Publication Summary

A Total Maximum Daily Load (TMDL) study was conducted to evaluate dry season water quality in the Black River, a tributary of the Chehalis River. Past studies have documented low dissolved oxygen (D.O.) levels during the summer in the mainstem Black River between Littlerock and Rochester. The Black River and tributaries were evaluated for loading sources and other physical, chemical, and biological river conditions that might contribute to the oxygen deficit.

A link to the appendices for this report is provided below.

(Previous to November 2003, the title to this report was: Black River Dry Season Total Maximum Daily Load .)

A Total Maximum Daily Load (TMDL) study was conducted to evaluate dry season water quality in the Black River, a tributary of the Chehalis River. Past studies have documented low dissolved oxygen (D.O.) levels during the summer in the mainstem Black River between Littlerock and Rochester. The Black River and tributaries were evaluated for loading sources and other physical, chemical, and biological river conditions that might 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 of the middle Black River (RM 9.6 to RM 15.3) during the summer months, with hypoxic and anoxic conditions in the hypolimnetic layer. Dissolved oxygen below the water quality criterion of 8.0 mg/L was widespread in surface waters of the Black River. Chlorophyll a levels indicated conditions ranging from mesotrophic to eutrophic. Temperatures in surface waters often exceed the water quality criterion of 18.0°C.

Based on a modeling analysis using the WASP5 water quality model, D.O. for background conditions was found to be less than the water quality criterion of 8.0 mg/L. Dissolved oxygen under existing conditions in some locations is degraded compared to background conditions. Also, total phosphorus (TP) is close to levels that produce eutrophic conditions, which would degrade both D.O. and aesthetic water quality standards.

A TMDL is recommended for TP, for oxygen-demanding materials (BOD), and for temperature. The TP loading capacity (LC) is defined as an instream criterion of 0.05 mg/L, which applies to waters of the mainstem Black River from the surface to two meters depth between RM 9.6 and RM 15.3 during the period May 1 to October 31. The BOD LC is defined as an antidegradation criterion of no significant degradation of D.O. due to any loading source or combination of loading sources as compared to existing or background levels. The BOD LC applies to the mainstem Black River for the period May 1 to October 31. The temperature TMDL recommends preservation and restoration of the riparian shade canopy on the mainstem Black River downstream from RM 10.0.

Wasteload Allocations (WLAs) and Load Allocations (LAs) are proposed for inclusion in the TP and BOD TMDLs. Implementation strategies are suggested that include modification of existing NPDES permits and improved Best Management Practices to control nonpoint sources. A monitoring strategy is proposed to evaluate the effectiveness of the TMDL and WLA/LAs. Additional studies are recommended to increase understanding of Black River water quality, specifically in the areas of ground water interactions, and aquatic macrophyte dynamics.

Link to EIM data for User Study ID PPIC0001