密歇根新闻广播 弗林特已测试超过3万个水样

Last month, the state of Michigan declared Flint's drinking water quality "restored." To get to this point, it's taken, among other things, more than 30,000 water tests.

Since the crisis, Flint's drinking water system has been one of the most closely monitored in the country. That's left us with a whole lot of data, which we compiled into one graph.

More than 30,000 water samples have been tested from homes in Flint since the water crisis made national news. Today, Flint's lead levels are well below federal action limits. Click image to enlarge.

Most of the data comes from the first half of 2016, just after the crisis became national news. Testing has slowed down significantly since then, but it's still going: more than 350 samples have been tested this year so far.

When you remember that every data point represents somebody sending off a sample bottle of their water and waiting to hear back about whether or not it was safe, it really hammers home what the city went through.

When you look at the full range of measurements (top panel of the graphic¹), it's hard not to notice the sheer ridiculous amount of lead in some of the samples. Dozens have measured more than 1,000 parts per billion (ppb) - the highest was almost 23,000, from a sample taken in April of 2016. Those numbers are outliers: less than one percent of all the samples collected, but they're still shocking. While there's no safe level of lead, the federal standard is just 15 parts per billion.

That said, most of the measurements (more than 90%) taken during this time period were less than 45 parts per billion, so the bottom panel of the graphic zooms² in on that range to show more individual points and the overall progression of water quality.

From a regulatory standpoint, the water quality of a city is defined by its 90th percentile; that's the lead level that 90% of samples test below for a given time period. The samples used to calculate this number, called "Tier 1" samples, come from the highest risk homes.

Things have definitely gotten better. For the first half of 2016, the 90th percentile was 20 ppb -- so it was above the federal standard. For the second half of 2017, it came in at six parts per billion. It's been below the federal standard for 18 months now. So, when you hear the state saying Flint's water quality has been "restored," that's what they mean.

1 graphic   

adj.生动的，形象的，绘画的，文字的，图表的

参考例句：

• The book gave a graphic description of the war.这本书生动地描述了战争的情况。
• Distinguish important text items in lists with graphic icons.用图标来区分重要的文本项。

2 zooms   

n.嗡嗡声( zoom的名词复数 )；隆隆声；（车辆等）疾驰的声音；变焦

参考例句：

• A sneeze zooms out of your mouth at over 600 m.p.h. 从你口中打出的喷嚏速度可达每小时600里。 来自互联网
• When the camera zooms in, the whole world watches out. 当镜头对准（汶川），全世界都在关注。 来自互联网