Predicting depth to sulfidic sediments in the Coastal Plain of Virginia

Abstract

Construction through sulfidic materials in the Coastal Plain province of Virginia has resulted in localized acid sulfate drainage that threatens water quality, fill stability, integrity of building materials, and vegetation management. Information regarding the likelihood of encountering sulfide-bearing sediments in construction zones can help minimize the negative impacts that result from the exposure of these materials. The objectives of this study were to evaluate field relationships between depth to sulfide-bearing sediments and landscape parameters, and to test models for predicting depth to sulfides. A study area in Hanover County, Virginia was evaluated using landscape parameters including elevation, slope, distance to streams, and surficial geology to predict depth to reduced sediments (depth-rs). Actual depth-rs values were interpreted from stratigraphic data for 408 well logs obtained from the Hanover County Health Department. A regression model could not be developed to accurately predict depth to sulfidic sediments based on the landscape parameters. Similarly, interpolation using a random subset of the well log data was unsuccessful at predicting depth-rs for the remaining points. However, since excavation depths in the study area are typically less than 9 m a procedure was developed to evaluate the likelihood of encountering sulfidic sediments within this depth based on two risk factors - elevation and soil type. This procedure accurately described the likelihood of encountering depth-rs within 9 m for 90% of 58 test points. Samples collected from twenty-three deep borings all had relatively high sulfur values and did not contain calcium carbonate, indicating that exposure of Tertiary sediments would always present a high risk of acid production.