Transit times link pollution sources to drinking water quality in a "One Water" system

Area of Science:

• Environmental Science
• Hydrology
• Water Quality Management

Background:

• Communities increasingly rely on recycled wastewater and urban stormwater for drinking water (One Water systems).
• Managing water quality in these systems presents chronic and emerging challenges.
• Effective source tracking and transformation rate analysis are crucial for water safety.

Purpose of the Study:

• To apply unsteady transit time theory to a large One Water system (Occoquan Reservoir).
• To link pollution sources to reservoir water quality using hydrologic and water quality data.
• To evaluate the model's predictive skill and parameter parsimony.

Main Methods:

• Utilized 11 years of hydrologic and water quality data from the Occoquan Reservoir.
• Applied amended unsteady transit time theory accounting for upstream and distributed inflows.
• Employed Bayesian Markov Chain Monte Carlo for parameter estimation and Bayesian Information Criterion for model evaluation.

Main Results:

• The model successfully identified distinct sources and transformation rates for nitrate, sodium, and chloride.
• High predictive skill was achieved with a minimal number of parameters (NSE = 0.65-0.76 for sodium/chloride, NSE = 0.55 for nitrate).
• Seasonal denitrification linked to stratification and hypolimnetic processes was captured for nitrate.

Conclusions:

• Unsteady transit time theory provides a parsimonious and effective framework for managing One Water systems.
• The model's simplicity facilitates rigorous evaluation and supports stakeholder engagement.
• This approach aids in developing collaborative solutions for complex water quality challenges.