Risk Assessment for Chemical Spills in the River Rhine

Abstract

Drinking water production based on riverbank filtration has a long tradition along the River Rhine. The river water quality plays a crucial role in ensuring the safe supply of drinking water. Despite the successful restoration of the Rhine, the risk of future chemical spills still remains. A prediction of the contaminant breakthrough at the reach of the river used for riverbank filtration can be performed using a computer model. This knowledge enables the waterworks to prevent the infiltration of polluted river water into the aquifer using an adapted approach to well management. Nevertheless, preventing water pollution must remain the goal of the environmental authorities and local industry.

1 Introduction

Riverbank filtration (RBF) is a well proven treatment that is part of a multi-barrier concept in drinking water supply (Grischek et al. 2002). In the Düsseldorf area, for instance, bank filtrate has been discharged from wells close to the banks of the Rhine since 1870 (Eckert and Irmscher 2006). RBF is a process whereby surface water is subjected to subsurface flow prior to extraction from vertical or horizontal wells. From a water resources perspective, RBF is characterised by an improvement in water quality, which in general is based on a combination of sorptive-filtration and biodegradation (Kühn and Müller 2000, Ray et al. 2002).

Until the middle of the last century the water quality of the Rhine was so good that the bank filtrate could be used as drinking water. Only biodegradable organic carbon, turbidity and microorganisms had to be removed or significantly reduced during bank filtration. In the 20th century, population growth and increasing industrialization along the Rhine resulted in extensive water pollution, necessitating technical water treatment in order to produce safe drinking water. Friege (2001) reports about incentives beginning in the 1970s, which led to a significant improvement in river water quality. These incentives included the effective combination of numerous regulations, and monitoring programs. Today, while waterworks along the Rhine are no longer threatened by a polluted river, the risk of chemical spills still remains. This paper presents measures to ensure the safe supply of drinking water in case of contamination events along the Rhine.

1.1 Site Description

The River Rhine, which is 1,320 km long and has a catchment area of 185,000 km², is the third largest river and the largest source of drinking water in Europe. The city of Düsseldorf is situated in the North-West of Germany, in the lower Rhine valley (Figure 5.1). The mean discharge from the Rhine in the Düsseldorf area is 2,200 m³/s of which the waterworks use less than 2 m³/s. During times of flood, the discharge increases to 10,000 m³/s. The width and dynamics of the Rhine, which has its source in the Alps, allow the sustainable application of RBF for drinking water supply.

Figure 5.1. The Rhine catchment and the location of Düsseldorf.

The raw water, containing between 50 and 90% bank filtrate, is pumped from a quaternary aquifer. At present three waterworks supply 600,000 inhabitants with 50 million m³ of treated bank filtrate annually, meeting demand of up to 200,000 m³/day.

The vertical wells and the horizontal collector wells are situated between 50 m and 300 m from the river bank. Figure 5.2 shows a line of vertical wells at Flehe waterworks that have been in operation since 1870. The raw water is discharged using a siphon system. Depending on the hydraulic situation, the residence time of the bank filtrate in the aquifer varies between 1 week and several months (Schubert 2002a, Eckert et al. 2005). In general, the flow path and the retention time are long enough for the purification processes to work, resulting in the complete degradation of biodegradable organic carbon and the effective elimination of pathogenic bacteria, viruses and protists (Schubert 2002b).

Figure 5.2.

Vertical wells at Flehe waterworks and the River Rhine.

2 Development of River Water Quality at the Rhine

Levels of contamination in the Rhine increased rapidly after World War II. In the 1950s and 1960s, sewage systems in the cities destroyed during the war were built prior to waste water purification plants. As a result, a lot of untreated sewage was discharged into the river leading to increasing pollution levels (Friege 2001). For a long time, local industries were successful in opposing pressure from the public and local government to construct wastewater treatment facilities.

In the 1950s the pollution of the Rhine reached such a high level that the RBF purification processes were no longer able to ensure good quality drinking water. The taste and odor of the bank filtrate became so bad that the waterworks were forced to develop and apply sophisticated new treatment steps. In addition to the application of technical treatment methods, the waterworks reinforced their efforts to achieve better river water quality by forming a common organization. The International Association of Waterworks in the Rhine Catchment Area (IAWR) was founded in 1970 in Düsseldorf, Germany. Its goal was to demand measures for water protection. In 1973, the IAWR published its first “Memorandum” on raw water quality that served as a “yardstick” for local government bodies and for the public debate on Rhine water quality. Together with other stakeholders, such as environmental groups, the IAWR promoted a public discussion on water protection. At the end of this process the federal government issued its first program for environmental protection, which included measures to ensure that river water quality would attain a high standard within 20 years. Target values for surface water were, for example, 8 mg/L for oxygen, 0.2 mg/L for ammonia and 0.1 µg/L for pesticides. State authorities started to control effluents thoroughly and levied a charge for certain pollution parameters. These measures forced local industries and communities to meet high purification standards in a very short time (Friege 2001). For example, in the state of North Rhine-Westphalia with 17 million inhabitants, the amount of effluents treated biologically increased from less than 22% in 1965 to 90% in 1985.

As a consequence of this collaboration between the waterworks and the government, the role of the chemical industry evolved from that of an opponent to that of a key partner, which now publishes its efforts and successes in reducing industrial effluents. Many actions were necessary to reduce nutrients and pollutants. The numerous measures taken to reduce nutrients and pollutants were consistent with the best available technology in wastewater treatment and production along the Rhine. Consequently, river water quality has improved significantly since the mid-1970s with the return of salmon to the river in 2000.

The historical development of water pollution of the Rhine can be illustrated by the concentration-time plot of oxygen (Figure 5.3). The oxygen concentration in the Rhine decreased continuously until the beginning of the 1970s. One of the many negative consequences of this decrease was the occurrence of manganese in the anaerobic well water, which increased the cost of treatment. Then, as a consequence of the restoration efforts, the oxygen concentrations returned to saturation level at the beginning of the 1990s. The higher oxidation capacity, combined with the lower oxygen demand of the infiltrating river water, led to more efficient natural attenuation processes within the aquifer. This, in turn, enabled the waterworks to reduce their treatment expenses (Eckert and Irmscher 2006). However, the occurrence of chemical pollutants in the river water, like pesticides and pharmaceuticals, remained an issue. (Verstraeten et al. 2002).

Figure 5.3.

Rhine oxygen concentrations and well water manganese.

In addition to the issue of reliable water quality, periodic contamination caused by chemical spills has implications for the supply of drinking water using RBF. Ship accidents and problems with industrial wastewater treatment are the primary causes of periodic river pollution. For example, the accidental release of the insecticide Endosulfan by the chemical industry in 1969 resulted in a huge spike in the death rate of fish in the river. This was one of the most serious pollutant accidents. However, together with the aforementioned efforts for greater water protection in the 1970s the risk of chemical spills was reduced during subsequent years.

Nevertheless, in 1986, the so called SANDOZ accident caused the death of large numbers of eel in the Rhine. The pollution was the result of a warehouse fire at the chemical production or plant SANDOZ, Switzerland, where some 1,200 tons of deadly agricultural chemicals were stored. Firemen attempting to put out the blaze accidentally washed some 20 tons of highly toxic pesticides into the river, where they soon formed a 50 km-long trail that moved downstream at 3 km/h. One of the measures taken was to close the drinking water wells along a 900 km stretch of the Rhine and as far as the Netherlands. As a consequence, the International Commission for the Protection of the Rhine (ICPR) formulated recommendations for the “Prevention of accidents and security of industrial plants.”

3 Risk Assessment

3.1 Rhine Alarm Model

Water protection is a high priority for waterworks in the Rhine valley. Ship accidents and problems with industrial wastewater treatment are still the primary reasons for periodic river pollution. When source water becomes extremely polluted, effective forecasting is essential for water supply companies in order to be able to take the necessary preventive measures in time. Along the Rhine, water quality data is provided by the ICPR.

If, despite all preventive measures, an accident occurs and large amounts of hazardous substances flow into the Rhine, the water suppliers depend on quick and accurate information about the chemical composition, the pollution source, and the estimated concentration and breakthrough at the reach close to the supply wells. This data is particularly needed to undertake a risk assessment of the drinking water supply.

The ICPR monitoring stations, as well as those in border countries where the Rhine flows, constantly check the chemical composition of the river water using, inter alia, biological tests. Based on this data and the information provided by the polluter, the international Warning and Alarm Plan (WAP) comes into effect providing reliable information about the accident. The WAP data enables water supply companies to forecast pollutant breakthrough at the relevant reach of the river using the Rhine alarm model (Mazijk et al. 2000).

The River Rhine Alarm Model was developed by the “International Commission for the Hydrology of the River Rhine” (CHR) and the ICPR. For this kind of predictive model, a great deal of effort and money must be spent on calibration using extensive in-situ tracer measurements (van Mazijk 2002). In the case of the River Rhine Alarm Model, which uses a two-dimensional analytical approximation for the travel time and concentration curve, a dispersion coefficient and a lag coefficient have to be calibrated. The alarm model covers the river from Lake Constance to the North Sea, including the Aar, Neckar, Main, and Moselle tributaries. The model calculations take into account the location and conditions of the initial pollution, decomposition and drift capacity of the harmful substances released, discharges and/or water levels, geometry and dispersion. The calibration was performed using tracer tests.

For calculation purposes, the following input data are required: location, time and duration of the discharge, and the amount and biodegradable fractions of the harmful substances discharged. Information on whether the discharge is a floating harmful substance (such as oil) or a substance that mixes with the river water is also required. Finally, data on the discharge of the Rhine must be entered since the discharge largely determining the speed at which the pollution is carried downstream. The model then calculates how the pollution is transported downstream.

Figure 5.4 shows a series of calculated breakthrough curves for the Rhine at Düsseldorf calculated from different locations where the pollution occurred. The calculation was performed based on a virtual pollutant of 1,000 kg over a period of 24 h during average river water level. The peak concentration and the arrival time varied depending on the location of the accident. For example, an accident in Leverkusen would result in a peak concentration of almost 5µg/L after 2 days while the same accident in Basel produces a peak concentration of just over 3 µg/L after 8 days.

Figure 5.4.

Calculated breakthrough curves at the Rhine near Düsseldorf.

3.2 Well Management

Based on the expected river water concentration an estimate is needed if the maximum thresholds for drinking water are exceeded. In some cases the amount of pollution is not very significant and the attenuation process in the river results in a low concentration, so that no action is necessary. Alternatively, if the expected concentration exceeds acceptable limits for drinking water and a significant decrease through attenuation processes within the aquifer can’t be accurately predicted, it becomes necessary for the water supplier to react. Even if technical treatment steps are taken in order to ensure a decrease in the level of pollution, well protection remains the main priority.

The drinking water supplier may react to a pollution accident by an adapted well management to prevent contaminated river water from reaching the drinking water wells. The presence of an extremely toxic and concentrated substance should result in the switching off of all wells while the pollutant passes the adjacent reach of the river. Depending on the length of time the river is polluted, this may negatively impact the water supply. Since pollution breakthrough often lasts for several days (see Figure 5.4) thereby exceeding the drinking water storage capacity of approximately 12 h, a concept was developed to safeguard well operation during this period.

An appropriate and adequate approach involves switching off only half of the wells shortly before the pollutant plume arrives. As shown in Figure 5.5, the reach where the wells are switched off becomes effluent and infiltration only occurs close to the wells that are still in operation. Since the pollutant breakthrough usually only lasts hours or in some cases days, the pollutant can only infiltrate a small section of the aquifer. In the case of Düsseldorf, the infiltration velocity is about 2 m/day and the wells are more than 50 m away from the river bank.

Figure 5.5.

Well Management in case of a chemical spill in the River Rhine (left: period of river pollution; right: period after the pollutant breakthrough).

Following the breakthrough of the pollutant plume, the wells that were in operation before must be switched off and the water supply is then drawn from the other wells (Figure 5.5). Without the drawdown the system becomes quickly effluent so that the contaminated groundwater flows back towards the river.

This approach depends on a reliable prediction of the pollutant breakthrough by the Rhine alarm model. Usually the pollutant plume is approaching so quickly that chemical sampling and analytical results are not available in time.

3.3 The Chloracetophenon Case in 2003

In 2003, contaminated wastewater containing 0.7 tons of chloracetophenone was released untreated into the river Main, a tributary of the Rhine. The treatment process of a local industry had failed, but the problem was recognized very quickly and the environmental authorities were informed within a short time of the accident. The polluter provided information about the duration of the discharge and the mass of the contaminant. This data made it possible to develop a reliable prediction of the pollution breakthrough in the Düsseldorf area (Figure 5.6).

Figure 5.6.

Measured and modeled chloracetophenon data.

The arrival of the pollutant plume was expected on 30 January 2001, about 60 h after the accident occurred 260 km upstream. During the breakthrough half of the production wells were switched off per the well management concept described above. After the plume had left Düsseldorf on 2 February, the wells that were switched off initially were switched back on. Simultaneously, the other wells were shut down for a 1-week period to ensure that the contaminants flowed out of the aquifer. Measured river water concentrations of chloracetophenone confirmed the model results (see squares in Figure 5.6) and showed that the emergency well management measures were applied at the correct time.

4 Conclusions

In the Rhine valley RBF has proved a reliable method for ensuring the supply of safe drinking water for more than 130 years though sophisticated technical treatment methods were necessary to overcome the massive river pollution levels that existed in the middle of the last century. The commitment of key stakeholders to water protection was a particularly important step towards re-establishing a healthy ecosystem. Nowadays, the risk of water pollution by chemical spills remains relatively low.

In case of accidental spills, due to the intensive monitoring by the environmental authorities and the good collaboration with the industrial concerns along the Rhine, the waterworks are generally given sufficient advance warning to react. Water supply companies and water boards use the Rhine alarm model to accurately predict pollutant breakthrough and implement preventive measures. The model results enable the water suppliers to decide on appropriate steps for well management or drinking water treatment. Monitoring and modelling based on data provided by/included in the international Warning and Alarm Plan help the waterworks ensure the provision of safe drinking water even in critical situations.