There are still a number of questions about that one.
Part of it seems to be changing farm practices. Initially we used farm practices where there was a lot of tillage on the land, and that tillage resulted in the soil particles being loosened. When we had a heavy snowfall and that melted or we had a rain event, these heavy particles were carried into the streams, the soil particles with the phosphorus fairly tightly bound onto them.
In some cases, because we've changed our farming practices to what's now referred to as “no till”, we have more liquid forms of phosphorus, these dissolved ones seeping.... That's not to say that's the only source of dissolved phosphorus, because a lot of what comes in from pipes, be they industrial or sewage, is also in dissolved form.