New Mexico Geological Society Annual Spring Meeting — Abstracts

Per- and polyfluoroalkyl substances (PFAS) are human-made, environmentally recalcitrant contaminants that include six chemicals newly regulated under the Safe Drinking Water Act. PFAS chemicals commonly detected in natural waters include perfluoroalkyl acids (PFAAs), which have a wide variety of industrial and commercial applications owing to their hydrophobic and lipophobic properties and ability to withstand high temperatures.

Chains of carbon atoms bonded to fluorine at most or all available bonding sites chemically define PFAS compounds, which otherwise vary tremendously in terms of chain length, molecular weight, structure, and functional groups. PFAS exhibit a corresponding array of fate and transport behaviors. Reported solubilities for PFAAs detected in the vadose and saturated zones range from 10^-9 to 10⁰ mol/L and organic carbon-normalized sediment/water partition coefficients range from log K_oc = -0.8 to 5.3. PFAAs with long carbon-fluorine chains tend to have lower aqueous solubilities and higher partitioning coefficients and can therefore adsorb more strongly to mineral surfaces than short-chain compounds. The presence of anionic PFAS precursor chemicals can further complicate the occurrence of PFAAs in the critical zone and aquifer systems because they do not significantly adsorb onto mineral surfaces under circumneutral pH and can transform to terminal PFAA products under aerobic conditions.

Despite their complex fate and transport properties, recent studies have identified robust predictor variables for PFAS occurrence across the U.S. (Tokranov et al., 2024). Monitoring data from aquifers used as drinking water sources across New Mexico were analyzed to evaluate Spearman rank correlations (ρ) between land use, population density, well depth, well construction age, soil clay content, basic water quality parameters, and PFAA occurrence. Case studies in two areas of known PFAS contamination and two areas with few detections in groundwater to date were evaluated to assess drinking water standard exceedances in the context of this statistical framework.

Exceedances of U.S. Environmental Protection Agency drinking water standards from drinking water sources included in the case studies range from 0% for hexafluoropropylene dimer acid (HFPO-DA) to 2.4% for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). Results indicate non-monotonic to weak correlations (with Spearman rank correlation -0.22 < ρ < 0.20, and significance of p < 0.05) among predictor variables and PFAA detections. The exception was nitrate concentrations, which had a high Spearman rank correlation but was not statistically significant at the p < 0.05 threshold (ρ = 0.68). These findings appear to contrast national patterns in predictions of PFAS occurrence, which include few data points from the Southwest. This could reflect differences in aquifer host geology, recharge, contamination sources, and/or analyses for finished versus untreated water.

References:

1. Tokranov, A.K. et al., 2024, Predictions of groundwater PFAS occurrence at drinking water supply depths in the United States: Science, v. 386, p. 748–755, https://doi.org/10.1126/science.ado6638.