A combined geological, hydrochemical, and geophysical approach to understanding a disease contamination hazard in groundwaters at a state fish hatchery

Abstract

At the Midway, Utah, USA fish hatchery, a groundwater development program was conducted to help transition the facility from surface to groundwater in response to contamination by whirling disease, which is caused by a trout parasite. The unconfined aquifer system that provided the hatchery water became infected through the recharge of infected irrigation water obtained from the Provo River. Whirling disease was first discovered in Utah in 1991 at a private fish farm. Infected fish from the farm quickly infected many of Utah’s waterways and infected the hatchery in 2000. Because the parasite completes its life cycle in multiple organisms and can survive for decades in a variety of harsh environments, a comprehensive study of the hydrostratigraphy and hydrodynamics at the hatchery was critical in order to understand the hazard and avoid further contamination. Drilling revealed the presence of a shallow unconfined (surface to 10 m) and two deeper confined aquifer systems (~20–35 m and >40 m bgs). Confinement is related to tufa layers, detected both by drilling and reflection seismology. The tufa layers are associated with past discharge of the thermal system. Vertical leakage is apparent from upward hydraulic head gradients and incrementally increasing unconfined aquifer discharge into downstream on-site drainage canals. High-resolution seismic profiles reveal small-offset faults that provide pathways for upward flow. Analysis of water quality data demonstrates an inverted geochemical gradient in that apparent ¹⁴C ages, solute concentrations, and temperatures decrease with depth. The origin of the inverted geochemical gradient is related to mixing of upwelling thermal, high-TDS waters with cold, low-TDS systems several kilometers up-gradient from the hatchery. Thermal upwelling appears to be fault controlled. Up-gradient of the hatchery, near-surface groundwater mixes with a larger proportion of thermal groundwater than does deeper groundwater. As these mixed systems flow toward the hatchery, a major locus of groundwater discharge, they are segregated into confined and unconfined compartments. Our study requires integration of hydrological, geochemical, and geophysical strategies in order to understand a complex natural hazard and thus may serve as a model for other similarly complex hydrological environments.