservoirs are on the major tributary (Maotiao River) . In this study, four reservoirs on the main-stream of the Wujiang River (Wujiangdu(WJD), Dongfeng(DF), Suofengying(SFY) and Hongjiadu(HJD)) and four reservoirs(Hongfeng(HF), Baihua(BH), Xiuwen(XW) and Hongyan(HY)) on the Maotiao River were investigatedThe main features of these reservoirs are described in table1.

Sampling and methods

Sampling

In each reservoir, water was collected before and after the dam monthly from July 2007 to June 2008. The site before the dam is about 500 m from the dam, and the site after the dam is the downstream side of the dam. Sampling sites are shown in Fig. 1. Samples downstream of the dam were collected 0.5 m under the water surface, while water samplings along the water column were collected in the central part of the reservoir using a Niskin bottle.

Water temperature, dissolved oxygen (DO) saturation, and pH were measured at the sampling sites with a pH, DO and salt conductivity meter (YSI-6600v2). \mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{-} was titrated by HCl in situ. All water samples were filtered through 0.45 μm acetate membrane filters and a small portion of filtrate was stored in the icebox for measuring anions, while another portion was acidified with ultra-purified hydrochloric acid to pH<2 for cation determination. Major cations (Ca²⁺, K⁺, Na⁺ and Mg²⁺) were analyzed by ICP-OES, and the anions (\mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-},Cl^- and \mathrm{N}{\mathrm{O}}_{\mathrm{3}}^{-}) by ion chromatography (ICS-90, DIONEX). Dissolved Si was measured by silicon molybdenum blue spectrophotometry. Repeat measurements show that in general the precision is±2% for cations and±5% for anions. Data in Table 2 is the annual average of the monthly monitoring data. TZ⁺ refers to the total cation concentration. TZ^- is the total anions concentration.

Methods

The chemical weathering rate (CWR) calculation process

Step 1: Atmospheric correction

This step assumes that all of the major elements from atmospheric sources remain in the river water.

(1)F{X}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}=F{X}_{\mathrm{a}\mathrm{t}\mathrm{m}}.

With FX_cycl the flux of the element X from atmospheric inputs exported by the river and FX_atm the rain flux of the element X

Where

(2)F{X}_{\mathrm{a}\mathrm{t}\mathrm{m}}=X_{\mathrm{a}\mathrm{t}\mathrm{m}}\times P,

(3)F{X}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}=X_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}\times R.

And X_cycl is the concentration of the element X in the river derived from atmospheric inputs (mmol/L); X_atm is the concentration of the element X in the rain (mmol/L); P is the precipitation rate, and R is the runoff (mm/yr).

The P/R is the ratio of the precipitation and runoff. To determine the cyclic concentration of the major elements X (Cl^-, \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}, Ca²⁺, Na⁺, Mg²⁺, and K⁺) in the river, we applied the following calculation:

(4)X_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}=X_{\mathrm{a}\mathrm{t}\mathrm{m}}\times P/R,

where X_atm is the average concentration of the element X from rain data for each area.

In all cases:

\mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}^{-}=\mathrm{S}{\mathrm{i}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}=\mathrm{0}

mmol/L

The residual concentration is calculated following this equation:

(5)X_{\mathrm{1}}=X_{\mathrm{r}\mathrm{i}\mathrm{v}}-X_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}},

where X = Cl^-, \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}, \mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{-}, Na⁺, Ca²⁺, Mg²⁺ ,and K⁺(mmol/L)

(6)\mathrm{S}{\mathrm{i}}_{\mathrm{1}}=\mathrm{S}{\mathrm{i}}_{\mathrm{r}\mathrm{i}\mathrm{v}}

where Si_riv is from the silicate weathering.

Step 2: Saltrock, pyrite, and hydrothermal correction

Depending on the basin characteristics, all remaining Cl^- and \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} are assumed to be derived from evaporite dissolution.

(7)\mathrm{C}{\mathrm{l}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{-}=\mathrm{C}{\mathrm{l}}_{}^{-}

(8)\mathrm{S}{\mathrm{O}}_{\mathrm{4}\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{\mathrm{2}-}=\mathrm{S}{\mathrm{O}}_{\mathrm{4}\mathrm{1}}^{\mathrm{2}-}

(9){\mathrm{N}}_{\mathrm{a}\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{+}=\mathrm{C}{\mathrm{l}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{-}(\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{L})

\mathrm{C}{\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{\mathrm{2}+}=\mathrm{S}{\mathrm{O}}_{\mathrm{4}\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{\mathrm{2}-}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{M}{\mathrm{g}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{\mathrm{2}+}=\mathrm{0}\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{L}(\mathrm{S}{\mathrm{O}}_{\mathrm{4}\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{\mathrm{2}-}\mathrm{i}\mathrm{s}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{d}\mathrm{f}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_{\mathrm{4}})

Where X_evap is the X concentration derived from evaporate inputs.

Residual concentration (mmol/L):

(10)\mathrm{N}{\mathrm{a}}_{\mathrm{2}}^{+}=\mathrm{N}{\mathrm{a}}_{\mathrm{1}}^{+}-\mathrm{N}{\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{}

(11)\mathrm{C}{\mathrm{a}}_{\mathrm{2}}^{\mathrm{2}+}=\mathrm{C}{\mathrm{a}}_{\mathrm{1}}^{\mathrm{2}+}-\mathrm{C}{\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{\mathrm{2}+}

(12)\mathrm{M}{\mathrm{g}}_{\mathrm{2}}^{\mathrm{2}+}=\mathrm{M}{\mathrm{g}}_{\mathrm{1}}^{\mathrm{2}+}-\mathrm{M}{\mathrm{g}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{\mathrm{2}+}

(13)\mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}\mathrm{2}}^{-}=\mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}\mathrm{1}}^{-}

(14)\mathrm{S}{\mathrm{i}}_{\mathrm{2}}=\mathrm{S}{\mathrm{i}}_{\mathrm{1}}

Step 3: Silicate weathering

The silicate inputs are calculated following these equations:

(15)\mathrm{N}{\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{+}=\mathrm{N}{\mathrm{a}}_{\mathrm{2}}^{+}

(16)\mathrm{C}{\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{\mathrm{2}+}=\mathrm{N}{\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{+}\times {(\mathrm{C}{\mathrm{a}}^{\mathrm{2}+}/\mathrm{N}{\mathrm{a}}^{+})}_{\mathrm{s}\mathrm{i}\mathrm{l}}

(17)\mathrm{M}{\mathrm{g}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{\mathrm{2}+}=\mathrm{N}{\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{+}\times {(\mathrm{M}{\mathrm{g}}^{\mathrm{2}+}/\mathrm{N}{\mathrm{a}}^{+})}_{\mathrm{s}\mathrm{i}\mathrm{l}}

(18)\mathrm{S}{\mathrm{i}}_{\mathrm{s}\mathrm{i}\mathrm{l}}=\mathrm{S}{\mathrm{i}}_{\mathrm{2}}

(19){\mathrm{K}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{+}={\mathrm{K}}_{\mathrm{2}}^{}(\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{L})

where (Ca²⁺/Na⁺)_sil and (Mg²⁺/Na⁺)_sil are the characteristic ratios of silicates. Previous studies (Han and Liu, 2004; Wu et al., 2005) were used to estimate the composition of the silicate end-member. In following the discussion, Mg/Na and Ca/Na are assumed to be close to 0.22 and 0.35 (Chetelat et al., 2008). The chemical weathering concentration of silicates (CWC_sil) is calculated as follows:

(20)\mathrm{C}\mathrm{W}{\mathrm{C}}_{\mathrm{s}\mathrm{i}\mathrm{l}}=\mathrm{N}{\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{+}+\mathrm{C}\mathrm{a}{\mathrm{2}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{+}+\mathrm{M}{\mathrm{g}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{\mathrm{2}+}+{\mathrm{K}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{+}+\mathrm{S}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}(\mathrm{m}\mathrm{g}/\mathrm{L})

Step 4: Carbonate weathering

Any remaining cations not accounted for by rain, evaporates, or silicates were attributed to carbonate weathering. The chemical weathering concentration of carbonates (CWC_carb) is calculated as follows:

(21)\mathrm{C}{\mathrm{a}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}^{\mathrm{2}+}=\mathrm{C}{\mathrm{a}}_{\mathrm{2}}^{\mathrm{2}+}-\mathrm{C}{\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{\mathrm{2}+}

(22)\mathrm{M}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}^{\mathrm{2}+}=\mathrm{M}{\mathrm{g}}_{\mathrm{2}}^{\mathrm{2}+}-\mathrm{M}{\mathrm{g}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{\mathrm{2}+}

(23)\mathrm{H}\mathrm{C}\mathrm{O}{}_{\mathrm{3}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{e}/\mathrm{d}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{t}\mathrm{e}}^{-}=\mathrm{C}{\mathrm{a}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}^{\mathrm{2}+}+\mathrm{M}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}^{\mathrm{2}+}(\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{L})

(24)\mathrm{C}\mathrm{W}{\mathrm{C}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}=\mathrm{C}{\mathrm{a}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}^{\mathrm{2}+}+\mathrm{M}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}^{\mathrm{2}+}+HC{O}_{\mathrm{3}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{e}/\mathrm{d}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{t}\mathrm{e}}^{-}(\mathrm{m}\mathrm{g}/\mathrm{L})

Step 5: CWR (Chemical weathering rate) calculation

(25)\mathrm{C}\mathrm{W}{\mathrm{R}}_{\mathrm{s}\mathrm{i}\mathrm{l}}=\mathrm{C}\mathrm{W}{\mathrm{C}}_{\mathrm{s}\mathrm{i}\mathrm{l}}\times \mathrm{R}/\mathrm{a}(\mathrm{t}/(\mathrm{k}{\mathrm{m}}^{\mathrm{2}}\cdot \mathrm{y}\mathrm{r}))

(26)\mathrm{C}\mathrm{W}{\mathrm{R}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}=\mathrm{C}\mathrm{W}{\mathrm{C}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}\times \mathrm{R}/\mathrm{a}(\mathrm{t}/(\mathrm{k}{\mathrm{m}}^{\mathrm{2}}\cdot \mathrm{y}\mathrm{r}))

where R is runoff (mm/yr) and a is drainage area (km²)

CaCO₃ saturation index (SIc) calculation method

SIc was calculated from the following Eq. (27):

(27)SIc=\mathrm{l}\mathrm{g}(({\mathrm{C}\mathrm{a}}^{2+})(\mathrm{C}{\mathrm{O}}_3^{2-})/K_C)

where Kc is the temperature dependent equilibrium constant for calcite dissociation. Ca²⁺ and \mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{\mathrm{2}-} are measured in mmol/L.

Result and discussion

Variations of major ions composition in river water of the study area

The average pH is 7.9 (with a range of 7.34-8.37) in the Wujiang River, indicating the geological background of limestone in the drainage basin. TDS values vary from 279.9 to 364.1 mg/L, except for a high value (380mg/L) of the sample (BH) (Table 2), which was polluted by waste water from a nearby aluminum factory.

TZ⁺ (K⁺ + Na⁺ + 2Ca²⁺ + 2Mg²⁺), TZ⁺ in meq/L and major element concentration in mmol/L, in river water has a range of 3.63-4.69 meq/L, with a mean value of 4.23 meq/L, which is higher than the average of the world rivers (1.25 meq/L, Meybeck, 1982). Because of draining a karst terrain, the TDS value in the Wujiang River is also higher than the Changjiang River (2.8 meq/L on average) (Han and Liu, 2004).

\mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{-} is the dominant anion for the majority of the samples (1.8-2.9 mmol/L). The second major anion is \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}, which has an average concentration of 0.93 mmol/L. Cl^- and \mathrm{N}{\mathrm{O}}_{\mathrm{3}}^{-}concentrations ranging from 0.07 to 0.22 mmol/L and from 0.06 to 0.25 mmol/L, respectively. \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{2-} and \mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{-} together account for 90% of the total anions in river water, while Ca²⁺ and Mg²⁺ together account for 80% of the total cations.

Variations of major ion compositions are shown in the anion and cation ternary diagrams (Figs. 2(a) and 2(b)). It is clear that Ca²⁺ and \mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{-} are the dominant ions in the water and an indication of the carbonate dissolution. Most of the samples are distributed around the Ca²⁺ apex and the \mathrm{H}\mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{-} apex in the ternary anion diagram (Fig. 2), revealing strong carbonate weathering and less silicate weathering in the drainage basin.

Reservoir effects on CWR evaluation

Our results showed that the difference between the chemical weathering rates calculated from reservoir surface water and water downstream the dam varied between -4.23% and 9.04%, and had a trend to be greater in a reservoir with longer water retention time (Figure.3). For example, in Fig. 3, it is clear that, the CWR above the dam is similar to the CWR below the dam, in XW, SFY, and HY. However, in BH, HF, and HJD reservoirs, which have a longer water retention time, the CWR showed large differences. Based on these data, a linear relationship between the rate of change and the retention time of reservoirs can be found (Fig. 3).

The rate of change= ((weathering rate below the dam) – (weathering rate before the dam)) / (weathering rate before the dam).

Table 2 shows that the dissolved Si and Ca²⁺ concentration in water downstream the dam were higher, compared with the surface water upstream of the dam, in reservoirs with longer residence time. For instance, the dissolved Si concentration in downstream water was two times that of the surface water upstream of the dam in the HF reservoir, and the Ca²⁺ concentration had a variation of 10%-20% (variation rate= ((concentration in downstream) – (concentration in surface water upstream the dam)) / (concentration in surface water upstream the dam)). Compared with Ca²⁺ and Si, the variations of K⁺, Cl^-, \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} were small. We conclude that the main factors influencing the CWR calculation should be Ca²⁺ and Si. In addition, because carbonate is dominant in the studied basin, Si has a quite low concentration in the river water. Consequently, the geochemical behavior of Ca²⁺ in the reservoir-river system should be the major contribution to the CWR variation.

Mechanism for the variation of water chemistry composition in reservoir Lacustrine reaction in reservoir

The water temperatures above and below the dam are shown in Fig. 4. From Fig. 4, the temperature differences above and below the dam are more obvious in the HJD, HF, and WJD reservoirs during warm seasons. This is because these reservoirs have longer residence time, and consequently easily form thermal stratification in warm seasons. Because of the deep water required for power generation, low temperature water was released downstream. In cold seasons, temperature differences become less as result of the mixing of water along the water column.

From Fig. 5, pH and DO in the water below the dam also are lower than that above the dam, but Ca²⁺ and Si concentrations show a reverse trend. The differences of pH, DO, Ca²⁺ and Si above and below the dam demonstrate that the influence of river damming on the downstream reaches are significant. Obviously, water quality shows a great discrepancy when river flow through a dam.

Generally, DO saturation in reservoir surface water is higher than that downstream of the dam. For example, in reservoir surface water, DO saturation can reach up to 126%, but its annual average value is only 60% in water downstream of the dam in the HY reservoir. Similarly, the pH of the surface water above the dam is higher than downstream water (Table 2). Obviously, water quality shows a great discrepancy when river flows through this dam. From Fig. 6, it is clear that the values of DO and pH decrease with the increase of water depth. Especially in the BH reservoir, bottom water had a quite low DO content.

The concentrations of Ca²⁺ and Si showed a reverse trend as compared to DO and pH. In Table 2, the two reservoirs with longer residence time (HF and HJD reservoirs) showed larger differences of Ca²⁺ and Si concentration between the surface water and the downstream water. There are two reasons for this phenomenon: (π) In reservoir surface water, during the growing season, dissolved silicon can be assimilated by diatoms, leading to the depletion of dissolved CO₂ in water and the resulting increasing of pH, which may increase SIc and favor the precipitation of CaCO₃. Both processes result in lower concentrations of Si and Ca²⁺ in reservoir surface water. (θ) The newly formed CaCO₃ and biomass will finally settle down into the reservoir sediment. Owing to organic matter decomposition, CaCO₃ and diatom frustules will be re-dissolved to some extent, and keep the high concentration of Ca²⁺ and Si in bottom water (Fig. 6). In reservoirs with longer residence time, the water column is easily forms thermal stratification in warm seasons. As a result, the exchange between the upper and bottom water is weak. Chemical stratification along the water column will then be maintained, consequently, such as in WJD, HJD, and HF reservoirs, while there is no obviously similar pattern in reservoirs with short residence time (Fig. 6). Furthermore, due to the deep water required for hydropower generation, water downstream of the dam has the similar chemical characteristics with the bottom water in the reservoir. Therefore, the river damming made an obvious difference of water chemical composition between the water upstream and downstream of the dam, and influenced the calculation of the chemical weathering rate.

Influence of reservoirs in series on the carbonate equilibrium system of river water

SIc means the calcite saturation index. If SIc>0,water is super saturated with respect to calcite, and calcium carbonate could deposit; if SIc<0, water is under saturated with respect to calcite, and calcium carbonate dissolution could happen; and if SIc = 0, the system reaches equilibrium (Wang et al., 2011).

In the HF reservoir, which is about 40 km from its headwaters (Fig. 7), SIc is greater than zero in its reservoir surface water and less than zero in downstream water. This phenomenon also could be found in the BH, XW, and HY reservoirs. Due to the deep water discharging from the dam, the downstream SIc value inherited that of the bottom water of the reservoir. Figure 7 reveals that CaCO₃ precipitation is favored in surface water of a reservoir, while water downstream of the dam becomes erosive to CaCO₃, particularly in hot seasons. This great difference can be explained as follows: in reservoir surface water, during the growing season period, dissolved CO₂ could be significantly taken up by phytoplankton, leading to the increase of pH and SIc. As a result, CaCO₃ would precipitate. At the bottom of the reservoir, because of organic matter decomposition, CO₂ was released into water, which lowered the pH, and then decreased SIc. Under this condition, CaCO₃ re-dissolution will occur. In addition, reservoirs with longer retention time developed thermal stratification in hot seasons particularly, which lead to the exchange between the upper and lower water becoming weak. As a result, both Ca²⁺ and \mathrm{C}{\mathrm{O}}_{\mathrm{3}}^{\mathrm{2}-} concentrations can vary widely along the water column in these reservoirs. Figure 7 also shows the interception effect of reservoirs in series. From upstream of the leading reservoir (HF), the SIc in the water gradually increased; however, it reduced downstream of the reservoirs. In a word, the interception of reservoirs in series can disrupt the continuity of riverine chemical composition. Therefore, the interception effect of reservoirs in series should be considered when calculate the chemical weathering rate of a drainage basin.

Conclusions

River damming has obvious impact on water chemical composition, particularly in reservoirs with longer residence times. The geochemical behavior of conservative ions, such as Ca²⁺, may become un-conservative when a river flows across a dam, because of changes in the carbonate equilibrium system.

Hydrological residence time of reservoirs has a certain relationship with the change of the water chemical composition, which may have some influences on the evaluation of chemical weathering rate. Due to the large number of reservoirs worldwide, these reservoirs may significantly change the original river characteristics. From the point of the whole basin and even the global chemical weathering rate, the influence of the reservoirs should not been ignored. Therefore, reservoir effect should be taken into account when calculating the CWR of drainage basins.

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