/ California's population increased 25% between 1980 and 1990, resulting in rapid and extensive urbanization. Of a total 123,000 ha urbanized in 42 of the state's 58 counties between 1984 and 1990, an estimated 13% occurred on irrigated prime farmland, and 48% on wildlands or fallow marginal farmlands. Sixty-six percent of all new irrigated farmland put into production between 1984 and 1990 was of lesser quality than the prime farmland taken out of production by urbanization. Factors dictating the agricultural development of marginal farmlands include the availability and price of water and land, agricultural commodity prices, and technical innovations such as drip irrigation systems that impact the feasibility and costs of production, The increasing amount of marginal farmland being put into production could have significant water quality consequences because marginal lands are generally steeper, have more erodible soils, poorer drainage, and require more fertilizer than prime farmlands. Although no data exist to test our hypothesis, and numerous variables preclude definitive predictions, the evidence suggests that new irrigated marginal lands can increase nonpoint source (NPS) pollution for a given size area by an order of magnitude in some cases.Large-scale land use conversion can have significant impacts on both surface water and groundwater quality. These impacts are especially well illustrated in California, where rapid population increase has resulted in conversion of prime agricultural land and undeveloped wildlands to urban uses. Less obvious but more importantly, many marginal lands have been placed into agricultural production with serious water quality implications because the marginal lands are typically more erodible and require greater fertilizer application than prime farmlands.In this study, we review the principal sources of water pollution in California, consider the probable long-term water quality impacts of land use conversion trends, and suggest possible water resource protection strategies.