Estimating the depth to sulfide-bearing materials in Upper Cretaceous sediments in landforms of the Maryland coastal plain

The unoxidized portions of Upper Cretaceous (UC) regolith in Marylandʹs inner Coastal Plain have been observed to contain significant amounts of microscopic pyrite. The exposure of these sulfide-bearing sediments initiates rapid acid sulfate weathering processes resulting in extremely acidic soils and runoff. Previous anecdotal observations have shown that the depth to the sulfide-bearing zone can be highly variable, ranging from less than 2 m to over 12 m. This depth appears to be primarily a function of landscape hydrology, which is in turn controlled by stratigraphic and geomorphological factors. Deep borings along 6 transects in three landscapes were made to characterize the boundary between oxidized and unoxidized materials and to determine the depth to sulfides. The boundary between the oxidized zone and the underlying unoxidized zone marked the depth to sulfides, and was easily distinguished in the field because of the sharp morphological contrast. The change from the brownish oxidized zone to the black or dark grey unoxidized material was abrupt and easily identifiable. Laboratory analysis showed that there were accompanying marked increases in chromium-reducible sulfide (CRS) across the morphological boundary (from essentially 0% above the boundary to >1% below the boundary, over distances generally ≤10 cm), indicating that the morphological boundary determined in the field was accurate. Changes in soil pH across the morphological boundary were much less pronounced. Moist incubation of the sulfide-bearing materials showed that they met the definition of sulfidic materials. Data were further analyzed to determine the relationship between depth to sulfides and a simple topographic variable, from which landscape models were developed for describing the depth to sulfides. Generally, sites with higher degrees of dissection had greater depths to sulfides, whereas at all sites, sulfides did not occur at depths shallower than 3 m. Strong correlations between depth to sulfides and point relief (the difference in elevation between the point of interest and the hydrologic base point in the landscape unit) were obtained, confirming that topographic characteristics are related to the depth of sulfide oxidation. The relationship between depth to sulfide-bearing materials and point relief in the UC sediments was best described by highly significant exponential models. Using these models and a digital elevation model, the depth to sulfides could be adequately predicted.