My first arsenic post last March reported on the discovery that there were high arsenic levels in groundwater near Tolono, IL. We recently finished a small study of the area and are preparing a short report. Well be presenting the results at a public meeting at the Tolono Town Hall on Tuesday, October 4, 2011, at 7 PM.

The initial sample sent to the Water Survey Public Service Laboratory (PSL) had an arsenic concentration of 344 μg/L, the highest level we had ever seen in Illinois and well above the recommended drinking water standard of 10 μg/L. After results from this and neighboring wells were reported in the local news, we got a flood of calls from concerned well owners. We decided to sample some of these wells ourselves, which allowed us to ensure sample integrity, measure certain unstable parameters on-site, filter and preserve the samples, and deliver them to the PSL the day they were collected.

We ended up sampling 17 wells during March and April. We supplemented these with samples from our Water Quality Database that had been previously analyzed by the PSL. Here’s a map of the results:

There’s a “hot” zone in the neighborhood where the well with the really high arsenic was found. Other than that, the results are spotty, with no real pattern. This is consistent with what we’ve found in other areas of Illinois. Also note that more than half of the wells have arsenic concentrations greater than the 10 μg/L standard.

Perhaps the most interesting result was the relationship between arsenic and well depth:

All the really high levels of arsenic are found in wells drilled to depths between 165 and 180 feet. Thanks to information provided by Andy Stumpf at the Illinois State Geological Survey, we were able to determine that this depth range corresponds to the Pearl Formation:

There are two aquifers in this area, the Pearl and Glasford Aquifers. These are both glacial sand and gravel deposits separated by a till (primarily clay) unit. They are also highly laterally discontinuous, thus are not always encountered during well drilling. (In the figure above, the Glasford Formation appears to be a continuous unit, but sand lenses that produce water within the unit are not.)

We’ll be discussing these and other topics at the public meeting October 4, and I’ll more to say on arsenic in future posts. Here’s the link to my first arsenic post, and here’s a link to the Water Survey’s PSL. The PSL can be reached at (217) 333-9234.