7.2.6 Ecoregion Historical Phosphorus

Fossil Diatom reconstruction indicate that lakes in this ecoregion historically (1750-1800 AD) have seen total phosphorus concentrations of .022-.026 mg/L.

Fig 7.7 Phosphorous indicator with historical lake Phosphorous concentrations.

See chapters 8 and 9 for site nutrient data and evaluation.

7.2.7 Solution

Phosphorus predominantly reaches surface waters via direct discharge and runoff from land application of fertilizers and animal manure. Once in receiving waters, the phosphorus can become available to aquatic plants. Land-applied phosphorus is much less mobile than nitrogen since the mineralized form (inorganic Phosphate) is easily adsorbed to soil particles. For this reason, most agricultural phosphorus control measures have focused on soil erosion control to limit transport of particulate phosphorus. However, soils do not have infinite phosphate adsorption capacity and with long-term over-application, inorganic phosphates can eventually enter waterways even if soil erosion is controlled.

The shorter the time between phosphorus application and receipt of rainfall, the greater the potential for phosphorus runoff. Similarly, the greater the intensity of rainfall, the greater the potential for phosphorus runoff.

Unless adequate conservation methods are employed, the greater the slope, the greater the potential for erosion and consequently for total phosphorus runoff. Practices such as no-till will reduce total phosphorus runoff, although it may increase the concentration of dissolved phosphorus. Other conservation practices such as terraces, strip cropping, contour farming, etc. will reduce the potential for phosphorus runoff.

One must remember that the magnitude of phosphorus runoff may or may not be important in terms of water contamination. For example, a site that has a high potential for phosphorus runoff may not contaminate water supplies if water from that site travels across soils that have a strong affinity to absorb phosphorus before entering the surface water body.

It is important to limit both the dissolved and the sediment bound phosphorus from entering surface waters.

Phosphorus generated by nonpoint source pollution such as soil erosion is dependent on highly variable and dynamic factors, making it extremely difficult to regulate in a similar fashion as the point source discharges. The amount of phosphorus contributed by soil erosion during any one year is dependent on the soil type, slope, the storm intensities, time between storms, total amount of precipitation, vegetative cover, season, soil phosphorus levels, soil mineralogy and chemistry, frequency of soil cover disturbance, and numerous other natural and human management variables.

7.2.8 Effects of Magnesium on TP Testing

In several tests of ortho phosphorous (OP) and total phosphorous (TP) conducted by RMB Environmental Laboratories in Detroit Lakes Minnesota, OP concentrations tested higher than TP concentrations. This is not possible since OP must be a fraction of TP.

In low oxygen water conditions, such as under ice in winter, Magnesium Oxide compounds can be separated from the soil and become dissolved in the water as Mn +2 . During TP testing the sample is heated to approximately 80 degrees Celsius and converts to Mn +4 . In some instances Mn +7 can also form. This variation of magnesium interferes with the samples color. The sample is treated with chemicals which turns the sample a shade of blue. The shade of blue relates to the TP concentration of the sample by a calibrated rating curve. Since the Mn +4 or Mn +7 interfere with the coloring, TP concentration can appear to be lower than actual.

Phosphorus is used in some applications by city sewer and water plants to remove high magnesium concentrations.

Fig 7.12 Effects of Mn on P testing. Visual representation only, does not show actual chemical reactions

Next page: Chapter 7.3 Chlorophyll-a (algae)