Many accidents of releasing toxic pollutants into surface water happen each year in the world. It is believed that dam cooperation can affect flow field in reservoir and then can be applied to avoiding and reducing spread speed of toxic pollutants to drinking water intake mouth. However, few studies investigated the effects of dam cooperation on the spread characteristics of toxic pollutants in reservoir, especially the source reservoir for water diversion with more than one dam. The Danjiangkou Reservoir is the source reservoir of the China’ South-to-North Water Diversion Middle Route Project. The human activities are active within this reservoir basin and cyanide-releasing accident once happened in upstream inflow. In order to simulate the spread characteristics of cyanide in the reservoir in the condition of dam cooperation, a three-dimensional water quality model based on the Environmental Fluid Dynamics Code (EFDC) has been built and put into practice. The results indicated that cooperation of two dams of the Danjiangkou Reservoir could be applied to avoiding and reducing the spread speed of toxic cyanide in the reservoir directing to the water intake mouth for water diversions.
Many accidents of releasing toxic pollutants into surface water happen each year in the world. For example, 1250 tons of toxic pollutants such as sulfur, phosphorous, and mercury flowed into Rhine River along with hydrating water after sudden explode of Sandoz Laboratory on November 1, 1986, in Basel Warehouse, Switzerland. This accident led fishes and other species within range of 160 km of the river to death in one night and the drinking water all polluted [
Accidental release of toxic pollutants into surface water would threat the safety of drinking water resource and human health tremendously [
The systems discussed above have successfully tracked pollutants in the surface waters. However, few studies have involved in scenario analysis and mitigation management on accidental release of toxic pollutants spreading to the drinking water intake mouth in reservoir based on dam operation [
The Danjiangkou Reservoir is the source reservoir of China’s South-to-North Water Diversion Middle Route Project (S-N-M Project). The water quality in this reservoir determines the fate of local ecosystem, human health in the receipt area (e.g., Beijing city and Tianjin city), and downstream of the great Danjiangkou Dam (e.g., Xiangfan City and Wuhan City). Unfortunately, human activities are active in this reservoir basin and toxic pollutants accidental release issue once happened in upstream inflow of this reservoir. For instance, 5.2 tons of toxic sodium cyanide was accidentally released into a tributary of the Han River (main tributary of the Danjiangkou Reservoir) in September 29 in year 2000 [
The objective of this study is to simulate the spread characteristics of cyanide in the Danjiangkou Reservoir based on the assumption that accidental release of cyanide into two main tributaries. The operations of the two dam including Danjiangkou Dam and Taocha Dam will be optimized to mitigate the toxic cyanide damage to the water quality for water diversion.
The Danjiangkou Reservoir, constructed in 1958 and located at the upstream of the Han River (as shown in Figure
Location of the study area and the sites for accidental release of cyanide.
The 3D water quality model was built based on the Environmental Fluid Dynamic Code (EFDC). The EFDC is a 3D software package with open source for the analysis of hydrodynamic and water quality [
In the above equations,
The governing mass-balance equation for the water quality state variable can be expressed as [
The governing mass-balance equation for water quality state variables (see (
The equation for kinetic processes only, which will be referred to as kinetic equation, is
which is expressed as
Orthogonal and curvilinear grids were built for the water quality model of the Danjiangkou Reservoir which totally has 60000 quadrilateral grids comprising 10000 grids in the horizontal plane and 6 layers in the vertical direction. The bathymetry of the water quality model was constructed by overlaying and interpolating the DEM data of the reservoir area with the field measurement data in the reservoir. The DEM data were downloaded from the Geospatial Data Cloud (
Flow field and water levels calibration for the hydrodynamic model of the Danjiangkou Reservoir.
In the water quality module, first order degradation of the cyanide was considered. The degradation rate was obtained from the experimental testing of the degradation of the sodium cyanide in static water [
Accidental release of cyanide was supposed to occur at two sites. One was at Yun City Bridge over the Han River and another was at the Dan River (see Figure
Daily water levels in Danjiangkou Dam area during the last 10 years.
In order to explore the effects of dam cooperation on the spread characteristics of cyanide in the Danjiangkou Reservoir, eight scenarios were set up. In each accident site, four scenarios were simulated. The first was normal water diversion in the Taocha Dam and normal outflow from the Danjiangkou Dam, the second was shutting down the Taocha Dam and normal outflow from the Danjiangkou Dam, the third was shutting down the Danjiangkou Dam and normal water diversion in the Taocha Dam, and the forth was shutting down the two dams. The detailed information of the scenarios setting was listed in Table
Scenarios setting for accidental release of cyanide into the Danjiangkou Reservoir.
Scenarios | Accident site | Outflow (m3/s) | |
---|---|---|---|
Danjiangkou Dam | Taocha Dam | ||
1 | Dan River estuary | Reference outflow | 350 |
2 | Dan River estuary | 0 | 350 |
3 | Dan River estuary | Reference outflow | 0 |
4 | Dan River estuary | 0 | 0 |
5 | Yun City Bridge | Reference outflow | 350 |
6 | Yun City Bridge | 0 | 350 |
7 | Yun City Bridge | Reference outflow | 0 |
8 | Yun City Bridge | 0 | 0 |
As Figure
Spread characteristics of accidental release of cyanide from the Yun City Bridge in the horizontal view.
As Figure
Spread characteristics of accidental release of cyanide from the Yun City Bridge in the vertical view (the concentration legend is the same as the figure of the horizontal view).
As it is presented in Figure
Time series of cyanide concentration in Taocha Dam area after accidental release of cyanide from the Yun City Bridge.
As Figure
Spread characteristics of accidental release of cyanide from the Dan River in the horizontal view.
As it is presented in Figure
Spread characteristics of accidental release of cyanide from the Dan River in the vertical view (the concentration legend is the same as the figure of horizontal view).
As Figure
Concentration of cyanide in Taocha Dam area after accidental release of cyanide from the Dan River.
Though 2 tons of cyanide release from two main tributaries would not be a great threat to the water quality for water diversion, it is still significant to study the effects of dam cooperation on the spread characteristics of cyanide in the reservoir for the uncertain amount of cyanide in a real accidental release.
It can be seen that in Figure
Time series of cyanide concentration at Hejiawan monitor site (a) and Taocha monitor site (b) after accidental release of cyanide from the Yun City Bridge at the scenarios of shutting down dams.
Figure
Figure
Time series of cyanide concentration at Hejiawan monitor site (a) and Taocha monitor site (b) after accidental release of cyanide from the Dan River at scenarios of shutting down dams.
In order to investigate the spread characteristics of toxic cyanide in the Danjiangkou Reservoir in the conditions of dam cooperation, a three-dimensional water quality model based on the Environment Fluid Dynamics Code (EFDC) was built and calibrated. The spread characteristics of cyanide in the reservoir were simulated based on eight scenarios related to the two dams cooperation including the Danjiangkou Dam and the Taocha Dam. The following conclusions were reached: two separately accidental release of 2 tons of cyanide from two main tributaries of the reservoir would be no threat to the water quality for water diversion in the hydrological year of 2009; the increasing outflow in the Danjingkou Dam would avoid cyanide spreading to the Dan part of the reservoir when accidental release of cyanide from Han River occurs; it indicated that the Taocha dam but the Danjiangkou Dam should be shut down to reduce the concentration of cyanide in water intake mouth for water diversion when accidental release of cyanide from Dan River occurs.
The conclusions are derived from analysing a number of scenarios, but this problem could be also studied from a viewpoint of mathematical optimization trying to find the best strategy for dams operation when an accidental release of cyanide occurs. However, the mathematical optimization has to be based on a huge database of different strategy that needs much more computational effort to achieve, as each successful run of the model costs more than 20 hours. Anyhow, it is a good option when computational resource meets its requirement.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The financial support of the National Key Technology Research and Development Program (2011BAC12B02) and the Creative Research Groups of the National Natural Science Foundation of China (no. 51121003) is gratefully acknowledged.