THE CHARACTERIZATION OF GYPSIC SOIL IN CULBERSON COUNTY, TEXAS

Dissolution of gypsic soils beneath and proximal to infrastructure in the northwestern Delaware Basin has caused extensive damage to RM 652 in the northern portion of Culberson County, Texas. Increased usage of RM 652 by larger vehicles has encouraged further damage when coupled with the effects of evaporite dissolution, both in gypsum bedrock and in indurated gypsic soils. The region is dominated by numerous evaporite karst manifestations, both epigene and hypogene in origin, and it is evident that evaporite dissolution has had a profound effect on both environmental and anthropogenic processes. Although the study site is located in an arid environment, monsoonal storms in late summer produce significant overland flow and groundwater recharge, resulting in increased suffosion promoting further surficial instability. The resulting eroded soil then flows through fractures and caves within the Castile Formation, producing unstable suffosion caves. These caves in indurated gypsic soil are of increased concern as significant potential geohazards when proximal to infrastructure.

A number of analyses have been implemented for characterization of gypsic soils proximal to RM 652. Field analyses include collection of soil cores. Laboratory analyses include geochemical analyses and soil solubility characterization: 1) soluble fraction of soil samples was based on gypsum solubility; 2) carbonate/bicarbonate analyses were conducted using an automatic titrator; and 3) chemical analyses of soluble soil extractions was conducted through ICP-MS to characterize major cation and anions. Based on these analyses, solubility in samples can exceed 75% with average solubility soluble fraction increasing with depth. Data plotted in piper diagrams indicate three defined geochemical trends with the soils: 1) Soils with elevated sulfate and chloride; 2) Soils with elevate of carbonates and chloride; 3) Soils dominated by sulfate. Current research is being used to refine soil characterizations based on soil solubility for improved infrastructure engineering within this complex karsted terrain.