Few Israelis have ever stopped to wonder if the water they are drinking is kosher or not. I discovered that the Haredim hadd spent an enormous effort in learning the details of Israel's water supply infrastructure just to be able to debate this issue. In fact, incredible as it sounds, the Haredim know more about water than the average engineer.
The problem is very complex. Most of Israel's water comes from the lake of Galilee, the Kinneret. If an Arab pisses in the Kineret, does it make the water non kosher? The answer is not. You can drink it without violating the Halacha. Now if a Jewish fisherman uses bread crumbs to trap the fish, will the water become Hametz (unfit for the Pessach when leavened bread is forbidden). The answer is, yes. The kashrut of whole the Israeli water supply system will be doubtful. קיים חשש לחמץ = a really pious Jew will not drink water during Pessach.
The solution? Jerusalem water system disconnects itself from the national network for the duration of the Passover. They dont let in even a drop of that suspicious impure water. So Jerusalem's water is fit for a Haredi during the Passover. But what about the poor Haredi who lives in Bney Brak near Tel Aviv? Drink bottled water? Dig a well in the yeshive? Collect rainwater?