Aquifers that lie deep under the "burbs" of many major cities can be permanently contaminated to highly toxic levels well in advance of the "burbs" even being built. In cases where agricultural land use is not the source of deep aquifer contamination the answer lies elsewhere. The mechanism at work is simple, direct, and very easy to understand. It works like this.
As cities build out & expand, building materials such as Rock, Sand, & Gravel are a basic requirement to make asphalt & concrete. Material pits were usually initially located out on the fringes of developed areas to keep cartage costs low, and pit usage continued as the urban sprawl eclipsed them. They were dug out to the leased, or owned boundaries, and went down in terraces much like an open pit mine. At some point digging into ground water was inevitable, and when they reached that depth, the usefulness as a Rock, Sand, & Gravel material source ended. At that point, material pits were retasked as "dumps", or landfills. Prior to the 1980's there were very few restrictions on either the Commercial, or Private sector with regard as to what any individual, or business could haul to the local dump. There was an amazing lack of foresight, and about the only thing that didn't get dumped locally was nuclear waste. Once they got filled back up with the worst stuff known to exist, they were topped off with some fill dirt, & declared Real Estate again. They were actually great methane generators, and full of surprises for anyone foolish enough to build on top of them. And all the time leaching their toxic load directly into the groundwater they have breached at their bottom.
Southern California is a good example of how massive this problem can be. The aquifer that lies under The San Fernando Valley was deemed too contaminated for use by the end of the Second World War. And since there were dozens of material pits retasked as dumps after the end of the war, the problem of remediation has been bigger than any solutions yet proposed. The former pits are still draining toxins into the groundwater today.
Technology might be up to a remediation attempt though. There was an article last year that announced a project that's aimed at reclaiming So Cals lost aquifer resources. Best of luck to them, but I have my doubts. Good article though. Most people I grew up with didn't have a clue there was even water down there, much less that it was poisoned. Here's a link to the article. And a pic of a wellhead at the corner of Vanowen & Laurel Grove in North Hollywood. It was painted green when I was a kid, no fence around it then, just a bike rack for the library. There are wellheads all over the valley, & I guess they use them to take samples from time to time. Wishful thinking at it's finest.
patch.com/california/studiocity/massive-project-aims-clean-valleys-contaminated-groundwater
