Isotopic evidence of natural perchlorate in the Principal Aquifer of Las Vegas Valley, Nevada (USA)

Jack M. Childress and Christopher S. Ashinhurst

ABSTRACT

In 2017, four wells completed in the Las Vegas Valley (LVV) Principal Aquifer for municipal supply, non-potable supply, and/or monitoring of the Principal Aquifer were sampled and analyzed for the stable isotopes of oxygen and chlorine present in perchlorate to better understand the origin of perchlorate in the groundwater system. The results indicate that the isotopic signature of perchlorate in groundwater obtained from the four wells is incompatible with the signature of synthetic perchlorate and is indistinguishable from the isotopic composition of natural perchlorate obtained from soil salts from Death Valley, California, roughly 80 miles (130 km) to the west of Las Vegas. Because of the similarity with the Death Valley perchlorate isotopes, the four LVV samples are likely natural perchlorate mobilized through the vadose zone via secondary recharge from irrigation systems (lawns, parks, and golf courses), pipe leaks, and/or septic tanks.

Perchlorate has been a constituent of interest in the LVV groundwater for over 30 years. The origin of perchlorate in groundwater from natural or synthetic sources has also been an area of interest. Synthetic perchlorate associated with historical industrial production and storage in Henderson, Nevada was released into the shallow groundwater system and mobilized through the Las Vegas Wash to eventually reach Lake Mead and the Lower Colorado River. When first detected downstream in California in 1997, detection limits for perchlorate were comparatively high and consistent identification of perchlorate below 100 µg/L was not possible until later that year. Investigators traced the source of the perchlorate upstream from Lake Mead through Las Vegas Wash to Henderson, and an intensive environmental clean-up and monitoring effort began.

Colorado River water (CRW) from Lake Mead was used as a water source for the LVV beginning in 1971. CRW provided a source of secondary recharge to the Principal Aquifer primarily through irrigation and infiltration of septic effluent. The Las Vegas Valley Water District (LVVWD) implemented a permitted aquifer storage and recovery (ASR) program beginning in 1987 utilizing excess CRW to stabilize subsidence and depleted groundwater levels in the valley and bank water in storage for future use. CRW introduced to the Principal Aquifer from the ASR program and secondary recharge from septic system effluent and irrigation sourced from Lake Mead locally influenced groundwater chemistry.

Keywords: perchlorate isotopes, forensic geochemistry, Las Vegas aquifer, water quality, oxygen anomaly
Authors: JACK M. CHILDRESS, P.G., Southern Nevada Water Authority, 100 City Parkway Drive, Suite 700, Las Vegas, Nevada 89106 (jack.childress@snwa.com); CHRISTOPHER S. ASHINHURST, Southern Nevada Water Authority, 100 City Parkway Drive, Suite 700, Las Vegas, Nevada 89106 (chris.ashinhurst@snwa.com)
Citation: Childress, Jack M. and Ashinhurst, Christopher S., 2025, Isotopic evidence of natural perchlorate in the Principal Aquifer of Las Vegas Valley, Nevada (USA). Journal of the Nevada Water Resources Association, Winter 2025, p. 4-26. DOI: 10.22542/jnwra/2025/1/1.
DOI: 10.22542/jnwra/2025/1/1.

Geochemistry and radiocarbon age of a fen in Big Smoky Valley, Nevada

Sade K. Cromratie Clemons, Geoff Moret, and Kathering J. Earp

ABSTRACT
Fens are groundwater-fed wetlands that can provide habitat for plants and animals. Due to anthropogenic activities and climate change, many fens around the world are at risk. This paper presents the results of a study of the hydrology and geochemistry of fens in Big Smoky Valley, central Nevada to support the Bureau of Land Management’s activities in the area. A water sample from the largest fen in the study area was analyzed for its water chemistry and compared to a nearby alluvial aquifer and hot spring. The high SiO2 concentration of the fen sample implies that the fen water may originate from geothermal water. A soil core was taken to analyze radiocarbon age and soil type. A majority of the core was composed of silt and clay interlayered with water-filled voids. Changes in the character of the clay with depth suggest that there may have been changes in the depositional environment over time. Radiocarbon dating of Ruppia seeds showed longevity of the fen, with the minimum 14C age of the core as 4,375±40 years. This paper provides reconnaissance-level information on the Big Smoky Valley fens, but further information would be needed to better understand the source of water to the fens or how the fen environment has changed over time with climate.
Authors: SADE K. CROMRATIE CLEMONS, U.S. Geological Survey, 500 Date Street Building 500, Boulder City, NV 89005 (scromratieclemons@usgs.gov), ORCID 0009-0002-2846-7158; GEOFF MORET, U.S. Geological Survey, 500 Date Street Building 500, Boulder City, NV 89005 (gmoret@usgs.gov), ORCID 0000-0002-6589-5699; KATHERINE J. EARP, U.S. Geological Survey, 500 Date Street Building 500, Boulder City, NV 89005 (kjearp@usgs.gov), ORCID 0000-0002-5291-6737
Citation: Clemons, Sade K. Cromratie, Moret, Geoff, and Earp, Katherine J., 2025, Geochemistry and radiocarbon age of a fen in Big Smoky Valley, Nevada. Journal of the Nevada Water Resources Association, Winter 2025, p. 27-40. DOI: 10.22542/jnwra/2025/1/2.
DOI: 10.22542/jnwra/2025/1/2