Nutrient budgets and river impoundments: Interannual variation and implications for detecting future changes

Reduced additions to river phosphorus for three years following implementation of a lawn fertilizer ordinance

“…The object was to hold Type I error reasonably low while seeking a credible level of power to detect environmental changes if they occurred. Statistical power calculations using this model (Ferris and Lehman 2008) indicated that weekly sampling should have a reasonable expectation of detecting 25% changes in DP and TP after 2-3 years. Our a priori expectation was that P concentrations would decrease, so we applied onetailed tests to the P data.…”

“…They were meant to represent a sampling of variables for which there was no a priori expectation of change and that were easy to measure with high precision. We adopted the statistical model by Ferris and Lehman (2008) for balancing type I error against type II error such that α = 0.1 and β = 0.75. The object was to hold Type I error reasonably low while seeking a credible level of power to detect environmental changes if they occurred.…”

“…Four sampling stations were established for this study (Fig. 1) Ferris and Lehman (2008) and is downstream of Ann Arbor at the site where the Huron River discharges into Ford Lake. Station F is also 9.5 km downstream from the outfall of the Ann Arbor wastewater treatment facility (AAWWTP); stations A and B are upstream of the outfall.…”

“…The SRP was measured as molybdate-reactive phosphate in filtrate; DP and TP were measured as SRP after first oxidizing filtrate (DP) or unfiltered water (TP) with potassium persulfate at 105 C for 1 h. Also measured were specific conductance at 25 C (K 25 , µS) and colored dissolved organic matter (CDOM) as UV absorbance at 254 nm. Ferris and Lehman (2008) showed that CDOM correlates strongly with both dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in the Huron River. All nutrient analyses were performed according to Ferris and Lehman (2007).…”

“…The prediction was made by estimating the lawn fertilizer runoff from one creek-shed within the city and extrapolating that result to all other city creek-sheds. Statistical modeling (Ferris and Lehman 2008) using historical Huron River water quality data indicated that reductions of that magnitude for total P (TP) could be detected within 2 years by sampling 4 times per month. Detection at similar percentage reductions in dissolved P (DP) was predicted to take 2-3 years, and for soluble molybdate-reactive P (SRP) as long as 8 years.…”