Differential effects of land use on nutrient concentrations in streams of Pennsylvania

Abstract

Nutrient pollution of surface waters is a widespread problem, calling for regional assessments of water quality conditions. In this study, we quantified long-term median nutrient concentrations of total nitrogen (TN) and total phosphorus (TP) in streams and rivers of Pennsylvania and explored relationships between stream nutrient concentrations and the land use of their watersheds. Our analysis is based on a synthesis of monitoring data from multiple agencies that included records of nutrient concentrations observed between 2000–2019. Across the state, stream nutrient concentrations observed in predominantly undeveloped areas (e.g., forests, shrubs, and grasslands) have median concentration values of 0.42 mg l−1 for TN and 0.011 mg l−1 for TP, reflecting background concentrations for minimally impacted watersheds. Median stream concentrations of TN in agricultural areas are about eleven times higher than in undeveloped areas; and are about five times higher in developed areas than in undeveloped areas. Median stream concentrations of TP in developed areas have about eight times higher concentrations than undeveloped areas; and are about four times higher in agricultural areas than in undeveloped areas. Concentrations of TN and TP increased substantially as the combined percentage of agricultural and developed land use increased. Fragmented data storage practices (e.g. incomplete metadata, ambiguous site names, and missing coordinates) and inconsistencies in monitoring protocols (e.g., differences in constituents measured, parameter names, and measurement methods) made leveraging the secondary use of multiple sources of data challenging. Nonetheless, our integrated dataset is robust, represents the best data available, and provides a new window into the nutrient status of Pennsylvania’s surface waters. The long-term median nutrient concentrations reveal the magnitude of variability in TN and TP concentrations across the state’s diverse environmental settings of land use, physiography, and geology. This information is useful for interpreting additional monitoring data, informing evaluation of water quality conditions, and guiding watershed management.