Utilization of Stable Water Isotopes and Geochemistry for Detecting Seepage Pathways within North Texas Dam

Identifying geologic and geotechnical related potential failure modes for dam safety risk assessments (backward erosion piping, suffusion, suffosion, etc.) rely on characterizing inherent variabilities and uncertainties within the constructed embankment and geological foundation. Subsurface physical and geochemical reactions from changing hydrologic loads on the dam will affect foundation and/or embankment soil performance over extended periods of time, necessitating dam risk analysis on a regular basis. The geochemical and isotopic compositions of water from groundwater, reservoir, piezometers, spillway, and seepage sites were comprehensively investigated at a North Texas dam. This survey utilized isotopic fractionation processes of deuterium and oxygen-18 to provide quantifiable “fingerprints” of water molecules from each water source. Combined with additional geochemical analysis of the groundwater, where suspected hydraulic pathways were tested against the possible cations/anions contributed by mineral dissolution in the soil, the source and degree of mixing of dam seepage could be approximated. Baseline isotopic signatures were established for groundwater (δD -25.6‰, δ¹⁸ O -4.5‰) along with reservoir isotopic ranges (δD -9.10 to -20.00 ‰ and δ¹⁸ O -3.37 to -1.00 ‰). By using a binary mixing model, these values were compared to the isotopic signatures from piezometers and suspected seeps to find subsurface hydraulic connections. At the site, a connection between a seep and the reservoir was found. Isotopically, the two samples were located on the same trendline with similar fractionations (seep δD -18.3‰, δ¹⁸ O -3.04‰; reservoir δD -20.0‰, δ¹⁸ O -3.37‰). Comparing the two samples geochemically a high amount of sulfate contribution (from 31 mg/L to 626 mg/L, a 2,019% increase) lead to the discovery of a potential seepage path through gypsum bearing embankment clay construction fill.