Missed Steps on the Road to the Radium-SGD Connection

Abstract:

Scientific discoveries are often preceded by missteps that lengthen the road to discovery. Here I recount missteps on my journey that found a strong link between high radium concentrations in coastal waters and the discharge of submarine groundwater (SGD). In 1968 Wally Broecker suggested to me that groundwater could be a source of Ra to the ocean. That summer Wally measured Ra in wells on Martha’s Vineyard; I measured wells on the Mississippi Gulf Coast. Neither of us found significant Ra concentrations; all of the water we measured was fresh. In 1977 Telu Li and others reported the desorption of Ra from riverine particles entering the ocean. I did not think to translate this observation to the release of Ra as salt water intruded into coastal aquifers; instead when Bob Elsinger and I could not balance the Ra budget for the Pee Dee River estuary, we called on upstream migration of the salt wedge during low river flow to release Ra from sediments. Similar budget problems in the Chesapeake Bay led me to propose cliff erosion as a source of Ra. JoLynn Carroll and I used the erosion mechanism to explain Ra budget problems at the mouth of the Ganges-Brahmaputra Rivers. Working with Tom Church, Marsha Bollinger measured significant activities of Ra in groundwater entering Canary Creek, DE. However, we did not consider groundwater a significant source of Ra to Delaware Bay. In 1993, after Marsha and I developed a robust data set for North Inlet, SC, I realized the upstream migration and erosion mechanisms were not adequate. Fortunately Rama, one of my post-doc advisors, was visiting my lab. I presented him with the dilemma of excess 226Ra fluxes. After digesting the data for a few days, he stated: “It must be groundwater”. We confirmed this by measuring salty groundwater samples from North Inlet that were loaded with 226Ra. Extending this concept to the coastal ocean was simple. We had recently completed a cruise to use Ra isotopes to study mixing rates and water ages. Upon inspection I found these coastal waters contained high 226Ra activities that could not be explained by riverine or sedimentary inputs. Taking the line from Rama, I proposed that brackish groundwater must be the answer. Tom Church reviewed the paper for Nature and wrote a news article, “An underground route for the water cycle”. It tool 25 years, but we finally completed the journey.