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In areas of water scarcity around the globe, made worse by climate change and pollution of groundwater, rainwater harvesting remains an important source of water supply for rural communities.
This is especially true in the Bolivian altiplano, where drought and mining work together to create pockets of severe water stress in what is generally considered a water-rich country. I lived among these communities high in the Andes for two years working with an organization called the Centro de Ecología y Pueblos Andinos (Center for Ecology and Andean Peoples, or CEPA). I assisted CEPA with a small-scale rainwater harvesting project for rural communities with high needs.
Over the course of the project, CEPA monitored the quality of harvested rainwater through consecutive wet and dry seasons. Surprisingly, we detected arsenic in every tank we monitored, 18 in total, whereas no microbial contamination was found.
This finding alerted CEPA to the risk of rainwater contamination in the region. Further testing identified roof dust that flushes into the tanks from the roof catchment as the principle source of arsenic in the rainwater. No arsenic was detected in raw rainwater before it interacted with the roof or tank. The source of the arsenic in the dust, whether naturally elevated in the altiplano soil or mobilized due to mining activity and released into the environment, is unknown, but widespread mining contamination in the area is likely a contributor.
Given these findings, the implementation of rainwater harvesting as an alternative drinking water supply by nonprofit groups and charitable organizations without adequate monitoring and evaluation of water quality is a potential concern. Since rainwater is presumed to be arsenic-free, rainwater harvesting has been promoted as an alternative drinking water source in other areas affected by arsenic contamination of groundwater as well, such as Mexico, parts of Central America, and Bangladesh. It is not safe to assume rainwater will be entirely arsenic-free, however. The levels found in collection tanks in Bolivia were double the WHO health guideline of 10 parts per billion.
As a result, arsenic and other metals should be included as standard monitoring parameters in rainwater projects. Groups implementing rainwater harvesting projects should seek additional partners with the tools and knowledge to perform thorough water quality testing.
This can be accomplished either through basic field tests, which provide semi-quantitative information for initial screening, or through laboratory analysis. Research done at North Carolina State University found that the standard field method—where inorganic arsenic in a water sample is reduced to arsine gas, which then reacts with a mercuric bromide strip to turn color—tends to underestimate the actual arsenic concentration as verified by ICP-MS (a sophisticated method that detects counts of atoms in a sample at specific molecular weights, allowing for a precise quantitative measurement). However, these low-cost and easily transportable kits still offer an accessible and simple screening tool for the presence of arsenic. The ITS Econoquick, for example, provides 300 tests with a 0.3 ppb detection limit for less than $200. For more precise measurements and longer term use, the Palintest Arsenator includes a standardized digital reading of the colorimetric output for $1,200. Both kits were field tested by CEPA and were easy to use for untrained operators.
In addition to greater testing, practitioners should also consider the required first flush volume for their project. First flush systems are essential for any rainwater harvesting scenario to mitigate both microbial and chemical risks. This is especially true when used as a drinking water source. One rule of thumb is that first flush systems should be able to capture at least 4 liters of water for every 10 square meters of roof. The tanks monitored in Bolivia did not meet this standard. Thus, the risk of arsenic contamination of rainwater and simple controls for system design and monitoring should also be communicated widely through knowledge platforms such as RWSN and the RAIN Foundation.
The results of this monitoring study were compiled by CEPA and a Belgian organization, the Comité Académico Técnico de Asesoramiento a Problemas Ambientales (CATAPA). The full results have been published and are accessible through the journal Science of the Total Environment. This work has also been featured previously by EngineeringforChange.org.
About the author
Riley Mulhern is a PhD student at the University of North Carolina Chapel Hill Gillings School of Global Public Health. He worked previously as a technical water quality adviser for a Bolivian environmental justice nonprofit addressing issues of mining contamination in rural indigenous communities in Oruro, Bolivia. He is from Denver, Colorado and received his B.S. in physics and geology from Wheaton College in Wheaton, IL and M.S. in Environmental Engineering from the University of Colorado Boulder. He has worked previously on water projects in Nicaragua and Haiti.
Photo: Rainwater tank monitored for the study being installed. Photo credit: Maggie Mulhern.