So, there are two types of chromium that we need to think about at the moment: the benign, natural form of the chemical, a.k.a. trivalent chromium, which helps people break down glucose; and the toxic form, hexavalent chromium, which is used for industrial purposes like electroplating, steel manufacturing, leather tanning and wood treatment, and can cause stomach and intestinal cancer. The key characteristic to understand about the difference between these two forms of chromium is its ability or lack thereof to move through the environment. Trivalent chromium does not move through the environment easily, instead sticking to minerals in soils and sediments; meanwhile, hexavalent chromium is quite soluble and moves easily within the environment.
That ability of hexavalent chromium (chromium-6) to move through the environment has raised concern in the past on the topic of groundwater contamination. But scientists previously were under the belief that chromium-6 was only present due to industrial leaks. Now a team of Stanford researchers are questioning that.
After analyzing almost 90,000 chromium values from California's Groundwater Ambient Monitoring and Assessment Program (GAMA) database, the scientists discovered that from the roughly half of these measurements that monitor California's public drinking water supply, 15% contained higher levels of hexavalent chromium than 10 parts per billion (CA’s drinking water standard). This was concerning, expressed co-author Scott Fendorf, but also intriguing. "While the highest concentrations of chromium in any single well are clearly from industrial leaks, the greatest number of wells and the largest geographic distribution are all from this naturally occurring chromium,” he said.
Published in the journal, Environmental Science & Technology, the results from the new study do indeed suggest that industrial leaks of chromium-6 aren’t the only contamination to groundwater; naturally occurring sources of chromium are also contaminating drinking water resources. But the bigger question is what is triggering the natural, benign form of chromium to transform into the toxic chromium-6. The authors have a feeling that the transformation is anthropogenically driven by activities like groundwater pumping.
"We have to start considering the pathways by which we might inadvertently create the bad form of chromium from the good and end up contaminating our aquifers," said Fendorf. "The best thing we can do is identify the processes that accelerate natural oxidation, and then try to develop management practices that will minimize them.”
Based on their analyses, the researchers determined specific regions where the transformation from trivalent to hexavalent chromium is occurring (mainly in California's coast and Central Valley), and where there are high concentrations of hexavalent chromium (near industrial LA and the Bay Area). The authors of the study hope that this information will highlight the complexities of groundwater management in order to better inform policies and regional practices.
"We have really been focused on industrial contamination, but this paper provides a visual representation of all of the different processes that are responsible for chromium-6 contamination, and it highlights how regional these controls can be," said lead author Debra Hausladen. "It's really important to have this holistic view as we develop groundwater management strategies."