Tuesday, December 27, 2005

Phosphorus and TMDL

Today I am pulling together conceptual information related to phosphorus (P) and Total Maximum Daily Load (TMDL) in anticipation of meeting with Walt Edelen and Rick Noll, water quality folks with the Spokane County Conservation District (SCCD). The problem we will be putting our heads together on is the relationship between river and stream P levels (ug/l) relevant to TMDL and soil P levels (mg/kg) in adjacent, contributing areas. More specifically, we are going to lay out what methods and approaches are available for gathering soil P information that will produce data that can be used to measure P loading, measured in pounds of phosphorus, lost to the river. We want tools to quantify the effects of implementing various Best Management Practices (BMPs) for controlling streambank sluffing and farm field erosion.

The motivation for specifically defining pounds of phosphorus delivered has to do with the concept of phosphorus load allocation. It has to be conceptual, rather than actual, because, when it comes to soil contributions, we don't have the studies needed to characterize the complex causes and effects in these systems. Nor do we have the luxury of time of waiting for the studies to be designed, funded and conducted. The TMDL beast is at the door.

At first glance, it seems like a simple question of mass balance. How many tons of soil are discharged to the river, where and when, and how much phosphorus did it have in it. A mass balance approach certainly makes sense for wastewater dischargers, considered the most significant source of P in the Spokane river. But that approach doesn't fit as easily for soil. How much soil phosphorus converts to water phosphorus. It can't be 100% efficient, some will end up dissolved in the water, some will end up involved with the suspended sediment load but the rest will end up on the stream bottom where it will contribute P to water only over a long period of time. Complicating this is seasonal changes in capacity of a stream to carry suspended sediment. Clean water is hungry water and it picks up material fairly easily. Muddy water is different and it drops it's sediment load whenever it loses energy, the insides of curves for instance. You can bet that sediment delivered to a clean stream is going to affect total phosphorus levels more efficiently than sediment delivered to a muddy stream. Considerations of efficiency mean testing for available phosphorus may not be enough. The next most obvious analytes to add into the mix are total phosphorus, texture, pH and organic matter content. I'm looking forward to a lively discussion.

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