We examined two sediment cores (0–6 m bgs) emphasizing As solubility and mineralogy. As content was elevated in the sediment and pore water throughout the cores. Mineral association of As(III) and As(V) in the sediment cores changes with depth. As solubility was the highest in the sediments that are rich in carbonates. As occurs in groundwater above the MCL in the basin-fill aquifers of southwest US.