Indicators of Arsenic in Groundwater
Tom Holm, Center for Groundwater Science, Illinois State Water Survey

Most of my presentation will be on arsenic (As) in groundwater. Some wells in Tazewell County have As concentrations above the USEPA maximum contaminant level (MCL) of 10 ppb. The source is dissolution of iron oxide coatings on sand grains which releases sorbedAs, the same process that has poisoned thousands of wells in south Asia. In some areas, As is removed from groundwater by co-precipitation with ferrous sulfide. There are characteristic relationships between As and redox-sensitive substances (Fe, Mn, SO42-, etc.). A simple two-step procedure with parameters derived from data collected in a recent project accurately predicts which wells studied in other recent projects exceed the As MCL. The procedure may be useful in screening water samples. Does it work for other parts of the state? Come to the seminar and find out.

The second part of the seminar will deal with oxidation-reduction potential (ORP) measurements. The ORP is sometimes a useful, if qualitative, indicator of redoxconditions. An ORP probe is typically calibrated using a redoxbuffer with a standard potential of +428 (ZoBell) to +675 (Light) mV, which is outside the range of potentials measured in anoxic groundwaters, typically -50 to +150 mV. I am working on a redoxbuffer with a well-defined potential of ~60 mV, which is within this range.