GROUNDWATER QUALITY ASSESSMENT BASED ON SAMPLES COLLECTED FROM SHALLOW DOMESTIC WELLS IN THE CENTRAL ARKANSAS AREA OVERLYING THE FAYETTEVILLE SHALE NATURAL GAS PLAY

One hundred five groundwater samples were collected from shallow domestic wells in six counties across central Arkansas overlying the Fayetteville Shale Natural Gas Play in order to establish a spatially distributed data set to define groundwater quality across this geologically heterogeneous area. Residents across the region expressed concerns about potential impacts to groundwater quality resulting from the extensive development and extraction of natural gas. Shallow domestic groundwater wells throughout the region are generally completed in the Western Interior Plains confining system characterized by multiple lithologies dominated by low permeability interbedded sandstone and shale. Groundwater storage is primarily developed in secondary porosity including fractures, faults and bedding planes, which also represent the main permeability, resulting in flow systems that are not laterally extensive. Groundwater wells are plagued with extremely variable yields and poor water quality. Because of the very low permeability and lack of a regionally significant aquifer throughout the region, little historic water-quality data were available. Pre-existing groundwater quality data consisted of 43 samples collected over an extended period (1951-1997), and the data reported were not uniform for all the samples. Our samples therefore represent the best background groundwater quality data set available for the region. Standard water quality field parameters were collected and samples were analyzed for major inorganic chemistry and selected trace elements. Groundwater chemistry across the study area was as heterogeneous as the aquifer, controlled by ion exchange and redox processes, and directly related to the lithology of the specific borehole. Groundwater quality throughout the area is impacted by elevated iron, manganese, and sulfate, and some areas of elevated total dissolved solids (TDS), among others. However, all 105 samples were within pre-existing groundwater quality parameters for central Arkansas as compared to the historical data, and there was no observed impact related to natural gas development.