San Diego is known for its temperate Mediterranean climate and beaches; however, human activities that compromise water quality degrade environmental conditions and impose economic and social costs. Urban runoff is one of the most serious threats to San Diego's coastal water quality, contributing to harmful algal blooms, low dissolved oxygen concentrations and impacts on economically important marine and estuarine taxa. Nutrients and toxins originate from residential, agricultural and industrial sources, and the land usage within watersheds determines their loadings. Economic development that improves human well being may either benefit water quality or further contamination of coastal ecosystems. The purpose of this study was to identify relationships between chemical and isotopic indicators of water quality and land usage and watershed socioeconomics composition. I found that areas with low socioeconomic status (SES), mostly in the southern watersheds, exhibited lower water quality scores based on composite indices. Interestingly, coverage of open, developed, impermeable or agricultural land uses was not directly correlated with water quality as I expected. Overall, water quality declined in southern watersheds where residents have the poorest socioeconomic status. My thesis suggests that anthropogenic activity and low socioeconomic status contribute to poor water quality and contamination of coastal environments. The role of economic development in transforming land use and affecting water quality needs to be evaluated in relation to other hydrologic factors that determine export of nutrients from watersheds to the ocean.