A GEOLOGICAL FRAMEWORK FOR THE OCCURRENCE AND DISTRIBUTION OF NATURALLY-OCCURRING ARSENIC IN THE PERMIAN (LEONARDIAN) GARBER-WELLINGTON AQUIFER, CENTRAL OKLAHOMA

The concentration of arsenic that occurs naturally in drinking water produced from portions of the central Oklahoma aquifer commonly exceeds EPA mandated standards (currently 50 ppb, falling to 10 ppb by 2006). In order to assist in development of remediation practices, we are working closely with EPA and USGS for the express purpose of providing a geological framework that can be used to better define the controls on the occurrence and distribution of arsenic in the aquifer system.

In south central Oklahoma, the Garber Sandstone and Wellington Formation are continental red beds that contain sandstone, mudstone, and rare intrabasinal conglomerate. Lithofacies, lithofacies associations, and analysis of bounding stratal surfaces from outcrops and cores suggest that the best-developed aquifer sandstones were deposited by braided-to-meandering fluvial systems that prograded from the east and south across a broad and muddy low relief alluvial plain. Correlation of stratal packages internal to the Garber-Wellington is difficult because of an absence of stratigraphic markers. Primitive soil structures and hardpans are common, though good evidence for flora and fauna is rare.

Integration of observations from outcrops, cores, and subsurface well logs with whole-rock geochemical data collected by the USGS indicate that arsenic is present in the sedimentary rocks that make up the aquifer. Moreover, the data demonstrate that the arsenic concentration in the red beds varies strongly with the lithology and grain size of the red beds; finer-grained lithofacies are enriched in arsenic while coarser-grained lithofacies contain much less arsenic. The enrichment of arsenic in the finer-grained lithofacies is because of the affinity of arsenic for iron oxide and clays, both of which are abundant in finer-grained lithofacies (such as mudstone).

In response to this finding, we have prepared maps of aquifer lithofacies (gross sand, net sand, shaley sand) and paleodepositional environments based on the outcrop, core, and well log data. These maps can be used to identify geographic areas where the potential for the occurrence of arsenic in the red beds is low. Based on the net-sand maps, we can identify several undrilled areas that appear to contain thick, well-developed sandstones with low arsenic potential.