Spatial Variability of Arsenic in Groundwater

Approximately 50 Illinois public water systems have source water with arsenic (As) concentrations that exceed the maximum contaminant level of 10 micrograms per liter. Some of these systems may consider drilling one or more new wells in an attempt to locate low-arsenic water. Recent research by the Illinois State Water Survey and other agencies has found that As concentrations can vary dramatically among wells that are separated by distances of 1-10 kilometers (km). The objective of this research was to characterize the variability of As concentrations over distances of tens to hundreds of meters and determine the feasibility of a process that a small water system could use to site a new well with low-As water. Two clusters of 10 to 20 private wells of 1 to 2 km in diameter in Tazewell County and 10 private wells in Wonder Lake in McHenry County were sampled for As and supporting geochemical data. The maps of As concentrations show the complexity of As spatial distribution in these areas and the process a water utility may follow to locate low-As water.