This report presents the results of a service-area wide investigation conducted by the South County Regional Wastewater Authority (Authority) to identify the sources of Total Dissolved Solids (TDS) in the municipal effluent. This investigation is in response to the Central Coast Regional Water Quality Control Board's (Regional Board) concerns with increasing TDS concentration trends in the shallow groundwater monitoring wells located at the Authority's Gilroy-Morgan Hill Wastewater Treatment Plant (WWTP). The goal of this effort is to identify the steps necessary for the Authority to achieve compliance with the revised municipal effluent TDS discharge limit of water supply TDS concentration + 370 mg/L for the new treatment plant. The previous municipal effluent TDS discharge limit was water supply TDS concentration + 425 mg/L. -- Executive summary. 1. Introduction -- 2. TDS investigation -- 3. Best management practices -- 4. Findings and recommendations -- Appendix A. TDS work plan, April 14, 1993 -- Appendix B. Total dissolved solids study, July 1994 -- Appendix C. TDS investigation proposal, November 1, 1994; digital conductivity and TDS meter specifications -- Appendix D. TDS investigation data summary; TDS investigation flow summary; TDS investigation sampling data sheets -- Appendix E. Water softener information -- Appendix F. Best management practices.