| Title |
Analysis on the Water Quality Change of Kyeongan TMDL Target Site Using Probability Distribution Models |
| DOI |
https://doi.org/10.15681/KSWE.2025.41.4.256 |
| Keywords |
Kyeongan stream; Water quality; Total maximum daily load; Probability distribution models; Differential entropy |
| Abstract |
This study examined water quality trends in the Kyeongan Stream, which is a tributary of the Paldang Reservoir?a crucial drinking water source for the Seoul metropolitan area. Monitoring focused on a designated Total Maximum Daily Load (TMDL) site and was conducted in two phases: Phase 1 (2005?2011), after TMDL implementation, and Phase 2 (2012?2023), during which water quality showed improvement. To describe water quality variables, ten shifted and truncated probability distribution models were utilized, with least squares estimation demonstrating better goodness-of-fit compared to the method of moments and maximum likelihood estimation. Among the goodness-of-fit tests, the Anderson?Darling test proved to be the most stringent, followed by the chi-square and Kolmogorov?Smirnov tests. The modeling results indicated that electrical conductivity (negatively skewed) was best represented by zero-truncated Weibull or two-sided truncated beta distributions. Dissolved oxygen (slightly positively skewed) was modeled using a truncated beta distribution, while pH was best described by a shifted logistic power model. BOD? and TOC were modeled using shifted generalized exponential or lognormal distributions, and truncated logistic or gamma distributions, respectively. Total nitrogen and total phosphorus were aligned with shifted lognormal or gamma models, and truncated beta or logistic power models. Total suspended solids (TSS) followed shifted Weibull or logistic power distributions. In Phase 2, median, mean, standard deviation, and differential entropy increased for electrical conductivity, pH, and dissolved oxygen, while other parameters showed a decline. These changes were attributed to variations in rainfall (which impacted pH, conductivity, TSS, and DO) and policy-driven improvements (affecting BOD?, TOC, total nitrogen, and total phosphorus). Notably, the strengthened phosphorus effluent standards in 2012 led to significant reductions and altered flow-dependent behavior. |