Development of a risk-based TMDL assessment approach using the integrated modeling system GIBSI

Area of Science:

• Environmental modeling
• Water quality assessment
• Risk analysis

Background:

• Water quality standards (WQS) are crucial for protecting water uses.
• Assessing the impact of both point (dry weather) and nonpoint/diffuse (wet weather) pollution sources is complex.
• Total Maximum Daily Loads (TMDLs) require robust assessment methods linking sources to WQS attainment.

Purpose of the Study:

• To develop and present a risk-based TMDL assessment approach.
• To link wet and dry weather pollution sources to the probability of exceeding WQS.
• To evaluate the attainability of WQS for recreational uses under different wastewater treatment and agricultural management scenarios.

Main Methods:

• Utilized the integrated modeling system GIB SI for simulations.
• Conducted a case study involving a small town's wastewater effluent and agricultural nonpoint source (NPS) loads.
• Assessed bacteriological and aesthetic impairment, considering different fertilization rates for NPS load reduction.

Main Results:

• Treating wastewater and reducing NPS loads by 27% achieved 100% bacteriological WQS attainment during summer.
• The probability of exceeding aesthetic WQS decreased from 0.32 to 0.19 (30 to 18 days).
• The risk-based assessment approach proved suitable for establishing TMDLs.

Conclusions:

• The developed risk-based TMDL approach effectively links pollution sources to WQS exceedance probabilities.
• Wastewater treatment and targeted NPS load reductions are viable strategies for improving water quality.
• Further evaluation using long meteorological series is recommended for robust probability assessments.