Cyanotoxin transport from surface water to groundwater: Simulation scenarios for Lake Erie

Abstract

Harmful algal bloom (HAB) and cyanotoxin studies in the Great Lakes region have been typically focused on surface-water issues, with few investigating or reporting on groundwater. This study aims to theoretically explore whether groundwater can be contaminated by microcystins from HABs in surface water due to surface-water and groundwater interaction. Specifically, a 3-D MODFLOW/MT3DMS model was developed to simulate pumping-induced reverse groundwater flow and solute transport from Lake Erie to the aquifer underneath the South Bass Island in Ohio. Our simulation results based on typical, base case settings showed that after microcystins were detected and released from the lake, it would take about two, three, and 13 months for the water in a well on the island to reach the EPA advisory levels of microcystin for detection (0.1 µg/l), infants and children (0.3 μg/l), and school-age children to adults (1.6 μg/l), respectively. Furthermore, our scenario analyses showed that, as expected, higher pumping rate and higher lakebed leakance would accelerate the microcystin transport to the well. However, higher hydraulic conductivity would increase the time to reach the EPA levels due to mixing and dilution effects. The 3-D modeling scheme developed in this study was suitable to simulate the complex surface-water and groundwater interaction and transport processes occurring in the Great Lakes. This theoretical study provides useful insight for managing coastal groundwater aquifers and resources under threat from HABs in the Great Lakes. Future improvements to the model would include incorporating reactions and fractures and obtaining water-quality data for model calibration.