Publication Summary

A Total Maximum Daily Load (TMDL) study was conducted to evaluate dry season water quality in the Upper Chehalis River (from the Porter bridge upstream). Past studies have documented areas of low dissolved oxygen (D.O.) during the summer in the "Centralia reach" of the Chehalis River (between the Newaukum and Skookumchuck Rivers). The Chehalis River and tributaries were evaluated for loading sources and other physical, chemical, and biological river conditions that contribute to the oxygen deficit.

A Total Maximum Daily Load (TMDL) study was conducted to evaluate dry season water quality in the Upper Chehalis River (from the Porter bridge upstream). Past studies have documented areas of low dissolved oxygen (D.O.) during the summer in the "Centralia reach" of the Chehalis River (between the Newaukum and Skookumchuck Rivers). The Chehalis River and tributaries were evaluated for loading sources and other physical, chemical, and biological river conditions that contribute to the oxygen deficit.

Several surveys were conducted in the study area during the dry seasons of 1991 and 1992. Survey results showed widespread thermal stratification in the Centralia reach during the summer months. Stratification was associated with hypoxic and anoxic conditions in deeper waters. Dissolved oxygen below water quality criteria was widespread in surface waters and tributaries of the Chehalis River. Temperatures in mainstem and tributary surface waters often exceed the water quality criterion of 18.0°C. Violations of fecal coliform bacteria criteria were also found in the mainstem and some tributaries.

Modeling analysis using the WASP5 water quality model predicted that D.O. for existing critical conditions would fall below water quality criteria over much of the mainstem study area in both mid-summer and early fall. When loading was reduced to background levels (no point source and significant nonpoint loading), modeling predicted D.O. below criteria during mid-summer in the upper study area (Ceres Road to the Newaukum River) and in the lower study area (Galvin Road to Porter), and during early fall in the Centralia reach.

Loading capacities (LC) for carbonaceous biochemical oxygen demand (CBOD) and ammonia nitrogen (NH3-N) were evaluated, based on meeting D.O. water quality criteria where background conditions met criteria, and allowing no significant degradation of D.O. where background conditions fell below criteria. A phased TMDL is recommended for CBOD and NH3-N during the period May 1 to October 31. Wasteload Allocations (WLAs) are proposed for the existing Pe Ell discharge and the proposed Grand Mound discharge. No CBOD or NH3-N loading capacity above background exists in the Centralia reach above the Mellen Street bridge. Two potential WLA alternatives are proposed for either a reduced Centralia discharge, or for a combined discharge for Centralia, Chehalis, and Darigold between Mellen Street and Galvin Road. Load Allocations (LAs) are proposed for existing levels of nonpoint sources from Galvin Road to Porter. No LAs above background are proposed for nonpoint sources upstream of the Galvin Road bridge and for future growth. The phased TMDL will allow the reassessment of the TMDL after implementation of nonpoint source controls.

Implementation strategies are suggested that include modification of existing NPDES and state permits to meet WLAs and LAs. Best management practices should be implemented to control nonpoint sources, particularly for livestock impacts and for urban stormwater from Galvin Road upstream. A monitoring strategy is proposed to evaluate the effectiveness of the TMDL. Additional studies are recommended for the following: eutrophication in the mainstem Chehalis River and Scatter Creek, ground water interactions, and sediment oxygen demand.

Link to EIM data for User Study ID PPIC0002