Are deep, drilled wells really safer than shallow, sandy wells?

Another excellent Science Cafe last night - this one about arsenic in the environment, notably in New Hampshire's drinking water, due to a variety of geological factors. (Arsenic also shows up in rice, the only plant that easily takes up the element from the soil. So if you have a rice-heavy diet ... well, be careful.)

One of the things that came up is that arsenic is generally more prevalent in water from deep wells drilled into bedrock as it is in shallow wells, which get water from soil. That's because conditions within bedrock - less oxygen, more acidity - make it easier for arsenic atoms to get free and enter the water.

I have both types of well; we paid for the drilled well after the shallow one ran dry one year. A consolation for the several thousand dollar cost is that my well, which went down 500 or so feet before it hit water, is less liable to surface contamination from, say, spilled gasoline.

Well, maybe not. One of the surprising things I learned last night is a study of New Hampshire wells found no significant difference between drilled and shallow wells when it came to the presence of MBTE, a well-known pollutant in gasoline that showed up in a lot of NH groundwater before it was banned.

It's not clear why this is so. It's partly because water in bedrock fractures move faster than it does which percolating through the "saturated overburden" of shallow wells, but that's not all. Interesting, nonetheless.