January 6, 1997
Improved techniques needed for accurate measurements of trace metals in groundwater
Standard techniques for measuring metals in groundwater may drastically overestimate their levels, forcing owners of supposedly contaminated sites to "remediate" problems that may not exist.
Graduate student Carol Creasey and professor of earth sciences Russell Flegal, both of UCSC, studied the groundwater from two wells at a reportedly contaminated site in central California. A consulting company also monitored the wells for trace-metal contamination using standard sampling and analytical techniques. The differences were striking: The concentrations of trace metals detected by the UCSC scientists ranged from 2 to 1,000 times lower than those reported by the consultants. In particular, the consultants claimed that the levels of cadmium and chromium in the groundwater exceeded the state's clean-water guidelines, whereas the UCSC tests revealed no such infractions.
Two key factors accounted for those discrepancies. First, the UCSC team used "low-flow" techniques to purge stagnant water from the wells and collect their samples, extracting less than one liter of water per minute. Pumping more quickly than that, Creasey says, can generate groundwater that is not representative of the natural system.
Second, Creasey and Flegal used rigorous "trace-metal clean" techniques at the site and during their laboratory analysis. These techniques prevent inadvertent contamination by ensuring that all materials touching the groundwater are thoroughly cleaned with acids and isolated from metallic objects or dusts. Also important, says Creasey, is to have more than one person assist with the water sampling so that samples are not contaminated.
Analysis of seven metals in the samples at UCSC showed that none of the concentrations exceeded federal or state standards, except for nickel in the most heavily impacted well. However, the consultants--who monitored the wells both five months before and one month after the UCSC team--found that levels of nickel, cadmium, and chromium violated the standards in one or both of the wells. For example, the consultants measured 51.4 parts per billion of chromium in one well (compared to a state maximum of 50 ppb), but the UCSC researchers detected just 0.0734 ppb, a 700-fold difference.
Creasey and Flegal acknowledge that not all consultants will have access to the precise analytical tools found in their UCSC lab. However, low-flow and trace-metal clean techniques are feasible for everyone, they say. Further, those techniques would raise sampling and analytical costs only moderately.
"Consultants should try these methods to see whether they make a difference," Creasey says. "Extensive monitoring and remediation of a site that may not be contaminated is certainly more costly than conducting the measurements right the first time with more careful techniques."
Return to the Currents home page.
Go to UCSC's home page.