A hydrochemical investigation and socioeconomic assessment in Rio Zapomeca river basin focusing on arsenic contamination

Abstract: The awareness of problems concerning arsenic contaminated drinking water sources has during recent years increased. WHO (World Health Organization) decided in 1993 to lower the recommended limit of arsenic in drinking water from 50 μg/l to 10 μg/l, mainly due to observations of its carcinogenicity. New places where the WHO limit for arsenic is exceeded are constantly discovered all over the world. Several areas in Nicaragua in Central America have since 1996 been discovered to have arsenic concentrations above the recommendations. This Minor Field Study aims to investigate the origin and triggers of the arsenic that is contaminating the drinking water sources in the river basin of Rio Zapomeca located in central Nicaragua. The study also evaluates different drinking water distribution methods and assesses the need of arsenic filters, so called Kanchan filters, by doing an analysis according to the principles of cost-benefit-analysis. Nicaragua is located close to a subduction zone resulting in tectonically active geology including earthquakes, active volcanoes and geothermal activity. The river basin of Rio Zapomeca is no exception with both hot springs and strong faulting. The geology in the river basin is dominated by two Tertiary groups named Matagalpa and Coyol. This study includes hydrochemical measurements of pH, temperature, salinity, conductivity and total dissolved solids. Arsenic analysis of drinking water sources and collected rock samples were also carried out. Furthermore, resistivity surveys and groundwater level measurements were done. The arsenic analysis showed that 15 of the 20 analysed drinking water samples had an arsenic concentration above the WHO limit (10 μg/l). The highest value of 104 μg/l was found in Los Negritos. The results of the investigations showed that the correlation between arsenic and the hydrochemical parameters pH, temperature, salinity and total dissolved solids is weak. The correlation with conductivity the highest showing an R2-value of 0.47. The decrease of arsenic in relation to increase of elevation shows a clear trend in the project area but the reason is unclear, perhaps the increased elevation results in shorter retention time due to higher hydraulic gradient and thereby less time to dissolve arsenic. The clearest connection found is that elevated values of arsenic, both in the rock samples and in the water, occur along the contact zone between the rock type groups Coyol and Matagalpa in the southern to south-western part of the Zapomeca basin. The geothermal activity does probably affect the arsenic concentration in the groundwater, since the solubility of arsenic in water increases with temperature. However, to conclude the geothermal involvement concerning the arsenic contamination in the river basin of Rio Zapomeca further studies are needed. The decrease in life expectancy for persons living in the river basin of Rio Zapomeca due to drinking the arsenic contaminated water was calculated using two different methods. The first method which included calculations regarding the percentage of life reduced resulted in an average decreased life expectancy of 3.2 years. The second method was calculated with the relative risk methodology that 26% of the deaths in the river basin can be linked to arsenic contaminated drinking water. Assuming that each person that dies of arsenic contaminated drinking water loses 15 years of lifetime makes the life expectancy decreases with 3.9 years. The value of statistical life (VSL) of a Nicaraguan was estimated to a value of $260 000. By using the VSL it was proven that it is economically valid to invest in Kanchan filters for the population living in the Rio Zapomeca river basin. The payback for each dollar invested varies depending on which discount rate that is used. Calculating conservatively and with a high but reasonable discount rate the payback per invested dollar is at least $2.6 thus making it a sound investment.