Using Solute Age Distribution to Assess the Potential for Future Degradation of Groundwater Quality and to Prioritize Remediation Activities

A groundwater sample contains a collection of water and solute molecules that entered the aquifer at different times. The distribution of times that the solute molecules entered the aquifer is called a solute age distribution. Assuming ergodicity, a solute age distribution is equivalent to the backward travel time probability density functions (PDF) between the sample location and the land surface. A backward travel time PDF represents the random travel time from an upgradient point of interest (e.g., the land surface) to the sample location, and can be calculated efficiently by solving the adjoint of a forward transport equation. The solute age distribution can be used to assess the potential for future degradation of groundwater quality, and to prioritize remediation activities. We calculate solute age distributions for a stream in Colorado that has been contaminated due to mining activity. We use the solute age distribution to prioritize mine sites for remediation. The highest priority is given to the site whose remediation would lead to the most substantial improvement in the water quality of the stream.