Excess nutrients resulting in eutrophication of surface waters has become one of the greatest water quality challenges of our time. The development of an effective nutrient management strategy is essential to protecting surface water quality, public health, aquatic ecosystems and economic interests. The complexity of cultural eutrophication and the influence of nutrients, especially in streams and rivers, has delayed the development of an effective regulation and a nationwide management strategy. Variations in hydrologic conditions, geology and both urban and agricultural land use can dramatically influence phosphorus loads to receiving waters. Furthermore, several complex mechanism exist within a river or stream (e.g. the phosphate buffer, light availability, hydraulic retention time, phosphorus spiraling, etc.) that change the concentration and impact of nutrient concentrations and resulting eutrophication. Temporal and spatial variations result in changing and often imprecise threshold between healthy and unhealthy ecosystems.

For this reason, it is important for policy makers to understand how the assimilative nutrient capacity of waterways varies with environmental, seasonal and loading conditions, and that it is not the same for every watershed or even within the same waterway. A one-size-fits all technology solution or a state-wide numeric standard that does not account for these variations is misguided and will result in costly upgrades with minimal improvements to water quality. The most efficient nutrient management method is one that best matches the nutrient load delivered with the maximum assimilative capacity of the receiving water.

This study provides an in-depth analysis of the Cache la Poudre River Watershed in Northern Colorado over the course of a year to examine the influence of different sources, transport pathways and hydrologic regimes on phosphorus concentrations along an urban-agricultural gradient. An extensive and comprehensive design of sampling locations was used to best capture the anthropogenic influence (e.g. wastewater treatment plants, concentrated feeding animal operations, land uses) and transport pathways (e.g. irrigation ditches, overland transport, streams and rivers) of phosphorus within the watershed. Exploratory models were used to better understand the influence of geospatial variables on the occurrence and transport of phosphorus within the watershed.

The influence of phosphorus from wastewater treatment plants (WWTPS) to the Cache la Poudre River was examined in detail. A mass-balance of the phosphorus load in the river and the effluent from WWTPs was used to best estimate the influence of WWTPs. Projections of the influence proposed regulations that reduce WWTP effluent concentrations were made as well as the resulting impact to the river and water quality. The role of sediment was investigated to better characterize and explain the temporal variations of phosphorus concentrations within this complex system. A brief economic analysis and associated improvements to water quality are discussed as well as effective management strategies in the Cache la Poudre River Watershed.

The objective of the study is to aid in the development of an efficient and effective nutrient management strategy for the Cache la Poudre River Basin and other similar mixed land use watersheds, as well as providing a foundation for creating a decision support system for water quality analysis, monitoring and management.