Radioactive substances in tap water

Tap water from Yokohama City does not interfere with drinking.

The management target value of radioactive materials in tap water is "10 becquerels / kg" (total value of cesium 134 and 137).
The test results of radioactive cesium in tap water and raw tap water at three water purification plants (Kawai, Nishiya, Kojaku) in Yokohama city are not detected (detection limit value of 1 becquerel / kg or less) and are below the management target value. I am.

From April 1, 2012, a new standard under the Food Sanitation Law has been set for radioactive substances in foods including drinking water, and radioactive cesium in drinking water has been changed from 200 becquerels / kg to "10 becquerels / kg". Was changed to
In response to this change, "10 becquerels / kg" (total value of cesium 134 and 137) was set as a management target value for radioactive substances in tap water according to the notification of the Ministry of Health, Labour and Welfare. For radioactive iodine, no new management target has been set because the half-life is short (about 8 days).

※Notification of Waterworks Section Manager, Health Bureau, Ministry of Health, Labour and Welfare, dated March 5, 2012 ("Settings of management target values for radioactive materials in tap water (PDF: 172KB)" "Details (PDF: 226KB)")

Even if the concentration of radioactive materials in tap water exceeds the management target value (10 becquerels / kg), it does not mean that it is not suitable for drinking.
However, if radioactive cesium exceeds the target value, the cause will be investigated immediately and water users will be notified for safety and security.

※Notification of Waterworks Section Manager, Health Bureau, Ministry of Health, Labour and Welfare, dated March 5, 2012 ("Settings of management target values for radioactive materials in tap water (PDF: 172KB)" "Details (PDF: 226KB)")

Since March 19, 2011, City of Yokohama, Water Works Bureau has been conducting radiological inspections of tap water at three water purification plants in Yokohama (Kawai, Nishiya, and Kojaku).
In June 2012, we introduced a germanium semiconductor detector capable of more precise measurement, and for each of cesium-134 and cesium-137, the detection limit (minimum concentration that can be detected by a device that inspects radioactive materials) of 1 becquerels / kg or less has been carried out.
Since 2023, we have been conducting inspections by outsourcing with the same accuracy, and currently inspect tap water and tap water at three water purification plants in Yokohama City once every three months. I am.

※Please refer to Q5 for the germanium semiconductor detectors.

This is an inspection device that can perform precise inspection with a detection limit of 1 becquerel/kg or less.

By using a germanium semiconductor detector, a detection limit of 1 becquerel / kg or less is secured for each of cesium-134 and cesium-137, and precise inspection is possible.

Testing with a germanium semiconductor detector

Samples are set inside the detector

According to a notification from the Ministry of Health, Labour and Welfare, the inspection frequency is "in principle, at least once a month, but until sufficient information is collected during high turbidity such as rainfall, thaw, etc., the inspection will be conducted at least once a week depending on the inspection system of water utilities, etc." In addition, "If radioactive cesium is not detected in tap water or raw tap water for three consecutive months by water quality inspection with sufficient sensitivity, subsequent inspections can be reduced to once every three months." It has been.

※Notification of Waterworks Section Manager, Health Bureau, Ministry of Health, Labour and Welfare, dated March 5, 2012 ("Settings of management target values for radioactive materials in tap water (PDF: 172KB)" "Details (PDF: 226KB)")

In response to the accident at the Fukushima Daiichi Nuclear Power Station in 2011, City of Yokohama, Water Works Bureau has conducted a sufficient number of inspections, but radioactive cesium has never been detected in tap water or raw water in Yokohama, including when the quality of tap water deteriorates due to typhoons. In light of these circumstances, we believe that the safety of tap water can be confirmed by conducting periodic inspections at frequency based on the notification from the Ministry of Health, Labour and Welfare.

As indicated in the notification from the Ministry of Health, Labour and Welfare, at present enough time has passed since the accident at the Fukushima Daiichi Nuclear Power Station in 2011, the inflow of radioactive cesium is contained as a component of turbidity in tap water, so it is possible to remove it as turbidity in the process of water purification treatment at water purification plants.
In the future, we will continue to thoroughly manage turbidity and conduct periodic inspections to confirm the safety against radioactive cesium.

※Notification of the Ministry of Health, Labour and Welfare
About setting of management target value to affect radioactive material in tap water (0305 second from healthy water on March 5, 2012) (PDF: 393KB)

City of Yokohama, Water Works Bureau conducts inspections in accordance with the frequency indicated in the notification issued by the Ministry of Health, Labour and Welfare.
In the event that a large-scale radioactive material is released in the future, or if any abnormality occurs in the water purification process at a water purification plant, we will increase the frequency of inspection and strengthen monitoring of radioactive materials.

Inspection of radioactive cesium allows the safety of all radioactive substances subject to regulation, so we do not inspect substances other than radioactive cesium.

According to a survey by the Ministry of Education, Culture, Sports, Science and Technology, etc., radioactive substances subject to regulation other than radioactive cesium have much lower concentration than radioactive cesium.
The target values for radioactive cesium management set by the Ministry of Health, Labour and Welfare (10 becquerels/kg) are set in consideration of other radioactive materials. Inspection of radioactive cesium can confirm the safety of all radioactive substances subject to regulation, so there is no need to re-examine substances other than radioactive cesium.

※Notification of Waterworks Section Manager, Health Bureau, Ministry of Health, Labour and Welfare, dated March 5, 2012 ("Settings of management target values for radioactive materials in tap water (PDF: 172KB)" "Details (PDF: 226KB)")

Radioactive substances have never been detected in tap water since March 25, 2011.

Since March 19, 2011, City of Yokohama, Water Works Bureau has been conducting radiological inspections of tap water at three water purification plants in Yokohama.
According to the results so far, all radioactive cesium is below the detection limit and has never been detected.
In addition, radioactive iodine was detected from tap water of some water purification plants supplying water to Yokohama City for three days from March 22 to 24, 2011 immediately after the earthquake, but March 25 It has not been detected since then.

"Not detected (detection limit value II)" means that the concentration of radioactive material in the inspected tap water is lower than the detection limit (minimum concentration that the device that inspects radioactive material can detect). I am.
If the test result is "non-detection (detection limit value ○)," the radioactive material concentration is "less than ○".

It varies slightly each time depending on the condition of the inspection equipment and the surrounding environment.

Although the inspection is strictly performed, the detection limit varies slightly each time depending on the condition of the inspection equipment and the surrounding environment, etc. Each time an inspection is performed, the detection limit at that time (the minimum detectable concentration) is calculated.

It is a collection of impurities such as earth and sand and turbidity in tap water.

Tap water is made based on water from lakes and rivers. The water of this lake and river contains earth and sand and turbidity.
Water purification soil is impurities such as earth and sand and turbidity that are removed when tap water is purified by tap water (water from lakes and rivers) and made.

This is because impurities such as turbidity in raw tap water are concentrated at high concentrations.

Most of radioactive cesium is found in tap water as a turbid component, and is removed along with turbidity during the water purification treatment process at the water purification plant.
By the time 1 kg of purified soil is generated, 20,000 to 26,000 liters of tap water is treated (the average amount of water used by ordinary households in a month is 15,000 liters). In other words, impurities such as turbidity in raw tap water are concentrated 20,000 to 26,000 times.
City of Yokohama, Water Works Bureau has been measuring radioactive materials in the soil generated at Nishiya Water Purification Plant and Kawai Purification Plant. Since FY2012, the measurement of radioactive materials in water purification soil has been completed in FY2021, since the clearance level (100 becquerels / kg), which is a dose that can ignore the effects on human health.

Water purification soil generated at Kawai Purification Plant and Nishiya water purification plants is reused as raw materials for cement and horticultural soil. In addition, clean water generated in Kosuzume Purifiction Plant is properly disposed of at the Nambu Sludge Treatment Plant River Bureau, Yokohama City.