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a Synagro West, Inc., Box 7027, Corona, CA 92878-7027
b College of Forest Resources, Box 352100, Univ. of Washington, Seattle, WA 98195
* Corresponding author (mgrey{at}synagro.com)
Received for publication February 14, 2001. Municipal biosolids are typically not used on the steepest of forested slopes in the U.S. Pacific Northwest. The primary concern in using biosolids on steep slopes is movement of biosolids particles and soluble nutrients to surface waters during runoff events. We examined the pattern and extent of P and N runoff from a perennial stream draining a small, forested 21.4-ha watershed in western Washington before and after biosolids application. In this study, we applied biosolids at a rate of 13.5 Mg ha-1 (700 kg N ha-1 and 500 kg P ha-1) to 40% of the watershed following nearly 1.5 years of pre-application water sampling and 1.5 years thereafter. There was no evidence of direct runoff of P or N from biosolids into surface water. Elevated surface water discharge did not change the concentration of PO4P, biologically available phosphorus (BAP), bioavailable particulate phosphorus (BPP), or total P nor did it affect the concentrationdischarge relationship. Some instances of total P concentrations exceeding the USEPA surface water standard of 0.1 mg L-1 were observed following biosolids application. However, total P in 27 Creek was predominately in particulate form and not labile, suggesting that detritus moving into the main creek channel and ephemeral drainage courses may be the principal P source. Ammonium N concentrations in runoff water were consistent before and after biosolids application, ranging from below detection limits (0.01 mg L-1) to 0.1 mg L-1; no concentrationdischarge relationship existed. Biosolids application changed the 27 Creek concentrationdischarge relationship for NO-3N. Before application, no relationship existed. Beginning nine months after biosolids application, increases in discharge were positively related to increases in NO-3N concentrations. Nitrate concentrations in runoff following biosolids application were approximately 10 times less than the USEPA drinking water standard of 10 mg L-1.
Abbreviations: BAP, biologically available phosphorus BPP, bioavailable particulate phosphorus PNW, Pacific Northwest
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