A comparative analysis of two methodologies to estimate well protection zones for transport of viruses from septic tanks in volcanic aquifers in Costa Rica

Abstract

High groundwater demand in the Great Metropolitan Area (GMA) of Costa Rica, combined with the presence of many septic systems, imposes the necessity to define adequate dimensions for setback distances or protection zones around wellheads to prevent contamination of aquifers by pathogenic organisms. Numerical 3-D simulations of transport of viruses under natural hydrogeological conditions of the northwest section of GMA were performed, considering both porous and fractured porous media with variable saturation. Based on the sensitivity analysis, principal transport parameters for the conditions in the study area are injection concentration, hydraulic conductivity, inactivation rate and attachment rate. Furthermore, a comparative analysis of the arrival of viruses to the water table and the modeling domain’s boundary was carried out using two approaches: maximum permissible concentration criteria and travel time criteria (TT) of 100 days frequently applied in Costa Rica for fractured rocks aquifers. The analysis indicates that under conditions of low mobility and high removal rates (typical for soils of volcanic origin), the TT approach overestimates the dimension of the protection zone. On the other hand, when conditions of high mobility and low removal rate are considered, the TT approach usually underestimates that dimension. It is recommended to perform more field and laboratory studies to obtain better representative parameters for soils of volcanic origin.