Analysis of Phosphorus Load in Sediment Collected from the Laguna de Santa Rosa Watershed

Abstract

Although phosphorus is a crucial element in plant growth, an excess of phosphorus in riparian ecosystems can lead to algal blooms and eutrophication. Due to an accumulation of phosphorus in the Laguna de Santa Rosa over the past century, the current allowable amount of phosphorus that can be discharged into the Laguna de Santa Rosa is zero. Wastewater treatment plants in Santa Rosa and Windsor do not have the capacity to completely remove phosphorus from their effluent, which they discharge into the Laguna de Santa Rosa during unusually high rain events.

A new Water Quality Trading Credit program allows the cities to purchase phosphorus credits from Sonoma Water when needed. To generate these credits, Sonoma Water removes large volumes of phosphorus-rich sediment from the watershed. In addition to generating water quality trading credits, sediment removal projects provide an opportunity to improve flood control channel morphology to improve habitat for native species.

In 2019, 15,000 phosphorus credits were created by removing 11,104 cubic yards (1,388 truckloads) of sediment from Laguna de Santa Rosa at Stony Point Rd in in Rohnert Park. One credit (1lb phosphorus) had a value of approximately $50, therefore generating ~$750,000 in water quality trading credits.

Roseland Creek and Colgan Creek in Santa Rosa are candidates for future sediments removal projects to generate additional trading credits. We have taken samples of sediment from Roseland Creek, Colgan Creek, the Laguna de Santa Rosa, and from Copeland Creek in order to determine the relative levels of phosphorus concentrations in different channels to allow Sonoma Water to determine which site could potentially remove the most phosphorus.

Results

Key words:  sediment, phosphorus, urban, water trading credits, Copeland Creek, creek maintenance