Phosphorus (P) is a labile naturally occurring element. It is one of a few essential elements required for the health and growth of many plants and animals; both terrestrial and aquatic. P comes in many forms, and is generally measured in the form of Total Phosphorus (TP) which includes all forms that P in in a given space. TP is broken down into multiple forms including Soluble Reactive Phosphorus (SRP), Particulate Phosphorus (PP), Inorganic Phosphorus, and Organic Phosphorus. The Environmental Protection Agency and other agencies typically measure P concentration of a space through TP in pounds of loading or in concentration of milligrams per liter.
The concentration of P will shift naturally based on a variety of factors. This can include precipitation patterns, agricultural land use practices, and the exposed bedrock of an area. One of the most influential factors is the agricultural land use. Many farms utilize fertilizer to grow healthy and profitable crops. This fertilizer provides extra nutrients to stimulate plant growth including P. Terrestrial plants on average need higher concentrations of P than aquatic plants. This is important in considering another important factor: precipitation patterns. The rainfall patterns over an agricultural area will result in run off to local tributaries within the watershed and the run off from fertilizer will wind up in those streams and creeks carrying high concentrations of P. Ultimately, within the Red Cedar Watershed, these P concentrations will wind up in the impoundment of Lake Menomin.
The EPA recommends a concentration of P to be 0.1 mg/L in a healthy stream. Of the two streams studied this summer, Tiffany Creek on the west side of the watershed hovers at around this concentration for most of the summer months with occasional spikes with increases in Total Suspended Solids (water’s cloudiness) after a rain event. 18 Mile creek on the east side of the watershed is more than double this recommendation. However, the EPA recommends streams and tributaries that flow to an impoundment to be half of this concentration at 0.05 mg/L which neither creek come close to. These are just two of many tributaries to the Red Cedar River. To get a full picture of where these creeks measure in concentration of P, additional studies should be done. They will be helpful in determining geographically where the largest concentrations of P are coming from and by determining the source, may lead to finding the most effective way to mitigate it.
The 2013 predictive P sources and their proportions mentioned the following categories: cropland, forest, barnyards, grassland, point sources, and urban. This left no potential source of the naturally occurring P of the bedrock that the streams are running over and constantly interacting with. This is a source of P that will always be present and cannot realistically be prevented or reduced. As a result, the man-made contributions of P concentrations need to be reduced even further to make up for what can’t be reduced by the natural sources. These reductions are made through best management practices and continued policy work that reflects the research being done in the area. A Total Maximum Daily Load of P needs to reflect and be proportionate to the catchment size of sub-watersheds, the bedrock of the area, the topography and precipitation patterns, and the land use of the surrounding area. These factors and the research surrounding them need to be consciously considered in determining a TMDL that is realistic and achievable for the area. Having a one-size-fits-all TMDL will result in sporadic changes in different parts of the watershed as reductions in some areas will be very effective where in other areas, the equilibrium concentration of P will prevent the reductions from being very effective. Thus, the TMDL and subsequent reductions need to be reflective of the area its trying to assist. Additional studies done by my colleagues determining P concentrations of rain water and base flow and EPC (Equilibrium P Concentrations) of these waters when they interact with various bedrock will need to be continued in order to help propel policy work forward.