OCCURRENCE OF ELEVATED SULFATE IN HANFORD SITE GROUNDWATER - RECOGNITION OF NATURAL VADOSE ZONE SOURCES

Elevated sulfate concentrations have been observed in groundwater samples collected on or in the vicinity of the Hanford Site through years of monitoring activities. Flow pathways and the nature of sources are related to this sulfate signature. The relative concentration of sulfate to other major cations and anions thus provides a clue to the history of the local groundwater system and can be used in some cases to help distinguish natural versus anthropogenic (waste site) sources. This is especially important in cases where the concentration of sulfate exceeds a drinking water or other regulatory standard.

Several areas of elevated sulfate concentrations in groundwater have been recently recognized on or near the Hanford Site that are believed to be due to natural sources. Elevated sulfate levels were observed in groundwater about seven years ago in conjunction with disposal of clean water from a facility to the vadose zone and became a regulatory concern. It was determined through laboratory leaching studies of sediment core that the source of the sulfate was the vadose zone, probably owing to minor amounts of gypsum in the sediment. More recent sampling of groundwater to the south of the Hanford Site has also indicated levels of sulfate elevated relative to other ions (e.g., a sulfate enrichment trend versus the normal sodium enrichment trend associated with rock/water interaction). This occurrence of sulfate enrichment in groundwater appears to be associated with dilute irrigational water released to the vadose zone and, again, is ascribed to the dissolution of minor amounts of gypsum present in sediments.

Evaluation of groundwater chemical data has thus resulted in the identification of areas of elevated groundwater sulfate levels in the vicinity of the Hanford Site. In some instances, these data indicate an evolutionary pathway distinct from that associated with normal rock-water interaction and are related to releases of large volumes of water to the vadose zone in association with agricultural or uncontaminated water disposal practices. In other cases, chemical trends indicate that sulfate enrichment is associated with past waste disposal practices.