Fogo is an island with a surface of around 237 km², located in the northeast of central Newfoundland. Around 2,400 people live on the island. The geology mainly consists of granites with an age of around 410 million years. Some dolerite intrusions as well as ash-flow tuffs appear locally.
It is known that a statistically high number of residents on Fogo Island suffer from cancer (personal communication with the local hospital), although the cause for high cancer rates is not confirmed. However, it is assumed that consumption of untreated water might be the cause, if high uranium (U) and/or arsenic (As) levels are present. This assumption justifies the investigations presented here.
A water sampling campaign was conducted in August/September 2018 to assess concentrations in different water supply wells on Fogo. Analysis revealed that 6 out of 20 groundwater samples in Central Fogo and Joe Batt´s Arm show exceedances of the maximum admissible concentration (MAC = 10 ppb) and 3 out of them exceeded 20 ppb. Additionally, the MAC for uranium (20 ppb) is exceeded in 9 out of 20 groundwater samples. Except one sample, all wells with high arsenic concentrations show concurrently elevated uranium concentrations. No arsenic or uranium exceedances, however, are found in any of the surface water samples from brooks and springs or in Joe Batt´s Arm in the north. In Central Fogo, As is elevated east of Highway No 333, while samples in the west do not show values exceeding MAC, potentially indicating different groundwater flow paths, here.
Curie (2003) as well as Sandemann and Malpas (1993) analyzed uranium concentrations in 53 and 9 rock samples (mainly granites), respectively, in different areas on Fogo Island. In contrast to concentrations found in groundwater, these measurements reach maximum values of 9 ppb in Shoal Bay granite and 7 pbb in ash-flow tuffs, only. Different processes are known to enhance accumulation of radionuclides in groundwater (physical recoil, chemical weathering and milieu changes, e.g. low redox potential fronts). Long groundwater residence times, and thus contact times with the bedrock, are crucial for these processes.
The wide range of As and U concentrations over a short distance of around 2 km in Central Fogo suggests that different groundwater flow paths exist within this limited area. Despite no MAC exceedances were found in treated water samples, e.g., by reverse osmosis, it cannot be concluded that such water samples are always free from As or U. Monitoring of U and As on Fogo island in drinking water is strongly recommended in future.